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PDD_18_EDM2 - Process Design Decisions(PDD Process Design...

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Process Design Decisions (PDD) Prof B V Babu Process Design Decisions Copyright © 2008-2009 B V Babu, BITS-Pilani LECTURE - 18
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Process Design Decisions (PDD) Prof B V Babu Engineering Economics Recap Economic Decision Making Example : Design of a solvent recovery system Design of Gas Absorber System Flow sheet Alternatives Design Heuristic Copyright © 2008-2009 B V Babu, BITS-Pilani Material Balances Trade-offs for fractional recovery in Absorber & Distillation columns Trade-offs for solvent flow rate in Absorber Rules of thumb (Heuristics) Importance of Solute-free basis quantities Summary
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Process Design Decisions (PDD) Prof B V Babu Design of Gas Absorber System Flow Sheets & alternatives for gas absorption system Before any calculations, it is necessary To invent a flow sheet for process The simplest possible flow sheet we might imagine is: Copyright © 2008-2009 B V Babu, BITS-Pilani
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Process Design Decisions (PDD) Prof B V Babu Design of Gas Absorber System: Flow sheet TION BER Air Solvent (Water) (77 0 F = 25 0 C or less) Acetone COOLANT Copyright © 2008-2009 B V Babu, BITS-Pilani Fig. 1 Flow sheet of gas absorption HEAT DISTILLAT ABSORB Acetone + Water Acetone + Air (Gas Feed having solute) Water + traces of Acetone COOLANT POLLUTION TREATMENT Sewer
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Process Design Decisions (PDD) Prof B V Babu Design of Gas Absorber System: Flow sheet We use water as a solvent (because it is cheap) to recover acetone from air stream in the gas absorber, and Then we distill the acetone product from the water And throw away the water Environmental constraints may preclude this possibility So, we include cost for pollution treatment Copyright © 2008-2009 B V Babu, BITS-Pilani Of course, we could recycle the process water to the gas absorber And in this way we would avoid any environmental problem If we used any thing other than water as a solvent , we would always recover and recycle the solvent
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Process Design Decisions (PDD) Prof B V Babu Design of Gas Absorber: Alternate Flow sheet Acetone COOLANT TION BER Air Solvent COOLANT Copyright © 2008-2009 B V Babu, BITS-Pilani Fig. 2 Alternate flow sheet of gas absorption HEAT DISTILLAT 212 0 F ABSORB Acetone + Water Acetone + Air (Gas Feed having solute)
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Process Design Decisions (PDD) Prof B V Babu Design of Gas Absorber: Alternate Flow sheet It is reasonable to question, whether discarding the process water, as shown in Fig. 1 can ever be justified Even when a pollution treatment facility is available The only justification is based on the temperature of the process water entering the gas absorber If a recycle process (Fig 2) is used, we cool the recycle Copyright © 2008-2009 B V Babu, BITS-Pilani stream with cooling water Normally we assume that cooling water is available from the cooling towers at 90 0 F (32 0 C) (on the hot summer day) and that it must be returned to the cooling towers at a temperature less than 120 0 F (49 0 C) to prevent excessive scale formation on the exchanger surface
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Process Design Decisions (PDD)
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