Process control, automatic pressure

An extraction plant should operate at steady state in accordance with the flow-sheet design for the process. However, fluctuation in feed streams can cause changes in product quaUty unless a sophisticated system of feed-forward control is used (103). Upsets of operation caused by flooding in the column always force shutdowns. Therefore, interface control could be of utmost importance. The plant design should be based on (/) process control (qv) decisions made by trained technical personnel, (2) off-line analysis or limited on-line automatic analysis, and (J) control panels equipped with manual and automatic control for motor speed, flow, interface level, pressure, temperature, etc. 

A high pressure gel permeation chromatograph (GPC) has been used to monitor the performance of the reactor. A novel aspect of the GPC is that, it too, has been put on-line to the process control computer and both data collection and analysis have been made automatic while giving the operator full interactive facilities. 

The most utilized and reliable process control in the petrofeum and related industries is human observation and surveillance. Local pressure and level gages along with control room instrumentation are provided so that human observation and actions can occur to maintain the proper process conditions. First stage process alarms are provided to alert operators to conditions that they may not have already noticed. Typically when secondary alarm stages are reached, computer control systems employed to automatically implement remedial actions to the process. 

A control system or scheme is characterized by an output variable (e.g., temperature, pressure, liquid level, etc.) that is automatically controlled through the manipulation of inputs (input variables). Suppressing the influence of external disturbances on a process is the most common objective of a controller in a chemical plant. Such disturbances, which denote the effect that the external world has on a process, are usually out of reach of the human operator. Consequently, a control mechanism must be introduced that will make the proper changes on the process to cancel the negative impact that such disturbances may have on the desired operation of the process. Control engineers usually refer to the combination of a sensing element and a control device with a set point as a control loop. ... 

It has been stated (J.S.D.C., 1968, 84, 66) that the productivity of one man looking after a group of five high pressure dyeing machines was increased by 37 per cent by the introduction of automatic process control. The reason is that there are a number of occasions when more than one operation should be carried out simultaneously and this cannot be done manually. Another factor is the human inability to move and react mentally with instantaneous speed. Many more examples could be quoted to demonstrate the advantage of the elimination of human memory and manual manipulation in the operation of dyeing processes. 

The great majority of automatic process control systems involve one or more of only five process variables, namely, pressure, temperature, flow rate, composition, and liquid level. Many of these variables are measured by the same kind of instrument, and indeed, all of them under certain circumstances can be evaluated in terms of pressures. Thus temperature can be measured by the pressure exerted by a confined gas in the gas thermometer the differential pressure across a restriction in a flow line is a measure of flow rate the pressure exerted by a boiling liquid mixture... 

As defined here, automatic process control always implies the use of a feedback. This means that the control instrument is continuously monitoring certain output variables of the controlled process, such as a temperature, a pressure, or a composition, and is also compeuing this output with some preestablished desired value, which is considered a reference, or a set point, of the controlled variable. An error that is indicated by the compeuison is used by the instrument to compute a correction to the setting of the process control valve or other final control element in order to adjust the value of the outpirt variable to its desired level and maintain it there. 

Modern cryogenics plants are well-instrumented to indicate process temperatures and pressures, flow rates, liquid levels, and contaminant concentrations. These instruments are used for measurement, control, and safety if a particular variable (temperature, pressure, flow rate) falls outside the control range, a corrective action is initiated automatically and, if necessary, an alarm may also be actuated. Complete alarm systems are now available commercially for use with all the common safety devices. Further, many are designed on the failsafe principle and sound an alarm in the event a power failure occurs. 

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