- Email: [email protected]
Integrated crude distillation design
Computers them. Engng Vol. 19, Suppi.,pp.S119-S124,1995 Copyright 0 1995 Elsevier Science Ltd Printed in &eat Britain. All rights reserved 0098-1354(95)00162-X 009%1354/95 $9.50 + 0.00
Pergamon
INTEGRATED CRUDE DISTILLATION DESIGN K. LIEBMANN AND V. R. DHOLE Department of Process Integration, UMIST, P.O. Box 88, Manchester M60 1QD
ABSTRACT
This paper presents a systematic and integrated approach to the design of energy efficient crude distillation systems. Integrated implies simultaneous consideration of the options in the distillation system and the heat exchanger network. The proposed procedure is applicable for grass-roots as well as revamp situations. The approach is based on a combination of insights in distillation and Pinch Analysis. First, the complex refinery towers are decomposed into a sequence of simple columns. Then, the procedure explores the trade-off between steam stripping and reboiling and the impact of thermal coupling and pump arounds. The tower design changes are considered in simulation mode while the preheat train modifications are evaluated simultaneously based on Pinch Analysis targets. This combination allows quick identification of promising configurations at an early stage in the design. Using the proposed approach for new designs it is possible to identify radically different designs than typically used in the industry today.
KEYWORDS
Crude Distillation, Petroleum Refining, Pinch Analysis, Distillation Design, Refinery, Energy Efficiency, Refinery Revamp
INTRODUCTION
In 1993 about 700 refineries were operating world-wide (Rhodes, 1993). Although each of them processes a different crude oil and separates this crude oil into different products. the general configuration of the crude distillation systems is similar in all refineries. The configuration, consisting of the main tower with side strippers, has been constructed since more then 65 years (Miller and Osborne, 1938). Over the years the trade offs between energy and capital have changed drastically. It is, therefore, important to check the validity of this ‘traditional’ configuration. Furthermore, the crude distillation systems are configurations with highly interlinked columns handling complex multi-component mixtures. The distillation columns are closely connected to the preheat train. Due to these complexities the design of crude distillation systems is still carried out based on experience, some design guidelines and simulation trials. In addition, the crude tower is usually designed first followed by the design of the heat exchanger network as a second step with some iteration. Within the published literature there is yet no systematic approach that addresses optimum distillation configurations, number of stages in different sections of the tower, optimum pump around and feed locations and the interactions between these options and the design of the crude preheat train. s119
European Symposium on Computer Aided Process Engineering-5
S120
BASIC PRINCIPLES The following principles form the cometstones of the proposed procedure. wi In essence any crude distillation system can be considered as a sequence of simple cohmms. Typically, the crude distillation system involves a main tower lied to several side strippers. A crude tower with side strippers is thermodynamically equivalent to an indirect sequence of single columns, i.e. every column in the sequence splits the heaviest product only off its feed (Fig.11 (Glinos and Malone, 1985). For a single column of this sequence it is possible to calculate the appropriate munber of stages and the feed stage by using standard methods. The conventional approach mainly relies on trial and error to set the number of stages in different sections of the tower. In comparison the proposed approach identifies a better tray distribution with reduced design time. Gther advantages of using a sequence of simple columns are the easier simulation, convergence and the improved transparency regarding effects of various column design changes. Several sequences different from the typical indirect sequence (the main tower with side strippers1 can accomplish the same separation task. I
DEGREE OF VAPORISATION 1 01 O# aA w
owlcuLT
@we1: Sequence
0
EASY
of Columns
?lgure 2: Steam vs. Reboiler Efficiency
(ii1 Steam Striouinn vs. Reboilinn. The stripping of the light constituents from a liquid can be accomplished by the injection of live steam into the column bottom (reducing the partial pressure of the hydrocarbons1 or by reboiling. There are inherent differences between the two options. The results of using a t&oiler are: less vapour travellmg up the column lower condenser duty, higher condenser temperature and the installation cost of the &oiler. If live steam is used, the heat of vaporisation comes from the liquid inside the column, thus resulting in lower bottoms temperature. The relative efficiency of the two options is dependant on the degree of vaporisation desired due to the different mechanisms involved. Figure 2 shows the different degrees of vaporisation obtained if equal amounts of steam are used in a reboiler and as live stripping steam. Therefore, for high strip out ratea, a &oiler is more efficient than stripping steam and vice versa. (iii) Thermal Couolinq is defined here as the flow of material from ‘a downstream column INDIRECT SEQUENCE DIRECT SEQUENCE to an upstream column (Fig.3). If, for instance, liquid is withdrawn from a f@ column and returned to the previous column (FIg.3-a), the second column is practically a condenser to the first and the first acts as a side reboiler to the second. The major effects are lower pump around duty in the first and larger condenser duty in the second column. COUPLINQ Therefore, some of the heat previously (a) @I I available at higher temperature in the first igure 3: Thermal Coupling column (PA) is now degraded (condenser of column 2). In addition, the second column will possibly have a lower reboiler duty because of its increased vapour-liquid traffic. The same logic applies to thermal coupling in the direct sequence as shown in Fig.3-b. -
European Symposium on Computer Aided Process Engineering-5
s121
It is usual practice to design the crude tower first and then design the heat exchanger network for it. However, even if these two steps are done optimally, the result might still not be the best. Specifications made during the column design may induce restrictions for the heat exchanger network design. Only when the two design tasks are done simultaneously, these restrictions can be removed. The targeting tools provided by the Pinch Analysis (Linnhoff, 1993) allow the quick evaluation of decisions made in the column design with sufficient accuracy.
(iv)
NEW DESIGN PROCEDURE To obtain an energy efficient grassroots design, the above mentioned principles are applied in a coherent manner. Starting from a given sequence of single columns, set. up to have the beat heat recovery opportunities, the separation system is .developed to improve the separation efficiency and therefore, reduce the energy consumption. The method involves rigorous column simulations in parallel,with Pinch Analysis targets for the heat exchanger network. 1. Sequence of Columns. The design of a crude tower is initiated with a given sequence of simple columns. For all the columns the number of stages and feed stage location can be set according to the feed composition to the columns and the required product purities. Note, that there are several possible start-sequences that will produce the required products. If no direct thermal coupling is introduced yet, all the heat sources (i.e. pump arounds, condensers) are at their highest possible temperature level (Fig.3). Initially, steam stripping is considered for all the columns. Therefore. all the heat sinks are at the lowest temperature level. This represents the beat heat recovery possibilities within the system., Starting from there, the separation system can be evolved to improve the separation efficiency without significant penalty to the heat recovery. 2. Satisfv Heat Sinks. The availability of all the heat sources at their highest possible temperature level provides the possibility to satisfy some of the heat sinks. There are two options to accomplish that, either the introduction of thermal coupling between columns that are in direct sequence @ig3-bl or the replacement of some of the stripping steam with reboiling. The latter implies the installation of t&oilers. If this step is taken the above mentioned trade-offs between steam and reboiling (Fig.21 are explored. Every design change is checked against its related change in the Pinch Analysis targets for energy consumption. Temperature constraints due to cracking have to be considered in this step. ,
WATER LN
3. Relax Heat Sources. After all the :heat-sinks are satisfied (all the decisions on thermal coupling by vapour recycle have been made and all possible reboilers are in place) one can relax the excess heat sources i.e. allow for thermal coupling between columns in indirect sequence (Fig.3 left). The excess heat from a pump around can now be rejected to a lower temperature level (Fig.3). Due to the improved separation efficiency, the boil up requirements will reduce resulting in further energy savings. 4. Merge. Finally, with the same logic that allows to decompose a tower into single columns, these single columns can be reunited to form refinery towers. Case Studv. Figure 4, 5 and 6 show the results of applying the above procedure to a case from Watkins 0979). Figure 4 shows the original design.
HN-ST HN
LD 24 .............
HBST HO ..............
RES-ST . RES
FURNACE
52.57 MW
UTILITY COST
6.55 MM$/YR
lgure 4: Watkins (1979) Design
s122
European Symposium on Computer Aided Process Engineering-5
If the procedure is started with an indirect sequence of single columns, the same configuration as proposed by Watkins and usually found in refineries will result (Fig.5). A reduction of l!J% in utility cost, involving a 14% decrease in furnace load and a 42% reduction of stripping steam import, could be obtained without any significant change in capital expenditure due to the synergic effects of the simultaneous consideration of column and preheat train and the better tray distribution.
v
_RES
FURNACE
45.33 MW
FURNACE
34.96 MW
UTILITY COST
5.28 MM$/YR
UTILITY COST
4.95 MM$/YR
I
igure 5: Usual Configuration
Figure 6: Different Configuration
If, however one starts with another sequence of simple columns, radically different designs will emerge. In the case of the direct sequence as the starting point, a crude tower as shown on Fig.6 is the result of the application of the above procedure. The feed is at the top of the main tower. Therefore, there is now no need to heat the whole feed to high temperatures. Only already reduced crude is drawn as side stream from the column and heated. Instead of side strippers this tower has side rectifiers. This design features a further 5% reduction of the cost of the utilities. As compared with the original design from Watkins the flue gas demand is reduced by 33% and the stripping (low pressure) steam consumption is increased by 8%. The technical data supporting the figures shown and more detailed explanation can be found in Liebmann (1995). From the different results developed based on different initialisations possible one with a low overall cost, allowing for flexibility and operability considerations, should be chosen.
REVAMP DESIGN PROCEDURE The majority of projects in crude oil distillation will nowadays be revamps. The revamp procedure uses the basic insights described above and simultaneous simulation and targeting. It is divided into three steps, each involving a marginally decreasing return on investment. Analyze Oriainal Confiauration. The first step involves a conventional analysis of the existing system to remove any losses due to mixing in the separation system and preheat train. This kind of losses are mostly the result of the historic development of the plant and the system boundaries. A simple example would be a stream that is being cooled and again reheated. Other examples that can be found are the remixing of already separated streams and the mixing of streams with different composition. Pinch Analysis will provide a quantification of the losses in the heat exchanger network. The pay out for the removal of these inefficiencies has to be checked by targeting methods such as
European Symposium on Computer Aided Process Engineering-S
S123
Pinch Analysis. Usually, it proves to be economical to undertake these modifications. However, practical constraints have to be considered. For the further analysis the original configuration is decomposed into sections. Thereby, the number of stages and feed stages are taken from the existing design. That implies that the internals of the columns remain unchanged after the analysis and, therefore, the investment is relatively low. All reboilers are initially replaced by steam stripping and all thermal coupling is removed. That leads to the configuration with the best possibilities for heat recovery - heat sources at high temperature and heat sinks at low temperature. From there the system is evolved in a similar fashion as described in steps two and three of the procedure for grassroots designs. The heat sinks are satisfied first and then the excess heat sources are relaxed. The result of this part of the study will be adjusted duties for pump arounds and reboilers and possibly suggestions for the proper placement of these items of equipment. The tray distribution and feed nozzles remain unchanged. Construct Ootimum Grassroots Confinuration and Comuromise. The next step involves more investment. Here, changes in the internals of the tower and the nozzles in addition to changes in the reboiler and pump around locations and duties will be considered. At first, the optimum grassroots design for the existing configuration is developed. Therefore, the starting point has to be the sequence corresponding to the existing system, usually an indirect sequence of simple columns plus the perhaps existing pre-fractionator or stabiliser or naphtha splitter. Then a number of stages according to the separation difficulty is assigned to every single column and feed stages are adjusted. In the same manner as described previously, the decisions about reboiler and pump around duties are made. If this sequence is then merged back into the original towers, the general configuration of the columns is unchanged. Only stage numbers in sections and nozzles are changed. Now, a compromise between the existing and the derived system has to be constructed. The objectives are to keep the existing nozzles or the internals of the towers. Check Neinhbouring Confinurations. As the next step we consider changes in the column configuration. One could think of a pre-fractionator as shown in Fig.7 or a post-fractionator to the existing tower (see Fig.8).
j:j If -IF
1)
:i:
2,I-F-
:I: ...
4)
3)
:j: j!
c2z Figure 7: Pre-Fractionator
/L!E
& Figure 8: Post-Fractionator Options
In selecting one of the options shown, one split is taken out of the main tower and performed in a separate column. With the same number of stages in the main tower and less separation load, the energy consumption and, at the same time, the vapour load in the main tower will be reduced. How to choose the right option is dependant on the objective of the actual study. If the throughput is to be increased and the hydraulics of the tower arc limiting, then the separation task of the limiting section of the tower has to be eased. There are two options: performing the separation originally done in the limiting section of the tower in a pre-fractionator or delay the split into a post-fractionator. If energy savings are the prime objective or the throughput is limited by the restrictions on one of the utilities, one has to identify and remove the energy bottleneck in the main tower by a pre- or postfractionator. This bottleneck is either the section that operates across the Pinch - usually an extended Pinch due to the utility Pinches - or the section that is most under-represented in its number of stages according to the number required for the separation task it performs. For the former case Pinch Analysis of the single columns will point towards the section that needs reduction of its energy consumption. In the latter case the section can be identified by comparing the tray numbers obtained for the grassroots
S124
European Symposium on Computer Aided Process Engineering-5
design with the existing number. The developed revamp design procedure has been applied successfully to two the atmospheric crude distillation case studies. One of the studies had a debottleneckiig objective while the other aimed at energy savings.
CONCLUSIONS A new methodology for the design of crude distillation systems has been developed. The main feature is the simultaneous design of the columns and the associated preheat train. Appropriate procedures for grass-roots as well as revamp situations have been developed. The approach uses insights in distillation principles and Pinch Analysis. After an initialisation that will result in a scheme with good heat recovery possibilities the Pinch Analysis Targets are used to evolve the distillation system and improve its separation efficiency. Typically, the crude distillation system involves a main tower linked to several side strippers. A crude tower with side strippers is thermodynamically equivalent to an indirect sequence of simple columns. In essence any crude distillation system can be considered as a sequence of simple columns. The approach clarifies the trade-off between steam stripping and reboiling. It also provides a clear understanding of the impact of thermal coupling, i.e. the flow of material from a downstream column to an upstream column, on column internal flows and energy consumption. In grassroots situations, the approach can be used to set the optimum number of stages in different sections of the main tower and the strippers. It can also determine the best feed and draw-off stages. In addition, location of the pump arounds and reboilers as well as their duties can be specified. As mentioned earlier the tower and the pre-heat train are designed simultaneously. Each tower design change is simulated rigorously while the resulting pre-heat train modifications are evaluated using the Pinch Analysis targets. This combination bypasses the complete design of the HEN resulting in considerably reduced design time. It is expected that significant differences in crude characteristics, product purity requirements, price regulations etc. would result in different column configurations. Using the proposed approach it is possible to identify radically different designs than typically used in the industry today. The revamp procedure commences by decomposition of the existing tower into a sequence of simple columns. Appropriate design modifications are then identified in this sequence. The modified design is then evolved so as to maintain the integrity with the existing system. The approach can consider limitations in changing the existing system hardware and identifies key practical modifications along with incremental benefits in implementing them. The whole methodology is applicable to any complex distillation system involving heat integration as well as thermal coupling and not only to crude oil distillation. Future work will explore these possibilities.
ACKNOWLEDGEMENT K. Liebmann wishes to thank the ‘Friedrich Flick Fijrdentngsstiftung’ for the provision of a study grant.
REFERENCES Glinos, K., Malone, M.F. (1985) Ind Eng. Chem Proc. Des. Dev. 24. 1087 Liebmann, K. (1995). Integrated Crude Distillation Design. Ph.D. Thesis (to be published) Linnhoff, B. (1993). Tmns IChemE 71 Part A. September 1993. 503 Miller, W., Osborne, H.G. (1938). History and Development of some Important Phases of Petroleum Refining in the United States. in The Science of Petroleum. Oxford University Press, London, New York, Toronto. Vol. II. 14651477 Rhodes, A.K. (1993). Worldwide Refining Report. Oil & Gas Journal. Dec. 20; 37 Watkins, R.N. (1979). Petroleum Refinery Distillation. Gulf Publishing
Pergamon
INTEGRATED CRUDE DISTILLATION DESIGN K. LIEBMANN AND V. R. DHOLE Department of Process Integration, UMIST, P.O. Box 88, Manchester M60 1QD
ABSTRACT
This paper presents a systematic and integrated approach to the design of energy efficient crude distillation systems. Integrated implies simultaneous consideration of the options in the distillation system and the heat exchanger network. The proposed procedure is applicable for grass-roots as well as revamp situations. The approach is based on a combination of insights in distillation and Pinch Analysis. First, the complex refinery towers are decomposed into a sequence of simple columns. Then, the procedure explores the trade-off between steam stripping and reboiling and the impact of thermal coupling and pump arounds. The tower design changes are considered in simulation mode while the preheat train modifications are evaluated simultaneously based on Pinch Analysis targets. This combination allows quick identification of promising configurations at an early stage in the design. Using the proposed approach for new designs it is possible to identify radically different designs than typically used in the industry today.
KEYWORDS
Crude Distillation, Petroleum Refining, Pinch Analysis, Distillation Design, Refinery, Energy Efficiency, Refinery Revamp
INTRODUCTION
In 1993 about 700 refineries were operating world-wide (Rhodes, 1993). Although each of them processes a different crude oil and separates this crude oil into different products. the general configuration of the crude distillation systems is similar in all refineries. The configuration, consisting of the main tower with side strippers, has been constructed since more then 65 years (Miller and Osborne, 1938). Over the years the trade offs between energy and capital have changed drastically. It is, therefore, important to check the validity of this ‘traditional’ configuration. Furthermore, the crude distillation systems are configurations with highly interlinked columns handling complex multi-component mixtures. The distillation columns are closely connected to the preheat train. Due to these complexities the design of crude distillation systems is still carried out based on experience, some design guidelines and simulation trials. In addition, the crude tower is usually designed first followed by the design of the heat exchanger network as a second step with some iteration. Within the published literature there is yet no systematic approach that addresses optimum distillation configurations, number of stages in different sections of the tower, optimum pump around and feed locations and the interactions between these options and the design of the crude preheat train. s119
European Symposium on Computer Aided Process Engineering-5
S120
BASIC PRINCIPLES The following principles form the cometstones of the proposed procedure. wi In essence any crude distillation system can be considered as a sequence of simple cohmms. Typically, the crude distillation system involves a main tower lied to several side strippers. A crude tower with side strippers is thermodynamically equivalent to an indirect sequence of single columns, i.e. every column in the sequence splits the heaviest product only off its feed (Fig.11 (Glinos and Malone, 1985). For a single column of this sequence it is possible to calculate the appropriate munber of stages and the feed stage by using standard methods. The conventional approach mainly relies on trial and error to set the number of stages in different sections of the tower. In comparison the proposed approach identifies a better tray distribution with reduced design time. Gther advantages of using a sequence of simple columns are the easier simulation, convergence and the improved transparency regarding effects of various column design changes. Several sequences different from the typical indirect sequence (the main tower with side strippers1 can accomplish the same separation task. I
DEGREE OF VAPORISATION 1 01 O# aA w
owlcuLT
@we1: Sequence
0
EASY
of Columns
?lgure 2: Steam vs. Reboiler Efficiency
(ii1 Steam Striouinn vs. Reboilinn. The stripping of the light constituents from a liquid can be accomplished by the injection of live steam into the column bottom (reducing the partial pressure of the hydrocarbons1 or by reboiling. There are inherent differences between the two options. The results of using a t&oiler are: less vapour travellmg up the column lower condenser duty, higher condenser temperature and the installation cost of the &oiler. If live steam is used, the heat of vaporisation comes from the liquid inside the column, thus resulting in lower bottoms temperature. The relative efficiency of the two options is dependant on the degree of vaporisation desired due to the different mechanisms involved. Figure 2 shows the different degrees of vaporisation obtained if equal amounts of steam are used in a reboiler and as live stripping steam. Therefore, for high strip out ratea, a &oiler is more efficient than stripping steam and vice versa. (iii) Thermal Couolinq is defined here as the flow of material from ‘a downstream column INDIRECT SEQUENCE DIRECT SEQUENCE to an upstream column (Fig.3). If, for instance, liquid is withdrawn from a f@ column and returned to the previous column (FIg.3-a), the second column is practically a condenser to the first and the first acts as a side reboiler to the second. The major effects are lower pump around duty in the first and larger condenser duty in the second column. COUPLINQ Therefore, some of the heat previously (a) @I I available at higher temperature in the first igure 3: Thermal Coupling column (PA) is now degraded (condenser of column 2). In addition, the second column will possibly have a lower reboiler duty because of its increased vapour-liquid traffic. The same logic applies to thermal coupling in the direct sequence as shown in Fig.3-b. -
European Symposium on Computer Aided Process Engineering-5
s121
It is usual practice to design the crude tower first and then design the heat exchanger network for it. However, even if these two steps are done optimally, the result might still not be the best. Specifications made during the column design may induce restrictions for the heat exchanger network design. Only when the two design tasks are done simultaneously, these restrictions can be removed. The targeting tools provided by the Pinch Analysis (Linnhoff, 1993) allow the quick evaluation of decisions made in the column design with sufficient accuracy.
(iv)
NEW DESIGN PROCEDURE To obtain an energy efficient grassroots design, the above mentioned principles are applied in a coherent manner. Starting from a given sequence of single columns, set. up to have the beat heat recovery opportunities, the separation system is .developed to improve the separation efficiency and therefore, reduce the energy consumption. The method involves rigorous column simulations in parallel,with Pinch Analysis targets for the heat exchanger network. 1. Sequence of Columns. The design of a crude tower is initiated with a given sequence of simple columns. For all the columns the number of stages and feed stage location can be set according to the feed composition to the columns and the required product purities. Note, that there are several possible start-sequences that will produce the required products. If no direct thermal coupling is introduced yet, all the heat sources (i.e. pump arounds, condensers) are at their highest possible temperature level (Fig.3). Initially, steam stripping is considered for all the columns. Therefore. all the heat sinks are at the lowest temperature level. This represents the beat heat recovery possibilities within the system., Starting from there, the separation system can be evolved to improve the separation efficiency without significant penalty to the heat recovery. 2. Satisfv Heat Sinks. The availability of all the heat sources at their highest possible temperature level provides the possibility to satisfy some of the heat sinks. There are two options to accomplish that, either the introduction of thermal coupling between columns that are in direct sequence @ig3-bl or the replacement of some of the stripping steam with reboiling. The latter implies the installation of t&oilers. If this step is taken the above mentioned trade-offs between steam and reboiling (Fig.21 are explored. Every design change is checked against its related change in the Pinch Analysis targets for energy consumption. Temperature constraints due to cracking have to be considered in this step. ,
WATER LN
3. Relax Heat Sources. After all the :heat-sinks are satisfied (all the decisions on thermal coupling by vapour recycle have been made and all possible reboilers are in place) one can relax the excess heat sources i.e. allow for thermal coupling between columns in indirect sequence (Fig.3 left). The excess heat from a pump around can now be rejected to a lower temperature level (Fig.3). Due to the improved separation efficiency, the boil up requirements will reduce resulting in further energy savings. 4. Merge. Finally, with the same logic that allows to decompose a tower into single columns, these single columns can be reunited to form refinery towers. Case Studv. Figure 4, 5 and 6 show the results of applying the above procedure to a case from Watkins 0979). Figure 4 shows the original design.
HN-ST HN
LD 24 .............
HBST HO ..............
RES-ST . RES
FURNACE
52.57 MW
UTILITY COST
6.55 MM$/YR
lgure 4: Watkins (1979) Design
s122
European Symposium on Computer Aided Process Engineering-5
If the procedure is started with an indirect sequence of single columns, the same configuration as proposed by Watkins and usually found in refineries will result (Fig.5). A reduction of l!J% in utility cost, involving a 14% decrease in furnace load and a 42% reduction of stripping steam import, could be obtained without any significant change in capital expenditure due to the synergic effects of the simultaneous consideration of column and preheat train and the better tray distribution.
v
_RES
FURNACE
45.33 MW
FURNACE
34.96 MW
UTILITY COST
5.28 MM$/YR
UTILITY COST
4.95 MM$/YR
I
igure 5: Usual Configuration
Figure 6: Different Configuration
If, however one starts with another sequence of simple columns, radically different designs will emerge. In the case of the direct sequence as the starting point, a crude tower as shown on Fig.6 is the result of the application of the above procedure. The feed is at the top of the main tower. Therefore, there is now no need to heat the whole feed to high temperatures. Only already reduced crude is drawn as side stream from the column and heated. Instead of side strippers this tower has side rectifiers. This design features a further 5% reduction of the cost of the utilities. As compared with the original design from Watkins the flue gas demand is reduced by 33% and the stripping (low pressure) steam consumption is increased by 8%. The technical data supporting the figures shown and more detailed explanation can be found in Liebmann (1995). From the different results developed based on different initialisations possible one with a low overall cost, allowing for flexibility and operability considerations, should be chosen.
REVAMP DESIGN PROCEDURE The majority of projects in crude oil distillation will nowadays be revamps. The revamp procedure uses the basic insights described above and simultaneous simulation and targeting. It is divided into three steps, each involving a marginally decreasing return on investment. Analyze Oriainal Confiauration. The first step involves a conventional analysis of the existing system to remove any losses due to mixing in the separation system and preheat train. This kind of losses are mostly the result of the historic development of the plant and the system boundaries. A simple example would be a stream that is being cooled and again reheated. Other examples that can be found are the remixing of already separated streams and the mixing of streams with different composition. Pinch Analysis will provide a quantification of the losses in the heat exchanger network. The pay out for the removal of these inefficiencies has to be checked by targeting methods such as
European Symposium on Computer Aided Process Engineering-S
S123
Pinch Analysis. Usually, it proves to be economical to undertake these modifications. However, practical constraints have to be considered. For the further analysis the original configuration is decomposed into sections. Thereby, the number of stages and feed stages are taken from the existing design. That implies that the internals of the columns remain unchanged after the analysis and, therefore, the investment is relatively low. All reboilers are initially replaced by steam stripping and all thermal coupling is removed. That leads to the configuration with the best possibilities for heat recovery - heat sources at high temperature and heat sinks at low temperature. From there the system is evolved in a similar fashion as described in steps two and three of the procedure for grassroots designs. The heat sinks are satisfied first and then the excess heat sources are relaxed. The result of this part of the study will be adjusted duties for pump arounds and reboilers and possibly suggestions for the proper placement of these items of equipment. The tray distribution and feed nozzles remain unchanged. Construct Ootimum Grassroots Confinuration and Comuromise. The next step involves more investment. Here, changes in the internals of the tower and the nozzles in addition to changes in the reboiler and pump around locations and duties will be considered. At first, the optimum grassroots design for the existing configuration is developed. Therefore, the starting point has to be the sequence corresponding to the existing system, usually an indirect sequence of simple columns plus the perhaps existing pre-fractionator or stabiliser or naphtha splitter. Then a number of stages according to the separation difficulty is assigned to every single column and feed stages are adjusted. In the same manner as described previously, the decisions about reboiler and pump around duties are made. If this sequence is then merged back into the original towers, the general configuration of the columns is unchanged. Only stage numbers in sections and nozzles are changed. Now, a compromise between the existing and the derived system has to be constructed. The objectives are to keep the existing nozzles or the internals of the towers. Check Neinhbouring Confinurations. As the next step we consider changes in the column configuration. One could think of a pre-fractionator as shown in Fig.7 or a post-fractionator to the existing tower (see Fig.8).
j:j If -IF
1)
:i:
2,I-F-
:I: ...
4)
3)
:j: j!
c2z Figure 7: Pre-Fractionator
/L!E
& Figure 8: Post-Fractionator Options
In selecting one of the options shown, one split is taken out of the main tower and performed in a separate column. With the same number of stages in the main tower and less separation load, the energy consumption and, at the same time, the vapour load in the main tower will be reduced. How to choose the right option is dependant on the objective of the actual study. If the throughput is to be increased and the hydraulics of the tower arc limiting, then the separation task of the limiting section of the tower has to be eased. There are two options: performing the separation originally done in the limiting section of the tower in a pre-fractionator or delay the split into a post-fractionator. If energy savings are the prime objective or the throughput is limited by the restrictions on one of the utilities, one has to identify and remove the energy bottleneck in the main tower by a pre- or postfractionator. This bottleneck is either the section that operates across the Pinch - usually an extended Pinch due to the utility Pinches - or the section that is most under-represented in its number of stages according to the number required for the separation task it performs. For the former case Pinch Analysis of the single columns will point towards the section that needs reduction of its energy consumption. In the latter case the section can be identified by comparing the tray numbers obtained for the grassroots
S124
European Symposium on Computer Aided Process Engineering-5
design with the existing number. The developed revamp design procedure has been applied successfully to two the atmospheric crude distillation case studies. One of the studies had a debottleneckiig objective while the other aimed at energy savings.
CONCLUSIONS A new methodology for the design of crude distillation systems has been developed. The main feature is the simultaneous design of the columns and the associated preheat train. Appropriate procedures for grass-roots as well as revamp situations have been developed. The approach uses insights in distillation principles and Pinch Analysis. After an initialisation that will result in a scheme with good heat recovery possibilities the Pinch Analysis Targets are used to evolve the distillation system and improve its separation efficiency. Typically, the crude distillation system involves a main tower linked to several side strippers. A crude tower with side strippers is thermodynamically equivalent to an indirect sequence of simple columns. In essence any crude distillation system can be considered as a sequence of simple columns. The approach clarifies the trade-off between steam stripping and reboiling. It also provides a clear understanding of the impact of thermal coupling, i.e. the flow of material from a downstream column to an upstream column, on column internal flows and energy consumption. In grassroots situations, the approach can be used to set the optimum number of stages in different sections of the main tower and the strippers. It can also determine the best feed and draw-off stages. In addition, location of the pump arounds and reboilers as well as their duties can be specified. As mentioned earlier the tower and the pre-heat train are designed simultaneously. Each tower design change is simulated rigorously while the resulting pre-heat train modifications are evaluated using the Pinch Analysis targets. This combination bypasses the complete design of the HEN resulting in considerably reduced design time. It is expected that significant differences in crude characteristics, product purity requirements, price regulations etc. would result in different column configurations. Using the proposed approach it is possible to identify radically different designs than typically used in the industry today. The revamp procedure commences by decomposition of the existing tower into a sequence of simple columns. Appropriate design modifications are then identified in this sequence. The modified design is then evolved so as to maintain the integrity with the existing system. The approach can consider limitations in changing the existing system hardware and identifies key practical modifications along with incremental benefits in implementing them. The whole methodology is applicable to any complex distillation system involving heat integration as well as thermal coupling and not only to crude oil distillation. Future work will explore these possibilities.
ACKNOWLEDGEMENT K. Liebmann wishes to thank the ‘Friedrich Flick Fijrdentngsstiftung’ for the provision of a study grant.
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