I
D. E. LUPFER and M. W. OGLESBY Phillips Petroleum Co., Bartlesville, Okla.
Automatic Control of Distillation Columns Control system for heat inputs have been developed to provide economically feasible means for achieving column operation improvement
IMPROVED
AUTOMATIC
CONTROL
SYS-
for distillation columns are needed for higher purity products and for economic optimization of new plant designs and operation. Operating results obtained by conventional distillation column control systems leave much to be desired. I t is difficult, for example, to predict the quality of control and operation which will be achieved by a given control system. One particular arrangement of controls may prove acceptable on a given column while the same system on a like column may be unsatisfactory. Extensive plant tests and analog simulation studies have shown that many column operating problems exist because the major heat inputs are not regulated. Interactions between various sections of the column tend to mask the heat input effects. There are two major requirements for the design of column control systems.
TEMS
0 Adequate controls to exclude disturbances which can be controlled. 0 Controls to compensate the operation for changes in those variables over which direct control cannot be applied. Composition analyzers are normally applied as a means to fulfill the second requirement. Although many types of analyzers can be applied for primary control of the separation, the chromatographic analyzer is probably the most important. Application of this device presents new problems to the control designer because the chromatograph obtains measurements periodically. This period, or sampling rate, is important and the minimum sampling rate which can be tolerated must be determined. Of prime importance are the effects of unregulated heat inputs on the column composition control system. These effects must be well understood if improved systems are to result. Continuous distillation systems are arranged in many ways (7). For illustrative purposes, one system (Figure 1) has been selected for analysis in this report. Many of the points to be made for this system are equally applicable to other arrangements. This system is typical of modern day superfractionators.
The automatic controls shown represent the recognized state of the art. The distillation system includes an “economizer” heat exchanger which is used to recover heat from the bottom product stream by exchange to the feed. Economics of operation dictate this design for many columns. The exchanger is included because of its widespread application throughout the petroleum and chemical industry. Good regulation and control of the three major heat inputs to distillation columns is important in achieving improved operations. Isolation of interacting parts of a column can result in pronounced improvement in operation. The systems to be presented have been proved in practice and offer economically practical solutions to the problems presented.
input involves the control of internal reflux ( 3 ) . An approximate measure of internal reflux flow rate, at the column top tray, can be calculated from measurements of external reflux temperature, overhead vapor temperature, and external reflux flow. An internal reflux computer such as shown in Figure 2 continuously supplies the internal reflux measurement to a controller, which manipulates the flow of external reflux to maintain internal reflux flow at the desired level. Application of the internal reflux computer control (IRCC) system serves to reduce the number of variables which can affect the operation. Since internal reflux flow is several degrees more specific to the separation process than is external reflux flow, a better operation results. The internal reflux computer control system was originally developed to stabilize the operation of distillation columns which have air-fan overhead vapor condensers. The problem in operating such columns results from drastic reflux temperature changes caused by ambient conditions. Since load changes which affect reflux temperature are severe, it is very difficult to achieve adequate temperature control of the reflux in an economical manner. The application of the internal reflux computer has exceeded expectations on many such columns. Any distillation column which is equipped with air-fan overhead
Effects and Control of Reflux Heat Input Some of the steady state and dynamic effects of a changing reflux heat input on the operation and control of distillation columns have been reported (3, 5). Whether or not an unregulated reflux heat input is particularly important depends upon the column involved. I n general, the importance of proper control and regulation increases as the separation becomes more difficult. Internal Reflux Computer Control. One of the best and most economical methods for regulating the reflux heat
ERHEAD VAPOR
Figure 1. Typical distillation column control system STEAM SUPPLY
SUPPLY
BOTTOM
ECONOMIZER HEAT EXCHANGER
FEED
VOL. 53, NO. 12
0
DECEMBER 1961
963
Justification for Improved Column Controls Economic justification is usually obtained by fulfilling one or more o f the following: 0 0 0 0 0
Better primary control of the separation Lower operaling costs Higher throughputs Smoother terminal stream flows A more predictable operation with less human attention
In general, i m p r o v e d controls make it possible to o p e r a t e a c o l u m n to produce products whose purities are closer to m i n i m u m specifications. Either t h e t h r o u g h p u t c a n t h e n be increased or if increased t h r o u g h p u t is not desired, a saving i n utilities will result. O n o n e large c o l u m n t h e combination of controls described resulted i n reducing the utility costs while also reducing t h e a m o u n t of heavy key component lost i n t h e overhead p r o d u c t s t r e a m by 0.3%. P a y o u t for this case was six months. O n another large c o l u m n t h e combination of controls described allowed an increase in t h r o u g h p u t of 3%. The payout for this case was three months. T h e r e are many benefits to be gained by improved c o l u m n control that a r e difficult to e v a l u a t e ; for example, t h e p a y o u t d u e to prevention of upsets. The benefits as such c a n n o t be assessed because upsets are unpredictable. Controls applied to o b t a i n a smoother operation of one column also result i n better operation of o t h e r units downstream. O p e r a t i o n of distillation columns in a more precise m a n n e r also results i n operating records m o r e useful for future design of similar systems.
vapor condensers is an immediate candidate for such control. A growing number of internal reflux computer control systems being applied to distillation columns have watercooled overhead vapor condensers. Composition Control Improved by Internal Reflux Computer. Conventional instrumentation schemes used for control of the separation involve either a temperature or composition measurement. Figure 1 shows an analyzer which measures composition near the column top. Conventionally, this analyzer measures the heavy key component
Figure 2. Pneumatic version of internal reflux computer
964
INDUSTRIAL AM)
which is the overhead product impurity. The analyzer supplies the composition measure to a controller (ARC) which manipulates the flow of reboiler heat to maintain the composition at the desired control point. This control loop is known as the primary control loop because it is the most specific control function performed with respect to the separation process. The quality of control which can be achieved by the analyzer conwol system is a function of many things. For the system as arranged in Figure 1, changes in reflux heat affect the dynamic character of the process contained within the analyzer loop. A so-called transient response of the process will reveal the dynamics in question. The transient response of the process is obtained when the analyzer-controller is taken off control, and the reboiler steam flow is step-changed. The resulting composition response (composition us. time) at the analyzer sampling point is the transient response. The response may, for example, appear as shown in Figure 3, Curve C. The shape of this response is a function of the process and governs the quality of control which can be achieved by the analyzer-controller. I n general, the faster the process response, the better the quality of control. A faster response implies shorter lags. The transient response is the sum of many effects. T o understand the transient influence of the refluxing system on
ENGINEERING CHEMISTRY
the dynamic character of the primary control loop, consider the events which occur when the reboiler heat is increased. The vapor flow u p the column will increase. The increase in vapor flow will cause the heavy key component concentration to start to increase a t the analyzer sample point. The internal reflux will also increase since the overhead vapor temperature increases with an increase in heavy key component. Another reason for the increase in overhead vapor temperature is the result of a pressure increase which accompanies the higher vapor flow rate. If the column pressure control does not immediately respond, a relative fast rise in overhead temperature will result. The increase in internal reflux flow resulting from the step increase in reboiler heat is in the direction to decrease the heavy component concentration at the analyzer location. Thus the observed response is the sum of two effects acting in opposite directions (Figure 3). With internal reflux control, Curve A is the observed response rather than Curve C. Since Curve A exhibits shorter lags, and particularly less apparent dead-time, a better analyzer-control system results. Internal Reflux Computer Reduces Operating Costs. All fractionators controlled in the manner shown in Figure 1 are subject to variations caused by ambient temperature changes. As the condenser cooling water becomes colder the internal reflux flow increases and the separation performed by a distillation column will improve. The analyzer will maintain composition at its location by increasing reboiler heat to compensate. If the column was not operating at full load before the reflux cooled, the other terminal stream purity will improve. Under these conditions,
-
TIME-
Figure 3. Open loop composition response of analyzer for Figure 1 system for step increase in reboiler heat Curve A i s the composition response which would have resulted h a d the internal reflux remained constant. Curve 8 is the composite response caused b y the increase in internal reflux. The sum of A and B responses then i s the observed response C
A U TOM AT1 C CONTROL better-than-specification products are being produced. This is costly. If the reflux should cool when the column is being operated at full load it is possible that the system will flood. For this reason columns must be operated at a throughput level which allows a margin of safety for such situations. In any event, a change in reflux temperature should not be compensated for by a change in reboiler heat. The proper correction is to raise the reflux temperature or change the reflux flow. Internal Reflux Computer Smooths Terminal Stream Flows. Automatic control of reflux temperature a t the reflux accumulator location will not necessarily accomplish the same degree of control as can be achieved by the internal reflux computer system. Many reflux lines are uninsulated and are 300 to 400 feet in length. On such a line, a 6 " F. drop in reflux temperature was observed between the accumulator and column entry point at the top. For a 30" F. drop in ambient temperature, the reflux temperature at the column entry point dropped 2" F., even though the accumulator temperature was constant. Such a change can be significant. The total effect of an unregulated reflux heat input will depend upon the nature of the change. For example, the analyzer controller may be capable of holding overhead composition if the reflux temperature changes slowly. In this event the only bad effect is a shift in bottom product purity. However, fast reflux temperature changes will result in a sudden change in internal reflux flow. Before the analyzer can compensate, the terminal stream flows may deviate. The analyzer will eventually apply a correction, and the terminal stream flows will return to a level consistent with terminal stream compositions. Many fractionators experience severe swings in the terminal stream flows as a result of reflux temperature changes. This may be a serious problem if the fractionator supplies the feed stream to another plant unit. The amount that the terminal streams will deviate for each O F . change in reflux is dependent upon the reflux-flow to feed-flow ratio, and upon the terminal stream-flows' to feed-flow ratios. The deviation is also dependent upon the components involved in the separation, but to a lesser degree. Figure 4 illustrates a typical case of steady state deviation in terminal stream flows for each degree variation in external reflux temperature if no analyzer controller is used. Even with analyzer control, there will be a dynamic deviation in terminal stream flows almost equal in magnitude to the steady state deviations shown in Figure 4 if the reflux temperature changes suddenly.
Effects and Control of Feed Enthalpy
I n general, the feed enthalpy should be held at a level which will result in the least operating costs while producing minimum specification products. Assuming that the optimum heat quantity can be chosen, the problem of regulating it at the selected value becomes important. Although the heat input to a column via the feed may be small compared with the total heat required for the separation, the importance of good control should not be discounted. Conventional Arrangements for Regulating Feed Enthalpy. There are two schemes in wide use for setting the feed heat input to a column. The first is illustrated in Figure 1. A temperature controller (TRC) manipulates flow of steam, or other heating medium, to the feed preheater to maintain temperature constant. The second scheme does not provide any automatic manipulation of the steam flow. The flow is manually set to achieve the degree of feed heat desired. Both arrangements have major disadvantages, and the latter arrangement is particularly bad. If the feed must enter the column at its bubble point or partially vaporized, temperature will not be a good measure of heat content, especially if the feed composition does not remain constant. If the heat requirement is such that the feed may enter subcooled, temperature will be a good measure of heat content, but temperature control fails to produce satisfactory results for many installations. The reason for this concerns the dynamic
W
a
0
5
IO
AB
15
20
25
character of the process contained in the temperature control loop. The process in question is contained between (TRC) output, X,and the measurement transmitter output, Y. Temperature controller (TRC) must contend with the process between X and Y. How Y responds as X is changed defines the dynamic character of the process. This in turn determines the quality of control which can be achieved by TRC. Good control with a conventional feedback system requires that the dynamic character of the process remain fixed. This requirement is not fulfilled by the system of Figure 1 . To illustrate this it is convenient to observe the transient response of the process at different feed flow conditions. The transient response is obtained by step-changing X and observing how Y responds with respect to time. If the feed flow through the preheater is high, the response of Y to a step-change in X will be characterized by relatively short lags and a low steady state gain. (Steady state gain is given by the change in Y divided by the change in X.) When the feed flow is low, the process is characterized by longer lags and a higher steady state gain. Both the dynamic and steady state character of the process is a function of feed flow. To understand why a varying process character affects a closed loop control system in an adverse manner, assume that T R C is adjusted for best response when the feed flow is high. If, with this adjustment, the feed flow should be reduced, the control system will oscillate to a degree dependant upon how much the process character has changed. It is necessary to readjust the controller to
a
Figure 4. Approximate steady state change in terminal stream flows with change in external reflux temperature To illustrate the use of these graphs, consider an LPG fractionator which i s operated with a reflux to feed flow ratio of 5, and an overhead to feed flow ratio of Y = 0.6. A shows that the bottom flow will d e viate approximately 6.5% for each degree variation in reflux temperature. B shows the accompanying charge in overhead product flow to be approximately 4.070 AB = Percentage change in bottom product flow per OF. change in temperature of reflux stream A 0 = Percentage change in overhead product flow per O F . change in temperature of reflux stream 0.H: Product flow Y = Split ratio = Feed flow
VOL. 53, NO. 12
DECEMBER 1961
965
+-
FEED TO COLUMN
STEAM SUPPLY
STEAM PREHEATER
r
Figure 5. Arrangement for controlling feed heat content when feed exists in all liquid phase When a feed temperature disturbance appears, the temperature controller will immediately respond to position the three-way valve to hold the feed temperature. Since m will have a new value, the VPC will manipulate the steam flow in the proper direction to return m to 9 p.s.i.g., which i s the three-way valve center position. In this arrangement, the three-way valve i s always being forced t o its center position so that i t can respond full travel in either direction
FE'ED
achieve stability. Eventually, the controller must be tuned for the lowest feed flow condition. Should the feed flow now be increased, the system will become very sluggish if the controller is not readjusted. Seldom, in practice, is the controller readjusted when the change occurring is not in the direction to produce instabilities. The temperature controller as used in the Figure 1 arrangement is not adaptive to the changing process condition. The application results in inadequate control unless given frequent attention. Bypass-Type Temperature Control for Regulating Feed Enthalpy. I n those cases where it is desirable to enter the feed subcooled, good temperature control can be achieved under all conditions of feed flow by employing the bypass-type control system illustrated in Figure 5. A temperature controller (TRC) manipulates a three-way control valve to maintain the feed temperature constant a t its entry point into the column. A valve position controller (VPC) measures the output, m, from the T R C and manipulates the flow of steam to the preheater. The signal, (A) ARRANGEMENT OF FEED ENTHALPY COMPUTER TO DISTILLATION COLUMN
m, from the TRC, is directly related to stem position of the three-way valve. For example, a pressure of 9 p.s.i.g. will locate the three-way valve at the center position. The arrangement shown in Figure 5 has an equipment advantage over conventional bypass-type heat exchanger controls which employ some type of cascade arrangement. The advantage is that only one measurement transmitter is used. Temperature control as shown in Figure 5 can be used in some cases when the feed must enter the column partially vaporized. A back pressure controller can be applied to maintain the feed in the all-liquid state at the point where temperature is measured. Computer Control of Feed Enthalpy. One of the best methods for regulating the feed heat content involves a feed enthalpy computer. This approach gives excellent results on those feed streams which must enter at their bubble point or partially vaporized. This condition covers the majority of cases. A number of feed enthalpy computers of the form shown (schematically) in Figure 6 and
fl
(6) BLOCK DIAGRAM OF FEED ENTHALPY COMPUTER
(photographically) in Figure 7 have been applied. The total feed heat content is calculated from: Initial feed heat content above some reference temperature, To, B.t.u./lb. of feed. The heat given up by the bottoms product stream to the feed in the economizer exchanger, B.t.u./lb. of feed. The heat given up to the feed by the steam through the feed preheater, B.t.u./lb. of feed. The computer output is the feed heat content in B.t.u. per pound of feed at the column entry. A controller manipulates steam flow to the preheater to maintain feed enthalpy constant. Depending upon the column involved, the feed enthalpy computer will take different forms. There are two basic arrangements. The first is used when the feed leaves the economizer exchanger partially vaporized (Figure 6). The second arrangement is used when the feed is not vaporized in the economizer exchanger. For this case, temperature will be a good measure of heat content a t the economizer exit. I t then is only necessary to add the measured quantity of heat given u p by the preheater steam to the feed heat quantity obtained from the temperature measurement. Measurement of the heat added to the feed by the preheater steam is accomplished in different ways depending upon
FEED IN(F)
Figure 6.
Computer control of feed enthalpy
Heat a d d e d to the feed b y the preheater i s the difference in heat content of the entering steam and the condensote. Condensate heat content normally need not b e measured as the several degrees change in temperature will not affect its heat content to any appreciable degree. If the preheater steam supply conditions vary, it must b e measured
966
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Figure 7. Pneumatic version of feed enthalpy computer
AUTOMATIC CONTROL Figure 8. Open loop response of ARC loop with various qualities of feed heat control Curve A is the composition response, at the analyzer sample point, due to a step increase in reboiler heat when no feed heat regulation is present. The composition initially responds in the wrong direction due to the heat feedback via the economizer exchanger. Curve C i s the composition response when the feed heat i s instrumented for good control. Curve 6 i s obtained when a poor quality of feed heat control exists. The dynamic character of the analyzer-controller loop is dependent upon the quality of feed heat control
the steam supply conditions. Figure 6 shows the simplest arrangement. Effects of Feed Enthalpy on Column Primary Control System. Good regulation of the feed heat is very important to operation of the column primary control system. A changing feed heat will constitute a load change, or disturbance, to the primary control system. The primary control system includes the analyzer-controller (ARC) shown in Figure 1. The analyzer-controller will compensate by manipulating reboiler heat to hold Composition at the analyzer location. If the feed heat change is sustained, the internal loading within the column will shift even though the analyzer-controller maintains composition at its location. The bottom product steam composition will change. I t will either contain more or less light components depending upon the direction of feed heat change. Theoretically an unwanted change in feed enthalpy should not be compensated for by changing the reboiler heat. The proper correction is to return the feed heat to its proper value. More serious than the load disturbance just described is an interacting effect which results from the closed loop heat feedback path provided by the economizer heat exchanger. Any disturbance which affects the bottom level will trigger the undesirable interaction. This effect can be understood by following a change through the system. For example, suppose a disturbance occurs in the heat supply source to the reboiler, causing more reboiler heat input. The vapor flow within the column will increase causing the bottom level to drop. The level controller will decrease bottom flow to maintain level. With a decrease in bottoms flow through the economizer exchanger, less heat will be exchanged to the feed steam. This will cause the feed to cool. Unless this change is compensated for by the preheater temperature control, the feed will enter the column with a lower heat content. The effect here is that the flow of liquid from the feed tray down will increase, and the
which the analyzer-controller must compensate. Since the heat feedback loop via the economizer exchanger interacts with the primary control loop, the analyzer-controller must be detuned for stability. How well the analyzer-controller controls the process depends to a large extent upon the quality of control achieved on the feed heat. The dynamic character of the analyzer-controller loop is in fact determined to a large degree by the feed heat control. Observation of transient responses of the process contained in the analyzer loop will reveal this interaction. Figure 8 shows three different transient responses, obtained by analog simulation, designed to match plant experimental data. Two plant experimental transient responses were matched These were the two transient composition responses with and without good feed enthalpy control. Relation of Chromatographic Analyzer Sampling Rate and Feed Enthalpy Control. When applying chromatographic analyzers for primary control, the designer must decide the sampling rate necessary for a given application. Figure 9 indicates some of the answers to this question. Shown on the graph are two plots for analyzer sampling rate us. analyzer control quality. T o obtain such a test of performance it is necessary that a control criterion be used to provide a basis for comparison.
I-
z w a gg
=%
s~ +
2 :
5y ;
0
15
30 45 TIM E,MI NUTES
60
vapor flow from the feed tray up will decrease. The change in liquid flow inside the column will cause the kettle level to increase. The level controller will in turn increase the bottom product flow. This completes the loop. The loop has negative feedback because the bottom flow change, which results from the initial flow change, is in the direction to cancel. Even though the loop is characterized by negative feedback, it is possible for sustained cycling to exist as the result of phasing of the events. Coupled to the effect just described is the primary control loop (ARC loop). Since both liquid and vapor flows within the column have changed there will be accompanying composition changes for
I I I I I 2 3 4 ANALYZER SAMPLING PERIOD, MIN
)4 0
30 60 90 120
\.-.-j
30 60 90 120 TIM E, M I NUTES
TIME,MINUTES COMPOSITION RESPONSES TO LOAD CHANGE
Figure 9. Effect of chromatograph sampling rate and feed heat control on ARC control quality Con ideroble improvement in composition control can be obtained b y using a high speed chromatograph and a feed enthalpy computer Note: ARC tuned for each point on above curves for minimum
VOL. 53, NO. 12
0
DECEMBER 1961
967
There are many criteria which can be used. I n general, a control criterion should take into account the error which exists between the desired value (set point of ARC), and the actual measurement, when a transimt change is in progress. The criterion should treat both positive and negative errors alike, and should also take into account the time during which the error exists. Such a criterion for measurement of control quality is the integral,
LLI 1
E tdt
The larger the absolute error, IEl, and the longer the time, t, which the error exists, the poorer will be control quality. If a controller is able to maintain the measurement with no deviation in the face of disturbances, the integral would be zero since no error appeared. Such a system is said to have perfect control quality. For the results disclosed in Figure 9, the control integral was evaluated for the same step load disturbance. To add meaning to the graph the actual composition responses are shown for various points on the graph. Any change which can be made to reduce the integral, results in improvement in control quality. Several important conclusions can be made from Figure 9. First is that with good control of feed enthalpy, a big improvement in analyzer control quality can be achieved by applying a high speed chromatograph (2, 4 ) . With no automatic control of feed enthalpy, only marginal improvement in control quality will result by increasing the analyzer sampling rate. Very good analyzer control can be achieved by applying a feed enthalpy computer and a hiqh speed chromatograph. I t is important to achieve good control of feed enthalpy if for no other reason than to maintain a fixed dynamic character of the analyzer control loop. When the feed enthalpy quality varies, the analyzer control qualitv will also vary. Figure 10, response A. shows that with good feed enthalpy control, a good composition response results. If the same analyzer-controller settings are retained and the feed enthalpy control quality decreases, curve B will result. The analyzer-controller must be retuned to achieve an acceptable degree of stability. Eventually the analyzer-controller will be tuned for the worst condition. Should the feed heat control quality improve, the analyzer-control loop will become sluggish unless it is retuned. Many column primary control systems suffer from the nonlinear effects produced by feed enthalpy changes. Primary controllers as conventionally applied are not adaptive to the changing process condition. Any controls, such
968
as the feed enthalpy computer, which can be added to help linearize the process will result in better primary control. Applic,ation of a feed enthalpy computer to operating columns has shown more improvement than might be predicted from the Figure 9 graph. One such example concerns an analyzer control ciystem on a 13-foot diameter, 50-tray natural gas liquids debutanizer. The column was originally instrumented as shown in Figure 1. Throughout several years operation, periods of time existed when the analyzer had to be taken off control because of unstable operation of the control loop. I n fact, the analyzer could be operated on automatic less than half the time. Even when on automatic control, the performance was only marginally acceptable. Application of a feed enthalpy computer to this column resulted in marked improvement. Unstable periods of operation were eliminated, and the analyzer control loop dynamics were greatly improved. The improvement for this case was the difference between no automatic control and good automatic control of composition.
Reboiler Heat Control Proper regulation of the reboiler heat is important in achieving good distillation column operation. Conventional Reboiler Controls. In general, there are two considerations in the design of reboiler controls. The first concerns how specific the controls are to heat flow. The degree to which the heat input is specifically measured and controlled governs whether heat disturbances can enter the column I A
0
I
30 60 90 I20 TIME,MINUTES
Figure 10. Closed loop ARC response to load step-change A.
Closed loop with 1-minute sampling period chromatograph. Good feed heat control. Note:
B.
INDUSTRIAL AND ENGINEERING CHEMISTRY
ARC tuned for minimum
s:
/E/tdf
Same ARC settings as Curve A. Oneminute chromatograph sampling period. Poor quality of feed heat control
through the reboiler system. The second consideration is one of dynamics and is concerned with how the vapor flow out of the column kettle varies in time when the heat input is manipulated. This is important if the heat input is to be manipulated by the primary controller of the column. There are several techniques in wide use for regulating the heat input. The most general of these are shown below. Although internal reboiler arrangements using steam are shown, the points to be covered are equally applicable to external reboilers and other heating media. Analyses of these systems regarding the above two considerations show that there is much to be desired. The system below falls short in both respects. If the steam supply conditions change, a heat change will enter the column. More or less bottom product will be reboiled depending upon the direction of change. A column side temperature controller or analyzer can be applied to manipulate the reboiler condensate valve. This takes care of the heat disturbance under steady state, but is not satisfactory dynamically. The heat disturbance must enter well into the column before the correction can be applied. Any variation in steam supply conditions which affect heat input will cause the bottom product flow to deviate. Many distillation column product flows are erratic because of reboiler heat disturbances. This is important especially if the product stream is the feed to another unit. The dynamic relation which describes the kettle vapor flow as a function of condensate valve change is quite poor for this system. Beside the fact that the response is slow, the system is also nonlinear. A change in heat input can be accomplished only by changing the condensate level within the reboiler tubes. Unless the tubes are mounted vertically, the level change will cause a nonlinear change in heat flow. Satisfactory control is seldom achieved with this system.
STEAM SUPPLY MANIPULATE CONDENSATE FLOW TO CHANGE HEAT INPUT PRODUCT CONDENSATE The system below offers dynamic advantages over the first arrangement. Heat input change is not a function of condensate level but is a function of pressure change within the tubes. Pressure responds rapidly when the valve
AUTOMATIC C O N T R O L is manipulated.
The arrangement results in a more linear system and has a faster response. Steam supply disturbances can, however, still enter the column. The valve located on the inlet steam is not specific to heat flow and therefore gives no regulation to this input.
COMPOS IT1O N SET POINT
[_I--il
+$,q -
ANALYZER
-. . .__ .. I
COMPOSITION RESPONSE
In the improved system a t left, the temperature controller manipulates the bottom product flow,
1
while the kettle level controller
manipulates the reboiler steam flow.
Actual
plant records o f the temperature and composition responses are shown.
These re-
sponses resulted from a step increase in reboiler steam supply pressure.
MANIPULATE STEAM FLOW VALVE STEAM SUPPLY .cgb----l
BOTTOM
y
CONDENSATE ucc nLv
""'1 VI I r
.
ASSOCIAT ED CONTROL:5 CONDENSATE
occurred.
PRODUCT
Figure 1 1 . system
Improved reboiler control
within the column. ANALYZER
The arrangement below offers further advantage in that the control of steam volume flow will help regulate disturbances caused by steam supply pressure changes.
m
CONDENSATE
The pressure controller below is more specific to heat flow and is theol i M A N l P U L A T E STEAM PRESSURE CONTROL SETPOINT
BOTTOM PRODUCT
STEAM SUPPLY CONDENSATE KEG WITH ASSOCl ATED CONTROLS CONDENSATE
retically better, dynamically. The dynamic improvement is obtained because the volume of the reboiler tubes is contained in the closed loop control system of the pressure controller. T h e Use of Kettle Level Control for Improved Reboiler Heat Regulation. Further improvement in reboiler heat control can be achieved by changing the manner in which the kettle level controller is applied. The change is shown in Figure 11. The level controller manipulates the flow of reboiler heat while the bottom flow is manipulated either manually or by the column primary control system.
The kettle level control acted to
exclude the heat disturbance long before it could affect composition and temperature Hardly any deviation in
bottom product flow resulted
--
COMPOSITION RESPONSE I
MANIPULATE STEAM FLOW CONTROLLER J-SETPOINT
Very little
deviation in the temperature of composition
1
The conventional control system for the same column (left) uses the temperature controller to manipulate reboiler heat, with the kettle
A TEMPERATURE
level controller manipulating bottom product
RESPONSE
flow.
The temperature and composition re-
sponses shown resulted from the same step SUPPLY
-'
increase in reboiler steam supply pressure
1 &"
,BOTTOM PRODUCT
as before.
Considerable deviation in bot-
tom product flow occurred.
The primary
control does eventually remove the effect of
Figure 12. Conventional arrangement of reboiler controls
the disturbance
A number of experiments have been made on operating columns to show the advantage offered by the Figure 11 arrangement over the conventional system of Figure 12. Some designers of distillation column control systems disapprove of the Figure 11 arrangement because the primary control loop contains more dynamic elements than does the conventional system. This can be seen by inspection of both the Figure 11 and 1 2 T R C control loops. Take the conventional system first (Figure 12), and follow a change from the T R C output, e,, back to the T R C measurement, T. When 8, changes, the steam flow will respond. As a result, the vapor flow u p the column is changed which in turn causes the temperature, T , to respond. Few elements are involved in the dynamics of this loop. Take the Figure 11 arrangement next. When 8, changes, the bottom product flow is manipulated. The change in bottom flow causes the kettle level to respond. As a result, the level controller will manipulate heat input to hold level. The change in heat input will cause a change in vapor flow out of the kettle which in turn causes temperature T to respond. Even though the Figure 11 system has more dynamic elements, actual tests have shown that there is not a detectable difference in behavior of the
two systems. This is true because the dominating factor for either system is the dynamic character which relates the vapor flow, V, with temperature T. The process, V to T , is characterized by a multiorder lag. Therefore, if another lag is added to an existing multiorder lag system, it will have little effect upon the over-all dynamics. Acknowledgment The authors wish to express their appreciation to personnel of the Process and Operating Divisions at Phillips Petroleum Company Sweeny Refinery for their cooperation in conducting the experiments disclosed in this report. literature Cifed (1) Haines, H. W., Jr., IND.ENG. CHEM. 52, 662-70 (1960). (2) Loyd, R. J., Ayers, B. O., Karasek, F. W., Anal. Chem. 32, 698-701 (1960). 13)Jour&l Luofer. D. E..(Juney959). Berrer. D. E.. Z.S.A. 6, 34-9 \
I
(4) Wightman, R. E., Karasek, F. W., Fourroux, M. M., Jbid., 7, 76-80 (May 1960). (5) Williams, T. J., Harnett, R. T., Rose, Arthur, IND. ENC. CHEM. 48, 1008-19 (1956).
RECEIVED for review July 19, 1961 ACCEPTED October 6 , 1961 VOL. 53, NO. 12
DECEMBER 1961
969
