Ind. Eng. Chem. Res. 2006, 45, 2017-2025
2017
Recovery of Acetic Acid from Aqueous Solutions by Reactive Distillation Ajay Singh, Anand Tiwari, Sanjay M. Mahajani,* and Ravindra D. Gudi Department of Chemical Engineering, Indian Institute of TechnologysBombay, Powai, Mumbai 400 076, India
The recovery of acetic acid by reactive distillation through esterification with methanol in the presence of ion-exchange resin as a catalyst has been investigated in detail. A kinetic expression that is valid under the conditions that prevail in a continuous reactive distillation column has been developed. The insensitivity of reaction rate to catalyst loading under certain conditions has been addressed. The recovery as high as 80% was realized experimentally for the feed concentration of 30% (w/w) in a laboratory column. Various alternative scenarios such as effect of reboiler duty, feed flow rate, and feed composition were covered in the experiments. The equilibrium stage model for reactive distillation predicts the performance well in agreement with the experimental results. The effect of changing various design and operating parameters, over a wide range, was also studied through simulation. The model predicts close to quantitative recovery (∼100%) with a properly designed column. Some feasible alternative configurations for a complete recovery system have been suggested. Introduction Reactive distillation (RD), a combination of reaction and distillation in a single vessel, is receiving increasing attention because of its high potential for process intensification. It is applicable to certain reactions in which the maximum conversion is limited by chemical equilibrium in conventional reactors. It offers various advantages over the conventional approach of reactor followed by separation. Improved selectivity, increased conversion, better temperature control, effective utilization of reaction heat, scope for difficult separations, and the avoidance of azeotropes are some of the benefits realized with the use of RD.1-3 The aqueous solution of acetic acid is realized as a waste stream in many important processes, such as that for the manufacture of cellulose esters, terephthalic acid, and dimethyl terephthalate, destructive distillation of wood, reactions involving acetic anhydride, and so forth.4 The process for the manufacture of cellulose acetate is typically associated with a 35% (w/w) aqueous solution of acetic acid as a waste stream, and the terephthalic acid process involves the concentrations as high as 65% (w/w) of acetic acid in water. The synthesis of glyoxal from acetaldehyde and nitric acid has a relatively dilute acetic acid stream, typically 13-20% (w/w) as a byproduct. The wood distillate contains much lower concentrations (18%, w/w) of acetic acid. Thus, the recovery of acetic acid from these streams is a major problem in both petrochemical as well as fine chemical industries. Separating acetic acid from the aqueous solution by normal distillation is a difficult and expensive task because of the low relative volatility and requirement of high latent heat for water to be separated as the overhead product in the column. RD is a potentially important method of separation for the recovery of acetic acid. The recovery of acetic acid by esterification with methanol produces methyl acetate (eq 1). It can be sold as a useful product, or if
CH3COOH + CH3OH S CH3COOCH3 + H2O
(1)
necessary, it may be hydrolyzed back to pure acetic acid. Hence, a complete recovery system would have subsequent processing steps depending on the desired objectives. In any case, esteri* To whom correspondence should be addressed. Tel.: +91-2225767246. Fax: +91-22-25726895. E-mail:
[email protected].
fication with RD is the most important operation in this process, and the present work is aimed at investigating the same in detail. The objective of the present work is to evaluate the applicability of RD for the recovery of acetic acid solution in the range 5-30%, w/w, by reacting it with methanol in a RD column, through experiments and simulation. The performance of a RD unit is strongly influenced by the nature of the reaction involved. The reaction falls in either the category of very fast reactions, wherein the kinetics does not play an important role, or the category of relatively slow reactions. In the latter case, an appropriate kinetic model, based on the kinetic data generated in the composition region associated with the RD process, is necessary to predict the column performance reliably. In the present work, such a model has been proposed and successfully applied to the RD process. The parametric sensitivity with respect to feed flow rate, reboiler duty, molar feed ratio of reactants, and location of the acid feed point was examined. Previous Studies The literature on this reaction in a RD column may be broadly classified in two categories. The first one, which is directly related to the present work, aims at the recovery of acetic acid from aqueous streams while the second category deals with the production of methyl acetate from pure acetic acid. It would be appropriate here to review both of the studies. Recovery of Acetic Acid. The recovery of dilute acid by esterification reaction in a distillation column was first studied using a chemorectification column packed with an acidic organic polymer catalyst by Neumann and Sasoon.5 The detailed reaction kinetics, experimental, and theoretical studies were performed in the range of 20-60% (w/w) of acetic acid, and it was shown for the first time that RD can be successfully used for the recovery of acetic acid. About 80% recovery was realized. In another study, the separation of acetic acid from water by catalytic distillation using Amberlyst-15 as a catalyst was studied by Xu et al.6 For a feed that contains 2.5-10% (w/w) of acetic acid in water, more than 50% of acetic acid recovery as methyl acetate was realized. To the best of our knowledge, the simulation work explaining these observed results is not evident in the literature. Saha et al.4 have reported experimental results on the recovery of dilute acetic acid through esterification with higher alcohols such as n-butanol and iso-amyl alcohol in a RD column using
10.1021/ie0505514 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/10/2006
2018
Ind. Eng. Chem. Res., Vol. 45, No. 6, 2006
Figure 1. Experimental setup of the continuous RD column.
ion-exchange resin, Indion 130, as a catalyst. The different column configurations were studied by changing the reflux location, length of catalytic zone, and length of the total column height to get optimal results. The effect of various parameters, for example, feed flow rate, feed location, reflux ratio, molar ratio of reactant, and effect of recycle of water was studied experimentally. In a typical configuration, at a 1:2 molar ratio of acetic acid to butanol and by recycling the aqueous phase to the reboiler, 58% conversion of acetic acid was realized. Few patents also suggest RD to be a viable alternative for the recovery purpose.7,8 The feed concentrations, as low as 5-30% (w/w), have been successfully processed, and nearly quantitative conversions (∼98%) were realized.8 With substantial work in the literature, a detailed analysis of the recovery process through modeling and simulation is still missing, and, hence, the present work has been undertaken.
Production of Methyl Acetate from Pure Acetic Acid. The esterification reaction of pure acetic acid with methanol for the production of methyl acetate, and not for the recovery purpose, has been well-studied and has been a model reaction for the early investigations on RD.2,9 Production of high-purity methyl acetate via countercurrent RD was proposed by Agreda et al.10 The authors described the laboratory and bench-scale testing, pilot plant development, modeling and design, full-scale plant construction and operation, and final troubleshooting and optimization for the Eastman Kodak’s commercial process. Bessling et al.11 studied the theoretical and experimental analysis of methyl acetate synthesis via RD. The experiments were carried out with a supported ion-exchange resin in the form of Raschig rings as heterogeneous catalyst. A model based on the simultaneous chemical and phase equilibria was developed that described the experimental data well. Feasibility and design
Ind. Eng. Chem. Res., Vol. 45, No. 6, 2006 2019
studies were also carried out for the RD process. The conceptual design and process feasibility based on the residue curve map technique has been well studied for this reacting system, and the interested readers can refer the literature for further information.1,12 Song et al.13 studied the reaction kinetics as well as residue curve map for the methyl acetate synthesis. Kinetic experiments were preformed in the presence of Amberlyst-15, and the Langmuir-Hinshelwood-Haugen-Watson rate model was developed to represent the reaction kinetics for the methyl acetate synthesis. Recently, Po¨pken et al.14,15 have extensively studied the reaction kinetics and RD through experiments and simulation for the synthesis and hydrolysis of methyl acetate. The experiments were performed in a continuous RD column using a structured catalytic packing Katapak-S embedded with Amberlyst-15 as the catalyst. As mentioned before, the work was aimed at the production of methyl acetate with pure acetic acid and methanol as feed components. The literature shows that most of the kinetic studies that have been performed are for the methyl acetate synthesis or for hydrolysis of methyl acetate. Very few kinetic studies in the composition region that prevails in a recovery process have been reported. Therefore, in the present work a kinetic study was performed to obtain a suitable rate equation for the acid recovery in the presence of a commercial catalyst, Amberlyst CSP-2. The kinetic expression thus developed in this study was used in the simulations to predict the column performance which is also compared with experimental results. Experimental Section Material and Catalysts. Methanol and acetic acid, both AR as well as commercial grade, were supplied by s.d. Fine Chemicals, Ltd., India. The catalyst Amberlyst-CSP2 (Rohm and Haas, U.S.A.) was obtained from BPCL, India. The catalyst in the supplied form was wet, containing around 30% (w/w) moisture. It was washed and vacuum-dried further for 6 h before using in the kinetic studies. The structured catalytic packing KATAPAK-S used in the column was supplied by Sulzer, Switzerland. Apparatus and Procedure. Kinetic Studies. The esterification reaction was performed in a 750 mL glass reactor dipped in the constant-temperature oil bath. The reactor was equipped with the temperature indicator (Pt-100) and rpm measuring facility. It was also equipped with a condenser with a chilled water (275.5 ( 0.5 K) supply to avoid any possible loss of methyl acetate or methanol. The measured quantities of methanol, acetic acid, and water were charged to the reactor, and the reactor was dipped in the oil bath. Once the desired reaction temperature was attained, the catalyst was charged to the reactor, and this time was considered as the zero reaction time. RD Column Experiments. The experimental setup of a laboratory scale RD plant is as shown in Figure 1. A 3 m tall atmospheric distillation column of inside diameter 54 mm was used. The reboiler (3 L) was externally heated with the help of a heating mantle equipped with a watt meter. The nonreactive rectifying and stripping sections were packed with HYFLUX (Evergreen, India, Ltd.) low-pressure drop structured packing made up of fine metallic wires. The middle reactive zone was packed with Sulzer KATAPAK-S packing embedded with ionexchange resin Amberlyst-CSP2 as a catalyst. All three sections were 1 m tall in height. A proper insulation with an external wall heating arrangement was provided to minimize the heat losses to the surroundings. Methanol and acid solutions were fed to the column with the help of two separate dosing pumps.
Figure 2. Concentration profile in the composition space for batch kinetics and continuous RD experiments.
A feed mixture of acetic acid and water (5-30%, w/w) was fed at the top of the catalytic section (position 6) with the help of a flow metering pump (maximum capacity 2 L/h), and pure methanol was fed at the bottom of the catalytic zone (position 3) by using a peristaltic pump (maximum capacity 7 L/h). The acid feed was preheated before introducing it to the column. A Dean and Stark apparatus with condenser was used to provide the reflux to the column and to continuously withdraw the distillate. Temperature sensors Pt-100 were provided at different locations in the column to measure temperatures (positions 1-8). The measured temperatures were transferred to the computer through an ADAM module. Analysis The individual products were analyzed using a gas chromatograph (GC-911M; MAK Analytica, India, Ltd.) equipped with a thermal conductivity detector. The column used for analysis was Porapack-Q with hydrogen as a carrier gas at a flow rate of 30 mL/min and a pressure of 4 atm. The injector and detector were maintained at temperatures of 493 and 423 K, respectively. The oven temperature was suitably programmed to get the best resolution and the least time for analysis. The initial oven temperature was 483 K, which was then increased further with a ramp rate of 20 K/min to 513 K and maintained there for 2 min. The retention times of all the individual compounds were verified using authentic samples. The results obtained by GC were confirmed by independent titrations, using standard sodium hydroxide (NaOH) solution. The analytical relative uncertainties were
