CA1277744C - Material and energy balance reconciliation process control method and apparatus - Google Patents
Material and energy balance reconciliation process control method and apparatusInfo
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- CA1277744C CA1277744C CA000520501A CA520501A CA1277744C CA 1277744 C CA1277744 C CA 1277744C CA 000520501 A CA000520501 A CA 000520501A CA 520501 A CA520501 A CA 520501A CA 1277744 C CA1277744 C CA 1277744C
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Abstract
ABSTRACT OF THE DISCLOSURE
A method for controlling a system parameter based on controlling a setpoint for a process control variable includes measuring the rate of input to and output from a vessel. The difference between the input and output rates is determined to provide an expected rate of accumulation in the vessel. The actual rate of accumulation in the vessel is measured. The difference between the expected rate of accumulation and the actual rate of accumulation is determined to provide a bias term. A value for the setpoint for the process control variable is calculated by solving one of a material or energy balance equation which includes the bias term and a term representative of a predetermined time period during which the controlled parameter is to reach a desired value. The setpoint for the process control variable is then controlled based on the calculated setpoint value. An apparatus is disclosed for carrying out the disclosed method.
A method for controlling a system parameter based on controlling a setpoint for a process control variable includes measuring the rate of input to and output from a vessel. The difference between the input and output rates is determined to provide an expected rate of accumulation in the vessel. The actual rate of accumulation in the vessel is measured. The difference between the expected rate of accumulation and the actual rate of accumulation is determined to provide a bias term. A value for the setpoint for the process control variable is calculated by solving one of a material or energy balance equation which includes the bias term and a term representative of a predetermined time period during which the controlled parameter is to reach a desired value. The setpoint for the process control variable is then controlled based on the calculated setpoint value. An apparatus is disclosed for carrying out the disclosed method.
Description
12~744 BACKGROUND OF THE INVENTION
The present invention is directed generally to a method and apparatus for controlling a setpoint for a process control variable, and more particularly, to a 5 method and apparatus utilizing a dynamic reconciliation technique.
Numerous types of controllers and control systems are known which take advantage of process models for controlling a process parameter. ~n one type a simple 10 dynamic relationship is assumed between the manipulated procass variable and the controlled process parameter. The known value of the manipulated variable is then used with the model to estimate the controlled process parameter. The difference or bias between the predicted value of the 15 controlled parameter and the measured value of that parameter is used to ad~ust the manipulated process variable to move the controlled process parameter to the desired value. This technique is called ~'Internal Model Control" and was described by C.E. Garcia and M. Morari in 20 IEC. Proc. Des. and Dev., Volume 21 in 1982.
Another technique postulates a simple dynamic relationship between a process parameter which can be easily measured, such as a temperature, and the process parameter to be controlled, such as a concentration, which 25 is more difficult to measure. The model and the easily measured process parameter are then used to estimate the controlled process parameter. Once again the value of the controlled process parameter prédicted by the model is compared with the measured value of the controlled process 30 parameter and a difference or bias is calculated. This bias is then used to adjust the manipulated process variable to move the controlled process parameter to its desired value.
This technique is called Dynamic Reconciliation and was described in an article by Robert V. Bartman entitled "Dual Composition Control in a C3/C4 Splitter" appearing in the S September 1981 issue of CEP.
An alternative approach is to use a material or energy balance model to estimate the controlled process parameter. As in the above techniques the difference between the model estimate of the controlled process 10 parameter and the measured value of the controlled process parameter is used to adjust the manipulated variable to move the controlled process parameter to its desired value.
This is the approach described herein.
SUMMARY OF THE PRESENT INVENTION
The pre5ent inventlon is directed to a method for controlling a system parameter based on controlling a setpoint for a process control variable. The method includes the steps of measuring the rate of input to and output from a vessel. The difference is then determined 20 between the input and output rates to provide an expected rate of accumulation in the vessel. The actual rate of accumulation in the vessel is measured. The difference between the expected rate of accumulation and the actual rate of accumulation is determined to provide a bias term.
25 A value for the setpoint for the process control variable is calculated by solving one of a material or energy balance equation which includes the bias term and a term representative of a predetermined time period during which the controlled parameter is to reach a desired value. The 30 setpoint for the process control variable is then controlled based on the calculated setpoint value.
~77~
In one embodiment of the present invention it is anticipated that the step of measuring the rate of input to and output from a vessel includes the step of measuring the rate of flow of material input to and output from the 5 vessel. In such an environment, the difference between the input and output flow rates is determined to provide an expected rate of accumulation of material in the vessel.
The actual rate of accumulation in the vessel is measured.
The difference between the expected rate of accumulatlon 10 and the actual rate of accumulation is determined to provide a bias term. A material balance equation of the general type wherein the actual rate of accumulation of material in the vessel equals the expected rate of accumulation of material in the vessel plus the bias term 15 is solved to provide a value for the setpoint for the process control variable.
In another embodiment of the present invention the step of measuring the rate of input to and output from a vessel includes the step of measuring the rate of energy 20 added to and energy removed from the vessel. In such an environment, the difference between the energy input and energy output rate is determined to provide an expected rate of accumulation of energy in the vessel. The actual rate of accumulation of energy in the vessel is measured.
25 The difference between the expected rate of accumulation and the actual rate of accumulation is determined to provide a bias term. An energy balance equation of the general type wherein the actual rate of accumulation equals the expected rate of accumulation plus the bias term is 3Q solved to provide a setpoint for the process control variable. The process control setpoint is then controlled based on the calculated setpoint value.
12~7744 The present invention is also directed to ~r apparatus for controlling a system parameter based on controlling a setpoint for a process control variable. The apparatus is comprised of means for measuring the rate o~
i~put to and output from the vessel, means for determining the difference between the input and output rates to provide an expected rate of accumulation in the vessel, sensing means for sensing the actual rate of accumulation in the vessel and means for determining the difference 10 between the expected rate of accumulation and the actual rate of accumulation to provide a bias term. Means f~r calculating are provided for calculating a value for the setpoint for the process control variable by solving one of a material or energy balance equation which includes the 15 bias term and a term representative of a predetermined time period during which the controlled parameter is to reach a desired value. Output means control the setpoint for the process control variable based on the calculated setpolnt value.
t~ e~ ~o~ ~
The p~eser~ _invcn~n compensates for sensor errors by including a bias term in the equation which is representative of errors in the system. In an ideal system, the bias term would have a value of zero. However, under real world con~itions, the addition of this bias term to 25 the equation provides a very accurate indication of the control action necessary to achieve the desired result.
Additionally, by including a term in the equation representative of a predetermined time period during which the controlled parameter is to reach a desired value, the 30 system can be driven as quickly or as slowly as desired.
These represent substantial advantages over the prior art.
These and other advantages and benefits of the present invention will become apparent from the description of 2 preferred embodiment hereinbelow.
12~774A
_ESCRIPTION OF THE DRAWI~GS
In order that the present invention may be clearly understood and readily practiced, preferred embodiments will now be described, by way of example only, 5 with reference to the accompanying figures wherein:
Fig. 1 illustrates a vessel together with varlous sensors and control equipment controlled by a control system constructed according to the teachings of the present invention;
Fig. 2 is a flow chart illustrating the steps carried out by the control unit shown in Fig. l; and Fig. 3 illustrates a reactor vessel together with various sensors and control equipment controlled by a control system const~ucted according to the teachings of 15 the present invention.
DESCRIPTION OF A PREFERRED EMBODIMENT
The present invention will now be described in con~unction with a particular emobodiment. However, it should be recognized that the particular environment in 20 which the present invention is described, is intended for purposes of illustration only, and not limitation. There are numerous environments in which the Dynamic R ~ p ~
Reconciliation Process Control Method and Aff~r$us of the present invention can be used.
In Fig. 1, a vessel 10 contains a material 12 undergoing a process. The material 12 is inpu-t to the vessel 10 through an input pipeline 14 having an input valve 16. The position of the input valve 16 determines the flow rate of material 12 into the vessel 10. The flow rate 30 of material 12 through pipeline 14 is measured by an orifice plate 18 and a differential pressure gauge 20. The differential pressure gauge 20 produces an output signal representative of the flow rate of material 12 into the 1.~7 7744 ~, vessel lO which is input to a control unit 22 through line 21.
~ aterial 12 can be removed fro~ vessel 10 through an output pipeline 24 having an output valve 26. The 5 position of the output valve 26 determines the flow rate of material 12 through the output pipeline 24. ~he flow rate of material 12 through output pipeline 24 is m~asured by an orifice plate 28 and a differential pressure ~auge 30.
Pressure gauge 30 produces an output signal representati~
10 of the flow rate of material 12 through output pipeline 24 which is input to a control unit 22 through a line 31.
The level of material 12 in the vessel lO is measured by a differential pressuPe gauge 33. A signal representative of the level of material 12 in the vessel 10 15 is input to the control unit 22 from the pressure gauge 33 through a line 35.
Those of ordinary skill in the art should recognize that other devices for providing flow or level indications may be used. The illustration of orifice plates 20 and differential pressure gauges is for purposes of illustration only, and not intended as a limitation.
The control unit 22 performs a sequence of opera-tions, described more fully hereinbelow in conjunction with Fig. 2, to produce output signals for controlling the 25 process carried out in the vessel lO. An output signal may be input to a controller 37 through a line 38 for controlling the position of the input valve 16 or an outp~t signal may be input to a controller 40 through a line 41 for controlling the position of the output valve 26, or 30both. In this manner, control unit 22 can precisely control the flow of material 12 into and out of the vessel 10 as well as the level of material 12 in the vessel 10.
~27 774A
The control unit 22 working with the raw data received from pressure gauges 20, 30, and 33 solves, in the embodiment shown in Fig. 1, a material balance equatiOn.
Th,e equation solved by the control unit 22 is derived from 5 the following basic material balance equation:
in Fout + Fgen . . . . . . . (1) Where dM/dt is the actual measured rate of accumulation of material 12 in the vessel, Fin and F t are the measured flow rates of material 12 into and out of the vessel 10, 10 and Fgen represents any generation of material 12 within the vessel 10. Equation ~1) can be expressed as a difference equation:
M(t) - M(to) = Fin (t) ~ Fout (t) + Fgen (2) t - to I/l5/~ Equation (2) can b~ made into a basic material r ~ f>V~
15 balance ~y~h~ Peeo~c~ n equation by adding a bias term B(t) to compensaté for sensor as well as other errors in the system.
M(t) - M(to) = F. (t) - F (t) + F (t) + B(t) . . (3) 1n out gen t - to Rearranging equation (3):
20 B(t) = M(t) - M(to) *
t - to (F (t) - F (t) + F (t)). . . . (4) in out gen where the superscript * is used to indicate that the input and output flowrates have been adjusted (if necessary) to the same dynamic frame as the controlled variable.
At this point, it may be advisable to pass the 25 bias term, B, through a lead/lag algorithm. Experience indicates that a lead is seldom required but that filtering is often advisable. Thus a filtered bias term can be produced using Bf(t) = Bf(to) ~ a (B(t) - Bf(to)] . . . . . . . . . (5) Substituting the filtered bias term into equation (3) and solving for the setpoint of the manipulated variable, in this case FinsP (t):
sp sp pv F (t) = M(t) - M(t) + F (t) ~ F (t) - Bf(t) . . . (6) in Tau out gen where the abbreviations sp and pv stand for setpoint and present value, respectively, and the new parameter, Tau, represents the time allotted for the controlled parameter to reach the setpoint, i.e. desired value. Depending upon 15 the process involved, the generation rate Fgen(t) or the output flow rate setpoint FoUtsP (t) may be the manipulated variable.
Those of ordinary skill in the art will recognize that the solution of equation (6) for the setpoint value 20 for the manipulated control variable can be simply and quickly calculated by a suitably programmed microprocessor or microcomputer. It is anticipated that control unit 22 could include a commercially available microprocessor or microcomputer suitably programmed to solve equation (6).
A flow chart illustrating the steps carried out by the control unit 22 is illustrated in Fig. 2. The flow chart illustrated in Fig. 2 begins with decision step 44 wherein the control unit 22 determines if this is the initial pass through the program. If that question is 30 answered affirmatively, the control unit 22 reads the current, or new, level of material 12 in the vessel lO from ~277744 g pressure gauge 33 at step 46. This level reading is then stored in memory at step 47 for future use and the control unit 22 exits the program.
If at decision step 44 the control unit 22 determines that this is not the initial pass through the program, the control unit 22 then reads the current level of material 12 in the vessel lQ at step 48. At step 50, the inlet and outlet flow rates are read from pressure gauges 20 and 30, respectively.
The control unit 22 then determines the expected rate of accumulation by determining the difference between the input flow rate and the output flow rate at step 52 and adding any generation term. The value of the term Fgentt) is determined. This value may be determined from either a lookup table in which the control unit 22 merely lookc up an appropriate value depending upon such parameters as input flow, output flow, temperature, pressure, etc.
Alternatively, the value for the term Fgen(t) may be calculated in a separate subroutine depending upon the particular procegs being carried out in vessel 10.
At step 54, the old, or previous, level reading of material 12 in the vessel 10 is brought from memory. At step 56, the actual rate of accumulation of material 12 in the vessel 10 is determined by determining the difference between the current level reading and the old level reading.
At step 58, the bias term B is provided by determining the difference between the expected rate of accumulation of material in the vessel and the actual rate of accumulation of material in the vessel. If required, the bias term B may also be filtered. At step 60, the new or current level reading is input to memory for use in the next pass through the program.
~2~744 At step 64, the control unit 22 solves equation (6) to determine a new value for the setpoint of the controlled process control variable. In our example, the control unit 22 solves equation (6) for the value of the input flow control setpoint. When the control unit 22 reaches equation 64, it has all the information required for solving equation (6). The term (MSP(t) - MP (t))/Tau specifies that you want M to move from its present value (pv) to its desired value (sp) in time Tau. The output flow FoUt(t) was read at step 50,as was the generation term figure. The bias term Bf(t) was determined, and filtered if necessary, at step 58.
Once the calculated setpoint value is obtained, it is output at step 65 to controller 37 which controls the position of the valve 16. Clearly, the flow chart illus-trated in Fig. 2 could be modified such that the position of the output valve 26 rather than the input valve 16 is controlled.
The present invention may also be used in conjunc-tion with an energy rather than a material balance equation.The necessary instrumentation for effecting an energy balanced system is illustrated in Fig. 3. In Fig. 3 a vessel 68 is charged with a material or materials which will undergo a chemical reaction within the vessel 68. To effect the chemical reaction, the vessel 68 is to be maintained at a predetermined elevated temperature. In order to add heat to the vessel 68 a plurality of gas jets 70 are provided.
Gas is provided to the jets 70 through a pipeline 72 having a valve 74 and an orifice plate 76. The orifice plate 76 30 acts in combination with differential pressure gauge 78 to provide an indication of the rate of gas flow in the pipeline 72.
In order to remove heat from the vessel 68 a pipe-line 80 is provided to supply coolant to the vessel 68. The ~xm4~
--ll--pipeline 80 includes an input valve 82, a flowmeter 84, a temperature sensor 86 for measuring the temperature of the coolant flowing into the vessel 68, a temperature sensor 88 for measuring the temperature of the coolant leaving the 5 vessel 68, a flowmeter 90, and an output valv,e 92.
The vessel 68 is provided with a temperature sensor 94 which produces an output signal input to the control unit 22. The signals produced by the pressure gauge 78, flowmeters 84 and 90, and temperature sensors 8~
10 and 88 are also input to the control unit 22. The control unit 22 produces output signals input to a controller for controlling valve 82, a controller 96 for controlling valve 74, and a controller 98 for controlling valve 92.
In operation, the dynamic reconciliation process 15 control method operates subs~antially as described above except that energy in the form of heat, rather than material flow, is used to control the system. In other words, the actual rate of accumu}ation of energy, or heat, within the vessel 68 must equal the expected rate of 20 accumulation of energy plus the bias term.
Referring to equation (3), the left-hand side of the equation is the actual measured rate of accumulation of energy (heat) as determined by successive measurements of the temperature sensor 94. The energy input to the system 25, is determined by the amount of gas supplied to the jets 70 plus the temperature and flow rate of the coolant into the vessel 68. The removal of heat from the system is determined by the temperature and flow rate of coolant out of the vessel 68. The term Fgen (t) is replaced by a term 30 representative of any heat generated by the chemical reactions occurring within the vessel 68. The expected rate of accumulation of energy is determined by determining the difference between the rate of energy input to the vessel 68 and the rate of energy output from the vessel 68.
~2q7~
-~2-The bias term is obtained by determining the difference between the expected rate of accumulation of ener~y and the actual rate of accumulation of energy. The setpolnt of the variable which is to be controlled can be determined in a 5 manner similar to that described above in conjunction with Fig. 2.
S Example 1 B~ l)'~l9 The reconciliation process control system of the present invention can be implemented in a wide 10 variety of environments. For example, in one application ~ ~o~y~4 the bed level of a 'reactor was controlled. In this application, the setpoint of a product discharge rate controller was adjusted to maintain the desired bed level.
Unfortunately, neither the polymer production rate 15 (generation) nor the product discharge rate (output) were directly measurable. Instead, the production rate was inferred from a heat balance around th~ cycle gas cooler.
Product discharge was a contlnuous, batch operation. A P~X
drop rate controller was used to initiate the product 20 dlscharge sequence at a specified frequency. The amount of product removed in a single drop varied with reaction conditions as well as the mechanical condition of the discharge system. The discharge rate was inferred based upon the drop rate (discharges per hour) and fluidized bed 25 density using a simple correlation.
The actual reconciliation was performed on bed weight, not level. Thus the key variables for substitution into equation (3) were:
C(t) = Bed Wt (t) = Bed Level * Fluid Bed Dens * 0.1653 30 F ge~t) = Production Rate (t) F ou~t) = 0.1724 * Fluid Bed Dens * Discharge Rate The reactor level measurement was very noisy. In order to avoid excessive control action the bias was then i277744 passed through a filter, equation (5), with the filter factor set to 0.55. In addition the reconciliation calculation, equation (4), was performed only once every five minutes.
The best value for Tau (20 minutes) was determined by trial and error. The control calculation, equation (6), was performed once per minute. This system resulted in very accurate control of the bed level.
Example 2 Another example involved an application wherein the partial pressure of an ethylene feed gas was controlled. Once again there were a few complications.
First, ethylene w~s fed to the reactor on pressure denland.
Therefore its partial pressure could not be directly 15 controlled. Instead, the buildup of inerts in the reactor was controlled by periodic venting of the reaction mixture.
These inerts consisted of nitrogen (used to carry catalyst into the reactor) lsopentane (the co-catalyst carrier) and ethane (produced by a side reaction). Nitrogen and 20 isopentane feed rates were both measured but ethane generation was inferred using a correlation based upon hydrogen concentration and production rate. A final problem involved the product discharge system. During a product discharge sequence, reaction gas is lost. Most of this gas 25 is returned by a recycle compressor, but an unmeasured quantity is lost. Once again this flow was inferred.
The key variables for substitution into equation (3) were:
C(t) = (Nitrogen + Isopentane ~ Ethane) * Total Moles Gas 30 F ge~t) = Ethane Generation Rate (t) F ln(t) = Nitrogen Feed + Isopentane Feed F ou~t) = C(t)*(vent Rate ~ Discharge Losses)/Tota~as~oles ~2~7744 The resulting bias was then passed through the filter, equation (S), with the filter factor set to 0.2. This was a very heavy filter, but it was necessary due to analyzer signal to noise ratio. Again, the inventive control method worked well.
In addition to the aforementioned examples, the inventive teaching may be used for controlling the following processes:
distillation columns lhip and bottom reboiler level control, overhead condensor level control, steam addition to steam heated reboilers, and fuel rate to direct fired reboilers), steam systems (steam header pressure regulation and boiler feedwater level control)~ furnaces (regulation of fuel to maintain temperature), high pressure tu~ular or autoclave LDPE reactors (modifier inventory control, ethylene vinyl acetate inventory control, initiator addition rate control and hish and low pressure ~eparator polymer inventory control), and low pressure gas phase polyolefin reactors (polymer inventory control, comonomer inventory control, monomer inventory control, inerts inventory control, cataly~t inventory control, co-cata~yst inventory control, reaction temperature control and chain transfer agent inventory control).
The inventive process and apparatus represents a substantial advance over the prior art. Not only does it take into account various errors occurring throughout the system, but it also enables the system to be continuously controlled. The accuracy will enable higher production rates, improved transitions resulting in less offspec material and improved monomer utilization, and reduced offspec material due to lower steady state variability in resin density. While the present invention has been described in connection with exemplary embodiments thereof, it will be understood that many modifications and variations will be readily apparent to those of ordinary skill in the art. This application and the following claim~ are intended to cover those modifications and variations.
The present invention is directed generally to a method and apparatus for controlling a setpoint for a process control variable, and more particularly, to a 5 method and apparatus utilizing a dynamic reconciliation technique.
Numerous types of controllers and control systems are known which take advantage of process models for controlling a process parameter. ~n one type a simple 10 dynamic relationship is assumed between the manipulated procass variable and the controlled process parameter. The known value of the manipulated variable is then used with the model to estimate the controlled process parameter. The difference or bias between the predicted value of the 15 controlled parameter and the measured value of that parameter is used to ad~ust the manipulated process variable to move the controlled process parameter to the desired value. This technique is called ~'Internal Model Control" and was described by C.E. Garcia and M. Morari in 20 IEC. Proc. Des. and Dev., Volume 21 in 1982.
Another technique postulates a simple dynamic relationship between a process parameter which can be easily measured, such as a temperature, and the process parameter to be controlled, such as a concentration, which 25 is more difficult to measure. The model and the easily measured process parameter are then used to estimate the controlled process parameter. Once again the value of the controlled process parameter prédicted by the model is compared with the measured value of the controlled process 30 parameter and a difference or bias is calculated. This bias is then used to adjust the manipulated process variable to move the controlled process parameter to its desired value.
This technique is called Dynamic Reconciliation and was described in an article by Robert V. Bartman entitled "Dual Composition Control in a C3/C4 Splitter" appearing in the S September 1981 issue of CEP.
An alternative approach is to use a material or energy balance model to estimate the controlled process parameter. As in the above techniques the difference between the model estimate of the controlled process 10 parameter and the measured value of the controlled process parameter is used to adjust the manipulated variable to move the controlled process parameter to its desired value.
This is the approach described herein.
SUMMARY OF THE PRESENT INVENTION
The pre5ent inventlon is directed to a method for controlling a system parameter based on controlling a setpoint for a process control variable. The method includes the steps of measuring the rate of input to and output from a vessel. The difference is then determined 20 between the input and output rates to provide an expected rate of accumulation in the vessel. The actual rate of accumulation in the vessel is measured. The difference between the expected rate of accumulation and the actual rate of accumulation is determined to provide a bias term.
25 A value for the setpoint for the process control variable is calculated by solving one of a material or energy balance equation which includes the bias term and a term representative of a predetermined time period during which the controlled parameter is to reach a desired value. The 30 setpoint for the process control variable is then controlled based on the calculated setpoint value.
~77~
In one embodiment of the present invention it is anticipated that the step of measuring the rate of input to and output from a vessel includes the step of measuring the rate of flow of material input to and output from the 5 vessel. In such an environment, the difference between the input and output flow rates is determined to provide an expected rate of accumulation of material in the vessel.
The actual rate of accumulation in the vessel is measured.
The difference between the expected rate of accumulatlon 10 and the actual rate of accumulation is determined to provide a bias term. A material balance equation of the general type wherein the actual rate of accumulation of material in the vessel equals the expected rate of accumulation of material in the vessel plus the bias term 15 is solved to provide a value for the setpoint for the process control variable.
In another embodiment of the present invention the step of measuring the rate of input to and output from a vessel includes the step of measuring the rate of energy 20 added to and energy removed from the vessel. In such an environment, the difference between the energy input and energy output rate is determined to provide an expected rate of accumulation of energy in the vessel. The actual rate of accumulation of energy in the vessel is measured.
25 The difference between the expected rate of accumulation and the actual rate of accumulation is determined to provide a bias term. An energy balance equation of the general type wherein the actual rate of accumulation equals the expected rate of accumulation plus the bias term is 3Q solved to provide a setpoint for the process control variable. The process control setpoint is then controlled based on the calculated setpoint value.
12~7744 The present invention is also directed to ~r apparatus for controlling a system parameter based on controlling a setpoint for a process control variable. The apparatus is comprised of means for measuring the rate o~
i~put to and output from the vessel, means for determining the difference between the input and output rates to provide an expected rate of accumulation in the vessel, sensing means for sensing the actual rate of accumulation in the vessel and means for determining the difference 10 between the expected rate of accumulation and the actual rate of accumulation to provide a bias term. Means f~r calculating are provided for calculating a value for the setpoint for the process control variable by solving one of a material or energy balance equation which includes the 15 bias term and a term representative of a predetermined time period during which the controlled parameter is to reach a desired value. Output means control the setpoint for the process control variable based on the calculated setpolnt value.
t~ e~ ~o~ ~
The p~eser~ _invcn~n compensates for sensor errors by including a bias term in the equation which is representative of errors in the system. In an ideal system, the bias term would have a value of zero. However, under real world con~itions, the addition of this bias term to 25 the equation provides a very accurate indication of the control action necessary to achieve the desired result.
Additionally, by including a term in the equation representative of a predetermined time period during which the controlled parameter is to reach a desired value, the 30 system can be driven as quickly or as slowly as desired.
These represent substantial advantages over the prior art.
These and other advantages and benefits of the present invention will become apparent from the description of 2 preferred embodiment hereinbelow.
12~774A
_ESCRIPTION OF THE DRAWI~GS
In order that the present invention may be clearly understood and readily practiced, preferred embodiments will now be described, by way of example only, 5 with reference to the accompanying figures wherein:
Fig. 1 illustrates a vessel together with varlous sensors and control equipment controlled by a control system constructed according to the teachings of the present invention;
Fig. 2 is a flow chart illustrating the steps carried out by the control unit shown in Fig. l; and Fig. 3 illustrates a reactor vessel together with various sensors and control equipment controlled by a control system const~ucted according to the teachings of 15 the present invention.
DESCRIPTION OF A PREFERRED EMBODIMENT
The present invention will now be described in con~unction with a particular emobodiment. However, it should be recognized that the particular environment in 20 which the present invention is described, is intended for purposes of illustration only, and not limitation. There are numerous environments in which the Dynamic R ~ p ~
Reconciliation Process Control Method and Aff~r$us of the present invention can be used.
In Fig. 1, a vessel 10 contains a material 12 undergoing a process. The material 12 is inpu-t to the vessel 10 through an input pipeline 14 having an input valve 16. The position of the input valve 16 determines the flow rate of material 12 into the vessel 10. The flow rate 30 of material 12 through pipeline 14 is measured by an orifice plate 18 and a differential pressure gauge 20. The differential pressure gauge 20 produces an output signal representative of the flow rate of material 12 into the 1.~7 7744 ~, vessel lO which is input to a control unit 22 through line 21.
~ aterial 12 can be removed fro~ vessel 10 through an output pipeline 24 having an output valve 26. The 5 position of the output valve 26 determines the flow rate of material 12 through the output pipeline 24. ~he flow rate of material 12 through output pipeline 24 is m~asured by an orifice plate 28 and a differential pressure ~auge 30.
Pressure gauge 30 produces an output signal representati~
10 of the flow rate of material 12 through output pipeline 24 which is input to a control unit 22 through a line 31.
The level of material 12 in the vessel lO is measured by a differential pressuPe gauge 33. A signal representative of the level of material 12 in the vessel 10 15 is input to the control unit 22 from the pressure gauge 33 through a line 35.
Those of ordinary skill in the art should recognize that other devices for providing flow or level indications may be used. The illustration of orifice plates 20 and differential pressure gauges is for purposes of illustration only, and not intended as a limitation.
The control unit 22 performs a sequence of opera-tions, described more fully hereinbelow in conjunction with Fig. 2, to produce output signals for controlling the 25 process carried out in the vessel lO. An output signal may be input to a controller 37 through a line 38 for controlling the position of the input valve 16 or an outp~t signal may be input to a controller 40 through a line 41 for controlling the position of the output valve 26, or 30both. In this manner, control unit 22 can precisely control the flow of material 12 into and out of the vessel 10 as well as the level of material 12 in the vessel 10.
~27 774A
The control unit 22 working with the raw data received from pressure gauges 20, 30, and 33 solves, in the embodiment shown in Fig. 1, a material balance equatiOn.
Th,e equation solved by the control unit 22 is derived from 5 the following basic material balance equation:
in Fout + Fgen . . . . . . . (1) Where dM/dt is the actual measured rate of accumulation of material 12 in the vessel, Fin and F t are the measured flow rates of material 12 into and out of the vessel 10, 10 and Fgen represents any generation of material 12 within the vessel 10. Equation ~1) can be expressed as a difference equation:
M(t) - M(to) = Fin (t) ~ Fout (t) + Fgen (2) t - to I/l5/~ Equation (2) can b~ made into a basic material r ~ f>V~
15 balance ~y~h~ Peeo~c~ n equation by adding a bias term B(t) to compensaté for sensor as well as other errors in the system.
M(t) - M(to) = F. (t) - F (t) + F (t) + B(t) . . (3) 1n out gen t - to Rearranging equation (3):
20 B(t) = M(t) - M(to) *
t - to (F (t) - F (t) + F (t)). . . . (4) in out gen where the superscript * is used to indicate that the input and output flowrates have been adjusted (if necessary) to the same dynamic frame as the controlled variable.
At this point, it may be advisable to pass the 25 bias term, B, through a lead/lag algorithm. Experience indicates that a lead is seldom required but that filtering is often advisable. Thus a filtered bias term can be produced using Bf(t) = Bf(to) ~ a (B(t) - Bf(to)] . . . . . . . . . (5) Substituting the filtered bias term into equation (3) and solving for the setpoint of the manipulated variable, in this case FinsP (t):
sp sp pv F (t) = M(t) - M(t) + F (t) ~ F (t) - Bf(t) . . . (6) in Tau out gen where the abbreviations sp and pv stand for setpoint and present value, respectively, and the new parameter, Tau, represents the time allotted for the controlled parameter to reach the setpoint, i.e. desired value. Depending upon 15 the process involved, the generation rate Fgen(t) or the output flow rate setpoint FoUtsP (t) may be the manipulated variable.
Those of ordinary skill in the art will recognize that the solution of equation (6) for the setpoint value 20 for the manipulated control variable can be simply and quickly calculated by a suitably programmed microprocessor or microcomputer. It is anticipated that control unit 22 could include a commercially available microprocessor or microcomputer suitably programmed to solve equation (6).
A flow chart illustrating the steps carried out by the control unit 22 is illustrated in Fig. 2. The flow chart illustrated in Fig. 2 begins with decision step 44 wherein the control unit 22 determines if this is the initial pass through the program. If that question is 30 answered affirmatively, the control unit 22 reads the current, or new, level of material 12 in the vessel lO from ~277744 g pressure gauge 33 at step 46. This level reading is then stored in memory at step 47 for future use and the control unit 22 exits the program.
If at decision step 44 the control unit 22 determines that this is not the initial pass through the program, the control unit 22 then reads the current level of material 12 in the vessel lQ at step 48. At step 50, the inlet and outlet flow rates are read from pressure gauges 20 and 30, respectively.
The control unit 22 then determines the expected rate of accumulation by determining the difference between the input flow rate and the output flow rate at step 52 and adding any generation term. The value of the term Fgentt) is determined. This value may be determined from either a lookup table in which the control unit 22 merely lookc up an appropriate value depending upon such parameters as input flow, output flow, temperature, pressure, etc.
Alternatively, the value for the term Fgen(t) may be calculated in a separate subroutine depending upon the particular procegs being carried out in vessel 10.
At step 54, the old, or previous, level reading of material 12 in the vessel 10 is brought from memory. At step 56, the actual rate of accumulation of material 12 in the vessel 10 is determined by determining the difference between the current level reading and the old level reading.
At step 58, the bias term B is provided by determining the difference between the expected rate of accumulation of material in the vessel and the actual rate of accumulation of material in the vessel. If required, the bias term B may also be filtered. At step 60, the new or current level reading is input to memory for use in the next pass through the program.
~2~744 At step 64, the control unit 22 solves equation (6) to determine a new value for the setpoint of the controlled process control variable. In our example, the control unit 22 solves equation (6) for the value of the input flow control setpoint. When the control unit 22 reaches equation 64, it has all the information required for solving equation (6). The term (MSP(t) - MP (t))/Tau specifies that you want M to move from its present value (pv) to its desired value (sp) in time Tau. The output flow FoUt(t) was read at step 50,as was the generation term figure. The bias term Bf(t) was determined, and filtered if necessary, at step 58.
Once the calculated setpoint value is obtained, it is output at step 65 to controller 37 which controls the position of the valve 16. Clearly, the flow chart illus-trated in Fig. 2 could be modified such that the position of the output valve 26 rather than the input valve 16 is controlled.
The present invention may also be used in conjunc-tion with an energy rather than a material balance equation.The necessary instrumentation for effecting an energy balanced system is illustrated in Fig. 3. In Fig. 3 a vessel 68 is charged with a material or materials which will undergo a chemical reaction within the vessel 68. To effect the chemical reaction, the vessel 68 is to be maintained at a predetermined elevated temperature. In order to add heat to the vessel 68 a plurality of gas jets 70 are provided.
Gas is provided to the jets 70 through a pipeline 72 having a valve 74 and an orifice plate 76. The orifice plate 76 30 acts in combination with differential pressure gauge 78 to provide an indication of the rate of gas flow in the pipeline 72.
In order to remove heat from the vessel 68 a pipe-line 80 is provided to supply coolant to the vessel 68. The ~xm4~
--ll--pipeline 80 includes an input valve 82, a flowmeter 84, a temperature sensor 86 for measuring the temperature of the coolant flowing into the vessel 68, a temperature sensor 88 for measuring the temperature of the coolant leaving the 5 vessel 68, a flowmeter 90, and an output valv,e 92.
The vessel 68 is provided with a temperature sensor 94 which produces an output signal input to the control unit 22. The signals produced by the pressure gauge 78, flowmeters 84 and 90, and temperature sensors 8~
10 and 88 are also input to the control unit 22. The control unit 22 produces output signals input to a controller for controlling valve 82, a controller 96 for controlling valve 74, and a controller 98 for controlling valve 92.
In operation, the dynamic reconciliation process 15 control method operates subs~antially as described above except that energy in the form of heat, rather than material flow, is used to control the system. In other words, the actual rate of accumu}ation of energy, or heat, within the vessel 68 must equal the expected rate of 20 accumulation of energy plus the bias term.
Referring to equation (3), the left-hand side of the equation is the actual measured rate of accumulation of energy (heat) as determined by successive measurements of the temperature sensor 94. The energy input to the system 25, is determined by the amount of gas supplied to the jets 70 plus the temperature and flow rate of the coolant into the vessel 68. The removal of heat from the system is determined by the temperature and flow rate of coolant out of the vessel 68. The term Fgen (t) is replaced by a term 30 representative of any heat generated by the chemical reactions occurring within the vessel 68. The expected rate of accumulation of energy is determined by determining the difference between the rate of energy input to the vessel 68 and the rate of energy output from the vessel 68.
~2q7~
-~2-The bias term is obtained by determining the difference between the expected rate of accumulation of ener~y and the actual rate of accumulation of energy. The setpolnt of the variable which is to be controlled can be determined in a 5 manner similar to that described above in conjunction with Fig. 2.
S Example 1 B~ l)'~l9 The reconciliation process control system of the present invention can be implemented in a wide 10 variety of environments. For example, in one application ~ ~o~y~4 the bed level of a 'reactor was controlled. In this application, the setpoint of a product discharge rate controller was adjusted to maintain the desired bed level.
Unfortunately, neither the polymer production rate 15 (generation) nor the product discharge rate (output) were directly measurable. Instead, the production rate was inferred from a heat balance around th~ cycle gas cooler.
Product discharge was a contlnuous, batch operation. A P~X
drop rate controller was used to initiate the product 20 dlscharge sequence at a specified frequency. The amount of product removed in a single drop varied with reaction conditions as well as the mechanical condition of the discharge system. The discharge rate was inferred based upon the drop rate (discharges per hour) and fluidized bed 25 density using a simple correlation.
The actual reconciliation was performed on bed weight, not level. Thus the key variables for substitution into equation (3) were:
C(t) = Bed Wt (t) = Bed Level * Fluid Bed Dens * 0.1653 30 F ge~t) = Production Rate (t) F ou~t) = 0.1724 * Fluid Bed Dens * Discharge Rate The reactor level measurement was very noisy. In order to avoid excessive control action the bias was then i277744 passed through a filter, equation (5), with the filter factor set to 0.55. In addition the reconciliation calculation, equation (4), was performed only once every five minutes.
The best value for Tau (20 minutes) was determined by trial and error. The control calculation, equation (6), was performed once per minute. This system resulted in very accurate control of the bed level.
Example 2 Another example involved an application wherein the partial pressure of an ethylene feed gas was controlled. Once again there were a few complications.
First, ethylene w~s fed to the reactor on pressure denland.
Therefore its partial pressure could not be directly 15 controlled. Instead, the buildup of inerts in the reactor was controlled by periodic venting of the reaction mixture.
These inerts consisted of nitrogen (used to carry catalyst into the reactor) lsopentane (the co-catalyst carrier) and ethane (produced by a side reaction). Nitrogen and 20 isopentane feed rates were both measured but ethane generation was inferred using a correlation based upon hydrogen concentration and production rate. A final problem involved the product discharge system. During a product discharge sequence, reaction gas is lost. Most of this gas 25 is returned by a recycle compressor, but an unmeasured quantity is lost. Once again this flow was inferred.
The key variables for substitution into equation (3) were:
C(t) = (Nitrogen + Isopentane ~ Ethane) * Total Moles Gas 30 F ge~t) = Ethane Generation Rate (t) F ln(t) = Nitrogen Feed + Isopentane Feed F ou~t) = C(t)*(vent Rate ~ Discharge Losses)/Tota~as~oles ~2~7744 The resulting bias was then passed through the filter, equation (S), with the filter factor set to 0.2. This was a very heavy filter, but it was necessary due to analyzer signal to noise ratio. Again, the inventive control method worked well.
In addition to the aforementioned examples, the inventive teaching may be used for controlling the following processes:
distillation columns lhip and bottom reboiler level control, overhead condensor level control, steam addition to steam heated reboilers, and fuel rate to direct fired reboilers), steam systems (steam header pressure regulation and boiler feedwater level control)~ furnaces (regulation of fuel to maintain temperature), high pressure tu~ular or autoclave LDPE reactors (modifier inventory control, ethylene vinyl acetate inventory control, initiator addition rate control and hish and low pressure ~eparator polymer inventory control), and low pressure gas phase polyolefin reactors (polymer inventory control, comonomer inventory control, monomer inventory control, inerts inventory control, cataly~t inventory control, co-cata~yst inventory control, reaction temperature control and chain transfer agent inventory control).
The inventive process and apparatus represents a substantial advance over the prior art. Not only does it take into account various errors occurring throughout the system, but it also enables the system to be continuously controlled. The accuracy will enable higher production rates, improved transitions resulting in less offspec material and improved monomer utilization, and reduced offspec material due to lower steady state variability in resin density. While the present invention has been described in connection with exemplary embodiments thereof, it will be understood that many modifications and variations will be readily apparent to those of ordinary skill in the art. This application and the following claim~ are intended to cover those modifications and variations.
Claims (19)
1. A method for controlling a parameter of a system based on controlling a setpoint for a process control variable, comprising the steps of:
measuring the rate of input to and output from a vessel;
measuring the actual rate of accumulation in said vessel;
calculating a model bias factor by solving a first model equation by determining the difference between the input and output rates to provide a calculated expected rate of accumulation in said vessel and determining the difference between said expected rate of accumulation and said actual rate of accumulation to provide said model bias factor;
calculating a value for said setpoint for said process control variable by solving a second model equation, said second model equation being representative of the balance of material or energy in said system and including a term representative of (i) a predetermined time period during which said parameter is to reach a desired value and (ii) said model bias factor, said model bias factor being a function of error in said first model equation and providing for adjustment to said second model equation based upon said error, said error corresponding to the difference between said calculated expected rate of accumulation and the measured actual rate of accumulation; and controlling said setpoint for said process control variable based on said calculated setpoint value.
measuring the rate of input to and output from a vessel;
measuring the actual rate of accumulation in said vessel;
calculating a model bias factor by solving a first model equation by determining the difference between the input and output rates to provide a calculated expected rate of accumulation in said vessel and determining the difference between said expected rate of accumulation and said actual rate of accumulation to provide said model bias factor;
calculating a value for said setpoint for said process control variable by solving a second model equation, said second model equation being representative of the balance of material or energy in said system and including a term representative of (i) a predetermined time period during which said parameter is to reach a desired value and (ii) said model bias factor, said model bias factor being a function of error in said first model equation and providing for adjustment to said second model equation based upon said error, said error corresponding to the difference between said calculated expected rate of accumulation and the measured actual rate of accumulation; and controlling said setpoint for said process control variable based on said calculated setpoint value.
2. The method of claim 1 wherein said step of measuring the rate of input to and output from a vessel includes the step of measuring the rate of flow of material input to and output from the vessel.
3. The method of claim 2 wherein determining the difference between the input and output rates includes the step of determining the difference between the input and output flow rates to provide an expected rate of accumulation of said material in said vessel.
4. The method of claim 3 wherein said step of measuring the actual rate of accumulation includes the step of measuring the actual rate of accumulation of material in said vessel.
5. The method of claim 4 wherein said step of measuring the actual rate of accumulation of material in said vessel includes the steps of periodically measuring the level of material in said vessel to provide a series of level measurements, and comparing the current level measurement to the previous level measurement to provide an indication of the actual rate of accumulation of material in said vessel.
6. The method of claim 4 wherein said second model equation is formulated such that said actual rate of accumulation equals said expected rate of accumulation plus said model bias factor.
7. The method of claim 6 wherein said calculating step includes the step of calculating a value for an input flow control setpoint.
8. The method of claim 6 wherein said calculating step includes the step of calculating a value for an output flow control setpoint.
9. The method of claim 6 wherein said second model equation further includes a term representative of a substance generated within said vessel.
16 .
16 .
10. The method of claim 1 comprising the further step of filtering said model bias factor.
11. The method of claim 1 wherein said step of measuring the rate of input to and output from a vessel includes the step of measuring the rate of energy added to and energy removed from said vessel.
12. The method of claim 11 wherein determining the difference between the input and output rates includes the step of determining the difference between the energy input and energy output rates to provide an expected rate of accumulation of energy in said vessel.
13. The method of claim 12 wherein said step of measuring the actual rate of accumulation includes the step of measuring the actual rate of accumulation of energy in said vessel.
14. The method of claim 13 wherein said step of measuring the actual energy in said vessel includes the steps of periodically measuring the energy in said vessel to provide a series of energy measurements, and comparing the current energy measurement to the previous energy measurement to provide an indication of the actual rate of accumulation of energy in said vessel.
15. The method of claim 13 wherein said second model equation is formulated such that said actual rate of accumulation equals said expected rate of accumulation plus said model bias factor.
16. The method of claim 15 wherein said calculating step includes the step of calculating a value for an input fuel control setpoint.
17 17. The method of claim 15 wherein said calculating step includes the step of calculating a value for an input coolant flow control setpoint.
18. The method of claim 15 wherein said second model equation further includes a term representative of heat generated within said vessel.
19. An apparatus for controlling a parameter of a system based on controlling a setpoint for a process control variable of a process carried out in a vessel, comprising:
means for measuring the rate of input to and output from said vessel;
sensing means for sensing the actual rate of accumulation in said vessel;
means for calculating a model bias factor by solving a first model equation including means for determining the difference between the input and output rates to provide a calculated expected rate of accumulation in said vessel and means for determining the difference between said expected rate of accumulation and said actual rate of accumulation to provide said model bias factor;
means for calculating a value for said setpoint for said process control variable by solving a second model equation, said second model equation being representative of the balance of material or energy in said system and including a term representative of (i) a predetermined time period during which said parameter is to reach a desired value and (ii) said model bias factor, said model bias factor being a function of error in said first model equation and providing or adjustment to said second model equation based upon said error, said error corresponding to the difference between said calculated expected rate of accumulation and the measured actual rate of accumulation;
and output means for controlling said setpoint for said process control variable based on said calculated setpoint value.
means for measuring the rate of input to and output from said vessel;
sensing means for sensing the actual rate of accumulation in said vessel;
means for calculating a model bias factor by solving a first model equation including means for determining the difference between the input and output rates to provide a calculated expected rate of accumulation in said vessel and means for determining the difference between said expected rate of accumulation and said actual rate of accumulation to provide said model bias factor;
means for calculating a value for said setpoint for said process control variable by solving a second model equation, said second model equation being representative of the balance of material or energy in said system and including a term representative of (i) a predetermined time period during which said parameter is to reach a desired value and (ii) said model bias factor, said model bias factor being a function of error in said first model equation and providing or adjustment to said second model equation based upon said error, said error corresponding to the difference between said calculated expected rate of accumulation and the measured actual rate of accumulation;
and output means for controlling said setpoint for said process control variable based on said calculated setpoint value.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US78881485A | 1985-10-18 | 1985-10-18 | |
| US788,814 | 1985-10-18 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1277744C true CA1277744C (en) | 1990-12-11 |
Family
ID=25145639
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000520501A Expired - Lifetime CA1277744C (en) | 1985-10-18 | 1986-10-15 | Material and energy balance reconciliation process control method and apparatus |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1277744C (en) |
-
1986
- 1986-10-15 CA CA000520501A patent/CA1277744C/en not_active Expired - Lifetime
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