3902
Ind. Eng. Chem. Res. 2006, 45, 3902-3915
Comparative Study of Modified Simulated Moving Bed Systems at Optimal Conditions for the Separation of Ternary Mixtures under Nonideal Conditions Anjushri S. Kurup, K. Hidajat, and Ajay K. Ray* Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, 4 Engineering DriVe 4, Singapore 117576
Separation of a ternary mixture based on the simulated moving bed (SMB) technology was studied. The conventional four-zone SMB system was modified into two different configurations in order to facilitate the simultaneous collection of all three components. The performances of the two modified systems were compared at optimal conditions for varying adsorption selectivity, mass-transfer resistance, and nonlinearity in adsorptionisotherm parameters. Comparative studies were made at optimal conditions in each case based on rigorous multiobjective optimization using elitist nondominated sorting genetic algorithm with jumping genes as the optimization technique. Introduction Simulated moving bed systems1 (SMB), designed as a practical realization of a true moving bed (TMB) where the solid and fluid phases move countercurrently, are used widely for large-scale chromatographic separations. This technology has been implemented successfully as a separation technique in the petrochemical, biochemical, and fine chemical industries. The SMB is a well-established separation technology, particularly for a difficult (or nearly impossible) separation of binary mixtures. The four-zone SMB for binary separation (see Figure 1) has been extensively studied by many groups and is now well-understood.2-6 However, the use of SMB systems for multicomponent separations has not been studied extensively.7-9 The major limitation of SMB is the inability to purify a ternary mixture into three different pure fractions in a single unit. As a consequence, several concepts have been proposed to achieve this goal through various modifications keeping the advantages of SMB. Hashimoto et al.3 employed a four-zone conventional SMB, but packed with two different adsorbents, while Kearney and Hieb10 used variation of the working flow rates with respect to time within a given switching period. In the concept described by Hashimoto et al.,3 a cation-exchange resin and an ionretardation resin were used to separate starch, glucose, and NaCl. Nicoud11 developed an SMB concept consisting of five zones by adding a side stream for the collection of the component with an intermediate adsorption affinity. Nicoud12 also described extending the concept of five zones by additionally changing the desorbent strength in the five zones. Another developed concept is based on a novel operation technique of SMB wherein the feed is discontinuously added only during a part of the total cycle time while operating in the batch chromatographic mode. For the remaining cycle time ,the system is switched to the SMB mode of operation with no feed. This process is commercialized by the Japan Organo Company. Mata and Rodriguez13 developed a pseudo-SMB model for the above-mentioned process. Wooley et al.14 proposed a nine-zone system for the separation of two * Corresponding author’s present address: Department of Chemical and Biochemical Engineering, University of Western Ontario, London ON N6A 5B9, Canada. Tel.: +1 (519) 661-2111 ext 81279. Fax: + 1 (519) 661-3498. E-mail:
[email protected].
Figure 1. Schematic diagram of the simulated moving bed system.
sugars, glucose and xylose, from biomass hydrolyzate. Wankat15 developed seven cascades of SMB system for ternary separation based on linear isotherms and established minimum desorbent usage and productivity based on equilibrium model. Kim et al.16 developed three cases of single cascades, similar to the approach described by Nicoud11 and Beste and Arlt,17 and determined (using equilibrium theory) favorable conditions to achieve good separations for these designs. Kim et al.16 simulated the system using Aspen Chromatography and studied the effect of different feed compositions and the mass-transfer rates. However, they only considered linear isotherms. In this study, we extend the work of Kim et al.16 in the presence of several nonidealities such as high mass-transfer resistance, lower adsorption selectivity, nonlinearity in adsorption isotherm, etc. Besides, we have compared the performances of the systems at optimal conditions. Furthermore, the optimization studies were carried out to satisfy multiple objectives.18-20 There are several SMB systems in the open literature for ternary separations;7-9,13-17 however, to the best of our knowledge, there does not seem to be any detailed study to see their behavior in the presence of nonideal conditions, which are more often found in industries. Also, these systems, because of the addition of
10.1021/ie050452q CCC: $33.50 © 2006 American Chemical Society Published on Web 04/29/2006
Ind. Eng. Chem. Res., Vol. 45, No. 11, 2006 3903
more sections and/or product streams (compared to a conventional SMB used for binary separation), tend to be more complicated and have more design variables. As a result, the design and, consequently, the optimization, particularly multiobjective optimization, of these systems becomes rather difficult. It is not necessary to emphasize the importance of optimization of the system at the design stage.19 In such a scenario, nontraditional optimization techniques such as the evolutionary algorithms20 prove to be very useful. In this study, we used an adaptation of genetic algorithm, elitist nondominated sorting genetic algorithm with jumping genes (NSGA-II-JG)21,22 as the optimization algorithm, which has the capability of finding a Pareto-optimal set in a single run. NSGA has been successfully applied for binary separation23-25 in SMB systems as well as reactive SMB26-29 systems. Conventional and Modified SMB Systems The general concept of a classical four-zone SMB unit (illustrated in Figure 1) for binary separation consists of a number of columns of uniform cross section connected in a circular array, each of length Lcol and packed with adsorbent. There are two incoming streams: the feed mixture (F) to be separated and the desorbent (D). Two streams leave the unit, one enriched with the less-adsorbable component, raffinate (Ra), and one enriched with the more-adsorbable component, extract (Ex). The four streams divide the unit into four sections (P, Q, R, and S). It should be noted that the sections are named as P, Q, R, and S (as in our earlier reported work23-29) in contrast to sections I-IV used by other research groups.2,5 Each zone contains at least one fixed column (bed) and has to fulfill distinct tasks, i.e., in sections Q and R, countercurrent separation takes place, while in section P and S, the solid and the fluid phases are regenerated, respectively. The movement of the solid bed is simulated by switching of ports (or columns) in a specific time interval, ts. The separation is achieved by a simulated countercurrent contact between the mobile fluid phase and the stationary adsorbent phase. There are many advantages1,2 of the SMB technology compared to the classical preparative chromatography, namely, overcoming problems associated with solid handling, efficient utilization of adsorbent, continuous mode of operation, lowering solvent consumption, reducing downtime (as separation and regeneration take place concurrently), possibility of scaling up to a large-scale production unit due to simplicity in the mechanical design, etc. To achieve separation between the components, the internal flow rates of the fluid phases within the four sections, and the switching time (which defines the hypothetical solid-phase velocity), have to be specified appropriately. Petroulas et al.30 defined for a true countercurrent moving bed system a parameter, σi, called the relative carrying capacity of the solid relative to the fluid stream for any component i as
σi ) [(1 - )/]qmKi(us/ug) ) δi(us/ug)
(1)
They showed that, to achieve countercurrent separation between two components, one must set σ >1 for one of the components and 0 (species move with the fluid phase), and when σi > 1, Vi < 0 (species move with the solid phase). When σ ) 0, it represents a fixed bed. Ray et al.32 redefined the above parameter, σ, by replacing the solid-phase velocity, us, by a hypothetical solid-phase velocity, ζ, defined as ζ ) Lcol/ts. They found, both theoretically33 and experimentally,34 that simulation of the countercurrent movement between two components can be achieved when redefined σ's were set such that they are >1 for one and
