Whiting Equipment Canada Inc.Swenson Evaporators

Swenson® evaporator experience Swenson Technology, Inc. is named after a chemical engineering pioneer. Magnus Swenson (1854-1936), a Norwegian immigrant and engineer. Following his first evaporator invention in 1886 - a submerged, steam-heated, horizontal-tube evaporator - Swenson formed his own company to build evaporators, crystallizersand other process equipment for a number of diverse industries. Literally thousands of evaporators have been built bearing the Swenson name. Swenson has long maintained a fully equipped laboratory for development and feasibility testing. This facility includes rising and falling-film evaporators, a forced-circulation evaporator and other process equipment.Operation is typically around the clock’ and tests with centrifuges, filters and dryers can be made so that the laboratory process will duplicate the commercial process as closely as possible for customer evaluation. Swenson has regionally located sales offices staffed by chemical engineers and an in-house staff capable of designing and engineering complete evaporator installations including steelwork, instrumentation, utilities,

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What is an evaporator? An evaporator is used to evaporate a volatile solvent, usually water, from a solution. Its purpose is to concentratenon volatile solutes such as organic compounds, inorganic salts, acids or bases. Typical solutes include phosphoric acid, caustic soda, sodium chloride sodium sulfate, gelatin, syrups and urea. In many applications, evaporation results in the precipitation of solutes in the form of crystals, which are usually separated from the solution with cyclones, settlers, wash columns, elutriating legs, filters or centrifuges. Examples of precipitates are sodium chloride, sodium sulfate, sodium carbonate and calcium sulfate. The desired product can be concentrated solution, the precipitated solids, or both. In some applications, the evaporator is used primarily to recover a solvent, such as potable water from saline water. In any case, the relatively pure condensed water vapor from many evaporators is recovered for boiler feed makeup, salt washing, salt dissolving, pump seals, instrument purges, equipment and line washing and other uses.

Fig. 1. Quintuple-Effect, Forced-Circulation NaCl Evaporator

separation, drying and material handling equipment. In

EVAPORATOR APPLICATIONS The following is a partial list of applications for Swenson evaporators: LIV Rising- and Falling-Film Evaporators: Acid Sulfite Liquor Ammonium Nitrate Animal Blood Blow-Down Water Calcium Chloride Caestic Potash Caestic Soda. Diaphragm Cell Caestic Soda. Membrane Cell Caustic Soda, Mercerizing Caustic Soda, Mercury Cell Coffee Extract Fish Stickwater Gelatin Glue Green Liquor, Semi-Chum Hydrolyzate Liquor Kraff Liquor Magnesium Chloride N.S.S.C. Liquor Pectin Pentaerythritol Phosphoric Acid (Bright Dip) Pickle Liquor Plating Bath Radioactixe Waste Salte Brine (Below Saturation) Salt-Free Glycerine Sodium Aluminate Sodium Nitrate Sodium Sulfate Sorghum Soybean Oil Spin Bath Steepwater Stillage Sugars Sulfuric Acid Syrups Tankwater Tomato Juice Urea

addition, Swenson has the capability to build any size of process equipment any where in the world through its network of licensees and affiliates.

Choice of equipment Swenson designs and manufactures four general types of evaporators. The first two are used for concentrating solutions where no precipitates are formed. The other two are used for salting or scaling applications.

The long-tube vertical (LTV) rising-film evaporator is commonly used to concentrate many non-salting liquors. For liquors requiring evaporation at extremely low ∆T’s, and for critically heat-sensitive liquors, the long-tube vertical (LTV) falling-film evaporator is preferred. LTV rising-film and LTV falling-film evaporators provide maximum evaporativeperformance for the least capital investment.

For applications where precipitates are formed during evaporation, a forced-circulation evaporator. usually with a submerged inlet, is the most desirable choice.

The calandria. or short tube, evaporator is less popular today than in the past, though a number of companies still prefer this type of evaporator over other designs. The calandria can handle both salting-and scaling-type applications as well as those where no precipitates are formed.

All of these evaporators require a vapor-liquid separation device and a heat exchanger. Different separation devices, commonly called vapor bodies, will be described throughout the text.

Whey Forced-Circulation Evaporators: Ammonium Sulfate Calcium Chloride Caprolactum Caustic Potash Citric Acid Diaphragm Cell Caustic Soda Magnesium Chloride Mono-Sodium Glutamate N.S.S.C Liquor Red Liquor Sodium Carbonate Monohydrate Sodium Chloride Sodium Dichromate Sodium Sulfate Super-Phosphoric Acid Urea Wet Process Phosphoric Acid Calandria Evaporators: The calundria evaporator can be used for many of the applications described for both forced-circulation and LTV rising-and -falling-film evaporators.

Fig. 2. Three-stage, Forced-Circulation Evaporator used toconcentrate wet process phosphoric acid

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Swenson LTV Rising-Film Evaporators In a Swenson single-effect, LTV rising-film evaporator (seeFig. 5), evaporation occurs inside the evaporator tubes, so itis used primarily to concentrate non-salting liquors. To provide for good heat-transfer rates, the ∆T between the heat-transfer medium and the liquor should be greater than 15°F, and preferably greater than 20°F. Tubes are normally 3/4” to 2” in diameter and from 10 to 30 feet long.

Operation of the rising-film evaporator is straightforward. Liquor is fed into the bottom liquor chamber and then into the tubes. There it is heated with condensing steam or any other suitable heat-transfer medium, such as Dowtherm® or hot liquor. If the vapor pressure of the feed equals or exceeds the system pressure at the bottom tubesheet, vaporization will occur immediately. For colder feed, the lower portion of the tubesis used to preheat the liquor to its boiling point. Vaporization then begins at that height within the tubes where the vapor pressure of the feed liquor equals the system pressure.

As the liquor climbs up the inside of the tubes, additional vapor is generated and the velocity of the liquid-vapor mixture increases to a maximum at the tube exit. The outlet mixture impinges upon a deflector, mounted above the top tubesheet of the heat exchanger, where gross, initial separation of the liquid from the vapor occurs.

Additional liquor is separated from the vapor by gravity as the vapor rises in the vapor body. A mesh-type or centrifugal entrainment separator can be installed near the top of the vapor body to remove most of the remaining traces of liquid from the vapor. The exit vapor is conducted to the next effect of a multiple-effect evaporator, to a compressor or to a condenser. A Swenson vertical-tube surface condenser is shown in Fig. 5. The concentrated liquor is discharged from a connection near the bottom of the vapor body.

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heat euse thtubesthe vais sepFig. 7initialFig. 3. Typical Fluid-Temperature Profile of Rising-Film Evaporator

eat-transfer rates are enhanced in the non-boiling n by surface or local boiling and in the boiling sectioncleate boiling. As expected, the heat-transfer rates in iling zone are several times greater than those in the oiling zone, so it is important to reduce the non-g zone to a minimum. typical fluid-temperature profile taken with a

ing thermocouple for a rising-film evaporator is n in Fig. 3. The maximum temperature occurred at a eight feet above the bottom tubesheet where the g began. ifferent two-phase flow schemes are created in the

g zone, including slug flow, where a slug of liquor is ed by a slug of vapor, similar to the perking in a percolator; annular flow, where a ring of liquor

es a center core of vapor and liquid mist; and mist where vapor blankets the tube surface. Mist flow d be avoided because poor heat transfer results when is not enough liquid present to wet the tube walls. To mist flow, it is sometimes necessary to recycle ntrated product from the vapor body to the bottom chamber so as to supplement the feed liquor. (This ulation line is shown in Fig. 5.) The LTV rising-film evaporator can also be adapted centrate moderate scaling liquors when a pump is

to recycle concentrated product from the vapor body bottom liquor chamber. This is known as forced ulation.

The vapor body shown in Fig. 5 is integral with the xchanger. When the heat exchanger is too large to e integral configuration, when quicker access to the

is desired for maintenance, or when surge volume in por body is required for level control, the vapor body arated from the heat exchanger, generally as shown in. A skirt-type baffle replaces the deflector as the separator.

Fig. 4. Swenson Double-Effect, LTV Rising-Film Evaporator

Fig. 5. Swenson LTV Rising-Film Evaporator with Vertical-Tube Surface Condenser

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Swenson LTV Falling-Film Evaporators

A Swenson single-effect, LTV falling-film evaporator with separate vapor body and heat exchanger is shown in Fig. 7. Liquor is fed into the top liquor chamber of the heat exchanger where it is distributed to each tube. Swenson provides several different distribution devices for falling-film evaporators; a distribution plate is shown in Fig. 7.

The liquor accelerates in velocity as it descends insidethe tubes because of the gravity and drag of the vapor generated by boiling. Liquid is separated from the vapor in the bottom liquor chamber of the heat exchanger and with a skirt-type baffle in the vapor body. A supplemental entrainment separator can be installed in the upper portionof the vapor body to reduce liquid entrained with the vapor to a minimum - The Swenson direct-contact condenser (shown in Fig. 7) is used to condense the vapor with water. Concentrated liquor is discharged from the bottom liquor chamber and cone bottom of the vapor body.

The vapor body can also be provided as an integral component of the heat exchanger, similar to that shown in Fig. 5, except the heat exchanger would be located above the vapor body in the falling-film configuration.

Evaporation also occurs inside the tubes of the falling-film evaporator. The unit can be used to concentrate the same non-salting liquids concentrated in rising-film evaporators, and it is suitable for concentrating more viscous liquors. Tube sizes and lengths are comparable to those for rising-film evaporators.

The falling-film evaporator is particularly useful in applications where the driving force in temperature difference between the heat-transfer medium and the liquid is small (∆T’s of less than 15°F). The retention time for liquor in this evaporator is less than that for a rising-film evaporator. The combination of short liquid retention time and the ability to operate at a low ∆T makes the falling-film evaporator ideal for concentrating the most heat-sensitive materials.

High heat-transfer coefficients are attained in falling-film evaporators when a continuous film of liquid, preferably at its boiling point, flows down the inside tube wall with a vapor core in the tube center. For some applications, however, it is necessary to supplement an insufficient quantity of feed liquor with product liquor pumped to the top liquor chamber to avoid vapor blanketing of the inside tube surface.

Fig. 6. Triple-Effect, Membrane Cell, Caustic Soda Evaporator Fig. 6A. Two, 4-Stage, Mechanical Recompression, Falling-Film Evaporators

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Fig. 7. Swenson LTV Falling-Film Evaporator

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Swenson Forced-Circulation Evaporator As the name implies, the liquor in a forced-circulation evaporator is pumped through the tubes to minimize tube scaling or salting when precipitates are formed during evaporation. A Swenson forced-circulation evaporator (with a submerged inlet) complete with a single-pass vertical heat exchanger, elutriating leg, cyclone, and top-mounted barometric condenser is shown in Fig. 8. Slurry is pumped from the bottom cone of the vapor body through the tubes of the vertical heat exchanger, where heat is added, and back into the vapor body where evaporation occurs. Sufficient slurry height (submergence) is maintained above the tangential inlet on the vapor body and above the top tubesheet of the heat exchanger to suppress mass boiling in the inlet and prevent surface (local) boiling on the tube surface. This is necessary to preclude salt precipitation on the tangential inlet and tubes. A high circulation rate is provided for adequate tube velocity to achieve good heat transfer. Therefore, lower slurry temperature rises are assured which minimize supersaturation of the solution. A sufficient quantity of salt crystals is suspended in the circulating system to provide seed crystals in the boiling zone for salt growth. Adherence to these basic principles of crystallization results in coarse crystals and minimal wall and tube salting, so less equipment washing is required. This conserves energy because less steam is required to boil wash water and this increases on-stream time for the evaporator. The circulating pump is usually of the axial-flow, single-elbow design, well-suited for the high flow rates and low pressure drops in Swenson-designed circulating systems. These heavy duty pumps operate at low speeds, which reduce maintenance and minimize mechanical attrition of the salt crystals. Circulating piping interconnects the vapor body, the heat exchanger and the pump. Conical liquor chambers provide gradual, low pressure drop transitions from the circulating piping to the tube bundle which is particularly important for the establishment of uniform feed to the tubes. To provide for thermal expansion without expansion joints, the circulating pump base is spring-mounted. As an alternative, the entire pump may be hung from the circulating piping. The tangential inlet provides excellent mixing of slurry in the vapor body because of the circular motion it creates. Secondary vertical currents are also generated, mixing body slurry with the hotter slurry entering the vapor body to reduce the degrees of flash. This agitation minimizes salt buildup on the bottom cone of the vapor

body. A swirl beaker is provided in the circulating slurry outlet. The vapor body is conservatively designed both in diameter and height. It is important to have an adequate free space above the liquor level to allow the liquor droplets entrained in the vapor leaving the boiling surface to reach equilibrium and return by gravity to the circulating slurry. The large diameters result in low vapor velocities which minimize entrainment and provide adequate retention time for salt growth. A mesh-type entrainment separator may be installed inthe upper portion of the vapor body to reduce solids carryover to normally less than 50 parts per million parts of vapor. Swenson’s sturdy mesh container design has virtually eliminated mesh falls into the vapor body. Other types of entrainment separators are also available. The elutriating leg, attached to the bottom cone of the vapor body, is a convenient device for thickening the slurry it receives from the vapor body and for salt crystal washing and classification - Slurry enters the top of the leg through a unique slurry-inlet device, which improves washing efficiency by reducing agitation in the leg.* Salt crystals are fluidized and washed in the leg with a portion of the feed liquor which enters the bottom cone and is distributed with a perforated, dished head. Smaller crystals are washed into the vapor body for additional growth and the larger crystals are discharged from a connection near the bottom of the leg. A Swenson-designed, low-pressure-drop liquid cyclone is sometimes used to clarify liquor discharged fromthe evaporator. The driving force is the pressure drop across the circulating pump. Thickened slurry is returned through a wide-open cyclone underflow connection to the circulating piping before the pump suction. Another Swenson innovation is the direct-contact condenser mounted on the vapor body. A short piece of vertical pipe connects the vapor body with the condenser to minimize piping and pressure drop. This design also eliminates structural steel for support of a separate condenser. For cooling tower applications, the hotwell is elevated to permit gravity flow of water from the hotwell tothe top of the cooling tower, thus eliminating the need for a pump. A Swenson forced-circulation evaporator with horizontal heat exchanger and a top-mounted stripping column is shown in Fig. 9. Reflux liquid is introduced on the top tray of the column to strip one or more compounds from the water vapor. Stripping columns are used for special applications and are provided either integral with the evaporator or as a separate column. The columns are forthe recovery of valuable components from the water vapor and for the reduction of volatile pollutants. Swenson has supplied stripping columns with ballast and valve trays for caprolactam and boric acid service. Spray columns have been used for the recovery of fluorine generated along with the vapor during the concentration of phosphoric acid. *U.S. Patent No. 4,113,552

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Fig. 7. Swenson Forced-Circulation, Submerged-Inlet, Vertical-Tube Evaporator

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Heat exchangers can be located on the discharge or suction side of the circulating pump.* In some cases, both discharge and suction-side heat exchangers have been provided in the same circulating system to maximize heat-transfer surface.Multiple circulating systems are provided for the same vapor body on large evaporators. Swenson designs include special provisions to reduce tube plugging caused by salt lumps.

A variety of forced-circulation evaporator designs are available, with and without elutriating legs or cyclones. Different inlets and outlets are usedon the vapor body, tailored for the particular application.

For some applications, such as super-phosphoric acid and red liquor, it is necessary to use unsubmerged-inlet evaporators to reduce the volume in the system to a minimum or to minimize foaming. Because of the higher head created by the unsubmerged inlet, it is sometimes necessary to use a mixed-flow circulating pump instead of an axial-flow pump. *U.S. Patent No. 3,303,870

Fig. 9. Swenson Forced-Circulation, Submerged-Inlet, Horizontal-Tube Evaporator with Stripping Column

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The Swenson Calandria Evaporator The Swenson calandria evaporator (see Fig. 10) is applied less often today than it was years ago, though there are still a number of companies that prefer this evaporator for various applications. For most applications, however, the lower equipment costs for other designs has prompted the replacement of calandria evaporators with LTV rising-film, LTV falling-film and forced-circulation evaporators. The calandria evaporator has a heat exchanger (with tubes usually less than six feet long) integral with the vapor body. The level is maintained in the upper portion of the tubes or above the top tubesheet and the circulation pattern is up through the tubes and down through a central pipe called a “downcomer”. Circulation is created by the difference in specific gravity between the body liquor and the heated liquor and vapor generated inside the tubes, plus a vapor lift effect. The Swenson calandria evaporator can be used for salting-type applications; however, an agitator located inside or beneath the downcomer is recommended to suspend the salt crystals in the lower portion of the body. Although the agitator creates some flow through the tubes, most of the flow is still created by “thermo-syphon’. The calandria is also used for batch-type concentration of liquors.

Fig. 10. Swenson Calandria Evaporator

Heat Exchangers Shell and tube heat exchangers are used for the majority of Swenson evaporator installations. The heating medium is almost always steam which is normally condensed on the outside of the tubes to heat the liquor or slurry which flowsinside the tubes. The energy transferred is simply the product of the steam flow rate and the difference in enthalpy between the steam and its condensate. For saturated steam, the enthalpy difference is the latent heat ofvaporization.

The heat-transfer surface required for the energy exchange is calculated as follows:

Ai = Q / (Ui ∆T) = (Ws)( Hs - Hc) / (Ui ∆T) Where Ai = inside tube area, sq ft Q = heat transferred, BTU/hr Ws = steam rate, lb/hr

Hs = enthalpy of steam at a given pressure and temperature, BTU/lb

Hc = enthalpy of condensate at the condensing temperature of the steam, BTU/lb

Ui = overall heat-transfer coefficient, BTU/(hr) (sq ft) (°F) ∆T = temperature difference between condensing steam and liquor or slurry (usually determined as log mean), °F

Both vertical- and horizontal-tube heat exchangers are

utilized. The horizontal exchanger is used for installations with limited headroom or where maximum liquor submergence is needed to prevent surface boiling and subsequent salt precipitation on the tubes. A typical horizontal heat exchanger is shown in Fig. 9.

The majority of Swenson installations use vertical-tube heat exchangers (as shown in Figs. 5, 7 and 8). Steam enters the enlarged shell section and flows around and up through the annulus formed between the inner and outer shells. The steam enters the tube bundle uniformly around the entire circumference beneath the top tubesheet. The Swenson steam inlet prevents condensate impingement on the tubes, reduces tube vibrations, provides for uniform steam distribution and, for some applications, acts as an entrainment separator. The noncondensible gases are sweptdownward and removed near the bottom tubesheet. Special vent baffles are provided for large diameter heat exchangers.

Internal tube supports are not normally required for vertical heat exchangers. The tubes are usually fastened mechanically to the top and bottom tubesheets by expansion of the tube into the tube holes.

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Preheaters and Condensate Flash Systems Preheaters and condensate flash systems are often incorporated into Swenson process designs to reduce steam consumption. Preheaters are used to heat the feed to an evaporator with a heat-transfer medium, preferably of less energy value than that of the steam or vapor condensed in the evaporator heat exchanger. Typical heat-transfer fluids used in preheaters include evaporatorcondensate, evaporator vapor and process liquor. Heat that is added in preheaters replaces the heat that would have been added in the evaporator heat exchangers; therefore, less heat-transfer surface area is required for the evaporator heat exchangers when preheaters are used.

A Swenson preheater is designed either as a separate exchanger or as an integral part of an evaporatorheat exchanger or condenser. Preheaters can also improve heat-transfer coefficients for rising- and falling-film evaporators because the non-boiling zone is reducedor eliminated. For some scaling applications, separate preheaters, with bypass piping, provide an additional benefit because scale forms in the smaller, easier-to-clean preheater rather than on the tubes of the larger evaporator heat exchanger.

Condensate flash systems are sometimes employed in place of, or to complement, preheaters. Condensate from a heat exchanger is flashed into a heat exchanger that is operated at a lower pressure to provide supplemental energy.

Fig. 11. Swenson Double-Effect, Falling-Film Evaporator

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Energy Conservation Energy conservation is a primary consideration in the design of every Swenson evaporator. In order to conserve energy, Swenson evaporators are utilized in any one, or a combination, of the following configurations: multiple-effect, mechanical recompression, or thermal recompression. They are also frequently used in conjunction with preheaters and/or condensate flash systems for additional energy savings. These evaporator configurations will be described in greater detail in the following sections.

Multiple Effect The multiple effect configuration combines two or more evaporator bodies to conserve steam, which is condensed inthe first-effect heat exchanger only. Water evaporated in the first-effect vapor body is condensed in the second-effect heat exchanger, which provides energy for evaporation in the second-effect vapor body (and so on for additional effects). Vapor from the last effect flows to a condenser. The last-effect vapor body is maintained at a high vacuum with a steam-jet air ejector or mechanical vacuum pump. The driving force is the pressure drop from the first to the last effect.

For the same overall pressure differential and process conditions, the evaporator with more effects will require less steam. A rough “rule of thumb” for comparison purposes is that the evaporation in each effect will be approximately 0.7 to 0.9 pounds for each pound of steam condensed in the first-effect heat exchanger. Thus, for the same evaporation rate, a quintuple-effect evaporator will require about 20% of the steam required fora single-effect evaporator. In actual practice, the steam economy can vary widely because of differences in feed temperature and other energy requirements such as heats ofdilution and crystallization. Figs. 1, 4, 14 and 15, plus the cover photo, show multiple-effect evaporator configurations.

For some applications, single-effect evaporators are used because multiple-effect evaporators cannot be economically justified. In particular, single-effect evaporators are utilized for small evaporation rates, or for liquids that boil at high temperatures (high boiling-point elevation liquors).

Mechanical Recompression Increasing energy costs have justified the increased use of mechanical recompression evaporators. The principle is simple. Vapor from an evaporator is compressed (with a positive-displacement, centrifugal or axial-flow compressor) to a higher pressure so that it can be condensed in the evaporator heat exchanger. Various combinations are possible, including single-effect recompression, multiple-effect recompression, multiple-stage recompression, and single-effect recompression combined with a multiple-effect evaporator.

A simplified flowsheet of a single-effect recompression evaporator illustrates why mechanical recompression is energy efficient. All of the pertinent pressures and temperatures are given in Fig. 12 for this example.

Based upon a 75% isentropic (adiabatic, reversible) compressor efficiency and a combined electric drive motor and gear reducer efficiency of 92%, the energy required to compress a single pound of vapor from 14.1 to 22.8 psia is only 49.3 BTU. To produce the equivalent steam from one pound of 234 0F evaporator condensate requires 999 BTU. Therefore, the energy savings for a recompression evaporator are highly competitive with those of multiple-effect evaporators and depend upon the compression pressure ratio required and the relative cost of electric power and steam.

The compression ratio required is comprised of three components which are:

1. The boiling-point rise, i.e., the temperature of the boiling liquor minus the temperature of boiling water at the same pressure.

2. The ∆T required for heat transfer. 3. The pressure drop in the vapor pipe to and from the

compressor. Mechanical recompression is most practical for low

∆T’s (larger heat-transfer areas) and low boiling-point elevations.

In Fig. 13, a simplified flowsheet is shown for a single-effect recompression soda ash evaporator which has replacedthe traditional triple-effect evaporators used for this application. The vapor body shown has the alternate Swenson vertical-inlet baffle design, which has proven to be effective in minimizing short circuiting.* Vapor from the body is compressed with a single-stage centrifugal compressor and condensed in the vertical heat exchanger. Condensate is sprayed into the vapor discharged from the compressor to reduce super-heat. Some make-up steam is required to supplement the mechanical energy from the compressor. For some applications, make-up steam is not required.

Most submerged-inlet evaporators short circuit. That is, some of the heated liquor which enters the vapor body short circuits to the outlet instead of rising to the boiling surface. * U.S. Patent No. 3,873,275

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The boiling temperature of the liquor is increased above the equilibrium value (denoted as degrees of short circuiting), which decreases the ∆T available for heat transfer. It is particularly important to minimize short circuiting in recompression evaporators because short circuiting increases the compression ratio required, thereby increasing power consumption.

The following table compares the energy required forthis system versus that required for a triple-effect evaporator:

UTILITIES REQUIRED PER ONE TON OF SODIUM CARBONATE MONOHYDRATE

Recompression Triple Effect Evaporator Power, kwh Compressor 0 46.7 Circulating pumps 9.07 12.9 Total 9.07 59.6 Steam, lb 1,580 636 Condenser water, gpm 75.3 0 Steam & power cost $9.93 $6.80 (Based upon $6 per 1,000 lb steam & 5c per kwh power)

Mechanical recompression is not limited to single-

effect evaporation. It is sometimes economical to compress vapor from the last effect of a double- or triple-effect evaporator so that the vapor can be condensed in the first-effect heat exchanger.

Multiple-stage, LTV falling-film, mechanical recompression evaporators are shown in Figs. 6 and 11.

g. 12. Simplified Flowsheet of Swenson Single-Effect Recompression Evaporator

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The vapor recompression plus quadruple-effect salt evaporator (shown in Fig. 14) is an example where recompression is combined with multiple effect. High pressure steam (880 psia, 900°F) is fed through two turbines in parallel; one turbine drives a generator to produce electricity for the plant and the other turbine drives a multi-stage, axial-flow compressor for the single-effect recompression evaporator. The 44 psia steam discharged from both turbines is condensed in the first effect of the quadruple-effect salt evaporator and a small quantity is condensed as make-up steam in the recompression evaporator heat exchangers. Approximately 40% of the salt is precipitated in the recompression evaporator.

The average hourly production and utility requirements for this plant are as follows:

NaCl Production 138 tons Electricity 4,000 - 5,000 KW Steam 194,000 lb Lb Steam/Ton of Salt 1,406

For comparison, an efficient quadruple-effect salt evaporator requires 1,900 lb of 50 psia steam per ton of salt.

Fig. 14. Single-Effect Forced-Circulation Evaporator

Fig. 13. Swenson Single-Effect Recompression Soda Ash Evaporator

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Thermal Recompression To reduce energy consumption, water vapor from an evaporator is entrained and compressed with high pressure steam in a thermocompressor so it can be condensed in the evaporator heat exchanger. The resultant pressure is intermediate to that of the motive steam and the water vapor. A thermocompressor is similar to a steam-jet air ejector used to maintain vacuum in an evaporator.

Only a portion of the vapor from an evaporator can be compressed in a thermocompressor with the remainder condensed in the next-effect heat exchanger or a condenser.A thermocompressor is normally used on a single-effect evaporator or on the first effect of a double- or triple-effect evaporator to reduce energy consumption. As with mechanical recompression, thermal recompression is more applicable to low boiling-point rise liquids and low to moderate ∆T’s in the heat exchanger to minimize the compression ratio.

compevapotherm85 psHg toevaposolutiand asecon DoubDoubTherm

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Fig. 15. Swenson Double-Effect Thermal R

To illustrate the energy effectiveness of a thermo-ressor, compare the steam usage for a double-effect rator with that of the double-effect evaporator with a ocompressor (as shown in Fig. 15). Motive steam at ig is utilized to compress first-effect vapor from 19” 32” Hg absolute. The forward-feed, double-effect rator is used to concentrate a weak sodium sulfate on. A rising-film evaporator is used for the first effect forced-circulation evaporator is required for the d effect where sodium sulfate crystals precipitate.

Lb/hr of Lb evaporation/85 psig steam lb of steam

le effect 12,800 1.94 le effect w/ ocompressor 8,510 2.90

The double-effect evaporator with thermal recompress-equires 33% less steam than the conventional double

t. In essence, the steam usage for the double effect withal recompression is comparable to that of a triple-

t evaporator. The main advantage of thermal pression is improved steam economy for a moderate l expenditure - less than that for an additional effect.

ecompression Evaporator 15

Rotary Dryer

Spray Dryer

CONDENSERS Direct Contact Type Digester Blow Surface Type COOLERS Flash Fluidized Bed Rotary Spray CRYSTALLIZERS Batch Direct Contact Refrigeration Draft Tube Draft Tube Baffle Forced-Circulation Surface Cooled Reaction Decomposition Recompression Teflon Tube Multi-Stage Horizontal Spray Evaporators DRYERS: FLUIDIZED Closed Cycle Direct Fired Indirect Heated DRYERS: ROTARY Countercurrent Direct Fired Indirect Heated Parallel Flow Steam Tube DRYERS: SPRAY Closed Cycle Countercurrent Mixed Flow Parallel Flow Research Reverse Flow

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DRYERS: FLASH Direct Fired Indirect Heated LABORATORY FACILITIES Bench Scale Tests Crystallization Evaporation Flash Dryers Fluidized Bed Crystallizers Fluidized Bed Dryers - Coolers Rotary Dryers - Coolers Spray Dryers Steam Tube Dryers EVAPORATORS Calandria Forced-Circulation LIV Falling-Film LIV Rising-Film Natural Circulation Recompression FILTERS Top Feed HEAT EXCHANGERS Direct Contact Shell and Tube PROCESSING AND PROJECT ENGINEERING Ammonium Sulfate Crystallization & Drying Caustic Soda Systems Citric Acid Systems Fluorine Recovery Salt Crystallization & Drying Sodium Sulfate Recovery & Drying Sodium Chlorate System Wet Process Phosphoric Acid Potash Crystallization & Drying Soda Ash Calcining, Crystallization & Drying Others

SWENSON® EQUIPMENT FOR THE PROCESS INDUSTRIES

Crystallizer

i t i n g E q u i p m e n t C a n a d a I n c . Dave Neville 350 Alexander Street Welland, Ontario L3B 2R2 CDN Tel (+1) 905-732-7585 ext. 265 Fax (+1) 905-732-2366 Cell (+1) 905-733-0116 E-Mail dneville@whiting.ca