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A new strategy for product switchover and startup for a heat- and mass-integrated distillation system
Chemical Engineering and Processing 40 (2001) 295– 302 www.elsevier.com/locate/cep
A new strategy for product switchover and startup for a heat- and mass-integrated distillation system K. Lo¨we, G. Wozny * Institute of Process and Plant Technology, Technical Uni6ersity Berlin, Sekr. KF 1, Straße des 17. Juni 135, 10623 Berlin, Germany Received 11 March 2000; received in revised form 2 August 2000; accepted 2 August 2000
Abstract For most separations fully thermally coupled distillation columns are thermodynamically more efficient than conventional arrangements. In spite of the superiority of double-effect distillation systems to a single column concerning energy savings these systems are not often used in the industrial practice, because of the disadvantages in dynamic operability. Generally the product switchover is a complicated and protracted operation, where many process variables must be changed simultaneously. The economic and ecological interest is to minimize the time of switchover periods, which are generally non-productive and require long times especially for coupled systems. The aim of this study is to reduce the time needed for the product switchover of coupled distillation systems. This paper presents a new product switchover strategy, which will lead to a time saving of 57%. Furthermore, a new startup strategy is developed and verified in experimental studies. The results demonstrate that using the optimized startup strategy a time saving of 57% can be achieved too and thereby these unproductive periods can be reduced significantly. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Integrated distillation system; Startup; Product switchover; Optimization
1. Introduction Reduction of energy consumption as well as the better use of resources has been an important aspect in the design and operation of chemical processes. The use of integrated distillation systems can lead to a significant reduction of energy consumption in comparison with conventional distillation columns. With heat integrated distillation column systems energy savings up to 45% can be achieved [1]. In the complex configuration of a heat- and mass-integrated distillation system the condensing vapour from the top of the high pressure column (HPC) is used to heat the reboiler of the low pressure column (LPC). The double column system is superior to a single column in energy saving but disadvantageous in dynamic operability. Interactions and time delays will lead to a more complicated controllability, so a double column * Corresponding author. Tel.: +49-30-31423893; fax: + 49-3031426915. E-mail address: [email protected] (G. Wozny).
system needs a higher effort in the design and the control systems than a single column. Both the product switchover procedure and the startup of distillation represent some of the most complicated procedures in the industrial practice and are generally unproductive periods. Before the steadystate operating point is reached, non-specification products, if any, are produced and either has to be returned to the column or disposed of. The economic and ecological interest is to minimize the time of these non-productive periods. Especially for coupled systems the non-productive period takes a long time. Optimal reflux and reboiler duty policies should be developed to minimize the operation time of the startup procedure. The aim of this study is the development and experimental verification of a time-optimal product switchover strategy for coupled distillation systems. Additionally, based on these investigations, a time optimal startup strategy is deduced and verified in experimental studies.
0255-2701/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 5 - 2 7 0 1 ( 0 0 ) 0 0 1 2 6 - 4
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Fig. 1. Picture of the pilot plant.
2. Pilot plant Experiments are carried out on a fully automated methanol/water distillation column system of pilot plant scale. A picture of the plant is given in Fig. 1. In this fully thermally coupled distillation system external heat is introduced only in the reboiler of the HPC. For material integration the so called LS/R (light-split/heat integration reverse) configuration is selected. Fig. 2 shows the investigated configuration.
Fig. 2. Scheme of the double column distillation system.
Fig. 3. Bubble cap tray.
Thereby the bottom product of the low pressure column is fed to the high pressure column. Boilup is provided by an electrically heated thermosiphon reboiler with a maximum duty of 30 kW. In addition to the heat-integration condenser/reboiler the HPC is fitted out with an auxiliary condenser and the LPC with an auxiliary reboiler. A cooling water condenser is used for the LPC. The diameter of both columns is 107 mm. Twenty (LPC) and 28 (HPC) bubble cap trays with central downcomers are installed, tray spacing is 210 mm (LPC) and 150 mm (HPC). A sketch of the tray are shown in Fig. 3. The column system is equipped with an extensive technique of measurement. All measurements are digitalized on a decentralized control system TELEPERM M, Siemens. The column system is operated in the LV-configuration. Liquid levels are controlled by bottom and distillate flowrates. Product compositions are controlled by the two reflux flowrates (LHP, LLP) and the reboiler duty (QHP).
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3. Simulation model A dynamic model is used to simulate the double effect column system. The model is implemented within a SpeedUp environment. The model equations of each tray consist of three parts: mass and energy balances, phase equilibrium and tray hydraulics. The dynamic balances of mass, component and energy lead to a set of differential equations. The vapour – liquid equilibrium is described by the Wilson- and Antoine-equations. The tray hydraulics is modelled with the Francis –weir-formula to correlate the tray holdup. To describe the system pressure the dynamic of the vapour holup is considered [7]. The tray pressure drop is calculated by the gas and liquid fluid dynamics based on the tray geometric sizes. Tray efficiencies are estimated with measured data from experiments. In order to describe the dynamic behavior of the heat-integrated condenser/ reboiler system a rigorous model of the thermosiphon reboiler and the condenser was developed [7].
4. Preparatory work on a single column Lo¨we et al. [5] optimized a product switchover procedure with a mathematical optimizing method for both the bottom and the top product purity from 90–99.5 mol% for a single high purity distillation column. The column is operated in the LV-configuration, so the reflux flowrate and the reboiler duty are chosen as manipulated variables. The technical limits of these two variables have been determined according to the allowable vapour and liquid flow regimes of the column. Fig. 4 shows the optimal operation policies for the reflux and the reboiler duty. The manipulated variables will be at their maximum allowable value in the first period in order to increase the distillate and bottom purities as fast as possible. The steady state values of
Fig. 5. Product purities during a conventional product switchover strategy.
the manipulated variables are reached within 60 min. The corresponding product purities increase continuously, the new steady state operation point is reached within 60 min. From these optimal operation policies a simplified curve of the manipulated variables is deduced. Thereby, when the reference set-point is changing, the manipulated variables are set to their maximum values and reduced at time t= 43 min to the steady state values. It can be demonstrated that with the simplified control policy the new product specifications are reached within 1 h also. Furthermore, it can be shown that not only the values of the manipulated variables but also the period of time is very important for the results. A longer operation time with the increased manipulated variables will lead to a higher bottom product purity as specified. If the column is operated too briefly with the increased manipulated variables, on the other hand, the time needed to reach the new steady state operation point will be extended extremely.
5. Simulation results for coupled distillation systems
Fig. 4. Optimal operation strategy of the reboiler duty (solid line) and reflux (dashed line) for a switchover of the product purities from 90–99.5 mol%.
In the next step this optimized strategy for a single column is transferred to the heat- and mass-integrated distillation column system and compared to the conventional strategy. Using the conventional product switchover strategy all process variables are set to their new steady state values at the switchover time. Because the coupled distillation system was designed for product purities of 99 mol%, a product switchover from 90–99 mol% is considered. Fig. 5 shows the product purities (Bottoms product HPC, Distillate HPC and Distillate LPC) during a conventional product switchover procedure. At time t= 30 min the new steady state operation point should be set. The distillate composition of the HPC shows a long lasting increase over the specified composition. The HPC’s bottoms product and the
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comparable operation problem to a switchover of the product purities from 90–99 mol% results, considering only the product compositions [5]. Consequently the results from the investigations of a product switchover can be used for optimizing the startup procedure without the need for a more computational complicated startup model.
7. Startup strategies
Fig. 6. Product purities during the optimized product switchover strategy.
LPC’s distillate reach the desired compositions after 300 min. Using the optimized product switchover strategy the column system is operated with the increased manipulated variables from the beginning of the product switchover procedure (t = 30 min). The bottoms concentration is used as a switchover criterion. As soon as the bottoms concentration reaches 98.4 mol% the new steady state variables are set. Fig. 6 shows the product purities during the optimized product switchover procedure. Steady state will be reached within 130 min. Thus a product switchover with the optimized strategy will lead to a time saving of 57% comparing to the conventional strategy.
6. Analogy between startup and product switchover Fig. 7 shows the temperature curves of a single column during a conventional startup strategy. Considering that point of time during the startup of a distillation column, when the first vapour reaches the top of the column, temperatures from about 68.2°C in the top and 87.3°C in the bottom are corresponding. Thus a
In the literature the following startup strategies for single columns are described. Using the conventional direct setting strategy, the compositions increase continually but steady state will be reached slowly. Startup of distillation columns with total reflux is said to be one of the most common and often used startup techniques for single columns [3]. Kruse [4] describes a startup strategy with total removal of distillate. At a proper time the column is switched from total removal of distillate operation to steady state operation. Yasuoka et al. [8] propose the following procedure to detect the optimal switchover time for a total reflux startup strategy. This point of time is determined by the minimum of the Mx or MT -function NT
Mx = % xj − x SP j ,
(1)
j=0 NT
MT = % Tj − T SP j ,
(2)
j=0
where xj and Tj are the current values of the concentration or temperature on the j-th tray and x SP and T SP j j are the corresponding steady state variables. Flender [2] validate this procedure for the total removal of distillate strategy for a packed column. Studies about the startup of heat- and mass-integrated distillation systems have not been emphasized in the open literature. Commonly, in a two column process, the two columns are started separately and, after a certain period of time, the two columns are linked together. In the following chapter the startup behavior of coupled distillation systems will be investigated in experimental studies.
8. Startup of coupled distillation systems — experimental results
Fig. 7. Temperature curves during a conventional startup.
In all cases the pilot plant is started up from atmospheric pressure and ambient temperature. In a heat-integrated condenser only the liquid level is available for pressure control. Due to the insufficient sensitivity and slow response of the HPC’s pressure to variations of the condensate level in the heat-integrated condenser, the HPC is operated at floating pressure. The conden-
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Fig. 8. Temperature curves of the HPC.
sate level in the heat-integrated condenser is controlled to a fixed value, whereby the corresponding pressure results. The following strategies are compared: 1. Conventional Strategy. 1.1. Fully thermally coupled. 1.2. Thermally independent. 2. Startup with total removal of distillate. 3. Startup with increased reboiler duty.
8.1. Con6entional strategy Using the conventional strategy all process variables are set to their steady state values at the beginning of the startup procedure. These values can be calculated with a simulation program or be taken from practical experience.
8.1.1. Fully thermally coupled During startup procedure of the fully thermally coupled system the auxiliary reboiler of the LPC is not in use. The advantage of this strategy is that startup of the LPC needs no additional external heat. Fig. 8 shows the measured temperature curve for the bottom and the top of the HPC during startup operation. The steady state operation point is determined
Fig. 9. Temperature curves of the LPC.
299
Fig. 10. Temperature curves. Comparison between fully thermally coupled (A) and thermally independent (B).
after 340 min. Fig. 9 shows the temperature curves of the LPC. Reaching steady state takes about 200 min. LPC will reach the steady state long before the HPC. This strategy is used for the comparison of the different strategies.
8.1.2. Thermally independent Fig. 10 shows the comparison of temperature curves for fully thermally coupled and thermally independent startup operations. Using the thermally independent strategy, both columns suffer disturbances when the coupling is switched on at time t= 130 min. Approaching steady state not only takes the same time as in case Section 8.1.1 but will consume more energy. Therefore it is useful to pay special attention to the startup of the HPC. The following presented experimental work is examined on the fully thermally coupled system. 8.2. Startup with total remo6al of distillate With the total removal of distillate strategy the startup time for a single column can be reduced up to 70% [4]. In this experiment the HPC is started up with the total removal of distillate strategy. Due to the heat-integration the LPC warms up automatically. As soon as the first vapour reaches the top of the LPC, reflux is added. As a result of the non-availability of reflux the impurity of the HPC’s top concentration increases in the meanwhile. Adding reflux lowers the top temperature immediately and will lead to a rise in purity (Fig. 11, t= 180 min.), whereby the corresponding pressure rises (Fig. 12). This effects a higher temperature in the HPC and so a greater difference in temperature in the heatintegrating condenser/reboiler. Consequently the LPC’s bottoms product temperature and purity will increase. So, the HPC suffers a delayed disturbance in feedconcentration. Comparing to the conventional strategy, by which the steady state operation point is determined
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Fig. 11. Temperature curves of the HPC.
Fig. 13. Temperature curves of the HPC.
after 340 min, the period of the startup procedure takes longer. This strategy, which reduces the startup time for a single column, shows no advantage for this column system.
pressure of the top exceeds three bar, reboiler duty is decreased to steady state value. After 220 min the column system reaches steady state (Fig. 13). Fig. 14 shows the pressure’s transient behavior. Only a slight overshoot in pressure over its steady state value is observed. As soon as the thermosiphon circulation ignites (at 3.5 bar), the steep increase of pressure in HPC is stopped. Startup with this strategy will lead to a time saving of 35% comparing to the conventional strategy.
8.3. Startup with increased reboiler duty The startup procedure begins with an increased reboiler duty in HPC. Consequently the LPC also receives a greater heat duty compared to its steady state operation point. It must be avoided that the pressure in HPC exceeds the pressure of steady state at nominal heat duty. As soon as pressure in HPC reaches its steady state value, the heat duty in HPC must be reduced to normal operation value. Otherwise the reboiler of LPC would be heated up to a higher temperature than desired. This would lead to a lower concentration of the light component in the reboiler of the LPC, which would result in a disturbance in feedconcentration on the HPC. Reaching steady state after such disturbance is delayed. Startup of the HPC begins with the 1.9-fold heat duty of the steady state value. As soon as the first vapour reaches the top of the column, heat duty is decreased to the 1.3-fold steady state value. When the
In this chapter experimental investigations of the startup of heat- and mass-integrated distillation columns are presented. The comparison between the fully thermally coupled startup strategy and the thermally independent strategy shows the superiority of the fully thermally coupled startup strategy in energy savings and the expenditure of time. Using the fully thermally coupled strategy the LPC will reach the steady state before the HPC. The total removal of distillate strategy, which reduces the startup time for a single column, shows no advantage for this column system. Startup with an increased reboiler duty will lead to a time saving of 35%.
Fig. 12. Pressure curve of the HPC.
Fig. 14. Pressure curve of the HPC.
8.4. Summary
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Fig. 15. Temperature curves of the HPC during a conventional startup.
Fig. 17. Temperature curves of the HPC during the optimized startup.
9. Optimized startup strategy
As soon as it reaches 98.5 mol% the manipulated variables are set to their steady state values. This switchover criterion and the values of the increased manipulated variables were deduced from the simulation results (chapter 5). The bottom composition is calculated online with a mathematical model [6], pressure compensated temperatures could be utilized also. Fig. 15 shows the measured temperature curves for the top and the bottom of the HPC during a conventional startup strategy. Fig. 16 shows the corresponding measured temperature curves of the top and the bottom of the LPC. The LPC will reach the steady state operation point long before the HPC. The column system needs 350 min for startup. The measured temperature curves for the top and the bottom of the HPC
In the experimental work presented below it is examined whether with the help of an optimized operation policy, developed for a switchover of the product purities, the period of startup can be reduced too. For validation the optimized startup strategy is investigated in experiments and compared with the conventional strategy. Using the optimized startup strategy the column is operated with an increased reboiler duty from the beginning of the startup procedure. As soon as enough liquid is available on the top of the HPC as well as of the LPC reflux with the increased value is added. The bottom composition is used as a switchover criterion.
Fig. 16. Temperature curves of the LPC during a conventional startup.
Fig. 18. Temperature curves of the LPC during the optimized startup.
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Table 1 Period of time [min] needed for reaching the steady state operation point Conventional
Optimized
350 (min) 230 (min) 100%
150 (min) 140 (min) 43%
WO 565/10-2. The authors thank DFG for the financial support.
Appendix A. Nomenclature HPC LPC
during the optimized startup strategy are shown in Fig. 17, the corresponding measured temperature curves of the top of the LPC in Fig. 18. Reaching steady state in LPC and HPC takes about 150 min. Startup with the optimized procedure will lead to a time saving of 57% (Table 1).
HPC LPC NT T
high pressure column low pressure column number of trays temperature
Subscripts j
tray
Superscripts SP
set point
References 10. Conclusions In this paper results of a new product switchover strategy for a heat- and mass-integrated distillation column system are presented. The computational results demonstrate that with the help of the optimized strategy the period for the switchover procedure in product composition from 90 – 99 mol% can be reduced by 57%. Furthermore, from this knowledge a time reduced startup strategy for coupled distillation systems is deduced. The experimental results demonstrate the superiority of this strategy. Comparing to the conventional startup strategy a timesaving of 57% can be achieved. This allows also a significant reduction of energy and raw material consumption and consequently lead to a more economical process.
Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft (DFG) under the contract .
[1] H.-C. Cheng, W.L. Luyben, Heat integrated distillation columns for ternary separations, Ind. Eng. Chem. Process Des. Dev. 24 (1985) 707– 713. [2] M. Flender, Zeitoptimale Strategien fu¨r Anfahr- und Produktwechselvorga¨nge an Rektifizieranlagen, in: Fortschritt- Berichte Reihe 3: Verfahrenstechnik Nr. 610, VDI-Verlag, Dusseldorf, 1999. [3] H.Z. Kister, Distillation Operation, McGraw Hill, New York, 1990. [4] C. Kruse, Theoretische und experimentelle Entwicklung zeitoptimaler Anfahr- und Produktwechselstrategien fu¨r die Rektifikation, in: Fortschritt- Berichte Reihe 3: Verfahrenstechnik Nr. 420, VDI-Verlag, Dusseldorf, 1995. [5] K. Lo¨we, P. Li, G. Wozny, Development and experimental verification of a time– optimal startup strategy for a high purity distillation column, accepted in Chem. Eng. Technol. [6] K. Lo¨we, G. Wozny, Feedforward control based on online concentration calculation of a heat- and mass-integrated distillation system, in: Computer Aided Process Engineering, vol. 8, Elsevier, Amsterdam, 2000, pp. 151– 156. [7] A. Rix, Modellierung und Prozeßfu¨hrung wa¨rmeintegrierter Destillationskolonnen, in: Fortschritt- Berichte Reihe 3: Verfahrenstechnik Nr. 549, VDI-Verlag, Dusseldorf, 1998. [8] H. Yasuoka, E. Nakanisshi, E. Kunugita, Design of an on-line startup system for a distillation column based on a simple algorithm, Ind. Chem. Eng. 27 (3) (1987) 466– 472.
A new strategy for product switchover and startup for a heat- and mass-integrated distillation system K. Lo¨we, G. Wozny * Institute of Process and Plant Technology, Technical Uni6ersity Berlin, Sekr. KF 1, Straße des 17. Juni 135, 10623 Berlin, Germany Received 11 March 2000; received in revised form 2 August 2000; accepted 2 August 2000
Abstract For most separations fully thermally coupled distillation columns are thermodynamically more efficient than conventional arrangements. In spite of the superiority of double-effect distillation systems to a single column concerning energy savings these systems are not often used in the industrial practice, because of the disadvantages in dynamic operability. Generally the product switchover is a complicated and protracted operation, where many process variables must be changed simultaneously. The economic and ecological interest is to minimize the time of switchover periods, which are generally non-productive and require long times especially for coupled systems. The aim of this study is to reduce the time needed for the product switchover of coupled distillation systems. This paper presents a new product switchover strategy, which will lead to a time saving of 57%. Furthermore, a new startup strategy is developed and verified in experimental studies. The results demonstrate that using the optimized startup strategy a time saving of 57% can be achieved too and thereby these unproductive periods can be reduced significantly. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Integrated distillation system; Startup; Product switchover; Optimization
1. Introduction Reduction of energy consumption as well as the better use of resources has been an important aspect in the design and operation of chemical processes. The use of integrated distillation systems can lead to a significant reduction of energy consumption in comparison with conventional distillation columns. With heat integrated distillation column systems energy savings up to 45% can be achieved [1]. In the complex configuration of a heat- and mass-integrated distillation system the condensing vapour from the top of the high pressure column (HPC) is used to heat the reboiler of the low pressure column (LPC). The double column system is superior to a single column in energy saving but disadvantageous in dynamic operability. Interactions and time delays will lead to a more complicated controllability, so a double column * Corresponding author. Tel.: +49-30-31423893; fax: + 49-3031426915. E-mail address: [email protected] (G. Wozny).
system needs a higher effort in the design and the control systems than a single column. Both the product switchover procedure and the startup of distillation represent some of the most complicated procedures in the industrial practice and are generally unproductive periods. Before the steadystate operating point is reached, non-specification products, if any, are produced and either has to be returned to the column or disposed of. The economic and ecological interest is to minimize the time of these non-productive periods. Especially for coupled systems the non-productive period takes a long time. Optimal reflux and reboiler duty policies should be developed to minimize the operation time of the startup procedure. The aim of this study is the development and experimental verification of a time-optimal product switchover strategy for coupled distillation systems. Additionally, based on these investigations, a time optimal startup strategy is deduced and verified in experimental studies.
0255-2701/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 5 - 2 7 0 1 ( 0 0 ) 0 0 1 2 6 - 4
K. Lo¨we, G. Wozny / Chemical Engineering and Processing 40 (2001) 295–302
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Fig. 1. Picture of the pilot plant.
2. Pilot plant Experiments are carried out on a fully automated methanol/water distillation column system of pilot plant scale. A picture of the plant is given in Fig. 1. In this fully thermally coupled distillation system external heat is introduced only in the reboiler of the HPC. For material integration the so called LS/R (light-split/heat integration reverse) configuration is selected. Fig. 2 shows the investigated configuration.
Fig. 2. Scheme of the double column distillation system.
Fig. 3. Bubble cap tray.
Thereby the bottom product of the low pressure column is fed to the high pressure column. Boilup is provided by an electrically heated thermosiphon reboiler with a maximum duty of 30 kW. In addition to the heat-integration condenser/reboiler the HPC is fitted out with an auxiliary condenser and the LPC with an auxiliary reboiler. A cooling water condenser is used for the LPC. The diameter of both columns is 107 mm. Twenty (LPC) and 28 (HPC) bubble cap trays with central downcomers are installed, tray spacing is 210 mm (LPC) and 150 mm (HPC). A sketch of the tray are shown in Fig. 3. The column system is equipped with an extensive technique of measurement. All measurements are digitalized on a decentralized control system TELEPERM M, Siemens. The column system is operated in the LV-configuration. Liquid levels are controlled by bottom and distillate flowrates. Product compositions are controlled by the two reflux flowrates (LHP, LLP) and the reboiler duty (QHP).
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3. Simulation model A dynamic model is used to simulate the double effect column system. The model is implemented within a SpeedUp environment. The model equations of each tray consist of three parts: mass and energy balances, phase equilibrium and tray hydraulics. The dynamic balances of mass, component and energy lead to a set of differential equations. The vapour – liquid equilibrium is described by the Wilson- and Antoine-equations. The tray hydraulics is modelled with the Francis –weir-formula to correlate the tray holdup. To describe the system pressure the dynamic of the vapour holup is considered [7]. The tray pressure drop is calculated by the gas and liquid fluid dynamics based on the tray geometric sizes. Tray efficiencies are estimated with measured data from experiments. In order to describe the dynamic behavior of the heat-integrated condenser/ reboiler system a rigorous model of the thermosiphon reboiler and the condenser was developed [7].
4. Preparatory work on a single column Lo¨we et al. [5] optimized a product switchover procedure with a mathematical optimizing method for both the bottom and the top product purity from 90–99.5 mol% for a single high purity distillation column. The column is operated in the LV-configuration, so the reflux flowrate and the reboiler duty are chosen as manipulated variables. The technical limits of these two variables have been determined according to the allowable vapour and liquid flow regimes of the column. Fig. 4 shows the optimal operation policies for the reflux and the reboiler duty. The manipulated variables will be at their maximum allowable value in the first period in order to increase the distillate and bottom purities as fast as possible. The steady state values of
Fig. 5. Product purities during a conventional product switchover strategy.
the manipulated variables are reached within 60 min. The corresponding product purities increase continuously, the new steady state operation point is reached within 60 min. From these optimal operation policies a simplified curve of the manipulated variables is deduced. Thereby, when the reference set-point is changing, the manipulated variables are set to their maximum values and reduced at time t= 43 min to the steady state values. It can be demonstrated that with the simplified control policy the new product specifications are reached within 1 h also. Furthermore, it can be shown that not only the values of the manipulated variables but also the period of time is very important for the results. A longer operation time with the increased manipulated variables will lead to a higher bottom product purity as specified. If the column is operated too briefly with the increased manipulated variables, on the other hand, the time needed to reach the new steady state operation point will be extended extremely.
5. Simulation results for coupled distillation systems
Fig. 4. Optimal operation strategy of the reboiler duty (solid line) and reflux (dashed line) for a switchover of the product purities from 90–99.5 mol%.
In the next step this optimized strategy for a single column is transferred to the heat- and mass-integrated distillation column system and compared to the conventional strategy. Using the conventional product switchover strategy all process variables are set to their new steady state values at the switchover time. Because the coupled distillation system was designed for product purities of 99 mol%, a product switchover from 90–99 mol% is considered. Fig. 5 shows the product purities (Bottoms product HPC, Distillate HPC and Distillate LPC) during a conventional product switchover procedure. At time t= 30 min the new steady state operation point should be set. The distillate composition of the HPC shows a long lasting increase over the specified composition. The HPC’s bottoms product and the
298
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comparable operation problem to a switchover of the product purities from 90–99 mol% results, considering only the product compositions [5]. Consequently the results from the investigations of a product switchover can be used for optimizing the startup procedure without the need for a more computational complicated startup model.
7. Startup strategies
Fig. 6. Product purities during the optimized product switchover strategy.
LPC’s distillate reach the desired compositions after 300 min. Using the optimized product switchover strategy the column system is operated with the increased manipulated variables from the beginning of the product switchover procedure (t = 30 min). The bottoms concentration is used as a switchover criterion. As soon as the bottoms concentration reaches 98.4 mol% the new steady state variables are set. Fig. 6 shows the product purities during the optimized product switchover procedure. Steady state will be reached within 130 min. Thus a product switchover with the optimized strategy will lead to a time saving of 57% comparing to the conventional strategy.
6. Analogy between startup and product switchover Fig. 7 shows the temperature curves of a single column during a conventional startup strategy. Considering that point of time during the startup of a distillation column, when the first vapour reaches the top of the column, temperatures from about 68.2°C in the top and 87.3°C in the bottom are corresponding. Thus a
In the literature the following startup strategies for single columns are described. Using the conventional direct setting strategy, the compositions increase continually but steady state will be reached slowly. Startup of distillation columns with total reflux is said to be one of the most common and often used startup techniques for single columns [3]. Kruse [4] describes a startup strategy with total removal of distillate. At a proper time the column is switched from total removal of distillate operation to steady state operation. Yasuoka et al. [8] propose the following procedure to detect the optimal switchover time for a total reflux startup strategy. This point of time is determined by the minimum of the Mx or MT -function NT
Mx = % xj − x SP j ,
(1)
j=0 NT
MT = % Tj − T SP j ,
(2)
j=0
where xj and Tj are the current values of the concentration or temperature on the j-th tray and x SP and T SP j j are the corresponding steady state variables. Flender [2] validate this procedure for the total removal of distillate strategy for a packed column. Studies about the startup of heat- and mass-integrated distillation systems have not been emphasized in the open literature. Commonly, in a two column process, the two columns are started separately and, after a certain period of time, the two columns are linked together. In the following chapter the startup behavior of coupled distillation systems will be investigated in experimental studies.
8. Startup of coupled distillation systems — experimental results
Fig. 7. Temperature curves during a conventional startup.
In all cases the pilot plant is started up from atmospheric pressure and ambient temperature. In a heat-integrated condenser only the liquid level is available for pressure control. Due to the insufficient sensitivity and slow response of the HPC’s pressure to variations of the condensate level in the heat-integrated condenser, the HPC is operated at floating pressure. The conden-
K. Lo¨we, G. Wozny / Chemical Engineering and Processing 40 (2001) 295–302
Fig. 8. Temperature curves of the HPC.
sate level in the heat-integrated condenser is controlled to a fixed value, whereby the corresponding pressure results. The following strategies are compared: 1. Conventional Strategy. 1.1. Fully thermally coupled. 1.2. Thermally independent. 2. Startup with total removal of distillate. 3. Startup with increased reboiler duty.
8.1. Con6entional strategy Using the conventional strategy all process variables are set to their steady state values at the beginning of the startup procedure. These values can be calculated with a simulation program or be taken from practical experience.
8.1.1. Fully thermally coupled During startup procedure of the fully thermally coupled system the auxiliary reboiler of the LPC is not in use. The advantage of this strategy is that startup of the LPC needs no additional external heat. Fig. 8 shows the measured temperature curve for the bottom and the top of the HPC during startup operation. The steady state operation point is determined
Fig. 9. Temperature curves of the LPC.
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Fig. 10. Temperature curves. Comparison between fully thermally coupled (A) and thermally independent (B).
after 340 min. Fig. 9 shows the temperature curves of the LPC. Reaching steady state takes about 200 min. LPC will reach the steady state long before the HPC. This strategy is used for the comparison of the different strategies.
8.1.2. Thermally independent Fig. 10 shows the comparison of temperature curves for fully thermally coupled and thermally independent startup operations. Using the thermally independent strategy, both columns suffer disturbances when the coupling is switched on at time t= 130 min. Approaching steady state not only takes the same time as in case Section 8.1.1 but will consume more energy. Therefore it is useful to pay special attention to the startup of the HPC. The following presented experimental work is examined on the fully thermally coupled system. 8.2. Startup with total remo6al of distillate With the total removal of distillate strategy the startup time for a single column can be reduced up to 70% [4]. In this experiment the HPC is started up with the total removal of distillate strategy. Due to the heat-integration the LPC warms up automatically. As soon as the first vapour reaches the top of the LPC, reflux is added. As a result of the non-availability of reflux the impurity of the HPC’s top concentration increases in the meanwhile. Adding reflux lowers the top temperature immediately and will lead to a rise in purity (Fig. 11, t= 180 min.), whereby the corresponding pressure rises (Fig. 12). This effects a higher temperature in the HPC and so a greater difference in temperature in the heatintegrating condenser/reboiler. Consequently the LPC’s bottoms product temperature and purity will increase. So, the HPC suffers a delayed disturbance in feedconcentration. Comparing to the conventional strategy, by which the steady state operation point is determined
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Fig. 11. Temperature curves of the HPC.
Fig. 13. Temperature curves of the HPC.
after 340 min, the period of the startup procedure takes longer. This strategy, which reduces the startup time for a single column, shows no advantage for this column system.
pressure of the top exceeds three bar, reboiler duty is decreased to steady state value. After 220 min the column system reaches steady state (Fig. 13). Fig. 14 shows the pressure’s transient behavior. Only a slight overshoot in pressure over its steady state value is observed. As soon as the thermosiphon circulation ignites (at 3.5 bar), the steep increase of pressure in HPC is stopped. Startup with this strategy will lead to a time saving of 35% comparing to the conventional strategy.
8.3. Startup with increased reboiler duty The startup procedure begins with an increased reboiler duty in HPC. Consequently the LPC also receives a greater heat duty compared to its steady state operation point. It must be avoided that the pressure in HPC exceeds the pressure of steady state at nominal heat duty. As soon as pressure in HPC reaches its steady state value, the heat duty in HPC must be reduced to normal operation value. Otherwise the reboiler of LPC would be heated up to a higher temperature than desired. This would lead to a lower concentration of the light component in the reboiler of the LPC, which would result in a disturbance in feedconcentration on the HPC. Reaching steady state after such disturbance is delayed. Startup of the HPC begins with the 1.9-fold heat duty of the steady state value. As soon as the first vapour reaches the top of the column, heat duty is decreased to the 1.3-fold steady state value. When the
In this chapter experimental investigations of the startup of heat- and mass-integrated distillation columns are presented. The comparison between the fully thermally coupled startup strategy and the thermally independent strategy shows the superiority of the fully thermally coupled startup strategy in energy savings and the expenditure of time. Using the fully thermally coupled strategy the LPC will reach the steady state before the HPC. The total removal of distillate strategy, which reduces the startup time for a single column, shows no advantage for this column system. Startup with an increased reboiler duty will lead to a time saving of 35%.
Fig. 12. Pressure curve of the HPC.
Fig. 14. Pressure curve of the HPC.
8.4. Summary
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Fig. 15. Temperature curves of the HPC during a conventional startup.
Fig. 17. Temperature curves of the HPC during the optimized startup.
9. Optimized startup strategy
As soon as it reaches 98.5 mol% the manipulated variables are set to their steady state values. This switchover criterion and the values of the increased manipulated variables were deduced from the simulation results (chapter 5). The bottom composition is calculated online with a mathematical model [6], pressure compensated temperatures could be utilized also. Fig. 15 shows the measured temperature curves for the top and the bottom of the HPC during a conventional startup strategy. Fig. 16 shows the corresponding measured temperature curves of the top and the bottom of the LPC. The LPC will reach the steady state operation point long before the HPC. The column system needs 350 min for startup. The measured temperature curves for the top and the bottom of the HPC
In the experimental work presented below it is examined whether with the help of an optimized operation policy, developed for a switchover of the product purities, the period of startup can be reduced too. For validation the optimized startup strategy is investigated in experiments and compared with the conventional strategy. Using the optimized startup strategy the column is operated with an increased reboiler duty from the beginning of the startup procedure. As soon as enough liquid is available on the top of the HPC as well as of the LPC reflux with the increased value is added. The bottom composition is used as a switchover criterion.
Fig. 16. Temperature curves of the LPC during a conventional startup.
Fig. 18. Temperature curves of the LPC during the optimized startup.
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Table 1 Period of time [min] needed for reaching the steady state operation point Conventional
Optimized
350 (min) 230 (min) 100%
150 (min) 140 (min) 43%
WO 565/10-2. The authors thank DFG for the financial support.
Appendix A. Nomenclature HPC LPC
during the optimized startup strategy are shown in Fig. 17, the corresponding measured temperature curves of the top of the LPC in Fig. 18. Reaching steady state in LPC and HPC takes about 150 min. Startup with the optimized procedure will lead to a time saving of 57% (Table 1).
HPC LPC NT T
high pressure column low pressure column number of trays temperature
Subscripts j
tray
Superscripts SP
set point
References 10. Conclusions In this paper results of a new product switchover strategy for a heat- and mass-integrated distillation column system are presented. The computational results demonstrate that with the help of the optimized strategy the period for the switchover procedure in product composition from 90 – 99 mol% can be reduced by 57%. Furthermore, from this knowledge a time reduced startup strategy for coupled distillation systems is deduced. The experimental results demonstrate the superiority of this strategy. Comparing to the conventional startup strategy a timesaving of 57% can be achieved. This allows also a significant reduction of energy and raw material consumption and consequently lead to a more economical process.
Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft (DFG) under the contract .
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