Feedback loops chemical composition

Most disturbances in chemical engineering systems are load disturbances, such as changes in throughput, feed composition, supply steam pressure, cooling water temperature, etc. The feedback controller s Auction when a load disturbance occurs is to return the controlled variable to its set-point by suitable changes in the manipulated variable. The dosedloop response to a load disturbance is called the regulator response or the closed-loop load response. 

Figure 21.9 The two control procedures for on-line analysis. Taking a chemical reaction as an example, procedure (a) corresponds to an analyser situated after synthesis (closed loop feedback through comparison with a reference value). Alternatively, procedure (b) corresponds to an up-stream analyser (in an open loop with continuous adjustment of the reagents composition).

There is a traditional pattern of what is called instrumentation in the chemical and petroleum industries based on single-loop control (sometimes called SISO—single input, single output). Each process operation has a number of independent or single loops for feedback control of temperatures, pressures, flows, liquid levels, and sometimes compositions. The term single loop means there is one measurement, one controller, and one final control element, usually a valve. Many, but not all, of these loops are represented in the central control room (CCR) by control stations. 

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