High-velocity two-phaseJ7ow through
A phase high-velocity flow through a plate heat exchanger has been spurred by the need for a technique
plate heat exchanger allows concentration,
for evaporation from foamy, viscous, heat-scnsitive materials. Traditional approaches to this design problem have been based on compromises with regard to heating area, residence time, and temperature using conventional evaporator configurations. The< plate evaporator, designed and fabricated by Parkson Industrial Equipment Go. of Fort Lauderdale, Fla. (7, Z ) , is now used primarily for concentration, monomer stripping and drying of synthetic latices, polymer solutions, and emulsions, but has operating characteristics that suggest
novel approsch to evaporator design based on two-
and/or strtpfx'ng of hat-sensitive, fwmy, and viscous latices and solutions, as well as drying of liquid polymers
Plate Evaporator System VAPOR AUD CWHWlED mwcr OUT
D. RUTHS
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Figwc 7. Schamafic diagramfor apldc cuapmrrfor typical of those now in m n u w i d
wider application. Combined effects of high fluid velocity and uniformity of gas-liquid mixture flow result in low effective viscosities and high heat and mass transfer rates. The relatively high velocity of fluid flow tends to scour plates, thus protecting heating surface against fouling and heat-sensitive product against degradation. In addition, this high-velocity flow and a small degree of vapor superheat combine to preclude foaming which hampers most other such evaporating operations. Evaporator System Desinn
A multiplicity of hot-water heated stainless steel plates, generally of corrugated-chevron configuration with self-contained gaskets comprises the heart of the plate evaporator. The plates are arranged so that corrugations on adjacent plates cross at many evenly spaced points to effect plate rigidity. Gaskets contained in each plate channel are not easily damaged by plate handling, high internal pressure, or hydraulic shock. In typical units such as those designed for latex concentration (Figure l ) , latex to be concentrated is drawn into the feed tank by vacuum or under pressure, and is fed to the plate evaporator by a variable-speed, manually controlled, positive-displacement rotary pump at pressures from 3 W O p.s.i.g. Plates are arranged so that initial vapor formation occurs a t an early stage in the flow through the heated plates; the generated vapor 48
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
UJC
flows cocurrently with the concentrating latex. Total residence time in the evaporating zone is a fraction of a second. The vapor-liquid mixture discharges from the plate evaporator and flows tangentially into a vertical cyclone separator which is held at a 28-in. Hg. vacuum. Separated vapors are condensed in a condenser, generally of shell-and-tube configuration; condensate is usually pumped to atmosphere but may be removed by a barcmetric leg. A two-stage s t e a m ejector maintains the vacuum. Disengaged product, such as concentrated latex, is removed from the separator bottom by a variable-speed, positive-displacement pump controlled by diaphragmtype differential pressure transmitter which maintains a low liquid level in the separator. In many units, a controlled portion of concentrate is recycled and allowed to mix with the dilute feed to provide intermediate-concentration feed for the plate evaporator. Deslgn Variations
Adaptation to a wide variety of processing applications is effected by variations of a basic system design. For certain applications, two or more evaporating stages (not to be confused with multieffect) result in the most economic design. Steam heating of the standard evaporator plates sometimes is used in place of the more conventional hot-water heating, while heat-transfer
ate, acrylonitrile, butyl acrylate, and other acrylic monomers from latices. One unit now in operation with an evaporative capacity of 6500 lb./hr. strips residual styrene from up to about 20 g.p.m. of latex to 0.01 to 0.02% content. In monomer styrene stripping, the monomer level diminishes in relation to the amount of water evaporated per unit quantity of solids stripped, and to the prestripped styrene content. Figure 2 shows the linear semilogarithmic relation of residual styrene content in a given latex to the quantity of water evaporated per unity quantity of dry solid. Variables such as residence time, operating temperature, polymer structure, and solids particle size determine the slope of the lines shown ; all of these factors affect diffusivity, extent of diffusion, or diffusion path. The following example illustrates the use of a graph such as given in Figure 2 in operation of a plate evaporator. Conversion of a continuous latex feed of 5000 Ib./hr. (bearing 48.0% total solids and 0.1 9% styrene monomer) to latex bearing 54.0% total solids and 0.0470 styrene corresponds to an influent solids flow of: TOTAL
LE. WATER EVAPORATEO/LB. DRY
SOLIDS
Figure 2. Performance characteristics f o r a plate-evaporator system &+ping styrene monomer from latices
oil may be employed in modified-design units with welded plate bundles. This latter design modification is of use for concentration, deodorization, and drying of sensitive, high-boiling organic compounds. For products that are less heat sensitive, the effluent homogeneous vapor-product mixture may be directly discharged to a variety of external sink. Neat soap containing 30 to 33y0 moisture is dried to a controlled level between 9.5 and 12.0y0 moisture, and directly discharged onto a chilled roll from which it is doctored and conveyed to milling (4, 5). Generated steam is removed by a blower from the hood covering the discharge and chill roll to prevent recondensation. Applications
Developments in the textile, foam, and precoating fields and in other converting operations have continued to advance the range of preferred total solids content of latex. Plate evaporator systems are now commercially used in simultaneous monomer stripping and solids concentration, degassing, and mixing and pre-floc reduction in latex processing ; another commercial application is drying of emulsion-polymerized ( 3 ) ) and solution-polymerized liquid polymers. Several large Parkson plate-evaporator systems are in operation for production stripping of styrene, ethyl acrylDavid Ruths is Development Manager for Parkson Industrial Equibment Co., Fort Lauderdale, Fla., which company specializes in design and fabrication of industrial evaporator systems using standard plate heat exchangers. AUTHOR
5000 lb./hr. X
solids
=
2400 lb./hr. solids
To use Figure 2, correct styrene monomer content to 50% total solids base : 0.19% X 50/48 = 0.20% styrene in feed 0.04% X 50/54 = 0.04% styrene in product
Then from the graph, the straight line of slope equal to that determined experimentally, and starting at 0.20% styrene monomer content, read 0.96 lb. water evaporated/lb. dry solids for 0.04Y0 styrene monomer content (corrected to 50% total solids). Then the evaporation rate is determined: 0.96 X 2400 lb./hr. of solids =
2320 lb. water evaporatedhr. And dilution water is calculated: 4450 lb./hr.
+ 2320 lb./hr. - 5000 lb./hr.
=
1770 lb./hr. This method of calculation has proved workable for several applications of the Parkson plate evaporator system. Among the applications under current and planned development are concentration of solutionpolymerized stereopolymers, vaporization of unsulfonated and unsulfated detergents intermediates, and urea drying. REFERENCES (1) U. S. Patent 3,073,380. (2) British Patent 935,542. (3) de Decker, H. K., McCall, C. A. “Synergism of Emulsion Polybutadiene and Natural Rubber,” Rubber World, 5’4-62 (Nov. 1964). (4) McCutcheon, J. W “Process and Equipment Innovations in the Soap and Detergent Industry,”’betergent Age (Jan. 1965). (5) Palmason, E. H., “Modern Soap Dryer,” Soap and Chemtcal Specinittes (July 1964).
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