Ind. Eng. Chem. Res. 1987,26, 745-750 Kato, K.; Wen, C. Y. AZChE Symp. Ser. 1969, 66(105), 100-167. Keairns, D. L., Ed. Fluidization Technology; Hemisphere: Washington, DC, 1976; Vol. 1. Kelly, R. M. Ph.D. Dissertation, North Carolina State University, Raleigh, NC, 1981. Kossakowski, E. R. Ph.D. Dissertation, Trinity College, - . University of Cambridge, UK, 1981. Kutten. M. Ph.D. Dissertation.. Citv- Universitv of New York, New York, 1978. Laurendeau, N. M., Prog. Energy Combust. Sci. 1978,4, 221-270. Lee, M. K. A. M.S. Thesis, North Carolina State University, Raleigh, 1985. Lee, B. S.; Pyrcioch, E. J.; Schora, F. C. Chem. Eng. Prog. Symp. Ser. 1970, 66(105), 152-156. McDonald, F., Presented at the AIChE Annual Meeting, Los Angeles, CA, Nov 1982; Paper 80f. McFarlane, R. C.; Hoffman, T. W.; Taylor, P. A.; MacGregor, J. F. Znd. Eng. Chem. Process Des. Dev. 1983,22, 22-31. McGreavy, C. Comput. Chem. Eng. 1983, 7(4), 529-566. Purdy, M. J. Ph.D. Dissertation, North Carolina State University, Raleigh, 1983. Purdy, M. J.; Felder, R. M.; Ferrell, J. K. Znd. Eng. Chem. Process Des. Deu. 1981, 20, 675. Purdy, M. J.; Felder, R. M.; Ferrell, J. K. Znd. Eng. Chem. Process Des. Deu. 1984, 23, 287. Ray, W. H. Comput. Chem. Eng. 1983, 7(4), 367-394. Rhinehart, R. R. Ph.D. Dissertation, North Carolina State University, Raleigh, 1985. Rudisill, T. S. M.S. Thesis, North Carolina State University,Raleigh, 1984. Rutledge, K. L. M.S. Thesis, North Carolina State University, Raleigh, 1984.
745
Sargent, R. W. H. “New Challenges for Process Control”, Presented at the AIChE Annual Meeting, Washington, DC, Oct 1983. Schmal, M.; Monteiro, J. L. F.; Castellan, J. L. Znd. Eng. Chem. Process Des. Dev. 1982, 21,256-266. Solomon, P. R. DOE Contract Report ET-78-(2-01-3167, 1979; United Technologies Research Center, East Hartford, CT. Solomon, P. R.; Colket, M. B. Presented at the 17th International Symposium on Combustion; Pittsburgh, PA, 1978; pp 131-143. Staton, J. S.M.S. Thesis, North Carolina State University, Raleigh, 1983. Staton, J. S. Ph.D. Dissertation, North Carolina State University, Raleigh, 1985. Suuberg, E. M.; Peters, W. A.; Howard, J. B. Znd. Eng. Chem. Process Des. Deu. 1978, 1701, 37-46. Weimer, A. W. Ph.D. Dissertation, University of Colorado, Boulder, 1980. Weimer, A. W.; Clough, D. E. AZChE Symp. Ser. 1981,205, 51-65. Wen, C. Y.In Proceedings of the NSF Workshop on Fluidization and Fluid-Particle Systems Research Needs and Priorities; Littman, H., Ed.; Rensselaer Polytechnic Institute: Troy, NY,Oct 1979; pp 317-395. Wen, C. Y.; Chen, L. H. AZChE J. 1982,28(1), 117-128. Wen, C. Y.; Dutta, S. Coal Conversion Technology; Wen, C. Y., Lee, B. S., Eds.; Addison-Wesley: Reading, MA, 1979; p 1. Willis, W. E.Ph.D. Dissertation, North Carolina State University, Raleigh, 1981. Zand, A. M.S. Thesis, North Carolina State University, Raleigh, 1984. Received for review February 3, 1986 Revised manuscript received September 30, 1986 Accepted December 6,1986
Improvements in Batch Distillation Startup Juan R. GonzBlez-Velasco,*Miguel A. Gutierrez-Ortiz, Jose M. Castresana-Pelayo,and Jose A. Gonziilez-Marcos Departamento de Qutmica Tgcnica, Universidad del Pats Vasco, 48080 Bilbao, Spain
Numerical simulations show that both the energy and time requirements in the start-up phase of batch distillation processes can be reduced by decreasing the backmix in the condenser holdup. Furthermore, computed results in the simulation show that filling the holdups with feed liquid is better than the usual way of filling the plates and condenser with condensed vapors. The increasing significance of fine chemistry, the need of recovering profitable materials from waste products, and the great development of computers in process control have renewed interest in batch distillation. The rigorous mathematical model of batch distillation is complex due to the presence of liquid holdup in the overhead equipment (condenser and reflux drum) and plates and consists of a stiff simultaneous differential equation system that is analytically intractable and numerically unstable (Robinson, 1970,1971; Sadotomo and Miyahara, 1983). This instability increases for small but nonnegligible liquid holdups in relation to the still pot holdup. The mathematical models reported in the literature allow determination of the time-varying compositions and temperatures during the product take-off period. In the models with appreciable holdups, the compositions of the holdups (included that of the reboiler) at the end of the start-up period without distillate withdrawal preceding the overhead take-off period are usually considered as the starting points for that period. The time consumed in starting up the unit with appreciable holdups can be an important fraction of the total distillation time (Goldman, 1970; Luyben, 1971; Sadotomo and Miyahara, 1983), particularly for close boiling sepaOS88-5885/87/2626-0745$01.50/0
rations and systems with large holdups. Consequently, to optimize the whole process, the start-up period may have to be considered as part of the complete batch distillation cycle (Robinson, 1971). However, the startup has not been considered by most authors, with the exception of Huber (1964), Converse and Huber (1965), Mayur and Jackson (1971))and Domenech et al. (1977a,b). This reduces the validity of many optimization studies to cases where the take-off period is dominant. Obviously, the start-up period has to achieve a very quick composition change in order to reach the prescribed condenser hold-up composition as soon as possible. Variations in the start-up procedure and/or the equipment characteristics may be used to minimize the duration of this period. The models of batch distillation units reported in the literature consider a backmixing flow behavior within the condenser holdup (Domenech et al., 1977a,b; Guy, 1983; Sadotomo and Miyahara, 1983), as this assumption generally matches the equipment supplied by the manufacturers. In this paper we propose the modification in the overhead equipment of the unit in order to more closely approach a plug-flow behavior within the condenser. We 0 1987 American Chemical Society
746 Ind. Eng. Chem. Res., Vol. 26, No. 4, 1987
-
demonstrate that this behavior, instead of the complete mixing, reduces the inertia to composition changes and permits a startup of the unit more rapidly with the consequent energy savings.
The S t a r t u p The start-up period consists of the following consecutive three steps: (1)preheating of the still charge to its bubble point; (2) filling of the column and the condenser holdups; (3) running without distillate withdrawal and taking the unit to a steady state. The duration of the first step can usually be considered negligible in relation to the overall batch distillation time, whereas the duration of steps 2 and 3 is important because of the energy consumed in the process by the vapor boil-up rate (Gonzglez-Velasco et al., 1984). The filling of holdups in step 2 can be achieved in different ways: (2a) directly with still pot liquid at the boiling temperature (Huber, 1964; Converse and Huber, 1965; Goldman, 1970; Mayur and Jackson, 1971; Luyben, 1971; Domenech et al., 1977a,b); (2b) operating the unit without reflux-i.e., with only one rectification theoretical stage. Vapors from the reboiler are condensed and stored in the overhead equipment until liquid fills the condenser and the column holdups. This mode was suggested by Luyben (1971), but it has not been applied in simulation or in practice. Step 3, without product withdrawal, avoids having to exclude a previous distillate portion not satisfying the required specifications or having to maintain high reflux at the beginning of the take-off period to obtain on-specification distillate product. Step 3 can be achieved in two different ways: (3a) on total reflux, i.e., returning all of the overhead vapors as reflux; (3b) on finite reflux, Le., returning a portion of the condensed vapors as reflux and the rest to the reboiler. The former changes the compositions in the units more quickly and leads to a steady state with a higher condenser hold-up concentration. The take-off period starts when the condenser hold-up composition reaches a desired value that depends on the policy to be followed (Meadows, 1963; Goldman, 1970; Luyben, 1971), so the modeling equations of step 3 must describe the time dependence of the concentration in order to know the precise time a t which to begin the take-off phase. The most common method to carry out step 3 in practice (Huckaba and Danly, 1960) is at total reflux (Distefano, 1968; Luyben, 1971; Domenech, 1976; Sadotomo and Miyahara, 1983) until equilibration when the take-off period operates with constant reflux or with the optimal reflux policy (Robinson, 1969, 1970, 1971; Domenech, 1976). Mathematical modeling of the start-up phase began with the works of Coulson (1945), Berg and James (1948), and Jackson and Pigford (1956). The early models were limited to the case of total reflux in step 3 and also included simplifications about holdups, liquid and vapor flow rates, equilibrium data, etc. Domenech and Enjalbert (1973) considered finite plate efficiency, heat balances, variable holdups, and vapor-liquid equilibrium defined by a distribution coefficient. Simulation Procedure Batch Distillation Equipment. We have considered in this work the three batch distillation units shown in Figure I . The operation of the units with perfect mixing (Figure la, normally used) and with plug flow (Figure IC) in the condenser holdup is well-known and does not need explanation here.
H
I
.
o
bJ
(bl
(C)
Figure 1. Equipment studied: (a) usual equipment with backmix in the condenser; (b) partial backmix equipment; and (c) plug-flow equipment. Table I. Assumptions binary mixture with constant relative volatility constant liquid molar holdups constant molar boil-up rate total condenser constant plate efficiencies negligible pressure drops perfect mixing in the reboiler and on the trays constant molar vapor flow constant molar liquid reflux negligible vapor holdups negligible dynamics lags (quasi-stationary state)
In the equipment with partial backmixing, Figure lb, the condenser holdup is divided into two portions with perfect mixing arranged in series. Each portion has the same maximum capacity, M , coincident with that of the single deposit of usual unit (Figure la). The scheme of Figure I b runs as follows: when deposit 1 has been filled during step 2 of the startup, the intercommunication valve is closed and step 3 begins; the reflux is provided from deposit D1and the overhead condenser vapors stored in deposit Dz.As soon as deposit D1 is empty, D2is completely filled; a t this time the valve 1s opened and liquid is transferred from Dzto D1 during a period of time which we consider negligible. This cycle is repeated until the steady state or a prescribed average concentration in the condenser holdup is reached. The above policy can be easily extended to any number of reflux storage tanks, N , with an individual maximum capacity of M / ( N - 1). The extreme value, N 03) corresponds to plug-flow behavior. We must mention that the total liquid amount in all deposits at every instant is M so that the startup times for all of the different equipment can be compared. Modeling Equations. In order to simplify the mathematical calculus, we have considered in this work the standard assumptions of Table I. All of those assumptions, except the last five, can be removed from the simulations described in this work without a fundamental change in their structure. According to the tendency in the literature, we consider the duration of step 1 to be negligible since it can be considerably reduced by increasing the heat-transfer rate. Thus, we have to model only steps 2 and 3. The model of step 2 depends on the hold-up filling mode. If the holdups are filled with the still pot liquid (mode 2a), the only equation required is
-
n
Sf=So-M- CM, p=l
(1)
because the duration of this step could also be considered negligible. If the holdups are filled according to mode 2b, we have to use the Rayleigh equation
-dX, - --Y n - X, dS
S
(2)
Ind. Eng. Chem. Res., Vol. 26, No. 4, 1987 747 with
Y, =
1
i REA0 DATA
ax8
(3)
+ (a- l)X8 n
Sf=S,-M- c M p
(4)
p=l
CALCULATION OF STEP 2 (Eqna. 2-5)
and the duration is
M + ?Mp P=1
t =
(5)
V
1-
P A R T I A L BACKMIX
The modeling equations for step 3 are the equilibrium equation
Y p=
ax,
1
+ (a- l)XP
p = s,1,2,...,n
(6)
v +v-L -dX,- - L-XI- -Ys dt S S S XC
(8)
p = 1,2,...,n
(9)
For the plug-flow unit, Figure IC,considering the lag time we arrive at
M t I
(12)
7
Finally, for the partial backmix unit, Figure Ib, the reflux composition is constant during each cycle and equals the distillate average composition of the last cycle,
M V
M
k- < t I (k + l ) ~
k = 0,1,2,...,etc.
(13)
If the column holdup is ignored, eq 9 becomes indeterminate and the following algebraic equation must be substituted
L YPl = Y , + T ( X p - X , )
CALCULATION OF
9 or 14, 1 I and 12)
Figure 2. Algorithm of the main programs used.
For the condenser holdup, we have three different expressions corresponding to the three types of equipment we are considering. For the backmix unit, Figure la, a differential material balance yields
X,(t) = 8,
1-
WRITE RESULTS
-d =S o dt
x,,
COHPOSITION-TIHE CALCULATION OF STEP 3 (Eana. 6-8.
r"7
and the differential balances
X,(t) =
1
INITIAL VALUES ASS16NATION (Eqn. 1)
p = 1,2,...,n
(14)
Calculations. The described models (eq 1-14) have been simulated on the Pais Vssco University Perkin-Elmer 3220 digital computer. The ordinary differential equations have been solved by using the classic fourth-order Runge-Kutta integration algorithm which is inherently stable and does not need iterations. The calculation of the condenser hold-up average composition needed in the
limited mixing and plug-flow equipment has been made by means of Simpson's rule. Three simulation computer programs have been written in FORTRAN VI1 (version of FORTRAN 77 by PerkinElmer), and their simplified algorithms are shown in Figure 2, one for each considered unit. A detailed description of the computer algorithms and calculation procedure is available as supplementary material. In the programs for the units with holdup only in the condenser, the greatest part of the computer time was consumed with the iterative calculations of Y,. In all cases and operating with double precision, the required CPU time was below 1min/start-up hour for a unit with seven plates. Calculations when holdup in the condenser and plates exists are about 6 times faster for the same number of plates, since iteration is not necessary.
Results We have studied the cases shown in Table 11. Cases 1-4 were proposed previously by other authors (Robinson, 1969; Luyben, 1971), whereas cases 5-8 have been introduced by us in order to analyze the influence of parameters on the startup. We must note that we have introduced cases with negligible holdup in the column. According to Robinson (1970), the column holdup can be ignored in modeling when it is less than 4% compared to the still pot holdup; modern packed columns fit this threshold condition perfectly. We have not considered any example with negligible condenser holdup because these are not cases that commonly occur, although they can be found in the literature (e.g., Mayur and Jackson, 1971). Composition vs. start-up time trajectories have been calculated for the different cases shown in Table I1 for the units in Figure 1. In all cases, step 3 was carried out at total reflux, as this allows the decrease of startup time. As the final condenser composition, we used two usual values: (a) a mole fraction of 0.95, which is reasonable to obtain an adequate exit purity; (b) an X , close to the steady-state concentration, i.e., a mole fraction of X , - lo4. Figures 3 and 4 show the start-up curves (condenser hold-up composition vs. running time) for the different cases, equipments, and modes of achieving step 2, and the start-up times required to reach the two final composition
748 Ind. Eng. Chem. Res., Vol. 26, No. 4, 1987 Table 11. Cases Studied origin and case Robinson, 1969 4 10
Luyben, 1971 1
M
10 1 100 100
F SO
xso
3
2
10
10 1
o(
0.5 1.5
n
10
xe
0.99958
100 100 0.5 1.5 10 0.99897
2.0
70 0.99999
6
10 0
0 100
2
100 200 0.5
this work 5
50 100 0.5 2.0 7 0.99515
100 0.5 2.0 7
0.99515"
8
7
5
10
0 100
0
10 0 100
100 200 0.5 2.0 7 0.99568
100 0.5 2.0 7
0.99568
100 0.6 2.0 7
0.99689
Result according to Robinson, 1969.
Table 111. Final Times Obtained for the Equipment Studied and Mode 2a of Filling the Holdups case 1 2 3 4 5 6 Final Condenser Composition = 0.95 mole fraction final time, h, for" BME 1.150b 0.628b 1.465 0.559 1.118 0.278 0.229 0.461 0.922 0.553 1.393 1.078 PBE 0.922 0.229 0.461 1.389 1.071 0.540 PFE
7
8
0.556 0.458 0.458
0.480 0.385 0.385
1.810 1.375 1.375
1.735 1.297 1.297
Final Condenser Composition = X,- lo-* mole fraction final time, h, for" BME PBE PFE
3.661 3.421 3.394
5.317 5.188 5.175
2.037 1.711 1.681
0.905 0.687 0.687
3.632 2.762 2.762
1.816 1.381 1.381
"BME, backmix equipment; PBE, partial backmix equipment; PFE, plug-flow equipment. bResults according to Luyben, 1971.
Case 1
Case 5
Case 6
t
I
Case 7
C
05
10
15
20
i 0
I
05
10
15
0
20
05
10
15
20
I
50
150
m
1 0
BOIL-UP
50
0
05
10
'5
20
100
150
m
TIME hours
TIME hours 0
Case 8
100
150
200
I
mdes
0
50
100
150
0 BOIL -UP
200
50 males
Figure 3. Start-up curves for mode 2a of filling the holdups and for the equipment studied: (1)backmix, (2) partial backmix, (3) plug flow.
Figure 4. Start-up cuwes for mode 2b of filling the holdups and for the equipment studied: (1)backmix, (2) partial backmix, (3) plug flow.
values mentioned above are shown in Tables I11 and IV. The obtained results have been compared with those of Robinson (1969) and Luyben (1971) as far as possible, in
order to check the computer programs. Our equilibration criterium does not meet Luyben's one, so we have only been able to check the times when X , was 0.95 mole
Ind. Eng. Chem. Res., Vol. 26, No. 4, 1987 749 Table IV. Final Times Obtained for the Equipment Studied and Mode 2b of Filling the Holdups case 1
2
3
4
5
6
7
8
0.273 0.230 0.230
0.546 0.460 0.460
0.467 0.389 0.389
0.900 0.688 0.688
1.800 1.377 1.377
1.723 1.300 1.300
Final Condenser Composition = 0.95 mole fraction final time, h, for”
BME PBE PFE
1.298 1.228 1.226
1.313 1.240 1.251
1.780 1.710 1.721
0.552 0.465 0.465
Final Condenser Composition = X, -
1.104 0.930 0.930
mole fraction
final time, h, for”
BME
PBE PFE
3.803 3.563 3.543
2.720 2.393 2.389
5.603 5.476 5.475
1.809 1.385 1.385
3.618 2.770 2.770
BME, backmix equipment; PBE, partial backmix equipment; PFE, plug-flow equipment. fraction (see Table 111). Equilibration composition (Table 11) was checked with Robinson’s paper.
Discussion Start-up curves show us that the start-up time is shorter working with the partial backmixing equipment than with the perfect mixing one, for any X,. However, for smaller backmixing ( N > 2), the start-up time is practically constant, the start-up time for the plug-flow equipment being nearly the same as that of the partial backmixing. This small influence for N > 2 is due to the slow variation of the column head composition relative to the residence time of the liquid held up in the condenser. Probably, a higher M I S ratio and a smaller number of trays will produce a more important effect of N on the start-up curve. The influence of backmixing in the condenser holdup upon the startup can be explained quantitatively. As a matter of fact, the plug flow provokes a delay time in the condenser composition. This inertial effect permits X , to rise more quickly than with a backmix flow. Obviously, the partial backmix flow produces an intermediate effect. The influence of the mode of achieving the hold-up fiing in step 2 can be deduced from a comparison between the values in Tables I11 and IV. If there is no column holdup, both modes 2a and 2b are practically equivalent, although a very small advantage for the filling with condensed vapor (mode 2b) can be observed. On the contrary, if there exists a column holdup, the start-up times are considerably shorter when the filling is done with liquid from the reboiler (mode 2a). The higher the column holdup, the bigger the advantage for mode 2a. The percentage of reduction of the start-up time in the equipment studied ranges from 2.4 to 25.2, the highest value corresponding to case 8 which is characterized by negligible column holdup and the highest composition of the initial charge. The lowest value corresponds to case 3 which has a large number of plates and a large holdup per tray, which implies a small influence of the condenser holdup on the behavior of the equipment. In cases without column holdup, the influence of the initial charge quantity and the vaporization and the quantity of holdup in the condenser can be considered negligible, at least for the cases considered in this paper. However, increasing of the initial charge composition causes an appreciably larger reduction of the start-up time. In cases with significant column holdup, the start-up time reductions are much shorter than those of the cases with holdup only in the condenser. The percentage is smaller as a result of increasing the plate holdup (compare cases 1 and 3), decreasing the relative volatility, and decreasing the plate number (compare cases 1 and 2). Concerning the general effect of both the filling mode (2a or 2b) and the overhead equipment behavior, we
mention that the percentage of reduction of the start-up time is smaller with mode 2b, because of the influence of the equipment explained above and is identical for mode 2a. We must also mention that the start-up percentage reduction varies with the final and different values than those considered in this work could reduce the start-up time more but could be inadequate to provide the required quality for the exit product.
x,,
x,
Concluding Remarks The influence of the start-up mode and the hold-up amount on the time required for distillation is quite important. The start-up time increases with the holdup, but the duration of the take-off period decreases. Thus, the total operating time, sum of the start-up and take-off times, depends on the hold-up and the start-up modes. In addition, this time is related to the consumed energy (by the boil-up rate), and an energetic optimization could be sought. In terms of a contribution to the design and/or development of batch distillation systems, there appears to be an incentive: the addition of hold-up storage for the reduction of the start-up time. However, one ought to determine if the addition of hold-up storage implies an additional capital investment which can be justified on the basis of a reduction in start-up time and energy. Although a detailed economic study has not been made, the results obtained in this work seem to indicate that short payback times will be achieved. Finally, it must be pointed out that the increase in the number of storage drums has an important and beneficial effect on the flexibility of the batch processing, mainly for multicomponent distillation. The possibility of intermediate fractions or “riser cuts” is of primary interest; however, recycle of material and the coupling with the reactor system have to be studied simultaneously.
Acknowledgment This work was supported in part by a postgraduate fellowship obtained by J.M.C. from the Ministerio de Educacidn y Ciencia of the Spanish government.
Nomenclature k = integer number (eq 13) L = molar liquid flow in the column, mol/h M = molar holdup in the condenser, mol M p = molar holdup in plate p , mol n = number of theoretical plates
Ind. Eng. Chem. Res. 1987, 26, 750-753
750
S = still amount, mol t = time, h V = boil-up rate, mol/h X = liquid composition of low boiler component, mole fraction Y = vapor composition of low boiler component, mole fraction Greek Symbols (Y
= relative volatility
Subscripts c = relative to condenser e = steady-state condition
f = final value 0 = initial value p = relative to plate p s = relative to still pot
Supplementary Material Available: Detailed description of calculation algorithms (7 pages). Ordering information is given on sny current masthead page. Literature Cited Berg, C.; James, I. J. Trans. Am. Inst. Chem. Eng. 1948, 44, 307. Converse, A. 0.;Huber, C. I. Ind. Eng. Chem. Fundam. 1965,4,475. Coulson, E. A. J. SOC.Chem. Ind. 1945, 64, 101. Distefano, G. P. AIChE J . 1968, 14, 190.
Domenech, S. Ph.D. Dissertation, Universit6 de Toulouse, France, 1976. Domenech, S.; Enjalbert, M. Rev. Inst. Fr. Pet. 1973, 28, 201. Domenech, S.; Muratet, G.; Enjalbert, M. Presented at the Colloque Modelisation et Optimisation des Procedes Chimiques, Toulouse, France, 1977a. Domenech, S.; Muratet, G.; Enjalbert, M. Presented a t the 5th Symposium on Computers in Chemical Engineering, Praha, Czechoslovakia, 1977b. Goldman, M. R. Br. Chem. Eng. 1970, 15, 1450. Gonztlez-Velasco,J. R.; GutiBrrez-Ortiz,M. A.; Castresana, J. M. An. Quim. (Sp.) 1984, 80 Sup. 1, 607. Guy, J. L. Chem. Eng. 1983, 90(1), 99. Huber, C. I. M.S. Thesis, Carnegie Institute of Technology, Pittsburgh, 1964. Huckaba, C. E.; Danly, D. E. AIChE J . 1960, 6, 335. Jackson, R. F.; Pigford, R. L. Ind. Eng. Chem. 1956, 48, 1020. Luyben, W. L. Ind. Eng. Chem. Process Des. Dev. 1971, 10, 54. Mayur, D. N.; Jackson, R. Chem. Eng. J. 1971, 2, 150. Meadows, E. L. Chem. Eng. Prog., Symp. Ser. 1963,59,48. Robinson, E. R. Chem. Eng. Sei. 1969,24, 1661. Robinson, E. R. Chem. Eng. Sci. 1970,25, 921. Robinson, E. R. Chem. Proc. Eng. 1971, 5 2 , 4 7 . Sadotomo, H.; Miyahara, K. Int. Chem. Eng. 1983, 23, 36.
Received f o r review February 14, 1986 Revised manuscript received October 21, 1986 Accepted December 6, 1986
Phase Behavior of Mixtures of Ethylene, Methyl Acrylate, and Copolymers under High Pressures Gerhard Luft Institute of Chemical Technology, Technical University, 0-6100 Darmstadt, Federal Republic of Germany
N a r a y a n a n S. S u b r a m a n i a n * Research & Development Division, E. I . du Pant de Nemours & Co., Polymer Products Department, Wilmington, Delaware 19898
To avoid reactor fouling problems in the radical copolymerization of ethylene and methyl acrylate, the phase behavior of mixtures containing 5 wt % methyl acrylate, 75-85 wt 9% ethylene, and 15 wt % dipolymer was investigated at pressures up to 220 MPa and temperatures of 90-210 "C. The measurements were performed in an autoclave of 0.75-L capacity, equipped with sapphire windows and needle valves t o take samples from the phases. By visual observation of the phase behavior, the pressures at the cloud point using copolymers of different molar mass and composition were determined. By analysis of the samples taken from the dense and the light phases, the distribution of the components between the phases was evaluated. From the results, recommendations for the selection of the reactor pressure, which is necessary to perform polymerization in one phase, are given. Ethylene-methyl acrylate copolymers, having excellent heat- and oil-resistant properties, can be commercially producted by radical polymerization under high pressures. Ten to twenty weight percent polymer is formed from the ethylene-methyl acrylate feed. The mixture inside the reactor contains, in addition to the polymer, 75-85 wt % ethylene and approximately 5 wt % methyl acrylate. To carry out the polymerization in one phase, appropriate reactor temperature and pressure must be selected. From the study of phase equilibria for mixtures of ethylene and low density polyethylene a t high pressures, it is known that a homogeneous mixture separates into a light and a dense phase when pressure or temperature is reduced (Ehrlich, 1965; Liu and Prausnitz, 1980; Luft and Lindner, 1976; Spahl and Luft, 1981, 1982; Steiner and Horl6, 1972; Swelheim et al., 1965). The light phase is rich in ethylene, whereas the dense phase consists mainly of polymer. The phase behavior changes when a third com0888-5885/87/ 2626-0750$01.50/0
ponent is added. Therefore, in this paper, mixtures of ethylene, methyl acrylate, and an ethylene-methyl acrylate copolymer were investigated at pressures up to 220 MPa and temperatures of 90-210 "C. By means of optical and analytical methods, not only the conditions for a phase separation but also the distribution of the components between the phases could be determined. Experimental Section The measurements were carried out in a well-thermostated autoclave of 0.75-L capacity (Figure 1). The autoclave was designed for pressures up to 245 MPa and a temperature of 250 "C. It was equipped with two windows (2) of synthetic sapphire arranged one opposite the other. The windows, illuminated by a lamp (3), allowed visual observation of the phase behavior. Samples could be taken from the autoclave by means of needle values (VA) mounted on the top and the bottom side. During the Q 1987 American Chemical Society
