DESIGN PRINCIPLES OF EVAPORATORS FOR INTENSELY F O ~ I N G SOLUTIONS V. Z. Globa and S. I. Tkachenko
UDC [66.066.8+ 66.067.55]:664
An acute need arose in the food industry for evaporating soap-alkali solutions (soap stock) which have no parallel among the solutions evaporated; soap stocks have high viscosity and form such a stable foam that well-known evaporators are unsuitable for processing them. For designing evaporators for soap stock, the complex problem of foam suppression was resolved since the well known methods [i, 2] and their technoeconomic indexes were unsuitable. A working laboratory model of an evaporator was fabricated based on preliminary studies (Fig. i). It consists of a foam generator (FG), foam-suppressor-separator (FSS), ancillary equipment, and a device for determining the vapor content of foam. The three-stage FSS consists of a row of centrifugal separators joined successively along the flow direction~ The separator of the first stage was fabricated like an entrainment type single-stage-cyclone  with three tangential pipe connections. The second and the third stages are designed like centrifugal separators . The measurements provided data on foam properties and process parameters, A device was fitted in the FG to determine the vapor content of foam by the interception method ; measuring containers fitted in the discharge lines of separators measured the foam quantum. Piezometers heated by secondary steam were fitted to control the pressure distribution along the channel and the resistance of the FSS elements. Tests were made on soap stocks of concentration up to 45% at atmospheric pressure of the secondary steam when the viscosity of the solution rose by 400-600 times. The sharp rise of viscosity began as maximum concentration Cma x = 18-20% was attained. The output of the FSS with respect to the secondary steam M" = 1.6 • i0 -~ kg/sec and its total resistance did not exceed 22 kPa. The inner diameter of the outer separation pipes was 56 mm and of inner ones 38 mm. The other dimensions are: Stage Height, m Distance from the pipe connections or swirlers to the stage exit, m Main parameter of the stage*
first 1.5; 1.8
second 0.4; 0.43
0.7; 0.i i0; 15; 30
0.15; 0.2 20-30
third 0.47 0~ 30-48
When using nonfoaming solutions and water, the FG functioned as an evaporator with inertial and settling vapor separation ensuring a good quality of secondary steam. When boiling soap stocks, foam formed instead of the vapor layer whose life exceeds 200 sec. Under conditions of massive bubbling of steam from the heat exchanger surface, even in the dynamic two-phase bed of the FG as in a tubular evaporator, foam is formed in which the true volume content of vapor ~ ~ 0.92. Sucha foam is inseparable in an inertial separator. In settling separators, liquid flow arises from the layer (syneresis) and ~ increases. The initial syneresis stages occur in 10-30 sec [i, 2] and the vapor content of foam, e.g., from soap stock of concentration C = 10%, increases from 0.9-0.92 to 0.95-0.97. After this, the stock fluidity diminishes. The quality of the foam layer of the boiling soap stocks depends little on their composition, operational factors, and geometric characteristics of the heat exchanger and settling separator surfaces. The solution concentration exerts utmost influence on the foam quality. The dispersity and stability increase and vapor content decreases as the concentration increases. At Cma x, the soap stocks form a vapor emulsion with ~ = 0.5-0.7. *The ratio m of the areas of the effective section of the outer separation pipes and pipe connections of the cyclone or swirler channels .
Translated from Khimicheskoe i Neftyanoe Mashinostroenie, No. 12, pp. 5-6, December, 1987.
9 1988 Plenum Publishing Corporation
23 2~ 25
~26 ~27 ~28
I /7 Fig. i. Sketch of a laboratory evaporator: I) foam generator: i, 5, 8, 15) pipe connections, 2, 6) baffle plates, 3) tubular electrical heater, 4) holder, 7) body; II) device for determining the vapor content of foam: I0, ii) glass tubes, 14) measuring flask with cold water; III) ancillary equipment: 9, 20, 21, 22) measuring flasks, 12) solution heater, 13) condenser; IV) foamsuppressor--separator: 16) foam pipelines, 17, 18) pipes for draining the separator, 19, 31) tangential pipe connection and cyclone body, 23) trap, 24, 27) glass and steel outer separation pipes; 25, 28) axial swirlers, 26, 29) inner separation pipes, and 30, 32) cyclone glass tubes. The disintegration of the liquid films of the foam commences (Fig. i) in the pipe connections 19, proceeds intensely in swirlers 25 and 28, and ends in pipes 24, 27, and 30 of cyclone 3. It is here that phase separation and vapor and liquid stream formation arise. The liquid flows down from the cyclone along tube 32 into a loop on its wall and into the swirled axisymmetrical stream of highly dispersed foam or emulsion. The swirling of the flow and the foam in pipe 32 are rapidly suppressed. The unsuppressed foam of high density mixed with liquid enters the FG. Vapor with some foam moves upward along pipe 30 of the cyclone. The flow is stratified imperceptibly. Vapor predominates at the center of the section and foam along the tube walls. The swirling motion is rapidly suppressed and ceases almost completely at a height of 400-500 mm from the pipe connections. On transition from a single- to three-stage FSS, its foam output with ~ = 0.92-0.94 increases from 0.7.10 -3 to 1.6,10 -~ kg/sec. Thus, it was experimentally established that the suppression of highly moist foam should be effected in a multistage FSS. The withdrawal of the liquid and the reduction of moisture and density p of foam in stages (P3 < p2 < Pl) ensure the possibility of increasing stagewise the main geometric parameter m3 > m2 > ml and flow velocity. The characteristic dependences of resistance AP of the FSS elements on concentration C of the soap stock which could be used for designing industrial separators are shown in Fig. 2. The curves each have two sharp maxima. The first pertains to a very weak solution with C < 0.2% and the second to the maximum soap stock concentration forming the emulsion. The FSS resistance, phase separation, and the quality of the secondary steam depend on the amount of liquid L entrained with the foam and the liquid L i separated in the different stages. At fixed load M" the value L is determined by the moisture (i -- @) and the foam entering the FSS, and L i by parameter m i. Based on the results of experimental studies, the following values are recommended: LI = (0.7-0.9)L, L3 ! 0.01 L, and ml = 10-15. This pattern corresponds to the least resistance of the FSS. An analysis of the experimental data showed that on reducing the pressure, a significant amount of spontaneously generated vapors is formed in the stream. In the first stage of the FSS, at high moisture content of foam and resistance AP~ = i0 kPa, self-evaporation
Apz, kPa ]
/k ..... L"~-1~C....
0 0,3 5
20 25 30 C,%
Fig. 2. Dependence of resistance AP of the FSS elements on concentration C of soap stocks at productivity 1.2.10 -3 kg/ sec: a) first stage (m = i0), b) second stage (m = 24), and c) third stage (m = 39), --) sunflower oil, and ---) hydrogenated fats. could be one-half or more of the FG output. The steam flow from the FG and the velocity in the cyclone pipe connections correspondingly decrease; also, the centrifugal force acting on the foam is also reduced. On the other hand, since the heating of the liquid is not much, self-evaporation arises after the flow exits from the pipe connections, i.e., in the cyclone eddy, in a field of reduced swirling velocities not facilitating foam suppression. Herein lies the physical factor for the low output of a single-stage FSS. The results of investigations helped formulate the fundamentals of modeling evaporators for intensely foaming solutions and devices were fabricated for foam suppression and evaporating soap stocks. The settling separator type evaporator was reduced to the dimensions of a vapor collecting chamber since it is ineffective when working with foam. The FSS is rationally joined with a cyclone in the first stage and axial separators in the subsequent stages. An evaporator designed for soap-alkali solutions (soap stock) based on this scheme is working at the Vinnitsk Oil and Fat Combine. LITERATURE !o 2. 3. 4. 5.
V. K. Tikhomirov, Foams. Theory and Practice of Their Production and Disintegration [in Russian], Khimiya, Moscow (1983). K. B. Kann, "Some principles of the syneresis of foams. Expulsion," Koiloidn. Zh., 40, No. 5, 858-863 (1978). E. F. Buznikov, Cyclone Separators in Steam Boilers [in Russian], Energiya, Moscow (1969). Patent Ii0765 GDR, WPB 01/d 177397 (1975). S. I. Tkachenko, "A study of the characteristics of evaporators used in the sugar industry," Candidates Dissertation, Technical Sciences, Kiev (1966).