- Email: [email protected]
Investigating the differences in acid separation behaviour on weak base ion exchange resins
Chemical Engineering Science 55 (2000) 6197}6208
Investigating the di!erences in acid separation behaviour on weak base ion exchange resins Vinay M. Bhandari *, Toshikuni Yonemoto , Vinay A. Juvekar Department of Chemical Engineering, Tohoku University, Sendai 980-8579, Japan Department of Chemical Engineering, Indian Institute of Technology Bombay, Bombay 400 076, India Received 10 March 2000; received in revised form 30 June 2000; accepted 4 July 2000
Abstract Di!erences in acid separation behaviour on weak base ion exchange resins have been investigated in this work. Experimental data has been obtained on sorption equilibrium using various types of weak base resins and both inorganic and organic monobasic acids. A detailed analysis has been presented using the experimental data of present work and also that reported in the literature. Apart from acid concentration and type of acid, other factors such as resin type and basicity are also discussed. It was found that, resins with high basicity are most suitable for the acid removal applications. For resins with medium and weak basicity, large variation in the sorption capacity with concentration was observed. A new theoretical approach incorporating e!ect of resin basicity has been proposed and examined qualitatively and quantitatively using the sorption data. The results substantiate reversibility of sorption through lower values of protonation equilibrium constants and also indicate incomplete resin salt dissociation in some cases. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Ion exchange; Separation; Sorption equilibrium; Acid removal; E%uent treatment
1. Introduction Removal of acids from aqueous streams containing low concentrations of acids: organic/inorganic or both, is an important problem in chemical process industries. Ion exchange resins in general, and weak base resins in particular, are most commonly employed for this purpose. The weak base resins have large capacity for acid sorption and are easy to regenerate as compared to strong base resins. The two most important and widely practiced applications of acid removal are boiler feed water treatment and wastewater treatment. In the former application, acids are generated through the exchange of cations of salts with H> ions from resin. This acid is then removed by passing the aqueous stream through a bed of weak base anion exchange resin, which sorbs acid as a complete molecule. Wastewater streams containing low concentrations of acids are inevitably encountered in acid manufacturing plants, industries where acids are * Correspondence address. Chemical Engineering Division, National Chemical Laboratory, Pune 411 008, India. Tel.: #91-20-5893163; fax: #91-20-5893260. E-mail address: [email protected] (V. M. Bhandari).
used as raw material or as catalyst, fermentation processes, metal plating industry, etc. Wastewater usually contains single or mixture of acids from 0.5 to 4%, depending on the source of generation (Katzen, Aries & Othmer, 1945; Othmer, 1958; Matsarenko, Lebedinskaya, Dudkova & Gushchina, 1969; Helsel, 1977; Ricker, Pittman & King, 1980; Wadekar and Sharma, 1981; Kawabata, Yoshida Jun-ichi & Tanigawa, 1981; Parulekar, Sharma, Joshi & Shah, 1982). Since, large volumes are to be treated that contain low concentrations of acids, ion exchange is most convenient method of treatment in many cases. The primary goal in such cases is largely removal of acids from the solution and not separation. However, acid separation is important in acid manufacturing units where another acid is usually obtained as a byproduct, e.g. formic acid in the manufacture of acetic acid. Thus, the selection of ion exchange resin for any particular application, whether removal or separation, is an important consideration. A number of experimental and theoretical studies have been reported on the sorption of various acids on weak base resins (Kunin, 1958; Hel!erich, 1962; Adams, Jones & Miller, 1969; Holl and Sontheimer, 1977; Hubner and Kadlec, 1978; Rao and Gupta, 1982a,b; Hel!erich and
0009-2509/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 9 - 2 5 0 9 ( 0 0 ) 0 0 2 1 2 - 8
6198
V. M. Bhandari et al. / Chemical Engineering Science 55 (2000) 6197}6208
Hwang, 1985; Bhandari, 1998; Garcia and King, 1989; Bhandari, Juvekar & Patwardhan, 1992a,b, 1993, 1997; Bhandari, Sawarkar & Juvekar, 1996, Yoshida, Kishimato & Kataoka, 1994; Cloete and Marais, 1996; Takatsuji and Yoshida, 1994, 1997, 1998a,b; Juang and Chou, 1996; Husson and King, 1999a,b). In many instances, acid sorption was considered irreversible and the sorption equilibrium was interpreted in terms of simple Langmuir- or Freundlich-type isotherm. The application of Langmuir- or Freundlich-type isotherm was, however, not found satisfactory in some cases (Husson & King, 1999a) and anomalous nature of sorption equilibria was also indicated with polybasic acids (Bhandari et al., 1997). The experimental and theoretical information on acid sorption is largely incomplete due to the following: 1. Majority of the studies are reported on single-component acid and not on acid mixtures. 2. Selection of resin for any acid removal/separation application has not been discussed. 3. In terms of detailed investigations on sorption of acids and di!erences in sorption behaviour, previous studies have largely focused on the sorption dynamics of acid on weak base resins and less attention has been given to the experimental and theoretical aspects of sorption equilibria, especially with reference to the di!erences in the sorption behaviour. 4. Practically, very little information is available on selectivity in acid sorption and theoretical explanation for the same. The present work is an attempt to gain insight into the acid separation behaviour by experimentally and theoretically analyzing sorption data on various weak base resins. Both, single and two component acid systems have been discussed. A new, less rigorous theoretical treatment is proposed to explain selectivity behaviour in acid sorption. Considering the enormous di$culties in developing the entirely satisfactory approach to equilibria, the proposed simpli"ed treatment of equilibria could be useful in evolving a coherent and practical approach for data assimilation and for ion exchange design and practice.
2. Acid sorption on weak base resins Sorption of acid on weak base resin essentially follows a two-step mechanism. In the "rst step, the free ionogenic groups of the resin (denoted by R) are protonated by the H> ions of the acid (for convenience, the charge sign is omitted in writing the equations). Step 1. Protonation of the free ionogenic sites: R #H & RH.
(1)
Step 2. Anion association (Resin-salt Dissociation): RH#A & RHA. Q
(2)
Protonation results into the formation of positively charged surface due to the presence of protonated species (RH>) on the surface of pore walls of resin. The second step, anion association, is a result of electrostatic interaction between the positively charged RH> groups and negatively charged anions of the acid. Speci"c adsorption of the anions is believed to occur in a region close to the pore walls (Bhandari et al., 1992). The conventional approach to acid sorption assumes step 1 as a irreversible process. The equilibrium constant of this reaction for most commercial weak base resins is considered to be very high of the order of 10}10 (kmol/m)\ (Hel!erich & Hwang, 1985). However, no established values/procedures for the protonation equilibrium constant are available in literature. Further, complete dissociation of resin salt and uniform distribution of anions across the cross-section of pore is usually assumed. The later assumption practically leaves protonation step as the only controlling mechanism in sorption equilibrium. According to the reversible sorption theory based on electrical double layer, (Bhandari et al., 1992), the sorption is reversible, in general, and the distribution of anions across the cross-section of pore is according to the Stern model. The exclusion of coion, equilibrium characteristics and selectivity, etc. can be attributed to the complex nature of the double layer adjacent to the pore surface, which can be compact (strong anion sorption), di!used (large anion distribution) or simply non-existent (negligible sorption). For mixture of acids, the selectivity for a particular ion would depend on the equilibrium distribution of two counterions in the double layer. The reversibility in the protonation step is attributed to the partial exclusion of coion from the double layer. Here also, no separate estimation of the protonation equilibrium constant was made, although the authors indicated that the values could be substantially lower. Fig. 1 shows the schematic plots of sorption equilibria on weak base resins. Case A is a typical rectangular isotherm, which depicts case of irreversible sorption. Cases B and C represent the medium and strong reversibility in sorption, respectively. The assumption of irreversibility is justi"ed only for resins of high basicity and/or at very high acid concentrations. The extent of reversibility is entirely governed by the equilibrium constant, which depends on the basicity of the resin and the acid being sorbed/desorbed and should not, in principle, depend on solution concentration. Since, experimental data in many cases have shown signi"cant reversibility, it becomes imperative to evaluate extent of reversibility in Step 1 or at least obtain qualitative/quantitative estimate of protonation equilibrium constant and its dependence on polymer matrix or basicity of the resin. Further, the assumption of complete
V. M. Bhandari et al. / Chemical Engineering Science 55 (2000) 6197}6208
Fig. 1. Reversibility of sorption.
dissociation of resin salt may also need re-examination in the light of new information.
3. Experimental work Experimental work was carried out on sorption of strong monobasic acids, HCl and HNO , and weak monobasic organic acid, CH COOH. The weak base resins used in the study were Dowex WGR-2 (Dow Chemical Co., USA), Amberlite IRA-68 (Rohm and Haas, USA), Diaion WA-10, Diaion WA-20 and Diaion WA-30 (Mitsubishi Chemical Co., Japan). The resins were "rst pretreated by employing standard procedure (Hel!erich, 1962). The resin samples were partially dried at 603C to a known moisture content, to make resin-free #owing so as to eliminate errors due to sticking of resin beads. The capacity values of the resins (meq/g dry weight basis) were determined using the procedures al-
6199
ready well discussed in literature (Hel!erich, 1962; Bhandari et al., 1992). The properties of these resins and other resins, which are included in the present analysis, are given in Table 1. The sorption equilibrium studies for the acids were carried out by equilibrating known weight of resin with solutions of acid of known initial acid concentration. The equilibration was carried out at room temperature (253C) for more than 48 h with intermittent shaking. Approach to equilibrium was checked by analysing samples after 24 h, 48 h and 72 h and it was observed that the equilibrium could be reached within 48 h. At equilibrium, samples of extraparticle #uid were analysed for the acid concentration by measuring pH of the solution for lower acid concentrations and by titration with standard NaOH solution. For weak acid, the total acid concentration of the equilibrated solution was computed from pH measurements and using the dissociation constant of acid. The equilibrium concentration of the sorbed acid was estimated from the di!erence between the initial and "nal acid concentration in the extraparticle #uid. Experimental work was also carried out on sorption from mixtures of HCl and HNO on Dowex WGR-2. Acid mixtures of known individual acid concentration were prepared using predetermined ratio. Known quantity of resin was then added to each #ask containing acid mixture. Equilibrium studies were then carried out using the procedure as above. Total concentration of acid was obtained by titration while the HCl acid concentration was obtained independently from chloride ion measurement using Speci"c Ion Meter. The concentration of nitric acid was then obtained from material balance in solution and resin phase. Figs. 2}5 show the experimental data on the sorption of hydrochloric acid, nitric acid and acetic acid on
Table 1 Resin characteristics Name
Matrix
Functionality
Dowex WGR-2
Epoxyamine
Diaion WA-10
Polyacrylic
Diaion WA-20
Polystyrene
Diaion WA-30
Polystyrene
Amberlite IRA-68
Polyacrylic
Amberlite IRA-93
Polystyrene
Amberlite IR-4B
Phenol-HCHO
Dowex MWA-1
Polystyrene
Polyfunctionality }NH; }N Polyfunctionality }NH; }N Monofunctionality }NH Monofunctionality }N Polyfunctionality }NH; }N Polyfunctionality }NH; }N Polyfunctionality }NH; }N Monofunctionality }N
Capacity (meq/g, dry basis) 9.6 5.5 6.4 6.0 5.75 5.5 10.2 4.3
6200
V. M. Bhandari et al. / Chemical Engineering Science 55 (2000) 6197}6208
Fig. 2. Equilibrium data on Diaion WA-10.
Fig. 4. Equilibrium data on Diaion WA-30.
Fig. 5. Equilibrium data on Amberlite IRA-68. Fig. 3. Equilibrium data on Diaion WA-20.
di!erent weak base resins. It is evident that sorption reversibility is signi"cant for most weak base resins. The sorption reversibility is pronounced at low acid concentrations. However, there appears to be a large variation in the form of reversibility and hence the sorption equilibrium curve. Such a large variation in the sorption behaviour is not commensurate with the assumption of
very high value of protonation constant, K , obtained N using the conventional method of apparent pK of resin. ? Further, considering ionic distribution adjacent to the charged surface, there exists possibility of incomplete resin salt dissociation, which consequently will also in#uence the sorption characteristics along with the protonation equilibrium on the surface.
V. M. Bhandari et al. / Chemical Engineering Science 55 (2000) 6197}6208
4. Theoretical treatment of sorption equilibria In majority of the studies on ion exchange, the distribution of the counterions was obtained by using the ion exchange equilibria and the partitioning of the coions in the resin phase was predicted using the ideal Donnan principle. The application of ideal Donnan principle in such cases predicts near total exclusion of coions from the resin pores (Saunders, Vierow & Carta, 1989; Bhandari et al., 1992, Bhandari, 1998). The models in literature lack mainly in the following: 1. Majority of studies deal with strong base resins where protonation step is absent. 2. For weak base resins, sorption is usually considered irreversible, which is not the case. 3. E!ect of resin basicity and polymer matrix was not considered. Not all resins are suitable for all kind of separation applications. The factors, which di!erentiate these resins must be accounted for in the model. In the following section, a new theoretical treatment is proposed to overcome some of the above mentioned lacunae. The proposed model di!ers from conventional approach in one respect: It considers dissociation equilibria for both acid and resin species. Further, the model is less rigorous and the mathematical treatment is simpli"ed by de"ning the equilibria on the basis of concentrations rather than activities. The dissociation constant of acid in the resin phase is assumed to be identical to that in the solution phase. 4.1. Model equations Consider a system of weak base resin in free base form and mixture of any two acids: HA and HB. The system can be conveniently divided into two parts: (1) intraparticle #uid and (2) extraparticle #uid. (1) Intraparticle yuid: All the concentrations in the resin phase are based on the pore volume of the resin. Electrical double layer is assumed to exist at the walls of resin pores, followed by core region where local electroneutrality exists (Bhandari et al., 1992). The concentrations in the region between wall of the pore and the inner Helmholtz plane (IHP) of the double layer (where speci"c adsorption of the counterions occur) are denoted by subscript `sa and the concentrations in the core region of pore are denoted by subscript `ca. The species in the resin phase are denoted by an overbar. The following equations can be written for the region bounded by IHP: [H ] [A ] Q Q, HA & H #A , K " Q Q Q [HA] Q
(3)
[H ] [B ] Q Q, HB & H #B , K " Q Q Q [HB] Q
(4)
6201
[RH] R #H & RH, K " . (5) Q N [R ][H ] Q Reaction (5) is a protonation reaction. A very high equilibrium constant of this reaction (10}10 * Rao and Gupta, 1982a; Hel!erich & Hwang, 1985) implies equal capacity utilization for the sorption of any acid on di!erent weak base resins. However, di!erent values of capacity utilization for commercial resins have been experimentally observed. This indicates that there is another factor besides these, which contributes to the sorption capacity utilization. It is believed to be the salt dissociation constant, K . The e!ects of resin basicity and B polymer matrix are believed to in#uence both, the protonation equilibrium constant and the salt dissociation constant: [RH] [A] Q, RH#A & RHA, K " Q B [RHA]
(6)
[RH][B] Q. RH#B & RHB, K " Q B [RHB]
(7)
The electroneutrality condition can be written as [RH]#[H] "[A] #[B] , Q Q Q whereas the capacity relation can be expressed as
(8)
[R]#[RH]#[RHA]#[RHB]"Q.
(9)
The acid dissociation equilibria in the core region of pore can be represented by equations obtained by substituting subscript s with c in Eqs. (3) and (4). The electroneutrality condition in the core region is, however, di!erent and is given by [H ] "[A ] #[B ] . (10) A A A (2) Extraparticle yuid: The dissociation equilibria for the extraparticle can be represented by the similar equations of Eqs. (3) and (4), by removing overbar and the subscript. The electroneutrality condition here is given by [H]"[A]#[B].
(11)
The bridging conditions with the assumption of identical acid dissociation constant in resin and solution phase, can be expressed as [H ] [A ] "[H ] [A ] "[H][A], (12) Q Q A A [H ] [B ] "[H ] [B ] "[H][B]. (13) Q Q A A For monobasic acids, the separation factor or selectivity is then de"ned as [RHA]/[RHB] a " . [A]/[B]
(14)
6202
V. M. Bhandari et al. / Chemical Engineering Science 55 (2000) 6197}6208
For single-acid sorption on weak base resin, the following form of sorption equilibria is obtained: K [R ][H]"K +Q![R ] N B (15) !(K [R ][H]/(1#K [R ]),. N N The unused capacity of the resin ([R ]) can be obtained from the di!erence between the theoretical capacity of the resin and the sorbed acid concentration. The model is solved for single acid systems using Eq. (15) and for the mixture of acids using CONLES program (Shacham, 1986) for the solution of composite system of non-linear and linear equations. Sorption equilibrium data of present work and also that reported in literature were used in the evaluation of the protonation equilibrium constant and resin salt dissociation constant. It may be noted that the equilibrium is de"ned on the basis of surface interactions and although the thickness of the double layer could depend on the size of the pore, the "nal equilibrium relation does not contain the pore size (which is apparent from the model equations). Hence, the equilibrium relations should hold for any size of pore. The double-layer concept introduces only the surface charge and interactions thereof as the important variable governing the equilibrium.
Fig. 6. E!ect of resin basicity: Sorption of HCl X"(Q![RM ])/Q. Experimental data of this work is shown by bold lines.
5. Results and discussion 5.1. Ewect of resin basicity For resins with very high basicity, it is naturally expected that there would be near total sorption of acid on the surface corresponding to the theoretical capacity of the resin. As mentioned earlier, this is then re#ected in the rectangular form of the isotherm. Thus, as a rule, the extent of reversibility increases with the decrease in the resin basicity and decrease in the acid concentration. Since, most of the ion exchange applications are for dilute solutions, the crucial parameter for sorption and selectivity would then be resin basicity. Although, some attempts have been made in the past to characterize resin basicity on the basis of polymer matrix and functionality using the apparent pK values (Adams et al., 1969; Garcia ? & King, 1989; Tung & King, 1994; Husson & King, 1999a,b), the problems in quantifying true resin basicity probably limited its applicability to commercial systems. Also, an approach of assigning an e!ective sorbent donor number involves use of the donor number of the most basic functional group of the sorbent. This approach warrants caution because the surrounding polymer matrix in#uences the basicity of the functional group and these e!ects are inadequately represented (Husson & King, 1998). Alternatively, we propose to obtain gradation in the basicity from the experimental equilibrium data on various resins. Figs. 6 and 7 compare the equilib-
Fig. 7. E!ect of resin basicity: Sorption of acetic acid X"(Q![RM ])/Q. Experimental data of this work is shown by bold lines.
rium data of this work (indicated by bold lines) and also using similar data from some of the earlier works (Kunin, 1958; Adams et al., 1969; Bhandari et al., 1992, 1996; Takatsuji & Yoshida, 1997; Husson & King, 1999a). In contrast to the earlier reports on the individual sorption equilibrium curves and isolated treatment of the same, it is now possible for us to have a clearer picture of sorption behaviour on di!erent weak base resins. On the basis of the experimental data on acid sorption, it is proposed to have weak base resins grouped in the following three categories:
V. M. Bhandari et al. / Chemical Engineering Science 55 (2000) 6197}6208
6203
1. Resins with high basicity. 2. Resins with medium basicity. 3. Resins with weak basicity. It must be emphasized here that the above classi"cation is only qualitative, based on the conceptual representation of Fig. 1. Thus, Amberlite IRA-68 and Diaion WA10 belong to the class of high basicity resin. Resins: Diaion WA-20, Diaion WA-30, Dowex MWA-1 and Amberlite IRA-93 have medium basicity, whereas Dowex WGR-2 and Amberlite IR-4B show comparatively weak basicity. Further, this analysis reveals that, resins with high basicity are more useful for the removal of acids from aqueous solutions, as it gives high sorption capacity utilization even at low acid concentrations. However, weak acids such as acetic acid shows signi"cant reversibility even on resins having high basicity, implying acid type as an important parameter in the sorption process. However, the trends in the overall basicity structure of resins essentially remain unaltered even for weak acids. Thus, prior knowledge of resin basicity would help in the selection of resin for acid removal applications. Another observation evident from Fig. 6 and Table 1 is more interesting and reveals the possibility of correlating the resin type and basicity. It was observed that, polyacrylic resins with polyfunctionality comprising secondary and tertiary nitrogen exhibit high basicity. Resins with polystyrene matrix: both monofunctional secondary/tertiary nitrogen and polyfunctional with secondary and tertiary nitrogen, show medium basicity, whereas the matrix of epoxyamine and phenol}formaldehyde implies weak basicity. It can, therefore, be concluded that the polymer matrix of the resin has signi"cant e!ect on sorption characteristics. As mentioned earlier, the salt dissociation is assumed to be complete in all the previous works. In order to reexamine this assumption, an appropriate value of the resin}salt dissociation constant needs to be obtained. It was found that value of K above 10 kmol/m validates B the above assumption. Using this value of K , the value B of protonation equilibrium constant, K was regressed N using the experimental data on resin capacity utilization. A typical "t of the model to the experimental data of this
Fig. 8. Fit of model to the experimental data (Sorption of HCl; K "1;10 kmol/m). B
work, on sorption of HCl on Diaion resins, is shown in Fig. 8. It can be seen that the "t is good in all the cases. The computational results showed three types of sorption equilibrium behaviour. 5.1.1. Systems with constant K N The equilibrium data in this case could be "tted with good accuracy using a practically constant value of the protonation equilibrium constant in a wide range of acid concentration. The regressed values of protonation equilibrium constant with the assumption of complete salt dissociation and using experimental data of this work and also that reported by Adams and co-workers (1969) are listed in Table 2. While the values in Table 2 represent the best "t using the experimental data for which maximum error is less than 15% in most cases, the actual variation of K in the experimental sorption data is N shown in Fig. 9. It can be seen that the variation is less. Two important observations can be made from the results of Table 2: First, the regressed values of K are far N lower than those reported by Adams and coworkers. Second, for commercial weak base resins, the value of protonation equilibrium constant is of the order of
Table 2 K values: theoretical vs. experimental (constant K ) N N Resin
Acid
K (kmol/m)\, experimental N (Adams et al., 1969)
K (kmol/m)\, regressed N
Diaion WA-10 Amberlite IRA-68 Amberlite IRA-68 IP-MetoHA IP-DMA DVB-2DMA IP-DetoHA DVB-2VP
HCl HCl HNO HCl HCl HCl HCl HCl
* * * 2.0;10 1.0;10 1.0;10 3.1;10 7.9;10
2.5;10 5.6;10 2.2;10 7.3;10 2.6;10 1.4;10 3.9;10 1.9;10
6204
V. M. Bhandari et al. / Chemical Engineering Science 55 (2000) 6197}6208
Fig. 11. Variation of K in medium/weak basicity resins * Sorption of N HCl.
Fig. 9. Variation of K in high basicity resins. N
Fig. 10. Simulation of sorption equilibrium pro"le * E!ect of K . N
10}10 (kmol/m)\ similar to that suggested by Hel!erich and Hwang (1985), but only for resins with high basicity. The e!ect of protonation equilibrium constant on the sorption behaviour is theoretically shown in Fig. 10. Larger the value of K , greater is the sorption N capacity attainable. 5.1.2. Systems with K variation N The model could not be "tted using a single constant value of K to most systems exhibiting strong deviation N from the rectangular form of isotherm, thus signi"cant reversibility. Here, large variation in the values of K was N
Fig. 12. Variation of K * Sorption of acetic acid. N
observed with acid concentration. These results are shown in Figs. 11 and 12 for the sorption of HCl and acetic acid, respectively. Such a large variation in the protonation equilibrium constant cannot be explained on the basis of activity coe$cient alone. The variation in the values of K for weak acid, acetic acid is considerably N less as compared to that in HCl. 5.1.3. Systems indicating incomplete dissociation of resin salt An interesting observation was made in the case of sorption of acetic acid on Diaion WA-10. The resin is
V. M. Bhandari et al. / Chemical Engineering Science 55 (2000) 6197}6208
from the class of high basicity. Here, the sorption data could be "tted only using the value of protonation equilibrium constant of 10 (kmol/m)\ as obtained previously for HCl sorption, but using K value of B approximately 1.4;10\ for H> concentrations below 1;10\ kmol/m. The results, here, indicate possibility of incomplete dissociation of the resin salt. The sorption capacity of this resin above this concentration level was higher than the theoretical capacity of the resin, similar to the observations made by Husson and King (1999a) and, hence, no further evaluation was possible using the present treatment. Although sorption capacities higher than that of the theoretical capacity of resin have been reported in some cases, especially for weak acids, this anomalous sorption behaviour is not explained satisfactorily so far. Husson and King (1999a) attributed it to higher level of adsorption as against monolayer sorption and then correlated the sorption data using modi"ed Langmuir isotherm equation. The theoretical treatment of this work has potential to explain higher values of sorption on the basis of double layer. The results of simulation using di!erent values of K and K are shown in Fig. 13. It is evident N B that higher sorption capacity utilization is easy to achieve on the resins of high basicity (high K ) and with N lower extent of salt dissociation (lower K ). This implies B a very compact double layer adjacent to the resin surface wherein anions of the acid are held very strongly. Here, the short-range electrostatic forces of attraction are likely to be capable of further promoting adsorption of acid similar to multi-layer adsorption on surfaces. Consequently, sorption capacity in excess of theoretical resin capacity can be realized for these resins. This is in contrast to complete resin salt dissociation, where anions of acid are loosely attached to the surface and are distributed over the cross-section of resin pore as in the di!used layer. The argument is further supported by the simulation results indicating lower sensitivity to variation in resin salt dissociation for resins having very weak basicity (K &10), especially at low acid concentrations. N 5.2. Resin basicity and selectivity From the above analysis, it is expected that the resins with high basicity, although suitable for the acid removal applications, may not o!er high selectivity required for acid separations, unless the salt dissociation constant for two acids is substantially di!erent. To evaluate this aspect, sorption behaviour of hydrochloric acid and nitric acid on three di!erent resins with high, medium and low basicity was analyzed. The results indicated that for weak base resin, Dowex WGR-2, experimentally obtained value of selectivity was approximately 1.1, practically constant over the range of acid concentration. The selectivity values obtained with mixtures of nitric acid and hydrochloric acid (concentration ratio of the two acids in
6205
Fig. 13. Simulation of sorption equilibrium pro"le * Sensitivity to resin}salt dissociation.
solution approximately 1) are also compared (Fig. 14) with those predicted using single component sorption data. Fig. 14, within the narrow range of acid concentration studied in this work, indicates possibility of using single component data for predicting selectivity behaviour in mixtures. Similar selectivity analysis using sorption data on medium and strong basicity resins (Amberlite IRA-93 and Amberlite IRA-68), also indicated values close to unity. This is in agreement with the arguments made in the earlier section and also with previous observations, wherein Tsitovich, Konovalova and Tsarichenko (1965) reported lower selectivity values with anion exchange resins and suggested that the separation may be di$cult using these resins. Apart from resin basicity, selectivity is also dependent on the ratio of acid concentration. For a concentration ratio of near unity, the selectivity values are very close to unity for most resins. The separation is better for higher concentration ratios, [HNO ]/[HCl]. Since the exact values of K , K and K are not N B B known, the established experimental trends were checked
6206
V. M. Bhandari et al. / Chemical Engineering Science 55 (2000) 6197}6208
dilute aqueous stream has concentration of HCl with pH &4, then from Fig. 6 it is seen that the maximum capacity realized on Dowex WGR-2 is below 3 meq/g due to equilibrium fractional conversion of less than 0.3. However, employing high basicity resins such as Amberlite IRA-68 or Diaion WA-10, the acid removal capacity can be above 4.5 meq/g, even though its maximum capacity is just 5.7 meq/g as against 9.6 meq/g of Dowex WGR-2. It can be recommended that the resins with high basicity should be selected for the applications involving removal of acids from dilute solutions. Apart from signi"cant pH dependence, the basicity e!ect also appears to be function of acid type and is less pronounced for weak acids. Thus, extent to which basicity can in#uence removal of acids is an important consideration in the selection of resin. This has implications in the wastewater treatment problems involving very weak acids such as phenol, cresols, etc. where, the selection of weak base resin can be on the basis of its capacity and basicity, both. Fig. 14. Separation from acid mixtures * Separation of HNO and HCl.
6. Conclusions using the model and arbitrary values of these constants for a mixture of acids HA and HB. As expected, the ratio [AM ] /[A] increases with increase in K and decrease in Q B K , indicating stronger adsorption of anion A in double B layer as compared to anion `Ba. It was also found that, for obtaining same capacity utilization, the salt dissociation constant for weaker acid is less than that for stronger acid. This result agrees with our observation on the sorption of HCl and acetic acid}both theoretical and experimental. Further, for the same values of K and B K , increase in the ratio [A]/[B] (either by changing B acid dissociation constant or by changing acid concentration) improves selectivity. Thus, the model qualitatively explains the experimentally observed sorption behaviour. 5.3. Resin basicity and acid removal from aqueous solution Weak base resins are most commonly employed for the removal of acids from aqueous streams. In these applications, obtaining high sorption capacities for acid removal is most desirable. The equilibrium dictates limiting capacity of the resin for design of industrial operations. The prediction of equilibria is still a di$cult task in most cases and such data are normally obtained by carrying out laborious experiments in the laboratory, since no clear directions exist for the selection of resin. From the results of this work, it is evident that the resin basicity has a large e!ect on the sorption capacity. Higher the basicity, better is the equilibrium capacity utilization. Thus, it may not be just su$cient to select a weak base resin for the removal of acids on the basis of its high theoretical capacity for sorption. For example, if the
The important conclusions of the present study are 1. Resin basicity has signi"cant impact on sorption equilibria. 2. A qualitative analysis using the experimental data on sorption equilibria can lead to clearer understanding of the resin basicity e!ect. 3. The weak base resins, in general, can be grouped into three di!erent categories on the basis of experimental observation of sorption equilibria- Resins with high basicity, medium basicity and weak basicity, each corresponding to a speci"c form of sorption isotherm. 4. Resins with high basicity are most suitable for the acid removal applications from aqueous solutions since they yield high resin capacity utilization even at low acid concentrations. 5. A theoretical treatment which accounts for the dissociation equilibria for both, resin and solution species, can satisfactorily explain the sorption behaviour on weak base resins. 6. The results of this work indicate that the conventional approach of de"ning basicity using apparent pK @ values of resin has limitations and the high values of protonation equilibrium constant (&10}10) are found to be appropriate only to resins having high basicity. 7. Substantial variation in the protonation equilibrium constant has been observed in some cases and even incomplete resin salt dissociation is indicated. Although the present study highlights di!erences in the sorption behaviour of various acids on di!erent weak base resins and implications of the same, further
V. M. Bhandari et al. / Chemical Engineering Science 55 (2000) 6197}6208
experimental and theoretical work is required to substantiate some of the "nding of this work.
Notation A B H HA HB K K K B K B K N Q R RH RHA RHB X
anion of acid HA anion of acid HB H> species acid with anion A acid with anion B equilibrium constant for acid HA, kmol/m equilibrium constant for acid HB, kmol/m dissociation constant for resin salt, RH>A\, kmol/m dissociation constant for resin salt, RH>B\, kmol/m protonation equilibrium constant, (kmol/m)\ resin capacity, kmol/m free base group of resin protonated species of resin resin salt of acid HA resin salt of acid HB equilibrium fractional conversion of resin ((Q![RM ])/Q)
Greek letters a
separation factor or selectivity
Subscript and superscripts c s *
concentration in core region of resin pore. concentration in the wall region bounded by IHP of the double layer pore phase
References Adams, G., Jones, P. M., & Miller, J. R. (1969). Kinetics of acid uptake by weak base anion exchangers. Journal of the Chemical Society, section A, 2543. Bhandari, V. M. (1998). Implications of weak Donnan potential in ion exchange reactions: An alternate strategy for modeling sorption process. Separation Science and Technology, 33, 2009. Bhandari, V. M., Juvekar, V. A., & Patwardhan, S. R. (1992a). Sorption studies on ion exchange resins. I. Sorption of strong acids on weak base resins. Industrial and Engineering; Chemistry Research, 31, 1060. Bhandari, V. M., Juvekar, V. A., & Patwardhan, S. R. (1992b). Sorption studies on ion exchange resins. II. Sorption of weak acids on weak base resins. Industrial and Engineering; Chemistry Research, 31, 1073. Bhandari, V. M., Juvekar, V. A., & Patwardhan, S. R. (1993). Sorption of dibasic acids on weak base resins. Industrial and Engineering Chemistry Research, 32, 200. Bhandari, V. M., Juvekar, V. A., & Patwardhan, S. R. (1997). Ion exchange studies in the removal of polybasic acids * anomalous sorption behaviour of phosphoric acid on weak base resins. Separation Science and Technology, 32, 2481.
6207
Bhandari, V. M., Sawarkar, C. S., & Juvekar, V. A. (1996). Ion Exchange Separations}Selectivity in acid sorption. In Varma et al. (Ed.), Advances in chemical engineering * Proceedings of international conference (P. 81) Allied Publishers, Madras. Cloete, F. L. D., & Marais, A. P. (1996). Recovery of very dilute acetic acid using ion exchange. Industrial and Engineering Chemistry Research, 34, 2464. Garcia, A. A., & King, C. J. (1989). The use of basic polymeric sorbents for the recovery of acetic acid from dilute aqueous solution. Industrial and Engineering Chemistry Research, 28, 204. Hel!erich, F. (1962). Ion exchange. New York: McGraw-Hill. Hel!erich, F.G., & Hwang, Y.L. (1985). Kinetics of acid uptake by weak base anion exchangers. Mechanism of proton transfer. A.I.Ch.E. Symposium Series, No. 242, vol. 81, (p. 17). Helsel, R. W. (1977). Waste recovery: Removing carboxylic acids from aqueous wastes. Chemical Engineering Progress, 73, 55. Holl, W., & Sontheimer, H. (1977). Ion exchange kinetics of the protonation of weak acid ion exchange resins. Chemical Engineering Science, 32, 755. Hubner, P., & Kadlec, V. (1978). Kinetic behaviour of weak base anion exchangers. A.I.Ch.E. Journal, 24, 149. Husson, S. M., & King, C. J. (1998). Regeneration of lactic and succinic acid-laden basic sorbents by leaching with a volatile base in an organic solvent. Industrial and Engineering Chemistry Research, 37, 2996. Husson, S. M., & King, C. J. (1999a). Multiple acid equilibria in adsorption of carboxylic acids from dilute aqueous solution. Industrial and Engineering Chemistry Research, 38, 502. Husson, S. M., & King, C. J. (1999b). Carbon dioxide sustained adsorption of lactic acid at pH ' pK of the acid. Industrial and Engineer? ing Chemistry Research, 38, 1625. Juang, R. S., & Chou, T. -C. (1996). Sorption of citric acid from aqueous solutions by macroporous resins containing a tertiary amine equilibria. Separation Science and Technology, 31, 1409. Katzen, R., Aries, R. S., & Othmer, D. F. (1945). Wood hydrolysis: Utilization of waste liquors. Industrial and Engineering Chemistry, 37, 442. Kawabata, N., Yoshida, Jun-ichi., & Tanigawa, Y. (1981). Removal and recovery of organic pollutants from aquatic environment. 4. Separation of carboxylic acids from aqueous solution using crosslinked poly(4-vinylpyridine). Industrial and Engineering Chemistry, Product Research and Development, 20, 386. Kunin, R. (1958). Ion exchange resins. London: Wiley. Matsarenko, V. A., Lebedinskaya, N. A., Dudkova, L. I., & Gushchina, L. I. (1969). Recovery of water soluble C}C acids from acid waste waters of the production of synthetic fats. Khimichiskaya Promyshlennost, 45, 12. Othmer, D. F. (1958). Acetic acid recovery methods. Chemical Engineering Progress, 54, 48. Parulekar, S. J., Sharma, M. M., Joshi, J. B., & Shah, Y. T. (1982). Separation processes for recovery of valuable chemicals from aqueous e%uents. Journal of Separation Process and Technology, 3, 2. Rao, M. G., & Gupta, A. K. (1982a). Kinetics of ion exchange in weak base anion exchange resins. A.I.Ch.E. Symposium Series No. 219, 78, 96. Rao, M. G., & Gupta, A. K. (1982b). Ion exchange processes accompanied by ionic reactions. Chemical Engineering Journal, 24, 181. Ricker, N. L., Pittman, E. F., & King, C. J. (1980). Solvent extraction with amines for recovery of acetic acid from dilute aqueous industrial streams. Journal of Separation Process and Technology, 1, 23. Saunders, M. S., Vierow, J. S., & Carta, G. (1989). Uptake of phenylalanine and tyrosine by a strong acid cation exchanger. AIChE J., 35, 53. Shacham, M. (1986). Numerical evaluation of constrained non-linear algebraic equations. International Journal for Numerical Methods in Engineering, 23, 1455.
6208
V. M. Bhandari et al. / Chemical Engineering Science 55 (2000) 6197}6208
Takatsuji, W., & Yoshida, H. (1994). Removal of organic acids from wine by adsorption on weakly basic ion exchangers: Equilibria for single and binary systems. Separation Science and Technology, 29, 1473. Takatsuji, W., & Yoshida, H. (1997). Adsorption of organic acids on weakly basic ion exchanger: equilibria. Journal of Chemical Engineering Japan, 30, 396. Takatsuji, W., & Yoshida, H. (1998a). Adsorption of organic acids on polyaminated highly porous chitosan: Equilibria. Industrial and Engineering Chemistry Research, 37, 1300. Takatsuji, W., & Yoshida, H. (1998b). Adsorption of organic acids on weakly basic ion exchanger: Equilibria for binary systems. A.I.Ch.E. Journal, 44, 1216.
Tsitovich, I. K., Konovalova, E. A., & Tsarichenko, B. F. (1965). Salt forms of cation exchange resins and separation of organic acids. Khim.i Khim. Tekhnol., 8, 60. Tung, L. A., & King, C. J. (1994). Sorption and extraction of lactic and succinic acids at pH ' p K . Factors governing equilibria. Indus? trial and Engineering Chemistry Research, 33, 3217. Wadekar, V. V., & Sharma, M. M. (1981). Dissociation extraction. Journal of Separation Process and Technology, 2, 1. Yoshida, H., Kishimoto, N., & Kataoka, T. (1994). Adsorption of strong acid on polyaminated highly porous chitosan: Equilibria. Industrial and Engineering Chemistry Research, 33, 854.
Investigating the di!erences in acid separation behaviour on weak base ion exchange resins Vinay M. Bhandari *, Toshikuni Yonemoto , Vinay A. Juvekar Department of Chemical Engineering, Tohoku University, Sendai 980-8579, Japan Department of Chemical Engineering, Indian Institute of Technology Bombay, Bombay 400 076, India Received 10 March 2000; received in revised form 30 June 2000; accepted 4 July 2000
Abstract Di!erences in acid separation behaviour on weak base ion exchange resins have been investigated in this work. Experimental data has been obtained on sorption equilibrium using various types of weak base resins and both inorganic and organic monobasic acids. A detailed analysis has been presented using the experimental data of present work and also that reported in the literature. Apart from acid concentration and type of acid, other factors such as resin type and basicity are also discussed. It was found that, resins with high basicity are most suitable for the acid removal applications. For resins with medium and weak basicity, large variation in the sorption capacity with concentration was observed. A new theoretical approach incorporating e!ect of resin basicity has been proposed and examined qualitatively and quantitatively using the sorption data. The results substantiate reversibility of sorption through lower values of protonation equilibrium constants and also indicate incomplete resin salt dissociation in some cases. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Ion exchange; Separation; Sorption equilibrium; Acid removal; E%uent treatment
1. Introduction Removal of acids from aqueous streams containing low concentrations of acids: organic/inorganic or both, is an important problem in chemical process industries. Ion exchange resins in general, and weak base resins in particular, are most commonly employed for this purpose. The weak base resins have large capacity for acid sorption and are easy to regenerate as compared to strong base resins. The two most important and widely practiced applications of acid removal are boiler feed water treatment and wastewater treatment. In the former application, acids are generated through the exchange of cations of salts with H> ions from resin. This acid is then removed by passing the aqueous stream through a bed of weak base anion exchange resin, which sorbs acid as a complete molecule. Wastewater streams containing low concentrations of acids are inevitably encountered in acid manufacturing plants, industries where acids are * Correspondence address. Chemical Engineering Division, National Chemical Laboratory, Pune 411 008, India. Tel.: #91-20-5893163; fax: #91-20-5893260. E-mail address: [email protected] (V. M. Bhandari).
used as raw material or as catalyst, fermentation processes, metal plating industry, etc. Wastewater usually contains single or mixture of acids from 0.5 to 4%, depending on the source of generation (Katzen, Aries & Othmer, 1945; Othmer, 1958; Matsarenko, Lebedinskaya, Dudkova & Gushchina, 1969; Helsel, 1977; Ricker, Pittman & King, 1980; Wadekar and Sharma, 1981; Kawabata, Yoshida Jun-ichi & Tanigawa, 1981; Parulekar, Sharma, Joshi & Shah, 1982). Since, large volumes are to be treated that contain low concentrations of acids, ion exchange is most convenient method of treatment in many cases. The primary goal in such cases is largely removal of acids from the solution and not separation. However, acid separation is important in acid manufacturing units where another acid is usually obtained as a byproduct, e.g. formic acid in the manufacture of acetic acid. Thus, the selection of ion exchange resin for any particular application, whether removal or separation, is an important consideration. A number of experimental and theoretical studies have been reported on the sorption of various acids on weak base resins (Kunin, 1958; Hel!erich, 1962; Adams, Jones & Miller, 1969; Holl and Sontheimer, 1977; Hubner and Kadlec, 1978; Rao and Gupta, 1982a,b; Hel!erich and
0009-2509/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 9 - 2 5 0 9 ( 0 0 ) 0 0 2 1 2 - 8
6198
V. M. Bhandari et al. / Chemical Engineering Science 55 (2000) 6197}6208
Hwang, 1985; Bhandari, 1998; Garcia and King, 1989; Bhandari, Juvekar & Patwardhan, 1992a,b, 1993, 1997; Bhandari, Sawarkar & Juvekar, 1996, Yoshida, Kishimato & Kataoka, 1994; Cloete and Marais, 1996; Takatsuji and Yoshida, 1994, 1997, 1998a,b; Juang and Chou, 1996; Husson and King, 1999a,b). In many instances, acid sorption was considered irreversible and the sorption equilibrium was interpreted in terms of simple Langmuir- or Freundlich-type isotherm. The application of Langmuir- or Freundlich-type isotherm was, however, not found satisfactory in some cases (Husson & King, 1999a) and anomalous nature of sorption equilibria was also indicated with polybasic acids (Bhandari et al., 1997). The experimental and theoretical information on acid sorption is largely incomplete due to the following: 1. Majority of the studies are reported on single-component acid and not on acid mixtures. 2. Selection of resin for any acid removal/separation application has not been discussed. 3. In terms of detailed investigations on sorption of acids and di!erences in sorption behaviour, previous studies have largely focused on the sorption dynamics of acid on weak base resins and less attention has been given to the experimental and theoretical aspects of sorption equilibria, especially with reference to the di!erences in the sorption behaviour. 4. Practically, very little information is available on selectivity in acid sorption and theoretical explanation for the same. The present work is an attempt to gain insight into the acid separation behaviour by experimentally and theoretically analyzing sorption data on various weak base resins. Both, single and two component acid systems have been discussed. A new, less rigorous theoretical treatment is proposed to explain selectivity behaviour in acid sorption. Considering the enormous di$culties in developing the entirely satisfactory approach to equilibria, the proposed simpli"ed treatment of equilibria could be useful in evolving a coherent and practical approach for data assimilation and for ion exchange design and practice.
2. Acid sorption on weak base resins Sorption of acid on weak base resin essentially follows a two-step mechanism. In the "rst step, the free ionogenic groups of the resin (denoted by R) are protonated by the H> ions of the acid (for convenience, the charge sign is omitted in writing the equations). Step 1. Protonation of the free ionogenic sites: R #H & RH.
(1)
Step 2. Anion association (Resin-salt Dissociation): RH#A & RHA. Q
(2)
Protonation results into the formation of positively charged surface due to the presence of protonated species (RH>) on the surface of pore walls of resin. The second step, anion association, is a result of electrostatic interaction between the positively charged RH> groups and negatively charged anions of the acid. Speci"c adsorption of the anions is believed to occur in a region close to the pore walls (Bhandari et al., 1992). The conventional approach to acid sorption assumes step 1 as a irreversible process. The equilibrium constant of this reaction for most commercial weak base resins is considered to be very high of the order of 10}10 (kmol/m)\ (Hel!erich & Hwang, 1985). However, no established values/procedures for the protonation equilibrium constant are available in literature. Further, complete dissociation of resin salt and uniform distribution of anions across the cross-section of pore is usually assumed. The later assumption practically leaves protonation step as the only controlling mechanism in sorption equilibrium. According to the reversible sorption theory based on electrical double layer, (Bhandari et al., 1992), the sorption is reversible, in general, and the distribution of anions across the cross-section of pore is according to the Stern model. The exclusion of coion, equilibrium characteristics and selectivity, etc. can be attributed to the complex nature of the double layer adjacent to the pore surface, which can be compact (strong anion sorption), di!used (large anion distribution) or simply non-existent (negligible sorption). For mixture of acids, the selectivity for a particular ion would depend on the equilibrium distribution of two counterions in the double layer. The reversibility in the protonation step is attributed to the partial exclusion of coion from the double layer. Here also, no separate estimation of the protonation equilibrium constant was made, although the authors indicated that the values could be substantially lower. Fig. 1 shows the schematic plots of sorption equilibria on weak base resins. Case A is a typical rectangular isotherm, which depicts case of irreversible sorption. Cases B and C represent the medium and strong reversibility in sorption, respectively. The assumption of irreversibility is justi"ed only for resins of high basicity and/or at very high acid concentrations. The extent of reversibility is entirely governed by the equilibrium constant, which depends on the basicity of the resin and the acid being sorbed/desorbed and should not, in principle, depend on solution concentration. Since, experimental data in many cases have shown signi"cant reversibility, it becomes imperative to evaluate extent of reversibility in Step 1 or at least obtain qualitative/quantitative estimate of protonation equilibrium constant and its dependence on polymer matrix or basicity of the resin. Further, the assumption of complete
V. M. Bhandari et al. / Chemical Engineering Science 55 (2000) 6197}6208
Fig. 1. Reversibility of sorption.
dissociation of resin salt may also need re-examination in the light of new information.
3. Experimental work Experimental work was carried out on sorption of strong monobasic acids, HCl and HNO , and weak monobasic organic acid, CH COOH. The weak base resins used in the study were Dowex WGR-2 (Dow Chemical Co., USA), Amberlite IRA-68 (Rohm and Haas, USA), Diaion WA-10, Diaion WA-20 and Diaion WA-30 (Mitsubishi Chemical Co., Japan). The resins were "rst pretreated by employing standard procedure (Hel!erich, 1962). The resin samples were partially dried at 603C to a known moisture content, to make resin-free #owing so as to eliminate errors due to sticking of resin beads. The capacity values of the resins (meq/g dry weight basis) were determined using the procedures al-
6199
ready well discussed in literature (Hel!erich, 1962; Bhandari et al., 1992). The properties of these resins and other resins, which are included in the present analysis, are given in Table 1. The sorption equilibrium studies for the acids were carried out by equilibrating known weight of resin with solutions of acid of known initial acid concentration. The equilibration was carried out at room temperature (253C) for more than 48 h with intermittent shaking. Approach to equilibrium was checked by analysing samples after 24 h, 48 h and 72 h and it was observed that the equilibrium could be reached within 48 h. At equilibrium, samples of extraparticle #uid were analysed for the acid concentration by measuring pH of the solution for lower acid concentrations and by titration with standard NaOH solution. For weak acid, the total acid concentration of the equilibrated solution was computed from pH measurements and using the dissociation constant of acid. The equilibrium concentration of the sorbed acid was estimated from the di!erence between the initial and "nal acid concentration in the extraparticle #uid. Experimental work was also carried out on sorption from mixtures of HCl and HNO on Dowex WGR-2. Acid mixtures of known individual acid concentration were prepared using predetermined ratio. Known quantity of resin was then added to each #ask containing acid mixture. Equilibrium studies were then carried out using the procedure as above. Total concentration of acid was obtained by titration while the HCl acid concentration was obtained independently from chloride ion measurement using Speci"c Ion Meter. The concentration of nitric acid was then obtained from material balance in solution and resin phase. Figs. 2}5 show the experimental data on the sorption of hydrochloric acid, nitric acid and acetic acid on
Table 1 Resin characteristics Name
Matrix
Functionality
Dowex WGR-2
Epoxyamine
Diaion WA-10
Polyacrylic
Diaion WA-20
Polystyrene
Diaion WA-30
Polystyrene
Amberlite IRA-68
Polyacrylic
Amberlite IRA-93
Polystyrene
Amberlite IR-4B
Phenol-HCHO
Dowex MWA-1
Polystyrene
Polyfunctionality }NH; }N Polyfunctionality }NH; }N Monofunctionality }NH Monofunctionality }N Polyfunctionality }NH; }N Polyfunctionality }NH; }N Polyfunctionality }NH; }N Monofunctionality }N
Capacity (meq/g, dry basis) 9.6 5.5 6.4 6.0 5.75 5.5 10.2 4.3
6200
V. M. Bhandari et al. / Chemical Engineering Science 55 (2000) 6197}6208
Fig. 2. Equilibrium data on Diaion WA-10.
Fig. 4. Equilibrium data on Diaion WA-30.
Fig. 5. Equilibrium data on Amberlite IRA-68. Fig. 3. Equilibrium data on Diaion WA-20.
di!erent weak base resins. It is evident that sorption reversibility is signi"cant for most weak base resins. The sorption reversibility is pronounced at low acid concentrations. However, there appears to be a large variation in the form of reversibility and hence the sorption equilibrium curve. Such a large variation in the sorption behaviour is not commensurate with the assumption of
very high value of protonation constant, K , obtained N using the conventional method of apparent pK of resin. ? Further, considering ionic distribution adjacent to the charged surface, there exists possibility of incomplete resin salt dissociation, which consequently will also in#uence the sorption characteristics along with the protonation equilibrium on the surface.
V. M. Bhandari et al. / Chemical Engineering Science 55 (2000) 6197}6208
4. Theoretical treatment of sorption equilibria In majority of the studies on ion exchange, the distribution of the counterions was obtained by using the ion exchange equilibria and the partitioning of the coions in the resin phase was predicted using the ideal Donnan principle. The application of ideal Donnan principle in such cases predicts near total exclusion of coions from the resin pores (Saunders, Vierow & Carta, 1989; Bhandari et al., 1992, Bhandari, 1998). The models in literature lack mainly in the following: 1. Majority of studies deal with strong base resins where protonation step is absent. 2. For weak base resins, sorption is usually considered irreversible, which is not the case. 3. E!ect of resin basicity and polymer matrix was not considered. Not all resins are suitable for all kind of separation applications. The factors, which di!erentiate these resins must be accounted for in the model. In the following section, a new theoretical treatment is proposed to overcome some of the above mentioned lacunae. The proposed model di!ers from conventional approach in one respect: It considers dissociation equilibria for both acid and resin species. Further, the model is less rigorous and the mathematical treatment is simpli"ed by de"ning the equilibria on the basis of concentrations rather than activities. The dissociation constant of acid in the resin phase is assumed to be identical to that in the solution phase. 4.1. Model equations Consider a system of weak base resin in free base form and mixture of any two acids: HA and HB. The system can be conveniently divided into two parts: (1) intraparticle #uid and (2) extraparticle #uid. (1) Intraparticle yuid: All the concentrations in the resin phase are based on the pore volume of the resin. Electrical double layer is assumed to exist at the walls of resin pores, followed by core region where local electroneutrality exists (Bhandari et al., 1992). The concentrations in the region between wall of the pore and the inner Helmholtz plane (IHP) of the double layer (where speci"c adsorption of the counterions occur) are denoted by subscript `sa and the concentrations in the core region of pore are denoted by subscript `ca. The species in the resin phase are denoted by an overbar. The following equations can be written for the region bounded by IHP: [H ] [A ] Q Q, HA & H #A , K " Q Q Q [HA] Q
(3)
[H ] [B ] Q Q, HB & H #B , K " Q Q Q [HB] Q
(4)
6201
[RH] R #H & RH, K " . (5) Q N [R ][H ] Q Reaction (5) is a protonation reaction. A very high equilibrium constant of this reaction (10}10 * Rao and Gupta, 1982a; Hel!erich & Hwang, 1985) implies equal capacity utilization for the sorption of any acid on di!erent weak base resins. However, di!erent values of capacity utilization for commercial resins have been experimentally observed. This indicates that there is another factor besides these, which contributes to the sorption capacity utilization. It is believed to be the salt dissociation constant, K . The e!ects of resin basicity and B polymer matrix are believed to in#uence both, the protonation equilibrium constant and the salt dissociation constant: [RH] [A] Q, RH#A & RHA, K " Q B [RHA]
(6)
[RH][B] Q. RH#B & RHB, K " Q B [RHB]
(7)
The electroneutrality condition can be written as [RH]#[H] "[A] #[B] , Q Q Q whereas the capacity relation can be expressed as
(8)
[R]#[RH]#[RHA]#[RHB]"Q.
(9)
The acid dissociation equilibria in the core region of pore can be represented by equations obtained by substituting subscript s with c in Eqs. (3) and (4). The electroneutrality condition in the core region is, however, di!erent and is given by [H ] "[A ] #[B ] . (10) A A A (2) Extraparticle yuid: The dissociation equilibria for the extraparticle can be represented by the similar equations of Eqs. (3) and (4), by removing overbar and the subscript. The electroneutrality condition here is given by [H]"[A]#[B].
(11)
The bridging conditions with the assumption of identical acid dissociation constant in resin and solution phase, can be expressed as [H ] [A ] "[H ] [A ] "[H][A], (12) Q Q A A [H ] [B ] "[H ] [B ] "[H][B]. (13) Q Q A A For monobasic acids, the separation factor or selectivity is then de"ned as [RHA]/[RHB] a " . [A]/[B]
(14)
6202
V. M. Bhandari et al. / Chemical Engineering Science 55 (2000) 6197}6208
For single-acid sorption on weak base resin, the following form of sorption equilibria is obtained: K [R ][H]"K +Q![R ] N B (15) !(K [R ][H]/(1#K [R ]),. N N The unused capacity of the resin ([R ]) can be obtained from the di!erence between the theoretical capacity of the resin and the sorbed acid concentration. The model is solved for single acid systems using Eq. (15) and for the mixture of acids using CONLES program (Shacham, 1986) for the solution of composite system of non-linear and linear equations. Sorption equilibrium data of present work and also that reported in literature were used in the evaluation of the protonation equilibrium constant and resin salt dissociation constant. It may be noted that the equilibrium is de"ned on the basis of surface interactions and although the thickness of the double layer could depend on the size of the pore, the "nal equilibrium relation does not contain the pore size (which is apparent from the model equations). Hence, the equilibrium relations should hold for any size of pore. The double-layer concept introduces only the surface charge and interactions thereof as the important variable governing the equilibrium.
Fig. 6. E!ect of resin basicity: Sorption of HCl X"(Q![RM ])/Q. Experimental data of this work is shown by bold lines.
5. Results and discussion 5.1. Ewect of resin basicity For resins with very high basicity, it is naturally expected that there would be near total sorption of acid on the surface corresponding to the theoretical capacity of the resin. As mentioned earlier, this is then re#ected in the rectangular form of the isotherm. Thus, as a rule, the extent of reversibility increases with the decrease in the resin basicity and decrease in the acid concentration. Since, most of the ion exchange applications are for dilute solutions, the crucial parameter for sorption and selectivity would then be resin basicity. Although, some attempts have been made in the past to characterize resin basicity on the basis of polymer matrix and functionality using the apparent pK values (Adams et al., 1969; Garcia ? & King, 1989; Tung & King, 1994; Husson & King, 1999a,b), the problems in quantifying true resin basicity probably limited its applicability to commercial systems. Also, an approach of assigning an e!ective sorbent donor number involves use of the donor number of the most basic functional group of the sorbent. This approach warrants caution because the surrounding polymer matrix in#uences the basicity of the functional group and these e!ects are inadequately represented (Husson & King, 1998). Alternatively, we propose to obtain gradation in the basicity from the experimental equilibrium data on various resins. Figs. 6 and 7 compare the equilib-
Fig. 7. E!ect of resin basicity: Sorption of acetic acid X"(Q![RM ])/Q. Experimental data of this work is shown by bold lines.
rium data of this work (indicated by bold lines) and also using similar data from some of the earlier works (Kunin, 1958; Adams et al., 1969; Bhandari et al., 1992, 1996; Takatsuji & Yoshida, 1997; Husson & King, 1999a). In contrast to the earlier reports on the individual sorption equilibrium curves and isolated treatment of the same, it is now possible for us to have a clearer picture of sorption behaviour on di!erent weak base resins. On the basis of the experimental data on acid sorption, it is proposed to have weak base resins grouped in the following three categories:
V. M. Bhandari et al. / Chemical Engineering Science 55 (2000) 6197}6208
6203
1. Resins with high basicity. 2. Resins with medium basicity. 3. Resins with weak basicity. It must be emphasized here that the above classi"cation is only qualitative, based on the conceptual representation of Fig. 1. Thus, Amberlite IRA-68 and Diaion WA10 belong to the class of high basicity resin. Resins: Diaion WA-20, Diaion WA-30, Dowex MWA-1 and Amberlite IRA-93 have medium basicity, whereas Dowex WGR-2 and Amberlite IR-4B show comparatively weak basicity. Further, this analysis reveals that, resins with high basicity are more useful for the removal of acids from aqueous solutions, as it gives high sorption capacity utilization even at low acid concentrations. However, weak acids such as acetic acid shows signi"cant reversibility even on resins having high basicity, implying acid type as an important parameter in the sorption process. However, the trends in the overall basicity structure of resins essentially remain unaltered even for weak acids. Thus, prior knowledge of resin basicity would help in the selection of resin for acid removal applications. Another observation evident from Fig. 6 and Table 1 is more interesting and reveals the possibility of correlating the resin type and basicity. It was observed that, polyacrylic resins with polyfunctionality comprising secondary and tertiary nitrogen exhibit high basicity. Resins with polystyrene matrix: both monofunctional secondary/tertiary nitrogen and polyfunctional with secondary and tertiary nitrogen, show medium basicity, whereas the matrix of epoxyamine and phenol}formaldehyde implies weak basicity. It can, therefore, be concluded that the polymer matrix of the resin has signi"cant e!ect on sorption characteristics. As mentioned earlier, the salt dissociation is assumed to be complete in all the previous works. In order to reexamine this assumption, an appropriate value of the resin}salt dissociation constant needs to be obtained. It was found that value of K above 10 kmol/m validates B the above assumption. Using this value of K , the value B of protonation equilibrium constant, K was regressed N using the experimental data on resin capacity utilization. A typical "t of the model to the experimental data of this
Fig. 8. Fit of model to the experimental data (Sorption of HCl; K "1;10 kmol/m). B
work, on sorption of HCl on Diaion resins, is shown in Fig. 8. It can be seen that the "t is good in all the cases. The computational results showed three types of sorption equilibrium behaviour. 5.1.1. Systems with constant K N The equilibrium data in this case could be "tted with good accuracy using a practically constant value of the protonation equilibrium constant in a wide range of acid concentration. The regressed values of protonation equilibrium constant with the assumption of complete salt dissociation and using experimental data of this work and also that reported by Adams and co-workers (1969) are listed in Table 2. While the values in Table 2 represent the best "t using the experimental data for which maximum error is less than 15% in most cases, the actual variation of K in the experimental sorption data is N shown in Fig. 9. It can be seen that the variation is less. Two important observations can be made from the results of Table 2: First, the regressed values of K are far N lower than those reported by Adams and coworkers. Second, for commercial weak base resins, the value of protonation equilibrium constant is of the order of
Table 2 K values: theoretical vs. experimental (constant K ) N N Resin
Acid
K (kmol/m)\, experimental N (Adams et al., 1969)
K (kmol/m)\, regressed N
Diaion WA-10 Amberlite IRA-68 Amberlite IRA-68 IP-MetoHA IP-DMA DVB-2DMA IP-DetoHA DVB-2VP
HCl HCl HNO HCl HCl HCl HCl HCl
* * * 2.0;10 1.0;10 1.0;10 3.1;10 7.9;10
2.5;10 5.6;10 2.2;10 7.3;10 2.6;10 1.4;10 3.9;10 1.9;10
6204
V. M. Bhandari et al. / Chemical Engineering Science 55 (2000) 6197}6208
Fig. 11. Variation of K in medium/weak basicity resins * Sorption of N HCl.
Fig. 9. Variation of K in high basicity resins. N
Fig. 10. Simulation of sorption equilibrium pro"le * E!ect of K . N
10}10 (kmol/m)\ similar to that suggested by Hel!erich and Hwang (1985), but only for resins with high basicity. The e!ect of protonation equilibrium constant on the sorption behaviour is theoretically shown in Fig. 10. Larger the value of K , greater is the sorption N capacity attainable. 5.1.2. Systems with K variation N The model could not be "tted using a single constant value of K to most systems exhibiting strong deviation N from the rectangular form of isotherm, thus signi"cant reversibility. Here, large variation in the values of K was N
Fig. 12. Variation of K * Sorption of acetic acid. N
observed with acid concentration. These results are shown in Figs. 11 and 12 for the sorption of HCl and acetic acid, respectively. Such a large variation in the protonation equilibrium constant cannot be explained on the basis of activity coe$cient alone. The variation in the values of K for weak acid, acetic acid is considerably N less as compared to that in HCl. 5.1.3. Systems indicating incomplete dissociation of resin salt An interesting observation was made in the case of sorption of acetic acid on Diaion WA-10. The resin is
V. M. Bhandari et al. / Chemical Engineering Science 55 (2000) 6197}6208
from the class of high basicity. Here, the sorption data could be "tted only using the value of protonation equilibrium constant of 10 (kmol/m)\ as obtained previously for HCl sorption, but using K value of B approximately 1.4;10\ for H> concentrations below 1;10\ kmol/m. The results, here, indicate possibility of incomplete dissociation of the resin salt. The sorption capacity of this resin above this concentration level was higher than the theoretical capacity of the resin, similar to the observations made by Husson and King (1999a) and, hence, no further evaluation was possible using the present treatment. Although sorption capacities higher than that of the theoretical capacity of resin have been reported in some cases, especially for weak acids, this anomalous sorption behaviour is not explained satisfactorily so far. Husson and King (1999a) attributed it to higher level of adsorption as against monolayer sorption and then correlated the sorption data using modi"ed Langmuir isotherm equation. The theoretical treatment of this work has potential to explain higher values of sorption on the basis of double layer. The results of simulation using di!erent values of K and K are shown in Fig. 13. It is evident N B that higher sorption capacity utilization is easy to achieve on the resins of high basicity (high K ) and with N lower extent of salt dissociation (lower K ). This implies B a very compact double layer adjacent to the resin surface wherein anions of the acid are held very strongly. Here, the short-range electrostatic forces of attraction are likely to be capable of further promoting adsorption of acid similar to multi-layer adsorption on surfaces. Consequently, sorption capacity in excess of theoretical resin capacity can be realized for these resins. This is in contrast to complete resin salt dissociation, where anions of acid are loosely attached to the surface and are distributed over the cross-section of resin pore as in the di!used layer. The argument is further supported by the simulation results indicating lower sensitivity to variation in resin salt dissociation for resins having very weak basicity (K &10), especially at low acid concentrations. N 5.2. Resin basicity and selectivity From the above analysis, it is expected that the resins with high basicity, although suitable for the acid removal applications, may not o!er high selectivity required for acid separations, unless the salt dissociation constant for two acids is substantially di!erent. To evaluate this aspect, sorption behaviour of hydrochloric acid and nitric acid on three di!erent resins with high, medium and low basicity was analyzed. The results indicated that for weak base resin, Dowex WGR-2, experimentally obtained value of selectivity was approximately 1.1, practically constant over the range of acid concentration. The selectivity values obtained with mixtures of nitric acid and hydrochloric acid (concentration ratio of the two acids in
6205
Fig. 13. Simulation of sorption equilibrium pro"le * Sensitivity to resin}salt dissociation.
solution approximately 1) are also compared (Fig. 14) with those predicted using single component sorption data. Fig. 14, within the narrow range of acid concentration studied in this work, indicates possibility of using single component data for predicting selectivity behaviour in mixtures. Similar selectivity analysis using sorption data on medium and strong basicity resins (Amberlite IRA-93 and Amberlite IRA-68), also indicated values close to unity. This is in agreement with the arguments made in the earlier section and also with previous observations, wherein Tsitovich, Konovalova and Tsarichenko (1965) reported lower selectivity values with anion exchange resins and suggested that the separation may be di$cult using these resins. Apart from resin basicity, selectivity is also dependent on the ratio of acid concentration. For a concentration ratio of near unity, the selectivity values are very close to unity for most resins. The separation is better for higher concentration ratios, [HNO ]/[HCl]. Since the exact values of K , K and K are not N B B known, the established experimental trends were checked
6206
V. M. Bhandari et al. / Chemical Engineering Science 55 (2000) 6197}6208
dilute aqueous stream has concentration of HCl with pH &4, then from Fig. 6 it is seen that the maximum capacity realized on Dowex WGR-2 is below 3 meq/g due to equilibrium fractional conversion of less than 0.3. However, employing high basicity resins such as Amberlite IRA-68 or Diaion WA-10, the acid removal capacity can be above 4.5 meq/g, even though its maximum capacity is just 5.7 meq/g as against 9.6 meq/g of Dowex WGR-2. It can be recommended that the resins with high basicity should be selected for the applications involving removal of acids from dilute solutions. Apart from signi"cant pH dependence, the basicity e!ect also appears to be function of acid type and is less pronounced for weak acids. Thus, extent to which basicity can in#uence removal of acids is an important consideration in the selection of resin. This has implications in the wastewater treatment problems involving very weak acids such as phenol, cresols, etc. where, the selection of weak base resin can be on the basis of its capacity and basicity, both. Fig. 14. Separation from acid mixtures * Separation of HNO and HCl.
6. Conclusions using the model and arbitrary values of these constants for a mixture of acids HA and HB. As expected, the ratio [AM ] /[A] increases with increase in K and decrease in Q B K , indicating stronger adsorption of anion A in double B layer as compared to anion `Ba. It was also found that, for obtaining same capacity utilization, the salt dissociation constant for weaker acid is less than that for stronger acid. This result agrees with our observation on the sorption of HCl and acetic acid}both theoretical and experimental. Further, for the same values of K and B K , increase in the ratio [A]/[B] (either by changing B acid dissociation constant or by changing acid concentration) improves selectivity. Thus, the model qualitatively explains the experimentally observed sorption behaviour. 5.3. Resin basicity and acid removal from aqueous solution Weak base resins are most commonly employed for the removal of acids from aqueous streams. In these applications, obtaining high sorption capacities for acid removal is most desirable. The equilibrium dictates limiting capacity of the resin for design of industrial operations. The prediction of equilibria is still a di$cult task in most cases and such data are normally obtained by carrying out laborious experiments in the laboratory, since no clear directions exist for the selection of resin. From the results of this work, it is evident that the resin basicity has a large e!ect on the sorption capacity. Higher the basicity, better is the equilibrium capacity utilization. Thus, it may not be just su$cient to select a weak base resin for the removal of acids on the basis of its high theoretical capacity for sorption. For example, if the
The important conclusions of the present study are 1. Resin basicity has signi"cant impact on sorption equilibria. 2. A qualitative analysis using the experimental data on sorption equilibria can lead to clearer understanding of the resin basicity e!ect. 3. The weak base resins, in general, can be grouped into three di!erent categories on the basis of experimental observation of sorption equilibria- Resins with high basicity, medium basicity and weak basicity, each corresponding to a speci"c form of sorption isotherm. 4. Resins with high basicity are most suitable for the acid removal applications from aqueous solutions since they yield high resin capacity utilization even at low acid concentrations. 5. A theoretical treatment which accounts for the dissociation equilibria for both, resin and solution species, can satisfactorily explain the sorption behaviour on weak base resins. 6. The results of this work indicate that the conventional approach of de"ning basicity using apparent pK @ values of resin has limitations and the high values of protonation equilibrium constant (&10}10) are found to be appropriate only to resins having high basicity. 7. Substantial variation in the protonation equilibrium constant has been observed in some cases and even incomplete resin salt dissociation is indicated. Although the present study highlights di!erences in the sorption behaviour of various acids on di!erent weak base resins and implications of the same, further
V. M. Bhandari et al. / Chemical Engineering Science 55 (2000) 6197}6208
experimental and theoretical work is required to substantiate some of the "nding of this work.
Notation A B H HA HB K K K B K B K N Q R RH RHA RHB X
anion of acid HA anion of acid HB H> species acid with anion A acid with anion B equilibrium constant for acid HA, kmol/m equilibrium constant for acid HB, kmol/m dissociation constant for resin salt, RH>A\, kmol/m dissociation constant for resin salt, RH>B\, kmol/m protonation equilibrium constant, (kmol/m)\ resin capacity, kmol/m free base group of resin protonated species of resin resin salt of acid HA resin salt of acid HB equilibrium fractional conversion of resin ((Q![RM ])/Q)
Greek letters a
separation factor or selectivity
Subscript and superscripts c s *
concentration in core region of resin pore. concentration in the wall region bounded by IHP of the double layer pore phase
References Adams, G., Jones, P. M., & Miller, J. R. (1969). Kinetics of acid uptake by weak base anion exchangers. Journal of the Chemical Society, section A, 2543. Bhandari, V. M. (1998). Implications of weak Donnan potential in ion exchange reactions: An alternate strategy for modeling sorption process. Separation Science and Technology, 33, 2009. Bhandari, V. M., Juvekar, V. A., & Patwardhan, S. R. (1992a). Sorption studies on ion exchange resins. I. Sorption of strong acids on weak base resins. Industrial and Engineering; Chemistry Research, 31, 1060. Bhandari, V. M., Juvekar, V. A., & Patwardhan, S. R. (1992b). Sorption studies on ion exchange resins. II. Sorption of weak acids on weak base resins. Industrial and Engineering; Chemistry Research, 31, 1073. Bhandari, V. M., Juvekar, V. A., & Patwardhan, S. R. (1993). Sorption of dibasic acids on weak base resins. Industrial and Engineering Chemistry Research, 32, 200. Bhandari, V. M., Juvekar, V. A., & Patwardhan, S. R. (1997). Ion exchange studies in the removal of polybasic acids * anomalous sorption behaviour of phosphoric acid on weak base resins. Separation Science and Technology, 32, 2481.
6207
Bhandari, V. M., Sawarkar, C. S., & Juvekar, V. A. (1996). Ion Exchange Separations}Selectivity in acid sorption. In Varma et al. (Ed.), Advances in chemical engineering * Proceedings of international conference (P. 81) Allied Publishers, Madras. Cloete, F. L. D., & Marais, A. P. (1996). Recovery of very dilute acetic acid using ion exchange. Industrial and Engineering Chemistry Research, 34, 2464. Garcia, A. A., & King, C. J. (1989). The use of basic polymeric sorbents for the recovery of acetic acid from dilute aqueous solution. Industrial and Engineering Chemistry Research, 28, 204. Hel!erich, F. (1962). Ion exchange. New York: McGraw-Hill. Hel!erich, F.G., & Hwang, Y.L. (1985). Kinetics of acid uptake by weak base anion exchangers. Mechanism of proton transfer. A.I.Ch.E. Symposium Series, No. 242, vol. 81, (p. 17). Helsel, R. W. (1977). Waste recovery: Removing carboxylic acids from aqueous wastes. Chemical Engineering Progress, 73, 55. Holl, W., & Sontheimer, H. (1977). Ion exchange kinetics of the protonation of weak acid ion exchange resins. Chemical Engineering Science, 32, 755. Hubner, P., & Kadlec, V. (1978). Kinetic behaviour of weak base anion exchangers. A.I.Ch.E. Journal, 24, 149. Husson, S. M., & King, C. J. (1998). Regeneration of lactic and succinic acid-laden basic sorbents by leaching with a volatile base in an organic solvent. Industrial and Engineering Chemistry Research, 37, 2996. Husson, S. M., & King, C. J. (1999a). Multiple acid equilibria in adsorption of carboxylic acids from dilute aqueous solution. Industrial and Engineering Chemistry Research, 38, 502. Husson, S. M., & King, C. J. (1999b). Carbon dioxide sustained adsorption of lactic acid at pH ' pK of the acid. Industrial and Engineer? ing Chemistry Research, 38, 1625. Juang, R. S., & Chou, T. -C. (1996). Sorption of citric acid from aqueous solutions by macroporous resins containing a tertiary amine equilibria. Separation Science and Technology, 31, 1409. Katzen, R., Aries, R. S., & Othmer, D. F. (1945). Wood hydrolysis: Utilization of waste liquors. Industrial and Engineering Chemistry, 37, 442. Kawabata, N., Yoshida, Jun-ichi., & Tanigawa, Y. (1981). Removal and recovery of organic pollutants from aquatic environment. 4. Separation of carboxylic acids from aqueous solution using crosslinked poly(4-vinylpyridine). Industrial and Engineering Chemistry, Product Research and Development, 20, 386. Kunin, R. (1958). Ion exchange resins. London: Wiley. Matsarenko, V. A., Lebedinskaya, N. A., Dudkova, L. I., & Gushchina, L. I. (1969). Recovery of water soluble C}C acids from acid waste waters of the production of synthetic fats. Khimichiskaya Promyshlennost, 45, 12. Othmer, D. F. (1958). Acetic acid recovery methods. Chemical Engineering Progress, 54, 48. Parulekar, S. J., Sharma, M. M., Joshi, J. B., & Shah, Y. T. (1982). Separation processes for recovery of valuable chemicals from aqueous e%uents. Journal of Separation Process and Technology, 3, 2. Rao, M. G., & Gupta, A. K. (1982a). Kinetics of ion exchange in weak base anion exchange resins. A.I.Ch.E. Symposium Series No. 219, 78, 96. Rao, M. G., & Gupta, A. K. (1982b). Ion exchange processes accompanied by ionic reactions. Chemical Engineering Journal, 24, 181. Ricker, N. L., Pittman, E. F., & King, C. J. (1980). Solvent extraction with amines for recovery of acetic acid from dilute aqueous industrial streams. Journal of Separation Process and Technology, 1, 23. Saunders, M. S., Vierow, J. S., & Carta, G. (1989). Uptake of phenylalanine and tyrosine by a strong acid cation exchanger. AIChE J., 35, 53. Shacham, M. (1986). Numerical evaluation of constrained non-linear algebraic equations. International Journal for Numerical Methods in Engineering, 23, 1455.
6208
V. M. Bhandari et al. / Chemical Engineering Science 55 (2000) 6197}6208
Takatsuji, W., & Yoshida, H. (1994). Removal of organic acids from wine by adsorption on weakly basic ion exchangers: Equilibria for single and binary systems. Separation Science and Technology, 29, 1473. Takatsuji, W., & Yoshida, H. (1997). Adsorption of organic acids on weakly basic ion exchanger: equilibria. Journal of Chemical Engineering Japan, 30, 396. Takatsuji, W., & Yoshida, H. (1998a). Adsorption of organic acids on polyaminated highly porous chitosan: Equilibria. Industrial and Engineering Chemistry Research, 37, 1300. Takatsuji, W., & Yoshida, H. (1998b). Adsorption of organic acids on weakly basic ion exchanger: Equilibria for binary systems. A.I.Ch.E. Journal, 44, 1216.
Tsitovich, I. K., Konovalova, E. A., & Tsarichenko, B. F. (1965). Salt forms of cation exchange resins and separation of organic acids. Khim.i Khim. Tekhnol., 8, 60. Tung, L. A., & King, C. J. (1994). Sorption and extraction of lactic and succinic acids at pH ' p K . Factors governing equilibria. Indus? trial and Engineering Chemistry Research, 33, 3217. Wadekar, V. V., & Sharma, M. M. (1981). Dissociation extraction. Journal of Separation Process and Technology, 2, 1. Yoshida, H., Kishimoto, N., & Kataoka, T. (1994). Adsorption of strong acid on polyaminated highly porous chitosan: Equilibria. Industrial and Engineering Chemistry Research, 33, 854.