Abstract: For parallel heat exchangers, one using a more preferred heat exchange medium as a heat exchange source and another using a less preferred heat exchange medium, maximum use of the more preferred heat exchange medium is used prior to use of the less preferred heat exchange medium in achieving a desired total heat exchange through the parallel heat exchange system. In employing such a heat exchange system in a cracking process it is combined with individual control of flows of feed fluid and diluent fluid to a plurality of individual cracking tubes in a cracking furnace to maintain a desired mass flow of feed fluid and ratio of mass diluent fluid to steam fluid to each such cracking tube.

Abstract: An integrated control system for a vinyl acetate process. A desired production rate of vinyl acetate is achieved by maintaining a desired reactor temperature. At the same time, the concentration of oxygen in the vinyl acetate reactor and condenser are maximized and the reactor pressure is maximized, all in a safe manner, so as to increase the selectivity of the process to the production of vinyl acetate. Also, the overall process is controlled so as to improve the efficiency of energy and materials usage.

Abstract: Control of an isomerization process is based on a known relationship between the octane number and temperature of a reaction at equilibrium; and a predetermined relationship between octane number and temperature of the reaction product of a reactor using a specific catalyst. In use, measurements of actual octane number and temperature are compared with the octane number and temperature of a reaction at equilibrium to determine if the reaction is at equilibrium. If the reaction is not at equilibrium, a control signal is developed to restore equilibrium. Thus control of an isomerization process is accomplished so as to substantially maximize the efficiency of the isomerization process both from a production standpoint and from an energy usage standpoint by maintaining the isomerization reaction at desired equilibrium conditions. Also, for a dual reactor system, the differential temperature between the two reactors is manipulated so as to maintain more favorable isomerization conditions in the tail reactor.

Abstract: In an aromatic extraction process, the flow rate of aromatics to the extraction column is utilized to control the downstream stripping column so as to maintain a desired purity of the aromatic extract stream withdrawn from the stripping column. Also, the flow rate of solvent to the extraction column is manipulated so as to maintain a desired aromatic concentration in the raffinate stream withdrawn from the extraction column. This control, together with other interactive control functions, results in a control of the aromatic extraction process which substantially maximizes the profitability of the extraction process.

Abstract: A reforming furnace having a radiant section containing cracking tubes, a convection section and a stack is controlled so as to maintain a desired synthesis gas temperature unless such maintenance would result in the violation of a process constraint. The desired excess oxygen concentration in the combustion gases withdrawn from the radiant section is also maintained while maintaining the desired synthesis gas temperature. As for the convection section, the temperature of a fluid stream passing through the convection section is maintained while still maintaining a desired excess oxygen concentration in the stack gas.

Abstract: In an absorption process in which an absorption medium is utilized to absorb H.sub.2 S from a feed gas containing H.sub.2 S, the flow rate of the lean absorption medium is manipulated so as to maintain the actual H.sub.2 S concentration in the product gas stream from the absorption process substantially equal to a desired value for the H.sub.2 S concentration. However, if the H.sub.2 S concentration in the rich absorption medium stream exceeds a high limit, the control of the lean absorption medium flow rate so as to maintain a desired H.sub.2 S concentration in the product gas stream is overridden until such time as the H.sub.2 S concentration in the rich absorption medium stream is below the high limit.

Abstract: In a process in which a heating fluid supplied from a heater is utilized to supply heat to a plurality of processes, method and apparatus is provided for substantially optimizing the operation of the heater by minimizing the temperature of the fluid flowing from the heater while continuing to satisfy the heat demand of the processes being served.

Abstract: Measurements of process variables associated with a cracking furnace are utilized to calculate the actual maximum tube skin temperature for the cracking furnace. The thus calculated actual maximum tube skin temperature is then utilized to derive a control signal which may be utilized to manipulate the rate at which heat is supplied to the cracking furnace. The accurate prediction of the actual tube skin temperature based on actual process measurements enables the conversion rate for the cracking furnace to be substantially maximized while preventing damage to the cracking tubes.

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 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: A fluid catalytic cracking unit (FCCU) is controlled in such a manner that load is automatically shifted from the wet gas compressor to the air blower for the catalyst regenerator if the loading on the wet gas compressor becomes a limiting factor on the yield of a desired product from the FCCU. Also, the preheat temperature for the feed flowing to the reactor is automatically increased if the desired temperature in the reactor cannot be maintained by increasing catalyst flow to the reactor without exceeding an air blower limitation. This automatic load shifting provides for a substantially maximum production of a desired product without exceeding a process limitation.

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: In a process in which a turbo-expander is utilized to drive a compressor, the pressure of the gas flowing to the turbo-expander is maintained at a desired level by manipulating the speed of the turbo-expander. If the turbo-expander should tend to overspeed, the discharge output of the compressor is recycled to the suction inlet of the compressor to thereby increase the loading on the compressor which will result in a reduced turbine speed. More gas can then be fed through the turbo-expander which will maintain the desired pressure. If the pressure of the gas flowing to the turbo-expander should continue to increase, a bypass valve is utilized to bypass the turbo-expander to thereby maintain a desired pressure of the gas flowing to the turbo-expander.

Abstract: A control system for a multiple feed fractional distillation column is disclosed in which the flow rate signals of the multiple feed streams are delayed and combined for use in feedforward predictive control of the upper reflux flow, intermediate reflux flow and/or reboiler fluid flow to the fractional distillation column. Temperature measurements of the upper vapor stream, the upper reflux stream, the intermediate vapor stream and the intermediate reflux stream as well as analysis of a product stream can be utilized to provide feedback control of the fractional distillation column.

Abstract: The heat supplied to a cracking furnace is controlled in response to a calculation of the heat required to maintain a desired conversion rate for the material being cracked in the cracking furnace. The calculated required heat for the cracking furnace is corrected by a comparison of the actual conversion rate to a desired conversion rate. The corrected required heat signal is utilized to manipulate the flow of fuel to the cracking furnace to thereby control the heat supplied to the cracking furnace.The flow of steam to the cracking furnace is also manipulated so as to prevent damage to the cracking furnace caused by a loss of feed flow to the cracking furnace. A loss in the flow of the feed to the cracking furnace is detected and the flow of the steam is increased to compensate for the loss in the flow of the feed to the cracking furnace so as to prevent damage to the cracking tubes of the cracking furnace.

Abstract: A control system is provided for a fractional distillation column where at least a portion of the overhead vapor stream from the fractional distillation column is utilized in a refrigeration and heating system for a process with which the fractional distillation column is associated. The liquid level is an overhead accumulator associated with the fractional distillation column is maintained at a level sufficient to insure that a sufficient supply of the overhead vapor stream from the fractional distillation column is available to supply the refrigeration and heating needs of the process with which the fractional distillation column is associated.

Abstract: A compression system is controlled so as to maintain the actual suction pressure as close as possible to a desired suction pressure without exceeding the operational limitations of the compression system. The speed of the compression system is manipulated so as to maintain the actual suction pressure equal to a desired suction pressure if no compression system constraint is encountered. If a compression system constraint is encountered, the compression system speed is manipulated so as to operate against the particular constraint encountered without exceeding that particular constraint.

Abstract: Control of the main gas stream from a compressor is accomplished by automatically adjusting the amount of gas vented from the compressor output by a variable relief valve or other flow restriction means, with the speed of the compressor being automatically adjusted to maintain the compressor output at a level permitting effective control within the useful range of the venting system. In a preferred embodiment, the amount by which the position of the means used to restrict flow through the vent conduit departs from a desired operating range is used to generate a compressor acceleration signal which is in turn used to generate a compressor speed set point signal from which the compressor speed is controlled.

Abstract: A method and apparatus is provided for controlling the operation of a reforming furnace used to decompose at least a portion of a feed stream. A by-product fuel from subsequent process steps, which is a result of decomposition of the feed in the furnace, is used as one fuel source to fire the furnace. The control system monitors the byproduct fuel stream and the feed stream to a decomposition side of the furnace and determines the feed rate of additional fuel required to fire the furnace, which feed rate is correspondingly controlled. The control system includes means for determining various selected variables as sensed by suitable analyzers and flow rate meters and comparing certain variables to determine the flow rate of the additional fuel required to fire the furnace.