Combined reactor

Thus, the combination reactor gives intermediate performance. The fraction unreacted from the composite reactor will be lower than that from a single CSTR with F = q + F2 but higher than that from a single PFR with F = Fi + F2. 

An example of integrated heat-transfer modehng and reactor design is shown in Figure 11.6. A predicted thermal profile for the reactor section of a combined reactor-heat exchanger is the solid line, while the discrete points are experimentally measured temperatures along the reactor length. The thermal profile is controlled... 

Figure 1. Block diagram showing the subsystems of the combined reactor and surface analysis system.

Chemat and his coworkers [92] have proposed an innovative MW-UV combined reactor (Fig. 14.7) based on the construction of a commercially available MW reactor, the Synthewave 402 (Prolabo) [9[. It is a monomode microwave oven cavity operating at 2.45 GHz designed for both solvent and dry media reactions. A sample in the quartz reaction vessel could be magnetically stirred and its temperature was monitored by means of an IR pyrometer. The reaction systems were irradiated from an external source of UV radiation (a 240-W medium-pressure mercury lamp). Similar photochemical applications in a Synthewave reactor using either an external or internal UV source have been reported by Louerat and Loupy [93],... 

In the Mohil-Badger vapor-phase process, fresh and recycled benzene are vaporized and preheated to the desired temperature and fed to a multistage fixed-bed reactor. Ethylene is distributed to the individual stages. Alkylation takes place in tile vapor phase. Separately, file polyethylbenzene stream from the distillation section is mixed with benzene, vaporized and heated, and fed to the transalkylator, where polyethylbenzenes react with benzene to form additional ethylbenzene. The combined reactor effluent is distilled in the benzene column. Benzene is condensed in the overhead for recycle to the reactors. The bottoms from the benzene column are distilled in the ethylbenzene column to recover the ethylbenzene product in the overhead. The bottoms stream from the ethylbenzene column is further distilled in the polyefitylbenzene column to remove a small quantity of residue. The overhead polyethylbenzene stream is recycled to the reactor section for transalkylation to ethylbenzene. 

More recently Peng and Song reported the rapid synthesis of a library of hydrazides in a MW-US combined reactor (Scheme 9.20)139. Unlike the aforementioned system that employed decalin as an energy transfer medium for the ultrasound irradiation, in their modified domestic oven, the horn was immersed directly into the reaction mixture. 

Chemat, F., Poux, M., Di Martino, J.-L. and Berlan, J., An original microwave-ultrasound combined reactor suitable for organic synthesis application to pyrolysis and esterification,. Microwave Power Electromag. 

Lagha, A., Chemat, S., Bartels, P.V. and Chemat, F., Microwave-ultrasound combined reactor suitable for atmospheric sample preparation procedure of biological and chemical products, Analusis, 1999, 27, 452. 

The construction and operation of a continuous rotating annular chromatographic reactor are described. Experimental data for the dehydration of cyclohexane over a Pt/Al Oj catalyst are presented, and the performance of the apparatus as a combined reactor-separator is discussed. A mathematical model is developed, and the results of numerical simulation of reactor performance are presented. 

The plug flow reactor is increasingly being used under transient conditions to obtain kinetic data by analysing the combined reactor and catalyst response upon a stimulus. Mostly used are a small reactant pulse (e.g. in temporal analysis of products (TAP) [16] and positron emission profiling (PEP) [17, 18]) or a concentration step change (in step-response measurements (SRE) [19]). Isotopically labeled compounds are used which allow operation under overall steady state conditions, but under transient conditions with respect to the labeled compound [18, 20-23]. In this type of experiments both time- and position-dependent concentration profiles will develop which are described by sets of coupled partial differential equations (PDEs). These include the concentrations of proposed intermediates at the catalyst. The mathematical treatment is more complex and more parameters are to be estimated [17]. Basically, kinetic studies consist of ... 

As shown in Fig. 18.23, dry liquid feed containing olefins and isobutane is charged to a combined reactor-settler. In this example, the reactor uses the principle of a differential gravity head to circulate through a cooler before contacting a highly dispersed hydrocarbon feed in the reactor pipe. The hydrocarbon phase, generated in the settler, is sent to a fractionator, which separates LPG-quality propane, isobutane recycle, n-butane, and alkylate products. A small amount of dissolved catalyst is also removed from the propane product by a small stripper tower. 

Fig. 18.23. Alkylation—Technology Solutions Division of ConocoPhillips. Include combination reactor/settler (1), main fractionator (2), and small propane stripper (3). (Source Hydrocarbon Processing, 2004 Refining Process Handbook. CD-ROM. September 2004 copyright 2004 by Gulf Publishing Co., all rights reserved.)...

The feed to a hydrofluoric acid alkylation unit is desiccant dried and then sent to the combined reactor settler (Figure 2.8). The reaction occurs at 90 to 100°F (32 to 38°C), at 250 psia (1,725 kPa), and in the presence of 90% hydrofluoric acid. The effluent from the combined reactor settler is fed to the main fractionator. The hydrofluoric acid goes overhead with the light ends and is condensed and collected in the accumulator. Part of the condensed overhead fluid is recycled from the feed to the combined reactor settler, part is used for reflux to the main fractionator, and the remainder is fed to the hydrofluoric acid stripper. The overhead of the stripper is returned to the main fractionator overhead condenser. The bottom product of the stripper is caustic washed to remove all traces of hydrofluoric acid. The bottom product of the main fractionator is often fed to a debutanizer fractionator column. 

The next step in the analysis is to seek another functional relationship between the reactor exit temperature and the reactor feed temperature resulting from the heat exchange, bypass, and influence of the furnace. Once we find the second relation we can superimpose it on top of the reactor temperature rise expression shown in Fig. 5.20. Intersections between the two curves constitute open-loop, steady-state solutions to the combined reactor-FEHE system. 

Reaction conditions Excess reagent Heat removal method Shift equilibrium Add diluent Combine reactor-separator ... 

Chemat, F., M. Poux, J. L. Martino, and J. Berlan, An Original Microwave-Ultrasound Combined Reactor Suitable for Organic Synthesis Apphcation to Pyrolysis and Esterification, J. Microwave Pofwer and Electromag. Energy, 31, 2P(1996). 

Thus the combination reactor gives intermediate performance. There wiU be less conversion than for a PFR with t = ti +J2 but more conversion than for a single CSTR with t =t +t2-... 

This is an example, taken from Dumez and Froment [141], combining reactor simulation on the basis of a slightly simplified version of the model considered in this section but accounting for transients resulting from start up and from catalyst deactivation due to coke deposition. 

V decreases with increasing conversion of A. Thus, if it is possible to remove small amounts of V cheaply from large volumes of the reaction mixture, the optimum reactor configuration and mode of operation would involve the use of a plug flow reactor with low conversions of A per pass coupled with a separator to remove the product V and to recycle unconverted reactants. The exact conversion level to be employed will depend on an economic analysis of the combined reactor-separator system. 

We made a reference to combined reactors in the introduction to this chapter. In these reactors, instead of a series of MFRs, different combinations of a PFR and an MFR are used to minimize the total reactor volume. The theory of these reactors was developed by Douglas (1964), Baburao and Doraiswamy (1965) and Aris (1969). Because they are rarely if ever used industrially, they are not considered in this chapter. But a simple method of calculating the volume of an MFR-PFR combination is included in Example 10.1. 

Reactor design for complete conversion may be impossible thermodynamically or undesirable because of reduced yields when byproducts are formed. In such cases, an economic alternative is to design a combined reactor-separator-recycle system, as illustrated in the simple example in Figure 20.4 and discussed in Chapter 8. Here, the reaction A B is carried out in a CSTR, whose liquid feed is a stream containing pure A. In the event that B is sufficiently more volatile than A, the separation can be performed using a flash vessel and unreacted A is recycled to the reactor. As will be seen in the plantwide control examples at the end of this chapter and in the quantitative analysis in Chapter 21, the presence of the recycle comphcates control of the process and requires special attention. ... 

The use of process simulation, in conjunction with optimization, as discussed in Chapter 18, allows one to determine optimal values of reactor conversion, entering temperature, mode of operation, pressure, molar ratio of reactants in a combined reactor feed, diluent ratio, and purge-to-recycle ratio. 

In this chapter the focus is on reactors. First we introduce the general factors that affect the selection of the reactor. In Section 6.2 are given general guidelines. Section 6.3 considers details for different types of reactions that affect the size of the reactor. The rest of the chapter discusses some specifics about the different types of reactor. Section 6.4 considers burners. Plug flow tubular reactors, PFTR, are considered in Sections 6.5 to 6.26. Stirred tanks reactors, STR, are considered in Section 6.27 to 6.33. Finally Sections 6.34 to 6.37 explore combining reactors with other unit operations such as distillation, extmsion, membranes and vacuum pumps. 

B.3.2.3 Reactions Occurring within D Since C , is formed as a result of one or many reactions occurring in D, a nonnegative reactor volume must also be present. Let V denote the combined reactor volume obtained from all reactive sections belonging to D. In a manner similar to how inflows and outflows of concentration vectors are expressed, a combined reaction rate vector may be written for D. We denote by r the combined average reaction rate associated with D. Since outgoing concentrations must ultimately originate from D, then the rate function must also be evaluated with points from D ... 

Researchers have also combined reactor types. Guo et al. (1997), for example, designed an external loop airlift reactor that incorporates a fluidized bed within the downcomer section, shown in Figure 10.1. The fluidized bed section is used to immobilize microorganisms on carrier particles in order to protect them from damage. The design is meant for the production of enzymes, biofluidization, and wastewater treatment. Although shear rates were minimized, bubbles were not entrained within the downcomer. Furthermore, the gas-liquid mass transfer coefficient was observed to increase with gas holdup. The result was that the gas-liquid mass transfer was limited due to the fact that global gas holdup for the reactor was strictly defined by the riser gas holdup without any addition by the downcomer. 

Figure 17.1 shows that it is possible to combine reactors and vary their residence times or volumes so that the final volume is equivalent or equal to the volume of a single reactor. The main advantage of combining reactors in series or parallel is to utilize less volume to yield the same efficiency, yield, selectivity, and final conversion as much as larger reactor yields. Two reactors in series are represented in Figure 17.1. If one assumes a PFR model for both reactors, the shaded areas under the kinetic curve ABEA and BCDE represent the two PFRs in series. The conversion at the outlet of the first and second reactors is X i and Xa2, respectively. Note that the area is proportional to the volume of each reactor, and therefore the total volume is the sum of the volumes Vi-Fy2=VppR.

---