Reactor configurations and designs

The simplest reactor is the stirred autoclave reactor. In polyolefin production, this reactor is operated as a CSTR and is used in slurry, bulk and solution processes. The main advantages of this configuration are that the reactor is easy to build and to run, and provides a relatively uniform reaction medium with proper stirring. Its principal disadvantage is that the heat transfer area-to-volume ratio is relatively low and heat removal is, therefore, limited. This limitation is especially difficult to overcome for new plants with increasing production capacity. 

Many modifications to the basic stirred autoclave design have been made to improve its heat transfer characteristics those commonly used in polyolefin production processes include the use of external coolers, overhead condensers or internal cooling coils. 

When external coolers are used, a portion of the polymer slurry in the reactor is circulated through one or more external heat exchanger to remove the heat of polymerization. This is an efficient method of heat removal, but puts stringent requirements on the morphology of the polymer particles being produced. Polymer fines are undesirable as they tend to deposit on the heat exchanger walls and require frequent shutdowns to clean out. An example 

A very efficient alternative for heat removal is to use overhead condensers. This modification uses the latent heat of evaporation of the monomer to remove the heat of polymerization. Monomer is evaporated in the reactor, condensed in the overhead condenser, and the cooled liquid monomer is returned to the reactor. This design works well for propylene polymerization, but it is not a good option for ethylene because of its much lower boiling point. Overhead condensers are used in the El Paso bulk polypropylene process [72]. 

Finally, internal cooling coils are also used to increase the heat transfer area inside the reactor but they are generally less efficient that the other two methods mentioned above, since they are often subject to fouling. 

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Photoconversion of air contaminants in photocatalytic reactors has been considered through a diversity of reactor configurations and designs. The high quantum yields observed, sm passing the value of 1, suggest that this approach will become a major area for futm e applications of photocatalysis. 

It is always important to choose an optimum design configuration of the hydrodynamic cavitation reactor so as to maximize the cavitational effects and achieve cost effective operation. In this section, we will discuss available reactor configurations and give some guidelines, based on theoretical analysis coupled with experimental results, for selection of optimum design and operating parameters for hydrodynamic cavitation reactors. 

The remainder of this text attempts to establish a rational framework within which many of these questions can be attacked. We will see that there is often considerable freedom of choice available in terms of the type of reactor and reaction conditions that will accomplish a given task. The development of an optimum processing scheme or even of an optimum reactor configuration and mode of operation requires a number of complex calculations that often involve iterative numerical calculations. Consequently machine computation is used extensively in industrial situations to simplify the optimization task. Nonetheless, we have deliberately chosen to present the concepts used in reactor design calculations in a framework that insofar as possible permits analytical solutions in order to divorce the basic concepts from the mass of detail associated with machine computation. 

The provision of the right amounts of educts at the reaction site, the establishment and the maintenance of the adequate reaction conditions and the in-time removal of the reaction products are tasks that are not necessarily solved optimally in standard fixed-bed reactor configurations Novel designs to improve the interaction of transport and reaction have therefore attracted considerable interest m recent years They can be placed under the heading multifunctional reactors [50-52]... 

Froment and Bischoff analyze this problem as follows (G. F. Froment K. B. Bischoff, Chemical Reactor Analysis and Design, Wiley, 1979). Let the PFR and CSTR have space times of Tj and T2, respectively. The overall RTD for either system will be that of the CSTR but with a delay caused by the PFR. Thus, a tracer experiment cannot distinguish configuration (I) from (II) in Figure 8.3.1. 

The main considerations are the reaction kinetics, equilibrium relations, and heat of reaction. The relationship of these factors will determine the reactor configuration and the cooling water requirements. Obviously, physical and thermal data for all the flow streams will be needed. Catalyst activity with information on its decline with time and reactivation time will be needed for regeneration design purposes. 

Ultradeep desulfurization approaches include 1) improving catalytic activity by new catalyst formulation for HDS of 4,6-DMDBT 2) tailoring reaction and process conditions 3) designing new reactor configurations and 4) developing new processes. One or more approaches may be employed by a refinery to meet the challenges of producing ultraclean fuels at an affordable cost. 

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