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# PDD_26_IOS - 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 - 26

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Process Design Decisions (PDD) Prof B V Babu Input Output Structure of the Flow Sheet Decisions for the Input-Output Structure Flow Sheet Alternatives Level-2 Decisions Purification of feeds Recover & Recycle reversible products Gas Recycle & Purge Copyright © 2008-2009 B V Babu, BITS-Pilani Do not Recover & Recycle some reactants Number of Product streams Design variables & Economic trade-offs Summary
Process Design Decisions (PDD) Prof B V Babu Decisions for the Input-Output Structure To understand the decisions required to fix the input- output structure of a flow sheet We merely draw a box around the total process Thus, we focus our attention on what raw materials are fed to the process and what products and byproducts are removed Copyright © 2008-2009 B V Babu, BITS-Pilani Since, the raw materials costs fall normally in the range of 33-85% of the total processing costs , we want to calculate these costs before we add any other detail to the design

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Process Design Decisions (PDD) Prof B V Babu Flow Sheet Alternatives Almost every flow sheet has one of the two structures shown below: PROCESS Product By-product Feed Streams Fig. 1 (a) Copyright © 2008-2009 B V Babu, BITS-Pilani PROCESS Product By-product Purge Gas Recycle Feed Streams Fig. 1 (b)
Process Design Decisions (PDD) Prof B V Babu Flow Sheet Alternatives For initial design calculations We use the order-of-magnitude argument to say that the rule of thumb That it is desirable to recover more than 99% of all valuable materials Is equivalent to requiring That we completely recover and then recycle all valuable reactants Copyright © 2008-2009 B V Babu, BITS-Pilani Thus Fig. 1(a) indicates that no reactants leave the system If air & water are reactants in a process They are sufficiently inexpensive compared to organic materials That it might be cheaper to lose them in an exit stream rather than to recover & recycle them Hence, in a few rare cases, Fig. 1(a) might not be complete

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Process Design Decisions (PDD) Prof B V Babu Flow Sheet Alternatives The other situation in which commonly reactants have been lost from a process occurs When we have a gaseous reactant , and Either a gaseous feed impurity Or a gaseous by-product produced by one of the reactions We want to recycle the gaseous reactant Copyright © 2008-2009 B V Babu, BITS-Pilani But the inert-gas components must be purged from the process so that they do not continue to accumulate in the gas-recycle loop In the past, it was so expensive to separate gaseous mixtures that some reactant was allowed to leave the process in a gas-recycle and purge stream [ Fig. 1(b) ]
Process Design Decisions (PDD) Prof B V Babu Flow Sheet Alternatives However, the new membrane-separation technology, such as Monsanto’s Prism Process , has cut the cost of gaseous separations

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PDD_26_IOS - Process Design Decisions(PDD Process Design...

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