96
I n d . Eng. Chem. Res. 1990,29, 96-100
Greek Symbols ml,cyz = frequency factors for reactions A
-
B and B
-
C,
min." O,, 0, = ratios of reactor wall heat capacity/reactor fluid heat capacity and jacket fluid heat capacity/reactor fluid heat
-
-
capacity A,, A, = heats of reaction for reactions A B and B C, Btu mol-' p j , pm, p = density of jacket, reactor walls, and reactor, respectively, mol/ft3 T = start-up time, i.e., time taken to reach Top,min rh = minimum feasible time to operating temperature, min rC = minimum safe time to operating temperature, min 4 = jacket space velocity, min-'
Literature Cited Astrom, K. J.; Wittenmark, B. Computer-Controlled Systems; Prentice-Hall: Englewood Cliffs, NJ, 1984; pp 180-181. Bilous, 0.; Amundson, N. R. Optimum Temperature Gradients in Tubular Reactors. Chem. Eng. Sci. 1956,5, 81-92, 115-126.
Hugo, P.; Steinbach, J. A comparison of the safe operation of a SBR and a CSTR. Chem. Eng. Sci. 1986,41, 1081-1087. LiptBk, B. G. Controlling and optimizing chemical reactors. Chem. Eng. 1986 (May 26), 69-81. Luyben, W. L. Process Modeling, Simulation and Control for Chemical Engineers; McGraw-Hill: New York, 1973; pp 16C-167. Luyben, W.L. Batch Reactor Control. Instrum. Technol. 1975, Aug, 25-34. Marroquin, G.; Luyben, W. L. Practical control studies of batch reactors using realistic mathematical models. Chem. Eng. Sci. 1973, 28, 993-1003.
Regev, 0.;Lewin, D. R.; Lavie, R. Exothermic Batch Chemical Reactor Automation via Expert System. Proceedings of the 20th Symposium on Computer Applications in the Chemical Industry, Erlangen, 1989. Rotstein, G.; Regev, 0.; Lewin, D. R.; Lavie, R. On Control of an Exothermic Batch Chemical Reactor. Proceedings of the IEEE International Conference on Control and Applications, Jerusalem, 1989.
Rewived for review July 29, 1988 Rwised manuscript received August 7 , 1989 Accepted September 26, 1989
SEPARATIONS Extraction of Ammonia from a Dilute Aqueous Solution by Emulsion Liquid Membranes. 1. Experimental Studies in a Batch Systems Chau J. Lee* and Chih C. Chanf Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan, ROC
The extraction of an alkaline substance from a dilute aqueous solution is of interest in practical water purification processes as well as in the system analysis of the facilitated mass-transport theory. Emulsion liquid membrane (ELM) technology is being utilized to study the ammonia removal process. The rates of ammonia extraction under various emulsification and operation conditions were experimentally studied in a batch system. For industrial application of ELM technology in tertiary treatment of water containing ammonia, a scheme of the continuous operation system is visualized and recommended for further process development. A mass-transfer model for analyzing the experimental data in the batch system, which incorporated the leakage effect of the internal phase due to membrane rupture, was also considered. A detailed discussion of the mass-transfer modeling and analysis of ammonia extraction by the ELM method is given in part 2. 1. Introduction The extraction of ammonia from a dilute aqueous solution has been a practical problem in industrial or municipal wastewater treatment processes. Usually, the genesis of ammonia in wastewater is due to enzymatic breakdown of urea, proteins, and other nitrogen-containing substances. For the removal of ammonia from clean water, there are several industrial processes extensively studied in the literature (Patoczka and Wilson, 19841, e.g., (1) biological nitrification and denitrification, (2) breakpoint chlorination, (3) air stripping, (4) selective ion exchange, and (5) physical chemical treatment. However, the newly developed "liquid membrane separation technology" has found great potential for application
* To whom correspondence should be addressed
Presented at PACHEC '88, Oct 19-22, Acapulco, Mexico. Present address: Department of Chemical Engineering, Feng-Chia University, Taichung, Taiwan, ROC. t
0888-5885/90/2629-0096$02.50/0
in the tertiary treatment of wastewater to remove ammonia (Chan, 1987; Frankenfeld and Li, 1977; Halwachs and Schugerl, 1978; Kopp, 1982; Li and Schrier, 1972; Maugh, 1976). The liquid membranes, in general, are formed by first making an emulsion of two immiscible phases and then dispersing the emulsion in a third phase (continuous phase). The liquid membrane phase refers to the phase in between the encapsulated phase in the emulsion and the continuous phase. Usually, the internal encapsulated phase and the external continuous phase are completely miscible, but they are not miscible with the membrane phase. In general, the emulsion consists of the encapsulated droplets, about 110-pm diameter, stabilized by surfactant added to the liquid membrane phase. The emulsion is dispersed in the continuousphase by agitation to yield globules of the order of 0.1-2.0-mm diameter. The solute, NH,, is selectively transported from the external continuous phase to the 0 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 1, 1990 97 Direct I r t e r n a l Sol"
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Figure 1. W/O emulsion droplet and extraction mechanism.
@-El
Figure 3. Procedures for preparation of emulsions. 100
80
ammonium 0
B D E F G
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ORION 9 0 1 IONALYZER ORION 811 pH Meter Disk-Turbine Impeller Sampling Device Membrane F i l t e r Rubber Packing lvater J a c k e t P e r i s t a l t i c Pump
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LI
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DC S t i r r e r Baffle ORION 9 5 1 P r i n t e r Voltage Stabilizer Thermometer Vessel Water Bath
Figure 2. Experimental setup for batch system operation.
internal encapsulated droplets containing acid, as shown in Figure 1. At the end of extraction operation, the emulsions are separated from the continuous phase by settlement, and the encapsulated phase may be treated or recovered by breaking the emulsions. In the present paper, emulsion liquid membrane (ELM) technology is being utilized to study the ammonia removal process. For purposes of process development, both an experimental study and theoretical model analysis were conducted. Part 1of this paper presents the results of the experimental studies in a batch system. The theoretical analysis and mass-transfer model will be presented in part 2.
2. Experimental Section 2.1. Experimental Setup. Fig. 2 presents the experimental setup for batch-type operations. V is a cylindrical vessel made of Plexiglas (acrylic plastics) with i.d. = 13.8 cm and height = 18 cm, which is used to contain the mixture of external dilute ammonia solution and the emulsified acidic extractants. The vessel is jacketed (for temperature control) and equipped with a stirrer and a six-bladed disk-turbine impeller. The stainless steel impeller (D) is situated 5 cm above the bottom of the vessel. There are four straight baffles of 1.3-cm width (P) symmetrically placed in the vessel to assure good mixing with minimum breakage of emulsions. For continuous sampling of the time-varying concentration of the external solution, the bottom of vessel is flanged with a membrane filter, (F) which is a sartorius cellulose-nitrate membrane of pore size = 0.45 pm and thickness = 130 pm. The filtered clear external solution is continuously recycled at 13 mL/min
Figure 4. Effect of solution p H on molecular ammonia and ammonium ion shift.
with a peristaltic pump (K), whose concentration is analyzed and recorded with an Orion 901 ionalyzer (A and R). Also the sampling port (E) is used to detect the degree of leakage of internal solution from the water/oil (W/O) emulsion droplets. The lithium ion, Li+, is used as a tracer for leakage detection. Other accessories of the experimental setup are self-explained in Figure 2. 2.2. Experimental Procedures. The water/oil extractant emulsions are prepared following the formulation and procedures as outlined in Figure 3. A 20 wt % H$04 solution is selected as the internal solution for extraction of ammonia from the external aqueous solutions. The W/O emulsionsare prepared by formulating with paraffin oils, surfactants, and the 20 wt 70H2S04solution in various fractional compositions and agitating in a high-speed emulsifier (IKA-Ultra-Turrax disperser, Model T 10/ 18, Janke & Kunkel GmbH & Co., W. Germany). The paraffin oils and surfactants selected for use in this study are as follows: paraffin oils, (1)PO7174 (MW = 355, viscosity = 25.98 cP), (2) PO7160 (MW = 410, viscosity = 92.8 cP); surfactants, (1) ECA4360 (nonionic polyamine, MW = 3400), (2) SPAN 80 (Sorbitan monooleate, MW = 428). The prepared W/O emulsions (after relaxation and stabilization) are poured into the vessel to mix with the external dilute NH, solution. The volume of extractant W/O emulsion ( Vwl0) is generally kept at 200 mL and that of external solution (V,) at 1800 mL. The initial NH3 concentration of the external solution is fixed at lo00 ppm in terms of the total N content or, equivalently, to 7.143 X mol of NH3/L. Under acidic conditions, the ammonia in the external solution exists in the form of the ammonium ion (NH4+);however, it exists as the stable molecular ammonia form (NH,) when the solution becomes basic (see Figure 4). Thus, the pH of the external solution must be adjusted t@pH 1 1 2 during the experiment in order to facilitate the extraction of ammonia from the external solution, since NH4+ is not soluble in the oil membrane and only NH,, which is soluble, can be trans-
98 Ind. Eng. Chem. Res., Vol. 29, No. 1, 1990 lo3 5
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t!
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Figure 7. Effect on amount of surfactant on the rate of ammonia extraction.
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Figure 5. Effect of stirring rpm on the rate of ammonia extraction.
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....
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2 0 ~ r % H ~ S O ~ / P 0 7 1+7ECA4360 4 q 1 = 0 . 5 , o e = o . l , N = 600 rpm 0 s = 1 wt% 0 S = 6 wt% A S = 4 wt% 0 s = 8 wr4 I
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Figure 8. Effect of the internal solution content, Qi, on the rate of ammonia extraction.
enhanced as the rate of stirring was increased. On the other hand, the higher stirring rate, the more the emulsion droplets became unstable and the more leakage of the internal solution occurred. It thus adversely affected the ammonia removal. Figure 5 reveals that, as the stirring exceeds 400 rpm, E, fell to a minimum and then bounced back and increased. This indicates that there are two competing processes involved in the ammonia transport, i.e., the diffusion of NH3 through the emulsion membranes into the internal solution and the leakage of internal solution due to the breaking of some emulsions. The latter
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Ind. Eng. Chem. Res., Vol. 29, No. 1, 1990 99
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A/P07160 +ECA4360
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Figure 9. Effect of acidity of internal solution on the ammonia extraction rate.
process would inhibit or retard the extraction of NH3 from the external solution. Figure 6 shows the effect of 4, (volume fraction of the W/O emulsion in the W/O/W emulsion)on the extraction of NH3. A large value of 4, means that there exist more extractant W/O emulsions in the mixture and thus more mass-transfer surfaces for NH3 removal. However, large 4, values also lead to more leakage of the acidic internal solution into the external solution (Chan and Lee, 1987; Chan, 1987); thus, it results in retardation of ammonia extraction. The volume fractions, 4i and $, density, and viscosity of the emulsion mixtures are defined by the following equations:
4i = volume fraction of the internal phase in the W/O emulsion =
W/O/W
+ v, emulsion = vi + v, + v, VW/O + v, P W ~ O / W=
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Ih3IO= 1000
Figure 10. Effect of initial ammonia concentration on extraction efficiency.
Y
vi (1) vi + v,
N = 6 0 0 rpm
[NH310=100 ppm [NH3lO= 500 ppm
(2)
(3)
2t 0
where Vi, V,, and V, are the volume of the internal solution, the membrane, and the external solution, respectively; and pe and pe are density and viscosity of the external solution, respectively. The effect of the amount of surfactant used in the formulation of the W/O emulsion (in extractant preparation) on the rate of ammonia extraction is shown in Figure 7. Adding more surfactant would lower the surface tension and result in a smaller droplet size of the W/O emulsion, which gives a larger mass-transfer area and thus more
0 0
a A 10
P07160+ECA4360 PO7160 + SPANRO . ... - PO7160 + ECA4360(3.5%)+ SPAN80(0.5%) P07174+ECA4360 PO7174 + SPAN80
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i 90
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Figure 11. Effect of different formulations of oil and surfactant on the rate of ammonia extraction.
efficient ammonia extraction, though a large amount of surfactants also increases the viscosity and lowers the diffusivity of the oil membrane formed. Thus, the rate of ammonia extraction may be decreased. Figure 7 indicates
100 Ind. Eng. Chem. Res., Vol. 29, No. 1, 1990
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.
f
Figure 12. Schematic diagram of continuous operation process for ammonia extraction using ELM technology
that the surfactant content in excess of 6% may have an adverse effect on the overall rate of ammonia extraction. Figure 8 examines the effect of the internal solution content, 41,on the rate of ammonia extraction. A large value of means more extractant is obtained and there is also less diffusion resistance due to the thinner oil membrane, thus, a higher rate of ammonia extraction. The best results of ammonia extraction were obtained with 4, = 0.4 as is shown in the same figure. The effect of the acidity of internal solution on the ammonia extraction rate is shown in Figure 9. It is obvious that higher acidity enhances the ammonia extraction rate. There is also a comparison between the capability of ammonia extraction with strong acid (H2S04)to that with weak acid (H,PO,). The results reveal that H2S04and H3P04 have practically the same ammonia extraction characteristics provided that their solutions are prepared to the same acidity, namely, 20 wt % H2S04has the same [H+] = 4.65 mol/L acid concentration as that of 14.1 wt % H3P04. Figure 10 compares the effects of initial NH, concentration of the external solution on extraction efficiencies. It is concluded that the emulsion liquid membrane method is more suitable for removal of ammonia from dilute aqueous solution. Figure 11 examines qualitatively the different formulations of oil and surfactant on the rate of ammonia extraction. Generally, PO7160 has a higher viscosity than P07174; SPAN 80 is a better and more active surfactant than ECA4360. The optimal combination of paraffin oil and surfactant is to look for a formulation that will give the best synergistic effect and meet the criteria of proper droplet sizes and stability of extractant emulsions throughout the process operation. Further research must be pursued with respect to this. 3. Discussion A method based on emulsion liquid membrane technology is being proposed for extraction of ammonia from a dilute aqueous solution. From a series of experiments with different formulations for acidic extractant emulsions, and under various operational conditions in the batch system, we are able to understand somewhat better the basic mechanism of mass transfer in such system. The experimental results (Figures 5-11) offer some insight into,
a t least, the following two aspects: (i) There is a mechanistic basis for a mass-transfer model for further process analysis in batch system operations. The model must incorporate the reverse transport in the continuous (external) phase due to membrane rupture (Figures 5-8). If membrane rupture occurs, the internal phase leaks into the continuous phase and the rate of extraction is affected. Stability of the emulsion globules may be one of the most serious problems in the application of the emulsion liquid membrane method to industrial separations. The effect of leakage must be incorporated in the treatment of experimental data and in an approach to the theoretical model analysis. A theoretical masstransfer model for analysis of the extraction of ammonia from dilute aqueous solution by the emulsion liquid membrane method is presented in the following paper (part 2) in this issue. (ii) For batch system operation, the following process and operational conditions are recommended for preliminary scale-up and process design purposes. Extractant emulsions (dispersed phase): materials, PO7160 + SPAN 80; surfactant, at S = 4%; acid, A = 20 wt % H2S04; volume fractions, 4i = 0.4-0.5, 4e = 0.05-0.1. Water to be treated (continuous phase) in tertiary water treatment: initial ammonia content, [NH,], = 1000 ppm; stirring speed at 400 rpm; operation time for one batch = 60-90 min. For actual industrial applications of this ELM method in tertiary treatment of water containing ammonia (or other alkaline substances), the process must be further developed to enable continuous operations. A schematic diagram of a continuous system is visualized and shown in Figure 12. Further process and engineering development of such continuous operation systems using ELM technology must be encouraged. Acknowledgment The authors express their appreciation for financial support in part by Grant NSC75-0402-E007-04 from the National Science Council of Taiwan, R.O.C. Registry No. ECA 4360, 74433-64-0; SPAN 80, 1338-43-8; NHB, 7664-41-7; HZS04, 7664-93-9.
Literature Cited Chan, C. C. Emulsion liquid membrane separation technology and its application in ammonia removal from dilute aqueous solution. Ph.D. Thesis, National Tsing Hua University, Taiwan, R.O.C., 1987. Chan, C. C.; Lee, C. J. A mass transfer model for the extraction of weak acids/bases in emulsion liquid-membrane systems. Chem. Eng. Sci. 1987, 42(1), 83-95. Frankenfeld, J. W.; Li, N. N. Wastewater treatment by liquid ion exchanger in liquid membrane systems. Recent Developments in Separation Science; CRC Press: Boca Raton, FL, 1977; Vol. 3, p 285. Halwachs, W.; Schugerl, K. The liquid-membrane technique-a promising extraction process. Chem. Ing. Tech. 1978, 50, 767. Kopp, A. Liquid membrane technology-a survey of phenomena, mechanisms, and models. Znt. Chem. Eng. 1982, 22(1), 44-60. Li, N. N.; Shrier, A. L. Liquid membrane water treating. Recent DeueloDment in SeDaration Science: CRC Press: Boca Raton. 1972; Vol. 1, p 163.' Maueh. T. H. Liauid Membranes-new techniaues for seoaration. purification. science 1976, 193, 134-150. Patoczka, J.; Wilson, D. J. Kinetics of the desorption of ammonia from water by diffused aeration. Sep. Sci. Technol. 1984, 19(1), 77.
Received for review February 14, 1989 Revised manuscript received August 29, 1989 Accepted September 8, 1989
