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Dynamics and control during startup of heat integrated distillation column
ELSEVIER
Computers &Chemical Engineering
Computers and Chemical Engineering 24 (2000) 1091-1097
www.elsevier.com/locate/compchemeng
Dynamics and control during startup of heat integrated distillation column Mario R. Eden, Arne Koggersbol, Louis Hallager, Sten B. Jorgensen * CAPEC, Department of Chemical Engineering, Technical University of Denmark, DK-2800 Lyngby, Denmark
Abstract
To advocate the usage of process integration in industrial practice, it is important to be able to guarantee not only robust control during near steady state operation, but also to provide procedures for generating fast and reliable startup sequences. This contribution concentrates on describing a systematic procedure for development of plant startup sequences. The basis for the startup procedure development is available qualitative process knowledge. Application of the startup sequence generation procedure is demonstrated upon a heat integrated distillation plant. This plant illustrates some of the inherent effects of process integration upon a startup procedure. In particular, the effect of energy recycle upon the possible startup sequences and the effect of using an actuator for control purposes, i.e. the heat pump, which in itself requires significant startup time. The experimental application of two generated startup sequences illustrates that safe and reliable startups are ensured. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Process integration; Distillation column dynamics; Startup procedure generation
1. Introduction
Distillation column dynamics and control have been considered as areas, where research has been completed in some sense. Although, the dynamics of distillation column startup have been intensively studied using dynamic simulation, e.g. Ruiz, Cameron and Gani (1988), very few experimental results have been reported. Simulations and experimental measurements have shown that the startup operation involves complex transient responses in hydraulic and thermodynamic variables, yielding a highly non-linear behavior of the product compositions. A division of the startup operation into three phases (Ruiz et al., 1988) has achieved wide range acceptance among process engineers. 1. I. The discontinuous phase
This is the initial phase of the startup, where the plates are weeping and heating is in progress. Pumps and flows are started. Low-level control o f secondary variables such as levels, flows and feed temperature is * Corresponding author. Fax: +45-4588-2258. E-mail address: [email protected] (S.B. Jorgensen)
applied. This phase defines the time needed for the initial heating. 1.2. The semi-continuous phase
This phase represents the most important part of the startup operation, since it is the slowest and hence most time-consuming. During this phase, the hydraulic variables reach their steady state values and the column is shifted from total reflux to the desired reflux rate. Several disturbances may destabilize the column operation at this stage, e.g. changes in reboiler steam pressure, feed temperature and composition and reflux flow rate etc. As the compositions slowly approach the desired level, they tend to get easily disturbed. This highly non-linear transition requires very good control of the process variables. 1.3. The continuous phase
In this final phase, the column reaches the vicinity of the desired steady state. In this phase, higher-level control structures may be applied. The challenging control problem during startup of distillation columns is located in the semi-continuous
0098-1354/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S0098- 1354(00)00488-9
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M.R. Eden et al./ Computers and Chemical Engineering 24 (2000) 1091-1097
phase, i.e. the non-linear transition from total reflux to specified reflux in the presence of several disturbances. For simple binary distillation columns, several control strategies have been implemented, e.g. Venkateswarlu and Gangiah (1997). However, for heat integrated distillation plants, experimental data is limited. The generation of startup sequences for chemical plants is usually far from trivial, due to the complexity of the plant, which may give rise to several possible startup sequences. One of the complicating factors is the application of recycle, which severely constrains the possible startup sequences. The startup procedure generation problem may be expected to become more difficult as the trend within process design continues to move towards increased energy and mass interaction for economical as well as for environmental protection reasons. Searching for optimal startup strategies would be a very large computational and modeling task, if carried out using standard simulation techniques. Furthermore, models, which can describe the plant behavior during startup, are normally not available. During startup, the plant is operated far from normal production conditions, and therefore may display very disparate behavior, which is unaccounted for in the models for normal production conditions. An additional obstacle for development of detailed dynamic models during startup is the lack of parameter correlations for such unusual operating conditions. Thus, it would seem most suitable to base the generation of startup procedures upon qualitative process knowledge. The purpose of this paper is to describe a systematic procedure for startup sequence generation, to demonstrate startup sequence generation for an energy-integrated pilot plant and to experimentally investigate the application of such a startup sequence. The paper is organized as follows, first the experimental pilot plant is briefly described along with the basic control structures on the plant. Then a procedure for generating startup sequences is presented. Subsequently, two alternative
m
:+_ m
Fig. 1. Distillation column and heat pump schematic.
startup sequences generated for the heat integrated distillation column plant are presented. Experimental applications of the generated procedures are described, illustrated and discussed.
2. Process description A schematic of the main pieces of equipment is given in Fig. 1. Feed can be introduced via two pumps (PF1 and/or PF2), mixed and preheated in a heat exchanger (HEFS). It may be fed into the column at five different positions. At the bottom of the column, liquid is withdrawn partly as bottoms product via a pump (PB) and partly evaporated in the thermosiphon reboiler (HERB) and reintroduced into the column as vapor. From the top of the column, the vapor is led to the condenser (HECOND), where it is condensed. The liquid is pumped to a decanter (DEC) via a pump (PC). In this study, the decanter simply is an accumulator. From the decanter, a part is withdrawn as top product (via the pump PT) and a part is returned as reflux to the top of the column (via the pump PR). For binary distillation experiments, a methanol-isopropanol system is currently used. The heat pump system depicted in Fig. 1 has been designed for large heat transfer load variations. It uses Freon 114 as the heat transfer medium and utilizes two 8-cylinder piston compressors. The heat pump worked as follows: most of the Freon vapor was condensed in the reboiler (HERB). An extra condenser (HESCOND) removed an amount of heat roughly corresponding to the heat introduced by the compressors (COMP). Three air fin coolers (ACOOL) passed on this heat to the environment. The condensed Freon passed to a receiver (REC) and through a heat exchanger (HECI) to the Freon evaporator (HECOND). The heat exchanger (HECI) served to superheat the Freon gas before it entered the compressors, to prevent condensation during compression. After the evaporator, a demister (GLSEP) prevented liquid from passing on to the compressors. The pilot plant was instrumented in accordance with the longterm objectives of the research project, of which the process dynamics and model identification for control design aspects were demanding in terms of direct measurements. The structure of the basic control loops relevant for startup purposes is described in Table 1. From this table, it may be observed that the top section of the distillation column may be operated in both D and in L configuration. After completing the startup, i.e. the system is close to normal operating conditions, it is possible to introduce P - V control in the form of an algorithm called multi input multi output selftuning controller (MIMOSC) (Brabrand, Jensen & Jorgensen, 1991).
M.R. Eden et al. / Computers and Chemical Engineering 24 (2000) 1091-1097
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Table 1 Relevant control loops for startup operation Loop
Sensor
Actuator
Setpoint
1 2 3 4 6 8 9 11 18 19
FM1 FM2 FM3 FM4 FM6 PI0 P8 FMSM DP1 L1
PF1 PF2 PB CV2 PR CV8 CV9 CV5 SP FM3 SP FM6/4
Feed flowrate 1 Feed flowrate 2 Bottoms product flowrate Distillate flowrate Reflux flowrate Heat pump high pressure Heat pump low pressure Preheater steam flowrate Reboiler level Accumulator level
3. Generating startup sequences In this section, a procedure for generating startup sequences is given and then applied to generate startup procedures for the process plant. The proposed startup generation procedure makes it possible to derive subgoals, which are consistent with the process constraints. The sequence of subgoals combines into achieving the overall goal of completing the startup. The procedure for generating a suitable subgoal structure is based upon usage of qualitative process knowledge. The first simple qualitative knowledge needed to develop a startup procedure is that of the basic limiting capacities in the system during startup. Often these capacities are relatively simple to identify. Thus, a relatively simple procedure for generating startup sequences follows: 1. identify limiting significant capacities in the plant; 2. identify suitable sequences to fill the capacities; and 3. select most promising procedures for satisfying safety and operational constraints. Below, this relatively simple procedure will be followed for the energy integrated distillation column. The qualitative process knowledge will be used especially in step 2 above to understand the dynamic interactions between process variables during startup. The resulting startup sequences will be presented schematically using GRAFCET (IEC, 1988) function diagrams to illustrate the significant process states and their transition conditions and the actions taken to achieve/ensure the process states. Once startup sequences are generated, the control related issues will be considered. For the energy integrated distillation, plant application of the above procedure led to the following key points: 1. one significant capacity is the energy content of the column. Another is the energy content in the heat pump. A third capacity is the mass holdup in the reboiler; 2. energy must be supplied in sufficient amount to establish necessary conditions for carrying out the
Remarks
Inner cascade to loop 18 Inner cascade to loop 19 Inner cascade to loop 19 Supervisory level setpoints Supervisory level setpoints Outer cascade to loop 3 Outer cascade to loop 6 or 4
energy driven separation process, before the distillation column concentration profiles may be established; 3. supplying energy to the plant requires careful supervision of the column pressure in order to ensure safe operation within the allowed pressure window. Secondly, the pressures in the heat pump must also be carefully supervised. In order to generate suitable startup procedures, qualitative process knowledge was formalized. The relations between significant capacities during startup are shown in Fig. 2. Considering, the physical couplings, i.e. constraints between the significant energy capacities, developed this diagram. Hangos, Cs~tki and Jargensen (1992) presented the assumptions made in setting up this diagram for the purpose of deriving startup procedures. Since the plant was initially cold, inspection of Fig. 2 revealed that the heat pump might not be started before the condenser was heated up. This initial heating may be achieved by feeding vapor high up into the column. The resulting initial phase of the startup operation is labeled phase I in Table 2. Once the column top section and the condenser were heated, the compressors might be started at a suitable speed to transfer heat from the column top section to the reboiler. During this second phase in Table 2, both energy inputs to the column were used. Once vapor production in the reboiler had ~
Iq~cd
c~P 0'-o~ o-,,
BV48
BV47
Fig. 2. Energy DYQUID with significant mass storages for distillation column and heat pump.
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M.R. Eden et al. / Computers and Chemical Engineering 24 (2000) 1091-1097
Table 2 Phase definitions for startup of energy integrated distillation column Phase Purpose
Initiating action
Indicator
I II III
Feed steam to preheater Start compressors Shut off help energy
OK for startup Condenser warm Vapor flow OK
Set desired setpoints in low level controls Selected setpoints specified by higher level algorithm
Purities acceptable
IV V
Heat condenser system Heat reboiler and heat pump Stop startup energy source Establish desired purities Bring process into desired operating form Introducesupervisorycontrol
started at a suitable rate, the startup energy supply to the feed preheater might be turned off or reduced. In the third phase of the startup, the column and heat pump were operating and the column purities might be allowed to reach the desired level before switching to production in phase IV. In phase V, higher-level regulator tasks may take control to assure quality and steady state optimality. To obtain a detailed view of a specific startup procedure, it is necessary to specify the macro steps further. This is done for two cases in Figs. 3 and 4. The major difference between the two cases is that in case A, the reboiler was initially empty or the cold contents were initially emptied out into a storage tank. The idea behind this procedure was to facilitate heating the reboiler by allowing vapor to condense in it. When the reboiler was full of liquid, as in procedure B, the heating had to take place from the heat pump. The two cases considered may be viewed as extremes. In case A, the emphasis is upon minimizing energy consumption, while in case B, the emphasis is upon minimizing material losses. An optimal startup procedure may well be a weighted average of the two depending upon the relative energy and material prizes. The startup sequences contain many common elements. Once clearance for starting up procedure A is received, a number of parallel states are initiated. The column is vented through a valve (CV10) and an effluent condenser, not shown in Fig. 1, mounted on the column top. When breakthrough of vapor is noted at a temperature measurement in the effluent condenser (TEC1), this outlet is closed. This outlet would normally not be opened again until plant was shutdown. Feed flow (fresh feed or recycled reboiler liquid) and steam supply to the feed preheater are started. Startup phase I was considered finished, when the condensate in the accumulator tank was warm as indicated by TC1. Then the compressors were started and full reflux conditions established. During phase II, energy was supplied by both the feed preheater and the compressors. Once the heat pump pressures were up to the initially desired level and the reboiler was producing a sufficient vapor flowrate, the desired number of active cylinders could be introduced and the preheater turned off. Once the vapor flowrate, column pressure and product purities were around the
desired levels, then the production may be initiated by implementing the selected control configuration for phase IV. The main differences between procedures A and B are firstly that the cold reboiler liquid in procedure B is recycled to column feed through the bottoms product cooler (HEBW) and the preheater (HEFS) during phases I and II. Secondly, all condensate was refluxed to the column. Thus material was kept inside the column until production might start. In both cases, the column was operated at full reflux in phases two and three. The control tasks during the startup procedure are also indicated in Figs. 3 and 4.
~
lltum,
X i JtGt~toz
• , ACtUIt~
I~ xn ~ i i lomp In
iuto
Fig. 3. GRAFCET function diagram for startup sequence A, empty reboiler. ~
~
B i
+.tur*, , aetvator loop a~rt'vl ~ r / ~ n t ~ l
Fig. 4. GRAFCET function diagram for startup sequence B, full reboiler.
M.R. Eden et al./ Computers and Chemical Engineering 24 (2000) 1091-1097
1095
Table 2, and the specific breakpoints presented. A detailed outline of the startup operation is given by Eden, L~ppenthien and Skotte (1999). In the plots presented, the curves are given by: Case A: thin solid and dotted lines (both black). Case B: thick solid lines (black and grey). ,
,
,
°
°
"nm,g~
•
,
•
,
4. I. Phase I
Fig. 5. Temperature in effluent condenser.
"rime~0 Fig. 6. Condenser temperature.
4.2. Phase H
1: t,!
.,
,
,
:
.,
,
After completing the initial steps, such as tuming on cooling water, turning on power supplies and initializing the computer and security systems, liquid was fed to the column top through the feed preheater HEFS. As outlined in Table 2, phase I was initiated when the preheater steam supply was started, which was after approximately 1.2 h. During phase I, the column is vented through BV52 and CV10; once vapor breakthrough is noted by an increase in the effluent condenser temperature (TEC1), venting is stopped (Fig. 5).
0
,
Fig. 7. Vapor production.
It is seen that in the first phases, the actuators are often in manual, but as soon as possible they are switched to automatic. In phase IV, the control configuration for the column top was selected. The column bottom is a priori in a V-configuration. The column pressure was not controlled in closed loop before phase IV, when the heat pump was operating. Thus during phases I and II, the column pressure has to be monitored carefully, and too high pressures avoided, by manipulating the energy input/output by adjusting the preheater steam flowrate, starting the compressors or changing the number of active cylinders used in the heat pump.
4. Results and discussion
Two startup experiments have been conducted according to the macro steps presented in Figs. 3 and 4. In this section, the data were presented and discussed. The startup is divided into four phases according to
While the condenser was being heated by the hot vapor produced by the feed preheater, some cold condensate was produced. The end of phase I and consequently, the beginning of phase II was indicated by the temperature (TC1) of the condensate in the accumulator tank. An increase in TC1 indicated the need for more cooling, thus the compressors were started at this point. Full reflux conditions were established and the compressors as well as the feed preheater provided heat throughout phase II. The increase in TC1, i.e. the beginning of phase II was found after approximately 1.9 h as indicated by Fig. 6. The remaining part of phase II consists of heating the reboiler and the heat pump in order to establish a sufficient vapor production. 4.3. Phase I I I
Once a sufficient vapor production (V_EST) had been established, the feed preheater steam supply was turned off, indicating the beginning of phase III. Phase III was initiated after approximately 4.1 h for case A and after 3.9 h for case B. During the remaining part of phase III, the desired purities were established. The vapor production is presented in Fig. 7, while the compositions (given by methanol mole fractions) are presented in Fig. 8. At the initial stages of the startup operation, it would seem as if the bottom product composition (XPTT1) was considerably higher than the top product composition (XPTT19). Both compositions indeed have values above 1, however, the reason for this strange behavior is found in the way the compositions are estimated. The compositions are calculated
1096
M.R.
E d e n et a l . / C o m p u t e r s a n d C h e m i c a l E n g i n e e r i n g 2 4 ( 2 0 0 0 ) 1 0 9 1 - 1 0 9 7
using Raoult's law, the Antoine equation for isopropanol and the measured values of temperature and pressure. No constraints are applied to the composition estimates; hence the combinations of temperature and pressure encountered during the initial stages of the startup operation may yield strange results. Since the column is fed at the top, the temperature will be significantly higher at this tray than at the bottom, yielding the erroneous composition estimates. As the column trays were filled with liquid, the temperature
and composition profiles became more reasonable. Once vapor was produced at a reasonable rate in the reboiler, the majority of the heat input was supplied in the bottom section, thus the profiles were inverted and the estimates got better. Once the desired product compositions were obtained, the column was switched from total reflux to finite reflux, the recirculation of reboiler liquid was stopped (case B only), fresh feed was entered at the middle of the column and the products were recovered. At this point, phase III was considered finished, which was after approximately 4.6 h for case A and 4.9 h for case B. 4.4. Phase I V
"l'mw~,) Fig. 8. Product compositions.
,
i
•
~
•
"nnmeaO
•
r
0
i
Fig. 9. Differential pressures in distillation column.
)
/" J
In this phase, the system was very close to steady state operating conditions, which could be seen from the differential pressures in the column. The values of DP2 and DP3 give a qualitative overview of how the column is operating, i.e. the separation capability between the three column sections, e.g. bottom section, feed section and top section. The differential pressures are presented in Fig. 9. In case A, the experimental goal was different than in case B, thus the steady state operating point was at a different level. Furthermore, the heat pump had reached the desired setpoints for the low and high pressure loops, which were presented in Fig. 10. It is apparent from these figures that the process is well under control during the production phase. The duration of phase IV was determined by the desired purities. The time required to achieve the desired product composition was determined by the external flows. However, since this experiment was designed to introduce the plant during startup, production and shutdown, the plant was shut down after approximately 9.0 h of operation. Phase V as presented in Table 2 was not entered as it involved the application of high-level control algorithms, e.g. adaptive control. 4.5. Discussion
). J
I
,
l
i
,
i
o
"nm0~ Fig. 10. Heat p u m p pressures.
llnm 0nO Fig. 11. Top and bottom temperatures during startup.
The startup operations outlined in the previous sections took a total of 3.7 h for case B and slightly less for case A, not including the time required for initializing the computer system and peripheral utilities. When evaluating the thermodynamic variables, i.e. temperature and pressure during the entire startup operation, the plots presented in Figs. 11 and 12 are obtained. These results demonstrated that the dynamics during the first two phases of the startup procedures were dominated by the time required to supply energy, first to the condenser section during phase I, and then to the heat pump and reboiler during phase II. During phase III, the product purities were established relatively faster due to the operation under full reflux, provided the energy transient from the first two phases was near
M.R. Eden et al./ Computers and Chemical Engineering 24 (2000) 1091-1097
1097
required to complete phase III, i.e. achieve the desired product purities.
5. Conclusions
Fig. 12. Top and bottom pressures during startup. Table 3 Phase duration for primary startup operation Phase ID
I
II III Total
Duration (h)
Total startup time (%)
A
B
A
B
0.6 2.3 0.8 3.7
0.6 2.1 1.0 3.7
16.2 62.2 21.6 100.0
16.2 56.8 27.0 100.0
completion. During phase IV, the external flows were determining the time required to reach the desired operating conditions, thus the desired purities played a significant role for the time required for completion of phase IV. Two relatively sharp peaks were apparent during the initial stage of the pressure plot presented in Fig. 12. One was caused by the vapor produced by the feed preheater breaking through the condenser, and one caused by the heating of the condenser. During this part of the startup operation, the pressure must be carefully supervised, since no pressure control was applied at this stage. Pressure control was not applied until the beginning of phase V, where e.g. an adaptive controller was activated. Although a total startup time of 3.7 h was quite good, it would be interesting to determine if and how the startup could be accomplished faster. Assuming the end of phase III to be the end of the primary startup operation, the duration of the individual phases is given in Table 3. It is apparent from these values that most of the time required to complete the startup operation is located in phase I and II. During these two phases, one thing primarily sets the limit to how much the time can be reduced, i.e. the energy input to the system to heat the condenser and the heat pump. Thus, if significant reductions in startup time are to be achieved, the initial heating of the system has to be accomplished faster. However, the thermodynamic variables settle significantly slower than the hydraulic variables (Ruiz et al., 1988), thus a reduction in the time required for the initial heating might result in an increase of the time
The paper demonstrated the usage of qualitative process knowledge for generating startup sequences. The procedure was illustrated on an heat integrated distillation column system. Two different sequences were investigated. It was demonstrated how control was used to facilitate the operation of the plant during startup. However, due to the operating conditions during startup being very different from production conditions, it is not possible to control the column pressure until rather late in the startup. Hence, this variable must be under careful surveillance, especially towards the end of phase II, when the heat pump may be started. This behavior is a result of the energy integration of the plant, where the heat pump during normal operation is used as an actuator to control column pressure and vapor flow. The total amount of time required before production conditions were reached was approximately 3.7 h, regardless of the chosen sequence, of which roughly 75-80% was used for the initial heating of the condenser and the heat pump. Thus, if the startup time is to be reduced further, it would seem that the initial heating of the system should be achieved faster. However, it must be emphasized that faster heating of the condenser and the heat pump, i.e. shortening phase I and II, might result in a longer phase III, since the effects of the energy transient would be more apparent in this phase. The impact of the transient energy could make it more difficult to obtain the appropriate column profile.
References Brabrand H., Jensen, N., & Jorgensen, S. B. (1991). MIMOSC - - a tool for real-time multivariable identification and adaptive control of chemical processes. Nordic process control workshop. Copenhagen, Denmark. Eden M. R., Loppenthien, & Skotte, R. (1999). Distillation column startup manual. Department of Chemical Engineering, Technical University of Denmark. Hangos, K. M., Cs~iki, Z. S., & Jorgensen, S. B. (1992). Qualitative model-based intelligent control of a distillation column. Engineering Applications and Artificial Intelligence, 5, 431-440. IEC (1988). Preparation of function charts for control systems. International Electrotechnical Commission, Publication 848. Ruiz, C. A., Cameron, I. T., & Gani, R. (1988). A generalized dynamic model for distillation Columns-III. Study of startup operations. Computer and Chemical Engineering, 12, 1-14. Venkateswarlu, C., & Gangiah, K. (1997). Comparison of nonlinear controllers for distillation startup and operation. Industrial Engineering and Chemical Research, 36, 5531-5536.
Computers &Chemical Engineering
Computers and Chemical Engineering 24 (2000) 1091-1097
www.elsevier.com/locate/compchemeng
Dynamics and control during startup of heat integrated distillation column Mario R. Eden, Arne Koggersbol, Louis Hallager, Sten B. Jorgensen * CAPEC, Department of Chemical Engineering, Technical University of Denmark, DK-2800 Lyngby, Denmark
Abstract
To advocate the usage of process integration in industrial practice, it is important to be able to guarantee not only robust control during near steady state operation, but also to provide procedures for generating fast and reliable startup sequences. This contribution concentrates on describing a systematic procedure for development of plant startup sequences. The basis for the startup procedure development is available qualitative process knowledge. Application of the startup sequence generation procedure is demonstrated upon a heat integrated distillation plant. This plant illustrates some of the inherent effects of process integration upon a startup procedure. In particular, the effect of energy recycle upon the possible startup sequences and the effect of using an actuator for control purposes, i.e. the heat pump, which in itself requires significant startup time. The experimental application of two generated startup sequences illustrates that safe and reliable startups are ensured. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Process integration; Distillation column dynamics; Startup procedure generation
1. Introduction
Distillation column dynamics and control have been considered as areas, where research has been completed in some sense. Although, the dynamics of distillation column startup have been intensively studied using dynamic simulation, e.g. Ruiz, Cameron and Gani (1988), very few experimental results have been reported. Simulations and experimental measurements have shown that the startup operation involves complex transient responses in hydraulic and thermodynamic variables, yielding a highly non-linear behavior of the product compositions. A division of the startup operation into three phases (Ruiz et al., 1988) has achieved wide range acceptance among process engineers. 1. I. The discontinuous phase
This is the initial phase of the startup, where the plates are weeping and heating is in progress. Pumps and flows are started. Low-level control o f secondary variables such as levels, flows and feed temperature is * Corresponding author. Fax: +45-4588-2258. E-mail address: [email protected] (S.B. Jorgensen)
applied. This phase defines the time needed for the initial heating. 1.2. The semi-continuous phase
This phase represents the most important part of the startup operation, since it is the slowest and hence most time-consuming. During this phase, the hydraulic variables reach their steady state values and the column is shifted from total reflux to the desired reflux rate. Several disturbances may destabilize the column operation at this stage, e.g. changes in reboiler steam pressure, feed temperature and composition and reflux flow rate etc. As the compositions slowly approach the desired level, they tend to get easily disturbed. This highly non-linear transition requires very good control of the process variables. 1.3. The continuous phase
In this final phase, the column reaches the vicinity of the desired steady state. In this phase, higher-level control structures may be applied. The challenging control problem during startup of distillation columns is located in the semi-continuous
0098-1354/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S0098- 1354(00)00488-9
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M.R. Eden et al./ Computers and Chemical Engineering 24 (2000) 1091-1097
phase, i.e. the non-linear transition from total reflux to specified reflux in the presence of several disturbances. For simple binary distillation columns, several control strategies have been implemented, e.g. Venkateswarlu and Gangiah (1997). However, for heat integrated distillation plants, experimental data is limited. The generation of startup sequences for chemical plants is usually far from trivial, due to the complexity of the plant, which may give rise to several possible startup sequences. One of the complicating factors is the application of recycle, which severely constrains the possible startup sequences. The startup procedure generation problem may be expected to become more difficult as the trend within process design continues to move towards increased energy and mass interaction for economical as well as for environmental protection reasons. Searching for optimal startup strategies would be a very large computational and modeling task, if carried out using standard simulation techniques. Furthermore, models, which can describe the plant behavior during startup, are normally not available. During startup, the plant is operated far from normal production conditions, and therefore may display very disparate behavior, which is unaccounted for in the models for normal production conditions. An additional obstacle for development of detailed dynamic models during startup is the lack of parameter correlations for such unusual operating conditions. Thus, it would seem most suitable to base the generation of startup procedures upon qualitative process knowledge. The purpose of this paper is to describe a systematic procedure for startup sequence generation, to demonstrate startup sequence generation for an energy-integrated pilot plant and to experimentally investigate the application of such a startup sequence. The paper is organized as follows, first the experimental pilot plant is briefly described along with the basic control structures on the plant. Then a procedure for generating startup sequences is presented. Subsequently, two alternative
m
:+_ m
Fig. 1. Distillation column and heat pump schematic.
startup sequences generated for the heat integrated distillation column plant are presented. Experimental applications of the generated procedures are described, illustrated and discussed.
2. Process description A schematic of the main pieces of equipment is given in Fig. 1. Feed can be introduced via two pumps (PF1 and/or PF2), mixed and preheated in a heat exchanger (HEFS). It may be fed into the column at five different positions. At the bottom of the column, liquid is withdrawn partly as bottoms product via a pump (PB) and partly evaporated in the thermosiphon reboiler (HERB) and reintroduced into the column as vapor. From the top of the column, the vapor is led to the condenser (HECOND), where it is condensed. The liquid is pumped to a decanter (DEC) via a pump (PC). In this study, the decanter simply is an accumulator. From the decanter, a part is withdrawn as top product (via the pump PT) and a part is returned as reflux to the top of the column (via the pump PR). For binary distillation experiments, a methanol-isopropanol system is currently used. The heat pump system depicted in Fig. 1 has been designed for large heat transfer load variations. It uses Freon 114 as the heat transfer medium and utilizes two 8-cylinder piston compressors. The heat pump worked as follows: most of the Freon vapor was condensed in the reboiler (HERB). An extra condenser (HESCOND) removed an amount of heat roughly corresponding to the heat introduced by the compressors (COMP). Three air fin coolers (ACOOL) passed on this heat to the environment. The condensed Freon passed to a receiver (REC) and through a heat exchanger (HECI) to the Freon evaporator (HECOND). The heat exchanger (HECI) served to superheat the Freon gas before it entered the compressors, to prevent condensation during compression. After the evaporator, a demister (GLSEP) prevented liquid from passing on to the compressors. The pilot plant was instrumented in accordance with the longterm objectives of the research project, of which the process dynamics and model identification for control design aspects were demanding in terms of direct measurements. The structure of the basic control loops relevant for startup purposes is described in Table 1. From this table, it may be observed that the top section of the distillation column may be operated in both D and in L configuration. After completing the startup, i.e. the system is close to normal operating conditions, it is possible to introduce P - V control in the form of an algorithm called multi input multi output selftuning controller (MIMOSC) (Brabrand, Jensen & Jorgensen, 1991).
M.R. Eden et al. / Computers and Chemical Engineering 24 (2000) 1091-1097
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Table 1 Relevant control loops for startup operation Loop
Sensor
Actuator
Setpoint
1 2 3 4 6 8 9 11 18 19
FM1 FM2 FM3 FM4 FM6 PI0 P8 FMSM DP1 L1
PF1 PF2 PB CV2 PR CV8 CV9 CV5 SP FM3 SP FM6/4
Feed flowrate 1 Feed flowrate 2 Bottoms product flowrate Distillate flowrate Reflux flowrate Heat pump high pressure Heat pump low pressure Preheater steam flowrate Reboiler level Accumulator level
3. Generating startup sequences In this section, a procedure for generating startup sequences is given and then applied to generate startup procedures for the process plant. The proposed startup generation procedure makes it possible to derive subgoals, which are consistent with the process constraints. The sequence of subgoals combines into achieving the overall goal of completing the startup. The procedure for generating a suitable subgoal structure is based upon usage of qualitative process knowledge. The first simple qualitative knowledge needed to develop a startup procedure is that of the basic limiting capacities in the system during startup. Often these capacities are relatively simple to identify. Thus, a relatively simple procedure for generating startup sequences follows: 1. identify limiting significant capacities in the plant; 2. identify suitable sequences to fill the capacities; and 3. select most promising procedures for satisfying safety and operational constraints. Below, this relatively simple procedure will be followed for the energy integrated distillation column. The qualitative process knowledge will be used especially in step 2 above to understand the dynamic interactions between process variables during startup. The resulting startup sequences will be presented schematically using GRAFCET (IEC, 1988) function diagrams to illustrate the significant process states and their transition conditions and the actions taken to achieve/ensure the process states. Once startup sequences are generated, the control related issues will be considered. For the energy integrated distillation, plant application of the above procedure led to the following key points: 1. one significant capacity is the energy content of the column. Another is the energy content in the heat pump. A third capacity is the mass holdup in the reboiler; 2. energy must be supplied in sufficient amount to establish necessary conditions for carrying out the
Remarks
Inner cascade to loop 18 Inner cascade to loop 19 Inner cascade to loop 19 Supervisory level setpoints Supervisory level setpoints Outer cascade to loop 3 Outer cascade to loop 6 or 4
energy driven separation process, before the distillation column concentration profiles may be established; 3. supplying energy to the plant requires careful supervision of the column pressure in order to ensure safe operation within the allowed pressure window. Secondly, the pressures in the heat pump must also be carefully supervised. In order to generate suitable startup procedures, qualitative process knowledge was formalized. The relations between significant capacities during startup are shown in Fig. 2. Considering, the physical couplings, i.e. constraints between the significant energy capacities, developed this diagram. Hangos, Cs~tki and Jargensen (1992) presented the assumptions made in setting up this diagram for the purpose of deriving startup procedures. Since the plant was initially cold, inspection of Fig. 2 revealed that the heat pump might not be started before the condenser was heated up. This initial heating may be achieved by feeding vapor high up into the column. The resulting initial phase of the startup operation is labeled phase I in Table 2. Once the column top section and the condenser were heated, the compressors might be started at a suitable speed to transfer heat from the column top section to the reboiler. During this second phase in Table 2, both energy inputs to the column were used. Once vapor production in the reboiler had ~
Iq~cd
c~P 0'-o~ o-,,
BV48
BV47
Fig. 2. Energy DYQUID with significant mass storages for distillation column and heat pump.
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M.R. Eden et al. / Computers and Chemical Engineering 24 (2000) 1091-1097
Table 2 Phase definitions for startup of energy integrated distillation column Phase Purpose
Initiating action
Indicator
I II III
Feed steam to preheater Start compressors Shut off help energy
OK for startup Condenser warm Vapor flow OK
Set desired setpoints in low level controls Selected setpoints specified by higher level algorithm
Purities acceptable
IV V
Heat condenser system Heat reboiler and heat pump Stop startup energy source Establish desired purities Bring process into desired operating form Introducesupervisorycontrol
started at a suitable rate, the startup energy supply to the feed preheater might be turned off or reduced. In the third phase of the startup, the column and heat pump were operating and the column purities might be allowed to reach the desired level before switching to production in phase IV. In phase V, higher-level regulator tasks may take control to assure quality and steady state optimality. To obtain a detailed view of a specific startup procedure, it is necessary to specify the macro steps further. This is done for two cases in Figs. 3 and 4. The major difference between the two cases is that in case A, the reboiler was initially empty or the cold contents were initially emptied out into a storage tank. The idea behind this procedure was to facilitate heating the reboiler by allowing vapor to condense in it. When the reboiler was full of liquid, as in procedure B, the heating had to take place from the heat pump. The two cases considered may be viewed as extremes. In case A, the emphasis is upon minimizing energy consumption, while in case B, the emphasis is upon minimizing material losses. An optimal startup procedure may well be a weighted average of the two depending upon the relative energy and material prizes. The startup sequences contain many common elements. Once clearance for starting up procedure A is received, a number of parallel states are initiated. The column is vented through a valve (CV10) and an effluent condenser, not shown in Fig. 1, mounted on the column top. When breakthrough of vapor is noted at a temperature measurement in the effluent condenser (TEC1), this outlet is closed. This outlet would normally not be opened again until plant was shutdown. Feed flow (fresh feed or recycled reboiler liquid) and steam supply to the feed preheater are started. Startup phase I was considered finished, when the condensate in the accumulator tank was warm as indicated by TC1. Then the compressors were started and full reflux conditions established. During phase II, energy was supplied by both the feed preheater and the compressors. Once the heat pump pressures were up to the initially desired level and the reboiler was producing a sufficient vapor flowrate, the desired number of active cylinders could be introduced and the preheater turned off. Once the vapor flowrate, column pressure and product purities were around the
desired levels, then the production may be initiated by implementing the selected control configuration for phase IV. The main differences between procedures A and B are firstly that the cold reboiler liquid in procedure B is recycled to column feed through the bottoms product cooler (HEBW) and the preheater (HEFS) during phases I and II. Secondly, all condensate was refluxed to the column. Thus material was kept inside the column until production might start. In both cases, the column was operated at full reflux in phases two and three. The control tasks during the startup procedure are also indicated in Figs. 3 and 4.
~
lltum,
X i JtGt~toz
• , ACtUIt~
I~ xn ~ i i lomp In
iuto
Fig. 3. GRAFCET function diagram for startup sequence A, empty reboiler. ~
~
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+.tur*, , aetvator loop a~rt'vl ~ r / ~ n t ~ l
Fig. 4. GRAFCET function diagram for startup sequence B, full reboiler.
M.R. Eden et al./ Computers and Chemical Engineering 24 (2000) 1091-1097
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Table 2, and the specific breakpoints presented. A detailed outline of the startup operation is given by Eden, L~ppenthien and Skotte (1999). In the plots presented, the curves are given by: Case A: thin solid and dotted lines (both black). Case B: thick solid lines (black and grey). ,
,
,
°
°
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•
,
•
,
4. I. Phase I
Fig. 5. Temperature in effluent condenser.
"rime~0 Fig. 6. Condenser temperature.
4.2. Phase H
1: t,!
.,
,
,
:
.,
,
After completing the initial steps, such as tuming on cooling water, turning on power supplies and initializing the computer and security systems, liquid was fed to the column top through the feed preheater HEFS. As outlined in Table 2, phase I was initiated when the preheater steam supply was started, which was after approximately 1.2 h. During phase I, the column is vented through BV52 and CV10; once vapor breakthrough is noted by an increase in the effluent condenser temperature (TEC1), venting is stopped (Fig. 5).
0
,
Fig. 7. Vapor production.
It is seen that in the first phases, the actuators are often in manual, but as soon as possible they are switched to automatic. In phase IV, the control configuration for the column top was selected. The column bottom is a priori in a V-configuration. The column pressure was not controlled in closed loop before phase IV, when the heat pump was operating. Thus during phases I and II, the column pressure has to be monitored carefully, and too high pressures avoided, by manipulating the energy input/output by adjusting the preheater steam flowrate, starting the compressors or changing the number of active cylinders used in the heat pump.
4. Results and discussion
Two startup experiments have been conducted according to the macro steps presented in Figs. 3 and 4. In this section, the data were presented and discussed. The startup is divided into four phases according to
While the condenser was being heated by the hot vapor produced by the feed preheater, some cold condensate was produced. The end of phase I and consequently, the beginning of phase II was indicated by the temperature (TC1) of the condensate in the accumulator tank. An increase in TC1 indicated the need for more cooling, thus the compressors were started at this point. Full reflux conditions were established and the compressors as well as the feed preheater provided heat throughout phase II. The increase in TC1, i.e. the beginning of phase II was found after approximately 1.9 h as indicated by Fig. 6. The remaining part of phase II consists of heating the reboiler and the heat pump in order to establish a sufficient vapor production. 4.3. Phase I I I
Once a sufficient vapor production (V_EST) had been established, the feed preheater steam supply was turned off, indicating the beginning of phase III. Phase III was initiated after approximately 4.1 h for case A and after 3.9 h for case B. During the remaining part of phase III, the desired purities were established. The vapor production is presented in Fig. 7, while the compositions (given by methanol mole fractions) are presented in Fig. 8. At the initial stages of the startup operation, it would seem as if the bottom product composition (XPTT1) was considerably higher than the top product composition (XPTT19). Both compositions indeed have values above 1, however, the reason for this strange behavior is found in the way the compositions are estimated. The compositions are calculated
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M.R.
E d e n et a l . / C o m p u t e r s a n d C h e m i c a l E n g i n e e r i n g 2 4 ( 2 0 0 0 ) 1 0 9 1 - 1 0 9 7
using Raoult's law, the Antoine equation for isopropanol and the measured values of temperature and pressure. No constraints are applied to the composition estimates; hence the combinations of temperature and pressure encountered during the initial stages of the startup operation may yield strange results. Since the column is fed at the top, the temperature will be significantly higher at this tray than at the bottom, yielding the erroneous composition estimates. As the column trays were filled with liquid, the temperature
and composition profiles became more reasonable. Once vapor was produced at a reasonable rate in the reboiler, the majority of the heat input was supplied in the bottom section, thus the profiles were inverted and the estimates got better. Once the desired product compositions were obtained, the column was switched from total reflux to finite reflux, the recirculation of reboiler liquid was stopped (case B only), fresh feed was entered at the middle of the column and the products were recovered. At this point, phase III was considered finished, which was after approximately 4.6 h for case A and 4.9 h for case B. 4.4. Phase I V
"l'mw~,) Fig. 8. Product compositions.
,
i
•
~
•
"nnmeaO
•
r
0
i
Fig. 9. Differential pressures in distillation column.
)
/" J
In this phase, the system was very close to steady state operating conditions, which could be seen from the differential pressures in the column. The values of DP2 and DP3 give a qualitative overview of how the column is operating, i.e. the separation capability between the three column sections, e.g. bottom section, feed section and top section. The differential pressures are presented in Fig. 9. In case A, the experimental goal was different than in case B, thus the steady state operating point was at a different level. Furthermore, the heat pump had reached the desired setpoints for the low and high pressure loops, which were presented in Fig. 10. It is apparent from these figures that the process is well under control during the production phase. The duration of phase IV was determined by the desired purities. The time required to achieve the desired product composition was determined by the external flows. However, since this experiment was designed to introduce the plant during startup, production and shutdown, the plant was shut down after approximately 9.0 h of operation. Phase V as presented in Table 2 was not entered as it involved the application of high-level control algorithms, e.g. adaptive control. 4.5. Discussion
). J
I
,
l
i
,
i
o
"nm0~ Fig. 10. Heat p u m p pressures.
llnm 0nO Fig. 11. Top and bottom temperatures during startup.
The startup operations outlined in the previous sections took a total of 3.7 h for case B and slightly less for case A, not including the time required for initializing the computer system and peripheral utilities. When evaluating the thermodynamic variables, i.e. temperature and pressure during the entire startup operation, the plots presented in Figs. 11 and 12 are obtained. These results demonstrated that the dynamics during the first two phases of the startup procedures were dominated by the time required to supply energy, first to the condenser section during phase I, and then to the heat pump and reboiler during phase II. During phase III, the product purities were established relatively faster due to the operation under full reflux, provided the energy transient from the first two phases was near
M.R. Eden et al./ Computers and Chemical Engineering 24 (2000) 1091-1097
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required to complete phase III, i.e. achieve the desired product purities.
5. Conclusions
Fig. 12. Top and bottom pressures during startup. Table 3 Phase duration for primary startup operation Phase ID
I
II III Total
Duration (h)
Total startup time (%)
A
B
A
B
0.6 2.3 0.8 3.7
0.6 2.1 1.0 3.7
16.2 62.2 21.6 100.0
16.2 56.8 27.0 100.0
completion. During phase IV, the external flows were determining the time required to reach the desired operating conditions, thus the desired purities played a significant role for the time required for completion of phase IV. Two relatively sharp peaks were apparent during the initial stage of the pressure plot presented in Fig. 12. One was caused by the vapor produced by the feed preheater breaking through the condenser, and one caused by the heating of the condenser. During this part of the startup operation, the pressure must be carefully supervised, since no pressure control was applied at this stage. Pressure control was not applied until the beginning of phase V, where e.g. an adaptive controller was activated. Although a total startup time of 3.7 h was quite good, it would be interesting to determine if and how the startup could be accomplished faster. Assuming the end of phase III to be the end of the primary startup operation, the duration of the individual phases is given in Table 3. It is apparent from these values that most of the time required to complete the startup operation is located in phase I and II. During these two phases, one thing primarily sets the limit to how much the time can be reduced, i.e. the energy input to the system to heat the condenser and the heat pump. Thus, if significant reductions in startup time are to be achieved, the initial heating of the system has to be accomplished faster. However, the thermodynamic variables settle significantly slower than the hydraulic variables (Ruiz et al., 1988), thus a reduction in the time required for the initial heating might result in an increase of the time
The paper demonstrated the usage of qualitative process knowledge for generating startup sequences. The procedure was illustrated on an heat integrated distillation column system. Two different sequences were investigated. It was demonstrated how control was used to facilitate the operation of the plant during startup. However, due to the operating conditions during startup being very different from production conditions, it is not possible to control the column pressure until rather late in the startup. Hence, this variable must be under careful surveillance, especially towards the end of phase II, when the heat pump may be started. This behavior is a result of the energy integration of the plant, where the heat pump during normal operation is used as an actuator to control column pressure and vapor flow. The total amount of time required before production conditions were reached was approximately 3.7 h, regardless of the chosen sequence, of which roughly 75-80% was used for the initial heating of the condenser and the heat pump. Thus, if the startup time is to be reduced further, it would seem that the initial heating of the system should be achieved faster. However, it must be emphasized that faster heating of the condenser and the heat pump, i.e. shortening phase I and II, might result in a longer phase III, since the effects of the energy transient would be more apparent in this phase. The impact of the transient energy could make it more difficult to obtain the appropriate column profile.
References Brabrand H., Jensen, N., & Jorgensen, S. B. (1991). MIMOSC - - a tool for real-time multivariable identification and adaptive control of chemical processes. Nordic process control workshop. Copenhagen, Denmark. Eden M. R., Loppenthien, & Skotte, R. (1999). Distillation column startup manual. Department of Chemical Engineering, Technical University of Denmark. Hangos, K. M., Cs~iki, Z. S., & Jorgensen, S. B. (1992). Qualitative model-based intelligent control of a distillation column. Engineering Applications and Artificial Intelligence, 5, 431-440. IEC (1988). Preparation of function charts for control systems. International Electrotechnical Commission, Publication 848. Ruiz, C. A., Cameron, I. T., & Gani, R. (1988). A generalized dynamic model for distillation Columns-III. Study of startup operations. Computer and Chemical Engineering, 12, 1-14. Venkateswarlu, C., & Gangiah, K. (1997). Comparison of nonlinear controllers for distillation startup and operation. Industrial Engineering and Chemical Research, 36, 5531-5536.