- Email: [email protected]
Energy-use analysis and improvement for delayed coking units
Energy 29 (2004) 2225–2237 www.elsevier.com/locate/energy
Energy-use analysis and improvement for delayed coking units Q.L. Chen a,, Q.H. Yin a, S.P. Wang b, B. Hua a a
The Key Lab of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, South China University of Technology, Guangzhou 510640, China b Department of Physics, Qingdao University, Qingdao 266071, China
Abstract This paper presents the energy and exergy analyses for a delayed coking unit in a Chinese refinery using the three-link energy structural model. According to the results of energy-use analysis and evaluation, the potentials of energy-use and improvements for energy-savings as well as detail procedures are suggested. The proposed improvements are expected to remarkably lower the energy consumption and to improve the economic profits of the delayed coking unit. Compared to the original unit, the energy consumption of the improved flowsheet decreased by 37.2%, which demonstrates that the applied strategies are suitable for the energy-use analysis and improvement of process systems. # 2004 Elsevier Ltd. All rights reserved. Keywords: Delayed coker; Energy-use analysis; Exergy analysis; Energy-saving
1. Introduction The delayed coking is one of the main processes for heavy oil processing in petroleum refining industry. It is essentially a high-temperature process involving extensive use of direct heat to up-grade products. The coking process, as a combined process of the severe thermal cracking and condensation reactions, needs to consume a large amount of high-grade energy [1]. Heavy oils, such as vacuum residue, cracking residue and catalyzed slurry oil, etc., undergo v severe thermal cracking and condensation reactions at a high temperature of 500 C, and such products as dry gases, liquefied petroleum gases, gasoline, diesel, heavy gas oils and cokes are produced. All heat required by the strong endothermic coking reactions is provided by the
Corresponding author. Tel.: +86-20-87113744; fax: +86-20-85511507. E-mail address: [email protected] (Q.L. Chen).
0360-5442/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2004.03.021
2226
Q.L. Chen et al. / Energy 29 (2004) 2225–2237
coking heater. The high-temperature reaction vapors at a superheated state are introduced into the main fractionating column from the coke drums. Different coking products are separated from the fractionating system in liquid or gas state at relatively low temperatures which carry a large quantity of low-grade heat. As with other refining plants such as crude topping, catalytic reforming, etc. [2,3], significant energy-conservation opportunities exist in a delayed coker. To efficiently lower the energy consumption of the coking unit, it is crucial to increase the efficiency of the coking heater and to utilize the low-temperature heat. In this paper the energy and exergy analyses of a delayed coker in a Chinese refinery are presented. Accordingly, some improvements and corresponding optimal measures are proposed in detail based on the threelink structural models for energy-use analysis and optimization of process systems [4,5].
2. Energy-use analysis for the delayed coking process 2.1. Description of the delayed coker A delayed coker in a Chinese refinery, which was designed and put into operation in 1996, is taken as an example to study the energy-use analysis and improvement. The simplified flow diagram of this delayed coker is shown in Fig. 1. The processing capacity designed was 500 kt/a (62.5 t/h), and the recycle ratio designed was 0.4. The feedstocks processed is the Daqing vacuum residue. Actual data are obtained from a typical operating day. Table 1 gives an overall
Fig. 1. Simplified flow diagram of the delayed coker being analyzed (1—heater; 2,3—coke drums; 4—4-way switch valve; 5—fractionating column; 6—gas–liquid separation drum; 7—steam generator; 8—heat exchanger; 9—cooler/ condenser; 10—pumps).
Q.L. Chen et al. / Energy 29 (2004) 2225–2237
2227
Table 1 Overall material balance for the delayed coking unit (kg/h) Stream
Input
Vacuum residue Naphtha Diesel Heavy gas oil Waste oil Coke Wet gas Total
62,500
Output 11,040 15,050 19,550 1720 9830 5370 62,500
62,500
material balance for the delayed coking unit. The specifications of the coking products are shown in Table 2. The practical energy consumption of this delayed coker in 2000 for processing a ton of fresh feedstocks was as high as 1500.0 MJ/ton feedstock due to the less usable heat sinks within the unit itself, and the lack of the integrated consideration of its energy use with the auxiliary systems, such as utilities, storage and transportation systems within the whole refinery. The delayed coker in which the fresh feedstocks is preheated, coked and fractionated, consists v of one heater and two coke drums. The fresh feedstocks, which is heated up to about 350 C via the heat exchange with the hot product flows and the flue-gas in convection section is introduced into the bottom of the fractionating column to quench the high-temperature superheated reaction vapors. A portion of the heavier ends in the reaction vapors is condensed for recycling, and a portion of lighter ends in the fresh feedstocks is vaporized meanwhile. The ratio of the condensed recycling heavier parts to the fresh feedstocks is defined as the recycle ratio of the coking unit. The preheated feedstocks from the bottom of the column, together with the condensed heavier ends from the reaction vapors, is pumped into the radiation house of the coking v heater and quickly heated to slightly below 500 C. After partially vaporized in the heater tubes and passed through a 4-way switch valve, the feedstocks are then introduced into one of the two coke drums where the coking reactions are taken place. Water with high pressure is therefore injected into the heater tubes to minimize the coke deposition and to delay the coking reactions in the tubes. The superheated reaction vapors drawn out from the top of the coke drums are then back to the base of the fractionating column, and are further separated into various products according to their boiling points such as wet gas, naphtha, light gas oil (diesel) and heavy gas oil. Three pumparounds are added to effectively recover the thermal energy of the column. Table 2 Specifications of the coking products v
v
v
Stream
ASTM D86 EP ( C)
Flashing point ( C)
Freezing point ( C)
Residual carbon (%)
Naphtha Diesel Heavy gas oil
6> 200 6> 350 –
– 6< 66 –
– 6> 0 –
– – 6> 0:80
2228
Q.L. Chen et al. / Energy 29 (2004) 2225–2237
Part of diesel is further cooled and used as the absorption oil in the unit of gasoline stabilization, and the rich absorption oil returns to the fractionating column after preheated by the diesel, which could be considered as a diesel pumparound. The coke from the condensation reactions is cooled and removed after the coke drums are filled to the safe margin from the top. 2.2. Division of the three links/subsystems The delayed coker being analyzed could be divided into three links/subsystems based on the functions and evolutions of energy according to the three-link energy structural model proposed by Hua, et al. [4,5], i.e. energy-conversion link/subsystem, energy-use link/subsystem and energy-recovery link/subsystem. The components such as fractionating columns, coke drums, etc., which perform the material transformation and separation via the role of energy, belong to the energy-use link. The components such as the coking heater, reboilers, and pumps, etc., which fulfill the conversion between different types of energy, belong to the energy-conversion link. The coolers, condensers, heat exchangers and steam generator, etc., which recover the heat or cool the target product flows, are affiliated to the energy-recovery link. Both the energy-conversion subsystem and the energy-recovery subsystem, as the auxiliary subsystems in process systems, serve mainly for the energy-use subsystem. Thermodynamic analysis is quite useful in identifying the true inefficiencies in a process. Based on the framework of three-link model, the energy and exergy analyses are conducted for all energy-use processes and equipment within the three subsystems of the delayed coker being analyzed. The coking heater, heat exchanger networks and distillation operations are among the major candidate locations for energy-use improvement in a delayed coker. On the basis of the energy-use analysis, evaluation and improvement, the energy-use schemes for both partial and total systems, therefore, can be optimized through the decomposition–coordination optimization strategies [5,6]. Many optimization methods such as pinch analysis, optimization techniques for energy-conversion systems, heuristic method, artificial intelligence, etc. may be used in analysis and optimization of the coking system being considered. 2.3. Energy-use analysis and evaluation 2.3.1. Energy-conversion subsystem The energy-conversion subsystem mainly consists of the coking heater and pumps, etc. The energy-use status in this subsystem depends mainly on the fuel oil/gas consumption by the coking heater. The results of energy and exergy analyses for the coking heater being analyzed are summarized in Table 3. It shows that the thermal and exergy efficiencies of the coking heater are 87.0% and 45.0%, respectively. The relatively low efficiencies can be attributed in part to the high stack temperature as well as the above normal excess air. The excess air coefficient of the v coking heater is as high as 1.5, and the temperature of the rejected flue-gas is 205 C, which results in loss of more flue-gas thermal exergy, decrease of the coking heater efficiency, and the increase of the fuel consumption accordingly. Moreover, the analysis of the pump operations shows that the unit efficiencies of part pumps such as the pump for feeds of the radiation house, and the pump of the heavy gas oil pump-
Q.L. Chen et al. / Energy 29 (2004) 2225–2237
2229
Table 3 Exergy efficiency of the coking heater Items
Value
Total supplied exergy (kW) Exergy destruction (kW) Heat losses (kW) Exergy losses (kW) Thermal efficiency (%) Exergy efficiency (%)
25,890 13,050 3350 1190 87.0 45.0
around, etc. in the coking unit are too low due to the corresponding large pressure losses, which consequently causes huge electricity consumption in the whole unit. 2.3.2. Energy-use subsystem The energy-use subsystem is mainly composed of the coke drums, fractionating columns, etc. In the coke drums several processes happen, such as the high-temperature thermal cracking and condensation reactions. Due to the restriction of the improving potentials, we do not emphasize here the energy-use analysis in the coke drums. The heat removal of different pumparounds in the fractionating column is shown in Table 4. From Table 4, it can be seen that the distribution of heat removals among different pumparounds in fractionating column is unreasonable. The heat removal from the low-temperature top and diesel pumparounds is relatively high under the existed operation conditions. Meanwhile, the heat removal from the high-temperature intermediate and heavy gas oil pumparounds is too low, which causes the exergy destruction of the fractionating column too large. Furthermore, the operational recycle ratio of the coking system is as high as 0.4, which increases the heat load and the fuel consumption of the coking heater. 2.3.3. Energy-recovery subsystem The energy-recovery subsystem in the coking unit being analyzed mainly relates to the coolers, condensers, heat exchangers and steam generators, etc. The status of the energy and exergy Table 4 Heat removal of the coking fractionating column Items
Top pumparound Diesel pumparound Intermediate pumparound Heavy gas oil pumparound Total
Flow rate (kg/h)
Temperature v in/out ( C)
Energy
35,500
70/156
2010
22.5
490
13.5
16,000
120/240
1250
12.5
460
12.6
44,850
200/290
3250
35.9
1440
39.6
29,000
220/330
2530
29.1
1250
34.3
–
–
9040
100.0
3640
100.0
kW
Exergy %
kW
%
2230
Q.L. Chen et al. / Energy 29 (2004) 2225–2237
Table 5 Exergy analysis results for the energy-recovery subsystem Items
Value
Total recoverable energy (kW) Total recoverable exergy (kW) Energy effectively recovered (kW) Exergy effectively recovered (kW) Exergy destruction (kW) Heat given to air and cooling water (kW) Exergy given to air and cooling water (kW) Energy recovery efficiency (%) Exergy recovery efficiency (%)
22,540 8227 12,805 4508 1524 9735 2195 56.8 52.0
recoveries in the coking unit is shown in Table 5. It can be found that the energy-recovery efficiency is 56.8% and the exergy recovery efficiency is 52.0%. The low recovery efficiencies indicate that the improvement of the energy recovery is the major focus for energy-saving retrofit of the delayed coker. At present, the delayed coker only produces a little steam of 1.0 MPa due to the unreasonable design and operation of the heat exchanger network, and a large quantity of low-temperature surplus heat is cooled by air and cooling water, and wasted. The relatively high average heat-transfer temperature differences for the effective recovery of the heat of the product flows cause a large exergy destruction in the heat-transfer processes. Furthermore, large exergy destruction and losses are also caused by the less usable heat sinks within the delayed coker itself, and by the lack of the heat integration of the delayed coker with other process units, steam and power systems and the storage and transportation systems, etc. in the whole refinery. It can also be seen that the thermal energy discarded by air or cooling water is 9735 kW, which demonstrates that the low-temperature heat recovery for re-use is crucial to lower the energy consumption of the whole unit. 2.3.4. Existing energy-use problems The energy and exergy analyses of the whole delayed coker indicate that the index of the energy consumption for processing a ton of fresh feedstocks in this coking unit is high at 1500.0 MJ/ton feedstock. The energy and exergy conversion efficiencies in energy-conversion subsystem are 84.2% and 43.2%, respectively, and the energy and exergy recovery efficiencies in energy-recovery subsystem are 56.8% and 52.0%, respectively. The total energy and exergy used in the whole process are 1956 MJ/ton feedstocks and 871.8 MJ/ton feedstocks, respectively. The energy-use indices outlined above demonstrate that the energy consumption of the coking unit is relatively high and the exergy destruction is mainly from the coking heater. The low-temperature energy losses in the energy-recovery subsystem are a little large. The energy-use problems of the delayed coker are mainly related to the following: (A) A large quantity of low-temperature heat is wasted: the low-temperature heat from the v v overhead vapors of the fractionating column (130–65 C), top pumparound (115–80 C), v v diesel (150–60 C) and heavy gas oil (130–80 C) etc. is given off to the cooling water or air, and wasted.
Q.L. Chen et al. / Energy 29 (2004) 2225–2237
2231
(B)
Poor design of the steam generating system: the present delayed coker which was designed to produce the steam of 1.0 MPa can only generate the steam of 0.6 MPa due to the unreasonable design and operation. It is found that the pressure of the steam generated is too low to enter the main steam networks of 1.0 MPa, and most of steam of 0.6 MPa generated has to be vented away to the ambient due to the lack of proper users. (C) Unreasonable design for the distribution of heat removals from different pumparounds in the fractionating column: The heat removal from the top pumparound and the diesel pumparound is relatively high under the present operation conditions. Meanwhile, the heat removals from the high-temperature heavy gas oil pumparound and the intermediate pumparound are too low. (D) Poor design of heat exchanger network (HEN): the relatively low-temperature fresh feedstocks is directly preheated by the high-temperature intermediate and heavy gas oil pumparounds, and the large temperature difference causes a large exergy destruction. Furthermore, the area of the heat exchanger for steam generating is too small, which makes the heat load of the subsequent heat exchanger of feedstocks and heavy gas oil too large. The high output temperature of the fresh feedstocks from the convection section makes the temperature at the bottom of the column too high, which has the influences on the whole heat balance within the fractionating column and the operation stability of the whole coking unit. (E) In accordance with integration and optimization process, the poor heat integration of the delayed coker with other process units, steam and power systems and the auxiliary storage and transportation systems, etc. in the whole refinery would result in the energy-use inefficiencies of the delayed coking system.
3. Energy-use improvement for the coking process Obviously, the retrofit for the coking process only can be done on the basis of the original flowsheet, which may involve the following process modifications: (1) adjustment of the operational parameters; (2) changes in the interconnections between process equipment; (3) replacement of the original equipment by other equipment; (4) change in the sizes of some original equipment in the existing process. Based on above energy-use analysis and evaluation, the three subsystems are optimized, respectively, and an optimal flowsheet is finally proposed. 3.1. Division of the improvement measures It is the major focus for energy-use optimization of petroleum refining processes to decrease the total energy used and the exergy destruction in process, and to improve the energy (exergy) recovery and conversion efficiency. Based on the three-link energy and exergoeconomic models, all measures to improve the energy-use situation in process systems can be summarized as the following four types in terms of their functions [7]: U–type: improving energy/exergy conversion efficiency gU (gXU) in energy-conversion subsystem.
2232
Q.L. Chen et al. / Energy 29 (2004) 2225–2237
N–type: decreasing total energy/exergy used in process EN (ExN) in the energy-use subsystem. K–type: decreasing the exergy destruction DKP in energy-use subsystem. R–type: improving energy/exergy recovery efficiency gR (gXR) in energy-recovery subsystem. The core energy-use process improvement decides, to a large extent, the boundary conditions for energy recovery of the energy-recovery subsystem and the energy supply by the energy-conversion subsystem. The energy use of a delayed coking process has typical characteristics of the high-grade energy consumed, the large process exergy destruction and the more low-temperature surplus heat produced. The low-temperature heat produced in the coking process can hardly be balanced within the unit itself. Nearly, 80% of the energy supplied from outside to the coking unit such as fuel oils/gases or electricity, etc., are used mainly to preheat the feedstocks and the water injected to radiation tubes for providing the endothermic reaction heat of the severe thermal cracking of heavy feedstocks [1,8]. Thus, it is very important for reducing energy consumption of the coking unit to increase the preheated load of the feedstocks, to lower the heat duty and improve the exergy efficiency of the coking heater, and to recover effectively the heat from the fractionating column, as well as to extensively utilize the heat sinks inside or outside the unit, etc. Based on above energy and exergy analyses, the improvement measures and corresponding optimal energy utilization schemes are proposed for different subsystems and the total system. 3.2. Process improvement 3.2.1. I The improvements of the coking techniques to lower the consumption of materials and energy simultaneously from the source. Optimizations of the operational recycle ratio and the quantity of water injected into the radiation tubes would lower the heat load of the coking heater and the fuel consumption from the source. When the operational recycle ratio decreased from 0.4 to 0.2, the flowrate of the feeds to the radiation house of the coking heater decreased from 87.5 to 75 t/h. The heat load of the coking heater decreased about 1400 kW. At the same time, by decreasing the water injection into the radiation tubes can also lower the heat load of the coking heater. 3.2.2. II Optimization for heat removal of pumparounds to decrease the exergy destruction of fractionating processes: in order to decrease the exergy destruction of the fractionating process, the distribution of the heat removals from different pumparounds in the fractionating column is optimized. After the optimization of the operational recycle ratio, the total heat entering the fractionating column is decreased, which makes the high-temperature heat removal more difficult. Under the present flowsheet conditions, the heat removals from different pumparounds are adjusted with the aid of the process simulation techniques. The influence of the modified operational parameters or input variations on the outputs of the unit is estimated through the process simulation software of PRO/II on the premise of the same mass flowrates and specifications of the products. The heat removals for the intermediate and the heavy gas oil
Q.L. Chen et al. / Energy 29 (2004) 2225–2237
2233
Table 6 Change of pumparound heat removals of the fractionating column (kW) Items
Before retrofit After retrofit
Top pumparound
Diesel pumparound
Intermediate pumparound
Heavy gas oil pumparound
Heat
Exergy
Heat
Exergy
Heat
Exergy
Heat
Exergy
2010
490
1250
460
3250
1440
2530
1250
1250
315
1960
620
3180
1410
2850
1410
pumparounds are reached to the maximum to produce more steam with pressure of 1.0 MPa after optimized. The pumparound heat removals before and after retrofit are shown in Table 6. 3.3. Improvement in energy-recovery subsystem The improvement of the energy-recovery subsystem in the delayed coking unit relates mainly to the heat exchanger network optimization. The thermal energy contained in the various product flows is mainly used to preheat the fresh feedstocks of the vacuum residue prior to final heating in the coking heater, and to generate the steam of 1.0 MPa. For the delayed coking unit, the cold process streams include the fresh feedstocks, softened water, deaerated water, and the water injected into radiation tubes of the coking heater, and the hot streams include the different product flows and the fluegas of the coking heater. Through the core energy-use process improvement, the relevant data of hot and cold streams for the subsequent HEN retrofit are shown in Table 7. 3.3.1. I Retrofit of the steam generating system and the heat exchanger networks (HEN). In accordance with the principles of the second law analysis, the high-temperature heat in the delayed coking unit should be recovered to produce more 1.0 MPa steam. Based on the adjustment of the heat removal of different pumparounds, the HEN is optimized through the pinch analysis, which mainly includes the adjustment of the preheating of the feedstocks. The original heat exchange between the rich absorption oil and the diesel is cancelled, and the rich absorption oil enters the fractionating column directly. The modified flowsheet for feedstocks preheating is shown in Fig. 2, in which the feedstocks are firstly preheated by the diesel. The high-temperature heat of the heavy gas oil, then, can be saved to generate more steams. About 8.5 t/h of the 1.0 MPa steam is generated after the optimization of HEN and the steam pipelines. More 4.5 t/ h steam of 1.0 MPa is exported to the main steam networks from the retrofitted coking unit due to the elimination of the steam venting. 3.3.2. II Heat integration with other installations to increase the heat outputs and to lower the heat cooling losses. There exists a large quantity of low-temperature heat in the delayed coker, and it is difficult to find the reasonable users to balance it within the unit itself. The cooling curve of the overhead vapors from the fractionating column through the simulation software of PRO/II
2234
Q.L. Chen et al. / Energy 29 (2004) 2225–2237
Table 7 Data of hot and cold streams in HEN retrofit of the coking unit Process streams
Heavy gas oil, heavy gas oil pumparound and quenching oil Heavy gas oil and heavy gas oil pumparound Heavy gas oil Diesel and lean absorption oil Lean absorption oil Top pumparound Intermediate pumparound Overhead vapors Fresh feedstocks Feeds to convective section Feeds to radiation house Steam of 1.0MPa Preheating of softened water Water injected into tubes Deaerated water Recycling medium water
Flowrate (kg/h)
v
Temperature ( C)
Heat load (kW)
Initial
Target
58,050
340
230
5297
50,050
230
220
368
19,550 30,550
220 228
65 60
2121 3467
15,500 29,000 50,500
60 145 287
40 75 210
185 1250 3180
21,540 62,500 62,500
124 90 215
40 215 325
4320 5540 5610
75,000
375
500
9720
8500 15,000
150 35
180 75
5065 698
3200
40
170
484
8500 250,000
104 40
150 65
455 7267
Fig. 2. Modified flowsheet for preheating of feedstocks (1—fractionating column; 2—coke drums; 3—convection section; 4—radiation house).
Q.L. Chen et al. / Energy 29 (2004) 2225–2237
2235
Fig. 3. Flow diagram of the low-temperature heat recovery through a closed recycling system. v
v
shows that the latent heat of the overhead vapor is mainly between 105 C and 55 C, about 3610 kW. At present, all the low-temperature heat is given off to air or cooling water. In this optimization, the low-temperature heat from overhead vapors, top pumparound, diesel and heavy gas oil of the fractionating column are all recovered and used to preheat the fresh water in a water treatment plant through a closed recycling medium water system. The flowsheet of the closed recycling medium water is shown in Fig. 3. A heater and a cooler should be added in this recycling flowsheet to maintain the steady operation of the heat recovery system. The annual average low-temperature heat recovered is about 5115 kW. 3.3.3. III Application of heat-transfer enhancement techniques: the heat-transfer enhancement techniques, heat exchanger optimal design techniques and the optimal synthesis softwares for HEN are extensively applied in this retrofit optimization of the delayed coker. The enhanced tubes, such as transversally corrugated tubes and low fin tubes, etc. are used in the renewed or newly added heat exchangers and steam generators so as to enhance the heat transfer and to lower the investment cost. 3.4. Improvement in energy-conversion subsystem Improvement in energy-conversion subsystem is mainly related to the retrofit of energy-conversion devices such as the coking heater, pumps, etc. Based on the optimal process parameters, a significant improvement in efficiency is possible for the coking heater if energy-saving measures such as applying the advanced combustion techniques, decreasing the excess air coefficient, adopting low oxygen content operation. Furthermore, some low-temperature heat sinks such as combustion air and feedwater preheating are introduced in this delayed coker energy-use retrofit for further decreasing the flue-gas rejected temperature and improving the heater efficiency. v When the flue-gas rejected temperature is lowered to 170 C, about 800 kW more heat may be effectively recovered. When the excess air coefficient of the heater is reduced to 1.25, about 500 kW fuel can be saved. Meanwhile, related electricity saving measures such as frequency control, etc. can be used to decrease the power losses from the high-pressure hydraulic pump for coke removal and other process pumps.
2236
Q.L. Chen et al. / Energy 29 (2004) 2225–2237
Table 8 Economic and energy consumption indices for retrofit of the delayed coker Items
Values
Total investment (103 $) Profit before tax (103 $) Payoff period (year) Energy consumption before retrofit (MJ/t) Energy consumption after retrofit (MJ/t)
658.5 462.6 2.73 1500.0 941.65
3.5. Energy consumption and main economic indices More steam with pressure of 1.0 MPa is exported, and more low-temperature heat is recovered through the above-mentioned energy-use retrofit of the delayed coker, which causes a remarkable decrease in energy consumption. The energy consumption and the main economic evaluation indices such as the investment and profit for this energy-use retrofit of the delayed coker are shown in Table 8. We can see from Table 8 that a substantial energy consumption decrease is achieved through the coking unit retrofit, and the payback time is less than 3 years for this energy-use improvement.
4. Conclusions Analysis and evaluation of energy use in the existing delayed coker studied in this paper reveal that there exist great potentials for energy-use improvements in three subsystems of energy-conversion, energy-use and energy-recovery, and the improvement of the core energy-use subsystem is decisive for the total system optimization. Significantly improved process efficiency is possible for the delayed coking unit being analyzed through the realization of the corresponding energy-saving measures proposed according to energy-use characteristics of the three subsystems which mainly include to optimize the operational recycle ratio of the unit and the heat removals from pumparounds of the fractionating column, to effectively utilize the heat from the high-temperature heavy gas oil to produce more 1.0 MPa steam, and to re-use the low-temperature heat from the coking process to preheat the fresh water from a water treatment plant. The economic profits of the delayed coking unit are increased remarkably with relatively low capital investment. The energy consumption of the improved coking unit decreased by 37.2%, which proves that the three-link analysis and optimization strategies applied are effective for energyuse improvement of process systems.
Acknowledgements The financial support from the Major State Basic Research Development Program (G2000026307), the Natural Scientific Foundation of China (20076018) and the Guangdong Provincial Natural Science Foundation of China (990638) is gratefully acknowledged.
Q.L. Chen et al. / Energy 29 (2004) 2225–2237
2237
References [1] Jones DSJ. Elements of petroleum processing. New York: J. Wiley & Sons; 1995. [2] Ballut AA. Heat recovery analysis of an existing crude distillation unit. Heat Recovery Systems & CHP 1986;6:361–7. [3] Ballut AA. Energy conservation in a catalytic reforming plant. Heat Recovery Systems & CHP 1989;9:493–7. [4] Hua B, Shen JF. The energy synthesis of fractionating tower networks. Proceedings of the 4th International Symposium on Process System Engineering, Canada. Design. Montebello, vol. I. 1991, p. I.12.1–I.12.14. [5] Chen QL, Hua B. Application of exergoeconomics to the analysis and optimization of process systems. Journal of Thermal Science 1996;5(1):1–10. [6] Chen QL, Yin QH, Hua B. An exergoeconomic approach for retrofit of fractionating systems. Energy—The International Journal 2002;27(1):65–75. [7] Hua B, Yin QH. Energy optimization through exergy-economic evaluation. Journal of Energy Resources Technology 1989;111(3):148–53. [8] Rodriguez-Reinoso F, Santana P, Romero Palazon E, et al. Delayed coking: industrial and laboratory aspects. Carbon 1998;36(1):105–16.
Energy-use analysis and improvement for delayed coking units Q.L. Chen a,, Q.H. Yin a, S.P. Wang b, B. Hua a a
The Key Lab of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, South China University of Technology, Guangzhou 510640, China b Department of Physics, Qingdao University, Qingdao 266071, China
Abstract This paper presents the energy and exergy analyses for a delayed coking unit in a Chinese refinery using the three-link energy structural model. According to the results of energy-use analysis and evaluation, the potentials of energy-use and improvements for energy-savings as well as detail procedures are suggested. The proposed improvements are expected to remarkably lower the energy consumption and to improve the economic profits of the delayed coking unit. Compared to the original unit, the energy consumption of the improved flowsheet decreased by 37.2%, which demonstrates that the applied strategies are suitable for the energy-use analysis and improvement of process systems. # 2004 Elsevier Ltd. All rights reserved. Keywords: Delayed coker; Energy-use analysis; Exergy analysis; Energy-saving
1. Introduction The delayed coking is one of the main processes for heavy oil processing in petroleum refining industry. It is essentially a high-temperature process involving extensive use of direct heat to up-grade products. The coking process, as a combined process of the severe thermal cracking and condensation reactions, needs to consume a large amount of high-grade energy [1]. Heavy oils, such as vacuum residue, cracking residue and catalyzed slurry oil, etc., undergo v severe thermal cracking and condensation reactions at a high temperature of 500 C, and such products as dry gases, liquefied petroleum gases, gasoline, diesel, heavy gas oils and cokes are produced. All heat required by the strong endothermic coking reactions is provided by the
Corresponding author. Tel.: +86-20-87113744; fax: +86-20-85511507. E-mail address: [email protected] (Q.L. Chen).
0360-5442/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2004.03.021
2226
Q.L. Chen et al. / Energy 29 (2004) 2225–2237
coking heater. The high-temperature reaction vapors at a superheated state are introduced into the main fractionating column from the coke drums. Different coking products are separated from the fractionating system in liquid or gas state at relatively low temperatures which carry a large quantity of low-grade heat. As with other refining plants such as crude topping, catalytic reforming, etc. [2,3], significant energy-conservation opportunities exist in a delayed coker. To efficiently lower the energy consumption of the coking unit, it is crucial to increase the efficiency of the coking heater and to utilize the low-temperature heat. In this paper the energy and exergy analyses of a delayed coker in a Chinese refinery are presented. Accordingly, some improvements and corresponding optimal measures are proposed in detail based on the threelink structural models for energy-use analysis and optimization of process systems [4,5].
2. Energy-use analysis for the delayed coking process 2.1. Description of the delayed coker A delayed coker in a Chinese refinery, which was designed and put into operation in 1996, is taken as an example to study the energy-use analysis and improvement. The simplified flow diagram of this delayed coker is shown in Fig. 1. The processing capacity designed was 500 kt/a (62.5 t/h), and the recycle ratio designed was 0.4. The feedstocks processed is the Daqing vacuum residue. Actual data are obtained from a typical operating day. Table 1 gives an overall
Fig. 1. Simplified flow diagram of the delayed coker being analyzed (1—heater; 2,3—coke drums; 4—4-way switch valve; 5—fractionating column; 6—gas–liquid separation drum; 7—steam generator; 8—heat exchanger; 9—cooler/ condenser; 10—pumps).
Q.L. Chen et al. / Energy 29 (2004) 2225–2237
2227
Table 1 Overall material balance for the delayed coking unit (kg/h) Stream
Input
Vacuum residue Naphtha Diesel Heavy gas oil Waste oil Coke Wet gas Total
62,500
Output 11,040 15,050 19,550 1720 9830 5370 62,500
62,500
material balance for the delayed coking unit. The specifications of the coking products are shown in Table 2. The practical energy consumption of this delayed coker in 2000 for processing a ton of fresh feedstocks was as high as 1500.0 MJ/ton feedstock due to the less usable heat sinks within the unit itself, and the lack of the integrated consideration of its energy use with the auxiliary systems, such as utilities, storage and transportation systems within the whole refinery. The delayed coker in which the fresh feedstocks is preheated, coked and fractionated, consists v of one heater and two coke drums. The fresh feedstocks, which is heated up to about 350 C via the heat exchange with the hot product flows and the flue-gas in convection section is introduced into the bottom of the fractionating column to quench the high-temperature superheated reaction vapors. A portion of the heavier ends in the reaction vapors is condensed for recycling, and a portion of lighter ends in the fresh feedstocks is vaporized meanwhile. The ratio of the condensed recycling heavier parts to the fresh feedstocks is defined as the recycle ratio of the coking unit. The preheated feedstocks from the bottom of the column, together with the condensed heavier ends from the reaction vapors, is pumped into the radiation house of the coking v heater and quickly heated to slightly below 500 C. After partially vaporized in the heater tubes and passed through a 4-way switch valve, the feedstocks are then introduced into one of the two coke drums where the coking reactions are taken place. Water with high pressure is therefore injected into the heater tubes to minimize the coke deposition and to delay the coking reactions in the tubes. The superheated reaction vapors drawn out from the top of the coke drums are then back to the base of the fractionating column, and are further separated into various products according to their boiling points such as wet gas, naphtha, light gas oil (diesel) and heavy gas oil. Three pumparounds are added to effectively recover the thermal energy of the column. Table 2 Specifications of the coking products v
v
v
Stream
ASTM D86 EP ( C)
Flashing point ( C)
Freezing point ( C)
Residual carbon (%)
Naphtha Diesel Heavy gas oil
6> 200 6> 350 –
– 6< 66 –
– 6> 0 –
– – 6> 0:80
2228
Q.L. Chen et al. / Energy 29 (2004) 2225–2237
Part of diesel is further cooled and used as the absorption oil in the unit of gasoline stabilization, and the rich absorption oil returns to the fractionating column after preheated by the diesel, which could be considered as a diesel pumparound. The coke from the condensation reactions is cooled and removed after the coke drums are filled to the safe margin from the top. 2.2. Division of the three links/subsystems The delayed coker being analyzed could be divided into three links/subsystems based on the functions and evolutions of energy according to the three-link energy structural model proposed by Hua, et al. [4,5], i.e. energy-conversion link/subsystem, energy-use link/subsystem and energy-recovery link/subsystem. The components such as fractionating columns, coke drums, etc., which perform the material transformation and separation via the role of energy, belong to the energy-use link. The components such as the coking heater, reboilers, and pumps, etc., which fulfill the conversion between different types of energy, belong to the energy-conversion link. The coolers, condensers, heat exchangers and steam generator, etc., which recover the heat or cool the target product flows, are affiliated to the energy-recovery link. Both the energy-conversion subsystem and the energy-recovery subsystem, as the auxiliary subsystems in process systems, serve mainly for the energy-use subsystem. Thermodynamic analysis is quite useful in identifying the true inefficiencies in a process. Based on the framework of three-link model, the energy and exergy analyses are conducted for all energy-use processes and equipment within the three subsystems of the delayed coker being analyzed. The coking heater, heat exchanger networks and distillation operations are among the major candidate locations for energy-use improvement in a delayed coker. On the basis of the energy-use analysis, evaluation and improvement, the energy-use schemes for both partial and total systems, therefore, can be optimized through the decomposition–coordination optimization strategies [5,6]. Many optimization methods such as pinch analysis, optimization techniques for energy-conversion systems, heuristic method, artificial intelligence, etc. may be used in analysis and optimization of the coking system being considered. 2.3. Energy-use analysis and evaluation 2.3.1. Energy-conversion subsystem The energy-conversion subsystem mainly consists of the coking heater and pumps, etc. The energy-use status in this subsystem depends mainly on the fuel oil/gas consumption by the coking heater. The results of energy and exergy analyses for the coking heater being analyzed are summarized in Table 3. It shows that the thermal and exergy efficiencies of the coking heater are 87.0% and 45.0%, respectively. The relatively low efficiencies can be attributed in part to the high stack temperature as well as the above normal excess air. The excess air coefficient of the v coking heater is as high as 1.5, and the temperature of the rejected flue-gas is 205 C, which results in loss of more flue-gas thermal exergy, decrease of the coking heater efficiency, and the increase of the fuel consumption accordingly. Moreover, the analysis of the pump operations shows that the unit efficiencies of part pumps such as the pump for feeds of the radiation house, and the pump of the heavy gas oil pump-
Q.L. Chen et al. / Energy 29 (2004) 2225–2237
2229
Table 3 Exergy efficiency of the coking heater Items
Value
Total supplied exergy (kW) Exergy destruction (kW) Heat losses (kW) Exergy losses (kW) Thermal efficiency (%) Exergy efficiency (%)
25,890 13,050 3350 1190 87.0 45.0
around, etc. in the coking unit are too low due to the corresponding large pressure losses, which consequently causes huge electricity consumption in the whole unit. 2.3.2. Energy-use subsystem The energy-use subsystem is mainly composed of the coke drums, fractionating columns, etc. In the coke drums several processes happen, such as the high-temperature thermal cracking and condensation reactions. Due to the restriction of the improving potentials, we do not emphasize here the energy-use analysis in the coke drums. The heat removal of different pumparounds in the fractionating column is shown in Table 4. From Table 4, it can be seen that the distribution of heat removals among different pumparounds in fractionating column is unreasonable. The heat removal from the low-temperature top and diesel pumparounds is relatively high under the existed operation conditions. Meanwhile, the heat removal from the high-temperature intermediate and heavy gas oil pumparounds is too low, which causes the exergy destruction of the fractionating column too large. Furthermore, the operational recycle ratio of the coking system is as high as 0.4, which increases the heat load and the fuel consumption of the coking heater. 2.3.3. Energy-recovery subsystem The energy-recovery subsystem in the coking unit being analyzed mainly relates to the coolers, condensers, heat exchangers and steam generators, etc. The status of the energy and exergy Table 4 Heat removal of the coking fractionating column Items
Top pumparound Diesel pumparound Intermediate pumparound Heavy gas oil pumparound Total
Flow rate (kg/h)
Temperature v in/out ( C)
Energy
35,500
70/156
2010
22.5
490
13.5
16,000
120/240
1250
12.5
460
12.6
44,850
200/290
3250
35.9
1440
39.6
29,000
220/330
2530
29.1
1250
34.3
–
–
9040
100.0
3640
100.0
kW
Exergy %
kW
%
2230
Q.L. Chen et al. / Energy 29 (2004) 2225–2237
Table 5 Exergy analysis results for the energy-recovery subsystem Items
Value
Total recoverable energy (kW) Total recoverable exergy (kW) Energy effectively recovered (kW) Exergy effectively recovered (kW) Exergy destruction (kW) Heat given to air and cooling water (kW) Exergy given to air and cooling water (kW) Energy recovery efficiency (%) Exergy recovery efficiency (%)
22,540 8227 12,805 4508 1524 9735 2195 56.8 52.0
recoveries in the coking unit is shown in Table 5. It can be found that the energy-recovery efficiency is 56.8% and the exergy recovery efficiency is 52.0%. The low recovery efficiencies indicate that the improvement of the energy recovery is the major focus for energy-saving retrofit of the delayed coker. At present, the delayed coker only produces a little steam of 1.0 MPa due to the unreasonable design and operation of the heat exchanger network, and a large quantity of low-temperature surplus heat is cooled by air and cooling water, and wasted. The relatively high average heat-transfer temperature differences for the effective recovery of the heat of the product flows cause a large exergy destruction in the heat-transfer processes. Furthermore, large exergy destruction and losses are also caused by the less usable heat sinks within the delayed coker itself, and by the lack of the heat integration of the delayed coker with other process units, steam and power systems and the storage and transportation systems, etc. in the whole refinery. It can also be seen that the thermal energy discarded by air or cooling water is 9735 kW, which demonstrates that the low-temperature heat recovery for re-use is crucial to lower the energy consumption of the whole unit. 2.3.4. Existing energy-use problems The energy and exergy analyses of the whole delayed coker indicate that the index of the energy consumption for processing a ton of fresh feedstocks in this coking unit is high at 1500.0 MJ/ton feedstock. The energy and exergy conversion efficiencies in energy-conversion subsystem are 84.2% and 43.2%, respectively, and the energy and exergy recovery efficiencies in energy-recovery subsystem are 56.8% and 52.0%, respectively. The total energy and exergy used in the whole process are 1956 MJ/ton feedstocks and 871.8 MJ/ton feedstocks, respectively. The energy-use indices outlined above demonstrate that the energy consumption of the coking unit is relatively high and the exergy destruction is mainly from the coking heater. The low-temperature energy losses in the energy-recovery subsystem are a little large. The energy-use problems of the delayed coker are mainly related to the following: (A) A large quantity of low-temperature heat is wasted: the low-temperature heat from the v v overhead vapors of the fractionating column (130–65 C), top pumparound (115–80 C), v v diesel (150–60 C) and heavy gas oil (130–80 C) etc. is given off to the cooling water or air, and wasted.
Q.L. Chen et al. / Energy 29 (2004) 2225–2237
2231
(B)
Poor design of the steam generating system: the present delayed coker which was designed to produce the steam of 1.0 MPa can only generate the steam of 0.6 MPa due to the unreasonable design and operation. It is found that the pressure of the steam generated is too low to enter the main steam networks of 1.0 MPa, and most of steam of 0.6 MPa generated has to be vented away to the ambient due to the lack of proper users. (C) Unreasonable design for the distribution of heat removals from different pumparounds in the fractionating column: The heat removal from the top pumparound and the diesel pumparound is relatively high under the present operation conditions. Meanwhile, the heat removals from the high-temperature heavy gas oil pumparound and the intermediate pumparound are too low. (D) Poor design of heat exchanger network (HEN): the relatively low-temperature fresh feedstocks is directly preheated by the high-temperature intermediate and heavy gas oil pumparounds, and the large temperature difference causes a large exergy destruction. Furthermore, the area of the heat exchanger for steam generating is too small, which makes the heat load of the subsequent heat exchanger of feedstocks and heavy gas oil too large. The high output temperature of the fresh feedstocks from the convection section makes the temperature at the bottom of the column too high, which has the influences on the whole heat balance within the fractionating column and the operation stability of the whole coking unit. (E) In accordance with integration and optimization process, the poor heat integration of the delayed coker with other process units, steam and power systems and the auxiliary storage and transportation systems, etc. in the whole refinery would result in the energy-use inefficiencies of the delayed coking system.
3. Energy-use improvement for the coking process Obviously, the retrofit for the coking process only can be done on the basis of the original flowsheet, which may involve the following process modifications: (1) adjustment of the operational parameters; (2) changes in the interconnections between process equipment; (3) replacement of the original equipment by other equipment; (4) change in the sizes of some original equipment in the existing process. Based on above energy-use analysis and evaluation, the three subsystems are optimized, respectively, and an optimal flowsheet is finally proposed. 3.1. Division of the improvement measures It is the major focus for energy-use optimization of petroleum refining processes to decrease the total energy used and the exergy destruction in process, and to improve the energy (exergy) recovery and conversion efficiency. Based on the three-link energy and exergoeconomic models, all measures to improve the energy-use situation in process systems can be summarized as the following four types in terms of their functions [7]: U–type: improving energy/exergy conversion efficiency gU (gXU) in energy-conversion subsystem.
2232
Q.L. Chen et al. / Energy 29 (2004) 2225–2237
N–type: decreasing total energy/exergy used in process EN (ExN) in the energy-use subsystem. K–type: decreasing the exergy destruction DKP in energy-use subsystem. R–type: improving energy/exergy recovery efficiency gR (gXR) in energy-recovery subsystem. The core energy-use process improvement decides, to a large extent, the boundary conditions for energy recovery of the energy-recovery subsystem and the energy supply by the energy-conversion subsystem. The energy use of a delayed coking process has typical characteristics of the high-grade energy consumed, the large process exergy destruction and the more low-temperature surplus heat produced. The low-temperature heat produced in the coking process can hardly be balanced within the unit itself. Nearly, 80% of the energy supplied from outside to the coking unit such as fuel oils/gases or electricity, etc., are used mainly to preheat the feedstocks and the water injected to radiation tubes for providing the endothermic reaction heat of the severe thermal cracking of heavy feedstocks [1,8]. Thus, it is very important for reducing energy consumption of the coking unit to increase the preheated load of the feedstocks, to lower the heat duty and improve the exergy efficiency of the coking heater, and to recover effectively the heat from the fractionating column, as well as to extensively utilize the heat sinks inside or outside the unit, etc. Based on above energy and exergy analyses, the improvement measures and corresponding optimal energy utilization schemes are proposed for different subsystems and the total system. 3.2. Process improvement 3.2.1. I The improvements of the coking techniques to lower the consumption of materials and energy simultaneously from the source. Optimizations of the operational recycle ratio and the quantity of water injected into the radiation tubes would lower the heat load of the coking heater and the fuel consumption from the source. When the operational recycle ratio decreased from 0.4 to 0.2, the flowrate of the feeds to the radiation house of the coking heater decreased from 87.5 to 75 t/h. The heat load of the coking heater decreased about 1400 kW. At the same time, by decreasing the water injection into the radiation tubes can also lower the heat load of the coking heater. 3.2.2. II Optimization for heat removal of pumparounds to decrease the exergy destruction of fractionating processes: in order to decrease the exergy destruction of the fractionating process, the distribution of the heat removals from different pumparounds in the fractionating column is optimized. After the optimization of the operational recycle ratio, the total heat entering the fractionating column is decreased, which makes the high-temperature heat removal more difficult. Under the present flowsheet conditions, the heat removals from different pumparounds are adjusted with the aid of the process simulation techniques. The influence of the modified operational parameters or input variations on the outputs of the unit is estimated through the process simulation software of PRO/II on the premise of the same mass flowrates and specifications of the products. The heat removals for the intermediate and the heavy gas oil
Q.L. Chen et al. / Energy 29 (2004) 2225–2237
2233
Table 6 Change of pumparound heat removals of the fractionating column (kW) Items
Before retrofit After retrofit
Top pumparound
Diesel pumparound
Intermediate pumparound
Heavy gas oil pumparound
Heat
Exergy
Heat
Exergy
Heat
Exergy
Heat
Exergy
2010
490
1250
460
3250
1440
2530
1250
1250
315
1960
620
3180
1410
2850
1410
pumparounds are reached to the maximum to produce more steam with pressure of 1.0 MPa after optimized. The pumparound heat removals before and after retrofit are shown in Table 6. 3.3. Improvement in energy-recovery subsystem The improvement of the energy-recovery subsystem in the delayed coking unit relates mainly to the heat exchanger network optimization. The thermal energy contained in the various product flows is mainly used to preheat the fresh feedstocks of the vacuum residue prior to final heating in the coking heater, and to generate the steam of 1.0 MPa. For the delayed coking unit, the cold process streams include the fresh feedstocks, softened water, deaerated water, and the water injected into radiation tubes of the coking heater, and the hot streams include the different product flows and the fluegas of the coking heater. Through the core energy-use process improvement, the relevant data of hot and cold streams for the subsequent HEN retrofit are shown in Table 7. 3.3.1. I Retrofit of the steam generating system and the heat exchanger networks (HEN). In accordance with the principles of the second law analysis, the high-temperature heat in the delayed coking unit should be recovered to produce more 1.0 MPa steam. Based on the adjustment of the heat removal of different pumparounds, the HEN is optimized through the pinch analysis, which mainly includes the adjustment of the preheating of the feedstocks. The original heat exchange between the rich absorption oil and the diesel is cancelled, and the rich absorption oil enters the fractionating column directly. The modified flowsheet for feedstocks preheating is shown in Fig. 2, in which the feedstocks are firstly preheated by the diesel. The high-temperature heat of the heavy gas oil, then, can be saved to generate more steams. About 8.5 t/h of the 1.0 MPa steam is generated after the optimization of HEN and the steam pipelines. More 4.5 t/ h steam of 1.0 MPa is exported to the main steam networks from the retrofitted coking unit due to the elimination of the steam venting. 3.3.2. II Heat integration with other installations to increase the heat outputs and to lower the heat cooling losses. There exists a large quantity of low-temperature heat in the delayed coker, and it is difficult to find the reasonable users to balance it within the unit itself. The cooling curve of the overhead vapors from the fractionating column through the simulation software of PRO/II
2234
Q.L. Chen et al. / Energy 29 (2004) 2225–2237
Table 7 Data of hot and cold streams in HEN retrofit of the coking unit Process streams
Heavy gas oil, heavy gas oil pumparound and quenching oil Heavy gas oil and heavy gas oil pumparound Heavy gas oil Diesel and lean absorption oil Lean absorption oil Top pumparound Intermediate pumparound Overhead vapors Fresh feedstocks Feeds to convective section Feeds to radiation house Steam of 1.0MPa Preheating of softened water Water injected into tubes Deaerated water Recycling medium water
Flowrate (kg/h)
v
Temperature ( C)
Heat load (kW)
Initial
Target
58,050
340
230
5297
50,050
230
220
368
19,550 30,550
220 228
65 60
2121 3467
15,500 29,000 50,500
60 145 287
40 75 210
185 1250 3180
21,540 62,500 62,500
124 90 215
40 215 325
4320 5540 5610
75,000
375
500
9720
8500 15,000
150 35
180 75
5065 698
3200
40
170
484
8500 250,000
104 40
150 65
455 7267
Fig. 2. Modified flowsheet for preheating of feedstocks (1—fractionating column; 2—coke drums; 3—convection section; 4—radiation house).
Q.L. Chen et al. / Energy 29 (2004) 2225–2237
2235
Fig. 3. Flow diagram of the low-temperature heat recovery through a closed recycling system. v
v
shows that the latent heat of the overhead vapor is mainly between 105 C and 55 C, about 3610 kW. At present, all the low-temperature heat is given off to air or cooling water. In this optimization, the low-temperature heat from overhead vapors, top pumparound, diesel and heavy gas oil of the fractionating column are all recovered and used to preheat the fresh water in a water treatment plant through a closed recycling medium water system. The flowsheet of the closed recycling medium water is shown in Fig. 3. A heater and a cooler should be added in this recycling flowsheet to maintain the steady operation of the heat recovery system. The annual average low-temperature heat recovered is about 5115 kW. 3.3.3. III Application of heat-transfer enhancement techniques: the heat-transfer enhancement techniques, heat exchanger optimal design techniques and the optimal synthesis softwares for HEN are extensively applied in this retrofit optimization of the delayed coker. The enhanced tubes, such as transversally corrugated tubes and low fin tubes, etc. are used in the renewed or newly added heat exchangers and steam generators so as to enhance the heat transfer and to lower the investment cost. 3.4. Improvement in energy-conversion subsystem Improvement in energy-conversion subsystem is mainly related to the retrofit of energy-conversion devices such as the coking heater, pumps, etc. Based on the optimal process parameters, a significant improvement in efficiency is possible for the coking heater if energy-saving measures such as applying the advanced combustion techniques, decreasing the excess air coefficient, adopting low oxygen content operation. Furthermore, some low-temperature heat sinks such as combustion air and feedwater preheating are introduced in this delayed coker energy-use retrofit for further decreasing the flue-gas rejected temperature and improving the heater efficiency. v When the flue-gas rejected temperature is lowered to 170 C, about 800 kW more heat may be effectively recovered. When the excess air coefficient of the heater is reduced to 1.25, about 500 kW fuel can be saved. Meanwhile, related electricity saving measures such as frequency control, etc. can be used to decrease the power losses from the high-pressure hydraulic pump for coke removal and other process pumps.
2236
Q.L. Chen et al. / Energy 29 (2004) 2225–2237
Table 8 Economic and energy consumption indices for retrofit of the delayed coker Items
Values
Total investment (103 $) Profit before tax (103 $) Payoff period (year) Energy consumption before retrofit (MJ/t) Energy consumption after retrofit (MJ/t)
658.5 462.6 2.73 1500.0 941.65
3.5. Energy consumption and main economic indices More steam with pressure of 1.0 MPa is exported, and more low-temperature heat is recovered through the above-mentioned energy-use retrofit of the delayed coker, which causes a remarkable decrease in energy consumption. The energy consumption and the main economic evaluation indices such as the investment and profit for this energy-use retrofit of the delayed coker are shown in Table 8. We can see from Table 8 that a substantial energy consumption decrease is achieved through the coking unit retrofit, and the payback time is less than 3 years for this energy-use improvement.
4. Conclusions Analysis and evaluation of energy use in the existing delayed coker studied in this paper reveal that there exist great potentials for energy-use improvements in three subsystems of energy-conversion, energy-use and energy-recovery, and the improvement of the core energy-use subsystem is decisive for the total system optimization. Significantly improved process efficiency is possible for the delayed coking unit being analyzed through the realization of the corresponding energy-saving measures proposed according to energy-use characteristics of the three subsystems which mainly include to optimize the operational recycle ratio of the unit and the heat removals from pumparounds of the fractionating column, to effectively utilize the heat from the high-temperature heavy gas oil to produce more 1.0 MPa steam, and to re-use the low-temperature heat from the coking process to preheat the fresh water from a water treatment plant. The economic profits of the delayed coking unit are increased remarkably with relatively low capital investment. The energy consumption of the improved coking unit decreased by 37.2%, which proves that the three-link analysis and optimization strategies applied are effective for energyuse improvement of process systems.
Acknowledgements The financial support from the Major State Basic Research Development Program (G2000026307), the Natural Scientific Foundation of China (20076018) and the Guangdong Provincial Natural Science Foundation of China (990638) is gratefully acknowledged.
Q.L. Chen et al. / Energy 29 (2004) 2225–2237
2237
References [1] Jones DSJ. Elements of petroleum processing. New York: J. Wiley & Sons; 1995. [2] Ballut AA. Heat recovery analysis of an existing crude distillation unit. Heat Recovery Systems & CHP 1986;6:361–7. [3] Ballut AA. Energy conservation in a catalytic reforming plant. Heat Recovery Systems & CHP 1989;9:493–7. [4] Hua B, Shen JF. The energy synthesis of fractionating tower networks. Proceedings of the 4th International Symposium on Process System Engineering, Canada. Design. Montebello, vol. I. 1991, p. I.12.1–I.12.14. [5] Chen QL, Hua B. Application of exergoeconomics to the analysis and optimization of process systems. Journal of Thermal Science 1996;5(1):1–10. [6] Chen QL, Yin QH, Hua B. An exergoeconomic approach for retrofit of fractionating systems. Energy—The International Journal 2002;27(1):65–75. [7] Hua B, Yin QH. Energy optimization through exergy-economic evaluation. Journal of Energy Resources Technology 1989;111(3):148–53. [8] Rodriguez-Reinoso F, Santana P, Romero Palazon E, et al. Delayed coking: industrial and laboratory aspects. Carbon 1998;36(1):105–16.