Vessels, process design example

The process design of a PFR typically involves determining the size of a vessel required to achieve a specified rate of production. The size is initially determined as a volume, which must then be expressed in terms of, for example, the length and diameter of a cylindrical vessel, or length and number of tubes of a given size. Additional matters to consider are effects of temperature resulting from the energetics of the reaction,... 

The basic adsorption process design. Sub-tasks within that include the adsorbent selection, made in view of aU of the requirements imposed on the dehydration process. The adsorption step time, regeneration and cooHng step times all need to be settled and these in view of mechanical details. The overall vessel configuration, for example, the vessel ID and length, which quantities are typically sized based on pressure drop. Finally we need to make some estimate of the expected service Hfetime for the adsorbent product. 

Once the vessel type has been selected and the rate expression established for the cleaning process, the design engineer must decide which variables are significant for the scaling process. Mathematical tools in the form of dimensionless variables are useful for determining these variables. Every chemical or physical process can be defined by a set of dimensionless ratios or variables intrinsic to the process. An example is the Reynolds number, which is defined for a process with internal flow through a tube by solvent properties, flow rate of the solvent, and the diameter of the tube. l... 

Solid-liquid mixing is used extensively throughout the chemical process industries. Examples include the suspension of solid catalysts in a reactor, the dissolution of solids to form a solution, the suspension of solids in crystallization, suspension polymerization, and fermentation. Crystallizers are usually designed to maximize flow, hence solids suspension, and to minimize shear to avoid crystal attrition. A common application is to provide solids-liquid mixing to feed a solids separator such as a centrifuge. Here the task is to provide a uniform solids feed as levels recede in the feed vessel. 

In cases in which the product of a chemical reaction or salt formation is insoluble, nucleation events and precipitation may start instantaneously. These events can be difficult to control, but nevertheless one should investigate the possibility of designing an addition-controlled process—for example, by adding the reaction partner slowly— which may be coupled with appropriate seeding once supersaturation is reached. Scale-up in this area is generally difficult, and the addition mode (e.g., using spray balls or other devices to optimize the distribution of the reactant in the crystallization vessel) plays an important role. For details on precipitation, refer to the work of Sohnel and Garside. ... 

Figure 8.12 shows the segmented ring quick closure system [12]. This special development of the Bredtschneider closure makes it possible to open or close the vessel in a very short time (3-4 minutes) by a computer-controlled hydraulic system. The technique has been operated successfully with discontinuous extraction processes for several years. Figure 8.13, for example, shows this type of closure on a pressure vessel used for the decaffeination of tea. The inner diameter of the vessel is t/i = 1100 mm. Closure and vessel were designed for a fluctuating load. 

As a process design engineer, I do not size relief valves. This is easily done by a computer program based on the vessel size, fluid properties, and maximum operating pressures. The location of the relief valve is a matter, however, that requires careful engineering judgment. I will illustrate the problem by several examples drawn from my own refinery experience. 

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