Membrane reactors for isoamyl acetate production
Graphical abstract
Annualized cost comparison for conventional, hybrid, and membrane reactor process schemes for isoamyl acetate production.
Introduction
One of the mostly used esters in food industry, due to its characteristic banana flavor, is isoamyl acetate (C7H14O2, E). It can be obtained from the liquid phase esterification of acetic acid (C2H4O2, HAc) with isoamyl alcohol (C5H12O, ROH). This reaction, in which water (H2O, W) is obtained as a byproduct, presents serious complications including limited chemical equilibrium and challenging product purification [1]. The combination of reaction and pervaporation in a single unit (membrane reactor) allows to shift the conversion for equilibrium limited reactions by removing, selectively, one (or more) products. This type of application is an example of relatively new use of membrane technology, which traditionally had been applied as a tail process solution, and not as an essential part of it. Wynn [2] referred to some examples in this regard. Additionally, membrane technologies are attractive since process efficiency is not limited by the phase equilibrium. Thus, both production costs, due to higher conversion, and efforts in the separation stage can be reduced (this last one is usually energyvore in conventional technologies).
Membrane reactors have been already used to perform many carboxyclic acid–alcohol reactions, such as acetic acid with methanol [3], ethanol [4], isopropanol [5], and n-butanol [6]. The reaction-pervaporation scheme can be implemented in various configurations [7], [8]. Among them, one can distinguish between the systems where the membrane and the reactor are one unit (integrated scheme) and those where pervaporation modules and the reactor are arranged in series (hybrid scheme). In our previous work [9], four process alternatives for isoamyl acetate production were evaluated. It included the transition from conventional to hybrid membrane processes. It was shown that hybrid membrane processes allows obtaining the specified product purity with lower energy duties and economical compromises than the conventional ones.
In this paper, our previous analysis [9] is extended for the integrated membrane reactor schemes. They were studied by simulation using ASPEN Plus® together with a homemade Excel®-MatLab® interface for membrane reactor modules simulation. The design procedure includes the optimization of membrane reactor performance, the minimization of distillation column energy consumption, and the energetic integration using the pinch point methodology [10]. Total annualized costs (TAC) are also estimated and compared with those of the conventional and hybrid schemes.
Section snippets
Isoamyl acetate production framework [9]
Conventional processes for isoamyl acetate production imply the use of one of the reactants in excess and/or the selective recovery of one of the reaction products. Nevertheless, the separation step presents serious complications. According to our previous study [1], the acetic acid–isoamyl alcohol–isoamyl acetate–water mixture presents three binary azeotropes and one ternary azeotrope. From the structure of the residue curve maps for the quaternary mixture, we had concluded that two
Models for simulation
The esterification under study is a very complex system. In consequence, for this thermodynamically non-ideal mixture, activities are needed in the description of phase behavior and transport (pervaporation) across the membrane. It is also well known that expressing reaction rates in terms of concentrations results in reaction rate constants which often depends on concentrations. Using activities not only corrects this problem, but also provides a unified approach in describing both reaction
Membrane reactor process design. Defining the operational mode
Initially, two operational (design) modes were analyzed by simulation. They involve two different operative constrains (criteria) for an isothermal membrane reactor: (i) the first one, define water molar fraction in the permeate side as equal to 0.99 and (ii) the second one, define a water free retentate stream (stream number 7 in Fig. 1). Each operational mode was iteratively solved until the operational criterion was reached, using the specific membrane area as parameter.
Fig. 2 summarizes the
Analysis of recycle loop
It is well known that recycle can present a dramatic effect on the performance of the reaction and separation units. Thus, in order to explore the sensibility of the isoamyl acetate process to the details of recycling, the following variables were defined for the recycle loop analysis (see also Eqs. (7)–(12)):
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The specific membrane area: the three following values will be considered: 0.1, 0.25, and 0.5 m2/kg (where the highest conversions were reached).
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The temperature drop (ΔT) in the membrane
Processes energy integration and economic evaluation
For each membrane reactor process, the hot stream resulting from the top of the distillation column can be used through energy integration in the preheating of membrane reactor and the distillation column feed streams. The integration was performed using Aspen Energy Analyzer® software. The integrated streams are shown schematically in Fig. 6 for MR1 and MR2 process schemes. The streams that cannot be integrated due to pinch point limitations are supplemented with the utilities available:
Economic evaluation of membrane reactor processes
The economic evaluation of the MR1 and MR2 processes was carried out using Aspen Process Economic Analyzer®. It includes services costs, tax values, interest rate, and inflation data for chemical plants in South America [9]. The equipment costs were updated to 2014 using the CE indexes [22]. considering a linear method of depreciation. The cost of membrane reactors was evaluated as these of conventional reactor plus the cost of pervaporation module. A value of 359.6 US$ per m2 and a durability
Conclusions
In this work, two different operational modes of membrane reactors were analyzed for the production of isoamyl acetate. The first one specifies 0.99 as molar fraction of water in permeate, while the second mode establishes that retentate stream must be water free. No total conversion was reached with any process configuration. Thus, the reagents recycle could not be avoided. This analysis allowed synthesizing the general membrane reactor process scheme. Several cases of membrane reactors with
Acknowledgments
The authors thank to ECOPETROL, COLCIENCIAS and Universidad Nacional de Colombia, Sede Manizales, for the financial support of this research through the project “Producción de acetato de amilo mediante un proceso intensificado utilizando tecnología de membranas”, code: 1119-490-26022, contract: RC no. 556-2009. Harold Norbey Ibarra Taquez is beneficiary of a COLCIENCIAS grant (Programa Convocatoria Nacional 567 para Estudios de Doctorado en Colombia año 2012).
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