VACUUM AND MECHANICAL CRYSTALLIZERS A COMPARISON
G . E. SEAVOY Swenson Evaporator Company, Harvey, Ill.
H. B. CALDWEXL Swenson Evaporator Company, New York, N . Y.-
The general factors affecting the design of mechanical and vacuum crystallizers are discussed, with particular reference to relative first cost and operating costs, and to batch as compared with continuous operation. For many crystallizing problems, particularly under corrosive conditions, the rubber-lined vacuum crystallizer is decidedly less expensive, both in first cost and operating cost, than the mechanical crystallizer using corrosion-resistant materials of construction.
HE main difference between batch and mechanical crystallizers is that heat is removed in the former by adiabatic vaporization of a portion of the solvent, whereas heat removal in the latter is the result of ordinary heat transfer through a metallic wall with a t least two film resistances. There are air-cooled crystallizers in which air is bubbled through or passed over the top of the solution. Some of these might be classed as mechanical crystallizers, but they will not be considered in this paper. The term "vacuum crystallizer" is used rather loosely, inasmuch as vaporization can take place a t any pressure level above or below atmospheric pressure. The vacuum crystallizers discussed here are the type where evaporation and crystal precipitation occur within one vessel as distinguished from the Krystal or Oslo crystallizer (4) where evaporation and crystallization are carried out in separate vessels.
T
General Characteristics I n order to establish the general principles of operation of a vacuum crystallizer consider a vertical cylindrical vessel with a dished head provided with a vapor outlet, and a cone bottom terminating in a discharge opening. Vapor piping leads from the top dished head to suitable condensing and vacuum-producing equipment. Assuming batch operation, the vessel is partially filled with the solution to be cooled, and the condensing and vacuum apparatus are started. When equilibrium vapor pressure is reached above the hot solution, boiling commences and sensible heat of solution together with heat of crystallization supplies the latent hest for vaporization. Cooling can continue in this manner with decreasing pressure until the limit of the condensing equipment is reached. The vacuum-producing apparatus causes the initial vacuum in the vessel and thereafter maintains the equilibrium vapor pressure by removing the noncondensable gases. Thus during batch operation the vapor pressure in the upper part of the vessel and the temperature of the solution are continually decreasing until the desired end temperature is reached.
Thus far i t is assumed that the cooling medium is of low enough temperature to condense the vapors corresponding t o the final solution temperature. With 85" F. cooling water available, a final solution temperature of 110" could be obtained, assuming a 5" terminal difference in the condenser, a 10" temperature rise of the cooling water through the condenser, and a 10" boiling point rise of the solution at its final concentration and temperature. If the same solution must be cooled to 50" instead of 110" F., it is necessary to interpose a vapor compressor, called a "booster", between the crystallizer and condenser; it starts operation when the solution reaches 110'. At this point the saturated vapor temperature in the vessel and in the condenser is 100" F. To accomplish further cooling, the booster must compress the lower temperature vapors to maintain a constant temperature of 100" in the condenser. At the end of the batch the booster will have to compress vapor through the maximum range of 40" to 100" F. (absolute pressures of 0.2478 to 1.932 inches of mercury). I n continuous operation the hot solution is continuously introduced into the vacuum crystallizer, and the cooled solution with crystals in suspension is removed in a continuous manner. The vapor pressure is constant and equal to that of the discharge solution. Therefore all of the hest is removed by compressing the vapor through the maximum range a t all times. To obtain better economy of energy for compression, multistage continuous operation is desirable. I n the preceding example the installation might consist of three stages in series: The first stage cools the solution t o 110" F., which does not require a booster, the second stage cools to 70", and the third cools to the desired 50" end solution temperature. With this arrangement only a portion of the vapor (that from the third stage) would be compressed from the 40" final saturated vapor temperature to 100' F. in the condenser, and the vapor from the second stage would be compressed from 60" to 100" F. saturated temperatures, corresponding to an absolute pressure range of 0.5214 t o 1.932 inches of mercury. Thus in any continuous vacuum crystallizer requiring boosters, it is evident that the greater the number of stages, the greater is the energy economy of compression and the more nearly the over-all economy approaches that of batch operation which is equivalent to an infinite number of continuous stages. Since the type of booster commonly employed for vacuum crystallizers is the well-known steam jet thermocompressor, this paper will not consider other types such as mechanical compressors of the reciprocating or turbine design. Mechanical crystallizer designs are about as numerous as the engineers who have been concerned with their development. However, there are some common types such as the agitated tanks with cooling coils, the jacketed tank, and the simple double-pipe exchanger. These make no effective attempt to prevent formation of crystals on the cooling surface. Others, such as the Swenson-Walker and the double-pipe exchanger with internal helical ribbons, keep one side of the
627
INDUSTRIAL AND ENGINEERING CHEMISTRY
628
cooling surface clean by means of scrapers; in the latter case this makes a close clearance with the cooling surface and in the former case actually makes contact with the cooling surface. Those in which the cooling surface is kept free of crystals maintain a constant rate of heat transfer; the others have a decreasing capacity between cleaning periods. 2- STAGE STEAM JET EJECTOR CONDENSER WATER
---!
/
VOL. 32, NO. 5
difference. The customary minimum terminal difference of a surface condenser is figured at 10" F. A smaller terminal difference would cause dangerous loss of capacity upon slight fouling of the condenser tubes. Surface condensers are used with refrigerated brine, and the latter lowers the final temperature to which the solution may be cooled without the use of boosters, in comparison to the ordinary sources of cooling water. When a surface condenser is used, it may be of the familiar shell-and-tube type, or the tubes may be installed directly in the vapor space of the crystallizer. I n this latter case the condensed vapor can be allowed to drip back into the solution, or a pan can be placed beneath the tube bundle to collect and drain it off. Generally the allowable rise in temperature of the cooling medium, in the case of water, is governed by the amount available or by the use to which it is t o be put after passing through the condenser. Sometimes cooled mother liquor from the final stage of a continuous crystallizer is used as the cooling medium in the first-stage surface condenser. I n such cases the quantity is definitely limited and its temperature rise is governed by the terminal difference of the condenser being used.
FEED.
STEAM
M U D P- - -&
VALVE
FIGURE 1. TYPICAL B-ATCH VACUUMCRYSTALLIZER WITHOUT BOOSTER
The Swenson-Walker mechanical crystallizer is satisfactorily used either as a batch or continuous machine, but is generally operated in a countercurrent continuous manner. The double-pipe exchanger with the internal ribbon scrapers is generally used only in a continuous-batch manner. It is placed in series with a large tank of solution to be cooled, the solution being pumped through it and returned to the tank a t a sufficient rate to maintain a reasonably good velocity through the exchanger. This method of operation is dictated by the ratio of cooling surface to cross sectional area.
Limit ations The ability of the vacuum crystallizer to obtain a certain final temperature satisfactorily without the use of a booster is governed by: (a) the temperature and nature of the cooling medium, (b) the boiling point rise of the solution and mother liquor, (c) the terminal difference required by the condenser, ( d ) the allowable rise in temperature of the cooling medium, (e) the use of a direct contact or a surface type condenser, and (f) the density of the final magma. With water as the cooling medium in a direct contact condenser, a 5" F. terminal difference is a common design factor, although efficient countercurrent condensers will operate a t 2" F. or less terminal difference. If a solution with an appreciable boiling point rise is the cooling medium in a direct contact condenser, advantage may be taken of the Konneman effect. For example, 100" F. water vapor can be condensed with a solution a t 110" temperature if the solution has a boiling point rise of 15". This assumes the same 5" terminal
FIGURE 2. SINQLE-STAGE CONTINUOUS VACUUMCRYSTALLIZER WITH BOOSTER
Magma densities too heavy to circulate freely limit the cooling range due to purely mechanical reasons, such as the difficulty of discharge and the impossibility of presenting fresh solution to the vapor-liquor boundary to be further cooled. The critical magma density is approximately 50 to 55 per cent crystals by weight when the density of the crystals is not greatly different from the density of the mother liquor, and 35 to 40 per cent by weight when the difference in densities is relatively large. Vacuum crystallizers invariably require more headroom than mechanical types and this is sometimes a limitation. When a vacuum crystallizer is provided with a booster, the temperature of the available cooling medium is no longer a limiting factor to the final solution temperature. Also the
MAY, 1940
629
SEPARATION OPERATIONS
type of condenser, its terminal difference, and the allowable rise in the cooling medium temperature, all more or less disappear from consideration. I n their stead is the practical limit of how rarefied a vapor the booster will compress. The commercial limit is probably about 20' F. saturated vapor (absolute pressure of 0.111 inch of mercury). The effective boiling point rise of the solution being cooled and the magma density are the other chief factors to be considered. Since the vacuum crystallizer can be completely rubberlined, with no metal parts in contact with the solution or vapor, materials of construction seldom create a limitation of its use. The vacuum crystallizer has two further limitations. One is a high boiling point rise of the solution to be cooled. A 50 per cent pure sodium hydroxide solution, for instance, would be a poor application. This has a 68' F. boiling point rise a t an absolute pressure of 2 inches of mercury. If we try to cool it to 85' F., we find that the final vapor will have to be compressed from an absolute pressure corresponding to 17" F. saturated vapor temperature, which is beyond the range of practical application for a booster. I n such cases the mechanical crystallizer must be used. The other limitation is hot acid solution a t such a high tJemperature that rubber lining is unsatisfactory. The practical limit today for such construction is about 200" F. Above this point the rubber lining life is short. With a homogeneously bonded lead lining or a glass lining this ternperature limit can be extended, but the initial cost is considerably increased. TO CONDENSING AND +VACUUMPRODUCING EQUIPMENT
\CIRCULATING
FIGURE 4.
PUMP
CRYSTALLIZER WITH AGIPUMPCIRCULATION
VACUUM
TATION BY
For capacities in excess of about 10,000 gallons of solution to be cooled in 24 hours, the mechanical crystallizer's initial cost is so high, even in steel construction, that it cannot compete. One of the fundamental drawbacks of the water-cooled mechanical crystallizer is the fouling of the cooling surface due to the use of hard or brackish water. Usually the water side of the surface is difficult or impossible to clean. Again, crystal growth on the liquor side of the cooling surface must be removed a t definite intervals m-hen no scrapers are included in the design to prevent these deposits. However, even when scrapers are used, they are subject to wear, and decreased capacity is the result. In the Swenson-Walker type there is no direct contact between the helical ribbon flights and the cooling surface, the principle of the design being to provide a close clearance. If this clearance is allowed to become as much as '/a to l//q inch, the capacity will be approximately cut in half as a result of the building up of crystals on the active cooling surface and increased resistance to heat transfer.
Construction of Vacuum Crystallizers FIGURE3.
VACUUM CRYSTALLIZER WITH VERTIC.AL PROPELLER AGIT.4TOR
The mechanical crystallizer, on the other hand, is frequently limited in successful operation by materials of construction. Unfortunately rubber is ruled out because it is not a favorable material for the transfer of heat. Lead is only partially satisfactory owing to its ease of erosion by the crystals and its poor structural strength. Thus when a crystallizer for acid solutions is required, the mechanical crystallizer is seldom indicated. However, the mechanical crystallizer, using a refrigerated cooling medium, can cool to much lower temperatures than even a vacuum crystallizer with a booster. If we add about 5' F. to the incoming temperature of the available cooling medium, we obtain the limit of its lowtemperature cooling ability. Magma density is less important in the mechanical crystallizer since free circulation is not particularly required.
Figures 1 to 4 show typical body and agitator arrangements of vacuum crystallizers. Figures 1 and 2 give usual arrangements of condensing and vacuum-producing equipment, without and with a booster, respectively. The design of Figure 1 is perhaps the simplest form of such equipment and is used exclusively for batch operation. The horizontal swirling agitation produced by the propellers ( 2 ) maintains a fairly uniform temperature and percentage of suspension of crystals throughout the mass. Salting of the body walls does not occur except in the band a t the liquor level. This is not an objection since the following batch to be cooled removes it. Figure 2 shows the former design adapted to continuous operation. Hot feed is introduced continuously through an insulated nozzle of Venturi design a t such a velocity and under such a hydrostatic head as to prevent vaporization and salting in the nozzle. The band of salt on the walls a t the solution level is prevented by filming a small amount of water down the
630
INDUSTRIAL AND ENGINEERING CHEMISTRY
VOL. 32, NO. 5
and p H values, all of which affect crystal size. I n a d d i t i o n , t h e volume of solution in the crystallizer controls the period of retention, which influences crystal size. I n batch operation the time of cooling can be varied by design, and usually longer cooling periods result in larger crystals. Inoculation and seeding may be practiced with their well-recognized results (8).
Figure 4 shows a design which can be operated in either batch or continuous manner and provides for further control of crystal size. Usually the agitators are omitted, horizontal swirling motion of the liquor being accomplished by tangential discharge from the circulating pump. The capacity of the circulating pump is sufficient so that, upon mixing the hot feed during continuous operation, the resulting temperature of the solution to be cooled does not exceed the stable band of supersolubility according to the Meir principle.
Batch vs. Continuous Operations When to use a batch or a continuous vacuum crystallizer generally depends upon t h e daily quantity and the desired final temperature of the solution to be cooled. When the final temperature can be obtained without a booster and when the cost of cooling water is not excessive, continuous operation is usually indicated regardless of the daily volume. I n amounts less than 50,000 gallons daily, batch operation without a booster may have some advantages of flexibility and ease of operation in certain cases. When a booster is not reI quired, it makes little difference in initial or in operating cost whether d! a batch or continuous crystallizer is used. VIEWSOF Two RUBBER-LINED VACUUM CRYSTALLIZERS, 11 FEET3 INCHES IN DIAMETER It is of utmost importance to re(Above) Boosters, condensers, and ejectors; (below) agitator drives in the cone bottoms member that batch operation with a booster necessitates compression of the vapor through the greatest range only a t the v e r i end of the walls (1). Usually 1 or 2 gallons per minute are sufficient cooling cycle. The compression range and the energy required for a 10-foot-diameter body. This is less than the vaporizafor compression increase toward the end of the batch. This is tion in such a body; hence no dilution of the liquor occurs. the fact that makes the cost of batch operation less than that Figure 3 illustrates a design with a central well and vertiof continuous operation, in so far as the energy for cooling is cal propeller agitation. This may be operated in either a concerned. When large quantities of solution are to be batch or continuous manner. If operation is continuous, it cooled, say 200,000 gallons or more daily, then the lower must be interrupted a t least once every 24 hours to remove energy consumption for batch operation is more than offset the accumulated salt on the walls. This design is effective in by the higher installation cost. Thus for large quantities preventing loss of temperature due to hydrostatic head during of solution, continuous vacuum crystallizers should generally continuous operation. be employed. The foregoing continuous designs make little or no attempt If a booster must be used, batch operation always gives to control crystal size from a given solution. However, the lower steam consumption regardless of the daily amount the characteristics of the feed solution itself may be varied of solution to be cooled. This becomes increasingly advanwith respect to impurities, dissolved or suspended solids,
-
S E P A R A T I O N O P E *AT I o N s
MAY, 1940
tageous with lowering of the final temperature. Hence, we must consider the various items of first cost of installation to decide at what point batch operation should be abandoned in favor of continuous. As previously stated, the dividing line should be about 200,000 gallons daily, above which continous operation becomes attractive. This statement assumes a final vapor temperature of 30' to 40' F. and corrosive solutions which require rubber-lined steel construction. With solutions that can be handled in steel or cast iron, the theoretical dividing line will be at a slightly higher daily amount.
85
75 70 b5 C O O L I N G WATER
SO
bO
55
TEMP.,'
50
45
40
F
Courteey, CroZl-Reynolds Company
FIGURE5. BOOSTER STEAMCONSUMPTION vs, COOLING WATERTEMPERATURE Conversely, continuous vacuum crystallizers should always be used for large quantities of solution to be cooled-namely, boosters are not reabout 50,000 gallons per 24 hours-if quired, and above 150,000 to 200,000 gallons per 24 hours if boosters are required. Batch operation 0 does not lend itself favorably to the ILbO control of crystal 4 size by application 2 1.50 of the Meir prin2 140 a ciple. InvestigaI30 tion has shown m that usually the $1 20 supersaturation band is relatively $ 5 I10 u narrow, but of ap; 100 proximately con0 0, stant width above I5 35 55 75 95 115 135 is5 175 the normal soluBOOSTER STEAM PRESSURE m J LBS. PER SQ. IUCH A B S O L U T E b i l i t y c u r v e of Courtesy, CroZl-Reynolds Company s a 1t s s c h a S FIQURE6. BOOSTER STEAM CONSUMP- Glauber, Epsom, TION us. BOOSTER STEAMPRESSURE and sodium chloride. I n a batch operation no thermal or chemical properties of the system remain constant, nor do their rates of change remain constant. At the start of a batch crystallizer operation, the inherent rate of cooling is always considerably faster than a t the end. This is true whether or not a booster is employed. Therefore, with a given mechanical design of recirculation, the supersaturation is likely to be exceeded at the start of a batch or the recirculation pump will be many times too large for the conditions corresponding to the end of the batch. Also, since relatively few crystals are present during the early part of the batch, the rapid cooling rate at this period, accomv)
'
631
panied by its high rate of crystal precipitation, usually exceeds the ability of the crystals already present to absorb the newly precipitated material. However, batch operation can be designed to produce favorable crystal sizes. Consider the simple experiment of cooling two identical beakers of sodium sulfate solution, one in 30 minutes and the other in 4 or 5 hours. Even \ though the agitation in both is the same, the longer cooling time produces the larger FIGURE7. TANKTYPEAGITATED CRYSTALLIZER crystals. Since the batch cooling time can be readily varied, so can crystal size within the limits of the rate of growth of the crystals. It has also been repeatedly shown that crystals grow better, all other factors being equal, with a higher density magma; in sugar operations this is referred to as a tight strike. Batch operation can be favorable to this condition. Therefore, regardless of the Meir principle, batch vacuum crystallization with its inherently high economy can also produce favorable crystal size.
First Cost I n comparing the first cost of vacuum and mechanical crystallizers, the rather sweeping statement can be truthfully made that the advantage is all in favor of the vacuum crystallizer except for small capacities in the order of magnitude of several thousand gallons of solution daily. A modern plant would hardly consider the old-fashioned tank and coil or the jacketed tank type of mechanical crystallizer for anything except the smallest capacities. Therefore, let us compare the standard Swenson-Walker type for a moderately large capacity, with the proper vacuum crystallizer design. EXAMPLE I. It is required to produce 150 tons per 24 hours of disodium phosphate crystals, NalHPOd.12H10. The conditions are as follows: feed to crystallizer, 80,000 gallons er 24 to hours; feed temperature, 190" F.; feed density, 32" BB.
6001
100' F. Available cooling water temperature, 80' F. (summer maximum); available steam at 125 pounds gage pressure, dry and saturated; initial crystallization temperature, 142' F. ; boiling point rise of feed solution, 10' F.; boiling point rise of final mother liquor, 8 ' F.; specific heat of solution and mother liquor, assume 0.85; specific heat of crystals, 0.40; heat of crystallization, 70 B. t. u. per pound of hydrated crystals:
Materials of construction Installation- cost,- G l u d i n g motors, drives, S ~ orts, P and piping, exolusive of b u i l 8 n p and foundations Cooling water a t SOo F gal./min. Connected h. p., i n h ~ d i n g water Dumuinn Steam 'conkmption, Ib./hr. Water evaporated daily lb Approx. space (length, k i d i h , height), it.
Swenson-Walker Continuous Crystallisers Steel and cast iron
Three-Stage Continuous Vacuum Crystallizer Steel
152,000 360
$28,000 300
70 None None
650 84,000
100,50,10
52
30, 10, 40
This example does not really tell the complete story because in this instance steel and cast iron are assumed to be satisfactory materials of construction for both cases. With solutions which
632
INDUSTRIAL AND ENGINEERING CHEMISTRY
require copper, Monel metal, stainless steel, or nickel, the comparison is more in favor of the vacuum crystallizer because it can be made of rubber-lined steel, whereas the mechanical crystallizer would have to employ the more expensive metals. EXAMPLE 11. It is required to cool 150,000 gallons of an average viscose spin bath per 24 hours, and crystallize out 225,000 pounds of Ka2SO~.lOH20crystals. The conditions are as follows: Feed solution consists of 8 per cent sulfuric acid (by weight), 14 sodium sulfate, 5 glucose, 1 zinc sulfate, and 72 water; specific gravity, 1.22; temperature, 105' F.; boiling point rise, 8' F. Cool to 41' F. Boiling point rise of final mother liquor, 9" F.; available cooling water temperature, 85" F. (summer maximum); available refrigerated brine at 10" F.; available steam at 125 pounds gage pressure; initial crystallization point, 58' F. ; specific heat of solution and mother liquor, assume 0.85; specific heat of crystals, 0.35; heat of crystallization, 105 R. t. u. per pound of hydrated crystals. AI echanical Crystallizer Type hlaterials of construction Installation cost, exclusive of building and foundations Cooling wate? a t 85' F., gal./min. Brine, gal./mm. Brine refrigeration t o d d a y H . P. for refrigdratioii and brine pumping H . P. for agitation drives etc. Av. steam consum&ion, lb:/hr. Water evaporated, lb./day Approx. space (length, width. height), ft.
Continuoub Nickel and nickel-clad steel $140,000 Sone5 535 335 630 50
INTERMEDIATE
\ OL. 32, NO. 5
BEARING -HANGER
X-acuum Crystallizer Batch R.uhber-lined steel %66,000 2,900 Xone Sone
Sone
None None
X0b 8,000 94,000
100,50,10
$ 5 , 20, 55
Does not include cooling for ammonia condenser. b Includes power for pumping condenser water.
EXAMPLE 111. It is required to cool 42,000 gallons per 24 hours of a sulfuric acid solution containing, among other ingredients, ferrous sulfate to be removed by crystallization. The conditions are as follows: feed solution, 50" BB. at 140" F. Cool to 50" F. Crystals, 125,000 pounds of FeS04.7H20 per 24 hours, initial crystallization point, 90" F. ; boiling point rise of feed solution, 11" F.; boiling point rise of final mother liquor, 15' F.; specific heat of feed and mother liquor, 0.85; specific heat of crystals, 0.3; heat of crystallization, 29 B. t. u. per pound of crystals; available cooling water at 80" F. (summer maximum) ; available brine at 20' F.; steam at 125 pounds gage pressure, dry and saturated.
Tbpe 1Iaterials of construction Installation cost. exclusive
Meohanical Crystallizer Double pipes Steel and lead $30,000 180& 100
Vacuum Crystallizer Batch Rubber-lined st.eel $18,000
85
675 Sone Sone
160 15 None None
Sone 30b 2,000 35,000
30, 10, 10 20, 1 5 , 5 5 Cool part way with water; does not include cooling water foi ammonia rondensers. b Includes power for pumping condenser water.
FIGURE 8. SWEKSON-iT.4LKER CRYSThLLIZER
basis of the maximum summer-time temperature of the cooling water. This may be 80" to 85" F. With normal weather conditions prevailing in this part of the United States, a much lower temperature is available over most of the year. This means that the main condenser can operate a t a higher vacuum and that the booster does not have to compress through such a large range. Less work by the booster means less steam. A typical curve showing this relation is given in Figure 5 . It is for a batch operation cooling a sulfuric acid solution of ferrous sulfate from 140" to 59" F. Boosters can also be operated with low- or high-pressure steam. With low pressures, more steam is naturally required since less energy is available to do work. Figure 6 shows this general relation for an average compression range of 0.35 to 2.0 inches of mercury absolute. Even atmospheric steam can be used in boosters, but about 65 per cent more is required than a t 100 pounds gage pressure.
a
Examples I1 and I11 bring into consideration another factor. They both require cooling to a lower temperature than is directly possible with any ordinary cooling water. In the past it has been customary to use mechanical refrigeration equipment with its ammonia compressor, condenser coils, brine system, pumps, etc., to obtain the low-temperature cooling medium for a mechanical crystallizer. The vacuum crystallizer, on the other hand, uses only a steam-jet thermocompressor or booster, which is much less expensive even in acid-resistant construction and occupies only a fraction of the space. Because there is no solid surface through which the heat must be transferred in a vacuum crystallizer, it is naturally smaller equipment than the mechanical type. I n any discussion of the operating cost of the vacuum crystallizer, it is interesting to point out that the steam consumption of the booster, in the case of either continuous or batch operation, will be about 80 per cent of the maximum quantity for the average yearly operation. Boosters, condensers, and steam-jet air ejectors are always designed on the
Design Factors Affecting Operating Cos L
d number of factors which affect the design of a vacuum crystallizer influence its operating cost and should be considered in all cases. Among these the most important are: 1. Amount of solution to be cooled daily. 2. Initial and final liquor temperatures.
3. Thermal data-specific heat of solution and crystals, latent heat of vaporization, heat of crystallization, and heat of concentration. Boiling point rise of the solution. 4.a. Available cooling medium temperature. 6. Batch operation. 7. Continuous operation. 8. Chemical data-initial crystallization temperature and solubility curve. 9. Corrosive or noncorrosive conditions. 10. Available steam pressure. 11. Vapor velocity affecting entrainment. 12. -4gitation to overcome hydrostatic head, prevent shortcircuiting, and maintain uniform suspension of crystals. 13. Prevention of salting on the side walls.
M A Y , 1940
ri
SEPARATION OPERATIONS
IO0 90
W
a
: eo a U
W
$ +Lu 0
70 60
3
9 J I
50 40
Some of these factors have already been discussed, such as the relation between the amount of solution to be cooled and whether batch or continuous operation should be employed. If batch operation is decided upon, it may be necessary to employ one or more bodies. The largest body which can be conveniently shipped on freight cars is 11 feet 3 inches in diameter and will properly hold 7000 to 10,000 gallons. If the cooling range is approximately 140" to 60" F. with a 10" boiling point rise, the cooling time should be a minimum of about 3 hours to maintain a vapor velocity that will not cause excessive entrainment losses. The initial temperature has only a slight effect on the overall capacity of the batch crystallizer since the rate of cooling a t the start is rapid. The final temperature is the one which really governs the design of the booster and condensing equipment. The amount of heat to be removed over the cooling range is calculated from the amount of solution per batch, its specific heat, the amount of crystals produced, their heat of crystallization and specific heat, heat of concentration, and the solubility curve of the crystals in the given solution. This last factor is of great importance; obviously the booster and condenser capacity must be greater because the majority of crystals comes out in the lower rather than in the high temperature range. When the salt crystallized is such that the greater portion of crystal yield is precipitated in the last 20 to 25 per cent of the cooling range, the heat of crystallization increases the heat load of the vacuum equipment appreciably and in the range of pressures where the booster capacity is inherently low. The following calculated heat quantities will illustrate this:
633
The careful designer will calculate the heat removal for each 2" or 3" F. decrement of temperature, taking into account the change in boiling point rise with respect to concentration and vapor pressure, the change in latent heat of vaporization, the evaporation from the previous decrement, and its amount of crystal precipitation. The heat of concentration is usually not a large factor and is lumped into a corrective multiplier along with radiation effects and the conversion of mechanical into heat energy due to the power input for agitation or circulation. Each design of crystallizer has its own multiple which is easily determined by blank runs on water. An increasing or decreasing boiling point rise with respect to the progress of cooling in a batch is an important design factor since it governs the vapor temperature for a given liquor temperature. In solutions of more than two comoonents. one of which is crvstallized out, theAconcentration of the others increases and thus gires an increasing boiling point rise on cooling. If a natural brine or solution is available for the condensing medium instead of water, the advantages of the Konneman effect may be realized by using a direct contact type of condenser. This is of great benefit when a booster is used since it materially reduces the range through which the booster must compress the vapor. Sometimes it means the difference between using and not using a booster; the latter case reduces both the first cost as well as the operating cost of the crystallizer. When continuous operation is indicated by the magnitude of the problem and the end temperature, it becomes necessary first to determine the optimum number of stages of cooling. For maximum steam and water economy the over-all cooling range should be divided into as many increments or stages of cooling as are consistent with investment and operating costs. The total heat for a chosen increment of cooling must necessarily be removed at the lower temperature level of that stage 300 275 s w
t p
250 225 200 175
0 i3 u
o w
(&SO0 a3000
ul-
>zl
02
55
Jn
%.
135 -115 115 - 95 95n- 85 85 75
0.792 0,781
1.040 0.985 75 - 65 0.929 Crystallization for 43v0 of cooling range
-
a
FeS04.7H20 from Aqueous HtSOr Soln. Liquor temp. B. t. u. range, O
F.
G.
140 -130 0.850 130 -120 0.845 120 -110 0.838 110 -100 0.830 100 93 0.822 930- 90 1.881 90 - 8.5 1.015 85 75 0.882 Crystallization for 2SY0 of cooling range
-
-
Initial crystallization temperature.
23.0 22.5 a,Lu 22.0 21.5 21.0 z
NatSO*.10Ht0 from Aqueous HlSO, Soln.
Liquor temp. range,
F.
105 -90 90 -80 80 -70 70 -60 60 -52
23 $5
B. t. u. ger
F.
$$
85000 o*x4500 V) $4000
mw
ZnSOa.7H20 from Slightly Acid Aqueous S o h . Liquor temp. B. t. u. range, ' F.
F
2, $6
u
0.845
0.835
0.830 0.823
0.818
524-48 1.730 48 -44 2.149 44 -41 2.098 Crystallization for 17% of cooling range
s z- e 45"
FINAL
40" LIQUOR
35°F. TEMP.
FIGURE 10. O P T I M ~ M ENDTEYPERATURE
INDUSTRIAL AND ENGINEERING CHEMISTRY
634 NaOW
VOL. 32, NO. 5
3%
4
FIGURE11. FLOWSHEETOF /
VISCOSE
RAYONP R O C E S S
I
of cooling in continuous operation. The greater the number of stages, the more nearly a multistage continuous crystallizer approaches the stedm and water economy of batch operation. I n batch operation, dumping, filling, and evacuating can be carried out simultaneously and the evacuation time can be reduced by employing a n auxiliary, single-stage priming ejector. It is customary to break the vacuum for dumping since headroom limitations usually prevent barometric discharge, and a large expensive pump for quick discharge is uneconomical for operation only 5 to 8 minutes per batch. I n order t o reduce the “time out”, the number of batches per day should be kept a t a minimum consistent with body size and number of units required. Five to seven daily batches are usual. For each solubility relation there is an optimum end temperature which must be predetermined by trial and error. For example, to obtain a given yield of crystals, two volumes of solution can be cooled to, say, 60’ or one volume cooled to 45’ F. When considering multicoinponent solutions, it is desirable to conduct laboratory vacuum-cooling tests a t various end temperatures in order to determine the effect of concentration by evaporation on yields. The optimum end liquor temperature of 41’ F. in the case of Glauber salt crystallized from a sulfuric acid solution is shown in Figure 10. The corrosive properties of the solution bear a direct relation to operating costs, particularly in regard to maintenance. Since no heat is transferred through a metallic cooling surface, the problem of providing corrosion-resistant construction is relatively simple for vacuum crystallizers because it involves only the inner surfaces of a vessel and auxiliary parts such as agitator, level gage, peepholes, etc. Rubber-lined steel, nickel, nickel-clad steel, stainless steel, stainless-clad steel, and lead-lined steel are the more usual and practical materials of construction. For the majority of cases rubber-lined steel construction has found wide application. With properly prepared surfaces and application, rubber lining will give long serviceable life under low and high vacuum operating conditions. The first rubber-lined batch vacuum crystallizer in this country has been in successful operation over 6 years. The relative unit cost of steam and cooling water should be taken into consideration in properly designing the vacuum and condensing equipment to obtain low operating costs. For
instance, if steam is relatively expensive and cooling water is cheap, the booster-condenser can be designed to use a minimum of steam and a maximum of water, or vice versa. The effect of using more water is to reduce the temperature rise of the cooling water and increase the condenser vacuum, and thereby reduce the compression range of the booster. Once installed for one or the other condition, the booster-condenser is rather inflexible and this consideration is an important preliminary economic factor.
TYPICALBATCHVACUUMCRYSTALLIZER SHOWING SIDE VIEW OF AGITATORAND DRIVE
SEPARATION OPERATIONS
hlAY, 1940
635
Operating Costs TABLE I. OPERATING COSTS Mechanical Vacuum Crystallizer Crystallizer Example I Annuala interest and depreciation a t 15% Water cost a t 16/1000 gal. Power a t lC/kw-hr Steam a t 306/1000 lb. Labor a t 70k/man-hr. Maintenance a t 6 and 2 5%, respectively Annual gross operating cost Credit for evapn. figured a s steam a t 0.4 Ib./lb. water (triple-effect economy) Annual net operating cost Example I1
$ 7,800
1,556 3,350 1,685 2,664 700
...
5,328
3,120 -
14,15j
22,626
... 22,626
Annual interest and depreciation a t 15% Cooling water a t lt/1000 gal. Refrigeration a t 1.4 kw-hr./ton Power a t l+/kw-hr. Steam a t 30+/1000 Ib. Labor a t 70#/man-hour Maintenance a t 6 and 37,, respectively Annual gross operating cost Credit for evapn. figured as steam a t 1.25 lb./lb. water (single-effect economy) Annual net operating cost Example I11
$2 1,000
Annual interest and depreciation a t 15% Cooling water a t l b / l O O O gal. Refrigeration a t 1.4 kw-hr./ton Power a t l+/kw-hr. Labor a t iO+/rnan-hr. Maintenance a t 10 and 3 7 , respectively Steam a t 306/1000 Ib. Annual gross operating cost Credit for evapn. figured as steam a t 1.25 lb./ lb. water (single-effect economy) Annual net operating cost
S 4,500
5
$ 4,200
1,808 4,510
$ 3,629 -
$10,526
$ 9,750
15,040
...
30;250 4,840 10:656
8,400 75,146:
... 75,146 933 8,225 1,450 2,664 3,000
...
020,772
...
$20.772
~
5,150 20,750 5,328 1,950 57,968
12,700 45,268 $ 2,700
3,500
...
2,160 2,664 540 5,184 16,748
First cost is always an important consideration, but operating cost is frequently more determinative in choosing the crystallizing equipment. With the foregoing general factors in mind, the quantitative operating cost figures, as applied to the three examples previously given, are shown in Table I. In this cost analysis there are no allowances for building rental, inventory cost of materials in process, or losses. Credit is given as a separate item for water evaporated during the vacuum cooling. I n those processes where evaporation is necessary, this credit is logical and should be taken on the basis of steam cost to accomplish the equivalent amount of evaporation in an evaporator. The importance of unit steam and water costs and the variation of steam consumption with cooling water temperature, as previously pointed out, is apparent from these cost analyses since the steam cost is the biggest single item when boosters are involved. Space requirements are sometimes vital and limiting considerations. I n general, the vacuum crystallizer requires relatively high headroom and small floor space, whereas mechanical crystallizers require low headroom and large ~ areas. Mechanical crystallizers are, as a rule, of 1 % capacity per unit, and a multiplicity of units is required.
4,730 ___
$12,018
360 days per year.
DISSOLVERS
Due consideration t o vapor velocity and its effect on entrainment must be taken in determining the body size. High vapor space is preferred rather than the use of baffles or other types of entrainment separators in combination with reduced vapor space. Baffles salt up and require frequent cleaning. Mechanical separators which usually employ change of direction or centrifugal force, inherently require some pressure drop, which imposes an equivalent increase in booster compression range. Agitation in vacuum crystallizers is an important item. I n batch operation, for instance, boiling occurs only a t the liquid surface and agitation is necessary to bring all parts of the liquid volume t o the surface periodically in order to cool the mass uniformly and overcome the effect of hydrostatic head. I n continuous operation agitation likewise serves to prevent short-circuiting. I n addition, agitation keeps the crystals in suspension, promotes crystal growth, and prevents excessive new nuclei formation resulting from local supercooling beyond the metastable range of solubility. On the other hand, the degree of agitation must be kept within certain limits since violent turbulence causes eddies, highvelocity currents, whorls, etc., with attendant low pressure areas which flash and cause the formation of excessive fine crystals. Rough wall surfaces, projections, etc., are to be avoided for similar reasons since they are points of salting which necessitate periodic cleaning. The propeller type of agitation as illustrated in Figure 2 has proved very satisfactory. The propellers cause the magma to rotate en masse. This swirling rotation in proper degree creates a pressure against the side walls and thereby prevents boiling a t the wall surface and salting on the walls below the liquor level. The cone bottom gives an upward component to the circulation so that the resultant spiral upflow serves to bring the liquor and crystals continually to the surface where it cools by adiabatic evaporation and thence flows downward in the central vortex.
HEATER SETTLER
t
4-STAOE CONTINUOUS VACUUM CRYSTALLIZER
FIGURE12. FLOWSHEETFOR RECOVERY OF POTASSIUM CHLORIDE FROM SYLVINITE The batch vacuum crystallizer requires more headroom than the continuous crystallizer since it is usual practice to provide a n agitated magma tank below the crystallizer to receive the batch contents and serve as a surge tank for the subsequent continuous filtration step. The low maintenance of vacuum crystallixers is reflected in high availability. The only moving mechanical parts are the agitators. If they fail, i t is always possible to operate to some degree by using air agitation for a while, usually a t the expense of higher entrainment losses. I n a vacuum crystallizer of proper proportions the loss by entrainment, based on the weight of vapor, is of the same order of magnitude as in vacuum evaporators. Since the quantity of vapor produced in a vacuum crystallizer is generally only 5 to 10 per cent of the weight of the feed solution, the material losses are usually negligible. To minimize entrainment, the essential requirement is to maintain the lineal vapor velocity as constant as possible. This is largely
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636
a matter of design, since it is a function of the booster characteristic curve and increasing specific volume of vapor as the vacuum increases. As previously indicated, high vapor space is more economical than the use of auxiliary entrainment separators. Vacuum cooling serves to remove dissolved and entrained gases effectively from the liquor. This is frequently des i r a b l e f o r instance, the removal of carbon disulfide and hydrogen sulfide from viscose spin bath.
Uses of Vacuum Crystallizers Up to a few years ago the only vacuum crystallizer installations in this country were on potash and borax in one western plant. Commercial installations now operating on copperas, Glauber salt, Epsom salt, ammonium sulfate, ammonium chloride, sodium potassium ferricyanide, ammonium fluotitanate, zinc sulfate, citric acid, and various other organics will give some idea of the acceptance by the
VOL. 32, NO. 5
chemical industry of vacuum type crystallizers. Figures 11 and 12 show approximate flow sheets of typical applications of batch and continuous crystallizers.
Bibliography A11 references are to United States Patents. (1) Caldwell, H. B., 2,067,043 (Jan. 5, 1937). (2) Connell, G. A , , Cramer, T. M.,and Caldwell, H . B., 1,972,730 (Sept. 4, 1934). (3) Heath, S. B., 1,815,735 (July 21, 1931). (4) Howard, Henry, 1,559,703 (July 5, 1923), 1,560,473 (Nov. 3, 1925) : Meynardie, Pierre, 1,661,489 (March 6, 1928) : Mumford, R. W., 1,676,277 (July 10, 1928); Caldwell. H. B., 1.865.614 (July 5, 1932); Grill, H. E., and Feldstein, H . H., 2,097,208 (Oct. 26, 1937). (5) Isaachsen, Isak, 1,478,337 (Oct. 12, 1921), 1,573,716 (Feb. 16, 1926), 1,646,454 (Oct. 25, 1927), 1,693,786 (Dec. 4, 1928): Jeremiassen, Finn, 1,704,611 (March 5, 1929), 1,151,740 (March 25, 1930).
“KRYSTAL” CLASSIFYING CRYSTALLIZER HANS SVANOE A/S Krystal, Box 134, Kennett Square, Penna.
To separate chemicals in a crystalline form from solutions, the driving force as a supersaturated solution must first be established. If the crystals are to be formed under controlled conditions, the solution must be supersaturated within the metastable field during the process of crystallization, To maintain this condition, the supersaturated solution must be exposed efficiently to a large quantity of crystals which will bring the solution back to the saturation point before the solution is again supersaturated. The features incorporated in the “Krystal” equipment (also called the “Jeremiassen” or “Oslo crystallizer”) are based upon these principles, and the result is close control of the crystallization process. The type of equipment used depends mainly upon the solubility characteristics of the chemical to be recovered. For a salt with a flat solubility curve, supersaturation of the solution is produced
T
HE separation of chemicals in the crystalline state from solutions has been used for centuries. It was recognized
early that the process of crystallization m-as in many cases the only practical method of manufacturing pure chemicals with economy This outstanding feature of crystallization-to be able t o produce pure chemicals from relatively impure solutions-has therefore been utilized by the chemical industry for numerous products on an extensive scale.
Metastable Field of Supersaturation In order to separate chemicals from solutions by crystallization, a driving force in the form of a supersaturated solution must first be established. To be able to control the process of crystallization, it is of prime importance that this factorsupersaturation of the liquor-be controlled during the entire process.
by evaporation. For a large-capacity installation several units can be connected to form a multipleeffect system. For a salt with a steep solubility curve, supersaturation can be produced by water cooling or more efficiently by the combined effect of vaporization and vacuum cooling. Regarding cost factors in a crystallization problem, i t is important to consider that crystallization is only a part of the total-to separate a chemical from a solution as a dry, marketable product. The efficiency of centrifuging and drying operations depends upon crystal size. Screening of the product to meet trade demands can be avoided when the desired grain size is produced in the crystallization process. Therefore in the crystallizer operation a grain size should be produced that meets the highest standard of efficiency for the entire process.
An important work regarding supersaturation of solutions was published by Miers some thirty years ago. Miers stated that under certain conditions solutions can be supersaturated to a considerable extent beyond the saturation point without the formation of any nuclei. The field where no nuclei are formed Miers called the “metastable field of supersaturation”. If a solution is saturated beyond this field the labile field of supersaturation is reached where nuclei will appear in a clear solution. I n the metastable field crystal growth takes place on nuclei or crystals already present in the solution. If a solution can be maintained in the metastable degree of supersaturation during the entire process, a most important step towards control of crystallization is made. The metastable field is not sharply defined, and factors such as the pH of the liquor or the presence of other chemicals
