Abstract: In a process in which liquid flows to a vessel and is then withdrawn from the vessel, method and apparatus is provided for implementing nonlinear liquid level control by providing a variable set point to a conventional level controller. The variable set point is a nonlinear function of the actual liquid level in the vessel. The variable set point enables the vessel to absorb disturbances in the flow rate of the liquid flowing to the vessel while the actual liquid level in the vessel remains between a desired high and low limit thus providing a smooth flow of liquid from the vessel.

Abstract: The efficiency of a parallel heat exchanger arrangement is substantially maximized by utilizing the differential temperature across a first heat exchanger or group of heat exchangers in series to derive a set point for the differential temperature across a second heat exchanger or second group of heat exchangers in series required to substantially maximize the efficiency of the parallel heat exchanger arrangement. Fluid flow to the heat exchangers is manipulated so as to force the differential temperature across the second heat exchanger or second group of heat exchangers to equal the derived set point.

Abstract: The heat supplied to an ethylene process cracking furnace is manipulated so as to maintain a desired ethylene production rate while the flow of feed to the ethylene process cracking furnace is manipulated so as to substantially maximize the selectivity of the ethylene process cracking furnace to the production of ethylene. In this manner a desired ethylene production rate is maintained while the process economics are improved in the case where the cost of the feed is a more important economic consideration than the cost of the energy required for the cracking furnace by substantially minimizing the conversion of the feed to undesired byproducts.

Abstract: In a process in which both a turbine or motor and an expander are utilized to drive a compressor, the use of the expander is substantially maximized while still insuring that the driving force applied to the turbine or motor does not go below a low limit. A variable limit is also utilized to prevent a stuck valve from being overdriven by a controller.

Abstract: The heat provided to a cracking furnace is manipulated so as to substantially maximize the value of the product stream flowing from the cracking furnace. An analysis of the product stream may be utilized to measure the ratio of a first constituent in the product stream to a second constituent in the product stream (severity equivalent). The severity equivalent required to substantially maximize the value of the product stream may be calculated in response to process operating conditions. The heat supplied to the cracking furnace is manipulated in response to a comparison of the actual severity equivalent to the desired severity equivalent to thereby substantially maximize the value of the product stream.

Abstract: A selective hydrogenation process wherein at least two catalyst beds in series is controlled by predicting the inlet temperature to the first catalyst bed in the series required to maintain the concentration of the component being selectively hydrogenated in the effluent flowing from the second reactor within a desired concentration limit. This prediction is biased by a comparison of the actual concentration to the concentration limit. The thus biased prediction is utilized to manipulate the inlet temperature to the first reactor in the series. The inlet temperature to the second reactor to the series is controlled by establishing a prediction of the differential temperature across the second reactor in the series.

Abstract: A control system for a fractionation column which separates a feed into a heavy bottoms product, which is used as fuel for a plant, and into a light overhead product, which is used in a process, is disclosed. The fractionation column is controlled in such a manner that sufficient bottoms product is supplied to meet the plant fuel requirements while maintaining a desired distillation temperature for the overhead product. The flow rate of fuel oil to the reboiler furnace associated with the fractionation column and the flow rate of the external reflux to the fractionation column are controlled in such a manner that the objectives of supplying sufficient bottoms product to meet plant fuel requirements and supplying an overhead product having a desired distillation temperature are met while still minimizing the external reflux flow rate and the flow rate of the fuel oil to the reboiler furnace to increase fuel economy.

Abstract: In a fractionation system in which pressure fluctuations within the fractionation vessel can result in alteration of the fractionator product composition, fractionation column pressure is monitored and converted to a temperature requirement signal representative of the temperature required within a preselected portion of the fractionation column in order to provide the desired product constituent distribution. The temperature requirement signal is utilized to provide a required heat signal which is representative of the direction and magnitude of any change necessary in the heat input to the fractionation column to maintain the required temperature. The heat input to the fractionation column is controlled in response to the required heat signal, corrected as necessary by actual analysis of the product.

Abstract: In a fractionation system in which pressure fluctuations within the fractionation vessel can result in alteration of the fractionator product composition, fractionation column pressure is monitored and converted to a temperature requirement signal representative of the temperature required within a preselected portion of the fractionation column in order to provide the desired product constituent distribution. The temperature requirement signal is utilized to provide a required heat signal which is representative of the direction and magnitude of any change necessary in the heat input to the fractionation column to maintain the required temperature. The required heat signal is utilized to provide fast changes in the heat input to the fractionation column in response to pressure fluctuations in the fractionation column.

Abstract: The flow rate of the absorption medium entering an absorption column and the flow rate of steam into a reboiler associated with a stripping column are controlled in response to the flow rate of a feed stream into the absorption column, the concentration of impurities in the feed stream, and the flow rate of the absorption medium containing the impurity removed from the feedstream into the stripping column in such a manner that the efficiency of the absorption process is optimized. A computer system is utilized to simulate the flow of materials inside the absorption and stripping columns. Feed forward control, based on the simulations, is utilized to maximize the energy efficiency of the absorption/stripping process by reducing the amount of steam required by the stripping process and reducing the energy consumption of the pumping system utilized to return the absorption medium to the absorption column.