Thermally Integrated Separation Flowsheets

At multicomponent distillation change of pressures in the columns to ensure heat exchange between their condensers and reboilers leads to the increase of total difference of temperatures between the hottest reboiler and the coldest condenser. This increase is hmited by temperatures of available heat carriers and refrigerants and in a number of cases also by thermolability of the mixture. 

The apphcation of thermally integrated separation flowsheets can give considerable economy of energy expenditures, but runs across the mentioned limitation. 

Besides that, usage of this flowsheet usually leads to the increase of capital costs for apparatuses and for the control system. 

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Thermodynamically Improved and Thermally Integrated Separation Flowsheets... 

In addition to recycle streams returned back to upstream units, thermal integration is also frequently done. Energy integration can link units together in locations anywhere in the flowsheet where the temperature levels permit heat transfer to occur. The reaction and separation sections are thus often intimately connected. If conditions are altered in the reaction section, the resulting changes in flowrates, compositions, and temperatures affect the separation section and vice versa. 

While synthesizing separation flowsheets, it is necessary to consider the pos-sibUity of thermodynamic improvement and thermal integration. Therefore, for each sequence, identified in the process of synthesis, it is necessary to realize possible thermodynanric improvements and thermal integration of the columns. The estimation of expenditures on separation is made taking into consideration these modifications, if these expenditures are smaller than for the sequence without modifications under consideration. This estimation of expenditures is used while comparing the sequence under consideration to all other possible sequences. 

After identification of several preferable sequences, choosing among the optimum sequences, taking into consideration possible thermodinamic improvements and thermal integration of columns, arises. This task is similar to the synthesis of separation flowsheets of zeotropic mixtures (see Section 8.3), and it should be solved by the same methods (i.e., by means of comparative estimation of expenditures on separation). The methods of design calculation, described in Chapters 5 7 for the modes of minimum reflux and reflux bigger than minimum, have to be used for this purpose. In contrast to zeotropic mixtures, the set of alternative preferable sequences for azeotropic mixtures that sharply decreases the volume of necessary calculation is much smaller. 

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