Purge control for ammonia plant

Abstract

Control of the synthesis loop pressure of an ammonia plant is accomplished by automatically changing the purge rate of the recycle gas to maintain optimum synthesis loop pressure. The control logic of this invention allows compressor speed to be set at a constant value and suction pressure to the first stage of the synthesis loop compressor to be maintained by changing the purge rate of the recycle gas. This allows the synthesis loop pressure to seek the highest possible value at the given feed rates of the synthesis gas and compressor speed assuring optimum compressor loading. If the feed rate of synthesis gas is low, additional control is achieved by using the synthesis loop pressure signal or the concentration of inerts signal to vary the compressor speed. A variation of this control logic for providing additional control where the feed rate of the synthesis gas is low is to use the synthesis loop pressure signal or the concentration of inerts signal to manipulate the purge rate of the recycle gas and to use the suction pressure to the first stage of the ammonia plant synthesis loop to manipulate compressor speed. Another variation of this control logic is to use the suction pressure to the first stage of the ammonia synthesis loop to vary the compressor speed and use either the synthesis loop pressure signal, the concentration of inerts signal, or the RPM of the compressors signal to vary the purge rate of the recycle gas. All of these schemes for providing additional control where the feed rate of the synthesis gas is low allow the synthesis loop pressure to seek the highest possible value at the low feed rate of synthesis gas assuring optimum compressor loading under all modes of operation.