Control of Continuous Distillation Columns

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Process Engineering Guide: GBHE-PEG-MAS-608

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Process Engineering Guide: Control of Continuous Distillation Columns

CONTENTS SECTION 0 INTRODUCTION/PURPOSE 4 1 SCOPE 4 2 FIELD OF APPLICATION 4 3 DEFINITIONS 4 4 GENERAL DESCRIPTION OF A DISTILLATION COLUMN 5 5 REGULATORY CONTROL 7 5.1 Composition Control 7 5.2 Mass Balance Control 12 5.3 Design of Feedback Control Systems 15 5.4 Pressure and Condensation Control 17 5.5 Reboiler Control 25 6 DISTURBANCE COMPENSATION 32

6.1 Feed-forward Control 32 6.2 Cascade Control 36 6.3 Internal Reflux Control 37 7 CONSTRAINT CONTROL 38 7.1 Override Controls 39 7.2 Flooding 39 7.3 Limiting Control 40



8 MORE ADVANCED TOPICS 43 8.1 Temperature Position Control 43 8.2 Inferential Measurement 44 8.1 Floating Pressure Control 45 8.2 Model Based Predictive Control 46 8.1 Control of Side-streams 47 8.2 Extractive/Azeotropic Systems 50 9 REFERENCES 51 TABLES 1 SYMPTOMS OF IMBALANCE AND THE REGULATORY

VARIABLES 12 2 PRACTICAL LINKAGES BETWEEN CONTROL

(P, R, B, C) AND REGULATION VARIABLES (h, r, d, b, c, v) 15

3 COMPOSITION REGULATION 16 4 COMPOSITION REGULATION - VERY SMALL FLOWS 17



FIGURES 1 DISTILLATION COLUMN SHOWING A SELECTION OF

POSSIBLE CONTROL AND REGULATION POINTS 4 2 SCHEMATIC OF TYPICAL SIEVE TRAY 6 3 COLUMN TOP AND BOTTOM TEMPERATURE CONTROL 8 4 TYPICAL TEMPERATURE PROFILE 9 5 PROFILE FOR A COLUMN WITH VERY PURE TOP

PRODUCT BUT MIXED COMPOSITION BOTTOM PRODUCT 9

6 PROFILE FOR A COLUMN WITH VERY PURE BOTTOM PRODUCT BUT MIXED COMPOSITION TOP PRODUCT 10

7 TEMPERATURE PROFILE SHOWING MULTIPLE SENSITIVE REGIONS 10

8 MASS BALANCE CONTROL 14 9(a) REGULATION OF VAPOR PURGE 18 9(b) REGULATION OF COOLANT 18 9(c) FLOODED CONDENSER 18 10 A HOT-GAS BYPASS IS THE MOST COMMON MEANS OF

CONTROLLING AIR-COOLED CONDENSERS 21 11 CONDENSATION CONTROL SYSTEMS FOR OPERATION

UNDER VACUUM 22 12 CONDENSATION USING A BOILING REFRIGERANT IN

CONDENSER SHELL 23 13 CONDENSATION USING A BOILING REFRIGERANT IN

CONDENSER TUBES 24 14 REFLUX DRUM MOUNTED LEVEL WITH OR ABOVE

CONDENSER 24



15 REFLUX DRUM MOUNTED LEVEL WITH OR ABOVE

CONDENSER BUT WITH REDUCTION OF PRESSURE DROP ROUND REFLUX LOOP 25

16 INSERTING A TUBE BUNDLE EITHER DIRECTLY IN THE

COLUMN BASE OR IN AN EXTERNALLY MOUNTED KETTLE 26 17 ALTERNATIVE ARRANGEMENTS OF THERMOSYPHON

REBOILERS 27 18 HEAT INPUT CONTROL 29 19 DIAGRAM SHOWING THE VALVE-POSITION CONTROLLER

(VPC) ADJUSTING THE TEMPERATURE SO THAT ONE VALVE IS ALMOST FULLY OPEN 30

20 HEATER PASS CONTROL- HOW THE VPC ADJUSTS FLOWS TO MAINTAIN ONE VALVE FULLY OPEN 31

21 HOW ENERGY INTEGRATION FORCES THE BOIL-UP OF ONE COLUMN TO BE DEPENDENT UPON ANOTHER 32 22 CONVENTIONALLY CONTROLLED COLUMN 33 23 AN EXAMPLE OF MORE COMPREHENSIVE FEED-FORWARD 34 24 FEED-FORWARD CONTROL WITH COMPOSITION

CORRECTION 35 25 A SUDDEN FEED CHANGE COULD CAUSE A REBOIL INVERSE RESPONSE 36 26 TEMPERATURE/FLOW CASCADE 37 27 TYPICAL SYSTEM FOR MAINTAINING INTERNAL REFLUX CONSTANT WHEN CONDENSER SUBCOOLING VARIES 38



28 THE OVERRIDE CONTROL OF REBOIL TO PROTECT BASE LEVEL DRYING OUT 39

29 EP OVERRIDE OF REBOIL TO PREVENT COLUMN FLOODING 40

30 LOW FEED FORWARD REFLUX FLOW LIMIT 41 31 COMPUTER SET BOTTOM OFF-TAKE WITH LEVEL

REGULATING FEED VALVE 42 32 FLOW CONTROLLER TUNED TO ENSURE THAT LONG TERM

DESIRED BOTTOM TAKE OFF-TAKE IS OBTAINED 42 33 SHARP TEMPERATURE PROFILE 43 34 POSITIONAL TEMPERATURE CONTROLLER 44 35 DIAGRAM SHOWING HOW THE VALVE-POSITION

CONTROLLER SLOWLY ADJUSTS THE PRESSURE SET POINT TO KEEP THE CONDENSER FULLY LOADED IN THE LONG TERM 46

36 DIAGRAM SHOWING HOW ALL THE HEAT USED IN CRUDE

OIL DISTILLATION ENTERS WITH THE FEED 48 37 DIAGRAM SHOWING HOW VIRTUALLY ALL THE METHANE

FROM THE BOTTOM OF THE DEMETHANISER LEAVES WITH THE LOW GRADE ETHYLENE 49

DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE 52



0 INTRODUCTION/PURPOSE As an industrial separation process, distillation has been in worldwide use since the 1920s. It is one of the leading unit operations in process engineering. A simple distillation column (see Figure 1) with only feedback control has over 700 possible control arrangements from simple pairing of variables alone. When other factors are taken into account such as choice of temperature measurement point, direct composition control, different possible methods of condensation and pressure control, different reboil arrangements, feed forward control and the question of the need for more advanced control techniques, the number of possible control systems for continuous distillation columns can be measured in the thousands. The choice of any control system should ideally be based on a quantitative understanding of the process and the comparative performance of different control structures. Mathematical modeling, simulation and controllability assessment are often used to design control systems where the design is not obvious a priori. In the case of distillation, modeling is difficult and time consuming. In addition, there are many different possible control arrangements. Fortunately, by a logical approach and argument it is possible in most cases to reduce the realistic possibilities to very few. This is the purpose of this Guide and the problem is tackled in the following way. (a) After a general description of a column, regulatory control for steady state

operation is addressed by considering the related questions of composition and material balance.

(b) Next, enhancements to cope with constraints and for superior control

during transients are described. (c) Finally, the relevance and application of more advanced techniques is

discussed. The Guide concludes with a number of references for further study.



1 SCOPE This Process Engineering Guide deals with the control of continuous distillation columns which are used widely in petrochemical and chemical manufacture and refining. It covers regulatory control, disturbance compensation, constraint control and more advanced topics. The purpose of the Guide is to display control options; it does not deal with control hardware. 2 FIELD OF APPLICATION This Guide applies to the process and control engineering communities in GBH Enterprises worldwide. 3 DEFINITIONS For the purposes of this Guide, the following definitions apply: VPC Valve Position Controller PIC Proportional, Integral and Derivative Control MBPC Model Based Predictive Control DMC Dynamic Matrix Control



FIGURE 1 DISTILLATION COLUMN SHOWING A SELECTION OF POSSIBLE CONTROL AND REGULATION POINTS



4 GENERAL DESCRIPTION OF A DISTILLATION COLUMN The main part of the column consists of a series of plates or a packed bed in which fractionation takes place (see Figure 2). The feed enters the column at some point between its ends chosen to minimize the mismatch between the feed and the material being processed at that point. Vapor from the reboiler enters at the lower end and passes upward through the column where it mixes intimately with the descending liquid. This liquid is that part of the condensed vapor returned as reflux from the condenser at the top of the column. In this way the excess enthalpy of the vapor is given up to boil the liquid on the plates of the column. FIGURE 2 SCHEMATIC OF TYPICAL SIEVE TRAY



Vapor thus produced gives up its latent heat to boil liquid in the plate above and so on. The rising vapor becomes richer in the more volatile component of the feed while the residual liquid becomes richer in the heavier component and is returned to the plate below. The overhead vapor is said to strip the more volatile component from the liquid while the reflux scrubs the less volatile component from the vapor. The action of a packed column is analogous with the distinction that the process is continuous in space. A pattern is thus set up such that the proportion of more volatile component increases with column height while the proportion of the heavier component decreases. If the column equilibrium conditions are upset the pattern is distorted and a measure of this distortion can be provided by composition analyses or temperature measurements within the column. In general, there are only two independent quantities which may be held constant within a column. The reflux and reboil rates are the two independent variables which have a significant effect on the separation and may be externally adjusted to compensate for disturbances in feed conditions. That part of the overhead vapor which is not returned in condensed form as the reflux is removed as top product or distillate to maintain a mass balance in the condensation system. A measure of a lack of mass balance may be detected from the pressure or the liquid level, depending on the particular form of condensation system used. If the top product is removed entirely in the liquid phase, the system is said to use a total condenser but if a substantial part of the top product is removed in the vapor phase it uses a partial condenser. Similarly, at the bottom of the column the down-flow liquid enters a reboil system and that part which is not returned to the column as reboiled vapor is removed as bottom product. The measurement which provides an indication of any lack of mass balance in the reboil system is usually the liquid level in the base of the column or in the reboiler itself. Some distillation columns feature multiple feeds and side-streams. Control of side-streams is covered in 8.5.



5 REGULATORY CONTROL 5.1 Composition Control Disturbances to column operation are most usually due to changes in feed-rate, composition or enthalpy but atmospheric conditions can also affect the column - particularly some condensation systems. There are normally only two quantities which can be independently adjusted by a control system to compensate for disturbances. Reference to Figure 1 should make this clear. Five independent adjustments (control valves) are shown. There are 3 mass balances to be maintained, viz: base liquid, tops liquid and vapor. This leaves two for composition control. This means that it is not possible to keep the composition profile in the column constant – only two quantities related to that profile. In practice, we are normally interested in the composition at the top and/or bottom of the column. The composition of an n-component mixture is determined by n-1 quantities and so, if n > 3 the column can never be controlled to give a constant top and bottom composition as feed conditions change. A degree of over-purification is always necessary in the steady state to ensure acceptable performance during transient conditions. 5.1.1 Temperature Control Temperature is most often used to infer composition. However, it should be remembered that while at constant pressure the composition of a multi-component mixture determines the boiling point uniquely, the converse is not true. As we are usually interested in the composition of products at the top and bottom of the column, it is sometimes suggested that the bottom temperature could be used to regulate reboil and the top to regulate reflux (see Figure 3). This can give rise to problems of interaction. If an analysis is carried out of the short term effects on temperature of changes in feed-flow, composition and enthalpy then it can be shown (see Ref.1) that: (a) the relationships between the temperatures and the flows they might

regulate change, dependent on the disturbance and the condition of the feed, and



(b) some short term effects are often in the opposite direction to the long term steady state requirements.

Note: When the words Ref.1 etc are shown in the text this refers to that number

document in the References in Clause 9. This suggests that dual temperature control regulating both ends of the column is fraught with difficulties and is likely to be highly interactive. This form of control is not recommended, therefore, for columns with simple feedback control systems. Dual composition control is feasible under some circumstances with mixed temperature and direct analysis (see 6.1) and with model based predictive control (see 8.4) where the column interactions and dynamics are built into the control system.



FIGURE 3 COLUMN TOP AND BOTTOM TEMPERATURE CONTROL



5.1.1.1 Temperature Profiles The temperature profile (Figure 4) shows a ''sensitive region'' (X) for temperature control where the rate of change of temperature is relatively high. It is in this region where temperature should be controlled to provide the maximum sensitivity for composition control. In the region of maximum sensitivity pressure variations are also likely to have a correspondingly small effect on the measured temperature. Generally, for good temperature control, the effect on temperature of normal pressure changes should be at least 5 times less than the effect of normal composition changes. In some instances there is a very large temperature change over only a few trays in the column and under these circumstances simple temperature control may be too sensitive. This problem is addressed in 8.1. In practice the column temperature profile is likely to be very different from that shown in Figure 4. It is necessary to distinguish carefully between the concept of high product purity and a product containing a constant proportion of a particular component. In the situation where a column is producing a very pure top product but a bottom product with different components the profile may well be as shown in Figure 5 while the opposite case is shown in Figure 6. In some instances the column temperature control may show multiple sensitive regions (see Figure 7) which are characteristic of particular internal separations. In all these cases it is important to be clear about the control objectives. Which of them need to be met under all circumstances and which might be relaxed and by how much? What are the key heavy and light components? What are the design feed rates? What is the normal turndown? Furnished with this information it is usually possible: (a) to make a reasonable decision on where to site the temperature

measurement point; (b) to consider whether on-line direct analysis is feasible for composition

control or as a supplement to temperature control, and (c) to decide whether the composition controller should regulate the reflux or

the reboil rate to the column.



FIGURE 4 TYPICAL TEMPERATURE FIGURE 5 PROFILE FOR A COLUMN WITH VERY PURE TOP PRODUCT BUT MIXED COMPOSITION BOTTOM PRODUCT

.



5.1.1.2 Choice of Reboiler or Reflux Regulation If temperature control is to be used and if the temperature profile has multiple sensitive regions, it is important to determine which corresponds to the key separation in the column and this is where the control temperature should be measured. In more complex azeotropic or extractive distillations (see 8.6), the presence of such a region can be dependent on maintaining other conditions. The choice of reboil or reflux regulation is usually decided on the basis of dynamic response. It is then usual to fix the other variable on flow control at a value to allow the column to cope with disturbances. Feed-forward compensation is dealt with later. In general, reboil changes affect the column temperature (composition) much more quickly than corresponding changes in reflux. It follows then that reflux regulation should be used only when the control measurement can be made very close to the top of the column. This is likely to be the case where one is interested in obtaining a top product containing a constant proportion of a given component, not a pure product. It is difficult to be



definitive about where the breakpoint should be between choice of reboil or reflux. F G Shinskey (Ref.4), argues that reflux should only be used if the control temperature is measured on the top tray. This is probably too restrictive but if the temperature has to be measured lower than, say, 20% from the top of the column then regulation by reflux is likely to be unsuitable. Because of uncertainty in the expected shape of the profile, and to provide flexibility for later changes in operation, there is a need in the design phase to provide an adequate number of optional temperature measurement points around the expected region of sensitivity. If analytical measuring instruments of sufficient reliability and speed are available, direct composition analysis is to be preferred. The control loop measurement delay and analyzer time constant should be short compared to the column dynamics. There is then no objection to controlling top product composition by varying the reflux rate when one is particularly interested in the composition of the top product or the bottom product composition by varying the reboil rate when one is interested in the composition of the bottom product. The control loop is very short in either case. The poor dynamics of reflux control are not then important. 5.1.1.3 Effect of Pressure As described above, temperature measurement used to infer composition is sensitive to pressure variations and this is why it is best to measure the temperature in a region of high sensitivity to composition changes. On occasions it is not possible completely to mask the pressure effect. A number of solutions are possible. (a) Column pressure may be controlled at the point of temperature

measurement. This assumes that it is desirable to hold column pressure constant (standard practice) but modern thinking suggests that a control system that allows pressure to float can be more economical (see 8.3).

(b) The temperature can be mathematically compensated for variations of

pressure so that the compensated temperature is seen by the temperature controller. The objective is to reference a temperature made at a variable pressure to one at some base pressure, e.g. atmospheric. The compensated temperature Tc is related to the measured temperature Tm, pressure p and base pressure P b as:



(c) Difference or double difference temperature measurements are

sometimes made which compensate for pressure variations. They are all affected by pressure changes to approximately the same extent. However if these arrangements are to be successful they need to be carefully matched to column characteristics (Ref. 2 & 3).

5.1.2 Direct Composition Control

Modern analytical devices are much more reliable than hitherto and have the capability to supplement temperature control in many applications. The advantage is that they can be sited to measure directly top or bottom compositions and to regulate reflux or reboil in fast loops as appropriate. In the case of azeotropic or extractive distillation, where it is necessary to maintain a minimum component composition at some point in the column, this concentration can be measured directly. Analyzers are regularly provided to monitor product quality in run down lines. There is an opportunity to use these analyzers to control quality directly, by siting them in the column, rather than to monitor the performance of a less effective inferential system. Unfortunately this option is rarely taken. The most common form of on-line analyzer is the chromatograph which has the advantage of being able to measure multiple components. It only performs an analysis at discrete intervals, however, and can be very slow. If it is possible to use IR or UV analyzers then their continuous output makes them much more suitable for control.



All analyzers are more expensive to install and maintain than temperature measurements. For this reason analyzer use is likely to be restricted to applications where there is a strong economic incentive to reduce costs, to minimize environmental pollution or to maximize recovery of a particularly high value product. Generally, but not exclusively, they will be used in conjunction with temperature based composition control. A particularly successful mode of operation is, for example, to trim reflux/feed ratio when the reboil is regulated by a conventional temperature control loop (see 6.1). In this way, a measure of dual composition control is possible. Care should be taken in the installation of all direct composition control loops to minimize the analyzer sample delay which is a pure dead time. The analyzer should be sited as close as possible to the column and a vapor sample is generally to be preferred since the higher velocity reduces the dead-time in the sample line. Chromatographs present additional problems since their analysis is produced at discrete intervals known as the "sample interval". When a new analysis is produced, the controller should apply immediate control action and then wait until a new analysis measurement is made. This sampled/data control has the benefit of introducing an integral action effect into the control loop without the normal disadvantages of increasing period. If a computer is used, performance will be improved if its sample period is keyed to the sample period of the analyzer so that the control can take immediate action on new information. 5.2 Mass Balance Control The choice of regulation variable to control composition has been discussed above. This leaves many other variables to be regulated from other measurements. The correct values of these can be obtained from mass balance considerations. Sufficient symptoms of mass imbalance are available to enable a conventional feedback control system to be set up. To reduce the options to a manageable level it is helpful to consider the various mass balances that it is necessary to achieve and those regulatory variables which can affect them directly or indirectly. The balances are: (a) Vapor balance in the column. (b) Liquid balance in the condensation system.



(c) Liquid balance in the reboil system. Table 1 shows the symptoms of imbalance and the regulatory variables. The situation is shown diagrammatically in Figure 8. TABLE 1: SYMPTOMS OF IMBALANCE AND THE REGULATORY

VARIABLES

Note: Not all of these measurements and regulatory variables may be available

on any given column. (a) Vapor balance in the column

Vapor is added to the system by the reboiler, by evaporation of liquid from the plates and possibly from the feed. It is removed by condensation in the condenser, by condensation on the plates and possibly as top product taken off in vapor form (non-condensables in the feed or air leakage in the case of a vacuum system may be additional contributors to the vapor balance). Any lack of balance between the sum of the rates of production and the sum of the rates of removal of vapor causes a change in the pressure in the system so pressure measurement at some point provides a symptom of lack of vapor balance. Inspection of Figure 8 shows that of the quantities mentioned above, the only ones available for pressure control are the rate of generation of vapor in the reboiler (h), the rate of condensation in the condenser (c) and the rate of removal of top product in the form of vapor (v). At least one of them should be left available to be adjusted either directly or indirectly by the column pressure controller.



The details of how the condensation rate can be adjusted in various types of condenser and how the reboil rate can be adjusted in various types of reboiler are covered in later sections of this Guide. The present indications are symbolic.

(b) Mass balance of the liquid in the condensation system.

The condensation system is that part of Figure 8 enclosed by the dotted lines and includes the condenser and reflux drum. Liquid enters this system by condensation of vapor in the condenser (C). It leaves as distillate in condensed form (d) and as reflux returned to the column (r). To maintain a liquid balance it is necessary, in the steady state, that (c) = (r) + (d). Any lack of balance is indicated by a rise in or fall in the reflux drum level (R). It is not necessary to hold a reflux drum level precisely. Indeed, it is usually best to use the reflux drum and the base of the column as surge capacities to smooth disturbances to downstream units (see 5.2.1). All three of the quantities (c), (d) and (r) are available for direct control of reflux drum level.



FIGURE 8 MASS BALANCE CONTROL

(c) Mass balance on the liquid in the reboil system.

Liquid enters this system from the column itself and leaves it as bottom product or by evaporation in the reboiler. The bottom product flow (b) and the rate of vaporization (h) are available to control the level directly.



From the preceding discussion it will have been apparent that certain variables affect more than one mass balance or composition. This is shown in the matrix in Table 2. Added to the matrix is an additional row showing possible linkages for composition control. Direct linkages between regulation variables and the symptoms of imbalance are shown by a cross in the appropriate location. An (x) shows an indirect, feasible but less desirable potential linkage.

At least one, or preferably both, of bottoms off take (b) and distillate (d) should be controlled by either a symptom of imbalance or composition. The composition link should only be considered if a sloppy split is adequate.

TABLE 2 PRACTICAL LINKAGES BETWEEN CONTROL (P, R, B, C) AND

REGULATION VARIABLES (h, r, d, b, c, v)

Sometimes adjustment of a column feed is used to control base level. This generally provides poor control due to excessive dead time and process delay before changes in flow can affect the level. All other flows in the column should also be adjusted in the same ratio to maintain separation conditions. This form of control should be avoided if possible. An example of how to design a feedback control system for a particular arrangement is given in 5.3.



5.2.1 Surge Capacity and Level Control It is a general rule that most capacities on a distillation column and its peripheral equipment should be used to the maximum extent to smooth out flow fluctuations and thereby help protect downstream equipment from disturbances. An exception to this general rule may be when, for example, reflux drum level is regulating reflux and it is important to increase reflux to match any increase in reboil rate. The reasons for setting up such a form of control is discussed later. In all other cases the level control system should be tuned to maximize the use of surge capacity. This means that a proportional only controller should be used tuned so that the capacity never empties or overfills. Sometimes proportional (error)2 control is suggested for such applications so that little control action takes place at the mid measurement point but more drastic action takes place as the measurement reaches its limits. In the experience of the author properly tuned proportional only control is normally acceptable. It is simpler and is better understood. 5.3 Design of Feedback Control Systems (a) Design Example

Consider a distillation column with the following specification:

Liquid feed (entering about half way up the column) - 10 te/hr

Distillate - 4 te/hr

Reflux ratio - 2 : 1, i.e. r/d The column has a total condensation system and operates at 20 bar g. The sensitive region for temperature control is 20% of the height above the base.

It can easily be deduced that the column reflux is 2 x 4 = 8 te/hr and the bottom off-take is 6 te/hr. The flow into the base will be approximately the feed rate + reflux = 18 te/hr. The bottom off take is 6 te/hr and so the reboil rate will be approximately 12 te/hr. There are no critical flows, so, from Table 2 first make a choice of composition regulation. This is fairly evidently reboil (h) and the other controls can be built up for the reasons given in Table 3.



TABLE 3 COMPOSITION REGULATION

This leaves reflux uncontrolled. In these circumstances it is normal to flow control reflux. This is discussed later.

We have not considered the type of condensation system provided. If, for example, it were a water cooled condenser it is quite possible that no regulation would be available because the necessary valves are likely to be very large and cooling water is relatively inexpensive. How should pressure be controlled?

In this circumstance there might be a temptation to control pressure by regulating reflux flow. But then there would be no independent way of setting the energy input to the column and, if the pressure and temperatures are mismatched (as they always will be), the reboil and reflux would ramp up or down until a balance is found. This might be maximum (or minimum) energy input.

In these circumstances the correct action is probably to let the pressure float to achieve the lowest possible value at any time. The problems of temperature control with the variable pressure are covered in 5.1.1.3. The reflux should be flow controlled or ratioed to feed. Then the reflux flow and its temperature sets the heat input to the column to balance the reflux flow to maintain steady compositions.

5.3.1 Very Small Flows On some occasions a top or bottom off-take will be very small since only trace impurities are being removed. Consider the following case, identical to the above example except that the top off-take is 9 te/hr with the bottom off-take 1 te/hr. Carrying out a similar calculation to previously, it can be shown that the ratio of reboil to bottoms off-take is 27:1. In this circumstance, it is very difficult to maintain a bottoms mass balance by regulating bottoms off-take; reboil has a much greater effect.



In cases like this it is not uncommon to regulate the bottoms level by adjusting reboil and to use the composition measurement, particularly if it is close to the bottom of the column, to regulate the bottoms off-take. Similar arguments apply to the control of distillate and reflux from a temperature control near the top of the column and the reflux drum level. Consider the following table. In this case the bottoms mass balance is the critical parameter to be measured so consider it first. TABLE 4 COMPOSITION REGULATION - VERY SMALL FLOWS

5.4 Pressure and Condensation Control 5.4.1 Principles of Condensation Control

The overhead vapor leaving the top of the column consists of a mixture of condensable vapors containing all the components of the feed in various proportions together with a certain amount of non-condensables which may be either true inerts or material of too high a vapor pressure to condense to any appreciable extent under the conditions prevailing in the condensation system.

Overhead condensers may be either total condensers where the condensable part of overhead vapor should ideally be completely condensed leaving only the inerts to be purged from a suitable point in the condensation system. Alternatively, they may be partial condensers where only a fraction of the condensable components are condensed, the remainder passing out with the inerts as a vapor top product. In any condenser the total rate of condensation depends on:



(a) The partial pressure of the condensable fraction of the vapor in contact with the cooling surface.

(b) The temperature on the cold side of the condenser. (c) The overall heat transfer coefficient. (d) The available surface area for condensation. (e) The effective latent heat of the mixture. Condensation control methods usually vary one or more of the first four of these quantities. They are considered in turn. Typical condensation arrangements are shown in Figures 9(a) to 9(c). In Figures 9(b) and 9(c) the vapor top off-take may not be present if condensation is total but there is always likely to be some means of manually venting inerts which have built up in the system. (1) Consider Figure 9(a). If it is required to reduce the rate of condensation,

the control valve closes and the fraction of non-condensables in the condenser will then build up.

The particular system shown can only be used with columns operating above atmospheric pressure but alternatives using the same principle can be used when operating at below atmospheric pressure. This is considered later. Strictly speaking, adjustment of the vapor purge valve also varies the overall heat transfer coefficient as well as the partial pressure of the condensables since the vapor side heat transfer coefficient is dependent on the fraction of the non-condensables in the vapor.

(2) Figure 9(b) shows a system where the flow of cooling medium is varied. At

high flow rates the average temperature of the coolant is reduced so the rate of condensation is increased. Sometimes boiling refrigerant is used as the cooling medium, absorbing the latent heat of condensation as its own latent heat of vaporization.



FIGURE 9 (a) REGULATION OF VAPOR PURGE

FIGURE 9 (b) REGULATION OF COOLANT



FIGURE 9 (c) FLOODED CONDENSER

In this case the refrigerant is in equilibrium with its own vapor so its temperature can be changed simply by changing the pressure in the vapor space above it. This can be used for condensation control (see 5.4.6). Manipulation of the coolant flow is rarely used to adjust heat transfer in water cooled condensers on distillation columns. Response of the condensing rate to water-flow variations is non-linear and slow. The speed of response also varies with coolant flow which can cause loop stability problems. Low velocity and also leads to high fouling and metal corrosion. For more details see (Ref.2). (3) The surface area available for condensation can be varied by operating

the condenser partly flooded with liquid as shown in Figure 9(c) where a control valve is placed in the line carrying the liquid away from the condenser. In this case only the part of the surface which is not submerged is available for condensation of vapor and the liquid will leave the condenser sub-cooled.

The basic condensation control systems differ in dynamic performance. For example, the speed of operation of the system shown in Figure 9(a) will depend entirely upon the proportion of inerts present in the overhead vapor from the column. If the proportion is very small then, even if the vent valve is closed, it could take a long time to build up the sufficient proportion of inerts in the condensation system to affect heat transfer. Sometimes a split range control system is provided where nitrogen can be added to the system to speed up the response.



The speed of operation of the system shown in Figure 9(b) depends very much on the thermal capacity of the tube wall separating the hot and cold sides of the condenser and on the residence time of the cooling medium when this is a cold liquid.

The system in Figure 9(c) has slow dynamics. Movement of the control valve affects the rate of change of the flooding depth in the condenser and this in turn affects the rate of change of condensation and hence pressure

5.4.2 Air Condensers In this system the air is directed upward by a fan through a horizontal bundle of finned tubes. The temperature of the condensing vapor tends to be constant if the tubes are not flooded with liquid and the following equation applies:

The rate of heat transfer is limited by the air film and so the 0.2 power in the exponent is justified. As a consequence, heat flow should be reasonably linear with air flow. Controlling air flow is another matter. Variable speed fans are rarely used because of the high cost of drive units. Most air coolers use multiple fans which can be energized in stages but this gives only incremental control. Some fans are equipped with variable pitch blades or adjustable louvers but they have a tendency not to work satisfactorily. A further problem with air condensers is the effect of ambient conditions. Rain tends to convert the dry condenser into a wet condenser which can have a marked affect on the temperature of the condensate and hence the internal reflux in the column. In critical conditions this may require the provision of internal reflux control computation. This is discussed later.



Adjustment of cooling in the air condenser can be effected by: (a) flooding with condensate, or, (b) bypassing. Most air cooled condensers are horizontal and hence flooding does not give smooth control. Sometimes the condenser is mounted at an angle and this helps but since the tubes are of large capacity and can contain a substantial amount of liquid, response can be slow. Bypassing hot vapors around the condenser is shown in Figure 10. Column or reflux drum pressure can be controlled by adjustment of the bypass valve. The sizing of the bypass valve is difficult but critical to the effective operation of the condensation system. In particular, it is necessary to decide what proportion of the vapor shall be bypassed under different operational circumstances, throughput and ambient conditions. For more information on air cooled heat exchangers see (Ref.3). 5.4.3 Internal Condensers In some columns a condenser is mounted inside the column above the top plate. There is then no reflux drum and the condensed liquid reflux falls directly from the condenser onto the top plate through some distributor system. Uncondensed vapor can be taken off directly as top product or an internal weir may be fitted, allowing the distillate to be withdrawn. This limits the options for control. Ideally, all condensate should be trapped and withdrawn for metering and control. If this is not done the system behaves as if the reflux flow were regulated via a very high gain reflux drum level controller. (R <-> r) should be assumed when examining Table 1. Large and sudden changes in top off-take can cause corresponding changes in reflux and, in extreme situations, reflux flow may be lost for a period.



5.4.4 Operation at Atmospheric Pressure with a Total Condenser Many columns are designed to operate at atmospheric pressure by linking the condenser to atmosphere through a vent and scrubbing system with a small pressure drop. In principle, this balances the rate of condensation to the overhead vapor flow by altering the proportion of inerts (air) present in the condensing vapor. Pressure control is rapid but varies with atmospheric pressure. From the point of designing an overall control system it is as if the pressure is linked to the condensation rate and (P <-> c) has to be assumed in Table 1. When a column is one of a number in a distillation train and a partial condenser is used, then the vapor top product may form the vapor feed to the next column in the train. If some method of pressure control is used on the second column, the first may be maintained at very nearly the same pressure as the second simply by ensuring that the line carrying the vapor offers little resistance to flow. Referring to the Table 1, this effectively means that the pressure is related to the vent rate (v) although physically there is no controller or control valve. Assume (P <-> v).



FIGURE 10 A HOT>GAS BYPASS IS THE MOST COMMON MEANS OF CONTROLLING AIR>COOLED CONDENSERS



5.4.5 Operation at Sub-Atmospheric Pressure

When the column operates at below atmospheric pressure, the proportion of inerts in the vapor entering the condenser is likely to be significant because of leaks of air into the system. The purge line is often connected to the suction side of some form of vacuum pump or steam ejector. With this arrangement it is possible to use any of the basic condensation control systems but, since the proportion of inerts in the overhead vapor may well be high, a modification of the system shown in Figure 9(a) is normally used. Three common arrangements are shown in Figures 11(a) to 11(c).

FIGURE 11 CONDENSATION CONTROL SYSTEMS FOR OPERATION

UNDER VACUUM



5.4.6 System Using a Refrigerant as Cooling Medium

When distillation is carried out at low temperature, a boiling refrigerant is frequently used as the cooling medium in the condenser.

The temperature of the boiling refrigerant depends on the pressure maintained in the vapor space above it which may very easily be regulated by manipulating a control valve in the refrigerant exit line. The refrigerant is often contained in the shell of a condenser as shown in Figure 12 and condensation of the process vapor takes place in the tubes. A constant refrigerant level is maintained by admitting a controlled flow of liquid refrigerant to the shell. The vapor condensation rate can be varied to a certain extent by altering the refrigerant level but the temperature of refrigerant boiling is usually used as the principal method of condensation control.



FIGURE 12 CONDENSATION USING A BOILING REFRIGERANT IN CONDENSER SHELL

When partial condensation is required, and the vapor velocity in the condenser tube is high, the condensing liquid is carried out of the tubes by the vapor flow. If the vapor velocity is low, as will be the case with a total condenser, it may not be easy to keep the tubes well drained. In this case, better heat transfer may be obtained by condensing the vapor in the shell with the refrigerant vaporizing in the tubes. A system of this type is shown in Figure 13. The refrigerant is held in equilibrium with its vapor in a separate accumulator to which a control flow of refrigerant liquid is added as again, the temperature of this liquid is determined by the pressure in the vapor space above it.

5.4.7 Systems Where the Reflux Drum is Mounted Level to or Above the

Condenser

Sometimes, with free standing distillation columns, both condensers and reflux drums are mounted at ground level to be easily accessible for maintenance and to avoid the need for expensive supporting steelwork. The reflux drum is then sited at the same level as the condenser or even slightly above it. Liquid formed by condensation cannot fall into the reflux drum under gravity. In practice, to avoid the need to pump liquid into the reflux drum, a pressure differential is maintained between condenser and reflux drum.



FIGURE 13 CONDENSATION USING A BOILING REFRIGERANT IN

CONDENSER TUBES

The basic method of operation is shown in Figure 14. The condenser works at almost the same pressure as the top of the column and the rate of condensation is varied by flooding part of the cooled surface with condensed liquid.



FIGURE 14 REFLUX DRUM MOUNTED LEVEL WITH OR ABOVE CONDENSER

The liquid entering the reflux drum is therefore cooled below its condensation temperature from contact with the submerged tubes. Its vapor pressure, which is the reflux drum pressure, is therefore lower than the pressure in the condenser. This pressure difference drives the condensed liquid into the reflux drum against the hydrostatic head due to the difference in level and the frictional losses in the piping. Pressure control is effected as shown in Figure 15 by bypassing some vapor from the column to the reflux drum. This alters the EP between condenser and drum and so alters the flow of condensate and thus the level in the condenser. This alters the condensation rate and so the column pressure.



FIGURE 15 REFLUX DRUM MOUNTED LEVEL WITH OR ABOVE CONDENSER BUT WITH REDUCTION OF PRESSURE DROP ROUND REFLUX LOOP

Experience suggests that there can be operational problems with these systems and great care needs to be taken with the hydraulic design. If the bypass valve is adjusted to control the column pressure there is an inverse response which can cause instability. Controlling the reflux drum pressure by adjusting the bypass appears to be more satisfactory.

5.5 Reboiler Control

Reboilers can be classified into two groups depending on whether the mechanism for transfer of heat to the column is natural or forced circulation. The different physical construction or heat sources can affect control.

Figure 16 shows an example of natural circulation where the reboiler tube bundle is immersed in liquid either in a kettle exterior to the column or



internally. In either case the heating medium may be steam or liquid within the tubes. Both achieve 100% vaporization.

FIGURE 16 INSERTING A TUBE BUNDLE EITHER (a) DIRECTLY IN THE

COLUMN BASE OR (b) IN AN EXTERNALLY MOUNTED KETTLE



Natural circulation or "thermosyphon" reboilers may be mounted either horizontally or vertically. Vertical thermosyphon reboilers are used primarily in the chemical industry and are typically steam heated (see Figure 17(a). Horizontal thermosyphon reboilers are more common in petroleum refining or similar operations in the chemical industry. They are usually heated with circulating oil, see Figure 17(b).

Forced circulation is used with vacuum distillation or when the heat input is obtained from oil or gas furnaces.

FIGURE 17 ALTERNATIVE ARRANGEMENTS OF THERMOSYPHON

REBOILERS



5.5.1 Steam Reboilers

Steam is the most common vapor used for heating. In some low temperature applications a refrigerant is used. Superheated steam is a relatively poor heat transfer medium, as are most gases. Saturated steam is an excellent medium because of good heat transfer and a high latent heat of vaporization. The boil-up rate may be controlled either by steam flow or shell pressure. The steam flow measurement is usually made upstream of the control valve where the pressure is normally constant.

The steam rate to the reboiler may be controlled either by a valve in the steam line or in the condensate off-take. With the control valve on the inlet to the reboiler, the saturation pressure in the shell varies with heat load. Since the heat is being transferred between a condensing and a boiling fluid, neither changes temperature greatly in the process. A steam trap or similar condensate seal is necessary to drain condensate without releasing steam. If pressure rather than flow is controlled, the boil-up automatically increases if the process fluid boiling point falls. This action compensates correctly for changes in bottom product composition but not for changes in column pressure. However, the rate of boil-up is not linear with steam pressure nor is steam pressure zero at zero boil-up. For these reasons, steam flow control is preferred to pressure control to establish and maintain boil-up rate, prevent flooding and control product quality. There are advantages and disadvantages in placing the control valve in the condensate line. In the first place the valve can be smaller - typically one third of the line size of a steam valve used for the same service. In addition, the steam reaching the reboiler is at a higher pressure than with a valve in the steam line and hence the maximum heat transfer rate is higher. The dynamic response characteristics of the two approaches differ significantly. Manipulating the control valve in the steam line causes steam flow to change immediately. Shell pressure, and hence rate of heat transfer, will lag behind for only a few seconds. By contrast, the condensate valve has no direct effect on steam flow and as the condensate level determines steam flow this level takes time to change. The slow response of a system with a valve in the condensate line usually means that it is not suitable if a boil-up is being used for base level control.



5.5.2 Hot Oil Reboilers When a liquid stream is used to boil-up a column there arises the question of how to measure and control the heat input. With steam heating, the rate of condensation and therefore heat input is directly proportional to the steam flow. With liquid media, however, the relationship is very non-linear since, as the flow of hot oil increases, the temperature difference between inlet and exit also changes because the heat transfer coefficient is dependent on flow rate. Heat input can be calculated from the equation:

The calculation of actual heat transfer can be implemented either with digital or analogue components as shown in Figure 18. This heat transfer equation assumes a steady state. In practice, the indicated heat flow always leads the true heat flow by the residence time of the liquid in the reboiler. Hot oil reboilers are often provided with heat from a fired heater (see Figure 19). A low oil flow requires higher reboiler inlet temperatures to transfer given flows of heat to the columns. Additionally, higher inlet temperatures require a higher flue gas temperature in the heater which causes higher stack losses. Maximum efficiency is realized when oil flow is at maximum and oil temperature is at its minimum. Conventional heater controls always include a bypass, recirculating hot oil back to the cold oil line, to protect against loss in flow through the heater. The bypass valve is usually manipulated to control differential pressure between hot and cold oil lines and, as less heat is required by the reboilers, the bypass valve opens to maintain a constant flow through the heater. A better arrangement is to ensure that the reboiler demanding the most heat, receives full oil flow at all times, with the oil temperature set to deliver that heat.



FIGURE 18 HEAT INPUT CONTROL

Flow to the other reboilers may then be throttled to match their requirements. This can be achieved by use of a valve position controller (VPC) as shown in Figure 19. Each column will have its own heat input controls manipulating hot oil flow. The valve position signals are compared in a high selector and the highest is sent to the VPC. This device then adjusts oil temperature until the highest valve signal is at or near full opening. The oil temperature will then be at its minimum acceptable value as will the hydraulic power loss through the control valves. The valves are free to be manipulated by the individual column controls for fast response in the short term while the slower acting VPC minimizes energy loss in the long term. If such as scheme is implemented bypassing is not normally required since one of the load control valves is always nearly full open. However, a bypass valve that fails open on high ΔP should still be used to protect the heater against controller failure. For more information on reboilers see (Ref.5).



5.5.3 Fired Heaters A fired heater may also be used as a reboiler. This is often done for the fractionation of crude oil and for other products having boiling points in excess of 150°C. The flow of bottoms product to the heater is nearly always controlled to maintain efficient heat transfer. In addition, the flow is usually conducted through several parallel passes in the furnace and some means of equalizing flow through these passes should be provided. This can easily be done by use of a VPC. FIGURE 19 DIAGRAM SHOWING THE VALVE POSITION CONTROLLER

(VPC) ADJUSTING THE TEMPERATURE SO THAT ONE VALVE IS ALMOST FULLY OPEN



The highest valve signal is selected as the input position controller which adjusts the set points of all flow controllers to hold that valve exactly at the fully open position (see Figure 20). This arrangement results in the minimum attainable pressure loss through the valves. 5.5.4 Inverse Response Inverse response can sometimes be a problem when heat input is used to control column base level. The percentage vaporization increases with heat input such that the volume of vapor bubbles in the reboiler liquid mass increases. This moves liquid from the reboiler to the column base or, in the case of an internal tube bundle, increases the "voidage" in the base. In either case this can cause the measured base level to rise. This is only a short term effect because the increased rate of boiling will drive off mass in the long term causing the level to fall. In these circumstances the base level controller should be tuned for the long term response where the level falls with increased heat input. The short term transient response in the opposite direction can cause severe stability problems. The situation is analyzed on page 165 of (Ref. 4).



FIGURE 20 HEATER PASS CONTROL> HOW THE VPC ADJUSTS FLOWS TO MAINTAIN ONE VALVE FULLY OPEN



5.5.5 Heat Integration It is attractive, for energy efficiency reasons, to integrate heat sources and sinks in distillation trains. This can be simply the use of a cooler on a bottom product to heat the feed to the column itself or the use of the top or bottom streams of some other column to provide feed heating. Such apparently innocent systems can sometimes transmit disturbances which may exacerbate an already difficult control problem. The reader should be aware of this possibility. On some distillation trains the condensers of one column provide the majority, if not all, of the reboil load for another column (see Figure 21). Such systems are difficult to operate efficiently since there can be a number of competing operating objectives but a distinct shortage of effective degrees of freedom, even when heater bypasses are provided. In practice, the control system should be set up to rank the objectives and where there is a limitation in, say, heat input, it should be remembered that some of the objectives may not be met fully. FIGURE 21 HOW ENERGY INTEGRATION FORCES THE BOIL-UP OF ONE

COLUMN TO BE DEPENDENT UPON ANOTHER



Ratio control and simple computation can go some way towards providing an efficient system but control based on conventional PID controllers is unlikely to be entirely effective. Model based predictive control, which takes account of measured process interactions, may well provide a better form of control in these cases and should be considered (see 8.4). 6 DISTURBANCE COMPENSATION 6.1 Feed-forward Control Up to now in this Guide we have discussed only feedback control systems. These are systems which feedback a correction as a result of a deviation from set point - an error. In other words, the process has to be disturbed before a feedback system can begin to make a correction. Feed-forward control, on the other hand, takes account of disturbances which can upset quality and, if properly designed, can apply corrective action to minimize deviations of the control variable. Feed-forward is theoretically capable of perfect control but in practice errors will always remain. Nevertheless a feed-forward calculation accurate to only ± 10% will reduce the sensitivity of product quality to these measured disturbances by a factor of 10; a good return for modest accuracy. It is difficult to understand why feed-forward is so little used; it is usually only considered when feedback systems fail. To a first approximation, to achieve constant separation, all flows in a distillation column should increase or decrease in sympathy with the feed rate. Feed rate changes are a common disturbance in distillation columns and so it is logical to feed-forward changes in feed in such a way as to adjust other column inputs to compensate for the feed change. Figure 22 shows a conventionally controlled column. Without feed-forward, the temperature controller will make the appropriate adjustments to the reboil but this will take time and the column will be subject to a larger disturbance than is necessary. Where reflux is flow controlled it would be set at the highest rate required for normal operation which could be costly in energy consumption.



FIGURE 22 CONVENTIONALLY CONTROLLED COLUMN

The most basic feed-forward system would ratio reflux to feed. A more comprehensive system would also ratio reboil to feed and compensate this ratio from the composition measurement (see Figure 23). As the feed rate changes, reflux and reboil will also change. This would speed up the column response to disturbances and minimize off specification product.



FIGURE 23 AN EXAMPLE OF MORE COMPREHENSIVE FEED>FORWARD



Feed composition changes are normally only compensated for by the standard feedback control system but, if they are very severe, it may be possible to analyze the feed and to make the appropriate changes to column operation. This is rare. Feed-forward allows a measure of dual composition control on a column because it helps to decouple feedback loops. For example, if the main separation is effected by a temperature controller near the base of the column, but it is important to maintain a fixed purity level in a top product, a system as shown in Figure 24 may be an option where an analyzer placed near the top of the column and monitoring a key tops impurity is used to modify the reflux ratio.



FIGURE 24 FEED-FORWARD CONTROL WITH COMPOSITION CORRECTION



6.1.1 Dynamic Compensation

It is often not sufficient that the slave flows be ratioed to the feed in a steady state relationship. The correct dynamic relationship should be established between the variables. Feed, reflux and reboil are physically located at different points in the column and their effect on product quality will differ in speed of response. When differences exist between the responses of the manipulated inputs, feed-forward correction by steady state calculation alone is likely to cause transient errors. If this is the case, particularly for rapid disturbances, a dynamic lag should be placed in the feed-forward loop to match as closely as possible the response of the feed forward loop to that of the column response to feed changes.

The need for dynamic compensation is clearly illustrated by examining the response of a column to feed and boil up variations. Consider the scheme shown in Figure 25. Here the bottom product flow is ratioed to the liquid feed through a feed-forward loop and the level in the column base is controlled by regulating heat input.

FIGURE 25 A SUDDEN FEED CHANGE COULD CAUSE A REBOIL

INVERSE RESPONSE



This is a common form of control where variation in the bottoms off-take is not sufficient to control the base level. If no dynamic compensation is applied, a step increase in feed will cause a step increase in bottoms flow causing a reduction in bottoms mass hold-up and therefore a decrease in boil-up. In the long run the boil-up will ultimately increase but the lack of dynamic compensation has caused a transient response in the wrong direction. Compensation with a simple first order lag in the feed-forward loop would moderate the inverse response but not remove it. Exact compensation in the requires a dead time equal to the difference in dead times of the effect of the feed and bottom flow on the bottom level. In addition, a lag equal to the lag of the feed on bottom level and a lead equal to the lag of bottom flow on the level is required. Since most responses can be characterized by a dead time followed by a first order lag, dynamic compensators provide dead time plus lead-lag. 6.2 Cascade Control Cascade control is commonly applied on distillation columns - often where it is not needed! A typical cascade loop is shown in Figure 26. Here the "master" loop, temperature, is regulating reboil via a steam reboiler. The flow control "slave" flow loop is provided to compensate for steam main pressure fluctuations. Any change in pressure is picked up by the flow loop and compensated before the reboil is affected. This implies two things: (a) The dynamics of the slave loop should be faster than the master loop. (b) There are disturbances which can be usefully compensated by the slave

loop. Another example of a good application of cascade control is with hot oil reboilers where a combination of flow and temperature to calculate heat input can be compensated for by the slave (see 5.5.2).



FIGURE 26 TEMPERATURE/FLOW CASCADE

Examples of often inappropriate applications of cascade are level/flow combinations where it may not matter if the level is affected by some disturbance in the flow line and the system can be simplified by removal of the slave loop. There are other cases where cascade can be useful. For example, if the valve has a very nonlinear installed characteristic the slave flow loop can, in effect, "linearized" the characteristic and in those special cases a level flow cascade may be appropriate. This is likely to be a way of fixing a poor installation rather than a way of designing an effective control system. The message is to be aware of the benefits of cascade control when it properly applied but to avoid the added complexity when it is not necessary.



6.3 Internal Reflux Control If the external reflux is sub-cooled compared to the column top temperature, it will cause additional condensation in the column and the internal reflux will be greater than the externally measured flow. So long as the sub-cooling is constant, this does not matter. If, however, a flooded condenser or air condenser is used the external reflux temperature may vary. If this is an important factor, a simple on-line calculation may be carried out to compensate for the effect, viz:

The calculated internal reflux is fed to a flow controller which regulates the external reflux flow valve (see Figure 27).



FIGURE 27 TYPICAL SYSTEM FOR MAINTAINING INTERNAL REFLUX CONSTANT WHEN CONDENSER SUB>COOLING VARIES

7 CONSTRAINT CONTROL In this Guide simple regulatory control for essentially steady state operation was first described. Then we discussed, under disturbance compensation, controls designed to provide better dynamic behavior. A more extreme situation is the need for constraint control. Control for constrained operation is not common but is essential if maximum performance is to be achieved since, if at least one variable is not being held at its limit, separation efficiency or productivity could still be improved.



Constraint control is particularly useful where, for whatever reason, column conditions have been pushed to an extreme or where a more important parameter should be safeguarded at the expense of a less important. An obvious example is override control of reboil from a measure of column flooding. Here it is more important to prevent flooding than to try to maintain column composition. Model based predictive control systems with constraint avoidance fall into this category. In all cases detection of an override is necessary to warn the operator that the control structure has, in effect, been changed. Examples of a number of different common override situations are given below. A comprehensive discussion of controlling within constraints can be found in (Ref.4). FIGURE 28 THE OVERRIDE CONTROL OF REBOIL TO PROTECT BASE

LEVEL DRYING OUT



7.1 Override Controls

A simple example of an override control would be that of a base level controller, normally regulating bottoms off-take overriding the composition control of the reboil in an extreme situation (Figure 28). It is necessary to provide the temperature controller with external reset from the output of the selector relay to prevent integral saturation of the controller when the level controller takes over regulation of the reboil heat input. A reboil deviation alarm is also well worth providing to warn the operators of the potential loss of composition control.

A similar arrangement could be provided for reflux inventory control but in both cases it is better to set up the mass balance controls to be self-sufficient in their operation. A more common example of override control is that to prevent flooding when a column is being pushed to its limits. This is covered in the next section.

7.2 Flooding

Flooding is the term used to describe various conditions which cause a loss of tray efficiency at high column throughput. These can be entrainment, foaming or downcomer flooding. The first two are most common and are due to poor vapor/liquid disengagement.

Downcomer flooding occurs where vapor velocity and hence pressure drop in the column is so high that the liquid is prevented from flowing down the column. It can be caused by an excessive reboil rate. To monitor this, overhead vapor flow can be measured directly but it is usually an expensive option on a large vapor line. An alternative, and more common arrangement, is to measure the differential pressure across the column using the trays themselves as orifices.

The differential pressure responds rapidly to heat input and can be used as an override control on the column reboiler (Figure 29). Here the temperature controller normally regulates the reboiler but if the ΔP rises too high the ΔP controller takes over. The temperature controller is fitted with external reset to prevent integral saturation when it is overridden, the ΔPC would normally be a narrow band proportional action only controller and so this is not appropriate.



FIGURE 29 EP OVERRIDE OF REBOIL TO PREVENT COLUMN FLOODING

7.3 Limiting Control

In 6.1 feed-forward control of reflux from feed was recommended to cope with feed rate changes. This works well. A linear dependence on total feed is usually good enough. Dynamic compensation may well be provided and, in some circumstances, feedback adjustment of the ratio from a composition measurement. A problem arises when the plant rates are reduced. It is important to maintain some minimum liquid (and vapor) load on the column to ensure separation. This can be done by providing a low limit on the reflux (or reboil) flow. This is easily provided by a computing element or a ratio relay and a low selector as shown in Figure 30.



Good descriptions of override limiting for reboilers, start-up, shutdown and standby operation are given in (Ref.4). There are many other examples of constraint control systems but most need to be tailor-made for the specific problem.

FIGURE 30 LOW FEED FORWARD REFLUX FLOW LIMIT



For example, consider the following. Part of a column control system was set up as shown in Figure 31. Here the bottoms off-take was set from a computer optimization and the mass balance was normally achieved by reflecting back changes in level to feed. This worked adequately under steady operation but when the column was disturbed control became chaotic. There was a significant dead time before changes in feed affected the level. In addition changes in feed upset the general operation of the column and the preceding column. The fact that there was additional heat integration between the two columns added to the difficulty of the situation. FIGURE 31 COMPUTER SET BOTTOM OFF>TAKE WITH LEVEL

REGULATING FEED VALVE

A better form of control, bearing in mind that the optimization requirements for a particular bottom off-take were long term steady state objectives, would be to implement the system shown in Figure 32.



FIGURE 32 FLOW CONTROLLER TUNED TO ENSURE THAT LONG TERM DESIRED BOTTOM TAKE OFF-TAKE IS OBTAINED

Here, short term changes in mass balance are compensated by changes in the bottoms off-take rate but, in the longer term (typically, -1 hr), the off-take is brought back to its desired value by slow adjustment of the column feed. If it were necessary to ensure a total quantity of bottoms off-take rather than a specific rate in the steady state, some form of flow integration could be provided.



8 MORE ADVANCED TOPICS 8.1 Temperature Position Control

Most distillation columns have a region of temperature sensitivity as shown in Figures 4, 5 and 6 In such cases, a carefully chosen temperature point located in the region of maximum rate of change of temperature is usually the best place for temperature control if direct composition measurement is not feasible.

Occasionally the temperature control profile is much "sharper" and virtually the full column temperature change can occur over only a few trays (see Figure 33). In this circumstance, control using a single temperature point can be very difficult because of the very great system gain change from the sensitive to insensitive regions. The consequence is often a sustained limit cycle oscillation with frequent complete loss of control.

FIGURE 33 'SHARP' TEMPERATURE PROFILE TEMPERATURE

HEIGHT



The answer is to invert the problem and, in effect, switch from controlling a temperature on a particular tray to controlling a tray at a particular temperature. Figure 34 illustrates the idea. A number of temperature points are installed around the region of sensitivity and linear interpolation is used to locate the position of the temperature of interest. This "position" is then fed to a conventional controller where the set point is the desired position. Clearly the technique will only work where there is an adequate provision of temperature points; at least one point should always be measuring within the region of profile sensitivity. An example application and a detailed discussion of the technique is given in (Ref.7), together with a software listing. FIGURE 34 POSITIONAL TEMPERATURE CONTROLLER



8.2 Inferential Measurement In this Guide, use of temperature as a substitute for direct composition control of columns has been discussed. It has been argued that where it is possible to use direct analysis with adequate safeguards, reliability, etc., this is to be preferred when it is possible to measure the specific quality control variable directly. Inferential measurement is a technique still in its infancy but gathering in applications and popularity. It means using easily made measurements to infer the value of a difficult-to-make (usually quality) measurement It is fundamentally different from using temperature as a substitute for composition measurement. Rather, a set of "secondary measurements", e.g. temperature, feed-flow, reflux are used to infer (estimate) the value of the desired or "primary" measurement. This might, for example, be a top composition. The situation usually is that the top composition is available by other means on an intermittent basis - perhaps from a chromatograph with a long sample delay time or by labs analysis. The method involves the derivation of a mathematical model which relates the infrequent primary measurement to the frequent secondary measurements. This model is then used to infer the primary measurement at the time frequency of the secondary measurements. The result can either be used open loop to inform the operator of likely trends in the primary measurement or, better, in a closed loop control system. This technique is potentially useful for the determination of difficult-to-make laboratory type quality measurements. A full description of the technique is given in (Ref.8). 8.3 Floating Pressure Control Precise control of column pressure is not necessary but it is common practice. While there needs to be an upper limit to column pressure because of vessel constraints, there is no similar lower limit and reboil heat transfer is enhanced. If it were possible to reduce the pressure excessively there might be a limitation on condenser heat transfer. Within these limits it is advantageous to allow column pressure to float to achieve the minimum possible under any circumstances. This maximizes the relative volatility of most components allowing an increase in recovery or reduction in energy in any given situation. Other advantages reported are reduced boiler fouling as well as increased boiler capacity.



Resistance to floating pressure operation is traditional, probably based on the necessity of holding pressure constant to achieve reliable temperature measurement for control purposes. The increasing use of analyzers for direct composition control makes this argument obsolete. Even where it is necessary to use temperature control, temperature can be compensated for pressure variations as discussed in 5.1.1.3. While it would be possible to operate a total condensing distillation column with no pressure control at all and with all condensation control valves removed, this is not recommended. Sudden changes in pressure should be avoided. For example, if a column with an air condenser is suddenly exposed to a rainstorm, the wetted surfaces of the air condenser transfer more heat and there can be a sudden drop in column pressure as a result. This fall in pressure may result in a marked transient increase in boil-up and vapor rate as sensible heat is converted into latent heat. In the extreme this can be enough to flood a column. Certainly high boiling components will be moved higher up the column at the expense of the tops specification. What is required is a combination of short term protection against rapid pressure changes combined with a control system which, in the long term, achieves the lowest possible pressure operation. One way is to instruct the operators at all times to reduce the set point of the pressure controller to the lowest possible achievable limit with the condensation system working at a maximum. There are two objections to this approach: (a) operators make small step changes which tend to upset unit operations;

and

(b) they always operate cautiously and will soon stop making continuous

adjustments if not regularly exhorted to do so! The correct solution is to provide a simple VPC to adjust the pressure set point to ensure that it operates at minimum at all times. This can be done with the system shown in Figure 35. Here, the VPC has, as its set point, a position of, say 90% or 10%, whichever corresponds to maximum condensation. The output of the VPC sets the pressure controller set point which, in turn, adjusts the condenser valve. The operators can easily revert to normal operation by breaking the cascade and setting the pressure control set point manually.



If this happens it may be difficult to restore fully automatic operation because the VPC would then integrate to an extreme. This is solved by feeding back, as external reset, the column pressure. The short and long term responses of the system are provided by the different settings of the PIC and VPC. The PIC is set up conventionally to control the pressure tightly while the VPC, which should be integral only, is set with the longer integral time constant to adjust the pressure set point over a period of, say, half an hour. FIGURE 35 DIAGRAM SHOWING HOW THE VALVE POSITION

CONTROLLER SLOWLY ADJUSTS THE PRESSURE SET POINT TO KEEP THE CONDENSER FULLY LOADED IN THE LONG TERM

8.4 Model Based Predictive Control

Under the headings of Regulatory Control, Disturbance Control and Constraint Control, methods have been described to circumvent problems with the control of highly interactive distillation columns. This has been necessary because of the use of single loop feedback controllers as the basic unit of control.



These controllers are usually very effective at maintaining particular variables at their desired values but they are operated in isolation one from the other. They do not take account of the effect on other loops of changes made to hold the variable at its set-point. Any system so controlled is always subject to continuous disturbances because of this interaction and the poor dynamic compensation provided by conventional feedback controllers. Feed-forward, ratio and dynamic compensation controls can be fitted to compensate for these interactions as described earlier. In most cases the normal less-than-perfect control is accepted as adequate and the ubiquitous single-loop PID controller is well established in the industry. In an increasing number of cases, particularly with more modern intensively designed plants or where tight operation against constraints is important, this form of control is definitely not adequate. Then, recognizing and compensating for interactions and abnormal dynamics is necessary and Model Based Predictive Control (MBPC) may be the answer.

The concept of MBPC is well described in (Ref.9). A particular version of MBPC, Dynamic Matrix Control (DMC) is probably the leading technology worldwide and has been very successfully applied in the refrigeration complex within of a European plant. This is described in (Ref.10). An alternative MBPC technology, An application to a reactor system is described in (Ref.11).

The fundamental concept of MBPC is not difficult to understand. A dynamic model of the process itself is derived from plant tests. This model links all the control variables (outputs) to all of the important regulation (controlled inputs) and disturbances (uncontrolled inputs). Thus it is possible, using a plant computer and knowing the past history of disturbances and the actions taken by the plant control system to compensate for them, to predict into the future the inevitable effect on all of the outputs (controlled variables). Since the future desired values of all of the set-points can also be known it is possible to calculate an error trajectory into the future. The purpose of the controller is then to minimize this error by making a series of future moves which will compensate, so far as is possible, this future error due to the historical actions. Clearly, in addition to this feed-forward effect, there needs to be some feedback correction to allow for model errors and unmeasured disturbances.



A well designed MBPC works well and some packages deal very effectively with constraints so that plants with very high cash flows can be operated very close to their real limitations. Interaction is also dealt with very well since the controller "knows" about the interaction in advance and makes the appropriate set of control actions to meet the control objectives.

There are many applications of DMC applied to distillation control. It has a particular value in multiple column applications where there are difficult interaction problems due, for example, to heat integration making PID control rather ineffective. Another significant success has been in dual column arrangements where, for example, quality control of the downstream column. The very long dead-times, process and analytical delays inherent in such a form of control make automatic control by conventional means virtually impossible. A successful application in this area is reported in (Ref 12).

8.5 Control of Side-Streams

Multi-component mixtures can be separated into multiple products in one column. This is economic in that only one column and a single heat source and sink are required, but the product specification which is achievable will be limited. Common applications of the multiple side-stream concept are crude towers in refineries (see Figure 36) and pasteurizing columns, as typically found on an olefin unit ethylene fractionator. A typical set-up is shown in Figure 37, in this case there are two final products - low and high grade ethylene. The top product contains virtually all the methane in the splitter feed, with the high grade ethylene being removed as a side-stream lower down the column. More methane could have been removed earlier in the chain in the demethanizer but at the expense of a higher boil-up and ethylene loss. In practice, the rate of tops off-take will be regulated to control the purity of the side-stream product - almost certainly an application for direct composition control.



FIGURE 36 DIAGRAM SHOWING HOW ALL THE HEAT USED IN CRUDE OIL DISTILLATION ENTERS WITH THE FEED



FIGURE 37 DIAGRAM SHOWING HOW VIRTUALLY ALL THE METHANE FROM THE BOTTOM OF THE DEMETHANISER LEAVES WITH THE LOW>GRADE ETHYLENE

Side-streams can be vapor or liquid. Changes in the flow of a vapor side-stream can have a marked effect on the vapor loading above the off-take point while changes in a liquid off-take can significantly affect the internal reflux below the off-take. The consequent effects should be considered when designing overall control systems. In practice, for vapor side-stream off takes a form of achieving constant vapor flow above the side-stream such as ΔP control of reboil may be indicated.



For liquid off-takes constant internal reflux below the side-stream may be desirable and the control system can be designed to achieve this. There can be severe interactions in multiple side-stream columns. Where this is experienced or expected a model based predictive control system may be well worth considering (see 8.4).

8.6 Extractive/Azeotropic Systems

The subject of extractive and azeotropic distillation is complex. It is discussed in some detail in (Ref.4).

8.6.1 Extractive Distillation

Extractive distillation uses a solvent to improve the relative volatility of the components of interest. The solvent is selected to have a particular affinity for a class of components and in this way acts very much like a selective absorbent. At least two columns are required. In the first column, in the case of a binary mixture, the extractant attaches itself to one component while the other is distilled. In the second column the captured component is stripped from the extractant which is then recycled. In many respects the process is similar to conventional absorber/stripper combinations.

In both columns, because of the presence of the extractant, temperature profiles can be confusing and care shall be taken to select the appropriate sensitive region if temperature is used as the means of composition control.

8.6.2 Azeotropic Distillation

An Azeotrope is a mixture of two or more volatile components of identical vapor and liquid compositions and equilibrium. The azeotropic mixture will boil at a higher or lower temperature than its components depending on the nature of the system. If one attempts to distil a mixture of two components forming an azeotrope only one of the components can be removed, the azeotrope becoming the other product. Since the azeotrope cannot be separated by conventional distillation, other methods such as extraction, need to be combined with distillation.



Azeotropes may also be either homogeneous or heterogeneous. The latter separates into two liquid phases when condensed from the vapor. This makes their separation easier. One method of separating homogeneous azeotropes is extractive distillation.

The heterogeneous azeotrope can be easier to separate than many ideal mixtures. The immiscibility of the liquid phases condensed from the vapor effectively breaks the azeotrope. This allows complete separation of the components. This property can be used to advantage to assist in this separation of close boiling mixtures by deliberately forming an azeotrope. The approach is to add a third component, form a low boiling heterogeneous azeotrope with one of the other difficult to separate components.

This often means introducing into the column the additional azeotroping stream, usually of a component already present, water perhaps. The additional control problem is then that of regulating this extra "feed" to maintain a sufficiently high, but not excessive, concentration of the azeotroping agent in the appropriate region of the column. If this is to be done properly, it is usually necessary to use direct composition control to regulate the azeotroping agent flow. This will be in addition to any other composition control system. Interaction is a real probability and a poorly set up or maintained control system can lead to very poor and inefficient operation. An example of an effective control system for an azeotropic separation is found in (Ref.13).



9 REFERENCES (1) Control of Distillation Columns by R Jackson & J S Anderson,

Billingham Division Central File Report B.125,196, 22 October 1962.

(2) PEG.HEA.103 "Selection and Design of Condensers" P.D.Hills,

1992. (3) GBHE-PEG-HEA-513"Air Cooled Heat Exchangers" P.D.Hills,

1991. (4) Distillation Control, F G Shinskey, McGraw Hill, 1977. (5) PEG.HEA.102 "Selection of Reboilers for Distillation Columns"

P.D.Hills, 1992. (6) Design of Distillation Column Control Systems, Buckley, Luyben,

Shunt, ISA, 1985. (7) Control of the Cumene Column Temperature Profile by T H C

Ankcorn, EDN7015, June 1989. (8) Inferential Measurement by M J Oglesby, Report No IC05895,

January 1991. (9) Model Based Predictive Control by M J Oglesby, IC05923, July

1991. (10) ICEE Award 1990-1991, "The Application of DMC to Olefines 6" by

J S Anderson, M J Oglesby and N J Cordingley, March 1991. (11) Application of Model Based Predictive Control to a Hydrocarbon

Processing Plant by H v Spreckelsen et al, IChemE Symposium, Advances in Process Control III, York, 23-24 September 1992.

(12) Dynamic Matrix Control on Benzene and Toluene Towers by D

Tran and C R Cutler, Instrument Society of America, Philadelphia, 22-27 October 1989.

(13) Problems in the Control of Distillation Columns by J S Anderson

and J McMillan, International Symposium on Distillation I.Chem.E., Brighton, September 1969.



DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE This Process Engineering Guide makes reference to the following documents: ENGINEERING GUIDES GBHE-PEG-HEA-513 Air Cooled Heat Exchangers

(referred to in Clause 9) GBHE-PEG-HEA-507 Selection of Reboilers for Distillation Columns

(referred to in Clause 9) GBHE-PEG-HEA-508 Selection and Design of Condensers

(referred to in Clause 9)

