2156
Ind. Eng. Chem. Res. 1997, 36, 2156-2162
Freeze Coagulation of ABS Latex Robert J. Adler† and Nelson Gardner Case Western Reserve University, Cleveland, Ohio 44106
Eugene R. Moore* SRI Consulting, Menlo Park, California 94025
Joseph M. Ceraso The Dow Chemical Company, Midland, Michigan 48667
Thermoplastic injection and extrusion resins based on acrylonitrile/butadiene/styrene (ABS) are widely used because of their toughness and solvent resistance. When high gloss is necessary, these resins must be produced by emulsion polymerization, which requires that they be coagulated and dewatered before sale. We review the coagulation techniques that are available, discuss the advantages and disadvantages of each, and then focus on coagulation by freezing. We examine two approaches to freeze coagulation: The first involves freezing against a cold surface. The second involves direct contact freezing of a suspension of ABS latex particles in contact with liquid CO2 at about 3340 kPa (485 psia). Introduction Dried polymers prepared by emulsion polymerization have many uses. These useful polymers range from poly(vinyl chloride) (Whelan and Craft, 1977), copolymers of vinylidene chloride (Wessling, 1970), a wide variety of rubbers both natural and synthetic (Bounty and Boyer, 1952), polymers and multipolymers of common addition polymers such as acrylonitrile/butadiene/ styrene (ABS) (Montanye, 1993), to the higher rubber content, grafted rubber concentrates (GRC) used for later preparation of ABSsthe subject of this study. All of these emulsion polymers must be converted from a stable emulsion in water to dried polymer before use. Review of Coagulation Techniques for Latex Several techniques have been developed to destabilize polymeric emulsions and cause the desired coagulation. Chemical Coagulation. The most common technique for coagulation has been the addition of chemicals (Bovey, 1955), including acids that convert stabilizing soaps to ineffective fatty acids. Widely used chemical coagulants are the trivalent sulfate salt of aluminum (alum), divalent salts of calcium or magnesium, and even sodium chloride in special cases. Coagulation efficiency increases markedly with an increase in the valence of the cation, causing alum to be the most widely used. The chemical coagulation route is both simple and effective. Chemicals are added to a simple agitated tank, and coagulation takes place within seconds. The coagulation is so effective that the filtrate is crystal clear, eliminating a possible disposal problem for “white water”. The major negative factors with chemical coagulation are the effect of residual coagulant both on the polymer and in the discharge water stream. The coagulant often promotes decomposition of the polymer or causes discoloration. Coagulant salts in the filtrate stream pose a * Author to whom correspondence should be addressed. E-mail:
[email protected]. Telephone: (415) 8593722. Fax: (415) 859-5134. Formerly with The Dow Chemical Company. † Deceased. S0888-5885(96)00695-1 CCC: $14.00
serious waste disposal concern, and it is not normally practical to concentrate and recycle the coagulant. Evaporative Coagulation. Coagulation by evaporating water is sometimes used for special applications. Spray drying appears to be the most practiced technique. A recent patent combines some aspects of spray drying with fluidized bed technology (Moore and Tong, 1989). Drying eliminates the need to dispose of a contaminated water stream, but the energy use and the capital investment for water evaporation are high. A further disadvantage is that all of the surfactant remains with the polymer. From our own experience, these surfactants usually accelerate aging. Shear Coagulation. Coagulation of emulsions may be induced by shear. Perhaps the longest practiced application of this technique has been the production of butter from sour milk. Shear coagulation technology, utilizing commercial butter churns, has been taught for coagulation of ABS resins (Pingel, 1981). Shear coagulation has the advantage of requiring no added chemicals, thus avoiding both accelerated polymer deterioration and some major wastewater disposal concerns. However, this technique does involve some waste with resulting yield loss. Yield loss occurs either if the latex is too shear sensitive or if the latex is not shear sensitive enough. If the latex is not shear sensitive enough, incomplete coagulation occurs and product is lost with the wastewater stream. Latex in the wastewater stream produces “white water” that generally cannot be discharged without clarification treatment. Such treatment typically requires chemical coagulation and associated waste disposal concerns for the salt. If the latex is made too shear sensitive, perhaps by reducing the surfactant, coagulum is generated during the polymerization and stripping of the latex. In the extreme, this coagulum can amount to a major loss in yield as well as a maintenance challenge requiring frequent vessel cleaning. One practical approach to solving this problem has been to use CO2 during the shear coagulation process to reduce the pH (Moore and Lefevre, 1986). This approach requires the use of pHsensitive fatty acid soaps for at least a portion of the © 1997 American Chemical Society
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surfactant but results in little coagulum in the reaction vessels while maintaining high yields in the coagulation process. Extruder Coagulation. A twin screw extruder manufacturer has commercialized a technique for using the combined shear field and high temperature of the extruder melt zone (with chemicals optionally added) to coagulate the latex in the extruder (Nichols et al., 1984), and another manufacturer has developed a process to remove the water from already coagulated latexes (Andersen and Kite-Powell, 1992). The water in both cases is primarily removed as a liquid. Often this dewatering, or coagulation and dewatering, is carried out at the same time as a compounding operation, which may involve the addition of pigments, antioxidants, or other additives. In the present study, a GRC, initially a latex, could be added to a styrene/ acrylonitrile (SAN) copolymer to produce ABS. When used to coagulate latex, this extrusion technique appears to have some of the same operational limits as shear coagulation discussed above: Namely, if the latex’s colloidal stability is too high, incomplete coagulation occurs; if the latex’s colloidal stability is too low, coagulation occurs in the reaction vessels. Freeze Coagulation. Freeze coagulation is useful for a wide variety of ABS and GRC resins. With this class of resins, freeze coagulation is complete with no trace of latex remaining in the crystal-clear filtrate. Depending on the rubber content, the nature of the dried coagulum has been found to vary widely. At rubber contents below about 50%, a dusty, fine particle size is obtained that can only be centrifuged down to 25-30% water. At higher rubber contents (between 55% and about 85%), however, a coarse particle is obtained that can be centrifuged to as low as 13% water (Moore, 1989) at room temperature. At increased centrifugation temperatures (96 °C), retained water is reduced to 7.4%. The use of a high-rubber GRC in a commercial operation confers economic advantages. Those advantages result because the capital cost for a conventional emulsion ABS plant approach is about double that for a plant to produce SAN by the solution (mass) process. Operating costs for the emulsion plant are also higher than for the mass process. The most favorable economics can thus be obtained with the highest rubber GRC product, and it has been found that these high-rubber products can be utilized without significant reduction in the key properties of high gloss, toughness, and melt flow rate. For freeze-coagulated, low-rubber GRC, additional steps are necessary to eliminate the problems associated with fine particle size. Fortunately, when the lowrubber GRC is exposed to steam under pressure for a short time (Pingel, 1981), the resin softens enough to allow controlled coalescence. This coalescence increases the “crumb” size. With larger crumb size, dusting is no longer a problem, and the water drains more freely in a centrifuge. One serious early problem encountered with freeze coagulation was the tendency for the coagulum to adhere to the freezing surface, causing difficulty in discharging the coagulated product. It was found, however, that this adhesion problem could be easily overcome by allowing a very thin film of water to freeze on the cold surface before contact with the latex (Moore, 1986). Using the frozen water film technique, conventional ice-making equipment can, with only slight modifications, effectively freeze, coagulate, and dis-
charge GRC. These pieces of equipment also require some modification because latex’s surface tension is lower than that of water (Moore, 1986). Three basic types of freezing equipment are commercially available. Tubular (Vogt, Henry Machine Co., 1988) and flat plate (Turbo Ice Machines, 1988) equipment types are of particular interest because they are designed to allow refrigerant to come in direct contact with the other side of the surface used to freeze the latex. These both require minor modifications (Moore, 1986) but provide the best energy efficiency. Both rotary drum freezers and continuous belt freezing systems (Sandvik Steel Belt Freezing Systems, 1990) accommodate the low surface tension of the latex and circumvent the sticking problem without the need for modification. They do not depend on surface tension to distribute the latex uniformly on the freezing surface, and the frozen solids are physically removed with a knife-edge scraper. Because containment of refrigerant is difficult with the rotary drum and impossible with the belt unit, an intermediate heat-transfer medium is required. Such units significantly reduce the thermal energy efficiency of both units, however, and they are generally unable to compete with tubular or flat plate freezers for ice manufacture. Nonetheless, they may still be attractive for higher value products such as ABS. In all of these cases, latex is frozen in direct contact with a cold surface, and the frozen layer must grow to a moderate thickness before removal. However, no data were available to use as a basis for predicting the rate of formation of this frozen layer, and such data were needed to predict the output rate of commercial equipment. To obtain the desired design rate data, to understand the latex freezing process better, and to define thermal properties of the frozen latex, The Dow Chemical Company and Case Western Reserve University undertook a joint research project. This paper reports the results of that research project. As the study progressed, the possibility of freezing in direct contact with liquid CO2 became of interest. Although other choices such as Freon or pentane pose environmental, flammability, or toxicity concerns, CO2 does not (CO2 does enter into the global-warming dispute, but its use in a closed system would allow for little to escape and would likely use reclaimed CO2). Except for freeze coagulation at the very high rubber levels discussed above, all forms of coagulation produce a fine powder that is difficult to extrude unless agglomerated in a separate unit operation. It was hoped that the plasticizing action of liquid CO2 might soften the small ABS coagulum particles and allow agglomeration to larger particles as coagulation occurs. Experimental Section Freezing against a Cold Surface. A cylindrical dilatometric cell 12.4 cm long and about 6 cm inside diameter was constructed with 0.16-cm wall aluminum tubing holding about 1/3 L of fluid (see Figure 1). The ends were sealed with relatively nonconductive, 2.54cm-thick, acrylic plates to minimize thermal end effects. A syringe affixed to the top was used to measure volume expansion as freezing caused the liquid latex to be expelled. The syringe was mounted on the centerline of the cylinder with a tapered entrance to avoid bubble entrapment as the cell was filled. The syringe volume measurements were good to (0.2 mL in both sensitivity and accuracy. Two additional ports were installed on
2158 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997
Figure 1. Experimental freezing apparatus.
the cell top, the first to fill the cell with latex and the second to insert a thermocouple. Temperature measurements were sensitive to changes of (0.1 °C and were probably accurate to (0.5 °C. The dilatometric cell was submerged in a wellagitated methanol bath cooled with pellets of dry ice. The bath was initially cooled to about 1 °C and held at that temperature until the latex temperature approached the bath temperature asymptotically. Then, the bath temperature was rapidly decreased by the addition of dry ice pellets. As the temperature of the methanol bath around the cell dropped, the solid phase grew radially inward from the wall in an annular shape until the cell contents were completely frozen. The expelled liquid level in the syringe, the bath temperature, and the temperature near the center of the cell were recorded as functions of time. The syringe liquid level indicated the amount of frozen solid formed at any given time. In all cases, the latex emulsion froze radially inward and, as expected, did so symmetrically and smoothly, with a uniform annular shape. The temperature of the liquid latex emulsion remained at about 0 °C throughout the freezing process in most instances. The time of freezing in a typical experiment was between about 10 min and 1 h, depending on the bath temperature. The latex emulsion froze about 20% faster than water under similar cooling conditions. On melting at room temperature, the frozen latex emulsion separated readily in all instances into crumblike coagulated latex solids and clear liquid. The data on expelled liquid volume, bath temperature, and liquid latex emulsion temperaturesall functions of timeswere subsequently analyzed further using a mathematical model derived for this process. The model predictions of the growth rate of the solid were matched with the data by adjusting parameters such as solid (frozen latex or water) thermal conductivity and solid heat capacity. The basic parameters identified can now be used to predict growth rates in new geometries and with different cooling temperatures. The dilatometric cell can also be used in the future to determine the rates of freezing of new types of latex emulsions relative to water. Water was used as a reference fluid in this study. The latex used was a 35% solids ABS latex containing styrene and acrylonitrile in a ratio of about 70/30, and the rubber (95% butadiene, 3% styrene, and 2% acrylonitrile) content was 85% of the total polymeric weight. Freezing by Direct Contact with Carbon Dioxide. The latex emulsion used was a 35% solids ABS
GRC containing about 50% rubber (95% butadiene, 3% styrene, and 2% acrylonitrile). This latex containing a lower percentage of rubber than above (50% rubber vs 85% above) was used because normal freeze coagulation of lower rubber latexes produces a very fine powder that is difficult to process. Liquid CO2 was obtained from a standard supply cylinder. Experiments were carried out in a Parr high-pressure vessel equipped with a 34.6 mPa (5,000 psig) pressure gauge, a 20.8 mPa (3,000 psig) blow-out safety valve, and two inlet/outlet tubes. One tube terminated near the top, and one near the bottom, of the vessel. The internal volume of the vessel was approximately 300 mL. A magnetic stirrer was used only to aid in loading the reactor with CO2. Latex stability in the presence of CO2 was explored both with the gas and with the liquid. For examination of stability, or lack of coagulation, of the latex in the presence of the gas-phase CO2, the high-pressure vessel was half filled with latex solution, sealed, and connected to the CO2 cylinder. No pressure regulator was used. CO2 gas from the supply cylinder entered the pressure vessel through a dip tube ending near the bottom of the Parr vessel. The CO2 bubbled through the latex until the pressures equalized. The pressure was about 4928 kPa (700 psi) at about 19 °C. After 12 h of connection to the supply cylinder, the vessel was disconnected, and CO2 was vented into the atmosphere through the other line from the gas space at the top of the vessel. The vessel was then opened and the latex examined. To ascertain the degree of stability in the presence of liquid CO2, the experiment was repeated, except that the pressure vessel was cooled to 0 °C with ice to assure that liquid CO2 would be in contact with the sample. After 4 h, the vessel was disconnected, removed from the ice bucket, warmed slightly in the atmosphere, and vented over a period of a few minutes. The vessel was then opened and the latex examined. In both cases, no coagulum was found. Freezing experiments were carried out in the same vessel; a magnetic stirrer was used during the CO2 addition to speed condensation while the vessel was maintained at 5 °C by immersion in a cold-water bath. Latex was loaded with the vessel lid removed, and the loaded vessel was frozen by placing it in the freezing section of a refrigerator. For the rapid freezing experiment, 140 g of latex and 64 g of CO2 were loaded into the vessel. The vessel was placed into the freezer at -22 °C for 9 h. The temperature trace as a function of time indicated that freezing was complete after 2 h by a plateau at -2.1 °C. The vessel was thawed in an agitated water bath for 2 h, and the vent valve was opened to bleed off the CO2 slowly. For the slow freezing experiment, 155 g of latex and 81 g of CO2 were loaded into the vessel. The vessel was placed in an insulating container formed from 1.27-cm foam rubber and then placed into the same -22 °C freezer. Freezing was complete after 8 h, as indicated by a temperature plateau, and the vessel was opened for examination as above. Development of a Mathematical Model for Freezing on a Cold Surface To convert the raw dilatometric data into meaningful and consistent values, we developed a mathematical model for freezing in the annulus of the cylindrical
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container that comprises two energy balances, including initial and boundary conditions: The following unsteady-state, differential energy balance in the frozen annulus, of unit length, expresses the temperature in the solid layer as a function of radial position r and time t, assuming angular symmetry when all dependent variables are functions of radial position and time only. Given a cylinder of cylinder radius ) rw, frozen annulus thickness in the cylinder ) ∆r, and distance of frozen interface from the cylinder center ) rf where qr ) heat flow outward at position r per unit height per unit time (cal/(cm s)), k ) conductivity of the solid (cal/ (cm °C s)), r ) radial position (rf < r < rw) (cm), t ) time (s), Tsol ) T(r,t) ) temperature of the solid (°C), Fsol ) density of the solid (g/cm3), cp,sol ) heat capacity of the solid (cal/(g °C), and h ) overall heat-transfer coefficient at the wall-coolant interface (cal/(cm2 °C s), then the heat entering is
qr ) -k2πr(δT(r,t)/δr)
(1)
the heat leaving is
qr+∆r ) -k2π(r+∆r)[δT(r+∆r,t)/δr]
(2)
and the accumulated heat is
δ[2πrFsol(cp,sol)T∆r]/(δt)
(3)
The equations are combined to yield the overall balance:
(1) + (2) ) (3) The appropriate initial and boundary conditions for T(r,t) are
δ[krδT(r,0)/δr]r ) 0
(initial)
T(rf(t)) ) Tf (boundary condition 1) -k(δT(rw,t))/δr ) h[T(rw,t) - Tc(t)] (boundary condition 2) An energy balance can also be written over the whole cylinder. Consider an integral control volume of unit length in the axial direction and bounded by 0, rw in the radial direction; the energy balance is
[δE(t)]/[δt] ) ksol2π(rw)[δT(rw,t)]/[δr]
(4)
where E(t) ) the internal energy content of the unit length of the entire vessel (the solid annulus, plus the liquid core) referenced to zero internal energy of the solid state at an arbitrary reference temperature, Tref (cal/cm). Because only liquid and solid phases are present, we made the approximation that the rate of change of internal energy, dE(t)/dt, is given by the rate of change of the enthalpy. Thus, E(t) is well approximated by
E(t) ) π(rf,t)2Fliq[cp,liq(Tliq(t) - Tf(t)) + ∆Hf + cp,sol(Tf(t) - Tref)] +
∫rr(t)2πrFsolcp,sol(T(r,t) - Tref) δr w
f
(5) The first term on the right-hand side of eq 5 is the energy content of the liquid core, and the second term is the energy content of the solid annulus where Tf(t) ) temperature at which the latex freezes (and of the inner
Table 1. Latex Emulsion Properties and Parameters Used in and Developed from the Computer Model value property
water
latex
cp,liq, cal/(g °C) cp,sol, cal/(g °C) ∆Hf, cal/g h, cal/(cm2 °C s) ksol, cal/(cm °C s) Fliq, g/cm3 Fsol, g/cm3
1.00 0.51a 80 2.4 × 10-2 b 4.0 × 10-3 c 1.000 0.915e
0.80 0.46 60 2.4 × 10-2 b 3.6 × 10-3 c 0.970d 0.919e
a 0.506 at 0 °C, 0.487 at -10 °C, 0.469 at -20 °C. b Equivalent to 1770 BTU/(ft2 °F h). c 5.54 × 10-3 at -10 °C, 5.80 × 10-3 at -20 °C, 6.09 at -30 °C, 6.35 at -40 °C. d Estimated. e Calculated from observed volume expansion (by the computer program).
annular surface), Tliq(t) ) temperature of the liquid inner core, ∆Hf ) heat of fusion (cal/g), Tref ) arbitrary temperature where E ) 0, and the initial condition of rf(t) is rf(0) ) rw. These equations were reduced to dimensionless form and converted to a finite difference version for solution by a digital computer. The time derivatives were forward differenced, and the space derivatives were central differenced, except at the boundaries for which one-sided differences were used. Comparison of Experimental Data with Modeled Data The major purpose of this study was to determine the freezing rate of latex compared with that of water. It was speculated that, depending on the nature of the frozen latex, the freezing rates of latex relative to water might be significantly reduced. Low rates were expected because of the relatively low thermal conductivity of the base polymer. In addition, the effect of polymer on the ice morphology was unknown. Data were collected for both water and the latex from repeat runs over a wide variety of cooling rates. Once the model was developed and the computer programmed to solve the model, it was a relatively simple, although time-consuming, task to vary the values for the thermal properties of the latex until values were found that fit the freezing curve data over a range of cooling rates. Table 1 presents these values for the latex and for water. Looking first at the data for the heat capacity of the liquid (cp,liq) of 0.80 cal/(g °C), we note that it is significantly lower than the value (1.00) for pure water. The heat capacity of the solid (cp,sol) is 0.46 cal/(g °C), which is close to the literature value of 0.506 at 0 °C (Dorsey, 1940) for pure water. The heat of fusion (∆Hf) value of 60 cal/g is significantly lower that the 80 cal/g value of pure water. The conductivity of the frozen latex (ksol) of 3.6 × 10-3 cal/(cm °C s) is somewhat lower than that of the value for ice projected from the available data (from 5.54 × 10-3 for ice at -10 °C and 5.80 × 10-3 cal/ (cm °C s) at -20 °C). Both in the laboratory tests and in the resulting computer simulations, the latex consistently freezes about 20% faster than does water. The thermal conductivity of the frozen latex is in fact reduced significantly by the presence of the polymer. The decrease in heat capacity and reduced heat of fusion, however, more than compensate for this drop in conductivity. Table 2 presents a typical set of data (along with the calculated values from the model) for freezing of latex. These same data are plotted in Figure 2. In this data
2160 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 Table 2. Typical Freezing Curve Data for Latex syringe liq vol, cm3 time, s
model
data
coolant temp, °C
0.00 300.00 446.50 627.40 822.68 1030.47 1241.68 1463.17 1692.53 1920.87 2133.60 2324.86 2465.38 2502.25
0.00 2.00 4.95 7.44 9.69 11.70 13.47 14.99 16.26 17.29 18.08 18.63 18.93 18.99
0.00 2.00 5.90 8.20 10.20 12.04 13.53 15.05 16.18 17.33 18.10 18.75 18.97 19.00
-6.20 -11.00 -12.68 -15.08 -16.94 -19.04 -20.32 -19.52 -19.83 -20.36 -20.10 -20.20 -20.20 -20.20
Figure 3. Comparison of the data for freezing of water to the data for freezing of latex.
the machine harvests ice mechanically from a stationary freezing surface. All capital estimates are based on 1991 costs. Direct Contact Freezing with CO2
Figure 2. Comparison of the model for the freezing with the dilatometric data for a typical latex.
set and the multiple sets examined, the measured volumetric expansion, as indicated by the volume in the syringe, comes within less than (1 mL of the calculated data from the model. The freezing rate of latex relative to the freezing rate of water is shown in Figure 3. This dilatometric technique could be used to screen a wide variety of latex types and concentrations relatively quickly. This type of apparatus would lend itself well to automation in data recording and could be coupled to a digital computer to carry out the necessary iterative calculations. Preliminary Economics We obtained capital cost information for a large flat panel, direct refrigerant contact, ice machine from Turbo Ice Machines. The machine (Model CF144SC) was rated for 30 000 kg/day of polymer product (a nominal 20 × 106 lb/year). At 70 kW‚h/ton of ice and electricity at 6¢/kWh, the cost of electrical energy is about 0.4¢/kg of polymer product. Calculated equipment amortization and interest-carrying charges range from 1.3¢/kg of polymer product (3-year amortization, 10% interest) to 0.4¢/kg of polymer product (15-year amortization, 10% interest). This analysis does not take into account the increase in capital (estimated at about 20%) needed to modify the equipment to accommodate the low latex surface tension and tendency to stick to the frozen surface (Moore, 1986). Similar economics appear to apply to the much smaller Model F229 manufactured by Crystal Tips Ice Products (1988), which states that
Direct contact heat transfer is potentially the lowest cost method of freezing and melting latex emulsion on a large scale. In this method, a liquid refrigerant is mixed with the liquid latex in an agitated vessel, and the refrigerant is flashed to absorb heat and freeze the latex and thereby form frozen latex particles suspended in liquid refrigerant. The slurry is transferred to a second agitated vessel where the solid latex particles are melted by condensation of slightly compressed refrigerant gas flashed from the first vessel. Because the latex droplets and particles have high surface area, the temperature and pressure differences are small between the flashing and melting vessels, and the power needed to compress the gas is a very small fraction of the thermal energy transferred between the flasher and melter. This freezing-melting operation is in effect a classical heat pump where the heat source and heat sink differ in temperature by only a few degrees. Additional power is needed in auxiliary operations to recover refrigerant dissolved in the water. Possible refrigerants include Freon substitutes, propane, and CO2. Although many factors should be considered in selecting the most suitable from these and other possibilities, we chose CO2 for process evaluation in this initial study because it appeared to be outstanding from the standpoints of safety and potential environmental contamination. Conceptual Process and Economics We designed a conceptual process to allow preliminary calculation of energy requirements for freezing and thawing in the presence of liquid CO2. Latex is contacted with liquid CO2 in one of two agitated vessels at -1 °C and 3340 kPa (485 psia) where flashing causes freezing. Vapor is slightly compressed and added to the second vessel which is operated in the thawing mode at +1 °C. We initially assumed that the decrease in pH caused by the presence of CO2 would not cause coagulation (now confirmed) and that the plasticizing
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effect of CO2 would allow a large enough crumb to grow to permit the use of common screw press devices (Pingel, 1981). Periodically, the melt vessel becomes the freezing vessel, and the freezing vessel becomes the melt vessel. After thawing, the resulting three-phase mixture (liquid water, liquid CO2, and coagulated latex), still under pressure, is passed through the screw press where liquid is removed to produce a solid crumb containing about 5% residual liquid. Liquid CO2 remaining on the solids is flashed off at 101.3 kPa (14.7 psia) in a recovery vessel, and the solid product is removed with a small amount of gaseous CO2. Liquid CO2 is decanted from the water and recycled to the freezing vessel. The water content of the CO2 phase is reduced from about 7% to about 0.2% by flashing to 1 atm. The gas is recompressed and also returned to the freezing chamber. Although this analysis is not precise in all respects, and certainly not optimized, it is suitable for rough estimates of energy requirements and as a starting point for process refinement and equipment sizing. We calculated the energy requirements for a highpressure latex pump and the four CO2 compressors by assuming 80% efficiency. The total energy requirements for these operations calculate to be 0.0126 kW‚h/ kg of product. With electricity at 6¢/kW‚h, this translates to the low value of 0.076¢/kg (0.035¢/lb). If the latex is not coagulated as soon as it contacts the CO2, the vessel will freeze suspended particles of latex to form a stable, clean suspension. If coagulation occurs during the initial mixing of latex with liquid CO2, immediate vessel fouling will occur, thereby making the whole process impractical. We have confirmed that coagulation does not take place in either liquid- or gasphase CO2swhich is important if the freezing vessel is to remain cleanswith the procedure described in the Experimental Section. When the latex was frozen in the presence of liquid CO2, either quickly or slowly, the particles recovered were relatively coarse, gritty, and distinct. They were not the talcum-powder-type, fine dust normally obtained from freeze coagulation of this low butadiene latex. Centrifuging for 10 min lowered the water content to 18.7% in the quickly frozen samples and to 20.0% in the slowly frozen samples. These percentages may be qualitatively compared with values of more than 30% for the fine powder from normal freeze coagulation. Although this work with freezing of ABS-type GRC in the presence of liquid CO2 is preliminary, it opens the way for additional research. Additional variables to be studied in the freeze/melt area include (1) ratio of CO2 to latex, (2) rate of flashing, (3) the intensity and type of agitation, and (4) the rate of condensation and melting. These experiments should be designed to reveal whether the solids produced by freezing and thawing reside in the liquid water phase or the liquid CO2 phase or perhaps concentrate at the interface between the two. Additional research on the direct contact freezing process could have favorable environmental and economic impacts on the large worldwide production of high-gloss ABS thermoplastics. Summary Freeze coagulation presents a nonpolluting method of coagulating ABS polymers and other polymers prepared in the emulsion state but useful in the solid state. Freeze coagulation offers an alternative to the common
technique that uses salts such as aluminum sulfate, magnesium chloride, or calcium chloride. This paper describes a simple, but precise, cylindrical dilatometric apparatus to measure the freezing rate. The thickness of a frozen layer of latex was continuously monitored, as freezing proceeded from the outside surface, by expansion into a calibrated syringe. The results of freezing a grafted rubber concentrate of ABS emulsion containing 85% rubber (95% butadiene, 3% styrene, 2% acrylonitrile) at 35% solids show that the freezing rate for this latex is about 20% faster than for pure water. This increase in rate indicates that the decrease in heat of fusion required due to the lower water content and the reduced heat capacity of the frozen cake has a greater influence on freezing rate than the decrease in thermal conductivity of the frozen latex. A mathematical model was presented that fits the data well and, in addition to predicting freezing rates, allows the calculation of the key latex parameters. The heat of fusion of the latex was found to be 60 cal/ g, compared to that for water of 80 cal/g, and the specific heat of the latex was found to be 0.460 cal/(g °C) compared with 0.506 cal/(g °C) for water, although the thermal conductivity of 3.6 × 10-3 cal/(cm °C s) was lower than the value that would be projected from available ice data (5.54 × 10-3 at -10 °C and 5.80 × 10-3 at -20 °C). Preliminary economics for freezing latex on a cold surface indicate combined energy and capital costs on the order of 0.8-1.7¢/kg (0.36-0.77¢/ lb). Freezing in direct contact with boiling liquid CO2 has been briefly explored. Preliminary experiments in which ABS-type GRC latex was frozen in the presence of liquid CO2 demonstrated the concept. Qualitative observations indicated that much larger particles of coagulated latex resulted, perhaps from the coalescence allowed by the plasticizing action of the CO2. Coagulation in the presence of liquid or gaseous CO2 did not occur, allowing a clean suspension of latex in the liquid CO2 to be formed before initiating freezing. Energy costs for conceptual process design were estimated at 0.076¢/kg (0.034¢/lb) of latex solids. Acknowledgment The Dow Chemical Company provided financial support of this work and gave permission to publish the results. Remily A. Clish and Christopher S. Kovach assisted in this work while completing BS requirements in chemical engineering at Case Western Reserve University. Nomenclature cp,sol ) heat capacity of solid, cal/(g °C) E(t) ) the internal energy content of the unit length of the entire vessel (the solid annulus, plus the liquid core) referenced to zero internal energy of the solid state at an arbitrary reference temperature, Tref (cal/cm) h ) overall heat-transfer coefficient at the wall-coolant interface, cal/(cm2)(°C)(s) ∆Hf ) heat of fusion, cal/g k ) conductivity of solid, cal/(cm °C s) qr ) heat flow outward at position r per unit height per unit time, cal/(cm s) r ) radial position (rf < r < rw), cm ∆r ) frozen annulus thickness in cylinder, cm rf ) distance of frozen interface from the cylinder center, cm rw ) cylinder radius, cm
2162 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 t ) time, s Tf(t) ) temperature at which the latex freezes (and of the inner annular surface), °C Tliq(t) ) temperature of the liquid inner core, °C Tref ) arbitrary temperature where E ) 0 and the initial condition of rf(t) is rf(0) ) rw, °C Tsol ) T(r,t) ) temperature of the solid, °C Fsol ) density of solid, g/cm3
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Received for review November 4, 1996 Revised manuscript received February 18, 1997 Accepted February 19, 1997X IE9606956
X Abstract published in Advance ACS Abstracts, April 15, 1997.
