Tubular reactor equilibrium reaction

Complete or very high conversion requires the study of catalyst at very low concentrations. At such conditions, close to equilibrium (Boudart 1968), all reactions behave according to first order kinetics. Study at very low concentrations is also helped by the very small heat generation, so these studies can be executed in small tubular reactors, placed in simple muffle furnaces. Such studies were made by Kline et al (1996) at Lafayette College and were evaluated by Berty (1997). 

The experiments were conducted in a down-flow tubular reactor with continuous feed and product withdrawal. For phosphine resins, establishment of equilibrium was exhibited by the fact that rhodium concentrations in solution were proportional to percent loading. The concentration was also dependent on solvent. As the solvent polarity increased, rhodium concentration increased. Typical concentrations in the effluent were 0.2-2.0 X 10-5 A/ Rh for reaction at 85°C, 1500 psi 1/1 H2/CO. An increase in CO pressure increased the concentration of rhodium in solution, and an increase in temperature sharply decreased the metal concentration. These are understood as factors that influence the equilibrium between phosphine and carbonyl complexes. 

A packed tubular reactor is used to produce a substance D at a total pressure of 100 kN/m2 (1 bar) utilising the exothermic equilibrium reaction ... 

It is well known that self-oscillation theory concerns the branching of periodic solutions of a system of differential equations at an equilibrium point. From Poincare, Andronov [4] up to the classical paper by Hopf [12], [18], non-linear oscillators have been considered in many contexts. An example of the classical electrical non-oscillator of van der Pol can be found in the paper of Cartwright [7]. Poore and later Uppal [32] were the first researchers who applied the theory of nonlinear oscillators to an irreversible exothermic reaction A B in a CSTR. Afterwards, several examples of self-oscillation (Andronov-PoincarA Hopf bifurcation) have been studied in CSTR and tubular reactors. Another... 

In designing a wall-cooled tubular reactor, we want to operate such that the trajectory stays near the maximum rate for all temperatures. Thus for an exothermic reversible reaction the temperature should increase initially while the conversion is low and decrease as the conversion increases to stay away from the equilibrium constraint. One can easily program a computer to compute conversion and T versus t to attain a desired conversion for rninimum T in a PFTR. These curves are shown in Figure 5-17 for the three situations. 

Another type of stability problem arises in reactors containing reactive solid or catalyst particles. During chemical reaction the particles themselves pass through various states of thermal equilibrium, and regions of instability will exist along the reactor bed. Consider, for example, a first-order catalytic reaction in an adiabatic tubular reactor and further suppose that the reactor operates in a region where there is no diffusion limitation within the particles. The steady state condition for reaction in the particle may then be expressed by equating the rate of chemical reaction to the rate of mass transfer. The rate of chemical reaction per unit reactor volume will be (1 - e)kCAi since the effectiveness factor rj is considered to be unity. From equation 3.66 the rate of mass transfer per unit volume is (1 - e) (Sx/Vp)hD(CAG CAl) so the steady state condition is ... 

The benzene yields given by the data of Figures 4 and 5, 87% at 204°C and 88% at 227°C, may be compared with computed equilibrium yields of 13% and 19%, based on inlet conditions. This clearly shows the advantage of the continuous annular chromatographic reactor over, say, a tubular reactor. The comparison is not entirely straightforward, because dilution of the cyclohexane by He carrier as it disperses circumferentially shifts the equilibrium toward products this would have to be taken into account in any quantitative comparison. The data show only partial separation of benzene and cyclohexane. This partial separation must result in partial suppression of the back reaction, and must also contribute to the observed yield enhancement (in addition to the dilution effect). ... 

This chapter focuses attention on reactors that are operated isotherraally. We begin by studying a liquid-phase batch reactor to determine the specific reaction rate constant needed for the design of a CSTR. After iilustrating the design of a CSTR from batch reaction rate data, we carry out the design of a tubular reactor for a gas-phase pyrolysis reaction. This is followed by a discussion of pressure drop in packed-bed reactors, equilibrium conversion, and finally, the principles of unsteady operation and semibatch reactors. 

This suggests a useful graphical presentation of the design of an adiabatic tubular reactor. In Fig. 8.8 the equilibrium line Fe and the curve of maximum reaction rate in an adiabatic bed, are shown. The broken lines are adiabatic paths. Let and be the extent and temperature at a point on Then the adiabatic path through such a point is... 

Next, it is necessary to identify the dominating factors that affect the chemical reactions and select the most suitable reactor configuration. For homogeneous chemical reactions, one of three factors often dominates (i) equilibrium limitation of the desirable reaction, (ii) the formation of undesirable products (by side reactions), and (iii) the amount of heat that should be transferred. For example, if a low concentration of the reactants suppresses the formation of the undesirable product, a CSTR is preferred over a tubular reactor even though a larger reactor... 

The conventional MTBE synthesis consists of a reaction of isobutene and methanol over an acidic sulfonated cation-exchange catalyst. This reaction is highly selective, equilibrium-limited, and exothermic in nature. Several types of industrial reactors such as tubular reactors, adiabatic reactors with recycle, and catalytic distillation configurations have been utilized to cany out the MTBE synthesis reaction. The factors considered in the optimal design of a MTBE unit include the following items [52]. 

The hydrolysis of esters is of technical interest therefore many different esters such as acetates [18], phthalates [19], natural fats [20] and others were investigated. A detailed investigation of the hydrolysis of ethylacetate (tubular reactor, 23-30 MPa, 250-450 °C, 4-230 s) [7] without the addition of a catalyst shows a lower activation energy at subcritical conditions than at supercritical conditions, indicating two different reaction mechanisms. Under subcritical conditions nucleophilic attack on a protonated ester is assumed to be the rate-determining step of the hydrolysis process. The formation of a protonated ester is favored in the subcritical region because here the self-dissociation of water and the dissociation of the acid, formed via hydrolysis, increase. At 350 °C, 30 MPa, 170 s reaction time, and without additional acid, the conversion to acid and alcohol was 96 %, which is the equilibrium value. In other cases, mostly with unsaturated esters, the acids formed undergo decarboxylation, which leads to poorer yields [12]. 

The technique involves injection of a quantity of pure reactant A into an inert carrier gas stream flowing at steady state through a tubular reactor. This injection is followed by subsequent injection of an identical amount of pure reactant B into the carrier gas. This procedure ensures identical contact times for both experiments. From a knowledge of the effluent compositions for these two trials, one can determine the equilibrium constant for the reaction. Subsequent experiments at different flow rates provide a means of determining individual rate constants. 

The problems of simultaneously treating spatial distributions of both temperature and concentration are currently the concern of the chemical engineer in his treatment of catalyst particles, catalyst beds, and tubular reactors. These treatments are still concerned with systems that are kineticaliy simple. The need for a unified theory of ignition has been highlighted by contemporary studies of gas-phase oxidations, many features being revealed that neither thermal theory, nor branched-chain theory for that matter, can resolve alone. A successful theoretical basis for such reactions necessarily involves the treatment of both the enorgy balance and mass balance equations. Such equations are invariably coupled and cannot be solved independently of each other. However, much information is offered by the phase-plane analj s of the syst (e.g. stability of equilibrium solutions, existence of oscillations) without the need for a formal solution of the balance equations. 

Extent of reaction specified Two-phase, chemical equilibrium Multiphase, chemical equilibrium Continuous-stirred tank reactor Plug-flow tubular reactor Pump or hydraulic turbine Compressor or turbine Pressure drop in a pipe Stream multiplier Stream duplicator... 

Phillips STAR process uses a tubular reactor to supply the heat of the reaction in a fired fiirnace. Steam is used to lower the partial pressure of the butanes (and thus increase the equilibrium conversion) and reduce coking. The STAR process operates on about an 8 hour cycle before regeneration (Brinkmeyer, et al, 1983). 

With strongly exothermic or endothermic reactions tubular reactors are used to control selectivity or prevent catalyst deactivation. Tnbes are either cooled or heated to maintain the catalyst temperature and approach to equilibrium. 

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