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Advances in Applied Science Research, 2012, 3 (3):1346-1352
ISSN: 0976-8610 CODEN (USA): AASRFC
Generalized Modeling and Simulation of Reactive Distillation: Esterification
1Kuldeep Bhatt and 2Narendra M. Patel
1Department of Chemical Engineering (CAPD), L. D. College of Engineering, Ahmedabad 2Govt. Engineering College, Valsad ______
ABSTRACT
There is an increasing trend of chemical industries toward new processes that should meet requirements such as generation of nearly zero waste chemicals, less energy, and sufficient uses of product chemicals in various applications. The reactive distillation provides an attractive alternative for reaction/separation processes with reversible reactions, especially for etherification and esterification. Esterification is the general name for a chemical reaction in which two reactants ethylene glycol and acetic acid form an ester as a product. Since the reaction was occurred in equilibrium and reversible manner, the reaction was become slowly without catalyst. Production of esters in a reactive distillation column is a promising alternative to the conventional sequential process. In the present work, the modeling and simulation of the reactive distillation column for the production of butyl acetate using acetic acid and n-butanol or i-butanol is shown. Thermodynamic aspects of considered system are discussed and UNIQUAC interaction parameters are given. The reaction was catalyzed heterogeneously by a strongly acidic ion-exchange resin (Amberlyst-15). The model incorporated reaction kinetics and vapor-liquid non- idealities into the MESH (Material balance, Equilibrium relationship, Summation equation and Heat balance) equations. The model was solved with the numerical method coupled with the relaxation method. To ensure the applicability and reliability of the proposed model, it was validated by comparing simulated results of esterification reaction (acetic acid and n-butanol) in a reactive distillation column with the pilot plant data i.e. published in literature. The model was capable of predicting the performance of a reactive distillation column for esterification reactions.
Keywords : modeling, simulation, reactive distillation, esterification reactions, heterogeneous catalysis. ______
INTRODUCTION
There is an increasing inclination of chemical industries toward new processes that should meet requirements such as generation of nearly zero waste chemicals, less energy, and sufficient uses of product chemicals in various applications. Chemical manufacturing companies produce materials based on chemical reactions between selected feed stocks. In many cases the completion of the chemical reactions is limited by the equilibrium between feed and product. The process must then include the separation of this equilibrium mixture and recycling of the reactants. The fundamental process steps of bringing material together, causing them to react, and then separating products from reactants are common to many processes. In recent decades, a combination of separation and reaction inside a single unit has become more and more popular. This combination has been recognized by chemical process industries as having favorable economics for carrying out reaction simultaneously with separation for certain classes of reacting systems, and many new processes (called reactive separations ) have been invented based on this technology.[2,4] Esterification is the chemical process for making esters, which are compounds of the chemical structure R-COOR', where R and R' are either alkyl or aryl groups. The most common method for preparing esters is to heat a carboxylic acid with an alcohol while removing the water that is formed. A mineral acid catalyst is usually needed to make the reaction occur at a useful rate. Esters can also be formed by various other reactions. These include the reaction of an alcohol with an acid chloride or an anhydride. The chemical structure of the alcohol, the acid, and the acid catalyst used in the esterification reaction all effect its rate. Simple alcohols such as methanol and ethanol react very fast
1346 Pelagia Research Library Kuldeep Bhatt et al Adv. Appl. Sci. Res., 2012, 3(3):1346-1352 ______because they are relatively small and contain no carbon atom side chains that would hinder their reaction. The most common acid catalysts are hydrochloric acid and sulfuric acid because they are very strong acids. At the end of the esterification reaction, the acid catalyst has to be neutralized in order to isolate the product.
Reactive Distillation[6]: The concept of reactive distillation is not new. This technique was first applied in 1920 to esterification process using homogeneous liquid phase catalyst.
Reactive distillation (RD) is a process in which a catalytic chemical reaction and distillation (fractionation of reactants and products) occur simultaneously in one single apparatus. Reactive distillation belongs to the so-called ‘‘process intensification technologies’’. From the reaction engineering view point, the process setup can be classified as a two-phase countercurrent fixed bed catalytic reactor. In the literature this integrated reaction – separation technique is also known as catalytic distillation (CD) or reaction with distillation (RWD). CD is a process in which a heterogeneous catalyst is localized in a distinct zone of a distillation column. RD is the more general term for this operation, which does not distinguish between homogeneously or heterogeneously catalyzed reactions in distillation columns.
Usually, a partially converted reaction mixture, close to chemical equilibrium, leaves the fixed-bed reactor section and enters the RD column in the fractionating zone to ensure the separation of products from feedstock components. The fractionated unconverted feedstock components enter the catalytic section in the RD column for additional or total conversion. The catalyst packing zone is installed in the upper or lower-middle part of the column, with normal distillation sections above and below.
Figure 1: Processing schemes for reaction where C and D are desired products
Let us considering a reversible reaction scheme: where the boiling points of the components follow the sequence A, C, D and B. The traditional flow-sheet for this process consists of a reactor followed by a sequence of distillation columns; see Fig. 1(a). The mixture of A and B is fed to the reactor, where the reaction takes place in the presence of a catalyst and reaches equilibrium. A distillation train is required to produce pure products C and D. The unreacted components, A and B, are recycled back to the reactor. In practice the distillation train could be much more complex than the one portrayed in Fig. 1(a) if one or more azeotropes are formed in the mixture. The alternative RD configuration is shown in Fig. 1(b).
1347 Pelagia Research Library Kuldeep Bhatt et al Adv. Appl. Sci. Res., 2012, 3(3):1346-1352 ______The RD column consists of a reactive section in the middle with nonreactive rectifying and stripping sections at the top and bottom. The task of the rectifying section is to recover reactant B from the product stream C. In the stripping section, the reactant A is stripped from the product stream D. In the reactive section the products are separated in situ, driving the equilibrium to the right and preventing any undesired side reactions between the reactants A (or B) with the product C (or D). For a properly designed RD column, virtually 100% conversion can be achieved.
Model for Reactive Distillation[8]: The following assumptions are made during the model formulation of catalytic distillation process. Figure 2 shows schematic diagram of a catalytic distillation unit. • The vapor and liquid are in equilibrium on each stage with negligible heat of mixing of liquid and vapormixtures. • The reactions occur only in the liquid phase, each stage in reaction section can be considered as a perfectly mixed stirred-tank reactor (CSTR). • The column is operating under adiabatic conditions. • The vapor holdup is assumed to be negligible.
The model equations including mass and energy balances, vapor-liquid equilibrium and summation equation (MESH equations) are
Mass Balance a) Overall material balance for equilibrium satage j:
… … 1 b) Component i material balance: , , , , , , … … 2 Where, j and i are the stage and component number respectively.
Energy Balance ∑ , , , ∑ , , , ∑ , , , ∑ ∆ , , ∑ , , , … … 3
Phase Equilibria … … 4
In the present study, the vapor phase is assumed to be ideal so that the entire fugacity coefficients for the system are equivalent to unity. The liquid phase non ideality is characterized by the activity coefficients ( γ) calculated from the UNIQUAC method. The saturated vapor pressure P0 is calculated from the Antoine equation and P is the total pressure of the system.
Summation For liquid phase,
, 1 … … 5 For vapor phase,
, 1 … … 6
1348 Pelagia Research Library Kuldeep Bhatt et al Adv. Appl. Sci. Res., 2012, 3(3):1346-1352 ______
Figure 2: Schematic diagram of RD and Equilibrium stage.
Reaction and Reaction Kinetics[13]: Reaction: In present study, n-butyl acetate synthesis by esterification of n-butanol with acetic acid in a reactive distillation column is examined.
Acetic acid (HOAc) + n-butanol (BuOH) n-butyl acetate (BuOAc) + Water (H 2O)
Reaction Kinetics Esterification reactions are the reversible reactions of second order. Therefore pseudohomogeneous model can be written as,
1 1
Temperature dependence of the rate constants is expressed by Arrhenius Law:
, … … 7 Solution of Model[8]: The mathematical model described the steady state behavior of a reactive distillation comprises of a set of nonlinear algebraic equations. In this work, a sequential solution procedure is proposed in order to solve the derived model equations. The detailed calculation algorithm is summarized as given in table 1.
1349 Pelagia Research Library Kuldeep Bhatt et al Adv. Appl. Sci. Res., 2012, 3(3):1346-1352 ______
Figure 3: Algorithm of solution model
RESULTS AND DISCUSSION
Table 1
Quantity Feed Acetic Acid n-butanol Feed stage 7 11 xAA 0.9947 0.000 xBuOH 0.000 0.999 xBuOAc 0.000 0.000 xH2O 0.0053 0.001 Column Pressure (KPa) 103.845 Number of Stages 28 Number of reactive stages 16 Catalyst weight (kg) 0.84 Reflux Ratio 1.05 Experimental Results Simulation Results
(Steinigeweg et. Al., 2002) (Present Study) XD (AA) 0.031 0.0399 XD (BuOH) 0.008 0.0099 XD (BuOAc) 0.003 0.0019 XD (H 2O) 0.958 0.9481 XB (AA) 0.003 0.0030 XB (BuOH) 0.008 0.0070 XB (BuOAc) 0.969 0.9782 XB (H 2O) 0.020 0.0117
A steady state process simulation reactive distillation model was developed from the unsteady state material and energy balance equations based equilibrium stage model. The model equations were solved numerically using the
1350 Pelagia Research Library Kuldeep Bhatt et al Adv. Appl. Sci. Res., 2012, 3(3):1346-1352 ______backward differential formula linear multistep method based on relaxation method. The proposed model has shown satisfactory results in simulating a reactive distillation column for the esterification reaction.
Notation • a activity of component i. • Cp heat capacity of ideal gas, J/mol·K. • EA activation energy, J/mol. • Hj liquid holdup on stage j in molar or volumetric quantity. • hL partial molar enthalpy of liquid, J/mol. • hV partial molar enthalpy of vapor, J/mol. • Kb backward rate constant, mol/g·s. • kf forward rate constant, mol/g·s. • Ki adsorption equilibrium constant for component i. • N number of stages in the column. • ni total molar of component i. • P total pressure of the system.
• Pc critical pressure, kPa. • P0 saturated vapor pressure, kPa. • Q molecular surface parameter. • Qy reboiler duty, kW. • Rg gas constant. • R molecular volume parameter.
• Rj total numbers of moles generated or consumed through reaction on stage j. • rj,r rate of reaction r on stage j, mol/s. • T temperature, K.
• Tc critical temperature, K. • t time, s.
• Vc critical volume, cm3/mol. • vr,i stoichiometric coefficient of component i for reaction r. • W weight of catalyst, kg. • X palmitic acid conversion, %.
• xj,i mole fractions of component i for liquid flow Lj on stage j. • yj,i mole fractions of component i for vapor flow Vj on stage j. • zj,i mole fractions of component i for feed flow Fj on stage j. • ∆Hv,0 standard heat of vaporization, J/mol. • ∆HR heat of reaction, J/mol.
Greeks Letters • δj (0 or 1) refers to reaction occurrence on stage j. When reaction occurs on stage j, δj is set to unity, otherwise δj is set zero. • Φ fugacity coefficient for the system. • γ activity coefficient for the system.
REFERENCES
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1351 Pelagia Research Library Kuldeep Bhatt et al Adv. Appl. Sci. Res., 2012, 3(3):1346-1352 ______[9] Svandova Z., Markos J., Jelemensky L., “Impact of Mass Transfer on Modelling and Simulation of Reactive Distillation Columns”, Mass Transfer in Multiphase Systems and its Applications 28, 649-676 [10] Schneider R., Noeres C., Kreul L.U., Gorak A., Computer and Chemical Engineering 25 ( 2001 ), 169-179. [11] Toor A., Sharma M., Kumar G., Wanchoo R.K., Bulletin of Chemical Reaction Engineering & Catalysis , 6 (1), 2011 , 23-30. [12] Alfradique M. F., Castier M., Computers and Chemical Engineering 29 ( 2005 ), 1875-1884. [13] Chen F., Huss R.S., Malone M.F., Dohetry M.F., Computers and Chemical Engineering 24 ( 2000 ), 2457-2472. [14] Steingeweg S., Gmehling J., Ind. Eng. Chem. Res. 2002 , 41, 5483-5490. [15] Sert E., Atalay F.S., Chem. Biochem. Eng . Q. 25 (2) 221-227 ( 2011 ). [16] Tang Y., Chen Y., Huang H., Yu C., AIChE Journal, J une 2005 , vol.51, No.6, 1683-1699. [17] Hanika J., Kolena J., Smejkal Q., Chemical Engineering Science 54 ( 1999 ) 5205-5209. [18] Buchaly C., Kreis P., Gorak A., Chemical Engineering and Processing 46 ( 2007 ) 790-799. [19] TANSKANEN J.P., “Phenomenon Driven Process Design”, Oulu University Liberary ( 1999 ). [20] Arpornwichanop A., Somrang Y., Wiwittanaporn C., “Numerical Simulation of a Catalytic Distillation Column for Ethyl Acetate Production”, 6th WSEAS International Conference on Simulation, Modelling and Optimization, Lisbon, Portugal, 2006 .
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