Control Structure with C4 Feedflow Controlled

The dynamic parameters in the Aspen Plus file are inserted. The volumes of the reflux drum (144 m3) and the column base (177 m3) are calculated so that the holdup times are 5 min when the levels are at 50 , based on the total liquid entering. Aspen Plus Tray Sizing gives a diameter of 7.4 m. Aspen Plus Packing Sizing gives a diameter of 5.4 m, and this Block TI Liquid Composition Pra les Block TI Liquid Composition Pra les Figure 15.2 Composition profiles in MTBE reactive column. Figure 15.2...

Llk Lk Hk

In terms of reactants and products, the boiling point ranking is just the opposite of type IIp Eq. (17.15) vs. Eq. (17.14) . Now the two reactants are lighter than the two products. Thus, these two reactants are more concentrated toward the upper part of the reactive distillation column, and the two products can be withdrawn from the bottom of the column. A typical example is the acetalization reaction for the synthesis of ethylal.8 The reaction can be expressed as formaldehyde + ethanol...

Methanol Recovery Column C3

Figure 14.8 shows the control structure used in the C3 column that is separating methanol and water. A constant reflux ratio is maintained in this column by adjusting the reflux flow-rate based on the distillate flowrate. The temperature on stage 7 is controlled by the reboiler heat input. There are two key plantwide material balance loops associated with column C3. The level in the reflux drum provides a good indication of the inventory of methanol in the system. If this level is going down,...

Neat Operation Versus Using Excess Reactant

The reactive distillation columns considered in previous chapters were all operated in neat mode. The two reactants are fed in exactly the correct amounts to satisfy the stoichiometry of the reaction. The control system must be able to detect any imbalance, which will inevitably result in a gradual buildup of one of the reactants and a loss of conversion and product purities. An alternative to operating neat is to operate the reactive column with an excess of one of the reactants. This...

Reactive Distillation Design

Tray Distillation

Figure 3.2 gives the reactive distillation column and the terminology used. The column is fed with two pure reactant fresh feedstreams F0A and F0B. The column has three zones. There are NS trays and a partial reboiler in the stripping section. Above this section, there is a reactive zone with NRX reactive trays. The third section is the rectifying section with NR trays and a total condenser. Figure 3.2 Reactive distillation column. Figure 3.2 Reactive distillation column. Light reactant A is...

Process Studied

Extractive Distillation Flowsheet

The reaction of methanol with unsaturated C5 isoamylenes 2-methyl-1-butene and 2-methyl-2-butene produces TAME. The liquid-phase reversible reactions are 2M1B MeOH TAME 2M2B MeOH , TAME This system is fundamentally a ternary system with inerts. The methanol fresh feedstream is essentially pure. The hydrocarbon feed comes from an upstream catalytic cracking unit. It is a mixed C5 stream that contains not only the two reactants 2M1B and 2M2B but also other C5s such as isopentane, n-pentane,...

Steadystate Design For Acetic Acid Esterification

The esterifications of acetic acid with five different alcohols, ranging from C to C5, are studied in this chapter. The purpose is to understand how process configurations vary as chemical systems change. First, qualitative relationships between process flowsheets and phase equilibria are established, and the process flowsheets are classified into types I-III for these five systems. Second, a systematic design procedure is devised to optimize the design based on the TAC, and dominant design...

Results For Temperaturedependent Relative Volatilities

In the previous section, the optimum economic steady-state designs of reactive distillation columns were quantitatively compared with conventional multiunit systems for a wide range of chemical equilibrium constants. Relative volatilities a 2 were assumed constant. Reactive distillation was shown to be much less expensive than the conventional process. In this section we explore how temperature-dependent relative volatilities affect the designs of these two systems. A fundamental difference...

Mtbe Control

The two fresh feedstreams to the column are a pure methanol steam and a C4 stream containing the reactive isobutene and the nonreactive n-butene. The bottoms from the column is mostly MTBE. The distillate is mostly n-butene. Figure 15.1 gives the steady-state conditions and equipment parameters found in Aspen Plus. The methanol and the C4 stream are both fed on stage 10 at the bottom of the reactive Reactive Distillation Design and Control. By William L. Luyben and Cheng-Ching Yu Copyright 2008...

Info

NINTB, NRE AL B,NIWORK,NWORK, N_COMP INTEGER K_ETOH,K_IC4, K_NC4, K_ETBE PARAMETER K_ETOH 1 PARAMETER K_IC4 2 PARAMETER K_NC4 3 PARAMETER K_ETBE 4 PARAMETER N_COMP 4 INTEGER IDX NCOMP , NBOPST 6 , INT NINT , IWORK NIWORK ,NSTAGE, KDIAG, IHLBAS,IHVBAS,NREAL, KPHI, KER, L GAMMA, J REAL 8 X NCOMP,3 , Y NCOMP , REALB NREALB ,WORK NWORK , B l , T, REAL 8 HLDLIQ,TIMLIQ,HLDVAP,TIMVAP,TZERO, FT REAL 8 REAL NREAL , RMISS, CI, C2, C3, Cb, DKA, DKR, RATNET, KETBE, KA, KRATE REAL 8 PHI N_COMP REAL 8 DPHI...

Control Of Mtbe And Etbe Reactive Distillation Columns

In Chapter 9 we explored the steady-state designs of both the MTBE and the ETBE reactive distillation columns using Aspen Plus. In this chapter we export the files into Aspen Dynamics as pressure-driven dynamic simulations and then look at dynamics and control. The control structures evaluated on both systems are based on those developed in Chapter 12 for ternary systems with inerts. In each of these systems, two alternative control structures are evaluated that use different production rate...

Conclusion

The impacts of a number of important kinetic, phase equilibrium, and design parameters have been explored in the chapter. Most of the parameters have effects that are logical and follow our experience with conventional distillation. However, there are two parameters that give counterintuitive results. The most important of these is the number of reactive trays. More trays does not necessarily improve the steady-state economics of a reactive distillation column. However, the effect of more...

Effect Of Holdup On Reactive Trays

We now investigate the impact of changes in various parameters from those used in the base case. The first parameter studied is the holdup of liquid on the reactive trays. As we would expect, the larger the holdup, the easier it is to achieve the desired conversion. Figure 2.3 shows how vapor boilup VS , reflux R , and product impurities change as reactive tray liquid holdup MRX is increased. Below a holdup of about 260 mol, the desired 95 conversion cannot be achieved in this reactive column...

Aspen Simulations

Aspen Plus is used for the steady-state designs of the real chemical systems. Convergence problems can occur because of the difficulty of trying to solve the large set of very nonlinear simultaneous algebraic equations. Another problem is that the current version of Aspen Plus does not permit the use of activities in the reaction rate expressions. User subroutines are used to incorporate this feature when necessary. Aspen Dynamics is used to study dynamics and control of the real systems. The...

Control Of Reactive Distillations For Acetic Acid Esterification

Steady-state design of the esterifications of acetic acid with five different alcohols C1-C5 was explored in Chapter 7. This chapter explores the control of these five reactive distillation systems using three different flowsheets. The degree of process nonlinearity is computed quantitatively based on the fraction of sign reversal for all tray temperatures or based on Allgower's nonlinearity measure.1'2 These measures provide useful information about potential problems in closed-loop control....

Design Of Mtbe And Etbe Reactive Distillation Columns

In this chapter we examine two reactive distillation column systems that are used for the production of real chemical components. The two systems are quite similar and are basically ternary systems with inerts that have characteristics similar to those discussed in Chapter 5 for ideal components. The first system is the production of MTBE from the reaction of methanol with iso-butene. The second is the production of ETBE from the reaction of ethanol with isobutene. The simulation tool used to...

Mtbe Process

The MTBE reactive distillation process was patented several decades ago, and the process was widely used in the petroleum industry. Many reactive columns were installed around the world to produce MTBE, which was blended into gasoline. This process was probably the largest application of reactive distillation in terms of the number of columns and total production capacity. Because MTBE presents groundwater contamination problems, it is gradually being phased out of use in gasoline. The reactive...

Basics Of Reactive Distillation

Reactive distillation is attractive in those systems where certain chemical and phase equilibrium conditions exist. We will discuss some of its limitations in Section 1.4. Because there are many types of reactions, there are many types of reactive distillation columns. In this section we describe the ideal classical situation, which will serve to outline the basics of reactive distillation. Consider the system in which the chemical reaction involves two reactants A and B producing two products...

Control Structure Design

In this section a systematic approach is proposed to design the control structures for these three types of reactive distillation flowsheets. Because all five reactive distillation systems Table 7.5 have almost equal molar feedflows neat flowsheet , the stoichiometric balance has to be maintained.3 Here we adjust the feed ratio to prevent accumulation of unreacted reactants attributable to stoichiometric imbalance. The next issue is, how many product compositions or inferred product purities...

Ternary System Without Inerts

In the two-product reaction system, the column has both bottoms and distillate products coming from the two ends of the column. The two reactant feedstreams are fed into the middle section of the column. With a one-product reaction system without inerts, the column has only a bottoms or a distillate product. If product component C is heavier than reactant components A and B, there is a bottoms stream but no distillate. The column operates at total reflux with all of the overhead vapor condensed...

Ternary System With Inerts

The previous section considered the case in which the fresh feedstreams of both reactants A and B are pure. In most of the real commercial reactive distillation systems, lighter reactant A is fed with other components that are inert in terms of the reaction but have volatilities that are quite similar to component A. We will assume that fresh feedstream F0A is a mixture of reactant A and an inert component I, which is not involved in the reaction. The volatility of I is assumed to be identical...

Aspen Dynamics Subroutines

Aspen Convergence

However, a kinetic model supplied by Aspen Technology is used in this chapter. It uses a kinetic expression with concentrations in terms of activities and calculates a chemical equilibrium constant using a more complex function of temperature. Simulation of reactive distillation using the standard models in Aspen Plus has some significant problems. The reactions can be specified to be either kinetic or equilibrium. In the former case, the choices of concentration units are limited to mole...

Ternary Reactive Distillation Systems

The chemical system considered in previous chapters featured the classical quaternary two-reactant, two-product A B , C D reversible reaction. Some interesting phenomena were discussed. In particular, the effect of the number of reactive trays on energy consumption was demonstrated to be counterintuitive, that is, there is an optimum number of reactive trays that minimizes energy consumption. In this chapter we explore a similar but somewhat different chemical system. The reaction is A B , C,...

Begin Executable Code

Chemical Equilibrium Model

Figure 9.19 User subroutine for ETBE. KETBE DEXP 10.387D0 406D.59D0 T-2 .890SSDO DLOG T -0.01915 lt HD0 T tfRITE MAXURT_HAXBUF 1 ,9010 FT, DKA, DKR C fugacity coefficient of components in the mixture DPHI, KER WRITE MAXWRT_MAXBUF 1 ,9000 T,P,KER WRITE MAXWRT_MAXBUF 1 ,9020 J,X J,1 ,ACTIV J RATE REALB 1 KRATE ACTIV K_ETOH 2 .dO WRITE MAXWRT_HAXBUF 1 ,9030 NS TAGE,RATE,RATNET 99.1 mol . The conversion of the isobutene fed is isobutene in C4 feed 1767 kmol h 0.40 The kinetics given above were used...

C3

Temperature Profile For Reboiler

Figure 8.24 The TAME process with extractive distillation methanol recovery. Block C1 Liquid Composition Profiles Block C1 Liquid Composition Profiles Figure 8.25 Composition profiles in reactive column. Figure 8.25 Composition profiles in reactive column. Figure 8.26 Temperature profile in reactive column. Figure 8.26 Temperature profile in reactive column. Reboiler heat input and condenser heat removal are 38.2 and 39 MW, respectively. The operating pressure is 4 bar, and the column diameter...

Extractive Distillation Methanol Separation Section 209

The distillate D2 composition 22.8 mol methanol is near the azeotropic composition at the 2 bar pressure. Reboiler heat input and condenser heat removal are 19.5 and 24.5 MW, respectively. The column diameter is 4.2 m. Distillate D2 is fed to the C5 recovery column C3. This column operates at a pressure of 10 bar, which shifts the azeotropic composition so that the distillate stream from column D3 has a composition of 34.2 mol methanol. Higher and lower pressures were explored to see their...

Extractive Distillation Methanol Separation Section

An alternative flowsheet is shown in Figure 8.24 in which water is used as an extractive agent in an extractive distillation column to remove the methanol from the distillate stream coming from the reactive distillation column. A second column separates the methanol water mixture coming from the base of the extraction column and recycles both methanol and water back to upstream units in the process. The reactor and column C1 are identical to those used in the pressure-swing process. The...

Design Of Tame Reactive Distillation Systems

Reactive Distillation

In this chapter we take a look at an important example of a reactive distillation column operating in a plantwide environment. The reactive column is part of a multiunit process that includes other columns for recovery of one of the reactants. The process may give the impression that the reactive column is not operating in neat mode because of the need for reactant recovery. We will show that this is really not the case. The recovery of reactant is made necessary by the presence of azeotropes...

Etbe Process

The reactive distillation column is essentially a ternary system with inerts. The liquid-phase reversible reaction is The heavy component is ETBE, which leaves the reactive distillation column in the bottoms. Most of the isobutene is consumed in the reaction, while the chemically inert n-butene in the C4 feedstream goes overhead in the distillate. The kinetic equations that we used are from an article by Sneesby et al.2 These were used by Al-Arfaj and Luyben to develop a Fortran simulation of...

Prereactor And Reactive Column

Swing Reactor

One is methanol. The other is a mixture of C5 components that contains reactive isoamylenes plus other C5 paraffins, naphthenes, and olefins. This C5 stream typically comes from a petroleum refinery light-ends unit that separates light hydrocarbon components generated in catalytic cracking into various streams. Because the boiling points of all of the C5 components are quite similar, it is uneconomical to separate out the isoamylene reactants from the other C5s....

Reactive Distillation

Reactive Distillation Images

Most chemical processes involve two important operations reaction and separation that are typically carried out in different sections of the plant and use different equipment. The reaction section of the process can use several types of reactors continuous stirred-tank reactor CSTR , tubular, or batch and operate under a wide variety of conditions catalyzed, adiabatic, cooled or heated, single phase, multiple phases, etc. . The separation section can have several types of operations...

Effects Of Feed Tray Locations On Design

CONTROL OF REACTIVE DISTILLATION 519 18.1 Process Characteristics 519 18.1.2 Steady-State Design 522 18.1.4 Feed Locations Versus Reactants Distribution 523 18.1.5 Optimal Feed Locations 527 18.2 Effects of Relative Volatilities 529 18.2.1 Changing Relative Volatilities of Reactants 529 18.2.2 Changing Relative Volatilities of Products 530 18.3 Effects of Reaction Kinetics 533 18.3.1 Reducing Activation Energies 533 18.3.2 Effects of Preexponential Factor 536 18.4 Operation and Control 538...

Steadystate Design For Acetic Acid

7.1 Reaction Kinetics and Phase Equilibria 147 7.1.1 Reaction Kinetics 147 7.1.2 Phase Equilibria 149 7.2 Process Flowsheets 153 7.2.1 Type I Flowsheet MeAc 153 7.2.2 Type II Flowsheet EtAc and IPAc 156 7.2.3 Type III Flowsheet BuAc and AmAc 157 7.3 Steady-State Design 158 7.3.1 Design Procedure 158 7.3.2 Optimized Design 160 7.4 Process Characteristics 168 7.4.2 Type II EtAc and IPAc 168 7.4.3 Type III BuAc and AmAc 170 8 DESIGN OF TAME REACTIVE DISTILLATION SYSTEMS 179 8.1 Chemical Kinetics...