Balance Flowsheet

The first objective of flowsheeting is to obtain a consistent description of the mate-rial-balance envelope. The reactor model should be of the kinetic type, at least for the main reactions, in order to account for the effect of variable flow rates and composition of recycles coming from separations. Stoichiometric or yield reactor models can be employed for describing secondary reactions and formation of impurities. In a first attempt the separators may be black-boxes provided with appropriate specifications. 

The treatment of conflicting specifications leading to convergence problems has been developed elsewhere [7]. For example, this situation arrives when the distillation columns are specified by fixed product flow rates. These specifications, correct for standalone columns, lead to nonconvergence when the units are placed in recycles. The explanation is that during the iterative solution it is impossible to 

The reader will observe that the direct sequence of separations leads to nested recycle loops, while the indirect sequence gives two independent recycle loops. Contrary to expectation, more favorable for the direct sequence, both schemes are equivalent in energy consumption. The explanation is that the largest energy consumer is the separation between the key components, cyclohexanone and cyclohexanol, which is the first split in the direct sequence, and the second split in the indirect sequence. 

The conclusion is that the indirect separation sequence should be more attractive, because it is equivalent in energy, but with fewer interactions in operation. In addition, the split in C2 is phenol-free, offering better catalyst protection in dehydrogenation. 

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In essence, two kinds of errors can occur. First, there are mistakes in setting-up the balance flowsheet (for exeunple some stream has been omitted), further gross and systematic errors of measurement due to malfunction of instrumentation, mistake of the staff and the like. 

Although the flowsheet shown in Fig. 4.7a is very attractive, it is not practical. This would require careful control of the stoichiometric ratio of decane to chlorine, taking into account both the requirements of the primary and byproduct reactions. Even if it was possible to balance out the... 

Preliminary Process Flowsheet. This will show major equipment and lines, preliminary equipment details (vessel diameter, number of trays, pump flow and driver horsepower, etc.), major instrumentation, and, it is hoped, have a material balance at the bottom of each drawing with flows keyed to a numbering system on the diagram. The process flowsheets should cover both the process and utility sides of the plant. 

Used to present the heat and material balance of a process. This may be in broad block form with specific key points delineated, or in more detailed form identifying essentially every flow, temperature and pressure for each basic piece of process equipment or processing step. This may and usually does include auxiliary services to the process, such as steam, water, air, fuel gas, refrigeration, circulating oil, etc. This type of sheet is not necessarily distributed to the same groups as would receive and need the piping flowsheet described next, because it may contain detailed confidential process data. 

Figure 1-11. Typical material balance process flowsheet.

Figure 1-36. Process engineering costs (1975), based on process engineering charged at 14 per manhour. Chemical plant engineering operations, includes flowsheet development and drafting, material and heat balances, equipment designs, ratings, checking, and bid reviews and selection of equipment. By permission, E. E. Ludwig [7].

This section is a general discussion of the techniques used for the preparation of flowsheets from manual calculations. The stream flows and compositions are calculated from material balances combined with the design equations that arise from the process and equipment design constraints. 

A ternary mixture of mole fraction ethanol of 0.15, ethyl acetate of 0.6 and methanol 0.25 is to be separated into relatively pure products. Sketch a system of distillation columns and mixer arrangements in the triangular diagram to carry out the separation by exploiting the shift in the distillation boundary with pressure. Sketch the flowsheet corresponding with this mass balance. 

The separation in Figure 12.21 can be carried out in an alternative sequence. Sketch the mass balance for the alternative sequence in a triangular diagram and the resulting flowsheet. 

Although the flowsheet shown in Figure 13.7a is very attractive, it is not practical. This would require careful control of the stoichiometric ratio of decane to chlorine, taking into account both the requirements of the primary and byproduct reactions. Even if it were possible to balance out the reactants exactly, a small upset in process conditions would create an excess of either decane or chlorine and these would then appear as components in the reactor effluent. If these components appear in the reactor effluent of the flowsheet in Figure 13.7a, there are no separators to deal with their presence and no means of recycling unconverted raw materials. 

In this case, only the material balance will be solved in order to keep the problem simple. If the material balance is to be solved, then a series of material balance equations can be written for the flowsheet in Figure 13.13a ... 

Water balance. Before an existing system can be studied from the point of view of water consumption and effluent treatment, the first step must be to establish a water balance. For a new design, this is a question simply of extracting the information relating to water streams from the flowsheets... 

Figure 5 A simplified flowsheet and materials balance for the recovery of copper from oxidic and transition ores by heap leaching, solvent extraction and electrowinning.

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