A pilot-scale distillation facility for digital computer control research

Computers & Chemical Printed in the U.S.A.

Engineering

Vol. 9, No. 3, pp. 301-309,

1985

0098-1354/E $3.00 + .w Pergamon Press Ltd.

A PILOT-SCALE DISTILLATION FACILITY FOR DIGITAL COMPUTER CONTROL RESEARCH? J. L. MARCHETTI,$ A. BENALLOU,~ D. E. SEBORG and D. A. MELLICHAMPII Department of Chemical and Nuclear Engineering, University of California, Santa Barbara (Revision received 29 May 1984; receivedjor

publication 11 September

1984)

Abstract-A new pilot-scale multicomponent distillation facility and associated computer control system are described. The control system design utilizes a hierarchical structure, with a microcomputer for direct digital control of secondary process variables and a general-purpose, real-time computer for high-level control of the primary variables. The facility is used for advanced process control research and real-time system instruction. Some typical results incorporating a new multivariable predictive control algorithm are presented. Scope-A pilot-scale distillation column designed to fractionate a ternary mixture of alcohols has been installed in the Department of Chemical and Nuclear Engineering at the University of California, Santa Barbara. The experimental unit serves four important functions: (1) provides a convenient demonstration unit for advanced control concepts of a general nature such as multivariable and adaptive control, (2) helps to critically evaluate promising control strategies for multicomponent distillation problems, (3) serves to develop and evaluate multicomponent distillation models, both for control studies and for the advancement of modeling methodology for distillation columns and large-scale systems generally, (4) plays an important role as a demonstration unit in applications of real-time computing, both for teaching and research. Conclusions and Significance-The investigation of advanced multivariable control techniques requires a sophisticated computer system. In the case of the UCSB distillation facility, a commercial microcomputer control unit has been interfaced directly to the process to provide direct digital control of the multicomponent distillation column, alarm checking for safe operation, data logging and start-up and shut-down operator assistance. In addition, a high-level computer is used for data storage, generation of mass and energy balances, and to calculate setpoints for two of the DDC control loops in a hierarchical control scheme. In the present system, any control strategy which uses reflux flow rate and reboiler steam pressure as manipulated variables, and distillate and bottoms compositions as controlled variables can be accommodated merely by changing the high-level control task. Other control configurations also can be implemented easily. Communication between the high- and low-level machines is presently accomplished through the system analog interface; however, a digital communication interface will be installed in the future. The computer system configuration used in the UCSB distillation facility is an example of a hierarchical control system which simplifies the application of advanced multivariable control schemes by separating high- and low-level functions between two or more control computers.

1. INTRODUCTION

In choosing a distillation pilot-plant unit for research studies, we have been guided by two main considerations: (i) previous successful experience with a dualeffect evaporator unit that was used for advanced process control studies at the University of Alberta by Fisher & Seborg [I], and (ii) the importance of distillation, both as a fundamental chemical engineering operation and as a major energy user in any ‘!An earlier version of this paper was presented at the IFAC/IFIP Symposium on Real-Time Digital Control Applications, Guadalajara, Mexico (January 1983). *Present address: Institute of Technological Development for the Chemical Industry, INTEC (CONICET, Universidad National de1 Litoral), 3000 Santa Fe. Argentina. §Present address: Department of Chemical and Energetics Engineering, National School of Mineral Industry, Rabat, Morocco. llAuthor to whom correspondence should be addressed. 301

modern, petroleum-based society. As the price of refined products and the cost of energy have increased, the economic incentives for improving the control of distillation columns have become more compelling [2,3]. Shinskey [4], in particular, cites the relationships of distillation control to productivity and energy conservation. The existence of a very large number of published articles dealing with various aspects of distillation column control also is ample evidence of the importance that industry and the academic control community have given to this subject. Top and bottom product composition control of even a binary separation has proven to be particularly difficult because of the interactions between control loops [S-7]. Because of these interactions, many studies have been concerned only with the control of overhead product composition [g-lo]. Multicomponent columns, such as the UCSB pilot unit, are much more typical of those found in industry; because stream analysis must be used to

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measure compositions of important streams, they also are much more difficult to control. Implementation of advanced control strategies on most large-scale processes, distillation columns included, requires a computer control system. Characteristics of the control systems are closely related to both the process design and to the control objectives. Kramer [ 1 l] cites three classes of computer control applications that conventionally have been used with distillation systems: (i) direct digital control, usually straightforward ;;;ptions of three-mode controllers plus feedfor(i;) supervisory control, including optimization for steady-state operations, mLtiihiL;,ptimal a n d o th er advanced dynamic control

As will be discussed in more detail below, a typical distillation system lends itself to a combination of classes (i) and (ii) in industrial situations and (i) and (iii) in academic control research. The reasons are that most columns typically involve a number of control loops which merely regulate secondary variables in the system, e.g. liquid levels in the reboiler and distillate receiver. These loops typically are implemented using analog (continuous) instrumentation or digital control equipment utilizing a fairly short sampling period, i.e. on the order of a fraction of a minute. Set points for primary variables, such as energy input at the bottom of the column or product returned as reflux at the top, are adjusted periodically. With supervisory control systems, the period is quite long, typically once per day, and the adjustments are based on optimization of a high-level economic model that yields optimum steady-state operating conditions to use as set points for the primary variables. With optimal and other dynamic control methods, the period usually is set as short as the cycle time (‘analysis time’) of the product stream analyzer, often a sampled output device such as a gas chromatograph. Digital control systems which require, at the same time, regulation of a number of routine secondary loops and high-level optimization or dynamic manipu-

lation of primary loops lend themselves admirably to a hierarchical type of organization [ 121. For this reason the UCSB distillation facility has been designed with a hierarchical configuration, one that easily permits the implementation and testing of advanced control algorithms in a high-level, real-time computer while handling routine control objectives in a low-level, microcomputer-based unit. Figure 1 gives an overview of the computercontrolled pilot plant, clearly showing the high/lowlevel computer organization. The control system consists of a microcomputer (Moore Industries MI 1002) which performs the low-level (DDC) control functions, and a host computer (Data General Eclipse S/ 130) used for implementation of advanced control algorithms, mass and energy balance calculations, and the generation of periodic reports. The control computations performed by the Eclipse computer generate the set points for several of the low-level microcomputer control loops. Each of the two computers has been equipped with a number of peripheral devices including operator consoles, graphics terminal, line printer, etc. A Hewlett Packard 5840A gas chromatograph with an automatic liquid sampling system is used to obtain periodic composition measurements of the two distillation product streams. Individual component concentrations are communicated directly to the high-level computer via a 600 baud ASCII link. At the present time, the MI 1002 digital communication interface is still under development; hence communication between the Eclipse S/130 and the MI 1002 necessary for implementation of the hierarchical system implementation is performed through the respective computer analog interfaces. This communications alternative and the detailed design and functions of each of the computer systems are discussed subsequently after the distillation column is described.

2. THE PILOT-PLANT

DISTILLATION

Preliminary criteria for the design of the multicomponent distillation system were not tied to a particular production rate or quality of the product mixtures. However, the desired system had to furnish the flexiASCII Data General

Real Time Laboratory

High-level

UNIT

S/130

Communication

Links

_

Computer l/z&y-i7

I

Process Area

Low-level Microcomputer

Termina’

-

Printer

-

Terminal Interface

Moore Industries MI 1002

El Terminal

A

Gas Chromatograph with Microprocessor Automatic Sampling

V Distillation

Column

Fig. 1. Hierarchical control configuration for the UCSB distillation facility.

t

Pilot-scale distillation facility for digital computer control research bility of operation needed for research equipment and to satisfy constraints due to limitations on space, available steam, cooling water consumption, safety considerations, and storage capacity for the chemicals, to mention only the most important ones. From the viewpoint of process dynamics, it was desirable to have small residence times in both the condenser/reflux-drum system and the bottoms-accumulator/reboiler system so as to reduce the overall dynamic response of these units vis-&-vis those of the trays as much as possible. The column was designed to fractionate a ternary system of alcohol isomers: n-butyl, s-butyl and t-butyl alcohols. After a careful screening of many alternatives, the above ternary mixture was selected based on the following key considerations: (a) ternary system equilibrium data available, as well as data for all three binary pairs, (b) absence of azeotropes, (c) suitable boiling point range (allows 20 psi steam to be used in the reboiler and cooling water in the condenser), (d) safe operation, i.e. low toxicity and relatively low flammability, (e) suitable relative volatilities (separation is neither too easy nor too difficult), (f) ease of on-line composition analysis (via gas chromatograph). 2.1. Column The installed column consists of 12 sieve trays, 6 in. in diameter, separated by 10 in. dividers. Each divider contains: (i) a 4 in. high stainless steel section with appropriate connections for sampling vapor and liquid streams, and (ii) a standard 6 in. glass pipe section to allow inspection of the internal process. The stainless steel section also has connections to locate thermocouples in both the vapor and the liquid phases. Moreover, a feed connection stub-in has been provided on each tray, increasing the flexibility of the unit for a variety of research studies. Sieve trays (perforated plates) with liquid crossflow were used in this unit. These trays are most commonly specified for new distillation column designs because they provide lO-20% more separation efficiency at optimal column loadings and cost 5&70% as much as bubble-cap trays. A circular overflow weir and tubular downcomer can be adjusted to accommodate different operating conditions, for instance to increase the tray liquid hold-up. The column and all heated auxiliary units are insulated with commercial fiberglass pipe insulation. A preheater in the feed line, a condenser and receiver for the distillate vapor, and a natural recirculation reboiler have been installed in order to provide for flexible operation. Three micropump magnetic drive gear pumps are used in the pilot plant: (i) the feed pump installed in the storage room approximately I50 ft from the column is dedicated exclusively to supplying the column feed line, (ii) the bottoms pump, located in the column area, returns the bottoms product to the storage room, and (iii) the distillate pump supplies pressure to return both the distillate product to the storage room and the reflux stream to the top of the column. A fourth pump in the storage room is used to transfer material from one storage tank to another

303

or to recirculate the contents of the feed tanks. Figure 2 gives a schematic view of the column and its main auxiliaries. Marchetti [ 131 gives complete details of the system.

2.2 Column auxiliaries The condenser installed in the distillation unit is located vertically with cooling water in the shell side and alcohol condensate in the tubes. Following current practice for vertical-tube condensers, the vapor and liquid flow cocurrently downward. Since pressure drop is not a limiting consideration, this configuration can result in higher heat transfer coefficients than shell-side condensation; it also has particular advantages for multicomponent condensation. A reflux drum (distillate receiver) with low capacity provides minimum hold-up of reflux/distillate liquid so as to minimize the associated time constant. A thermosiphon or natural recirculation reboiler is connected to the lower part of the column through a 2 in. diameter pipe which feeds the reboiler with the liquid bottoms mixture and a 4 in. diameter pipe that returns vapor and recirculating liquid to the column. In addition to the heat transfer considerations, the reboiler was designed to meet the capabilities for recirculation as well as the reduced hold-up criteria noted above in connection with the reflux drum. The feed preheater is used to heat the feed stream from room temperature to just below the mixture boiling point. This heat exchanger operates with steam in the shell side and the ternary feed mixture in the tube side. The feed preheater, condenser, and reboiler, were constructed of Type 3 16 stainless steel.

___ L___ v 7

Water

Alternative Feed Trays

Steam -

EL Reflux Drum

Reflux Line

~/sUlate

Steam oiler

Fig. 2. Pilot-scale distillation unit.

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Since the column is designed for operation at atmospheric pressure, a vent line and knock-out condenser are provided to vent noncondensable vapors to the environment safely. Because both distillate and bottom products are quite hot, these streams are cooled to 25°C before returning them to the storage area. A separate (isolated and explosion-proof) feed and product storage area is provided for the pilot-plant unit. The large capacity of the feed and product tanks permits the column to be operated continuously between 8 and 12 h, depending on characteristics of the operating point.

2.3 Process instrumentation Since this distillation pilot plant serves as an experimental unit for modeling studies and applications of advanced control techniques, it was heavily instrumented to meet the needs of future operations, including the requirements of any control tests. Every significant process variable is recorded, logged, or displayed to keep track of the operating conditions. Table 1 lists the key process variables. Figure 3 shows schematically the main facility and the installed instrumentation. Each of the stages except the top one has been fitted with a type J thermocouple to sense the liquid temperature. These signals are recorded using an analog multichannel recorder to give a continuous indication of column temperature profile. Temperature transmitters have been connected to the thermocouple installed on tray number eleven and to a resistance thermometer device (RTD) in the lower part of the column, respectively. These two temperature signals are extremely important from the control point of view because they can be used as controlled variables when direct composition analysis is not suitable for control purposes (as is discussed later in the experimental section). The lower part of the column is fitted with two additional instruments which should be mentioned: (i) a pressure transmitter connected below the first tray for measurement of column AP, (ii) a level transmitter that serves the bottoms sump level controller. Four primary process lines serve the needs of the distillation column: the feed line, the distillate product line, the bottoms product line, and the reflux line. Each of these lines has a flow transmitter connected to Table 1. Key process variables Variable

Sensor type

Transmitter (see Fig. 3) FTl FT2 FT3 F-T4

Feed flow Reflux flow Distillate flow Bottoms flow

Orifice meters

Top temperature

Thermocouple

TTl

Bottoms temperature Feed temperature Distillate temperature

Resistance Thermometer

TT2 TT3 TT4

Column pressure Reboiler steam pressure

Pressure

PTl PT2

Bottoms level Distillate drum level

Differential Pressure

LTl LT2

II H

Fig. 3. Low-level puter.

control

loops implemented

via microcom-

an orifice meter for measurement of flow rate and an automatic valve for control of flow. Steam available at 100 psig is reduced to 40 psig through a pressure regulator in the main supply line before it splits into two branches, one supplying the feed preheater and the other providing steam to the reboiler. A normally-closed solenoid valve installed in the main line is manually activated from a remote switch panel. This valve has proved to be extremely useful, not only during start-up and shut-down operations, but particularly for emergency situations. Elements common to these two lines are a control valve, pressure indicator, temperature indicator and steam trap. The single significant difference is the pressure transmitter connected to the reboiler inlet for measurement of steam pressure in the reboiler. Water is used as the cooling medium for three different heat exchangers: the column main condenser, the knock-out condenser, and the cooling tank for the product lines. The main condenser has an important effect on the column operating condition while the other two exchangers can be regarded as necessary auxiliary elements with no important effect on the process. Consequently, only the water line to the main condenser has been instrumented; a control valve, a rotameter and two dial thermometers satisfy the current needs. Available compressed air at 160 psig is reduced to about 20 psig through a pressure regulator. A manifold feeds the current-to-pressure transducers that provide the interface between the controllers and the final control elements, the seven pneumatic control valves shown in Fig. 3. Provisions for automatic sampling of the bottoms and distillate lines are included in the instrumentation, as noted above, in order to be able to characterize column operation completely during unsteady-state operation. Since the reflux stream has the same com-

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Pilot-scale distillation facility for digital computer control research position as the distillate stream, and since the feed stream does not change (and can be analyzed manually at the beginning and at the end of each run), there is no need to automate the sampling of these two lines. Each sampling line has a flow regulator attached to a rotameter to maintain constant flow and to allow visual inspection of its working condition. A Hewlett Packard 5840A Gas Chromatograph which includes an automatic sampling system is programmed to sample both product streams alternatively. Currently, the sampling period has been reduced to about 140 s, the minimum time required for chromatographic analysis and reporting of a single stream result. This period presently is small enough to implement most sampled data control methods without having to incorporate a continuous method to approximate composition, such as equilibrium temperature, during the period between CC analyses. The gas chromatograph can be operated manually from a terminal or automatically from the high-level computer.

2.4. Low-level control loops Seven low-level control loops are used during regular operation of the distillation column. Table 2 summarizes this multiloop configuration. All of these loops are serviced by the low-level microcomputer system, as is discussed below. A considerable amount of trouble initially was encountered in tuning the PI or PID controllers for several of these loops [ 131. Primary sources of difficulty were associated with: (a) valve stiction combined with oversized valve trim resulting in limit cycle behavior, (b) interactions between loops, particularly Loops 1 and 6 where the single distillate pump supplies upstream pressure for both the distillate and reflux control valves, (c) process noise, associated with steam supply pressure in Loop 2, (d) excessive thermal capacitance associated with the heat exchanger metal and cooling water in the distillate condenser. Problems (a) and (b) were eliminated by using smaller control valve trims. Problem (c) has been attributed to a defective steam supply pressure regulator. It has been replaced in a recent modification. Problem (d) was substantially eliminated by redesigning the cooling water supply line to reduce the lag associated with that stream. However, of the seven loops, the distillate temperature controller (Loop 7) remained the most difficult to tune and the one most subject to disturTable 2. Low-level computer control loops Control loop 1. (PI) 2. (PI)

Controlled

Manipulated

variable

variable

Reflux flow rate position

3. (PI)

Reboiler steam pressure Feed flow rate position

4. 5. 6. 7.

Feed temperature position Reboiler level flow rate Reflux drum level flow rate Distillate temperature

(PID) (PI) (PI) (PID)

Reflux valve Steam valve Feed valve Steam valve Bottoms Distillate Cooling water

bances. Eventually it was necessary to replace condenser with one of smaller capacity.

the

3. COMPUTER CONTROL SYSTEM

3.1 The low-level (DDC) computer The Moore Industries MI 1002 controller is a general-purpose process control microcomputer with a read/write bubble memory capable of handling up to 112 analog and/or digital inputs and outputs and up to 24 control loops. In addition, a number of algebraic manipulations can be performed using the 96 available function blocks. This microcomputer, which is located in the proximity of the distillation unit, is used for monitoring and controlling it in a dedicated fashion. As indicated in Fig. 1, the MI 1002 has two peripheral devices for operator communications, a color graphics terminal and a line printer. The color graphics terminal is used for routine MI 1002/operator communications. These communications are supported by full bar graph displays of the key process variables, alarm variables, and control loops. Several operator-selectable display modes can be used interchangeably depending on the level of information sought by the operator at a given time or depending on the state of the process. These display modes range from an overview of all the process and alarm variables to a bar graph display of a single control loop or process variable. The line printer is used to obtain periodic reports of operator-selected process variables or controller outputs and to produce a hard copy of any alarm reports during process operation. This type of information is particularly useful for analysis of nonstandard process conditions. In the present column control configuration, the MI 1002 microcomputer performs the following tasks: 3.1.1. Data acquisition. The key process variables are logged by the low-level microcomputer and are used for control computations, alarm checking, and generation of periodic reports. These variable are shown in Table 1; the data are made available to the MI 1002 through its analog interface (signal level: l-5 V, 16-bit converter, 0.0025% resolution). Some of the variables in Table 1 play an important role in the safe operation of the column. These variables are checked for alarm conditions at each sampling time. 3.1.2 Alarm checking. Table 3 shows the process variables which are regularly checked to insure safe operation of the distillation unit. During operation the column pressure is not allowed to exceed a certain limit and the reboiler and distillate drum levels are Table 3. Variables checked for alarm conditions Process variable Steam pressure Column pressure Distillate drum level Bottoms level Feed flow rate Distillate flow rate Bottoms flow rate Distillate temperature

Type of alarm

High High High High High

High High and and and and and High

Low Low Low Low Low

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kept between pre-set low and high limits. In addition the feed and distillate flow rates are also regularly checked in order to determine instrument malfunctions resulting in aberrant flow measurements. When a variable enters an alarm state and remains there for an operator-prespecified time interval, the digital output of the alarm function is set. The system then automatically transmits this information to the operator through the graphics terminal, and an alarm report is generated via the line printer. Typically, an alarm report shows the date and time when a given variable enters its alarm state or leaves it. In addition, the output of the alarm function can be used to take a control action which would prevent prolonged process operation under abnormal conditions. An example of such a control action would be to close the solenoid valve supplying steam to the reboiler if the column pressure remains in its alarm state for an extended time period. 3.1.3. Signal conditioning and direct digital control. In addition to data logging and alarm checking, the DDC microcomputer is responsible for lowlevel control. As discussed above, seven loops are used to control the key process variables shown in Table 2. Before beginning control computations, some of the measurements are filtered using the signal conditioning functions available on the MI 1002. A typical direct digital control loop arrangement is shown in Fig. 4, where the set point is entered directly by the operator, as during start-ups and shut-downs. Note that the output from the MI 1002 is a sequence of pulses which is sent to the direct digital interface (DDI). The DDIs are manual/automatic override stations that also can be thought of as the latching components for the final control elements. The DDI output is an analog signal in the 4-20 mA range which drives a current-to-pressure transducer connected to a pneumatic control valve. All control options available on the low-level microcomputer are versions of the PID discrete algorithm including several practical features such as: (1) (2) (3) (4)

velocity and position algorithms, anti-reset windup, anti-derivative kick, error limiting capability.

In addition, the sampling period is varied internally to keep it as small as possible, i.e. the system does not

DDC Microcomputer ------------

! I ; I

I

u

1st Order u 1 Filter 1_

remain idle between two control computations. When the number of control loops is large, a maximum sampling period of 5 s is imposed. 3.1.4. Operator assistance during start-up and shut-down. The DDC machine is also used to assist the operator during start-up and shut-down by supervising the operation and using the alarm functions discussed earlier. At the present time, the MI 1002 only reports process conditions and alarms in an ‘operator-assist’ fashion; decisions are left to the operator. However, the control system design provides the necessary hardware for automated start-ups and shutdowns, i.e. for on/off control of all pump drive motors and operation of the steam-line solenoid valve. TTLcompatible relay switches allow the DDC machine to start or stop any of the feed, distillate, or bottoms pumps, as well as to open or close the steam-line supply valve. Note that these electrical connections are made in such a way as to allow operator manual override via the DDIs. In the dual composition control scheme used by Benallou [ 141, the set points of five of the DDC microcomputer control loops are established by the operator (Loops 3-7). These would only be changed to simulate a disturbance to the column, e.g. a change in feed flow rate. Once the system start-up is complete, the set points of the primary loops (reflux flow rate and reboiler steam pressure) are driven by the highlevel or host computer described in the next section. 3.2 The high-level computer The host machine (Data General S/ 130 Eclipse) is located in the Real-Time Laboratory which is housed one floor above the distillation area. In addition to performing the high-level functions necessary to control the distillation unit, the host computer is responsible for a number of additional tasks. It is a generalpurpose machine connected to a number of peripheral devices and processes through standardized interfaces. Fig. 5 illustrates the present system. The system software consists of the Data General advanced operating system (AOS) supporting multiuser, multitask operations in a real-time environment. The signals available to the high-level computer are in the l-5 V range. If these signals were sent directly to the machine, the noise-to-signal ratio would not be adequate. Consequently, differential amplifiers and low-pass filters were installed at the host computer

System

-I 1

Variable ADC Block

LI

1 1

1 mittter

Fig. 4. Typical DDC control loop.

1-

I

Pilot-scale

distillation

facility

To Campus Computer Center

for digital computer

I -

i

307

control research

Remote

_

loi CRT

16-Line

System MGR

,

-

Multiplexer

1

Floating Point Processor AOS Paper Tape Punch

Sensor

I/O

4-16 Bit In 4 16 Bit Out .f 2 Pulse Out 32 A/D 20 D/A

Plotter

I

Experimental

Processes

Fig. 5. High-level

ADC ports. The purpose of the differential amplifiers is to raise the signal level from l-5 V to - 10 to 10 V, permitting utilization of the full ADC span in the host machine. A multitask program in the host computer contains several tasks dedicated to operation of the distillation unit. 3.2.1. Operation of stream analyser and logging of results. As shown in Fig. 1, the sampling system consists of a terminal interface (HP 18833A digital interface) which links the host computer to the stream analyser (HP 5840A Gas Chromatograph) equipped with an automatic liquid sampling valve. The host computer both commands the system to take a sample at operator- or program-specified time inter-

real-time

I computer.

vals and collects the results of the gas chromatograph analysis. In the present configuration, two streams, the distillate and bottoms, are sampled on an alternate basis. The analysis and reporting time for a single stream is 140 s. 3.2.2. Data acquisition and storage. The key process variables for the distillation unit (see Table 1) are made available to the host computer through its analog interface. The system also collects the distillate and bottoms composition measurements via the gas chromatograph terminal interface. Data collected by the host machine are stored and used to perform a number of operations including advanced control calculations and the generation of the column status reports.

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MARCHETTI

Moore 1002 -

Eclipse _&

I

et al.

I I 1

-b

Distillation

Overhead Composition

_

Bottoms Composition

Fig. 6. Arrangement of distillation column primary loops

3.2.3. Mass and energy balance computation and status reports. At operator request, the host computer uses the most recently logged data to compute stream densities and enthalpies, Material and energy balances are then calculated and the results are transmitted to the operator through the CRT terminal in the form of a column status report. The status report includes the temperature, composition, density, enthalpy, and flow rate of each column stream, as well as the results of mass and energy balance computations. Condition reports are used by the operator to check the consistency of measured data and the approach to steady-state operation. The operator can save the status reports on disk as data files for later analysis. 3.2.4. Advanced control calculations. In addition to the auxiliary functions discussed above, the multitask program also performs the advanced control algorithms. In this case, the set points for the low-level microcomputer reflux flow rate and reboiler pressure control loops are generated by the host computer periodically and sent to the low-level microcomputer as analog signals (Fig. 6). This type of communication between the high- and low-level machines is a straightforward alternative to digital communication (currently not available since the MI 1002 digital communication interface is still under development by Moore Industries). 3.2.5. Operator-host communications. The multitask program also handles routine communications between the operator and the high-level machine. These communications include requests to: ( 1) (2) (3) (4) (5)

4.

CURRENT RESEARCH

At the present time the distillation unit is being used by one M.S. and five Ph.D. students to verify their theoretical modeling and control developments. Table 4 summarizes the areas of study involved.

5.

EXPERIMENTALCONTROLSTUDIES

During the past five years, several research and development groups in both university and industry, have investigated predictive control techniques. This approach is characterized by the use of a multiinput/multi-output process model based on a convolution-type representation whose parameters can be obtained easily by step-input testing of the actual process. The resulting deterministic model typically requires from 20 to 40 parameters to represent each input/output pair; hence a computer is required to perform the matrix operations associated with prediction of the future process trajectory. Knowledge of the predicted trajectory, however, permits the manipulated inputs to be computed so as to optimize a particular transition from one operating state to another, to avoid hard constraints on the process outputs, etc. Unlike traditional multiloop and many modern multivariable control techniques, there is no requirement that the number of process inputs and outputs be equal; hence an additional advantage of

Controller -

Predictive p,

perform mass and energy balances, generate condition reports, save condition reports, open or close high-level control loops, change or display the value of a given parameter or variable.

Table 4. Research projects using the distillation column facility.

I. Modeling and control of bilinear systems 2. 3. 4. 5.

Multivariable predictive control Self-tuning control Control strategies for nonlinear systems Controller design for large-scale systems

107 0

I 400

I I BOO 1200 TIME (set)

I 1600

I 2000

Fig. 7. Comparison of MPC and multiloop PI control for a + 15°C step change in bottoms temperature set point.

Pilot-scale

distillation

facility

for digital computer

Predictive

1.

p,

2.

3.

2

+’

0

I

1200 TIME (set) 800

I

,

1600

2000

Fig. 8. Comparison of MPC and multiloop PI control - 1.5”C change in top temperature set point.

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REFERENCES

Controller -

control research

for a

such predictive methods is that many process inputs can be utilized for control, and a strict pairing of an input with an output is avoided. References concerning output-predictive techniques are predominately those related to model algorithmic control [ 151, to model predictive heuristic control [ 161, and to dynamic matrix control [ 171. Marchetti presents a complete review of recent activities in this field and describes several modifications to the methodology. In an experimental evaluation of multiple prediction control (MPC) techniques, Marchetti [ 131 compared an MPC algorithm with standard welltuned, multiloop (proportional integral) control of the pilot-plant distillation column for the case where temperatures at both top and bottom of the column are controlled. In this case the temperatures are approximately related to the product stream compositions and can be measured continuously. Hence implementation of the control algorithms is not restricted to the measurement cycle of the gas chromatograph. Figures 7 and 8 show a typical comparison of PI and MPC responses for a sampling period of 10 s. In Fig. 7 the bottoms temperature set point has been increased by l.5”C; in Fig. 8, the top temperature has been reduced by I .YC. In both cases the faster response of the multivariable algorithm (MPC) is apparent. The better decoupling obtained with the MPC algorithm is particularly apparent in Fig. 7 where there is relatively little change in top temperature (none is desired) compared to the multiloop PI control. These results illustrate the comparative advantages that advanced control methods can yield; in this case, however, a high-level digital computer is required to obtain them.

8

9

IO

II

12

13. 14. 15.

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