588
Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 4 , 1979
Separation of Urea and Ammonium Chloride by Temperature Cycle Process S. N. Vyas' and R. Venkataraman Department of Chemical Engineering, Indian Institute of Technology, Bombay, Powai, Bombay-400 076, India
Phase equilibrium data for the binary system urea-ammonium chloride and the ternary system urea-ammonium chloride-water have been collected at temperatures of 17.5, 26.5, 41.5, 61.5, 80.2, and 95 OC. Based on these equilibrium data a temperature cycle process has been proposed and experimentally verified.
Besides their basic value as fertilizers, urea and ammonium chloride are extensively used as raw materials in many chemical industries. Their large scale utilization, for instance, in dry cells and urea-formaldehyde resins, respectively, is well known. A process wherein urea and ammonium chloride are simultaneously produced has been reported by Werner and Carpenter (1918). This involves a reaction between ammonia and phosgene as described by the stoichiometric equation 4NH3 + COClz 2NHdC1+ NHpCONHz Since this process is carried out at normal pressure and moderate temperatures, it has an edge over the conventional high-pressure process for the production of urea. However, probably due to high toxicity of phosgene, this process has not been commercialized so far. Furthermore, this process poses the problem of separation of urea and ammonium chloride. The present work was undertaken to develop a process for the separation of urea and ammonium chloride based on fractional crystallization. Phase equilibrium data for the binary systems urea-water (Seidell, 1941) and ammonium chloride-water (Edward, 1928) are available in the literature. The data on the binary system urea-ammonium chloride and the ternary system urea-ammonium chloride-water are not available. Hence investigations were also carried out to obtain phase equilibrium data on these systems. Experimental Section Recrystallized urea and ammonium chloride were used in all the experiments. The purity of urea was checked by determining its melting point, and that of ammonium chloride by estimating its chloride content. The melting point of urea was 132 "C, and the purity of ammonium chloride was 99.9%. P o l y t h e r m a l S t u d y of t h e Condensed B i n a r y System Urea-NH4C1. In the polythermal study, urea and ammonium chloride were mixed in known quantities and the mixture was heated slowly with constant stirring in an oil bath until a homogeneous melt was obtained. The melt was allowed to cool slowly with stirring, and the temperature at which solid phase appeared was recorded. The temperature-composition data are listed in Table I and plotted in Figure 1. Isothermal S t u d y of t h e T e r n a r y System UreaAmmonium Chloride-Water. The ternary mixtures were prepared by heating a mixture of urea, ammonium chloride, and water, keeping one or both of the solids as the case may be in excess, to a temperature higher than that at which determinations were to be carried out. These
Table I. System: Urea-Ammonium Chloride no.
mol ?& of urea
1 2 3 4 5 6
100.0 88.9 84.0 80.0 74.0 68.9
mol % NH,C1
te,mp, C
0.0
132.0 104.0 94.0 104.0 136.0 185.0b
11.1
16.0 20.0 26.0 31.1
Decomposition of ammonium chloride section.
+
0019-7882/79/1118-0588$01.00/0
were then kept in a thermostated bath and allowed to attain equilibrium at the desired temperature. The minimum time required to attain equilibrium was determined separately. Solution phase samples at higher temperatures were taken by a jacketed tube, through which thermostatic bath liquid was circulated. This was necessary to prevent crystallization during sampling. Solid phases were identified by Schrienemaker's wet residue method (Ricci, 1951). Phase equilibrium data for the ternary system ureaammonium chloride-water obtained at temperatures 0, 17.5, 26.5, 41.5, 61.5,80.2, and 95 "C are tabulated in Table I1 and graphically presented in Figure 2. Discussion B i n a r y System Urea-Ammonium Chloride. In Figure 1,AC is the melting point curve of urea and CB is the solubility curve of ammonium chloride in urea. The latter could not be extended beyond 185 "C owing to appreciable decomposition of ammonium chloride. Both curves appear to be smooth and do not show any break, indicating the absence of any double compound. The analysis of the solid phase also did not show the presence of any double compound. A simple binary eutectic comprised of 0.84 mole fraction of urea is formed in the melt at 94 "C. Ternary System Urea-Ammonium Chloride-Water. This condensed ternary system has been studied at seven different temperatures: 0, 17.5, 26.5, 41.5, 61.5, 80.2, and 95.5 "C. In Figure 2 invariant points C represent the composition of solutions saturated with respect to both urea and ammonium chloride at different temperatures. The left-hand curves (AC) represent the compositions of solutions saturated with respect to urea, and the curves (BC) on the right of points C represent the compositions of solutions saturated with respect to ammonium chloride at different temperatures. From the solubility data a t different temperatures for the ternary system ureaNH4C1-water (Table I1 and Figures 2 and 3) it is clear that the solubility of urea increases by the addition of increasing
0 1979 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 4, 1979
200 I
I
Table 11. System: Urea-Ammonium Chloride-Water. Liquid Phase Composition no.
/
AMMONIUM
i
UREA
*Ot
601 0
I
10
+AMMONIUM (51
CHLORIOE~s)
-
1
I
I
1
20
30
LO
50
60
M O L E 'I. N H L C I
Figure 1. System: urea-ammonium chloride.
amounts of ammonium chloride to aqueous saturated solutions of urea. Similarly, the solubility of ammonium chloride also increases by adding various amounts of urea to the aqueous solution of NH4C1. As a general rule the solubility of a solute is supressed by the addition of a third component, but in this the increase in solubility by the addition of a third component a t a given temperature is attributed to the interaction of the two solutes in the liquid phase. Here too the nature of all the isotherms indicates the absence of formation of any double compound. In all cases, the nature of the solid phase in equilibrium with the solution phase was established by Schreinemaker's wet residue method. Cyclic Process f o r Separation of Urea a n d Ammonium Chloride from T h e i r Mixture. For the purpose of separation of individual components from a mixture, solubility isotherms were plotted on rectangular coordinates (Figure 3). ABC and EFG are two isotherms at 26.5 and 95.0 "C, respectively. In Figure 3, points A and E represent the solubility of ammonium chloride in pure water a t 26.5 and 95.0 "C, respectively. Similarly, points C and G represent the solubility of urea in water at these temperatures. Curves AB and E F in Figure 3 represent the composition of solutions saturated with respect to NH4C1 in the presence of urea a t 26.5 and 95.0 "C, respectively. The curves are similar to curves AC in Figure 2. Similarly, curves BC and FG represent the composition of solutions saturated with respect to urea in the presence of NH4C1 a t these temperatures; the corresponding curves in Figure 2 are labeled as BC. The invariant points B and F represent the solution composition saturated with respect to both urea and ammonium chloride. At 26.5 "C point F lies in a region saturated with respect to both urea and ammonium chloride. Hence on cooling to 26.5 "C the solution having composition F will precipitate urea and ammonium chloride. Though there is no double compound formation in the temperature range studied, the nature of all the isotherms indicates that an apparent strong interaction between the two solutes in the liquid phase exists. Now for the separation of the two solutes, if we start with point B a t 95 "C, and carry out isothermal evapo-
589
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42
temp, "C 0.0 0.0 0.0 0.0 0.0 0.0
17.5 17.5 17.5 17.5 17.5 17.5 26.5 26.5 26.5 26.5 26.5 26.5 41.5 41.5 41.5 41.5 41.5 41.5 61.5 61.5 61.5 61.5 61.5 61.5 80.2 80.2 80.2 80.2 80.2 80.2 95.0 95.0 95.0 95.0 95.0 95.0
%
%
urea
NH,Cl
solid phase
0.0 17.8 17.2 42.8 42.6 39.5 0.0 16.8 34.1 49.2 49.1 49.5 0.0 16.3 43.3 53.9 54.0 55.0 0.0 24.4 34.3 59.9 61.1 63.0 0.0 24.9 63.3 68.6 69.6 71.8 0.0 27.5 43.3 76.6 78.4 80 0.0 39.3 46.8 79.1 83.9 85.5
22.8 20.9 20.8 15.9 14.6 0.0 26.8 22.9 19.4 14.9 13.9 0.0 29.0 25.6 18.2 14.8 7.9 0.0 31.6 25.1 22.4 14.8 3.7 0.0 35.7 29.9 16.8 14.8 11.3 0.0 39.6 31.1 26.8 13.8 6.5 0.0 42.0 30.6 28.3 14.2 6.5 0.0
NH,C1 NH,Cl NH,CI NH,Cl, urea urea urea NH,C1 NH,Cl NH,CI NH,Cl - urea urea urea NH,C1 NH,Cl NH,Cl NH,CI + urea urea urea NH,Cl NH,CI NH,C1 NH,CI + urea urea urea NH,CI NH,CI NH,CI NH,C1 urea urea urea NH,CI NH,C1 NH,CI NH,CI urea urea urea NH,CI NH,CI NH,CI NH,CI + urea urea urea
-
ration, we would observe that pure ammonium chloride would start crystallizing from point PI onward. Isothermal evaporation is discontinued a t point P. Addition of a mixture of urea and ammonium chloride (1:2 molar) in sufficient quantity to a definite weight of the system P will change the composition of the system. The composition of the resultant system lies on the line FR, parallel to the Y axis. At 95 "C, any point on this line represents a system, consisting of a solution saturated with respect to both urea and ammonium chloride and excess ammonium chloride in the solid phase. The quantity of the solid urea-ammonium chloride (1:2 molar) added to P should be so adjusted that all the urea added dissolves and ammonium chloride added remains undissolved. Any solid ammonium chloride that has separated before addition of the mixture will go into solution. The remaining undissolved ammonium chloride is filtered. The residual solution F is diluted with water and cooled to 26.5 "C. The amount of water added should be such that the resultant system is given by point Q. In this process the same amount of urea (which was present in the solid mixture added), would precipitate out and the residual solution will be given by point B. This resultant solution is again evaporated isothermally at 95 "C to give a solution having composition P, and the cycle is repeated.
590
Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 4, 1979
A UREA
B
1 0 0 'I.
AMMONIUM CHLORIDE
loo -1.
Figure 2. System: urea-ammonium chloride-water. 640
R
560-
I
a c w
s LL
I
= I
450-
I
0
I I
W 0
s
I
400-
I
\ w
P a
2
I
I
I
320-
I
I I
I I I
I
V I
0'
I
'
'
. A I
I
'
,
I
G
OF
G
,
I
UREA
100
G.
OF
WATER
F
Figure 3. System: urea-ammonium chloride-water.
Thus starting with a sufficient quantity of the system of composition P, i.e., a system consisting of a solution saturated with respect to ammonium chloride but not with respect to urea, the cyclic process consisting of following stages can be operated: (1)addition of solid mixture of
urea and ammonium chloride (1:2 molar) a t 95 "C; (2) separation of undissolved ammonium chloride at 95 "C; (3) dilution with water and cooling to 26.5 "C;(4) separation of urea crystallized at 26.5 O C ; (5) evaporation of solution B to bring back the system to composition P at
Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 4, 1979 591
95 "C. Similar cyclic processes can be worked out for 61.5-26.5 "C and 61.5-0 "C temperature pairs. Based upon the calculations of the quantities required for the cyclic processes for the separation of urea and ammonium chloride from (1:2 molar) solid mixture at temperature ranges 95-26.5 "C, 61.5-26.5 "C, and 61.5-0 "C, experiments were carried out on a laboratory scale. In order to determine the feasibility of the three cycles, two sets of experiments were carried out. In the first set, for the separation of urea at lower temperature, the solution was allowed to stand for 2 h, whereas in the second set this solution was allowed to stand for 24 h. In all these experiments it was observed that the quantity of insoluble residue (remaining undissolved a t higher temperature) obtained by filtration was always higher than the calculated yield. This was due to the difficulty experienced in separating insoluble residue from the mother liquor a t higher temperature; Le., larger amounts of mother liquor always adhered with ammonium chloride separated. This difficulty was less at lower temperatures. Secondly, due to very high viscosity, filtration also posed a problem. The products of 95% purity could be obtained by washing the residues with saturated solutions of the respective component. On account of some mother liquor being lost along with undissolved ammonium chloride, the yield of the mother liquor was always less than the calculated value. The quantity of water added was accordingly adjusted, and on this basis the yield of urea was almost theoretical. Thus the separation of urea and ammonium chloride by a cyclic process was found to
be feasible with all three temperature pairs. Choice of a Temperature Cycle. From the foregoing discussion, it is clear that the separation of urea and ammonium chloride from a mixture of the two by a multiple temperature cycle process is feasible one. In the present work only three sets of temperature cycles have been worked out, but in principle, all the temperature combinations are possible. The selection of an actual working temperature cycle would depend on the following factors. The upper temperature of a given cycle should preferably be around 60 "C or so, as the solutions saturated with both or individual compounds at higher temperatures are highly viscous. The filtration and handling of such solutions would create many problems. This also results in contaminated products. The handling of saturated solutions of urea at higher temperatures mzy also enhance the possibility of biurate formation. It may be desirable to fix the lower temperature of the cycle near the ambient conditions. Considering these factors, a 60-26.5 "C cycle may be a better choice. Literature Cited Edward, W. W., "International Crnical Tables", Vol. 4, p 218, McGraw-Hill, New York, N.Y., 1928. Ricci, R. E., "The Phase Rule and Hetrogeneous Equilibrium", p 407, Van Nostrand, New York, N.Y., 1951. Seidell, A,, "Solubilitites of Inorganic and Organic Compounds", Vol. I , Van Nostrand, New York, N.Y., 1941. Werner, E. A,, Carpenter, G. K., J . Chem. Soc., 13, 694 (1918).
Received for review March 13, 1978 Accepted March 24, 1979
Pilot-Scale Synthesis of Macroporous Styrene-Divinylbenzene Copolymers James C. Watters" and Theodore G. Smith Department of Chemical Engineering, University of Maryland, College Park, Maryland 20742
Styrene-divinylbenzene copolymers were synthesized using a 50-gal stainless steel pilot-plant scale reactor. Use
of the polymers as the stationary phase in a gel permeation chromatography apparatus enabled the separation of alkanes and low to intermediate (less than 5 X lo4) molecular weight polystyrenes. Particle size distributions of the raw polymer particles were in the range of 5 to 150 pm and bimodal about the 53-pm (270 mesh) screen fraction. I t is recommended that a surfactant (PVA) concentration of 3 wt % of the aqueous phase and a prereaction mixing time of about 1.5 h be used.
Introduction Styrene-divinylbenzene cross-linked copolymers have been used as packing in liquid or gel permeation chromatography columns since the development of this separation technique by Moore (1964). Molecules are separated on the basis of their physical size relative to the pore structure of the gel. Molecules larger than the maximum pore size pass through the column via the particle interstices. Those smaller than the maximum pore size enter pores and are thus separated by requiring a longer elution time. *Address correspondenceto this author at the Department of Chemical Engineering, University of Louisville, Louisville, Ky. 40208. 0019-7882/79/1118-0591$01.00/0
Macroporosity in the polymer network is induced by performing the polymerization in suspension in the presence of an organic diluent. This diluent causes initial swelling of the gel and forms pores in the polymer matrix as the monomers polymerize around it. Pore size is controlled by varying the amount and nature of the diluent. Proper adjustment of particle and pore size is crucial to the optimization of the properties of the gel permeation chromatography (GPC) material. A series of gels of differing pore sizes is important to fully classify an unknown sample into its several constituents. A commercial GPC unit typically runs a cascade of five columns, each containing a material of different pore size. Particle size is important from the standpoints of residence time within the column, physical length of the column, and pressure 1979 American Chemical Society
