154328976 Nitrogen Blanketing

Nitrogen Blanketing

A computer program has been developed to size blanketing systems for chemical storage tanks. Here are the details.

T. J. DePaola and C. A. Messina, Linde Division, Union Carbide Corp., Tarrytown, N. Y. 10591

In many industries, inert gas blanketing is used in the storage, transport, packaging, and handling of materials such as foods, pharmaceuticals, petroleum products and chemicals. Vessels which are blanketed range in size from single-serving orange juice containers, to hundred thou- sand barrel (16,000 m3) tanks used for the storage of etro- leum derivatives. A computer ro ram has been x evel- oped to size blanketing systems P a or c emical storage tanks. NEED FOR BLANKETING

Blanketing is recommended or required in many situations. The most common reasons for blanketing are: reduction of operating hazards due to flammability, mainte- nance of stored product uality, reduction of corrosion,

Perhaps the most important reason for blanketing is to reduce operating hazards with flammable liquid products. Three elements are needed for a fire or explosion to occur. These elements are combustibles or fuel (flammable vapors), an oxidant or source of oxygen (usually air), and a source of ignition. Together, these three elements form the Fire Triangle. The source of ignition can be an open flame, a static electrical dischar e, or other ignition

tially eliminates oxygen and lessens the chance of a fire. Figure 1 shows a flammability diagram for the system methane, oxygen, and nitrogen.

and reduction of product 7 oss due to evaporation.

source within the tank. Blanketingwit 1 an inert gas essen-

OXYGEN, VOLUME %

n.pnn(.d rim p.rmiulon 01 t k u.8. ~ u n a u d win-.

Figure 1 . Flammability diagram for the system methane-oxygen-nitrogen at atmospheric pressure and 26C.

PlantlOperations Progress (Vol. 3, No. 4)

If only one element of the Fire Triangle is absent, or if the relative concentrations of combustibles and oxidant fall outside the flammable limits, a fire cannot occur. One exception is ethylene oxide, a chemical that needs no oxygen for a fire or explosion to occur ( the upper flammable limit of eth;ylene oxide is 100%).

In general, if the vapor concentration is too rich or too lean, a fire will not occur. However, using only vapor concentrations for flammability control is not com letely

centration into the flammable region. Increasing the temperature or pressure usually enlarges the flammable region [ I ] . Tabulated flammability limits are usually given for typical conditions (atmospheric temperature and pressure) that prevail in chemical storage tanks, and therefore may be incorrect in unusual situations.

The U.S. Bureau of Mines has published the Flamma- bility Characteristics of Combustible Gases and Vapors, which can be used as a general reference [Z] . A more recent publication by the NFPA lists flammability limits for many products [3] .

Other types of combustible materials include particles such as the fine dust often found in grain storage bins or metal powders. These materials present a special type of explosion hazard. In general, however, dusts with particle sizes above 400 pm will not explode [4].

Blanketing sensitive materials is often essential to maintain product quality. Some products degrade by reaction with oxygen or moisture in the air. Blanketing gases can be chosen such that they contain very small amounts of oxygen and moisture. Also, when a storage tank is kept under a positive pressure as rn blanketing, air infiltration, and therefore product contamination, is minimized. Table 1

failsafe. Air leaking into a rich mixture can bring t E e con-

TABLE 1. TYPICAL MATERIALS REQUIRING INERT GAS BLANKETING

Material HazardiProblem [SJ

Acetaldehyde Aniline 1, 3 Butadiene

1, 1 Dichloroethene Diethyl Zinc

Ethylene Oxide

Reacts with oxygen to form peroxides. Contact with oxygen causes darkening. Flammable; reacts with oxygen to form peroxides. Flammable liquid; explosive. Ignites on contact with air; reacts violently on contact with water. Extremely flammable liquid; explosive with wide range of flammable limits. Flammable liquid; explosive with wide range OF flammable limits. Reacts with moisture and oxidizingma- terials.

flammable.

flammable.

H ydrazine

Maleic Anhydride

Phosphorus Trichloride Reacts violently with water;

Trichlorosilane Reacts violently with water;

Octoober, 1984 203

contains some typical materials requiring inert gas blanketing, and the hazard or problem avoided by blanketing. Other products need some oxygen present to maintain product quality. For example, inhibited monomer storage requires a controlled amount of oxygen present for the inhibitor to function properly, otherwise the monomer will degrade by polymerization. High and consistent product quality is important to both manufacturers and distributors (specifications, price), as well as to end users (reactivity, purity).

Blanketing can also help minimize the formation of unwanted and potentially hazardous reaction products. For example, the reaction of acetaldehyde with ambient oxygen produces organic peroxides which are highly explosive.

Another related problem, the corrosion of storage vessels and piping, may be caused by acids formed by reaction ofcertain products with moisture. This situation is trouble- some, and may even be dangerous. Replacing corroded materials is expensive, labor-intensive and can slow or halt plant operations. Blanketing reduces corrosion by replacing moist air with dry inert gas.

In addition to reducing operating hazards, maintaining product quality and reducing corrosion, inert gas blanketing can reduce product loss of volatile materials resulting from evaporation. Blanketing systems reduce loss by allowing vapors to vent only when relief settings are reached, unlike tanks open to the atmosphere, where vapors are continually venting due to evaporation. Blan- keting is especially desirable with an expensive, volatile material. In addition, it is necessary with the storage of a noxious or toxic product whose vapors could present health or ecological hazards.

CHOICE OF BLANKETING GAS

Gases commonly used for blanketing include nitrogen, carbon dioxide, nitrogen-carbon dioxide mixtures, natural gas or methane, combustion-generated inert gas, and flue gas.

Combustion-generated inert gas and flue gas have several disadvantages. Being combustion products, they are essentially nitrogen and carbon dioxide. However, they can also contain significant amounts of carbon monoxide, uncomhusted hydrocarbons, hydrogen, oxygen, and water. These residual constituents reduce the purity of the blanket gas and may cause problems themselves. With sulfur containing fuels, sulfur dioxide can present corrosion problems as can oxides of nitrogen. In most cases, down- stream treatment of combustion-generated inert gas and flue gas is often required when a low moisture product is needed. In other casec, caustic scrubbers or adsorbents are used when the carbon dioxide must be removed.

Natural gas is used for blanketing when it is readily available and can be recovered for its heating value. How- ever, because it is combustible, its use is less desirable. Another potential disadvantage is the dissolution of methane in the stored product.

Carbon dioxide is soluble in many liquids and may con- taminate stored materials. The solubilities of nitrogen and carbon dioxide in various liquids are shown in Table 2.

TABLE 2. SOLUBILITIES O F NITROGEN AND CARBON DIOXIDE IN VARIOUS LIQUIDS AT ATMOSPHEHC PRESSURE AND 25C [6]

Nitrogen Carbon Dioxide (cm3/cm3) (cm3/cmS)

Amy1 Acetate 0.15 4.1 Carbon Disulfide 0.06 0.87 Methyl Alcohol 0.14 3.8 Petroleum (Crude) 0.12 1.2 Toluene 0.12 2.3 Water 0.016 0.82

This dissolved carbon dioxide may have a detrimental ef- fect on the end use of the product. Carbon dioxide reacts with moisture to produce carbonic acid which may form undesirable carbonates in stored products, and lower the pH. The price of carbon dioxide depends greatly upon availability, for it is produced mainly as a by-product stream from oil refineries and fertilizer plants.

For several reasons, nitrogen is the most commonly used blanketing gas. Because it is essentially non-reactive and has low solubility in liquids, the risk ofproduct contamination is virtually eliminated. Nitrogen is also extremely low in moisture and oxygen content. When the refrigeration value of liquid nitrogen can also he used, it becomes an especially attractive blanketing gas.

The type of nitrogen supply depends strongly upon volume and use patterns. Compressed gas cylinders are best suited for very low volume uses. As nitrogen requirements increase, gas cylinders become impractical and uneco- nomical to store and transport. Other methods used to supply nitrogen are bulk liquid storage, various forms of on- site plants such as a cryogenic nitrogen plant (N-plant), and pipeline supply from a nitrogen producing plant which serves more than one customer.

DESIGN CONSIDERATIONS OF A BLANKETING SYSTEM Components of a Blanketing System

A blanketing system provides a means of allowing the blanket gas in and out of the storage tank headspace on de- mand. A conventional inert gas blanketing system, shown in Figure 2, consists of the following equipment: gas blanketing regulator, pressure relief valve, vacuum relief valve, and rupture disk (optional).

One important aspect of specifying a blanketing s stem is sizing the equipment. The equipment and gctors determining their size are discussed below.

The gas blanketing regulator reduces the supply pressure to the desired blanket pressure. I t protects the storage tank from a vacuum condition by supplying blanket gas to the tank headspace, and is sized to satisfy the inbreathing requirement [7J Inbreathing is caused by: 1) the contraction of vapors in the tank headspace resulting from a decrease in atmospheric temperature; and 2 ) the outflow of liquid. To determine the inbreathing requirement due to the contraction of vapors, a condition was chosen which would represent the maximum rate at which the storage tank could cool [fl. The amount of blanket gas required is that which would prevent a vacuum condition when the storage tank is rapidly cooled by a rainstorm on a hot, sunny day. The amount of blanket gas required during liquid withdrawal is that necessary to replace the volume of liquid being removed.

The pressure relief valve protects the storage tank from becoming overpressurized during normal operation. The pressure reliefvalve capacity is determined by: 1) the maximum failed regulator capacity (includes expansion of vapors resulting from an increase in atmospheric temperature), and 2) outbreathing due to the inflow of li uid. The

the reliefvalve capacity needed to prevent the storage tank maximum failed regulator capacity is used to ? etermine

PRESSURE SENSING LINE PRESSUREIVACUUM RELIEF VALVE

n INERTGAS

SUPPLY

Figure 2. Inert gas blanketing system,

Plant/Operations Progress (Vol. 3, No. 4) 204 October, 1984

from being overpressurized if the regulator failed in a com- pletely open position. The maximum failed capacity can be found in the regulator sizing literature and may be the same as the maximum regulated capacity, depending on the manufacturer. The vapor vented, due to the inflow of liquid, is determined by the amount of vapor displaced by the liquid added. When the liquid has a flash point below 100F (37.8C), an adjustment must be made to account for the additional vapor vented due to the evaporation of some liquid [7J This is apparent especially when filling an empty tank.

The vacuum relief valve protects the storage tank from a vacuum condition by su plying air to the tank headspace

the regulator fails to open. The vacuum relief valve capacity must equal or exceed the regulator capacity.

The rupture disk protects the storage tank from becoming overpressurized during emergency situations. The vapor venting capacity is that required when the tank is exposed to fire [a. This venting capacity can be handled by oversizing the pressure relief device. However, this is not too common because the large gas volumes vented in emergencies would re uire an extremely large, and there-

when: 1) there is a blan E et gas supply interruption; or 2)

fore expensive, relief 4 evice. Selection of Blanketing Equipment

Storage tanks are blanketed at various pressures, depending on the application. For our purposes, low, medium, and high pressure blanketing is limited to the ranges defined below. The most common, low pressure gas blanketing, utilizes pressures in the range of 1/2 to 2 inches water column (124-497 Pa g). These pressures are required for field erected storage tanks, where the tank size ex- ceeds approximately 30,000 gallons (113.6 m3), but can also be used for certain applications in smaller tanks. Medium- pressure blanketing utilizes pressures in the range of 2 inches water column (497 Pa g) to 15 psig (103 kPa g) and high pressure blanketing exists when pressures exceed 15 psig. Medium and high pressure blanketing (sometimes referred to as tank padding when pressures exceed a few psig) are used when low pressures cannot be used due to product storage requirements.

There are generally three types of regulators used for blanketing: 1) external pilot operated; 2) internal pilot operated; and 3) direct operated. The external pilot operated system is a multi-regulator arrangement utilizing a sepa- rate sensing regulator to control the main blanket gas control valve. The internal pilot operated system is a one valve system, where the sensing regulator is part of the main valve assembly. The direct operated regulator is a spring or weight operated valve in which the opposing diaphragm assembly senses the tank pressure directly. All three types of re ulators can he used for low, medium, and high pressure Rlanketing. Direct operated regulators, however, re uire a relatively large diaphragm for low pressure gasxlanketing.

Pilot, spring, or weight loaded pressure and vacuum relief valves are used, depending on the ap lication and re-

valves usually have the ability to attain lower set pressures than spring loaded relief valves. Weight loaded relief valves are usually used for lower set pressures (inches of water), and spring loaded relief valves are usually used for higher set pressures (>1 psig, or 6.9 kPa g). Pilot operated relief valves have a wide range of set pressures, from inches of water to thousands of pounds, and usually re- quire less overpressure for full lift than spring or weight loaded relief valves.

A rupture disk or reuseable emergency relief vent can be used for emergency venting requirements, depending on the application.

Plantloperations Progress (Vol. 3, No. 4)

lief pressures needed. Pilot and weig hp t loaded relief

Compotibility

Another important aspect of specifying a blanketing sys- tern is compatibility.

The blanket gas must be compatible with the chemical to maintain safe operating conditions and prevent product degradation. This information was discussed earlier, under the heading, the Need for Blanketing.

The blanketing equi ment materials of construction

prevent product contamination and malfunction of the equipment due to material failure. If the blanket gas contains hydrocarbons or other corrosive components, the diaphragm material and other materials should be chosen ac- cordingly. Excessive amounts of moisture in the blanket gas may also cause corrosion if the proper materials are not chosen. The remote location of blanketing regulators, when possible, will prevent their contact with the chemical vapor. In this case, it is not necessary to make the regulator materials of construction compatible with the chemical. The relief valves, however, are almost always exposed to the chemical vapor and must be constructed of the proper materials.

The blanketing equipment materials of construction must comply with federal, state, and local codes [9].

must be compatible wit K the blanket gas and chemical to

NITROGEN BLANKETING PROGRAM

An interactive computer program has been developed by Linde to size blanketing systems for liquid chemical storage tanks. The program is written in FORTRAN level VS for the IBM 370 computer. The computer program de- termines the size, in SCFH air capacity, of each compo- nent in the blanketing system, including: the gas blanketing regulator, pressure relief valve, vacuum relief valve, and rupture disk. The equipment sizing calculations can be used for low, medium, and high pressure blanketingap- plications and are in compliance with industrial standards [7J Also determined are the nitrogen consumption and chemical evaporation loss using a model based on field data [lo, 111. The nitrogen consumption and chemical evaporation loss calculations can be used only for low pressure blanketing applications.

The computer program contains the necessary physical property data of over 50 chemicals which are most commonly blanketed. Also included is the necessary weather data for various locations in the United States.

The nitrogen blanketing program requires the input data described in Table 3. An example of the nitrogen blanketingprogram input and output is shown in Figure 3.

TABLE 3. INPUT DATA REQUIRED FOR NITROGEN BLANKETING PROGRAhl*

Name of Chemical Tank Location Tank Information:

Capacity Diameter or height Average liquid content Fill and discharge rates Desi n pressure Num a er of tank turnovers per year Paint color and condition Specify whether tank has cone or flat roof Specify whether tank is horizontal or vertical Specify if tank is heated or refrigerated and at what temperature

or temperature range Regulator, pressurehcuum relief and rupture disk set points Nitrogen supply pressure to regulator

* Chemical and weather data are also required when the chemical and tank location are not in the nitrogen blanketing program data hanks.

October, 1984 205

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . * * X * * X * X * X * * * * X * * * * X * X CASE NO. 1 X * * * X * * * X X * * * * * * * * k * X X * * * * * *

************ CHEMlCAL 1 N F O H m T l O N **************** B KN Z K N E CHEMlCAL STORED

CHEMlCAL FORHU1.A gH6 MOLECULAR WEIGHT - 7 8 . 1 FLASH P O I N T , F - 1 2 . 0 LATENT HEAT OF VAP. @ 60 F . B T U I L B - 2 1 0 . L I Q U I D S P E C I F I C GRAVlTY (3 60 P I 6 0 [z 0 . 8 8 4 ANTOZNE EQUATLON CONSTANT A - 1 4 . 2 5 1 ANTOINE EQUATlON CONSTANT B - 1 9 1 1 . 5 7 ANTOINE EQUATlON CONSTANT C - 1 0 2 . 1 7

( S E E NOTE 5)

* * * * * * * * * * A * * * * * TANK INFORMATION **************** TANK HAS CONE ROOF TANK CAPACZTY, GALLONS - 100000. TANK DlAMETER, F E E T - 30.0

TANK DESIGN P R E S S U R E , P S l G 2 .00 TANK F I L L RATE, GPM - 500. TANK DISCHARGE RATE, GPM - 500. AVERAGE L I Q U l D CONTENT, % OF CAPAClTY - 7 5 .O TANK TURNOVERS P E R MONTH - 1 . 0

TANK HEIGHT. F E E T 1 8 . 9

- -

( S E E NOTE 2 )

********X***** TANK PRESSURE I N F O **************** NITROGEN SUPPLY ( I N L E T ) PRESSUHE TO

REGULATOR, P S I G - 50.0 REGULATOR S E T P O I N T , INCHES W.C. - 0.500 PRESSURE R E L l E F S E T P O I N T , I N C H E S W.C. - 0.865 VACUUM R E L l E F S E T P O I N T , INCHES W.C. - 0.865 BURSTlNG D I S C S E T P O I N T , INCHES W.C. - 2.000

-

-

************** PRESSURE REGULATOR **************** INBREATH DUE T O TEW. DECREASE, SCFli 2384.

MIN. REGULATOR CAPACITY, SCFH 7 0 2 8 . INBREATH DUE T O L I Q U I D OUTFLOW, S C F l l - 4005.

( S E E NOTE 1)

************* PRESSURE R E L I E F VALVE * * * * * * * * * * * *A* MAX. REGULATOR CAPACITY, SCFt t - 7 0 8 8 . OUTBREATH DUE T O L I Q U l D INFLOW, S C F l l - 8590. TOTAL FLOW REQUIREMENT, SCFI1 156 7 8 .

( S E E NOTE 1)

2 6 2 O K 4 8 7 . 8 k J I k g

3 7 8 m3 9 . 1 m 5 . 8 m 13 .8 kPa g 1.89 m 3 / r n i n 1 .89 m 3 / m i n

3 4 4 . 8 kpa g 1 2 4 Pa g 215 Pa g 2 1 5 Pa g 4 9 7 Pa g

**m3 /hour** 63.9 1 0 7 . 3 188.3

189 .9 2 3 0 . 1 4 2 0 . 0

188.3

13886.6

ASPIRATOR BLANKETING In some blanketing ap lications, it

have a controlled level o P oxygen in complish this type ofhlanketing oped a design concept called aspirator blanketing. For this type of tank blanketing, a gas eductor would be used to vary the blanket gas composition according to process requirements.

206 October, 1984

Basically, the same equipment would be used for aspirator blanketing as for normal tank blanketing with the exception of the aspirator, or gas eductor. A simplified aspirator blanketing system is shown in Figure 4. The blanketing regulator maintains the storage tank headspace pressure at the set point via a sense line. The aspirator is installed between the blanket regulator outlet and the storage tank inlet. When the headspace pressure drops below

Plant/Operations Progress (Vol. 3, No. 4)

JANUARY FEBRUARY

MARCH APRIL HAY JUNE JULY

AUGUST SEPTEMBER OCTOBER NOVEMBER DECEMBER

BREATHLNG WORK 1 N G MEAN MEAN MEAN CHEMLCAL CHKMICAL TOTAL TEMP TEMP TEMP LOSS LOSS CHKMICAL NITHOGKN DEG F DEG F DEG F LBS LBS LOSS CONSUMP

(SEE NOTE 6) (SEE NOTE 7) (SEE NOTE 4) LBS SCF'

41.7 46 .O 53.4 63.1 71.1 78.6 82.3 81.4 75.1 64.2 52.4 45.0

52.6 55.3 60.8 68.0 75.1 82.7 86.7 85.4 79.8 69.3 59.5 51.1

30.3 35.7 44.9 58.9 66.0 15.4 79.0 77.9 68.3 56.8 45.1 36.7

74. 77. 82. 77. 91 1 96. 107. 103. 112. 92. 76. 64.

MEAN MONTHLY BREATHlNG CHEMlCAL LOSS, LBS = 88. MEAN MONTHLY WORKING CHEMlCAL LOSS, LBS MEAN MONTHLY TOTAL CHEMICAL LOSS, LBS MEAN MONTHLY NITROGEN CONSUMPTION. SCY

s LB = 0.4536 k DEG K = (DEG F + 459.67)/1.8 SCF = 0.0283 m

*ALL FLOWS ARE SCFH OF AIR Q 60 F.

~API STANDARD 2000, THIRD EDITION, JANUARY 1982.

2THE HElGHT WAS CALCULATED FROM THE TANK VOLUME AND DIAMETER.

3BURSTING DISC USED FOR EMERGENCY RELIEF, SUCH AS FIRE EXPOSURE.

4 ~ 1 BULLETlN 2523, FIRST EDITION, NOVEMBER 1969.

= 271. = 359. = 16167.

132. 152. 191. 256. 321. 394. 434. 424. 358. 264. 186. 147.

206. 229. 274. 333. 412. 490. 540. 527. 470. 356. 262. 211.

16131. 16430. 16S5 7. 16176. 16222. 15403. 15132. 15325. 16102. 16663. 16509. 17352.

~ANTOINE EQUATION OF THE FORM: LN(P) = A - B/(T t C) WHERE: P IS VAPOR PRESSURE, nun HG

(nun HG = 0.133 kPa) T IS TEMPERATURE, K.

THE TEMPERATURES ARE THE MEAN DAILY TEMPERATURES FOR THE HONTH. THE MEAN IS TAKEN OVER A PERIOD OF 30 YEARS.

7"BACKGROUND DQCUMUNTATION FOR STORAGE OF ORGANIC LIQUIDS",

TRW ENVIRONMENTAL, INC., RESEARCH TRlANGLE PARK, NC, HAY 1981.

EPA CONTRACT 68-02-3174,

Figure 3. (Continued from page 206) Example of nitrogen blanketing program input and output.

the set point, the regulator opens, flowing inert gas into the headspace via the aspirator. This flow through the aspirator causes a negative pressure to develop at the air inlet of the aspirator, which causes air to be aspirated into the inert gas stream. A fixed orifice and globe valve are used to modulate the air flow into the as irator. A check valve is installed to prevent backflow of ciemical vapors.

For applications requiring more precise control of the oxygen concentration in the blanket gas, a fixed orifice and control valve would be used to modulate air flow into the aspirator. The signal to operate the control valve would be derived from the variable inlet pressure to the aspirator via a proportioning relay. This system would provide an

essentially constant oxygen concentration in the blanket gas for a given application.

AIES!4URE/VACUUH PRESSURE SENSING UNE RELIEF VALVE

CHECK VALVE

FIXED GLOBE ORIFICE VALVE

INHIBITED MONOMER STORWE

Figure 4. Aspirator blanketing system.

Plant/Operations Progress (Vol. 3, No. 4) October, 1984 207

Evaluate: 1) Process

operating hazards 2) Product: flammability,

sensitivity, reactivity, corrosivity, value, health and ecological hazards, etc.

Evaluate Need for

Regulator, pressure/ vacuum relief and rupture disk capacities determined in SCFH of air

low and average I I temperatures Nitrogen consumption L,,---J and chemical

evaporation loss determined in SCF/month and

Aspirator blanketing would enable a blanketing system of moisture and excess oxygen in the storage tank to maintain the desired oxygen concentration in the blan- headspace. This enhances the safety aspect of blanketing ket gas without connecting air or oxygen lines to the inert when small amounts of oxygen in the headspace are gas supply line. This reduces the chances ofcontaminating required. the inert gas supply with oxygen. Aspirator blanketing, as Inhibited monomer storage is one application where opposed to blanketing with air, would reduce the amount aspirator blanketing could be used. For example, styrene

208 October, 1984 Plant/Opemtions Progress (Vol. 3, No. 4)

and acrylic ester monomers need some oxygen present for the inhibitor to function properly, otherwise the monomer will degrade by polymerization. Also, reduced moisture in the storage tank leads to enhanced monomer storage life and improvements in the polymerization process.

Actually, any blanketing application that could tolerate small amounts of oxygen and moisture could utilize aspirator blanketing to reduce the inert gas consumption.

GENERAL CONSIDERATIONS Safety

Inert gas blanketing does not eliminate the need for other safety measures, such as electrical rounding, proper

etc. The benefits of inerting are realized only in the storage tank headspace and not in the external surroundings of the tank. The flammable vapors vented from the inerted tank during outbreathin may be subject to ignition as they mix with air at the vent &charge opening. The vapors can also settle and accumulate in a trench or other poorly venti- lated area.

Tanks blanketed with an inert gas should be clearly marked to insure that no worker will enter without the proper equipment, such as an air supply mask, life line, etc. [ I ] . Additional care should be taken to disconnect the inert gas supply from the storage tank being entered.

location and spacing of tanks, fire fig a ting equipment,

Purging a Storage lank Into Service

When a new storage tank (which will store blanketed materials) is put into service, or an existing one changes service, it may be necessary to purge the tank with inert gas prior to filling. Three methods of purging commonly used are: 1) vacuum purging, 2) syphon purging, and 3) dilution purging.

In vacuum purging, a vacuum is pulled on the storage tank and it is then backfilled with inert gas [12]. This is probably the most efficient method, but least likely to be used since relatively few liquid storage tanks can with- stand a vacuum.

For syphon purging [I], the storage tank is filled with liquid (product or water). Inert gas is fed into the storage tank as the liquid is drained. Since there will be some residual moisture if water is used, this method cannot be used for moisture-sensitive chemicals.

Dilution purging is the simplest and most common method used to purge storage tanks [12]. The inert gas feed into the tank as well as the vent stream, containing inert gas and contaminant, are both continuous. Dilution purging is based on the assumption that the contents of the tank are well mixed. The feed and vent flow rates, as well as the location of the inlet and outlet nozzles, must therefore promote effective mixing of the storage tanks contents. For exam le, the inert gas feed nozzle should be located as

inert gas from venting before it has a chance to mix with the tanks contents. Analyses of the contents at different points in the tank should be performed to make certain that the contaminant concentration is below the required level. SUMMARY AND CONCLUSIONS

Inert gas blanketing is used in many industries, ranging from food to petroleum. It is used at various stages in pro- duction, from the raw material to the finished product, including transportation. The most common reasons for blanketing are: to reduce operating hazards, maintain product quality, reduce corrosion, and reduce product loss due to evaporation.

far from t B e vent nozzle as possible. This will prevent the

Selection of the inert gas and blanketing equipment varies from a lication to application. Proper equipment selection anxiesign can increase the safe operation of a storage facility, as well as save significant operating costs. The computer program developed by Linde will help the customer to select and size the proper equipment for each and any blanketing, application. A summary of the proce- dure used to select and design a nitrogen blanketing system is shown in Figure 5.

Remember, inert gas blanketing does not eliminate the need for other safety measures [ I ] . The benefits of inerting are realized only in the storage tank heads ace, not in the external surroundings. Extreme care m u s t t e taken when working on inerted tanks to insure that the pro er life support and safety equipment are used by workers fI].

LITERATURE CITED

1. 2.

3.

4.

5. 6.

7.

8.

9.

10.

11.

12.

NFPA 69, Explosion Prevention Systems (1978). Zabetakis, Michael G., Bureau of Mines Bulletin 627, Flammabilitv Characteristics of Combustible Gases and Vapors (1965). NFPA 325 M, Fire Hazard Properties of Flammable Liq- uids, Gases, Volatile Solids (1977). Bartknecht, W., Explosions-Course Prevention Program, Springer-Verlag, Berlin-Heidelberg (1981). NFPA 49, Hazardous Chemicals Data, (1975). Linke, William F., Ph.D., Solubilities of Inorganic and Metal-Organic Compounds, Washington, D. C. (1965), Vol. I

API Standard 2000, Venting Atmospheric and Low-Pressure Storage Tanks, Third Edition (January 1982). API Bulletin 25118, Evaporation Loss From Fixed-Roof Tanks (June 196:2). NFPA 30, Flamlmable and Combustible Liquids Code ( 198 1). EPA Contract #68-02-3174, Background Documentation for Storage of Organic Liquids (May 1981). API Bulletin 2523, Petrochemical Evaporation Loss From Storage Tanks, First Edition (November 1969). Nitrogen Purging and Inerting, L-5547, Union Carbide Corporation (1983,).

pp. 479-493, V O ~ . I1 pp. 575-590.

Thomas DePaola is a Development Engineer in the Bulk Gas products Development Department of the Linde Division, Union Carbide Corpora- tion. He is responsible for developing and evaluating industrial gas applications in the chemical industry. He holds Bachelor and Master De- grees in Chemical Engineering from Manhattan College, and is a member of the AIChE and ACS.

Since 1982, Celeste A. Messina has been Market- ing Manager, Chemical Industry, in the Bulk Gas Products Market Development Department of the Linde Division, Union Carbide Co oration Her responsibilities include evaluating xveloping industrial gas applications and investigating com- mercial uses of gas technology extensions. Previ- ously, she worked for 4 years in synthesis research for Union Carbides Molecular Sieve Department. She holds B.A. degrees in Mathematics and in French from the College of New Rochelle, and a B.S. degree in Chemistry and a M.S. degree in Physical Chemistry from the University of Paris. Hyolder of a M.B.A. degree in Finance/Marketing from Fordham University, she is amemberofACS.

PlandOperations Progress (Vol. 3, No. 4) October, 1984 209

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154328976 Nitrogen Blanketing