Ind. Eng. Chem. Res. 1994,33, 118-124
118
Kinetic Study on Thiourea Adduction with Cyclohexane-Methylcyclopentane System. 1. Equilibrium Study Kwang-Joo Kim' and Choul-Ho Lee Department of Chemical Engineering, Korea Research Institute of Chemical Technology, Taejon, 305-606, Korea
Seung-Kon Ryu Department of Chemical Engineering, Chungnam National University, Taejon, 305-764, Korea
Ternary solid-liquid equilibrium diagrams of the systems methanol, thiourea, and guests cyclohexane, methylcyclopentane (MCP), and their mixtures have been presented and discussed in the range between 30 and 60 OC. A study of phase diagrams reveals that thiourea could successfully be used for the separation of cyclohexane and MCP from nonaromatic raffinate by adductive crystallization. Solubility limits for thiourea-cyclohexane adduct, thiourea-MCP adduct, and thiourea-MCPcyclohexane adduct in ternary liquid solutions were measured over the entire feasible range of composition. Activity coefficients estimated for the saturation liquid solutions reflected strong positive deviation from ideality. Adduct solubilities increase with increase in the concentration of MCP guest and with increase in the temperature. In most cases adduct was found to be able to be formed near room temperature. The process of thiourea adduction, therefore, can be operated near room temperature. mthiourea(d) + guest(d) + adduct(s)
Introduction Many of the processes concerned with nucleation and crystal growth for the adducts formed by thiourea and hydrocarbons are not well understood. Naphthene, isoparaffins, and aromatic compounds have been isolated by adductive crystallization using thiourea (Fetterly, 1950; Redlich et al., 1950). Recent work has shown that nonaromatic raffinate derived from naphtha cracker can be separated by multistage adduction with thiourea (Lee et al., 1992; Kim, 1993). Previous studies of thiourea adduction (McCandlessand Handle, 1973;Smith, 1950)dealt with yields, purity, and stoichiometry. In contrast, both equilibrium and kinetic data have received no attention. For the analysis of the kinetics in adduction and the design of the adduction process, it is necessary to know the phase equilibrium of a ternary system and the solubility of the host (thiourea) and adducts in the solvent. To achieve rapid adduction rate in the thiourea adduction process, a polar solvent capable of dissolving both hydrocarbons and thiourea was required. Accordingly, the solvent selected in this study was methanol. Therefore the purpose of this study is to investigate the equilibrium of adduct by probing ternary systems in depth, i.e., the solubility of the adduct formed by the guest of the various concentrations in methanol. The methanol-thiourea-guest systems in which guests were composed of cyclohexane, methylcyclopentane, and their mixtures were selected for the study on the adduct solubility.
Theoretical Section Adduct Equilibrium. Equilibrium between adduct crystals and a solution containing the dissolved thiourea and the guest which contains cyclohexane,MCP, and their mixtures can be described by
* To whom correspondence should be addressed.
(1)
where m is a constant meaning the entrapping capacity of thiourea and is represented as the molar ratio of thiourea to guest entrapped in thiourea. d and s in eq 1 refer to dissolved phase and solid phase, respectively. However, the analysis of thermodynamic equilibrium in solutions involves some ambiguity when strong specific interaction occurs (Hildebrand and Scott, 1950; Lewis and Randall, 1961). Derivation of equilibrium constants by free energy minimization requires postulation of the species involved in the actual equilibrium. Usually a plausible system of components (perhaps including associated or dissociated species) is assumed to be involved in the equilibrium and an equilibrium constant based on this model is derived. In the urea adduction system, Tamarelli and Manning (1967) represented that specific interaction can be neglected. If no specific interaction is assumed, the dissociation constant is written as rn
K=
agueat(d)u
thiouredd)
'adduct(s)
(2)
where a represents the activity. For the adduction step, this involves as low a value as possible for K. The solvent plays an important role in this respect, in that it brings the thiourea and the guest into homogeneous solution, so that the adduction may proceed at an optimum rate. The nature and concentration of the solvent must be such that the equilibrium is maintained on the side of precipitated solid adduct. Solubility. The solubility of solid of adduct in a solvent may be expressed in a very general manner by (Prausnitz, 1979)
where X I , 71, AHml, AC,,,,
(3) T m l , and T stand for the mole
0888-588519412633-Ol18$04.50/0 0 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 1, 1994 119 3
5
t
Table 2. Adduct Solubility Limits for the Thiourea-Methanol-MCP System wt % guest at
wt%
4--i
1
Figure 1. Solid-liquid equilibrium apparatus. 1,equilibrium cell; 2, magnetic stirrer; 3, programming controller; 4, thermostatic bath and circulator; 5, temperature recorder; 6, vacuum line; 7, thermocouple; 8, condenser.
Table 1. Adduct Solubility Limits for the Thiourea-MethanolCyclohexane System wt % guest at
wt%
thiourea 1.00 2.00 3.00 4.00 5.00 7.00 9.00 11.00 12.00 12.50 13.50 14.50 15.50 16.50 17.50 18.50 19.50 20.50 22.00 23.00 24.00
30°C 62.19 38.67 24.05 14.96 9.30 3.60 1.39 0.54 0.33
35°C 64.40 41.48 26.71 17.20 11.08 4.60 1.91 0.79 0.51 0.41 0.26
40°C 67.17 45.11 30.30 20.35 13.67 6.17 2.78 1.26 0.84 0.69 0.46 0.31
45°C 69.49 48.29 33.55 23.32 16.20 7.82 3.78 1.82 1.27 1.06 0.73 0.51 0.35
5OoC 55°C 60°C 72.47 75.35 78.35 52.52 56.78 61.39 38.06 42.78 48.09 27.58 32.24 37.68 19.99 24.29 29.52 10.50 13.79 18.12 7.83 11.12 5.51 2.90 4.45 6.83 2.10 3.35 5.35 1.79 2.91 4.74 1.29 2.19 3.71 0.94 1.65 2.91 0.68 1.24 2.28 0.49 0.94 1.78 0.36 0.71 1.40 0.53 1.10 0.40 0.86 0.30 0.67 0.47 0.37 0.29
fraction, activity coefficient,enthalpy of fusion, solute heat capacity difference between the solid and the liquid at the melting point, melting temperature of solute, and equilibrium temperature, respectively. An ideal solubility value can be obtained by taking y1 = 1. For calculation the AHml and T,I values a t the triple point have been substituted for the enthalpy of fusion and the normal melting temperature. Moreover, the last term in eq 3 has little effect on the results and has therefore been omitted.
Experimental Section Materials and Apparatus. Thiourea was purchased from Wac0 Pure Chemical Industries Co., methanol of analysisgrade was purchased from Merck, and cyclohexane and MCP were purchased from Aldrich Chemical Co. Before use thiourea was purified further by recrystallization. Figure 1 shows a schematic drawing of the solidliquid equilibrium apparatus. The equilibrium cell is 12 cm in length and 5 cm in diameter with a total volume
thiourea
30°C
35°C
40°C
45°C
50°C
55°C
60°C
1.00 2.00 3.00 4.00 5.00 7.00 9.00 11.00 12.00 12.50 13.50 14.50 15.50 16.50 17.50 18.50 19.50 20.50 22.00 23.00 24.00
69.59 48.45 33.74 23.49 16.36 7.93 3.84 1.86 1.30
71.19 36.14 25.75 18.35 9.31 4.73 2.40 1.71 1.44 1.03
72.84 54.65 40.44 29.93 22.15 12.13 6.65 3.64 2.69 2.32 1.71 1.27
75.59 57.36 43.52 33.02 25.06 14.43 8.31 4.78 3.63 3.16 2.40 1.82 1.38
77.56 60.54 47.26 36.84 28.80 17.55 10.69 6.51 5.09 4.49 3.51 2.74 2.14 1.67 1.30
80.46 65.41 53.18 43.24 35.15 23.24 15.36 10.15 8.25 7.44 6.05 4.92 4.00 3.25 2.64 2.15 1.75 1.42
81.93 68.37 57.05 47.60 39.72 27.66 19.26 13.41 11.19 10.22 8.53 7.12 5.94 4.96 4.13 3.45 2.88 2.40 1.83 1.53 1.27
Table 3. Adduct Solubility Limits for the Thiourea-Methanol-MCPCyclohexaneSystem w t % guest at
wt%
thiourea
30°C
35°C
40°C
45°C
50°C
55°C
60°C
1.00 2.00 3.00 4.00 5.00 7.00 9.00 11.00 12.00 12.50 13.50 14.50 15.50 16.50 17.50 18.50 19.50 20.50 22.00 23.00 24.00
65.68 43.15 28.35 18.63 12.24 5.28 2.28 0.98 0.65
67.95 46.24 31.46 21.41 14.57 6.74 3.12 1.45 0.98 0.81 0.55
70.45 49.70 35.06 24.73 17.44 8.68 4.32 2.15 1.52 1.27 0.90 0.63
72.41 74.83 52.53 56.11 38.11 42.07 27.64 31.54 20.05 23.65 10.55 13.29 5.55 7.47 2.92 4.20 2.12 3.15 1.81 2.73 1.31 2.04 0.95 1.53 0.69 1.15 0.86 0.65
77.89 60.78 47.43 37.01 28.88 17.59 10.71 6.52 5.09 4.50 3.51 2.74 2.14 1.67 1.30 1.02 0.79 0.62
80.44 64.95 52.43 42.33 34.18 22.28 14.52 9.46 7.64 6.87 5.54 4.48 3.61 2.92 2.36 1.90 1.54 1.24 0.90 0.73 0.59
of 230 mL. The cell has a perforated rubber stopper, through which the copper-constantan thermometer is inserted. The contents are stirred with a magnetic spin bar. This cell is placed, by means of a ground glass joint, inside a triple-jacket vessel. The outer one is under vacuum and provides thermal insulation to allow the visual observation of the mixture. Through the middle passage, either cooling or heating medium is circulated from a water bath, with a control accuracy of 0.1 K. The inner jacket can be left empty, or it can be filled with ethylene glycol, in order to regulate the rate of heat transfer from the circulating medium to the cell. The thermometer outputs are also fed to a temperature recorder. Method and Procedure. Solubilities were determined by using a dynamic (synthesis) method. Mixtures of solute and solvent, which were cooled in a water bath until abundant amounts of crystal formed, were heated very slowly (at less than 2 K/h near the equilibrium temperature) with continuous stirring inside a Pyrex glass cell, placed in a thermostat. The crystal disappearance tem-
120 Ind. Eng. Chem. Res., Vol. 33, No. 1, 1994
90
90
I""
00
00
70
70
60
60
50
50
40
40
30
30
20
20
IO
IO
n "
ti
0 0
5
IO
15
20
25
0
5
Thiourea in liquid(wt.%)
15
5
t
10
IO
20
Figure 4. Thiourea-methanol-cyclohexane-MCP solution-adduct isotherms.
40
5
15
Thiourea in liquid(wt.%)
Figure 2. Thiourea-methanol-cyclohexane solution-adduct isotherms.
0
IO
20
25
I 30 50
20
peratures, detected visually, were measured with a calibrated thermostat coupled in the recorder (Yokogawa, 180Micro). Adduct Solubility. Since the solubility data were taken at many different temperatures, each data point was converted to the nearest integral multiple of 5 O C by determining
over the range of compositions. This procedure did not appreciably degrade the accuracy. Curves were then drawn through the resultant isothermal data points. Some coordinates of the smoothed isotherm for the systems thiourea-methanol-cyclohexane, thiourea-methanol-MCP, and thiourea-methanol-their mixtures are presented in Tables 1,2, and 3, respectively. Their plots are given in Figures 2, 3 and 4, respectively. The solubility limit was reached when only a very small remaining amount of minute adduct crystals could be visually detected when light was passed through the
60
70
Temperature (T)
Thiourea in liquid(wt.%)
Figure 3. Thiourea-methanol-MCP solution-adduct isotherms.
40
Figure 5. Solubility curves of thiourea in methanol.
mixture. At this point, the temperature and the concentration were calculated from the solubility of thiourea in methanol shown in Figure 5 (Kim, 1993).
Results and Discussion Dissociation Constant. In eq 1for the equilibrium of thiourea adduction, adduct@)is the thiourea complex with guest and so the phase of the thiourea complex is solid. In thiourea adduction, guest converts from liquid phase into solid phase of adduct, and homogeneous solution is supersaturated with respect to (thiourea + guest) complexes; therefore guest is in equilibrium between liquidus and adduct(s). The values of Yguest and Ythiouree in eq 2 were calculated from eq 3 and data of Tables 1-3 and are for the guests consisting of cyshown in Table 4. yguest clohexane,MCP-cyclohexane mixture, and MCP increases with increase in the thiourea concentration and decreases with increase in the temperature. Figure 6 shows the comparison of activity coefficients at 35 "C on the guest for the guest consisting of cyclohexconcentration. yguest ane, MCP-cyclohexane mixture, and MCP at 35 OC varies
Ind. Eng. Chem. Res., Vol. 33, No. 1, 1994 121 Table 4. Activity Coefficients Estimated from Adduct Solubility Data in the Thiourea-Methanol-Guest System ythiOurea at
wt%
thiourea
30°C
35°C
1.00 2.00 3.00 4.00 5.00 7.00 9.00 11.00 12.00 12.50 13.50
15.92 7.96 5.31 3.98 3.18 2.27 1.77 1.45 1.33
18.05 9.03 6.02 4.51 3.61 2.58 2.01 1.64 1.50 1.44 1.34
thiourea
30°C
35°C
1.00 2.00 3.00 4.00 5.00 7.00 9.00 11.00 12.00 12.50 13.50
1.56 2.24 3.22 4.62 6.64 13.69 28.25 58.26 83.68
40°C 20.33 10.17 6.78 5.08 4.07 2.90 2.26 1.85 1.69 1.63 1.51
40°C 1.51 1.55 2.17 2.04 2.76 3.05 4.28 3.73 6.00 5.04 11.83 9.20 23.30 16.80 45.90 30.68 64.43 41.45 76.33 48.18 107.13 65.10 YMCP
wt%
wt%
thiourea 1.00 .. 2.00 3.00 4.00 5.00 7.00 9.00 11.00 12.00 12.50 13.50 ~
30°C 35°C 40°C 1.65 1.62 1.58 2.25 2.52 2.38 3.83 3.50 3.18 4.51 5.83 5.15 6.40 8.87 7.56 20.55 16.33 12.86 47.60 35.27 25.85 110.25 76.19 51.96 167.80 111.96 73.66 135.73 87.70 199.47 124.32
+
55°C 27.98 13.99 9.33 7.00 5.60 4.00 3.11 2.54 2.33 2.24 2.07
60°C 30.78 15.39 10.26 7.69 6.16 4.40 3.42 2.80 2.56 2.46 2.28
thiourea 14.50 15.50 16.50 17.50 18.50 19.50 20.50 22.00 23.00 24.00
at 45°C 50°C 1.47 1.50 1.89 1.97 2.42 2.60 3.42 3.10 4.51 3.97 6.52 7.84 13.61 10.70 23.63 17.56 31.14 22.49 35.75 25.46 47.12 32.62
55°C 1.44 1.77 2.17 2.67 3.29 4.98 7.53 11.39 14.01 15.54 19.11
60°C 1.43 1.71 2.05 2.45 2.94 4.22 6.07 8.71 10.44 11.43 13.70
thiourea 14.50 15.50 16.50 17.50 18.50 19.50 20.50 22.00 23.00 24.00
-+
as
%hiourea
35°C
40°C 1.40
30°C
35°C
40°C 87.97
-
K was computed at a number of points with the experimental values of m (Kim, 1993) and the calculated values of Ygumt and Ythiourea in Table 4 for the temperature range of 30-60 "C and is shown in Table 5. The equilibrium constants of the adducts are shown in Figure 7. The values of K increased with increase in the temperature and decreased progressively with increase in
45°C 1.57 1.47
50°C 1.75 1.63 1.53 1.45
55°C 1.93 1.81 1.70 1.60 1.51 1.43 1.36
60°C 2.12 1.99 1.87 1.76 1.66 1.58 1.50 1.40 1.34 1.28
50°C 41.78 53.53 68.58 87.85 112.54
55°C 23.51 28.92 35.57 43.74 53.81 66.18 81.40
60°C 16.42 19.68 23.58 28.26 33.87 40.59 48.64 63.81 76.47 91.65
55°C 42.24 54.13 69.36 88.88 113.90 145.96 187.04
60°C 26.11 32.34 40.06 49.61 61.45 76.12 94.28 129.96 160.98 199.39
YMCP at
45°C 62.09 81.83 107.84 142.12
YMCP
+ cyclohexane at
30°C
35°C
40°C 45°C 176.25 118.99 164.03
30°C
35°C
40°C 358.24
~
wt%
50°C 55°C 60°C thiourea 1.58 1.53 1.49 14.50 2.18 2.04 1.90 15.50 2.43 3.01 2.70 16.50 4.15 3.10 17.50 3.59 4.76 3.96 18.50 5.72 6.45 19.50 10.90 8.38 20.75 14.77 10.50 20.50 22.00 39.51 26.01 17.11 23.00 54.51 34.52 21.84 24.00 64.04 39.76 24.67 88.36 52.77 31.49
from 1 to 418, 1 to 200, and 1 to 107, respectively, and 7thiourea varies from 65.2 to 1.34,where these values increase sharply. = 1.0 As shown in Figure 6, Xguest = 1.0 and hence at the guest-rich limit of adduct solubility curve. Thus, Yguest
30°C
wt%
at wt% 45OC 50°C 55°C 60°C thiourea 1.56 1.53 1.48 1.45 14.50 2.15 2.04 1.90 1.80 15.50 2.97 2.72 2.44 2.23 16.50 3.12 2.76 17.50 4.09 3.63 3.42 5.64 4.84 4.00 18.50 5.24 10.71 8.61 6.58 19.50 20.50 20.36 15.31 10.80 8.05 38.69 27.23 17.73 12.35 22.00 23.00 53.33 36.32 22.72 15.29 24.00 62.62 41.95 25.72 17.02 86.32 55.94 32.96 21.08
Ycyclohexane at 30°C 35°C 40°C 45OC 1.75 1.71 1.66 1.63 2.47 2.34 2.81 2.66 3.37 4.12 3.68 4.51 5.49 4.85 7.26 6.40 8.17 6.98 11.67 9.94 18.11 14.45 30.19 23.97 78.05 57.79 40.13 29.93 201.82 139.32 88.96 61.98 324.53 216.32 132.45 89.19 269.55 161.61 106.99 418.54 240.61 153.97
In
thiourea at
wt%
50°C 25.31 12.65 8.44 6.33 5.06 3.62 2.81 2.30 2.11 2.02 1.87
YMCP
wt%
thiourea 1.00 2.00 3.00 4.00 5.00 7.00 9.00 11.00 12.00 12.50 13.50
45°C 22.75 11.38 7.58 5.69 4.55 3.25 2.53 2.07 1.90 1.82 1.69
~
c
50°C 74.62 99.52 132.74 177.04
l at~
45°C 221.57 318.86
h
s
~
~
50°C 55°C 121.93 70.03 168.25 92.94 232.17 123.34 320.37 163.68 217.22 288.28 382.57
60°C 40.19 51.30 65.47 83.57 106.66 136.14 173.76 250.55 319.79 408.16
Table 5. Equilibrium Constants for Thiourea-Methanol-Guest Systems temp
("0 40 35 40 45 50 55 60
cyclohexane
guest cyclohexane + MCP
MCP
0.005 36 0.007 82 0.011 19 0.015 68 0.021 56 0.029 15 0.038 77
0.012 25 0.016 91 0.022 91 0.030 52 0.040 02 0.051 71 0.065 89
0.028 53 0.037 15 0.047 66 0.060 30 0.075 30 0.092 89 0.113 31
the concentration of cyclohexane in guest. These results represent that the tendency for the stability of thiourea adduct is cyclohexane > cyclohexane-MCP mixtures > MCP, and is in accord with Redlich's results (Redlich et
122 Ind. Eng. Chem. Res., Vol. 33, No. 1, 1994 7
Guest
-
6
Kay
0 0 0
GUWI Cycbhgrana
Cysbhaxona-MCP MCP
2
0 ,
Methonol
Liquid
+ Solid thiourea
Thiourea
Figure 8. Schematic isothermal phaw diagram for thioureamethanol-guest systems.
Thiourea in liquid(wt.%)
Figure 6. Comparison of activity coefficients at 35 O C . -20
I
I
-2.5 -3.0 -3.5 Y
E
-4.0 -4.5
-5.0
_.I
20
30
40
50
60
70
TernperataeO
Figure 7. Equilibrium constants for the adducts.
al., 1950). This tendency suggests that different interaction energy between the guest molecules and the walls of the thiourea channel exists. The ionic surface of the thiourea can interact with the methyl group of the hydrocarbon (guest) chain causing dispersion, short-range repulsion, and polarization energies between the guest molecules and the thiourea channel. Comparing the separation factor for cyclohexane and MCP (Lee et al., 19921, it was seen that the tendency for inclusion is cyclohexane > MCP. This result indicates that the selectivity of thiourea is related to thiourea structure and guest configuration. This is like the behavior of molecular sieves in which the size and configuration of the guest molecules relative to the size of their cavities are particularly important. The case appears different for the adduction of guest on thiourea where the physicochemical properties play a dominant role in adduction selectivity. In most cases adduct was found to be able to be formed near room temperature. The process of thiourea adduction, therefore, can be operated near room temperature, which is adequate for high recovery and convenient handling.
Ternary Phase Diagram. Figure 8 is a distorted-scale schematic of the phase diagram measured for cyclohexanethiourea-methanol (system I), MCP and cyclohexane mixture-thiourea-methanol (system 111,and MCP-thiourea-methanol (system 111)within the temperature range investigated. On the true-scale diagram, line BF for systems I, 11,and I11 would be only 12.0-24.0% of line BC (depending on temperature) for all cases and line EF for systems I, 11, and I11 would be only about 0.34,0.71, and 1.21 % of line AB, respectively. Point D for systems I, 11, and I11 represent thiourea adduct, which contains 28.0,24.5, and 21.3 wt % guest, respectively. Point F was determined from the thiourea solubility studies discussed earlier. Boundary AE was obtained from the adduct solubility experiment. The metastable curve GA in this study was not measured but will be measured in the succeeding nucleation study. Adduct exists with liquid in the region AEDA and is easily separated from liquid by the phase separation. The existence of the region AEDA was supported by the observation that the refractive index of the liquid phase of an equilibrium mixture within that region did not depend upon the overall composition. Effect of Guest. In the region AEDA of Figure 8,where a homogeneous solution is supersaturated with respect to adduct, only adduct crystallizes. This region is only of concern in the kinetic studies of adduct crystals. In this section, therefore, ternary systems with average cyclohexane concentrations of 0, 10, 20, 30,40, 50,60,70, 80, 90, and 100 wt % in the guest composed of cyclohexane and MCP were investigated. The solubilities of the adduct crystal formed by the various concentrations of guest and dissolved thiourea at a fixed ratio of methanol to thiourea (15 wt % thiourea:85 wt 76 methanol), which is shown as line AK in Figure 8, are experimentally obtained in the temperature range of 40-60 "C and are shown in Figure 9. Adduct solubility increases with increase in the temperature and the concentration of MCP in guest. This indicates that the increase in the crystallization temperature weakens the interaction between the methyl group in the hydrocarbon and the electrostatic field on the thiourea crystals. The fact that thiourea adduction decreases as a result of increasing the temperature may also be due to fast diffusivity of the guest molecules out of the thiourea channels. The diffusivity increases according to the Arrhenius equation, and the inclusion of guest decreases.
Ind. Eng. Chem. Res., Vol. 33, No. 1, 1994 123 Table 6. Values of Contants in Eq 4
-1.2
cyclohexane guest (wt %)
a1
0 10 20 30 40 50
0.000109 0.000379 0.001 44 0.00351 0.00609 0.00886
cyclohexane gueet(wt %) a1 0.0239 60 0.01194 70 0.01473 0.0198 0.015 5 80 0.01754 90 0.01997 0.0126 0.0108 100 0.02081 0.00961
81
81
0.00864 0.00795 0.00738 0.00695 0.00675
Table 7. Heat of Adduction for Cyclohexane-Thiourea Adduct, MCP-Thiourea Adduct, a n d Cyclohexane-MCP Mixtures-Thiourea Adduct cyclohexane guest (wt %) -1.81
308
'
'
'
'
313
'
'
38
'
'
323
0 10 20 30 40 50
'
328
333
TemperaturcO
