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What are the typical benefit one could expect from APC implementation in Alky (sulphuric acid based) unit? Also, has anyone used early event system to forewarn operator on reaction section operation?
 
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28/09/2009 A: keith bowers, B and B Consulting, kebowers47@gmail.com
The S/A alkylation unit is characterized by very long time delays between changes in feedstock, contactor condition, DIB tower operations AND measurable changes in alkylate quality and yield. These long time delays make conventional 'feedback' process control strategies of little use in optimizing or improving economics of the unit. Even 'feed-forward' process control is difficult to implement successfully with measurable results. A key difficulty is precise measurement of spent acid quality. The large circulating inventory and significant time delay in the acid settlers hinders using 'acid strength' as a direct control variable.
APC can help significantly reduce overall utilities consumption associated with the IC4 recycle rate, helping optimize the recycle compressor operations (effluent refrigeration) and reflux/reboil rates in the DIB column. The interaction of these two systems is not 'obvious' to a classical process control scheme since they are loosely coupled with long time lags. The overall goal of this 'system' is optimal energy use to achieve maximum economic value added in terms of octane barrels, considering ALL the costs, including acid use/regeneration costs.
In contrast to the 'slow' process response of the acid and IC4 recycle loops, the actual reaction is extremely fast, and fast, precise control of all the variables of each reactor are critical to maintaining maximum economic conditions. There is a fundamental philosophy difference between a 'Stratco Contactor' type reactor system and a cascade of 'mixed tank' reactors, each requiring different control schemes for optimization.