Parallel-column distillation

Packed-column distillation, while practiced much less frequently than gas absorption, still finds considerable use in medium- and small-scale applications. The model equations parallel those for the absorption case and result in the same HTU-NTU relations seen there, with mole fractions taking the place of mass ratios as the pertinent concentration xmits. We obtain... 

Chapter 8 deals with the configuration of bulk flows of two phases/regions (one of which may be solid) perpendicular to the direction of force(s). The directions of motion of the two phases may be parallel to each other in either countercurrent or cocurrent fashion, or they may be in crossflow. Figures 8.1.1-8.1.4 illustrate the counter-current flow vs. force configuration for all three classes of separation systems. Conventional countercurrent devices/ processes of gas absorption/stripping, column distillation... 

Distillation columns are expensive items in any plant, and are tricky to control. They should initially be built large enough to accommodate a proposed expansion. The reboilers, condensers, and pumps, however, do not need to be designed to handle any more than the initial throughput. Figure 5-1 shows how the auxiliary system may be expanded by placing similar equipment in parallel when the plant capacity is increased. 

Figure 11.3d shows a process where the manipulated variable affects the two controlled variables and in parallel. An important example is in distilla tion column control where reflux flow aSecte both distillate composition and a tray temperature. The process has a parallel structure and this leads to a parallel cascade control system. 

Optimization implies maximum profit rate. An objective function is selected, and manipulated variables are chosen that will maximize or minimize that function. Unit optimization addresses several columns in series or parallel. It is concerned with the effective allocation of feedstocks and energy among the members of that system. Plantwide optimization involves coordinating the control of distillation units, furnaces, compressors, etc., to maximize profit from the entire operation. All lower-level control functions respond to set points received from higher-level optimizers. 

Thermodynamic cost analysis relates the thermodynamic limits of separation systems to finite rate processes, and considers the environmental impact through the depletion of natural resources within the exergy loss concept. Still, economic analysis and thermodynamic analysis approaches may not be parallel. For example, it is estimated that a diabatic column has a lower exergy loss (39%) than that of adiabatic distillation however, this may not lead to a gain in the economic sense, yet it is certainly a gain in the thermodynamic sense. The minimization of entropy production is not always an economic criterion sometimes, existing separation equipment may be modified for an even distribution of forces or an even distribution of entropy production. Thermodynamic analysis requires careful interpretation and application. 

Step I Divide the process into separate blocks. Every block may contain a single processing unit or a small number of processing units with an inherent, common operational goal. For example, the block containing a distillation column should also contain the condenser and reboiler attached to the column two neighboring heat exchangers in series or in parallel should be contained in the same block a reactor and its feed preheater could be in the same block and so on. 

Whilst the time for the establishment of a stationary condition in normal distillations varies from a few hours to at most 24, it may amount to weeks or months in the fractionation of isotopes. Since the older formulae for calculating this time proved unreliable, the problem was studied anew. Kuhn and collaborators [59] evolved the following equation for the equilibration time of parallel-tube and packed columns ... 

The entire assembly is enclosed in a highly insulated cold box to conserve energy. The columns must be made as compact as possible to minimize capital investment as well as reduce heat leak. Up to 150 distillation trays are used in a double column system and tray spacings are kept small at 10 to 20 cm. The trays are typically multipass sieve trays with small diameter perforations. Because the distillation is an extremely clean service, perforations are typically as small as 1 mm. Many tray geometries are used, including multipass cross-flow, split cross-flow (parallel) and circular flow (race track) trays. Each has certain attributes which are used to optimize the column design for different design conditions. 

The oxidation of cumene with oxygen from air is nowadays performed in a series of bubble columns. The off-gas is treated by a combination of cooling and adsorption. The recovered cumene from the off-gas treatment is recycled to the oxidation unit. In the concentration unit, the product from oxidation with around 20 -40 wt% CHP is distilled under vacuum to increase the CHP concentration to 65-90wt%. The separated cumene is again recycled to the oxidation unit. In the cleavage, the CHP is converted into phenol and acetone using sulfuric acid as the catalyst. In parallel, the DMBA from oxidation is nearly quantitatively converted... 

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