Phillips Petroleum Company

James W Hobbs

Nancy K Stengle

William S Stewart

Dexter E Smith

Robert J DiBiano

David A Christie

Mary M Halliday

Grover R Campbell

Grover L Campbell

A supervisory computer is provided for controlling a compressor wherein a computer generated set point manipulates flow in a recycle line from the compressor outlet to inlet. The supervisory computer manipulates the recycle flow so as to prevent the compressor from surging due to changes in flow rate, pressure or the molecular weight of the gas being compressed. The molecular weight of the gas is calculated on-line from actual measurements of flow, pressure, temperature and speed along with compressor performance data that is prestored in the computer. In addition the supervisory computer automatically maintains a minimum flow for the compressor that is as close as possible to the surge limit without danger of putting the compressor into surge due to changes in flow, pressure or molecular weight of the gas being compressed.

Molecular weight determination for constraint control of a compressor

A supervisory computer for a centrifugal compressor control system is disclosed which manipulates the set point of a controller for a valve in a recycle line from the compressor systems outlet to inlet. The supervisory computer manipulates the set point so as to both prevent surging and substantially minimize the recycling of compressed gas. The computer first determines the surge flow limit for the compressor based on prestored data from the compressors manufacture performance curves such as constant speed compressor curves, and the actual speed of the compressor. The controller then outputs a set point signal which periodically moves the control valve in incremental amounts toward the fully closed position. The periodic movement towards the closed position continues until the flow is close to the surge flow limit, or until the actual deviation of the setpoint flow and the actual process flow exceeds a desired high limit for this deviation. When the constraints are exceeded the controller periodically moves the control valve toward the fully open position until the constraints are satisfied, and then the cycle repeats.

Constraint control for a compressor system

In a process in which a compressor feeds gas to a manifold which in turn supplies a controlled flow of gas to each of a plurality of users in parallel operation, method and apparatus are provided for controlling the back pressure acting against the compressor. The back pressure is manipulated to substantially optimize operation of the compressor. Since user demand for the gas varies over wide ranges, the control system automatically reduces back pressure acting against the compressor during periods of low loads by incrementally opening the control valve to one of the plurality of users. The reduced back pressure causes each on line control valve to move toward a more open position to maintain its set point. The pressure reduction continues in increments until one of the plurality of on line control valves reaches a maximum desired open position. The condition of a control valve exceeding a maximum desired open position is also sensed and the back pressure is then increased to cause the plurality of valves to move toward a closed position. The pressure increase cycle continues until all on line valves achieve a position less than the high limit. At this time the decrease/increase pressure is repeated so that the actual back pressure is floating, being dependent on user demand for the gas.

Floating pressure control for a gas distribution system

In a fractional distillation process in which a sidedraw product stream is withdrawn from the fractional distillation column, the flow rate of the overhead product stream is controlled so as to increase the flow of the upper external reflux so as to drop as much as possible of the overhead product into the sidedraw product stream. However, valve position control is utilized to override the control of the flow of the overhead product stream, which is being controlled so as to substantially maximize the flow of the sidedraw product stream, if the valve controlling the flow of the upper external reflux approaches fully open such that control of the liquid level in the overhead accumulator associated with the fractional distillation process is not lost.

Fractional distillation process control

The heat provided to a cracking furnace is manipulated so as to maintain the actual conversion of a first component in the feed stream flowing to the cracking furnace substantially equal to the desired conversion for the first component. The actual conversion of the first component is determined based on an analysis for the concentration of the first component in the feed stream flowing to the cracking furnace, an analysis of the concentration of the first component in the product stream flowing from the cracking furnace, an estimate of the conversion of the first component and a calculation of the expansion of the feed stream in the cracking furnace. The thus derived actual conversion is compared to a desired conversion with the results of the comparison being utilized in combination with the feed flow rate to manipulate the heat provided to the cracking furnace.

Control of a cracking furnace

A fractional distillation process, in which the flow rate of the external reflux is utilized to control the bottoms product composition, is controlled so as to maintain a desired bottoms product composition to the extent that maintaining the desired bottoms product composition does not cause a process constraint to be violated. A plurality of actual process conditions are compared to process constraints for those process conditions to determine if the flow rate of the external reflux should remain the same, be increased by an incremental amount or be decreased by an incremental amount. The order of the comparison of process constraints to actual process conditions is such that the most important comparison will control the external reflux flow rate if that comparison indicates that a process constraint has been violated.

Constraint control of a fractional distillation process

In a fractional distillation process in which it is desired to maintain a desired overhead product composition and bottoms product composition, a set point for the flow rate of the external reflux to the fractional distillation column is increased or decreased by an incremental amount or is left constant based on whether the concentration of the heavy key component in the overhead product stream and the concentration of the light key component in the bottoms product stream are above a high limit, below a low limit, between the high limit and low limit or some combination thereof. At the same time, a temperature set point for the fractional distillation column is increased or decreased by an incremental amount or is left constant depending on the same factors. Use of these set points for process control provides control of the overhead product composition and bottoms product composition without the use of analysis controllers.

Control of a fractional distillation process

In a fractional distillation process in which first (light key) and second (heavy key) components contained in a feed stream are separated with the major portion of the light key component being removed in an overhead product stream and the major portion of the heavy key component being removed in a bottoms product stream, a signal which is responsive to the concentration of the light key component in the bottoms product stream is summed with a signal which is responsive to the concentration of the heavy key component in the overhead product stream to derive a signal which is utilized to manipulate the flow rate of the external reflux (sigma control). Also, the difference between a signal which is responsive to the concentration of the light key component in the bottoms product stream and a signal which is responsive to the concentration of the heavy key component in the overhead product stream is utilized to control the flow rate of the bottoms product stream (delta control). The sigma-delta control results in decoupling of the overhead product composition control and bottoms product composition control, improved recovery from disturbances and increased stability of the process.

In a fractional distillation column in which at least a first component and a second component are at least partially separated with the first component being removed principally as an overhead product and the second component being removed principally as a bottoms product and in which the heat supplied to the fractional distillation column is used as the primary control for the overhead product composition with the flow rate of external reflux to the fractional distillation column being used to control the bottoms product composition, a comparison of the actual concentration of the second component in the overhead product to the desired concentration is utilized to bias the control signal which is utilized to manipulate the flow rate of the external reflux to the fractional distillation column. The bias term provides a quick response to an off-specification overhead product by changing the flow rate of the external reflux so as to maintain a desired overhead product composition until the heat supplied to the fractional distillation column can be changed to maintain the desired overhead product composition.

In a fractionation process in which at least two fractional distillation columns are cascaded in series, the vapor from the overhead accumulator associated with the first fractional distillation column in the series is provided to a feed tank for the second fractional distillation column in the series. The flow of the vapor from the overhead accumulator associated with the first fractional distillation column in the series is controlled by means of a control valve. An interactive control system is utilized to maintain the pressure in the overhead accumulator associated with the first fractional distillation column within safe operating limits, maintain the control valve at a substantially fully open position so as to minimize the pressure drop across the control valve controlling the flow of vapor from the overhead accumulator associated with the first fractional distillation column, and maintain the pressure in the feed tank to the second fractional distillation column at a desired value.

Fractionator feed tank pressure control

A non-linear level controller is provided in which the proportional term is a function of the actual liquid level in a vessel compared to the actual liquid level in the vessel at a previous time. The integral term is a function of the actual liquid level in the vessel compared to a set point which is representative of the desired liquid level in the vessel. Both the proportional term and the integral term are modified by a variable gain factor which is a non-linear function of the liquid level being controlled. The proportional term and integral term are combined to provide the controller output which is utilized to manipulate a flow of liquid from the vessel in which the liquid level is being controlled.

Non-linear level controller

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.

Compressor control

The flow rate of feed material containing a constituent to be absorbed and the flow rate of the absorption medium entering an absorption zone are controlled in response to the content and flow rate of feed material entering the absorption zone and the amount of a desired bottoms constituent in the overhead stream of the absorption zone so that the efficiency of the absorption process is optimized. Under circumstances necessitating maximum production from the absorption zone and its associated stripping zone, the absorption medium flow rate is maintained at a maximum value and the feed flow rate is controlled in response to feed composition and flow as well as the concentration of desired bottom product in the overhead stream. The output of the absorption unit is stabilized by alteration of the flow control apparatus associated with the output stream of the stripping zone in order to alter the amount of control over the output flow. A set point signal which controls the product output flow is generated in response to the predicted rate of delivery of product material to an accumulator and a modified signal related to the liquid level within the accumulator.

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Sweeny, TX, United States of America

James W Hobbs