Heat Integration in a Crude Distillation Unit Using Pinch Analysis Concepts (AIChE 2008 Spring Meeting 165b)

Transcription

1 Heat Integration in a Crude Distillation Unit Using Pinch Analysis Concepts (AIChE 2008 Spring Meeting 165b) PETROBRAS R&D Center CENPES Antonio V. S. de Castro*, M.Sc. Carlos Ney da Fonseca Claudio L. M. Kuboski Silvia Waintraub, M.Sc. Washington de O. Geraldelli, Ph.D.

2 Introduction Higher prices of energy and oil Crude Distillation Unit: Energy-intensive Process Heat Integration Fractionation Constraints Pumparound Design Number of Pumparound Sections Location of Pumparound Sections Pumparound Section Heat Duty

3 Outline for Simulation Approach Design procedure: Location of pumparounds (PA) Analyse Pumparound Duty concerning the Fractionation constraints Evaluate alternatives to improve Heat Recovery: global costs (Pinch Design Method) Evaluate PA heat duty distribution at atmospheric tower (vacuum constant) Evaluate changing pinch stream possibilities by process modifications (modify vacuum tower configuration, considering atmospheric tower best result fixed) Evaluate modifying pinch stream return temperature (if PA)

4 Pumparound Section Heat Recovery at higher temperature Maximum heat recoverable Heat of vaporization of the liquid from the tray above the pumparound section Trade-off: Pumparound Duty Fractionation above the pumparound Fractionation Quality: Internal reflux Gap and Overlap

5 Pumparound Section Max Heat Duty at PA: Zero Internal Liquid Reflux above PA return. By Simulation: Internal Liquid Reflux above PA return Enthalpy Difference at bubble and dew point; Simulate the tower specifying near Zero Internal Reflux above PA, varying PA duty. In all studies, products specification were a target. However, stripping steam optimization was not part of this present work.

6 Sketch TPA MPA BPA NAPHTHA KEROSENE LIGHT DIESEL HEAVY DIESEL LVGO MVGO HVGO SLOP WAX REDUCED CRUDE VACUUM RESIDUE

7 Pumparound Section - Example Flow rate (kmol/h) Temperature ( C) Theoretical stage Theoretical stage Liquid Internal Reflux - Max PA Liquid Internal Reflux 30 Temperature - Max PA Temperature These graphics compare both liquid internal reflux and temperature profile at atmospheric column, considering BPA is already defined. Data refering to Max PA are at near zero liquid reflux, while the other data refer to maximum liquid internal reflux above Mid PA section.

8 Simulation Basis 19 o API Brazilian Crude Kept Constant: Atm Furnace Outlet Temperature Vacuum Furnace Outlet Temperature Atm Ovhd Drum Temperature Overflash Rate Number of stages HVGO / LVGO ~ 1 Pumparound Withdraw at Product Drawoff Pans Fractionation Constraints: Naphtha Kerosene: 0 o C min gap Kerosene Light Diesel: 5 o C min gap Light Diesel Heavy Diesel: 30 o C max overlap Cost basis: Brent: US$ / bbl Fuel oil: US$ / 10 6 kcal Cooling water: US$ / m3 Equipment Cycle Life: 10 years

9 Fractionation vs Heat Recovery Gap between side products C Overlap at 6x10 6 kcal/h GAP5-95 Kerosene vs Naphtha GAP5-95 Light Diesel vs Kerosene GAP5-95 Heavy Diesel vs Light Diesel d(gap HDxLD)/d(BPA Duty) Gap 5-95 ( o C) Bottom Pumparound Duty (10 6 kcal/h)

10 Fractionation vs Heat Recovery Gap between side products C gap at 18x10 6 kcal/h GAP5-95 Kerosene vs Naphtha GAP5-95 Light Diesel vs Kerosene GAP5-95 Heavy Diesel vs Light Diesel d(gap LD x K)/d(MPA Duty) Gap 5-95 ( o C) -30 Inflection Point at Duty = 14x10 6 kcal/h Inflection Point at Duty = 17x10 6 kcal/h Mid Pumparound Duty (10 6 kcal/h)

11 Fractionation vs Heat Recovery Gap between side products C gap at 17x10 6 kcal/h 0 GAP5-95 Kerosene vs Naphtha -10 GAP5-95 Light Diesel vs Kerosene GAP5-95 Heavy Diesel vs Light Diesel d(gap N x K)/d(TPA Duty) Inflection Point at Duty = 18x10 6 kcal/h Gap 5-95 ( o C) -20 Inflection Point at Duty = 12x10 6 kcal/h Top Pumparound Duty (10 6 kcal/h)

12 Case Study 1 Case Study 1 max heat recovery Evaluate PA heat duty distribution in atmospheric tower (vacuum configuration constant) Duties in 10 6 kcal/h Base Case 16,20,0 16,14,6 Ovhd Condenser Top PA Mid PA Bottom PA Overall

13 Results Case Study 1 BASE 16,20,0 16,14,6

14 Results Case Study 1 Atmospheric Tower Duties in 10 6 kcal/h Base 16,20,0 16,14,6 Hot Utility Cold Utility Hot Utility (Base) Case 16,14,6: Bottom PA: 6x10 6 kcal/h; Tout = 338 C; Treturn = 303 C Pinch: HVGO; Tpinch = 312 C Bottom PA: Above the Pinch = = 26 C (74,3%) 6 x 0,743 = 4,45x10 6 kcal/h ~ 4,27x10 6 kcal/h ( )

15 Results Case Study 1 Atmospheric Tower Base 16,20,0 16,14,6 T optimum (ºC) Utility Cost Capital Cost Overall Cost Savings * For Case 16,20,0 at T =19.2 ºC Capital Cost = 5.637x10 6 US$/yr slightly lower than Case 16,14,6 caused by lower approach near pinch region, but process recovery lead to much lower Utility Cost

16 Case Study 2 Vacuum Tower MVGO Draw In case study 1: benefit on moving duty from below to above the pinch What about moving the pinch by changing process/configuration, keeping specification? LVGO LVGO MVGO HVGO HVGO Add MVGO draw HVGO : MVGO : LVGO ~ 1 : 4 : 1 (case: MVGO) High flow rate required to change pinch location. Atmospheric column configuration constant (best result previously achieved). process to process recovery above pinch Hot and cold utility approach - Capital cost (trade-off)

17 Results Case Study 2 16,14,6 MVGO

18 Results Case Study 2 Hot Utility at pinch (10 6 kcal/h) Cold Utility at pinch (10 6 kcal/h) Hot Utility (Base) (10 6 kcal/h) T optimum (ºC) Utility Cost Capital Cost Overall Cost Savings 16,14, MVGO (pinch Mid PA) As pinch is occurring at MVGO (much higher flow rate than HVGO), there is a large portion of Hot Composite Curve with few variation in flow above the pinch, resulting expressive increment on Capital Cost (penalty too high).

19 Results Case Study 2 Add MVGO withdraw didn t present good results, but : If products flow rates change? HVGO : MVGO : LVGO ~ 1 : 2 : 2 T HVGO PA Return = 285 ºC Named: Case MVGO 285 HVGO kept as pinch stream (same process recovery than Case 16,14,6) Higher approach (hot x cold composite) Capital Cost decrease MVGO 285 result only evaluating HEN (capital cost of tower changes not included)

20 Results Case Study 2 16,14,6 MVGO MVGO 285

21 Results Case Study 2 16,14,6 MVGO (pinch Mid PA) MVGO 285 (pinch HVGO) Hot Utility at pinch (10 6 kcal/h) Cold Utility at pinch (10 6 kcal/h) Hot Utility (Base) (10 6 kcal/h) T optimum (ºC) Utility Cost Capital Cost Overall Cost Savings If we keep pinch at HVGO, heat recovery is the same than Case 16,14,6, however the HEN approach is much higher, allowing more heat recovery.

22 Pinch Stream Pumparound T Case Study 3 Evaluate modifying pinch stream return temperature (if PA) HVGO: for low T high flow rate (pumping need to be evaluated) How will thermodynamics respond to flow variation?

23 Results Case Study 3 Pinch Stream Pumparound T MVGO 200 MVGO 230 MVGO 260 MVGO 285 T HVGO pan (ºC) T HVGO PA return (ºC) Hot Utility (10 6 kcal/h) Cold Utility (10 6 kcal/h) Heat moving across pinch arctan( ) arctan( ) Hot T 55 end cold o Utility T pinch 350ºC 326ºC 314ºC Hot utility Grand Compositve Curve

24 Results Case Study 3 MVGO 260 MVGO 285

25 Results Case Study 3 MVGO 200 MVGO 230 MVGO 260 MVGO 285 T optimum (ºC) Utility Cost Capital Cost Overall Cost Savings As HVGO flow rate increases, the HEN approach becomes higher, resulting less Capital Cost, allowing more heat integration.

26 Discussion Procedure constraints Pinch analysis assumes direct heat exchange Cost of new sections inside the tower need to be evaluated appart Modification on vacuum and atmospheric collumn simultaneously are not easily evaluated Non optimal design (but close to optimum)

27 Conclusion In Case Study 1, moving duty from below to above the pinch (transfering duty from MPA to BPA) reduced Utility Cost with almost no penalty in Capital Cost. In Case Study 2, moving the pinch stream by creating a new drawoff at vacuum tower did not bring benefit initially, as the increase on Capital Cost was too high. However, appropriate flow rate definition for this new stream lead to much higher approaches (lower Capital Cost). In Case Study 3, capital cost becomes higher for lower return PA temperature (lower flow rate).

28 Conclusion Appropriate variation of process streams observing thermodynamics may result in high process integration (grass root or revamp) Optimization taking into account these insights could improve the design.

29 Thank you very much! Antonio V. S. de Castro, Claudio L. M. Kuboski Carlos Ney da Fonseca Silvia Waintraub Washington de O. Geraldelli