Chromatographic Reactors with Reactive Desorbents - Industrial

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Chromatographic Reactors with Reactive Desorbents Davino Gelosa* and Andrea Sliepcevich Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta”, Politecnico di Milano, Via Mancinelli 7, 20131 Milano, Italy

Massimo Morbidelli† Swiss Federal Institute of Technology Zurich, Institut fuer Chemie und Bioingenieurwissenschaften, ETH-Hoenggerberg/HCI, CH-8093 Zurich, Switzerland

The possibility of introducing, as a desorbent in a chromatographic reactor, a reactive desorbent that removes the most strongly retained component by chemical reaction and not by purely physical desorption has been investigated. The principle is demonstrated in the case of the esterification of glycerol with acetic acid using a mixture of acetic acid and acetic anhydride as the desorbent and a polymeric acidic resin as the stationary phase. It is shown that reacting the most strongly retained component, water, with acetic anhydride allows for a significant improvement in the column regeneration process, thus improving the process performance in terms of both productivity and desorbent requirements. Introduction The possibility of conducting a chemical reaction together with the separation of the corresponding products is an attractive alternative in many processes. The advantage is not only in the reduction of expensive units, but also in the possibility of forcing the reaction beyond the limits imposed by chemical equilibrium. The difficulty is that, in this case, the reaction and separation have to be conducted at the same operating conditions, which, in general, are not the optimal conditions for both processes. Reactive distillation is probably the most common of such processes, but, particularly in the case of thermolabile chemical species, reactive chromatography can be the method of choice. Particularly convenient for applications are those cases where the stationary phase can act as both a catalyst of the reaction and a selective adsorbent for the products. Typical examples are the ion-exchange resins, which, in their acidic or basic forms, have catalytic properties with respect to various reactions. In addition, they exhibit significant adsorptive capacities with a specific selectivity toward polar as opposed to apolar species. Such properties can be tuned, to a certain extent, by changing the type and concentration of the functional groups on the resin, such as sulfonic, carboxylic, or others.1 The most efficient way of conducting a chromatographic reactive process with respect to productivity and desorbent requirements is to operate a continuous countercurrent unit. In practice, this is conveniently done using simulated moving bed reactor (SMBR). This technology has been applied over the years in a number of purely separative processes, such as those related to mixtures of xylene isomers; various fractions of sugars or other natural products; and, most recently, racemic mixtures, particularly in the pharmaceutical sector. The various applications and fundamental aspects related to SMBRs have recently been reviewed by Lode et al.2 The major component in the economy of this technology is the amount of desorbent utilized. Chromatography is, in fact, known to require * To whom correspondence should be addressed. E-mail: [email protected]. † Current address: Institute for Chemical and Bioengineering, ETH, CH-8093 Zurich, Switzerland.

the addition of significant quantities of desorbent, which eventually leads to quite diluted products. Because the desorbent eventually has to be separated from the products in order to be recycled to the SMBR, this contributes significantly to the total process cost. In addition, it is worth noting that, to avoid the introduction of a new species in the process, the tendency is to use one of the reactants as the desorbent. This has the additional advantage that this reactant is then present in excess in the unit, so that the other becomes limiting and is eliminated from the system simply by complete conversion. This approach, however, has the consequence that the choice of the desorbent is limited and its properties cannot really be optimized. Typical examples are esterification reactions in which the production of an ester and water is conducted on acidic ionexchange resins.3-5 The acid or the alcohol is typically used as desorbent, and the separation between water and the ester is relatively simple because of the large difference in polarity between the two. The problem is the regeneration of the resin, which requires the desorbent to desorb water. Because water is typically much more strongly retained than any of the reactants, this leads to the utilization of large amounts of desorbent, which are then responsible for the high costs of the process. Here is where the tuning of the properties of the resin becomes important. For example, Stroehlein et al.1 have shown that, in the case of the production of methyl acetate on sulfonic resins, the selectivity between methanol and water can be decreased by decreasing the concentration of sulfonic groups on the resin. This obviously also leads to lower ester/water selectivities and to lower catalytic activities, thus making the optimization problem rather complex. A particularly interesting process of this type is the esterification of glycerol to produce triacetine. This involves three esterification reactions, each with the production of one water molecule, and it is characterized by very small equilibrium conversions, which make the possibility of conducting this reaction together with product separation particularly attractive. The application of reactive chromatography to this system using acidic ion-exchange resins and acetic acid as the desorbent was investigated by Gelosa et al.6 It was demonstrated that reactive chromatography can produce triacetine with the required foodgrade purity in a single step, thus avoiding the multiple

10.1021/ie051069o CCC: $33.50 © 2006 American Chemical Society Published on Web 04/28/2006

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purification steps required by the current technology based on homogeneous catalysis with sulfuric acid. In addition to the observations reported above for esterification reactions in general, it has been found6 that the water content of acetic acid used to regenerate the column has a profound effect on the performance of the unit. In particular, the water left in the resin after regeneration reacts in breakthrough experiments with the purified triacetine to produce diacetine, thus reducing the efficiency of the process. The proposed solution, based on the use of anhydrous acetic acid, was shown to be effective but has a strongly negative impact on the economy of the process. In this work, we propose an alternative solution to this problem: the introduction of a reactive desorbent that more efficiently regenerates the column by eliminating the adsorbed water by chemical reaction.7 This procedure, which has a relatively general validity, is discussed herein with reference to the triacetine production process.

Figure 1. Experimental mass fractions at the outlet of a chromatographic reactor as a function of time for a reaction step starting with the column regenerated using acetic acid with a water content of 0.75 wt %: (b) acetic acid, (×) triacetine, (2) diacetine, (9) monoacetine, (/) water, ([) glycerol.

Experimental System The considered reaction system has the following stoichiometry

glycerol + acetic acid / monoacetine + water

(1)

monoacetine + acetic acid / diacetine + water

(2)

diacetine + acetic acid / triacetine + water

(3)

The experimental setup described by Gelosa et al.,6 consisting of a laboratory chromatographic reactor operating at ambient pressure, was used in the experimental investigation. The jacket glass column was 44 cm long, with an internal diameter of 1.5 cm, and it was packed with Amberlyst 15 resin, with an internal void fraction of 0.36 and an interparticle void fraction of 0.42. All experiments were conducted at 80 °C. In the breakthrough experiments, an acetic acid/glycerol feed (with a 4.5 molar ratio) was fed from the bottom of the column at a flow rate of 0.3 mL/ min. The initial part of the column, packed with inert material, was used to preheat the feed stream at the operating temperature. After each breakthrough experiment, the column was regenerated using pure desorbent, at a flow rate of 0.5 mL/ min, until the same composition entering the column was measured in the outlet. The outlet composition was monitored as a function of time by titrating acetic acid with NaOH, as well as by measuring the concentration of water by the Karl Fischer technique and the concentrations of all remaining components, i.e., glycerol, monoacetine, diacetine, and triacetine, by gas chromatography. Breakthrough Experiments Three experimental breakthrough runs with different desorbents were considered. In the first, acetic acid with a water content of 0.75 wt % was used to regenerate the column before the breakthrough experiment. The weight fractions of the various components in the column outlet are shown in Figure 1 as a function of time. As the mixture of glycerol and acetic acid moves along the column, esterification reactions 1-3 take place. The less adsorbable components, i.e., triacetine, diacetine, and monoacetine (in that order), move more rapidly along the column and break through at the outlet in the same order. Water is the most retained component and breaks through last, and glycerol is almost entirely consumed in the column. During process


operation, the column equilibrates with the reacted feed until adsorption equilibrium conditions are reached everywhere along the column. At this point, the unit reaches steady-state conditions, and the outlet composition essentially corresponds to reaction equilibrium conditions, because the residence time in the column is sufficiently long with respect to the characteristic times of the reactions. As noted earlier by Gelosa et al.,6 it can be seen that the peak of diacetine is strongly asymmetric, having a dispersed front that largely overlaps with the peak of the desired product, i.e., triacetine. This is due to the fact that, although the pure triacetine travels along the column ahead of all of the other components, it reacts back with the water left at the end of the regeneration step to produce diacetine, which therefore breaks through together with triacetine. The amount of diacetine is significant, as it is proportional to the water content of the acetic acid used to regenerate the column. It is worth stressing that this behavior is due not to the adsorption selectivity between diacetine and triacetine, but to the reverse of reaction 3 from triacetine to diacetine. To demonstrate this behavior in the second experiment, a drier acetic acid sample with a water content of 0.2% was used to regenerate the column, with all of the remaining operating conditions kept unchanged. The mass weight fractions in the outlet stream are shown as a function of time in Figure 2. It is seen that, in this case, the separation between diacetine and triacetine is much better. The amount of diacetine breaking through with triacetine is much reduced, and its breakthrough

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time is now much retarded with respect to that of triacetine, i.e., about 60 min instead of the ∼20 min value in Figure 1. This is clearly due to the much lower amount of water left in the resin at the end of the regeneration step preceding this breakthrough experiment. These results confirm the previous conclusion of Gelosa et al.6 that, in order to improve the process performance, highly dehydrated acetic acid should be used as the desorbent. This, however, has an obvious negative impact on the economy of the process. In the next experiment, a so-called reactive desorbent was used to regenerate the resin. In particular, an acetic acid stream containing 5 wt % of acetic anhydride has been used. The latter reacts with water to produce more acetic acid, which obviously does not affect the process in any respect, except that a corresponding purge of acetic acid has to be introduced before it is recycled back to the chromatographic column. It was seen that the amount of water in the outlet stream after regeneration was completed was less than 0.05 wt %. In the breakthrough experiment, a mixture of acetic acid, acetic anhydride, and glycerol (4.25/0.13/1 molar ratio) was used as the feed stream while all remaining operating conditions were kept unchanged with respect to the previous experiments. The corresponding mass weight fractions in the outlet stream are shown as a function of time in Figure 3. It is seen that the efficiency of the process is better than in all previous cases, with a period of time where only acetic acid and triacetine are collected at the column outlet before diacetine breaks through, which is now about 100 min. This confirms that the addition of the reactive desorbent (acetic anhydride) succeeds in keeping the resin anhydrous and then optimizing the process performance with respect to both its reactive and separating properties. Regeneration Experiments To further support the results of the analysis above, regeneration experiments were conducted. These were performed by feeding the pure desorbent stream to the column, equilibrated with the feed stream at the end of the breakthrough experiments described in the previous section. In Figure 4 are shown the weight fractions of all involved species as a function of the desorbent volume fed to the column. The desorbent in this case is acetic acid with 0.2 wt % water content. It is seen that, whereas triacetine, diacetine, and monoacetine are desorbed relatively rapidly from the column, water takes a much longer time. In particular, after 200 mL of

Figure 4. Experimental mass fractions at the outlet of a chromatographic reactor as a function of desorbent volume (acetic acid with 0.2 wt % water content) for a regeneration step following the reaction step shown in Figure 2: (b) acetic acid, (×) triacetine, (2) diacetine, (9) monoacetine, (/) water, ([) glycerol.

Figure 5. Experimental mass fractions at the outlet of a chromatographic reactor as a function of desorbent volume (acetic acid with 5% acetic anhydride) for a regeneration step following the reaction step shown in Figure 3: (b) acetic acid, (×) triacetine, (2) diacetine, (9) monoacetine, (/) water, ([) glycerol.

desorbent has been fed to the column, the water content in the outlet stream is still 1.2 wt %, and after 250 mL, the water content is equal to 0.8 wt %. This is still significantly larger than the 0.2 wt % in the desorbent fed to the column, which obviously represents the lower bound of the outlet water content. The corresponding results in the case in which an acetic acid stream with 5 wt % acetic anhydride was used to regenerate the column are shown in Figure 5. It is found that, whereas the kinetics of desorption of the three esters is essentially unchanged, that of water is now significantly faster. In particular, after 160 mL of desorbent has been fed to the column, the water content in the outlet is 0.5 wt %, and after 180 mL, the water content drops further to 0.02 wt %. This indicates that a much better regeneration of the resin was achieved with a substantially lower amount of desorbent. Concluding Remarks The possibility of operating a chromatographic reactor using a so-called reactive desorbent that desorbs the column by chemically reacting with the component to be removed has been investigated. This reaction is unrelated to the production process and is introduced solely to facilitate the regeneration process and then decrease the required amount of desorbent. This objective was achieved and demonstrated for the esterification reaction of glycerol with acetic acid on acidic

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polymeric resins. In this case, the main cost of the process lies in the regeneration of the resin from water using acetic acid as the desorbent. Because water is more strongly retained than acetic acid, this requires a large amount of desorbent. The addition of a small fraction of acetic anhydride to the desorbent stream significantly improves the process efficiency by increasing the productivity and reducing the desorbent requirements. One important aspect to be considered when introducing a reactive desorbent is its impact on the production costs. It is clear that the addition of a new chemical species to be handled in the process would have a rather negative impact in this respect. In the case discussed in this work, because acetic anhydride reacts with water to produce the desorbent itself, i.e., acetic acid, this problem does not arise. The only additional cost to be considered is related to the difference in price between acetic anhydride and acetic acid. It is clear that, when considering economic constraints, the possible application of reactive desorbents does not appear to be very general. However, acetic anhydride can be used as a reactive desorbent for the production of various acetates, which indeed represent a relatively wide class of products. Other applications could be identified for specific processes, keeping in mind that the most attractive situation is obtained when the reactive desorbent reacts with the most strongly retained component in the system to produce one of the reactants of the process.

Literature Cited (1) Stroehlein, G.; Lode, F.; Mazzotti, M.; Morbidelli, M. Design of stationary phase properties for optimal performance of reactive simulated moving bed chromatography. Chem. Eng. Sci. 2004, 59, 4951-4956. (2) Lode, F.; Houmard, M.; Migliorini, C.; Mazzotti, M.; Morbidelli, M. Continuous reactive chromatography. Chem. Eng. Sci. 2001, 56, 269291. (3) Sardin, M.; Villermax, J. Esterification catalysee par une resine echangeuse de cation dans un reacteur chromatographique. NouV. J. Chim. 1979, 3, 225-235. (4) Kawase, M.; Suzuki, T. B.; Inoue, K.; Yoshimoto, K.; Hashimoto, K. Increased esterification conversion by application of the simulated moving bed reactor. Chem. Eng. Sci. 1996, 51, 2971-2981. (5) Mazzotti, M.; Kruglov, A.; Neri, B.; Gelosa, D.; Morbidelli, M. A continuous chromatographic reactor: SMBR. Chem. Eng. Sci. 1996, 51, 1827-1836. (6) Gelosa, D.; Ramaioli, M.; Valente, G.; Morbidelli, M. Chromatographic reactors: Esterification of glycerol with acetic acid using acidic polymeric resins. Ind. Eng. Chem. Res. 2003, 42, 6536-6544. (7) Ruggieri, R.; Ranghino, G.; Carvoli, G.; Tricella, A.; Gelosa, D.; Morbidelli, M. Process for esterification in a chromatographic reactor. U.S. Patent 6,586,609, 2003.

ReceiVed for reView September 23, 2005 ReVised manuscript receiVed February 15, 2006 Accepted March 29, 2006 IE051069O

Chromatographic Reactors with Reactive Desorbents - Industrial

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