TECHNICAL BULLETIN SAFE HANDLING & STORAGE OF STYRENE MONOMER

DISCLAIMER

Sterling Chemicals Inc., 2000THE INFORMATION AND RECOMMENDATIONS SET FORTH HERIN ARE BEING PROVIDED AS AN ACCOMODATION ONLY. OF THE DATE HEREOF, STERLING, ALTHOUGH THE INFORMATION AND CHEMICALS INC. MAKES NO RECOMMENDATIONS SET FORTH HEREIN ARE BELIEVED TO BE CORRECT AS REPRESENTATION AS TO THE COMPLETENESS OR ACCURACY OF SUCH INFORMATION AND RECOMMENDATIONS. RESULTS OF THE APPLICATION FURTHER, SUCH INFORMATION AND OF SUCH INFORMATION AND RECOMMENDATIONS ARE BY NECESSITY OF A GENERAL NATURE AND THE RECOMMENDATIONS TO SPECIFIC FACTS AND CIRCUMSTANCES MAY PRODUCE VARIED RESULTS. ACCORDINGLY, YOU ARE STRONGLY URGED TO CONSULT WITH EXPERTS WITH RESPECT TO THESE MATTERS PRIOR TO APPLYING ANY OF SUCH INFORMATION OR RECOMMENDATIONS TO YOUR PARTICULAR FACTS OR CIRCUMSTANCES. NOTHING CONTAINED HEREIN IS TO BE CONSTRUED AS A RECOMMENDATION TO USE ANY PRODUCT IN CONFLICT WITH ANY PATENT. STERLING CHEMICALS INC. SHALL IN NO EVENT BE RESPONSIBLE FOR ANY DAMAGES OR LOSSES THAT MAY RESULT, DIRECTLY OR INDIRECTLY, FROM THE PUBLICATION OR USE OF OR RELIANCE UPON SUCN INFORMATION AND RECOMMENDATIONS. NO WARRANTY, EITHER EXPRESSED OR IMPLIED, OF MERCHANTABILITY OR FITNESS FOR PARTICULAR PURPOSE OR OF ANY OTHER NATURE WITH RESPECT TO ANY PRODUCT REFERRED TO, OR WITH RESPECT TO THE INFORMATION AND RECOMMENDATIONS CONTAINED HEREIN, IS MADE HEREUNDER.

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INDEX

Introduction Section A

Introduction Safety Health Hazards Fire and Explosion Hazards Polymerization Hazard References

Section B

Styrene Monomer Storage and Handling Quality Problems in Storage and Shipping Inhibition of Styrene Polymerization and Oxidation Storage Temperature Design of Styrene Storage Facilities Special Operating Practices in Styrene Storage & Transfer Facilities Sampling & Analytical Techniques Maintenance & Inspection Procedures Environmental Considerations Environmental Regulations

Appendix B-1 Appendix B-2 Appendix B-3 Section C Section D

Low Pressure Gas Blanket Control System Specs for Coating Tanks with Catalyzed Epoxy for Styrene Monomer Service Analytical Methods for Color, TBC, & Polymer in Styrene Monomer (SM) Physical Properties of Styrene Monomer Chemistry of Styrene Inhibition & Oxidation

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INTRODUCTION

Styrene, C6H5CH=CH2, is one of the most important monomers produced by the chemical industry today. World production exceeds 22 million metric tons. Styrene undergoes polymerization by all the common methods used in plastics technology to produce a wide variety of polymers and copolymers. The most important of these are polystyrene, rubber-modified impact polystyrene, styrene-acrylonitrile copolymer (SAN), acrylonitrile-butadiene-styrene terpolymer (ABS), and styrene-butadiene copolymer (SBR synthetic rubber). Other important uses of styrene are found in styrene-polyester resins, in latexes, and in styrenated oils and alkyls. The number of producers and users of styrene continues to increase. This increase has emphasized the need for a practical, readily available source of information describing the chemical and physical properties, the storage and shipping requirements, and the safety requirements of this relatively safe but perishable chemical. Although styrene is not considered to be a particularly hazardous substance, its reactivity is such that it must be handled within a range of specially prescribed conditions. This is necessary not only to avoid certain safety hazards but also to prevent deterioration in the quality of the styrene. However, it has been well-demonstrated that styrene, like any other chemical, can be used, handled, and stored without difficulty when its physical and chemical properties are understood and the precautions associated with these properties are observed.

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SECTION A SAFETYStyrene is not considered to be a serious industrial hazard. As with any chemical, however, its hazardous properties must be recognized and all necessary precautions must be taken to avoid accident and injury. The toxic, flammable, explosive and chemical properties of styrene are discussed in this section, along with both preventive and remedial steps to be taken with regard to its potential hazards.

HEALTH HAZARDSThe toxicity of styrene monomer is relatively low. Observance of a few fundamental precautions with respect to ventilation, protective equipment, and good engineering/work practices is normally all that is needed to prevent personnel injuries. Effects of Styrene on the Body The seriousness of exposure of body tissues to styrene vapor or liquid depends upon which region of the body is affected as well as upon the degree and duration of exposure.

InhalationOccupational Safety and Health Administration (OSHA) regulations on exposure to styrene vapors are found in the current issue of Title 29 Code of Federal Regulations 1910.1000. The current American Conference of Governmental Industrial Hygienists' (ACGIH) publication entitled "Threshold Limit Values (TLVs) for Chemical Substances and Physical Agents and Biological Exposure Indices" is an additional resource in determining what conditions should be maintained in an operating environment. 700 ppm. Concentrations of styrene above 400 ppm cause moderate irritation to the eyes and respiratory tract. Concentrations as great as 10,000 ppm depress the central nervous system, severely irritate the lungs, and can cause death within 30 to 60 minutes. However, the low odor threshold (0.15 ppm) and irritating quality of styrene vapor make it unlikely that anyone would inhale toxic quantities unless one were unable to escape from a vapor-filled space. The current (1998) OSHAThe current (1999) ACGIH TLV for styrene is 20 ppm and ACGIH Short Term Exposure Limit (STEL) is 40 ppm. The NIOSH IDLH is

Oral IntakeStyrene taken into the alimentary canal causes severe irritation of the mouth, esophagus, and stomach.

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EyesLiquid styrene and concentrated styrene vapor both cause severe eye irritation. permanent damage to the eyes by styrene has been reported. However, no

SkinStyrene contacting the skin for short periods may cause some irritation, while long exposures can cause severe irritation and blistering. Repeated contact with the skin over extended periods may cause a form of dermatitis normally associated with exposure to aromatic hydrocarbons.

Systemic EffectsThere have been no reported systemic effects in workers who have been exposed repeatedly to low concentrations of styrene. Mandelic acid or phenylglyoxylic acid in urine and styrene in blood or exhaled air are the biological indicators used in detecting styrene exposure.

First AidFirst aid measures are those, which may be taken in an emergency by non-medical personnel before regular medical aid can be obtained. If a patient has had only minor exposure to styrene, it is only necessary to move the patient to fresh air and keep the patient comfortably warm. However, if the exposure to styrene liquid or vapor has been severe or prolonged, further action is required to minimize injury or to preserve the life of the patient.

UnconsciousnessRemove the patient to fresh air. Lay the victim on back or side and remove any foreign objects from victim's mouth. If victim is not breathing, initiate artificial respiration immediately. Call a physician as soon as possible. Describe the nature of the case and follow physicians instructions. If a person trained in the use of oxygen is available, that person should begin administration of 100% oxygen immediately to prevent pulmonary edema. CAUTION: OXYGEN SHOULD NOT BE GIVEN BY THE UNTRAINED. If styrene has been swallowed, give two glasses of milk or water after the patient regains consciousness. CAUTION: NEVER INDUCE VOMITING.

Inhalationa. b. c. d. Remove the patient to fresh air. If the patient is conscious, place victim on back, with head level, and keep the victim comfortably warm. Call a physician immediately. instructions. Give the patient oxygen to facilitate breathing. This is particularly important if the victims breath is shallow or if the patient exhibits signs of cyanosis (blue lips, ears, or fingernails). However, oxygen should only be administered by someone trained in its use. Tell the nature of the case and follow physicians

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Oral Intakea. b. c. If the patient is conscious, give two glasses of milk or water. CAUTION: DO NOT INDUCE VOMITING. Call a physician immediately. instructions. Keep the patient lying down and keep victim warm and comfortable. Tell the nature of the case and follow physician's

Eyesa. b. Irrigate the eyes with clean running water for at least fifteen minutes. The eyelids should be held open to permit the water to contact both eyes and lids. Call a physician. Tell the nature of the case and follow physicians instructions.

Skina. b. c. Remove any styrene-contaminated clothing, shoes, etc. Wash the exposed area thoroughly with soap and water. Repeat until the irritation abates. Call a physician. Tell the nature of the case and follow physicians instructions.

Preventative MeasuresExposures and injuries can best be prevented by thorough personnel training, by furnishing and requiring the use of necessary protective equipment, and by using proper engineering practice both in the design and operation of industrial units.

TrainingEmployees who work in areas where styrene monomer is manufactured, handled, or stored should receive instruction in (1) the hazardous properties of styrene, (2) t e requirements for h proper protective equipment and its use, and (3) first aid measures required after exposure to styrene liquid and vapor.

Protective EquipmentNormally the type of protective equipment used should be governed by the type of hazard that exists. a. b. c. Safety goggles or face shields should be used wherever it is likely that styrene might be splashed into the eyes. Gloves and aprons or rainsuits made of chemical-resistant synthetic rubber or non-soluble plastic should be used when there is danger of styrene contacting skin or clothing. Shoes or boots of chemical-resistant synthetic rubber should be worn where there is danger of wetting the feet with styrene. Styrene passes easily through leather and can cause blisters on the skin of the feet. d. Respiratory protection is needed for entering areas having high concentrations of styrene vapor. An industrial organic vapor canister-type gas mask with half face-

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piece can be used in atmospheres containing 200 ppm (10 X current TLV) or less (by volume) styrene and at least 19.5% (by volume) oxygen. A full face cartridge However, a respirator can be used in atmospheres containing 1000 ppm (50 X current TLV) or less (by volume) styrene and at least 19.5% (by volume) oxygen. piece, must be used under the following conditions. when exposure time may be prolonged when the styrene concentration exceeds 1000 ppm (50 X current TLV) when the oxygen concentration in the air may be less than 19.5 volume-percent when the styrene or oxygen concentrations are uncertain. supplied-air respirator or self-contained bottled-air breathing apparatus, with full face-

NOTE: The explosive range for styrene in air is 1.1 to 6.1 percent by volume. No one should enter an explosive atmosphere except when it is necessary in order to sa ve a life.

Engineering Control of Health HazardsThe proper design and operation of equipment used in handling styrene can virtually eliminate hazards to employees' health.

VentilationVentilation in all normal work areas, whether natural or forced, should be adequate to keep the atmospheric concentrations of styrene vapor as low as is practical. Consult the current American Conference of Governmental Industrial Hygienists publication entitled, "Industrial Ventilation" to determine what conditions should be maintained in the operations. Local exhaust ventilation is preferred over pumping air in or relying on natural air currents. systems provide good airflow away from the worker and work area. Exhaust intakes should be located as close to the floor as possible since styrene vapor is denser than air. Forced ventilation

Tank and equipment cleaning and repairDuring the cleaning and repair of tanks and other equipment used for styrene monomer, all standard safety practices applicable to volatile, flammable hydrocarbons should be followed, including: Disconnecting or blinding pipelines into the tank or equipment Locking electrical switches and taking precautions to avoid accidental starting of any moving parts in or near the tank Emptying the liquid monomer and freeing the tank of all styrene vapor. Since polymer inside tanks will continue to evolve styrene vapor, it first must be hardened with steam or circulating air and then removed by chipping. This is especially important if welding is to be done on the tank. Providing adequate ventilation while personnel are in the tank, ensuring adequate oxygen content and styrene levels as low as possible, but below 10 ppm (one-half of the current TLV). Refer to applicable regional regulations.

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Providing and requiring use of all necessary safety equipment, including personal protective equipment.

Spillsany personnel splashed by styrene liquid or affected by styrene vapor should be given medical attention. Only personnel with adequate protective equipment should stay in a spill area.

Fire and Explosion HazardsStyrene monomer is a flammable liquid and its vapor may form explosive mixtures with air under ambient conditions. The same precautions that are taken with other flammable organics must be taken with styrene.

Flammability and Explosive PropertiesSome physical properties pertaining to the flammability of styrene are: Autoignition temperature Flashpoint (Tag. closed cup) Flashpoint (Tag. open cup) 1.1-6.1 volume-percent 490C (914 F) 31C (88 F) 37C (98 F)

Flammability (explosive) limits in air at atmospheric pressure

The approximate explosion limits of styrene in air at different pressures and temperatures are given in the following Figure A-1. These limits were established using a method similar to that previously described in the literature.1

1

R.H. Boundy and R.F. Boyer, Styrene: Its Polymers, Copolymers, and Derivatives, Reinhold, New York, 1957, pp. 51 and 196.

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Figure 1-A

PREVENTION OF FIRE AND EXPLOSIONPrecautions must be taken to avoid producing combustible or explosive mixtures. Even more

stringent safeguards should be used to prevent ignition of the mixtures if they are formed. Formation of hazardous mixtures can be minimized by: a. b. c. d. e. f. Utilizing inert gas blanketing in storage tanks and other process equipment. Providing adequate ventilation in all areas where styrene is handled. Gas-freeing styrene tanks before welding on them. which can continue to evolve styrene vapor. Inspecting equipment regularly to prevent leaks and spills. Repairing leaks as they are found. Removing spills rapidly. This includes removal of polystyrene,

A large styrene spill into an enclosed, paved area may be removed with a vacuum truck. If the area is unpaved, water flooding may be used to float up the styrene to facilitate its recovery and prevent its soaking into the ground. non-reactive absorbent. Ignition of hazardous mixtures should be avoided by the following precautions: a. b. c. d. Locate styrene-handling equipment away from open flames, local hot spots, and other sources of ignition. Protect vents in the storage system with flame arrestors. Avoid the use of any equipment or tools that may produce sparks. Styrene storage tanks should be well grounded to prevent build-up of static electricity. In let lines should discharge beneath the liquid surface and be in electrical contact with the tank. The interior of tanks should never be completely coated with epoxy or other non-conducting coating. charge. e. Construction and location of styrene storage tanks should conform to standards set by the National Fire Protection Association (NFPA No. 70) as well as to any local codes. The tank bottom and lower walls should be left uncoated to drain off electrical Other spills may be washed away with large amounts of water into a waste treatment facility. Small spills may also be taken up by sand or some other

FIRE FIGHTINGFires involving styrene can be combated with foam, dry chemicals, carbon dioxide, and water fog. Normal water streams are not effective because of the immiscibility of water and styrene. Foam should not be used in the proximity of electrical equipment because of shock hazard. If fire occurs near equipment containing styrene, the equipment should be sprayed with streams of water to keep it cool.

POLYMERIZATION HAZARD

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Styrene may undergo polymerization, which, if it becomes rapid and uncontrolled, can be dangerous.

POLYMERIZATION IN STORAGEStyrene polymerizes slowly at normal ambient temperatures but very rapidly at elevated temperatures. Polymerization can take place in storage as well as under more controlled of the monomer will conditions. The polymerization process is exothermic, evolving 288 Btu/lb. (17.8 kcal/gm-mole). If this evolved heat cannot be dissipated rapidly enough, the temperature rise. During a runaway polymerization, the temperature will reach and exceed the boiling point of styrene. The vapor may erupt violently from the tank vents or, if the vents are plugged or too small, it can create enough pressure to rupture the tank. As the liquid polymerizes and becomes more viscous, vapor bubbles may become trapped, expanding the liquid and causing spills or rupture of the tank. If a temperature rise or other indication of rapid polymerization is observed, it is imperative that the styrene be cooled, if possible, by spraying the tank with water or by circulating the monomer through cooling equipment. Inhibitor (4-tert-butylcatechol, usually abbreviated to TBC) should be added to the styrene with thorough mixing. Diluting the monomer with a non-reactive, miscible solvent such as Ethyl benzene or toluene will also tend to quench a runaway polymerization.

Precautions for Handling TBCSolid TBC or concentrated solutions of TBC are acidic and can cause severe burns to the eyes and skin. Dilute solutions (under 25% TBC by weight) may cause an allergic reaction in some people. When handling these materials, personnel should wear goggles or face shields along with gloves and aprons made of chemical-resistant synthetic rubber or non-soluble plastic. Fumes from hot TBC are irritating and should be avoided. If concentrated TBC contacts the skin, remove all contaminated clothing and wash the skin thoroughly with soap and water. If the eyes are affected, wash the eyes and lids thoroughly with clean running water for at least fifteen minutes. Contact a physician for any eye or skin exposure to TBC.

PREVENTION OF POLYMERIZATIONPolymerization in storage may be prevented by close attention to monomer temperature, inhibitor level, and oxygen content Determinations of inhibitor content, polymer content, and monomer temperature should be made on a routine basis. Styrene-containing vessels should be protected from external sources of heat. avoided. Running pumps against closed valves (dead-heading) must be Care should be taken that vents, valves, pressure-relief devices, gauges and controls

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do not become plugged with polymer. (Requirements for the preceding are discussed in detail in the Storage and Handling Section of this publication.)2

2

REFERENCES The current editions of the following should be consulted for more detailed information on safety aspects of handling styrene monomer. Title 29 Code of Federal Regulations 1910.1000 Title 49 code of Federal Regulations 174.67 Material Safety Data Sheet for Styrene Monomer CRC Critical Reviews in Toxicology; Review of the Toxicology of Styrene, Vol. 19, Issue 3 (1989): 227249 National Fire Protection Association No. 30 Flammable and Combustible Liquids Code National Fire Protection Association No. 77 Recommended Practice of Static Electricity Occupational Safety and Health Standards, Occupational Safety and Health Administration of the Department of Labor American Conference of Governmental Industrial Hygienists Threshold Limit Values

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SECTION B STYRENE MONOMER STORAGE AND HANDLINGThe two primary concerns in the storage and handling of styrene are the prevention of accidents and the maintenance of quality in the stored monomer. hazards. 1. 2. 3. 4. 5. 6. 7. These concerns are not incompatible since measures normally taken to maintain styrene quality also serve to lessen many of its Both safety problems and quality problems can normally be avoided by giving careful attention to: The use of inhibitors The oxygen level in styrene The atmosphere over the styrene The storage temperature The design and operation of storage tanks and receiving, transfer, and circulation systems Sampling and analytical techniques Maintenance and inspection procedures

Each of these topics will be discussed in this section. The storage and handling methods and practices described in this section are those considered to be advisable not only for styrene producers but also for styrene users who wish to minimize degradation of their monomer in storage. This information should enable the reader to adapt those methods and practices most suitable to his or her own storage requirements. It will be apparent that the use of all possible measures to maintain styrene quality is not needed in every installation. The economics of each case must be taken separately with consideration given to such factors as normal residence time, ambient temperature, and the quality requirements for the monomers end use.

QUALITY PROBLEMS IN STORAGE AND SHIPPINGStyrene has many end uses that require that rigid specifications be set on the monomer. Keeping the monomer within these specifications is referred to as maintaining its quality. Styrene quality is easy to maintain when the monomer is properly stored and handled. The major problems encountered in maintaining styrene quality are formation of polymer, formation of color, and contamination of the monomer by foreign materials especially particulate matter. The scope of these problems is outlined below. Detailed suggestions for minimizing these problems are given elsewhere in this section.

POLYMER FORMATIONStyrene monomer readily undergoes polymerization by both ionic and free radical mechanisms. Ionic polymerization can be avoided in storage by rigid exclusion of ionic catalysts such as strong Lewis acids. Free-radical polymerization, on the other hand, can occur thermally without the

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need for a catalyst or initiator.

Free-radical polymerization of styrene is normally avoided in This inhibitor requires a certain level of dissolved

storage by keeping the styrene cool and by using an inhibitor. The inhibitor universally used for this purpose is 4-tert-butylcatechol (TBC). oxygen (or air) in the styrene to enable it to inhibit effectively. The presence of more than 15 ppm polymer (all ppm concentrations in liquids refer to weight concentrations) in styrene monomer may affect the color, clarity, and toughness of polymer produced by some polymerization processes. blending with styrene having lower polymer. Since TBC has a very low volatility, polymer can form in styrene storage tanks on tank walls in the vapor space where uninhibited styrene vapor is condensed. Rough or rusty surfaces, internal roof supports, nozzles, and framing around manholes provide p laces where styrene vapor can condense and polymerize, often forming bands of polymer on walls and polymer stalactites on overhead structures. These bands and stalactites may break and fall into the monomer or be dissolved into it when the tank is filled, since polystyrene is readily dissolved by styrene monomer. This polymer may be highly colored by products of styrene oxidation and can thus adversely affect both the polymer and the color levels of the monomer. Polymer on the roof and its structure can re-dissolve in condensed monomer and cause major changes in polymer content in the tank. Formation of polymer in the vapor space of storage tanks can normally be minimized by proper tank design and maintenance. Styrene may become contaminated with polymer pumped into storage tanks from stagnant transfer lines, especially lines that have been exposed to elevated temperatures. Formation of polymer in transfer lines can be minimized by proper operation of transfer and circulation systems. Great care must also be taken to avoid pumping styrene against a closed valve as polymerization can occur. (See subsequent discussion of Sampling and Analytical Techniques.) The polymer level in styrene may be reduced either by redistilling or by

COLOR PROBLEMSStyrene in storage occasionally develops color that can be carried into the polymerization product. Color may develop in several ways: 1) Copper or copper-containing alloys can form soluble copper salts when contacted by styrene. These will impart a green or blue-green color to the monomer and may also inhibit its polymerization. (2) Highly-colored styrene oxidation products may form in the liquid monomer or be introduced by polymer falling or dissolving from the walls and roof. (3) TBC may be oxidized to form highly colored reaction products. (4) Iron, usually originating from rust in tanks or in piping, may react or complex with TBC at the ppm level to give styrene a yellow or yellow-green color. (5) Styrene lying stagnant in a line may develop color and, if flushed into a tank, throw the entire tank off-color.

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(6) Water will cause styrene to appear hazy if it is saturated. Color problems in storage can be minimized by: (1) Using no copper nor copper-alloyed material in contact with styrene monomer. (2) Paying careful attention to the oxygen level in the styrene and in the vapor space above the styrene. (3) Employing proper tank design and maintenance to avoid rust and polymer build-up on tank walls and roof. (4) Applying good transfer practices. Color in styrene monomer can be reduced to acceptable limits by: (1) Distilling the colored monomer. (2) Blending with non-colored styrene. always blend proportionately! (3) Passing the styrene over silica gel or alumina. This method has the disadvantage of removing the TBC inhibitor, which must be replaced. Acidic or highly activated alumina may cause the styrene to polymerize. For this reason, this procedure is only recommended if no other alternatives are available. Caution should be used, however, since color does not

PARTICULATE MATTERStyrene should be free of particulate matter when it is polymerized. Although some particulate matter in styrene originates from outside contamination via the receiving-transfer system, it is also formed by the reaction of concentrated TBC solutions with iron. This may happen in lines that have contained styrene but have been blown dry. Particulate matter in styrene may be avoided by: (1) (2) (3) Paying careful attention to cleanliness. Properly coating the inside of tanks and transfer lines which may contain high concentrations of TBC or which may be blown dry after carrying TBC-inhibited styrene. Filtering styrene to remove particulate matter before storing, shipping or use.

INHIBITION OF STYRENE POLYMERIZATION AND OXIDATIONUninhibited styrene may react with itself in a thermally induced polymerization, or it may react with oxygen to form styrene-oxygen copolymer, benzaldehyde, and formal-dehyde. Both polymerization and oxidation are normally prevented by using an inhibitor. The universally used inhibitor in styrene storage is 4 -tert-butylcatechol (TBC). This compound functions both as an oxidation and polymerization inhibitor. It is actually more effective at inhibiting oxidation than it is at inhibiting polymerization.

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In order for TBC to effectively inhibit polymerization, oxygen must be present to react with styryl radicals as they are formed. The growth of the resulting peroxy r dical is then inhibited very a efficiently by TBC. Normal levels of TBC will not effectively inhibit styrene polymerization in the absence of oxygen. Consequently, it is important to control the amount of oxygen in the styrene. Too little oxygen will hinder TBC from functioning as a polymerization inhibitor. Too much oxygen may increase the rates of free radical initiation and TBC uptake as well as increase the amount of oxygenated impurities in the styrene. More details on the chemistry of styrene inhibition are given in Section D of this publication.

TBC INHIBITORStyrene is usually inhibited with 10-15 ppm TBC, although any concentration between 10-55 is acceptable, the time in transit usually setting the requirements. The lowest effective limit of TBC appears to be about 5 ppm. It is common practice never to let the TBC level in styrene fall below 10 ppm without adding more inhibitor. Under ideal conditions, 10-15 ppm TBC is sufficient to insure styrene stability for 3 months or longer. The rate of TBC depletion varies with storage conditions, particularly with oxygen concentration and temperature, as well as with the amount of moisture, rust, and other impurities in the storage vessel. Consequently, the inhibitor level in the styrene must be checked periodically. (See subsequent discussion of Sampling and Analytical Techniques.) Whenever the TBC concentration falls below the desired level, it can easily be increased by adding to the storage tank small quantities of a concentrated solution of TBC dissolved in styrene. After addition of TBC, the contents of the tank must be thoroughly mixed. Only clean vessels should be used in preparing concentrated TBC solutions in order to avoid imparting color or other impurities to the monomer.

OXYGEN REQUIREMENTSIn order for TBC to effectively inhibit styrene polymerization, oxygen must be dissolved in the monomer. The minimum effective level of oxygen appears to be 8 -10 ppm. In order to have a safety margin, 15-20 ppm is preferred. Higher concentrations of oxygen do not enhance inhibition but only serve to initiate free-radical formation and increase the rates of styrene oxidation and TBC depletion. There are several methods by which enough oxygen can be dissolved in styrene monomer to enable TBC to function as a polymerization inhibitor. The user should select the method most suitable to their particular needs.

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Blanketing of Styrene in StorageThe oxygen needed for styrene inhibition may be obtained from an air blanket, or the styrene may be exposed to air before storage and then blanketed with an inert gas. Blanketing with air - Styrene, which is in equilibrium with air at 25C (77F), contains 60-65 ppm oxygen. Although an air-blanketing system is the easiest and cheapest to install and operate, it has certain disadvantages. First, the relatively high concentration of oxygen in air may cause oxidation of the uninhibited styrene vapor and condensate in the top of a storage tank. This can lead to problems with color and polymer, particularly in a tank in which there are internal members or where the roof and walls have not been coated to facilitate drainage of condensate back into the inhibited liquid styrene. (See subsequent discussion of Internal Coating). Second, blanketing with air increases the danger of fire or explosion. Care should be taken to maintain storage temperatures safely below the flash point of styrene (88F or 31C) when an air blanket is used. Blanketing under air usually is satisfactory if (a) the holdup time in the tank is short enough to prevent a significant buildup of oxygenated impurities, (b) the tank is designed to minimize styrene oxidation and polymerization, particularly in the vapor space over the liquid, and (c) all possible precautions have been taken to prevent explosions. Styrene may normally be stored under air at 70F (21C) for 8 -10 weeks without appreciable loss of quality. Higher temperatures can markedly decrease this storage time. A storage tank, which utilizes an air blanket, should be properly coated internally and it should be insulated. It should have sufficient liquid agitation to prevent buildup of a stagnant, warm, surface layer that may reach the flash point of the monomer. Agitation is also needed to insure that the oxygen from the surface is properly distributed throughout the monomer. Blanketing with inert gas - Tanks may sometimes be satisfactorily blanketed under an inert gas such as nitrogen. ("Inert", as used here, implies that the gas does not react with styrene.) Elimination of oxygen from the vapor space of the storage tank reduces the explosion hazard and eliminates problems associated with oxidation of the monomer. Without oxygen in the vapor space, the concentration of oxygen dissolved in the styrene decreases fairly rapidly by off gassing. When the oxygen level falls below 8-10 ppm, the TBC inhibitor will no longer be able to function effectively and polymerization will begin. In general, blanketing under an inert gas, with no air added to either the styrene or the vapor space, is only recommended for tanks having rapid turnover of monomer. The styrene going to such a tank should have been previously bubbled with air so that the level of oxygen is at least 25 ppm. The hold-up time in the tank should not exceed the time required for the oxygen level to drop to 10 ppm. This time is primarily a function of the monomer temperature. Storage under inert gas without degradation normally will not exceed a week at 70F (21C) under the best of conditions.

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Injection of Air into the StyreneThe most satisfactory method for maintaining the proper oxygen level in styrene in large tanks is by storing the monomer under nitrogen, with air added to the liquid via an immersed tube (a rate of 3-4 cubic feet per hour has been found adequate or a 540,000 gal. refrigerated tank). The f continual filling and emptying of tanks in service, along with the small continuous nitrogen purges needed on all openings into the vapor space, serve to keep the partial pressure of oxygen in the vapor space low. This in turn reduces styrene oxidation and, if the oxygen level in the vapor space is kept below 10-11%, also removes the danger of explosion. Agitation of the monomer is recommended, however, in order to avoid regions of oxygen depletion. This method is particularly useful for uncoated storage tanks having a slow turnover of styrene.

Controlled AtmosphereOxygen in styrene monomer may be maintained at the recommended 15-20 ppm level by blanketing the styrene with a nitrogen-air mixture containing about 5-7% oxygen. Oxidation should be no problem, and the oxygen level is below the 10-11% needed to support combustion. Agitation would be needed to distribute oxygen to all parts of the liquid. In most instances, the expense of a system such as this cannot be justified.

STORAGE TEMPERATUREControl of temperature is of prime importance in maintaining styrene quality. Thermal initiation of polymer formation increases with increasing temperature. High temperatures in the vapor space over styrene, as are often found in un-insulated storage tanks during hot weather, speed up formation of oxygenated impurities in the uninhibited styrene condensate when blanketed under air. In un-insulated tanks having no liquid circulation, a stagnant warm layer at the surface may exceed the flash point of styrene and, if the monomer is stored under air, pose a safety hazard.

EFFECT OF STORAGE TEMPERATURE ON STORAGE LIFEStyrene has been stored for six months and longer without loss of quality when proper temperature, inhibitor level, and tank management have been maintained. This storage time decreases markedly, however, with increasing temperatures. In order to minimize monomer degradation in large volumes of stored monomer, it is recommended that the maximum storage times listed in Table B-1 are not exceeded. TABLE B-1 Monomer Temperature (F) 95 85 75 65 Maximum Storage Time 3 days 2 weeks 5 weeks 3 months

The effect of temperature on the rates of TBC depletion in styrene stored under air at several temperatures is shown in Figure B-1. These rates should only be regarded as typical, since TBC

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depletion is governed not only by temperature but also by oxygen level and the concentration of other radical-initiating impurities in the styrene.

Figure B-1 TBC Depletion in Styrene Stored under Air

CONTROL OF STORAGE TEMPERATUREThe temperature of styrene monomer in storage should be maintained as low as is economically feasible. Ideally, the temperature should not be allowed to exceed 65F (18C) for long-term storage. Sterling experience indicates that essentially every aboveground styrene storage facility should be equipped with the following: (1) insulation (2) white or other reflective exterior surface coating (3) agitation. In addition, refrigeration is recommended for large, aboveground bulk-storage tanks in which the monomer temperature may exceed 75F (24C). The length of time during which it is practical to store small lots of styrene which are shipped in steel or galvanized pails or drums is limited mainly by temperature conditions and initial inhibitor levels. Small containers should be stored indoors, if possible, in well-ventilated areas that are removed from open flames, electric heaters, and other sources of ignition. They should not be

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located near steam radiators or other heat sources. If indoor storage is impractical and outdoor storage is necessary, the containers should be shaded from direct or reflected sunlight. In either case, the monomer should be used on a first-in, first-out basis, and inventories should be kept to a minimum.

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DESIGN OF STYRENE STORAGE FACILITIESThere is considerable leeway in designing storage facilities for styrene. However, the nature of the monomer does require that certain features be incorporated into such facilities. Some of these features will vary with the user's particular storage situation. Others may be specified by federal, local, state, and underwriters regulations. These must be considered before the design of a storage facility is completed. STORAGE TANKS A storage tank for styrene monomer may be vertical or horizontal, aboveground or underground, and have a wide range of sizes. The design of such a tank is governed primarily by the requirements of the user.

Size of TanksStyrene storage tanks should be the smallest compatible with shipping and receiving requirements. Storage times exceeding three months should normally be avoided.

Materials of ConstructionMost common metals such as carbon steel, stainless steel, aluminum, and galvanized steel may be used in handling styrene. Carbon steel is ordinarily used in tank construction for economic reasons and is satisfactory if it is either coated or kept clean and rust-free. Carbon steel used in new construction should be completely free of mill-scale. Neither copper nor copper containing alloys should contact liquid styrene since copper can discolor the monomer and tend to inhibit its subsequent polymerization.

Type of ConstructionIn general, tanks should conform to American Petroleum Institute (API) Standard 650. All-welded construction should be used. Internal welds and those around nozzles, vents, hatches, etc. should be ground smooth. The interior of the tank should be as smooth and clean as possible, with an absolute minimum of internal beams, pipes, and projections that can provide places for condensed styrene vapors to accumulate and polymerize. Large, upright cylindrical tanks should have self-supporting or externally supported roofs. Tank floors should be sloped so that all liquid can be drained without leaving puddles in low spots.

Filling and Withdrawal LinesFilling lines should enter tanks at the bottom. Withdrawal lines should also be attached at the bottom so as to draw from a point 8"-12" above the tank floor. Vortex breakers on withdrawal lines may be necessary to avoid losing pump suction at low tank levels. A separate small bottom outlet or sump should be provided in tanks to permit them to be completely drained.

Insulation and RefrigerationAll aboveground styrene storage tanks should be insulated. At Texas City, Texas, 2" of foam glass or 2"-3" of polyurethane foam has been found adequate for refrigerated tanks. Such insulation should be vapor-sealed and covered with sheet aluminum. Aluminum sheeting should be caulked wherever nozzles or support clips protrude through the sheeting. Since corrosion

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under insulation due to sweating can occur on refrigerated surfaces, tanks and piping should be properly coated prior to insulating. Refrigeration may be needed in some locations to keep styrene cool. Refrigeration is

recommended if the temperature of the monomer exceeds 75F (24C). (See preceding discussion of Storage Temperature.) Styrene is best cooled by circulating a portion of the tank contents to an external chiller. Cooling coils inside the tank are not recommended because they are difficult to maintain. Insulation and refrigeration can reduce losses of styrene vapor to the atmosphere by a factor of ten over un-insulated un-cooled tanks. (See subsequent discussion of Environmental Considerations.)

AgitationAgitation is recommended for all styrene storage tanks, presuming heat from the mixing can be removed, for the following reasons: (1) to maintain even temperature distribution and avoid stagnant warm layers in the monomer. (2) to maintain good distribution of oxygen throughout the styrene and avoid regions of localized oxygen depletion. This is important whether the monomer is stored under air or whether it is stored under an inert atmosphere and the oxygen level is maintained by air injected into the liquid. (3) to insure adequate mixing of added TBC with the monomer. (4) to insure adequate mixing of fresh styrene with old. Agitation is best achieved by using a mechanical stirrer. Typically, a stirrer is side-mounted with an offset entry. The motor has 7-10 h.p./million gallons of styrene and drives a 3-bladed propeller at 280 rpm. Wetted metal on stirrers should be aluminum or 304 stainless steel. For tanks with relatively high turnover rates and continuous circulation, properly designed eductors may be used for mixing.

Level and Temperature MeasurementsStyrene storage tanks should be equipped with a means for determining both the liquid level and the liquid temperature. An opening (gauge hatch) at the top of the tank will permit sampling and stock gauging. If some form of mechanical gauging is preferred, either a differential-pressure (D/P) cell or the equivalent should be installed. Float, tape, and reflex gauges are not recommended for this service. External sight glasses should be avoided because they provide a stagnant region and expose the styrene in the glass to heat and light, both of which may initiate polymerization. The temperature of the liquid in the tank may be measured either by a thermometer or by a temperature indicator reading-out at a central location. Temperature measurements are needed

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not only to help maintain the temperature desired in the tank but also to warn of the onset of rapid polymerization.

BlanketingStyrene may be blanketed under air or nitrogen depending on the user's particular storage requirements. The blanketing system should insure a slight positive pressure (~1" W.C.) under all conditions of operation, i.e., filling, emptying, or static. Such a system is described in Appendix B-1. (Also see the preceding discussion of Oxygen Requirements).

Manholes, Gauge Hatches, Vents and Foam ProtectionAll connections to the vapor space of a tank should be insulated insofar as practicable and have smooth internal surfaces. They should be designed to drain easily and to provide as little space as possible for accumulation and condensation of styrene vapor. Inert-gas purges are recommended on all openings into the vapor space of the tank to insure that they do not become plugged with polymer. Gauge hatches should be of the standard self-closing type. aluminum because of its non-sparking property. The covers should be made of

A conservation vent or the equivalent is recommended for all styrene bulk-storage tanks. (See API 2000.) All vents should be equipped with flame arrestors. The flame arrestor used with a conservation vent is normally installed between the vent and the tank, and an inert-gas purge is fitted below the flame arrestor. Elimination of the flame arrestors may be considered in facilities that have both 1) highly reliable inert gas blanketing systems, and 2) effective inspection and preventative maintenance programs. Foam protection is recommended when a tank is blanketed under air or natural gas, but s i normally not needed with a nitrogen blanket. A non-polar solvent mechanical foam is satisfactory for this service.

Back-up or Emergency Pressure ReliefNormally a conservation vent will provide all the pressure relief necessary for a styrene storage tank. However, it is good practice to have an additional means of relieving pressure in the event of a sudden pressure build-up or the failure of the primary relief device. Weak-seam roofs or relief manways are often used for this purpose. A relief manway should be fitted with a nitrogen purge to insure that styrene does not polymerize in the manway and defeat its protective function.

Internal CoatingThe build-up of rust and polymer inside a styrene storage tank can be minimized by coating the interior of the tank roof and walls with a suitable material, such as catalyzed epoxy. This coating provides a smooth surface, which facilitates drainage of condensed, uninhibited styrene droplets back into the main body of liquid before polymerization can occur. All storage tanks using an air blanket should be coated.

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The procedure, which has been used in coating styrene tanks with catalyzed epoxy, is given in Appendix B-2. Other materials, such as modified epoxy, inorganic zinc, and baked phenolic resin have also been reported to be satisfactory.

Electrical GroundingsA minimum of two grounding clips, or one for each 25 feet of circumference, is recommended to allow drainage of static electricity. If a tank is coated with epoxy or other non-conducting material, the bottom and lower wall below the normal liquid level should either be left uncoated or coated with a suitable conducting material such as inorganic zinc. Storage tanks constructed on the earth rather than on a concrete slab should be provided with cathodic protection.

Other ConsiderationsRequirements for diking, tank spacing, and other features pertaining to safety are detailed in guidelines set by the National Fire Protection Association (see NFPA No. 30). These, as well as local building codes and governmental regulations, should be consulted since some requirements vary with the size and configuration of an installation. All electrical equipment associated with the tank should conform to the National Electrical Code (NFPA No. 70). Typical Design for a Styrene Storage Tank The design for a typical styrene storage tank is shown in Figure B-2. Some features shown can be changed, depending on the user's requirements. For instance, a refrigeration unit might be added in warm climates. If nitrogen is used, then foam protection may not be needed. The styrene storage tanks at Sterling's Texas City, Texas plant are designed to store up to 2.5 million gallons of specification styrene, under Gulf Coast climatic conditions, for periods exceeding three months without a significant decrease in quality. These tanks are API Standard 650 carbon-steel tanks with self-supporting roofs. The interior roofs and walls have epoxy coatings. The tanks are insulated and are fitted with mixing adductors and external refrigeration units. The styrene is blanketed under nitrogen at 64F (18C), with air added to the liquid. As mentioned previously, all of the features used in bulk storage tanks at the Sterling plant are not needed in every installation. Each user should determine their own needs based on climate, Monomer turnover, quality requirements of the styrene end-use, and economics. PIPING Piping is normally of carbon steel, although stainless steel, aluminum, and galvanized steel may also be used. Copper and copper-containing alloys in contact with styrene must be avoided. Steel pipe, which may carry concentrated TBC solutions, may be lined with baked phenolic, Teflon or equivalent to reduce formation of particulate matter.3 3

Trademark E. I. DuPont de Nemours and Co.

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All piping should be sloped and have no pockets where styrene can become stagnant. All low points should be provided with drains or some other means of removing the monomer. Aboveground piping should be insulated and properly grounded. Provision should be made either for circulating styrene through all lines or for blowing them empty with nitrogen. For 1inch pipe or smaller, threaded or socket welded connections are suggested. For larger piping, butt-welded fittings and flanged connections are preferred. It is convenient to have sampling points at storage tanks and at all loading points.

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PUMPSCentrifugal pumps are preferred for styrene service. They should be fitted with closed impellers and mechanical seals. All pump motors should meet National Electrical Code standards.

VALVESBall valves with seats of Teflon are satisfactory for styrene service under ambient storage conditions. Gate valves are less satisfactory, but they are usually cheaper and may be used.4

GASKETSGarlock style 3200 or its equivalent is satisfactory for flanged connections at ambient conditions. Gaskets made of rubber or other styrene-soluble materials should be avoided.

FILTERSSince small amounts of foreign matter may enter storage tanks from various sources, a filter in the transfer piping between the tank and processing equipment is recommended. A cartridgetype filter with a fine or medium replaceable cartridge is suggested. The filter housing "O" ring should be VitonTM or TeflonTM. Filter cartridges should be inspected and renewed periodically.

HOSESIf hoses are needed for loading or unloading operations, they should be flexible and chemicalresistant. VitonTM cross-linked polyethylene and multi-layered polypropylene has also given good results in this service.

SPECIAL OPERATING PRACTICES IN STYRENE STORAGE AND TRANSFER FACILITIESThe nature of styrene requires that extra attention be given to monomer storage and transfer operations. Great care must be taken to avoid pumping styrene against a closed valve (dead-head). The styrene in the pump will become heated and polymerize, often resulting in contamination of entire tanks. Styrene tanks may also become contaminated with polymer pumped into the tank from stagnant transfer lines, particularly those that have been exposed to elevated temperatures. All lines and pumps in styrene service should be periodically or continuously circulated with styrene to prevent buildup of polymer or colored material. This may be required as frequently as once/day, depending on the ambient temperature. Lines, which cannot be circulated or are seldom used, should be drained.

4

Trademark E. I. DuPont de Nemours and Co.

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Tank levels should be kept as high as feasible. Tanks should be nearly filled as often as possible to minimize polymer build-up on the upper walls. In order to minimize accumulation of static electricity when pumping into air-blanketed tanks, barges, or ship compartments, the initial pumping rate of styrene should not exceed a linear velocity of 3 ft./sec. in the branch line to the tank. When the inlet opening is covered by at least 3 ft. of liquid, the pumping rate may be increased to no more than 20 ft./sec. Allow 30 seconds charge-relaxation time (in piping) downstream of filters. Allow 30 minutes relaxation time in tanks after filling before gauging or sampling. The requirements for cleanliness and materials of construction prescribed for styrene storage tanks also apply to shipping containers. Whenever possible, styrene should be loaded into tank cars, barges, and ship compartments which are dedicated to styrene service. In the instances when a prior cargo was not styrene, the container must be gas-freed and cleaned. This is especially important when the prior cargo was a strong acid, metal halide, peroxide, etc. that may initiate a violent polymerization reaction. Hoses should be drained and cleaned after use. They should be monitored for breakage and wear on a daily basis as they are used. Crimping and extreme stress should be avoided. Hoses should be pressure-tested at 1.5 times their working pressure at least annually. The precautions recommended in the Safety Section of this publication under Fire and Explosion Hazards and Polymerization Hazard should be carefully observed.

SAMPLING AND ANALYTICAL TECHNIQUESGood samples and careful analyses are necessary to properly evaluate the condition of styrene in storage. Standard sampling practice for styrene monomer should include: 1. 2. 3. 4. Using clean, dry bottles for sample collection. Insuring that representative samples are taken. Flushing sample taps thoroughly before sampling. Analyzing samples immediately after they are taken. If this is not feasible, the samples should be refrigerated until they can be analyzed.

Sample taps should be as short as possible. Tubing and valves should be of stainless steel. Styrene in storage should be analyzed for color, inhibitor content, and polymer content as often as experience indicates. A typical analytical schedule is given in Table B -2. More frequent sampling may be required if storage problems develop.

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TABLE B-2 Analysis Color Polymer Frequency 2-3 per week 2-3 per week Daily if the monomer temperature is 80F (27C) or higher 2-3 per week Daily if the monomer temperature is 80F (27C) or higher Only well-demonstrated and calibrated analytical techniques should be used. The methods used by Sterling for these analyses are given in Appendix B-3.

Inhibitor

MAINTENANCE AND INSPECTION PROCEDURESTanks should be thoroughly cleaned before being put into styrene service. This may require abrasive blasting, vacuuming, wiping, and coating. Moisture should be excluded from styrene tanks. All openings (vents, hatches, flame arrestors, foam nozzles, etc.) into the vapor space of tanks should be inspected at regular intervals. Polymer should be removed whenever the buildup becomes excessive. The interior of the tank roof, walls, and floor also should be checked periodically for polymer deposits. Polymer can be removed by chipping after it has been hardened with airflow or with steam. Since tank coatings and walls are often damaged by chipping, they should be repaired before the tank is used again. A typical inspection schedule is given in Table B-3. More frequent inspections may be indicated for some installations. TABLE B-3 Inspection Points Vents, hatches, flame arrestors, etc. connected to the vapor space Tank interior Frequency Annually, if purged with nitrogen. Monthly, if not purged Every 3-5 years, if coated. Annually, if uncoated

ENVIRONMENTAL CONSIDERATIONSThe effects of chemicals on the environment, as well as their regulatory status, must be carefully considered before designing and operating chemical storage and handling facilities. Although styrene is not generally considered to be a serious pollutant to either air or water, all federal, state, and local regulations regarding this class of chemical should be closely followed.

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ENVIRONMENTAL REGULATIONSEnvironmental regulation is a complex and rapidly changing field. This bulletin should serve only as a quick overview to the general regulatory requirements associated with styrene storage and handling in the United States. Facility operators should contact State and local authorities, environmental consultants, or seek legal advice concerning their specific obligations under the various regulatory programs.

Toxic Substances Control Act (TSCA)Styrene (CAS No. 100-42-5) is registered on the TSCA Chemical Substances Inventory, and is subject to periodic TSCA inventory updates.

AirIn the United States, styrene is regulated as a Hazardous Air Pollutant, Volatile Organic Compound, and a Volatile Organic Liquid under various air pollution control regulations issued by Federal, State, and local authorities. At this time, there is not a specific Federal air quality standard for styrene. However, many States have established health or odor-based air quality standards for styrene. Because of its low odor threshold, even small air discharges of styrene have the potential to create a nuisance condition for nearby neighbors. In the United States, sources of air contaminants may be regulated under many different programs. Sources may be regulated as new sources of air contaminants, as new or existing sources of Hazardous Air Pollutants, or as sources of ozone precursors. In addition, owners of new or modified sources of air contaminants may be required to obtain permits prior to construction or operation of the sources. Since the enactment of the Clean Air Act Amendments of 1990, under the authority to regulate emissions of Hazardous Air Pollutants, the Federal government has proposed or issued regulations (MACT Standards) that impact most large styrene users. Additional MACT controls may not be required for styrene storage tanks, since styrene has such a low vapor pressure under ordinary storage conditions. However, other styrene emissions sources, such as fugitive emissions from equipment leaks, process vents, and wastewater may be subject to controls under MACT standards. In areas where ozone exceeds national ambient air quality standards, styrene emissions sources may also be subject to regulations governing emissions of Volatile Organic Compounds. The scope and impact of these regulations is usually similar to those regulating styrene as a Hazardous Air Pollutant. In addition, most areas in the United States require stationary sources of air pollution to obtain a permit, or be exempted from permitting, before construction or operation. These permits may require certain technology-driven standards of construction, operation, maintenance, monitoring, record keeping, and reporting. Some areas also regulate fence-line concentrations based on air

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quality standards derived from toxicological concerns to protect public health or to prevent nuisance odors. These effects-based standards may require additional vent controls.

WaterA water discharge permit is required before wastewater or storm water can be discharged to surface waters or to a publicly owned treatment works. Accidental discharges of styrene to surface waters, to the air, or to soil should be reported to competent authorities for appropriate response. Discharges of styrene to the environment in amounts equal to or greater than its Reportable Quantity (1,000 lb) must also be reported to the National Response Center under CERCLA (Superfund) and the Clean Water Act.

SARAUnder the Federal SARA program, owners of certain facilities storing or handling styrene may be required to submit Material Safety Data Sheets and chemical inventory data to local and state emergency planning authorities. In addition, emissions above a threshold may be reportable annually to local, state, and federal authorities under the SARA Toxic Release Inventory (TRI) program.

Solid WasteSpilled or waste styrene will probably have a flash point below 140 F. If so, it should either be recycled or managed as an ignitable hazardous waste as specified under regulations implementing the Resource Conservation and Recovery Act (RCRA). Ignitable hazardous wastes must be incinerated at a licensed RCRA waste management facility. Generators of hazardous waste may also be subject to permitting or licensing under RCRA by state or federal authorities.

CONTROL OF AIR AND WATER EMISSIONS Effect of Storage Temperature on Vapor LossesIn a static styrene storage tank, the volume of vapor over the liquid changes with the temperature. This volume change results in breathing, which is most pronounced in un-insulated tanks where vapor head temperature changes with the daily changes between daytime and nighttime ambient temperatures. Breathing losses are negligible for insulated tanks where the vapor temperature changes are small. Conservation vents also help control breathing losses. Most of the vapor losses from styrene storage tanks occur when the tank is being filled, when the incoming liquid displaces the vapor through the tank vent. These losses are known as working losses. The working loss emission rate depends on the liquid temperature and the filling rate. The effect of temperature and filling rate on styrene storage tank working losses are shown in Table B-4.

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TABLE B-4Styrene Temperature F 60 75 90 C 16 24 32 Styrene Vented (lbs./hr.) @ Pump rate 100 gpm 1.0 1.6 2.6 1000 gpm 9.8 15.8 25.8

Control of Vapor LossesIf abatement of styrene vapor emission becomes necessary, either because of halogens in the atmosphere or because of environmental control regulations, storage tank vents can be routed for control to a refrigerated condenser or to a properly designed combustion unit such as a flare, boiler, heater, or incinerator. Storage tank working losses may also be controlled through a properly designed vapor-balance system. FATE AND EFFECTS OF STYRENE IN THE ATMOSPHERE The concentration of styrene in ambient air is unlikely to reach a level sufficient to pose a health hazard. However, significant losses of styrene vapor should be avoided because of its odor, its potential contribution to photochemical smog, and its possible reaction with halogens. (Chlorine or bromine react with styrene to form very potent lachrymators.) Styrene is transformed in the lower atmosphere to oxidized compounds such as benzaldehyde and formaldehyde. Half-life of these reactions is on the order of hours. Styrene is considered a moderately reactive precursor to ozone through photochemical reactions in the lower atmosphere. The relatively low vapor pressure of styrene tends to minimize vapor losses, particularly when styrene is stored and handled at temperatures below 70F (21C). FATE AND EFFECTS OF STYRENE IN WATER Styrene floats on water and is only slightly soluble in it (about 350 ppm maximum under ambient conditions). Styrene also tends to polymerize, forming a surface glaze. Both the monomer and the polymeric glaze may be removed by skimming or vacuum pick-up. Prompt removal of styrene spills is important in order to minimize the quantity of material dissolved. Aerobic biodegradation by microorganisms and volatilization to the atmosphere are the primary means of removal of dissolved styrene from surface waters. The half-life of styrene in surface water is on the order of a few hours to several days, depending on the amount of volatilization.

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Biodegradation does not occur in an anaerobic environment. Adsorption and deposition are not important mechanisms of attenuation of styrene from surface waters. The acute toxicity of dissolved styrene to fish and other aquatic species is relatively low. The mean acute tolerance limit, at which 50% lethality is observed, is tens of parts per million for several common species. Bioconcentration of styrene in fish flesh does occur to a minor degree. The flesh of fish or other marine life may be tainted if exposed to high concentrations of styrene resulting from a spill or discharge. In the event of a spill or discharge of styrene to surface waters, there is also some chance for a fish kill since biodegradation processes will deplete vital oxygen in affected waters. Microorganisms are inhibited only at concentrations of several tens to hundreds of parts per million of styrene in water. Styrene dissolved in groundwater is adsorbed strongly to the organic matter in soils. This strong adsorption slows its vertical transport downward towards groundwater aquifers. During this vertical transport, styrene is subject to aerobic biodegradation by microorganisms. From shallow, porous soils, volatilization may also be an important attenuation mechanism. In anaerobic or sterilized soils and groundwater, styrene may exist nearly indefinitely. Treatment of Contaminated Water Floating styrene and polymeric surface glaze may be removed by a skimmer or vacuum pick-up. Dissolved styrene in contaminated process water is best removed by a carbon adsorption system. Several other methods are effective in removing dissolved styrene, such as: 1. Steam or gas stripping 2. Biological treatment with acclimated microorganisms.

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APPENDIX B-1 LOW PRESSURE GAS BLANKET CONTROL SYSTEM

A suitable system for low pressure gas blanket control on liquid storage tanks and process tanks has been sought for many years. Some of the requirements in such a system are as follows: a) High speed of response to protect the tank in the event b) Accurate control at low pressure since many of these systems are designed for blanket pressure as low as water column (w.c.) c) Economical initial installation d) Simple, easy to maintain, reliable components

Through experience, we have found that self-operated diaphragm valves will not perform satisfactorily when gas blanket pressures of 2" w.c. or less is required. A pilot operated system, shown schematically in Figure B-3, was devised to handle these low pressure requirements. The components of this pilot operated system are: a) Main Valve - Type 133L with pilot tube plugged. Type 166 may also be used. It is recommended that the valve size be 1/2" to 1" oversize for fast, high capacity action. Valve incorporates 14" w.c. to 28" w.c. spring set at 10 ounces and is installed close coupled to the process or storage tank. Sensing Element - 3/4" Type Y600 regulator (heavy diaphragm head and spring #1B5584) with 1/2" orifice and set at 1/2" w.c. Type Y600 is installed upside down so that the weight of the parts will aid the regulator in maintaining the 1/2" w.c. setting. This regulator also acts as the variable orifice in the pilot system.

b)

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c) d) e)

Fixed Orifice - Type 111 lock shield needle valve. Pressure Regulator - Type 922 set at 12 ounces. Standard 3/4" pipe tee.

Principle of Operation The main valve, Type 133L, 166 or 66, is installed close coupled to the process or storage tank to eliminate all possible time lag of gas entry to the tank. On a decrease of pressure in the tank of 0.1 w.c., the Type Y600 regulator opens, relieving the pressure from the lower diaphragm casing of the Type 133L, 66, or 166 main valve. This causes the main valve to open, allowing gas to enter the tank. Since the maximum droop or offset of a Type Y600 on 0.4 w.c. to 1.0 w.c. control is 10%, the control range of the gas blanket is 0.1 w.c. At w.c. or slightly above, the Y600 closes, causing a buildup of gas pressure under the main valve diaphragm and closing the Type 66 or 166. The allowable inlet pressures for this system are as follows:

Type 66 2 size 10 psi 3 size 5 psi 4 size 5 psi

Type 166-1 125 psi

Type 166-2 60 psi

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APPENDIX B-2 SPECIFICATION FOR COATING TANKS WITH CATALYZED EPOXY FOR STYRENE MONOMER SERVICE Extent of CoatingAll internal surfaces should be coated except for the floor and bottom of the vertical walls below the normal liquid level (to prevent accumulation of static electricity). This includes, in addition to the tank, all internal surfaces of nozzles, manholes, manhole covers, internal fittings and all surfaces, which may contact the liquid contents of the tank or be exposed to vapors from the liquid. If any internal fittings are of non-ferrous metal, they should be removed and replaced when the lining is completed.

Preliminary PreparationAll sharp edges and high points should be ground smooth and rounded to a minimum radius of 1/8". Welded seams need not be ground flush; however, welds should be free from undercuts or pinholes. If either exists they should be ground out, filled with weld metal or epoxy putty, and ground smooth. Weld spatter beads should be removed by grinding or by the abrasive blasting which follows.

Surface PreparationAll internal surfaces to be lined should be abrasive blasted to the degree defined as "White Metal Blast" by Steel Structures Painting Council Specification SP5 (NACE 1). Anchor pattern depth should conform to a minimum of 1.5 mils and a maximum of 2 mils. Loose material such as sand, grit, dust or any foreign matter should be removed from the tank, preferably by use of an industrial vacuum cleaner.

MaterialMaterials that have been used by Sterling for lining styrene storage tanks are amine-adduct cured (two-component) epoxy primers and finishes, usually applied as two primer coats and two finish coats. Colors used in successive coats should be contrasting or different shades of the same color in order to insure complete coverage of each coat.

Application of CoatingFollowing surface preparation above, the first coating should be applied within eight hours after abrasive blasting is completed. It must be applied before any rust appears or "turning" occurs in the "White Blast." Otherwise re-blasting to the "White" condition will be necessary. 1. A brushed prime coat should be applied to all welds, rounded edges and other irregular surfaces, working the paint well into the metal.

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2.

Apply a full sprayed coat of primer over brushed surfaces. overnight. Dry film thickness should be 1.5-2.5 mils.

Allow to dry

3.

Apply one sprayed coat of intermediate primer to all surfaces. Allow to dry overnight. The total dry film thickness of this and the preceding coat should be 3-4 mils. (Note: some colored primers may dissolve in styrene and discolor it unless the primer is completely covered by sub-sequent coats of paint.) Apply one sprayed coat of finish paint to all surfaces. Allow drying overnight. The total dry film thickness following this step should be 4.5-6 mils.

4.

5.

Apply one sprayed coat of finish paint to all surfaces. Drying time for the coating normally is 4 hours at 120 F (49 C), or 7 days with air-drying. The total, finished dry film thickness should be 6-10 mils.

PrecautionsRecommendations of the paint manufacturer regarding mixing, thinning, etc. should be followed. Curing and drying times should be in accordance with the paint manufacturer's recommendations. The completed lining should be free of runs, sags, over spray, pinholes, abrasions or other breaks in film continuity. Each coat should be inspected after it has dried and before the following coat is applied. Film thickness should be determined with a thickness gauge. The completed lining should be inspected for breaks in film continuity using a holiday detector. No holidays should be permitted. The solvents used in these paints can be toxic if inhaled or absorbed in large quantities. All of the manufacturer's recommendations should be followed. Consequently, it is recommended that personnel wear supplied-air respirators and protective clothing while working. Forced-air ventilation should be provided during blast-cleaning and liningsapplication work since the solvents are also flammable. The ventilation should be maintained at all times while people are in enclosed areas. An adequate number of air changes must be provided to keep solvent vapors below lower explosive limits and levels that would be immediately dangerous to life and health.

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APPENDIX B-3 ANALYTICAL METHODS FOR COLOR, TBC, AND POLYMER IN STYRENE MONOMER (SM ) COLOR IN STYRENE MONOMER ScopeThe color of the sample is compared to the PtCo scale. This procedure is suitable for all solutions, which are clear and colorless to light yellow. If turbidity or suspended solid material is present, filter the sample before analyzing. measurement, are APHA and Hazen color. Other names, which are used for this color

PrincipleTransmitted light is focused from the sample onto a holographically lined concave diffraction grating with one objective lens. The light then illuminates a semiconductor masked multi-celled photodector diode array (32 points). By this means, the visible range of the spectrum is scanned from 400 nm to 700 nm. The measured values of transmitted visible light are integrated by the instrument software to arrive at X, Y, Z values are then converted to the APHA (PtCo) yellowness index equation per ASTM D1925 as follows: APHA = 20 Y1 (D1925)/(t in cm) Where Y1 (D1925 (1.274976795X - 1.058398178Z)]/Y and t = 5 cm

SafetyUse good laboratory practices and normal precautions in handling chemicals, glassware, and electrical equipment.

Apparatus1. Cells, 5 cm x 5 cm path length round screw cap cells. (Nicholas Scientific, 905 software Halik St., Pearland, TX 77581). 2. Colorimeter, Hunter Lab Color Quest 1200 (Reston, VA), with APHA or equivalent.

Reagents and MaterialsDeionized Water PtCo 500 ppm Standard

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Calibration Check/Sample Procedures1. Prepare standards by diluting PtCo 500 stock solution in volumes of 0, 1, 2, 3, 4 and 5 ml to 100 ml with distilled water. The standards represent 0, 5, 10, 15, 20 and 25 PtCo colors. Zero instrument a. Go into instrument standardization mode. b. Place a black card in front of lens and enter value. This will give a zero % transmission reading for the instrument. c. Remove card and fill a clean cell with distilled water and place in instrument. Enter this value which is 100% transmission referenced to distill water. d. Check APHA reading on the distilled water blank. Reading should be 0.5 or better. If not, clean cell and repeat procedure. Sample Analysis a. Fill cell with sample, clean outside faces of cell and place in instrument. See notes 1 and 2. b. Follow instrument procedure and read APHA color of sample.

2.

3.

Reporting ResultsReport results as observed on terminal (0.1 APHA unit).

Accuracy and PrecisionWithin-shift analyses of the same sample should not vary more than 0.5 APHA units.

Notes1. The instrument must be zeroed and samples run with the same cell to eliminate any problem with unmatched cells. However, if cells are matched within 0.5 APHA, one cell may be used for the zero blank and the other for the sample. Thoroughly rinse cells with methanol and acetone and air dry between samples of water and/or different samples.

3.

ReferencesHunter Lab Instruction Manual, Version 3.2, 4/88 ASTM D1925-70 ASTM E308-85 ASTM D1209-84

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TBC IN STYRENE MONOMER ScopeThis method measures the level of TBC inhibitor in styrene in the 1 to 100 ppm range. Color is developed in the sample by the addition of caustic in a methanol-octanol-water solvent mixture. The intensity of the color is measured with a colorimeter or spectrophotometer and compared to a calibration curve for quantification.

ApparatusSpectrophotometer - Capable of measuring at a wavelength of 490 nm (a colorimeter can also be used with a #54 green filter, 500-570 nm). 1 cm cells 5 ml pipette 500 ul syringe 100 ml volumetric flasks

Reagents and Preparation4-Tertiary Butyl Catechol, mp 52-55 C; available from Aldrich Chemical Company, Milwaukee, WI 53233. Toluene, AR grade Methanol, AR grade n-Octanol, Baker analyzed reagent, JT Baker Company, Phillipsburg, NJ 08865. Sodium Hydroxide pellets, AR grade Sodium Hydroxide, approximately 0.15N: Dissolve 2 pellets of sodium hydroxide (0.3g) in 25 ml of methanol. Add 25 ml of n-Octanol and 100 ul of water. TBC standard stock solution: Weigh 250 mg of TBC into a 100 ml volumetric flask and dilute to volume with toluene. This solution will contain 2,500 ug of TBC/ml. Pipette 1, 2, 3 and 4 ml aliquots into 100 ml volumetric flasks and dilute to volume with toluene. These will contain 25, 50, 75 and 100 ug TBC/ml. For ppm, divide the ug/ml by 0.9.

SafetyToxicity For complete toxicity information on all chemicals handled refer to the material safety data sheet.

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Personal Protection Use good laboratory practices and safety precautions and rules in handling all chemicals, equipment and glassware. Wear safety glasses and gloves while performing the analysis. First Aid Should any chemical come in contact with the skin, flush with water for at least 15 minutes. If a chemical should come in contact with the eyes, wash out the eyes in an eye bath for 15 minutes. Report to First Aid for treatment. Procedure 1. 2. 3. Pipette 5 ml of sample into a 25 ml scintillation vial. Add 100 ul (0.l ml) of sodium hydroxide solution to the vial and vigorously shake for a minimum of 45 seconds (See Notes 1, 2, 3 and 4). Add 200 ul (0.2 ml) of methanol to the vial and again shake for 15 seconds (See Note 2). 4. Zero the spectrophotometer using toluene in both the reference and sample one (1) cm cells. Empty the sample cell, rinse the cell with a small amount of the sample preparation, empty and fill with the sample preparation. Read and record the absorbance of the sample (See Note 5).

5. 6.

Calculation Read the concentration of TBC in ppm from the calibration graph or multiply absorbance by calibration factor. Calibration and/or Standardization 1. Pipette 5 ml of each standard of TBC into a vial. 2. Follow steps 2 through 6 in the procedure section. 3 Plot the ppm TBC versus absorbency (Note 5). slope of the curve. Precision 0.6 ppm at a level of 12 ppm TBC (95% conf.) Analytical Time 15 minutes analyst time Reference ASTM Method D4590 Notes 1. The turbidity formed as the sodium hydroxide solution ages does not appear to be harmful. 2. A vortex mixer should be used for agitation.

Calculate calibration factor from

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3.

4.

The reaction requires mixing with oxygen during the agitation time. Therefore, the agitation time should be adhered to as closely as possible to obtain reaction completion. If a blue color persists before the methanol is added, the concentration range of the method has been exceeded. Dilute the sample 1:10 with toluene and repeat analysis. The calibration curve is close to a straight line. Higher concentrations will definitely yield a curve up to about 300 ppm, and then it drops off.

5.

POLYMER IN STYRENE MONOMER Description of AnalysisPolystyrene is precipitated from styrene monomer by methanol. The solution increases in opacity with time. After 15 minutes, however, the growth rate tends to level off so that 1 minute does not greatly affect the results. Toluene is used as a solvent for the polystyrene in the standardization procedure since polymerfree styrene is extremely difficult to obtain. (In our initial standardization, toluene was compared to styrene as a solvent with no differences in calibration curves being observed.) The particle size and shape varies due to many influences; the procedure used for calibration should be followed as closely as possible to minimize the effects of these unknown influences. Apparatus 10-, 15-mil pipettes 50-ml beakers Stirring rod Colorimeter or spectrophotometer with 5.0-cm cells Reagents C.P. Methanol Toluene Procedure A. 0-100 ppm Polymer Into a beaker, pipette 15 ml of toluene for a blank. Into a second beaker, pipette 15 ml of methanol. Into both beakers, pipette 10 ml of the sample with stirring. Start a timer. At 15 minutes by the timer, zero the colorimeter against the blank in 5.0-cm cells, using a 420 nm wavelength. Read the %-T of the sample and determine the ppm polymer from a previously prepared calibration chart.

B. 100-1000 ppm Polymer

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Dilute, by pipette, 10 ml of sample to 100 ml in a volumetric flask using toluene for the dilutent. Stopper and mix well. Using this dilution, analyze for polymer. The result read from the chart and multiplied by ten is the ppm polymer in the original sample. Calculations Read ppm polymer from a previously prepared calibration curve or chart. Calibration and/or Standardization Weigh out 0.0905 gms of polystyrene and dissolve in 100 ml of C.P. toluene for a 1000-ppm polymer standard. This may be diluted 10 ml to 100 ml with toluene to make a 100-ppm standard. Make dilutions of the 100 ppm standard as follows: 5 ml, 10 ml, 15 ml, 25 ml, 50 ml and 75 ml, up to 100 ml total volume with toluene. Mix. These correspond to 5, 10,15, 25, 50 and 75 ppm polymer. Analyze these by the procedure previously described. Plot the %-transmittances versus the corresponding ppm-polymer. Draw in the best representative curve. Use this for a working curve or prepare a chart covering a range of %-T's for certain values of ppm polymer. Preparation of Reagents Methanol should be C.P. and dry. Toluene should be dry. 100 ppm polymer - prepare some polystyrene by scrubbing 50 ml of 99.7+% SM with 1 N NaOH twice, discarding the lower aqueous phase each time to waste. Then scrub the styrene twice with distilled water discarding the aqueous phase each time. Finally, filter the styrene through two layers of No. 12 filter paper. Place about 20 ml of the styrene, thusly prepared, in a test tube. Place in 100 C oven for 24 hours. Remove the polymer from the test tube by breaking the tube and discarding all glass. Grind the polymer plug to a fine powder in an agate mortar. Safety Precautions The method has no unusual dangers. Materials worked with are flammable and toxic. Handle chemicals, glassware, and electrical equipment with caution.

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SECTION C PHYSICAL PROPERTIES OF STYRENE MONOMERThe data contained in this section have been obtained or derived from the chemical literature. References to the literature are listed at the end of the section. Property Auto ignition temperature (in air) Boiling point(6) (5)

Value 490C (914F) 145.45C (293.25F) 122.5 101.1 82.2C 60.6C 33.4C -1.7C Colorless Non-corrosive to metals except to copper and alloys of copper

760 mm Hg 400 mm Hg 200 mm Hg 100 mm Hg 40 mm Hg 10 mm Hg 1mm Hg

Color Corrosivity Critical pressure (pc )(6) (6)

37.9 atm. 362.9C (685.2F) 3.378 ml/g -4 9.710x 10 at 20C 9.902 x 10 at 40C Temperature (C) 0 20 60 100-4

Critical temperature (t c )(6)

Critical volume (V c ) Cubical coefficient of expansion (per deg. C) ) (6) Density of liquid(2)

Density (g/cc) 0.9226 0.9050 0.8687 0.8306 2.4257 2.3884 2.3510

Dielectric constant of liquid

(3)

Temperature (C) 20 40 60 82.48 cal/(g-mole)-deg. 1.1 6.1 volume-percent 31C (88F) 37C (98F) 51.09 kcal/(g-mole) -30.6C (-23.13F) -1018.83 kcal/(g-mole) 35.22 kcal/(g-mole) 24.72 kcal/(g-mole)

Entropy of gas at 25C (S)

(1)

Flammable limits (in air) at atmospheric (4) pressure Flash point(2)

: (Tag. closed cup) (Tag. open cup)

Free energy of formation of gas at 25C (4) (Gf ) Freezing point Heat of combustion, gas at const. press., 25C, (2) to form gaseous products (H0 ) Heat of formation(4) (2)

gas at 25C (Hf ) liquid at 25C (Hf )

Property Heat of Polymerization at 90C (H) (6) Heat of vaporization at 145.1C Molecular weight Odor(6) (4)

Value -17.8 kcal/(g-mole) 8.86 kcal/(g-mole) 104.152 g/(g-mole) Characteristic, aromatic pleasant odor in very low concentrations; increasingly disagreeable at higher concentrations Liquid Temperature (C) 15 20 25 30 ND 1.5491 1.5465 1.5439 1.5413 1.5387 Solubility (g/100 g H20) 0.018 0.029 0.040 0.051 0.062 Solubility (g/100 g styrene) 0.020 0.060 0.100 0.140 0.180

Physical state at room temperature Refractive index:(4)

Solubility: Styrene in water

(4)

35 Temperature (C) 0 20 40 60 80

Solubility: Water in styrene

(4)

Temperature (C) 0 20 40 60 80

Solubility in:

(2)

Acetone CC14 Benzene Ether n-Heptane Ethanol(6)

Soluble in all proportions 0.9052 (25/25C) 0.8866 (60/60C)

Specific gravity

Property Heat Capacity (liquid)(6)

Value Temperature (C) 0 20 40 60 80 100 120 140 c p (cal/g-C) 0.402 0.414 0.428 0.443 0.459 0.477 0.496 0.518 c p (cal/gm-C) 0.256 0.271 0.280 0.292 0.340 0.385 0.462 0.593 Surface Tension (dynes/cm) 31.8 30.9 29.9 29.0 28.1 27.2 26.2 25.3 Viscosity (Centipoises) 1.05 0.751 0.567 0.455 0.375 0.31 0.27 0.233

Specific heat (vapor)

(1)

Temperature (C) 0 15.6 25.0 37.8 93.0 149.0 260.0 538.0

Surface Tension

(7)

Temperature (C) 0 20 40 60 80 100 120 140

Viscosity (liquid)

(8)

Temperature (C) 0 20 40 60 80 100 120 140

Vapor pressure (Antoines equation, t=C

(2)

Log10p (mm Hg) = 6.95711 1446.578 t + 210.2 17%

Volume change on polymerization (shrinkage)(24)

REFERENCES

(1) (2) (3) (4) (5) (6) (7) (8)

American Petroleum Institute Research Project 44, Selected Values of Properties of Hydrocarbons and Related Compounds. Kirk-Othmer Encyclopedia of Chemical Technology, (3 Ed.), Vol. 21, Interscience, New York, 1983 J. Petro and C.P. Smyth, J. Amer. Chem. Soc., 80, 73 (1958). R.H. Boundy and R.F. Boyer (Eds.), Styrene: Its Polymers, Copolymers, and Derivatives, Reinhold, New York, 1952, Ch. 3 Perry 7 Chilton, Chemical Engineers Handbook, (6 Edition), McGraw Hill, New York, 1984, p. 3-60. Design Institute for Physical Property Research (DIPPR) 801 Database (release of 1999), American Institute of Chemical Engineers and Brigham Young University. Kehde, H; Hiscock, B.F.; Coulter, K.E.; Styrene and Related Monomers; Vinyl and Diene Monomers, Part 2; Wiley-Interscience, New York, 1971 Edwards, D; Bonilla, C. Viscosity of Liquid Styrene and Butadiene; Ind. Eng. Chem. 1944, 36, p. 1038th rd

SECTION D CHEMISTRY OF STYRENE INHIBITION AND OXIDATIONThe principal chemical reactions that normally take place in styrene storage facilities are generation of styrene free radicals, inhibition of styrene free radicals, and oxidation of styrene. This section briefly discusses some aspects of these reactions and gives several references to the chemical literature. The information presented here is derived from the literature. FREE-RADICAL INITIATION Thermal initiation of polymer formation increases with increasing temperature. styrene, the initial rate for thermal polymerization is given by the equation: Log rate (%/hr.) = 11.55 (4170/T), Where T is the temperature in degrees K.(1)

For uninhibited

The thermal initiation of styrene free radicals is not completely understood. One scheme which has been proposed . involves the formation of a styrene dimer followed by hydrogen abstraction from the dimer by another styrene molecule to produce free radicals (Equations 1 and 2).(2,3)

Oxygen is also believed to initiate free radicals via hydrogen abstraction of the dimer (Equation 3).

Oxygen initiation has been found to be more important than thermal initiation at 50C(4)

.

ROLE OF TBC AND O2 IN INHIBITION In order for a growing styryl radical (R) to be terminated (inhibited) by TBC, the rate of reaction of TBC with the radical (Equation 4) must be much more rapid than the propagation step for polymerization (Equation 5).

The rate constant for Equation 4 is estimated to be about 63 1./mole-sec. at 60C, while that for Equation 5 is 176 1./mole-sec. . Since the concentration of styrene is normally much greater than that of TBC, the best that can be expected from TBC in the absence of oxygen is a slight (8) retardation of the polymerization process. The reaction of styryl radicals with oxygen (Equation 6) is very rapid (4x10 liter/mole-sec. at 50C). (4) This reaction is several orders of magnitude faster than either termination reaction 4 or propagation reaction 5.7 (5,6,7)

The resulting peroxy radical reacts very rapidly with TBC in a termination step (Equation 7).

The semi-quinoidal compound (II) will also react rapidly with peroxy radicals as shown in radicaltermination reaction 8.

The rate constant for reaction 7 is about 10 liter/mole-sec. at 50C. Reaction 8 is also quite rapid. Consequently, a very low concentration of oxygen will scavenge growing styryl radicals, and the resulting peroxy radicals will be trapped by the rapid reaction of Equations 7 and 8(9,10,11)

4

.

It has been suggested that each TBC molecule can terminate four radicals. Reactions illustrating this are shown in Equations 7 10.

EFFECTS OF OXYGEN Data showing the effect of oxygen depletion on polymer formation in styrene are given in Table D-1. The data were obtained at 100C in the presence of an excess of TBC. The danger of allowing the oxygen level to fall below 10 ppm is clearly shown. For this reaso