Process control chlorine compressor

In a plant producing liquid chlorine, the compressed gas goes next to the liquefaction system. Rather than impose a pressure drop between the processes, the gas is allowed to flow freely into liquefaction. A valve on the uncondensed gas venting from the liquefaction unit (Section 9.1.7.2) controls the pressure on both systems. When chlorine is sent to another process without liquefaction, it would be possible to withdraw it on downstream pressure control and let the compressor outlet pressure fluctuate. This approach leads to variability in the differential pressure across the compressor recycle valve. Fluctuations in this flow can cause fluctuations in the compressor suction pressure and therefore in the cellroom chlorine header. It is better to control the compressor outlet pressure itself, even at the cost of another pressure control loop at the destination. Section 11.3.2.6 describes instrumentation hardware and the problems of transferring chlorine to more than one destination. 

The two-phase process stream goes to a receiver that also serves as a phase separator. The liquefied chlorine leaves through a bottom connection. The uncondensed gas, containing some of the chlorine along with the noncondensable impurities, goes overhead. A valve on the outlet line maintains the desired pressure on the gas in the liquefier and also serves to control the discharge pressure of the chlorine compressor. The combination of the pressure and temperature on the process side of the liquefier determines the extent to which the chlorine can condense (see Achievable Degree of Liquefaction, below). 

Figure 11.29 shows a liquefaction unit. As with chlorine compressors, the refrigeration systems associated with liquefaction can be supplied as completely automated packages. Within their capacity, they will automatically adjust to the amount of chlorine fed to the system. The chlorine separator temperature is used to adjust the refrigerant evaporation pressure (and therefore, its temperature). With control by PLC, the only other signals to be passed to the main control center are the alarms and the shutdown status. Refrigerant flow is balanced with process demand by control of its level in the liquefier. 

Direct feed of compressed chlorine to another process is possible only when the quality of the chlorine meets the user s needs. The receiving process determines and controls how much chlorine is taken from the header, but another control valve is necessary at the compressor discharge header in case the user attempts to take more chlorine than is available. The liquefiers normally handle the chlorine not taken by the direct user. It has been common practice to design the liquefaction plant for full cell output, so that the cells can operate at full rate during short upsets in the user s process. This approach may be modified to suit restrictions on maximum chlorine inventory. In any case, the liquefiers should always have some chlorine gas fed to them to keep them operational and ready to handle full chlorine production should the direct user suddenly stop taking gas. In addition, a supply of liquid chlorine may be needed for a suction chiller. A bypass line around the control with a restricting orifice sized for about 10% of full capacity at the control valve drop can meet this requirement. Any chlorine not taken... 

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