Design considerations Flow distribution

In addition to flow, thermal, and bed arrangements, an important design consideration is the amount of catalyst required (W), and its possible distribution over two or more stages. This is a measure of the size of the reactor. The depth (L) and diameter (D) of each stage must also be determined. In addition to the usual tools provided by kinetics, and material and energy balances, we must take into account matters peculiar to individual particles, collections of particles, and fluid-particle interactions, as well as any matters peculiar to the nature of the reaction, such as reversibility. Process design aspects of catalytic reactors are described by Lywood (1996). 

Chapter 7, Reactor Design, discusses continuous and batch stirred-tank reactors and die packed-bed catalytic reactor, which are frequently used. Heat exchangers for stirred-tank reactors described are the simple jacket, simple jacket with a spiral baffle, simple jacket with agitation nozzles, partial pipe-coil jacket, dimple jacket, and the internal pipe coil. The amount of heat removed or added determines what jacket is selected. Other topics discussed are jacket pressure drop and mechanical considerations. Chapter 7 also describes methods for removing or adding heat in packed-bed catalytic reactors. Also considered are flow distribution methods to approach plug flow in packed beds. 

Considerable attention was given to the design of the reactor internals including the inlet distributor, outlet collector and thermocouple locations as shown in Fig. 2. Laboratory simulation and testing of catalyst characteris tics helped to develop catalyst bed support criteria and loading procedures to ensure uniform packing and bed density for good flow distribution. 

The present design consideration must concern itself with the question of just how this flow directionality can best be used to achieve a high sticking coefficient. It is necessary to determine the distribution of the flow in the tube and the probability of a nonsticking... 

Pipes and valves. No discussion of liquids handling would be complete without consideration of the selection of the necessary piping and valves. The satisfactory performance of any liquid handling depends on the material and design of the distributing pipeline and the type of valve which controls the rate of flow. The valves should be placed in positions easily accessible to the operator and to the fitter for maintenance. Reference may be made to the design and application of the various types of valves, pipes, and fittings in Chapter 3. 

There are three types of structures within the System 80+ Standard Design steam generators which support the tubes. These are the horizontal grid or "eggcrate , the vertical supports, and the diagonal supports, all of which are fabricated from Stainless Steel 409. One of the design considerations for these supports is prevention of dryout at support locations. With one exception, all tube supports in the System 80+ steam generator are constructed of flat strips which present a flat surface to the tube. The one exception is the flow distribution plate just above the entrance to the economizer section of the tube bundle. At this location secondary water is subcooled and, therefore, dryout will not be experienced. 

Taking into consideration all the different properties of cohesive to very cohesive powders tested (particle size distribution, moisture content, material properties etc.), the model fit can be characterised as satisfactory to good. Thus, the model has proved its effectiveness and can be accordingly applied in reliable silo design for flow and pressure calculation [26]. 

Coiled-tube heat exchangers frequently have flow distribution problems that include (1) tube distribution (2) two-phase tube distribution and (3) two-phase shell distribution. Good flow distribution within the tubes can be obtained by designing the headers in such a way that their pressure drop is considerably less than that for the frictional pressure drop in the tubes. To obtain good shell-side distribution one must use symmetric bundles and separately introduce the vapor and liquid phases to the bundles. It is also advisable to arrange for downflow of the shell-side fluid. For two-phase annular flow, the vapor will flow mostly in the space between the tube layers while the liquid needs to be carefully distributed in the radial direction for proportionate vapor-liquid flow normal to each tube layer. To avoid convection on the shell side due to density gradients, it is normal practice to use sufficiently large pressure drops on the shell side. 

These gases can be premixed but often they are mixed in the gas manifolding systems and the partial flow of each gas is measured separately. In reactive deposition, the reactive gas availability and plasma activation can be important variables that are sensitive to the fixture/ system geometry. If this is the case, the injection of gas into the system is an important design consideration (Ch. 4). Often gas manifolding with multiple inlets is used to obtain uiuform gas distribution in the deposition system. 

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