E. BRIGHT, S. ROY and S. AL-ZAHRANI, Saudi Aramco, Dhahran, Saudi Arabia
I
n this case history, a crude distillation unit (CDU) preheat train network in a Saudi Aramco refinery was simulated and analyzed for anticipated modifications to the network. This analysis helped eliminate inef- ficiencies in the network, and, based on the insights from the analysis, various options were generated and the existing network was reconfigured. The reconfiguration allowed the temperature of the crude preheat net- work, which processes Arab Light crude oil, to be increased to the maximum of 277°C from a previous temperature of 261°C.Existing configuration. Desalted crude from the tank is heated by the crude column top pumparound, light gasoil (LGO) product, heavy gasoil (HGO) prod- uct, LGO pumparound (LGO PA), HGO pumparound (HGO PA), heavy vacuum gasoil (HVGO) pumparound and vacuum residue (VR) product, as shown in Fig. 1 in exchangers E1 to E7, respectively. The current crude preheat temperature entering the CDU furnace is around 261°C. This
exchanger network is validated using heat exchanger design software and by adjusting the fouling coefficients.
Modifications required. The base- case network was altered for anticipated modifications in the future. The reasons
Top pumparound
Vacuum residue To CDU furnace
Crude from tank
Fuel oil storage tank Hot HVGO to hydrocracker
HVGO pumparound to column HVGO to storage Asphalt oxidizer All temperatures in °C Cooler Cooler HVGO pumparound plus product HGO pumparound LGO pumparound LGO product HGO product Desalter E7 E6 E5 175 203 236 45 85 113 143 339 249 281 370 152 245 339 261 245 217 E4 E1 E2 E3
Current configuration of CDU preheat train. FIG. 1
Fuel oil Fuel oil Solvent deasphalting Pitch DMO Vacslop, 380°C Low-viscosity feed in asphalt oxidizer unit VGO RCO Asphalt oxidizer unit Asphalt To ejectors Cooler
Current configuration of vacuum slop circuit. FIG. 2 Fuel oil Vacslop, 380°C VGO RCO Asphalt To ejectors Cooler New heater Crude Solvent deasphalting Pitch DMO Asphalt oxidizer unit Modifications in vacuum slop circuit. FIG. 3
CLEAN FUELS
SPECIALREPORT
70
I
FEBRUARY 2012 HydrocarbonProcessing.comfor the modifications are listed below: • Vacuum slop circuit. In the current
configuration (Fig. 2), the vacuum slop is recycled to the vacuum tower through the vacuum furnace. The purpose of this recycle is to recover the VGO components and send the VGO to the hydrocracker; however, this is not achieved in the cur- rent operation due to vacuum furnace limitations and insufficient separation in the wash section. As a result, this vacuum slop stream (which is lower in viscosity) goes with the vacuum tower bottoms. The
mingling of streams deteriorates the feed to the asphalt oxidizer and creates operational problems in meeting the penetration prop- erty of the asphalt.
To address this concern, the vacuum slop stream from the vacuum tower is available at a temperature of 380°C, which is withdrawn as a separate cut and is used to increase the preheat temperature of the crude. This proposed new exchanger is con- figured to be in parallel with the existing heat exchanger E4 in Fig. 1. Fig. 3 shows the rerouting of the vacuum slop.
• Future splitter configuration.
To meet the clean gasoline specification of 1% benzene in gasoline, the existing naphtha splitter must remove the benzene precursors in the catalytic reformer feed by increasing the initial boiling point of the heavy naphtha. This process requires a higher reboiler duty. In addition, the heavy naphtha from the hydrocracker needs to be processed in the naphtha splitter, as this feed also contains benzene precursors.
Currently, hydrocracker heavy naphtha is not part of the naphtha splitter feed. The hydrocracker heavy naphtha feed volume is 12,500 barrels per day (bpd), and the existing naphtha splitter capacity is 23,000 bpd. Figs. 4 and 5 show the naphtha sys- tem’s current and planned configurations, respectively. As the current naphtha splitter cannot handle this higher throughput with higher reboiler requirement, the existing naphtha splitter will be mothballed. The existing reboiler, which uses HGO PA flow and gives a duty of 10.4 million kilocalories per hour (MMkcal/hr), will also be moth- balled. High-pressure steam will be used in the reboiler of the new naphtha splitter to meet the higher reboiler requirements. For the column to be in heat balance, this 10.4 MMkcal/hr of heat removal is required. In the proposed exchanger network, this stream (HGO CR) will be used to preheat the crude.
Synthesis of crude preheat train.
A new, preliminary heat exchanger network (Fig. 6) was synthesized to accommodate the above modifications. While modifying the crude preheat train network, the following impact on the equipment was kept in mind:
• Prevention of vaporizations in the fur- nace pass-control valves, as it is difficult to control two-phase flows across pass-control valves. Inadequate flow in the furnace pass flows will also lead to coking.
• Column heat balance. • Column hydraulics.
• Impact of hot streams going directly to the other unit.
The changes made in the base-case net- work are listed below:
• Exchanger N1 was added parallel to E4 (see Fig. 6) using vacuum slop (vacslop) and vacuum residue ex-E7 as the hot fluid. This modification is required to improve the viscosity of the vacuum residue to the asphalt oxidizer. The current viscosity of the feed to the asphalt oxidizer is 1,500 centistokes (cst), and the required viscosity is 2,000 cst.
• Another exchanger N2 (E5-2, similar to E2) was added parallel to E2 using HGO
LPG + gas LN LN to storage Area D treater LN to storage LN NHT HN HN HCU HN LPG + gas FG CSF DEB feed
CDU stab feed
Drag Separator and stripper gas Rich oil CDU debut CSF debut CSF NS CDU NS GCU debut Lean oil HCU fractionator Absorber CSF treater Catalytic reformer Caustic wash HGO CR CDU NS will be mothballed in the future E29 LPG + gas C5/C6 to storage
Current configuration of naphtha circuit. FIG. 4 LPG + gas LN LPG + gas LN to isomerization unit LN+HN HN FG to area D Lean oil CSF DEB feed
CDU stab feed
Drag
CDU debut
New piping and equipment
GCU debut CSF debut CSF NS New CDU NS Steam HCU fractionator HCU HN HP SEP gas Absorber Rich oil Caustic wash Sulfur guard bed LPG + gas C5/C6 to storage Catalytic reformer NHT
Configuration of naphtha management system after clean-fuel implementation. FIG. 5
CLEAN FUELS
SPECIALREPORT
HYDROCARBON PROCESSING FEBRUARY 2012
I
71PA fluid ex-E5 (hereafter referred to as E5-1) as the hot fluid. This modification is per- formed to accommodate the 10.4-MMkcal/ hr duty in the HGO PA circuit.
• Increased area in E4 from the 2-par- allel-1-series arrangement to a 2-parallel- 2-series design and added cooler N3 down- stream of E4.
Due to the first two modifications, the inlet temperature to E4 has increased, which decreases the logarithmic mean tempera- ture difference (LMTD) available across the unit. Since E4 is the LGO PA exchanger, the column will not be in heat balance if the required heat removal is not performed. The required duty was 18.8 MMkcal/hr, and the available duty was 12.7 MMkcal/hr (see Table 1). Therefore, additional area and a cooler were added in the LGO PA circuit to meet the duty requirement of the column.
The required HGO PA duty is 26.8 MMkcal/hr, and the available duty is 29.8 MMkcal/hr. As the heat removed in HGO PA is higher by 3 MMkcal/hr, the requirement of LGO PA duty will come down by 3 MMkcal/hr. As both LGO and HGO are mixed outside of the column and go to the diesel hydrotreater (DHT), the splitting of the duty between LGO and HGO pumparound is not a concern from a separation point of view. However, it does impact the column draw temperature, which will slightly reduce the LMTD across E3 (HGO product/crude exchanger) and E5 (HGO PA/crude exchanger).
Results of network modification.
In the modified network, the obtained preheat temperature was 266°C. The duty, LMTD and area of each exchanger in the network are presented in Table 1. From Table 1, it can be observed that:
• Exchanger E6, which has a higher area, is experiencing the lowest LMTD; therefore, any modification that increases the LMTD will significantly increase the heat recovered from E6.
• The exchanger preceding exchanger E6 is heated by HGO circulating reflux (CR), which is at 337°C; this is higher than the hot stream (HVGO CR) temperature of E6, which has decreased the LMTD in E6.
TABLE 1. Performance of base-case network after modifications
Exchangers Duty, MMkcal/hr Area, m2 LMTD, °C Minimum approach, °C
E1 A/B/C (top PA) 13.0 1,300 50.6 34.0
E2 (LGO) 7.6 326 94.8 79.0
E5-2 (new N2—HGO PA) 9.1 326 98.6 83.0
E3 (HGO) 9.4 363 92.7 38.0
E4 (LGO PA) 12.7 1,379 48.1 35.0
Vacslop + FO (new N1) 3.5 270 99.5 84.0
E5-1 (HGO PA) 14.2 327 79.0 50.0
E6 (HVGO PA + HVGO) 10.9 1,458 24.0 14.0
E7 (VR) 10.4 1,027 39.0 8.6
Total area 6,776
Total duty 90.8
Note: Crude preheat temperature is 266°C. The required LGO PA duty is 18.8 MMkcal/hr; therefore, the new required LGO PA cooler duty is 6.1 MMkcal/hr. The required HGO PA duty is 26.8 MMkcal/hr, and the available HGO PA duty is 29.8 MMkcal/hr.
TABLE 2. Energy analysis of base-case network
Heat exchanger Percent
network of target
Heating, MMkcal/hr 67 145 Cooling, MMkcal/hr 21 N/A Total area, m2 9,491 56
TABLE 3. Performance data for modified network in Fig. 8
Exchangers Duty, MMkcal/hr Area, m2 LMTD, °C Minimum approach, °C
E1 A/B/C (top PA) 13.00 1,300 50.6 34
E2 (LGO) 8.60 326 86.0 68
E5-2 (New N2—HGO PA) 13.60 326 113.0 92
E3 (HGO) 9.40 363 77.0 26
E4 (LGO PA) 11.10 1,379 47.0 31
Vacslop + FO (new N1) 3.20 270 77.0 65 E6 (HVGO PA + HVGO) 14.53 1,458 33.0 20
E5-1 (HGO PA) 8.60 327 60.0 49
E7 (VR) 10.30 1,027 50.0 23
Total area 6,776
Total duty 92.30
Note: Crude preheat temperature is 269°C. The required LGO PA duty is 18.8 MMkcal/hr; therefore, the new required LGO PA cooler duty is 7.7 MMkcal/hr, and the required HGO PA duty is 26.8 MMkcal/hr.
Crude from tank
HGO pumparound HGO CR to column 186 45 83 Area: 123 127 151 326 m2 New N2 E26 A/B 6.5 MMkcal/hr HVGO product TO CDU furnace
To fuel oil storage tank through cooler Asphalt oxidizer
Existing exchanger New exchanger Existing exchanger with additional area Piping modifications All temperatures in °C HVGO pumparound to column New N1 Area: 270 m2 378 198 188 219 280 370 337 243
Vacuum residue HVGO PA
plus product 15 MMkcal/hr
266 251 New cooler N3 Vacuum slop Cooler 133 245 LGO product 335
HGO product Desalter 155 Top pumparound 245 LGO pumparound Additional area: 690 m2 E1 E2 E5-2 E29 E3 E4 E5 E6 E7
Base-case network after modifications. FIG. 6
CLEAN FUELS
SPECIALREPORT
72
I
FEBRUARY 2012 HydrocarbonProcessing.comThis preliminary network was analyzed for possible improvement in the preheat temperature. The analysis indicated that
heat recovery can be increased by 45% by boosting the area by 56% (see Table 2).
The analysis also indicated that the driv-
ing force across exchanger E7 further lim- ited the heat recovery. Fig. 7 displays the driving-force plot. The figure indicates that the driving force in E7 can be increased by decreasing the inlet temperature in E7. This temperature adjustment can be achieved by operating E5 in parallel with E7.
Case 1. Based on the insights derived
from Table 1 and Fig. 7, to improve the heat recovery, the crude stream in E7 and E5 was split by operating E5 in parallel with E7. The objective of this modification is to increase the LMTD across E7 and E6. However, it also decreases the LMTD across E5-1. The net effect is shown in Table 3, and the modified network is shown in Fig. 8. With this arrangement, the preheat temper- ature has increased from 266°C to 269°C.
Case 2. From LMTD and approach
data in Table 3, it can be inferred that heat recovery in E5-1 can still be improved by increasing the area. Hence, another case study was performed by adding two similar exchangers in a series in E5-1. The results are tabulated in Table 4. The preheat was found to be increased to 277°C.
The HGO PA is now providing an extra 4.2 MMkcal/hr more than required, which will reduce the LGO PA duty requirement by the same amount for the column to be in heat balance. Then, the required LGO PA cooler duty comes down to 2.6 MMk- cal/hr. HP
Said A. Al-Zahrani is the general supervisor in the process and control systems department at Saudi Aramco. He is the chairman of the multi-disciplinary product speci- fications committee, tasked with managing various issues related to Saudi Aramco products and fuel specifications. Al-Zahrani holds a degree in chemical engineering from King Fahd University of Petroleum and Minerals, and began his career at Saudi Aramco as a process engineer in the Ras Tanura refinery. He is a member of several local and international societies and an officer of the American Institute of Chemical Engineers, Saudi Arabian chapter. Edwin Bright has over 17 years of experience in the petroleum refining industry. Before joining Saudi Aramco, he worked for Reliance Industries Ltd., Indian Oil Corp., ATV Petrochemicals and Foster Wheeler India Ltd. He holds a bachelor’s degree in chemical engineer- ing and master’s degrees in petroleum refining and pet- rochemicals from AC Tech, Anna University, Chennai. He also earned a master’s degree in management from the Asian Institute of Management in Manila.
Samit Roy is an engineering consultant at Saudi Aramco’s downstream process engineering division under the process control and systems department. A graduate in chemical engineering, he has more than 33 years of broad experience in the process engineering and technical services areas of oil refining and gas processing plants. His experience includes 21 years in Saudi Aramco refining and engineering services and 12 years at Indian refineries. He has worked at most of the refinery process units associated with distillation, hydroprocessing and gas treating plants.
400 350 300 250 Hot temper atur e, °C 200 150 100 100 120 140 160 180 200 Cold temperature, °C Target driving force
E5-1 at main E6 at main y=x E7 at main
220 240 260 280 300 Driving-force plot for base-case network.
FIG. 7
Crude from tank
HGO pumparound HGO CR to column 186 48 156 83 264 134 Area: 326 m2 New N2 E26 A/B 6.5 MMkcal/hr Naphtha stabilizer bottoms out 14.1 MMkcal/hr 9.1 MMkcal/hr HVGO product TO CDU furnace
To fuel oil storage tank through cooler Asphalt oxidizer
Existing exchanger New exchanger Existing exchanger with additional area
Existing exchanger in new location Piping modifications All temperatures in °C HVGO pumparound to column New N1 Area: 270 m2 378 198 241 191 280 HVGO PA plus product Vacuum residue 271 269 New LGO or cooler N3 Vacuum slop Cooler 133 245 LGO product 335
HGO product Desalter 155 Top pumparound LGO pumparound Additional area: 690 m2 E1 E2 E5-2 E3 E4 E6 E7 E5 E29
Modified network based on E5 operating in parallel with E7. FIG. 8
TABLE 4. Performance data for modified network with increased area in E5-1 Exchangers Duty, MMkcal/hr Area, m2 LMTD, °C Minimum approach, °C
E1 A/B/C (top PA) 13.0 1,300 50.6 34
E2 (LGO) 8.6 326 86.0 68
E5-2 (new N2—HGO PA) 10.5 326 91.0 74
E3 (HGO) 9.7 363 80.0 27
E4 (LGO PA) 12.0 1,379 45.0 33
Vacslop + FO (new N1) 3.4 270 81.0 70 E6 (HVGO PA + HVGO) 15.0 1,458 34.0 21
E5-1 (HGO PA) 14.0 983 31.0 12
E7 (VR) 10.1 1,027 53.0 27
Total area 7,432
Total duty 96.3
Note: Crude preheat temperature is 277°C. The required LGO PA duty is 18.8 MMkcal/hr; therefore, the new required LGO PA cooler duty is 6.8 MMkcal/hr. The required HGO PA duty is 26.8 MMkcal/hr, and the available HGO PA duty is 31.0 MMkcal/hr.
In today's challenging economic climate, creative thinking, accurate cost estimates and a commitment to technical excellence are crucial to the success of capital projects. Mustang has a superior reputation for providing owners with total project delivery, from feasibility analysis and front-end engineering to facility startup.
Whether a project is grassroots, revamp, expansion or modernization, Mustang has the experience and capability to successfully execute your project to achieve the desired results. Contact Mustang today!