Unit Operations - Specific

Some examples and considerations for specific common unit operations follow. 

Reactor design is particularly critical, because reactors involve chemical transformations and often potentially significant energy releases. 

Not all reactions take place in a designated reactor. Some occur in a heat exchanger, a distillation column, or a tank. Understand the reaction mechanisms and know where the reactions occur before selecting the final design. 

Some batch reactions have the potential for very high energy levels. If all the reactants (and sometimes catalysts) are put into a kettle before the reaction is initiated, some exothermic reactions may result in a runaway. The use of continuous or semi-batch reactors to limit the energy present and to reduce the risk of a runaway should be considered. The term semi-batch refers to a system where one reactant and, if necessary, a catalyst is initially charged to a batch reactor. A second reactant is subsequently fed to the reactor under conditions such that an upset in reacting conditions can be detected and the flow of the reactant stopped, thus limiting the total amount of potential energy in the reactor. 

Additional discussion regarding reactor design strategies is covered in Section 3.1 on minimization (as an inherently safer design strategy), and in Section 4.4 on Design and Construction. 

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The use of the computer in the design of chemical processes requires a framework for depiction and computation completely different from that of traditional CAD/CAM appHcations. Eor this reason, most practitioners use computer-aided process design to designate those approaches that are used to model the performance of individual unit operations, to compute heat and material balances, and to perform thermodynamic and transport analyses. Typical process simulators have, at their core, techniques for the management of massive arrays of data, computational engines to solve sparse matrices, and unit-operation-specific computational subroutines. 

Process Rationale. The products of plasma fractionation must be both safe and efftcaceous, having an active component, protein composition, formulation, stabiUty, and dose form appropriate to the intended clinical appHcation. Processing must address a number of specific issues for each product. Different manufacturers may choose a different set or combination of unit operations for this purpose. 

The flow sheet can be tailored to the specific characteristics of the ore. The unit operations can be combined or modified in many ways to upgrade the total iron content in the ore from levels as low as 20—38% up to levels of 65—70%. 

Each basic operation can be divided into one or more unit operations. Size reduction involves cnishing and grinding depending on the size of material handled, and these may be carried out in stages. Separations can be either soHds from soHds, based on size or mineral composition, or soHds from Hquids, ie, dewatering (qv). Size separation or classification is an integral part of any flow sheet, not only to meet product size specifications, but also to ensure a narrow size distribution for subsequent minerals separation circuits and to decrease the load and improve the efficiency of size reduction units which are energy intensive. 

Table 3 fists cycloaliphatic diamines. Specific registry numbers are assigned to the optical isomers of /n t-l,2-cyclohexanediamine the cis isomer is achiral at ambient temperatures because of rapid interconversion of ring conformers. Commercial products ate most often marketed as geometric isomer mixtures, though large differences in symmetry may lead to such wide variations in physical properties that separations by classical unit operations are practicable, as in Du Font s fractional crystallisation of /n t-l,4-cyclohexanediamine (mp 72°C) from the low melting (5°C) cis—trans mixture. 

Contaminant Particle size Specific gravity Unit operation... 

Gaseous vent streams from the different unit operations may contain traces (or more) of HCl, CO, methane, ethylene, chlorine, and vinyl chloride. These can sometimes be treated chemically, or a specific chemical value can be recovered by scmbbing, sorption, or other method when economically justified. Eor objectionable components in the vent streams, however, the common treatment method is either incineration or catalytic combustion, followed by removal of HCl from the effluent gas. 

Steps 6 and 7 ate involved with inputting additional specifications about the process being simulated. It is necessary to give an adequate number of specifications for each unit operation, for each calculation unit, and for the overall process flow so that all the degrees of freedom ate taken away and a unique solution can be obtained from the simulator. On the other hand, if mote than the necessary number of specifications ate given, the problem becomes overconstrained for the simulator and no solution can exist. 

A wide array of general-purpose distillation packages are available to the engineer. Some of the distillation software is stand-alone, whereas other packages are a part of a general-purpose flow sheet or process-simulation system. Because distillation is so universal, all process simulators have one or more distillation program modules for this unit operation. Often the nature of the distillation modules determines the suitabiUty of or the preference for the use of a specific simulator for an appHcation. 

Performance of the power-recovery unit operating with a makeup driver (Fig. 29-56) is shown in Fig. 29-57 specific percentage values are shown, but the general characteristics and curwe shapes are typical. It should be noted that the flow scheme, the selection of... 

The review should emphasize developing an understanding of the processing sequence, the equipment, the equipment plot, the operating conditions, instrument and sample locations, the control decisions, and the operators perspectives. Wmle the preparation effort may be less for those who have been responsible for the unit for a long period of time, the purpose of the test requires that the types and locations of the measurements be different from those typically recorded and typically used. The condition of these locations must be inspected. Operating specifications may be different. Therefore, refreshment is always necessaiy... 

Focus For the purposes of this discussion, a model is a mathematical representation of the unit. The purpose of the model is to tie operating specifications and unit input to the products. A model can be used for troubleshooting, fault detection, control, and design. Development and refinement of the unit model is one of the principal results of analysis of plant performance. There are two broad model classifications. 

Parameter Estimation Relational and physical models require adjustable parameters to match the predicted output (e.g., distillate composition, tower profiles, and reactor conversions) to the operating specifications (e.g., distillation material and energy balance) and the unit input, feed compositions, conditions, and flows. The physical-model adjustable parameters bear a loose tie to theory with the limitations discussed in previous sections. The relational models have no tie to theory or the internal equipment processes. The purpose of this interpretation procedure is to develop estimates for these parameters. It is these parameters hnked with the model that provide a mathematical representation of the unit that can be used in fault detection, control, and design. 

Step 6 Write the Component Material Balances. The Phase II auditing steps define the pollutants and wastes that are among the team s focus. Its objective has always been to identify specific wastes or pollutants that the enterprise can reduce these are the components the team needs to assess in the material balances. It is important to note that once the material balance for each unit operation has been completed for raw-material inputs and waste outputs, it is necessary to repeat the procedure for each contaminant of concern. 

The term process intensification is used synonymously with minimization. Process intensification is also often used more specifically to describe new technologies which reduce the size of unit operations equipment, particularly reactors. Innovative process intensification techniques are receiving more and more attention. Interesting possibilities for a range of unit operations, including reaction, gas-liq-... 

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