Heat Integration in a Crude DistillationUnit 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. KuboskiSilvia Waintraub, M.Sc.

Washington de O. Geraldelli, Ph.D.

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

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 byprocess modifications (modify vacuum tower configuration, considering atmospheric tower best result fixed)

• Evaluate modifying pinch stream returntemperature (if PA)

Pumparound Section

Heat Recovery at higher temperatureMaximum heat recoverable – Heat ofvaporization of the liquid from the trayabove the pumparound section

Trade-off:– � Pumparound Duty– � Fractionation above the

pumparound

Fractionation Quality:– Internal reflux– Gap and Overlap

Pumparound Section

Max Heat Duty at PA:

– Zero Internal Liquid Reflux above PA return.

By Simulation:

– Internal Liquid Reflux above PA return EnthalpyDifference at bubble and dew point;

– Simulate the tower specifying near Zero InternalReflux above PA, varying PA duty.

In all studies, products specification were a target. However, stripping steam optimization was not part of this present work.

Sketch

NAPHTHA

KEROSENE

REDUCED CRUDE

LIGHT DIESEL

HEAVY DIESEL

HVGO

SLOPWAX

VACUUMRESIDUE

LVGO

MVGO

BPA

MPA

TPA

Pumparound Section - Example

0

5

10

15

20

25

30

35

0 500 1000 1500 2000 2500 3000 3500 4000 4500

Flow rate (kmol/h)

The

oret

ical

sta

ge

Liquid Internal Reflux - Max PA

Liquid Internal Reflux

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.

0

5

10

15

20

25

30

35

100 150 200 250 300 350 400

Temperature (°C)

Theo

reti

cal st

age

Temperature - Max PA

Temperature

Simulation Basis

19o API Brazilian CrudeKept Constant:– Atm Furnace Outlet Temperature– Vacuum Furnace Outlet Temperature– Atm Ovhd Drum Temperature– Overflash Rate– Number of stages

HVGO / LVGO ~ 1Pumparound Withdraw at Product Drawoff PansFractionation Constraints:– Naphtha – Kerosene: 0 oC min gap– Kerosene – Light Diesel: 5 oC min gap– Light Diesel – Heavy Diesel: 30 oC max overlap

Cost basis:– Brent: US$ 30.00 / bbl– Fuel oil: US$ 20.60 / 106 kcal– Cooling water: US$ 0.066 / m3– Equipment Cycle Life: 10 years

Fractionation vs Heat Recovery

Gap between side products

-70

-60

-50

-40

-30

-20

-10

0

10

20

-25-20-15-10-50

Bottom Pumparound Duty (106 kcal/h)

Gap

5-9

5 (o C

)

GAP5-95 Kerosene vs Naphtha

GAP5-95 Light Diesel vs Kerosene

GAP5-95 Heavy Diesel vs Light Diesel

d(Gap HDxLD)/d(BPA Duty)

30 C Overlap at 6x106 kcal/h

Fractionation vs Heat Recovery

Gap between side products

-40

-30

-20

-10

0

10

20

-35-30-25-20-15-10-50

Mid Pumparound Duty (106 kcal/h)

Gap

5-9

5 (o C

)

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)

Inf lection Point at Duty = 14x106 kcal/h

Inflection Point at Duty = 17x106 kcal/h

5 C gap at 18x106 kcal/h

Fractionation vs Heat Recovery

Gap between side products

-40

-30

-20

-10

0

10

20

-25-20-15-10-50

Top Pumparound Duty (106 kcal/h)

Gap

5-9

5 (o C

)

GAP5-95 Kerosene vs Naphtha

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 = 18x106 kcal/h

Inflection Point at Duty = 12x106 kcal/h

0 C gap at 17x106 kcal/h

Case Study 1

600Bottom PA

16160Top PA

59.058.758.8Overall

14200Mid PA

23.022.758.8Ovhd

Condenser

16,14,616,20,0Base Case

Duties in 106 kcal/h

Case Study 1 – max heat recoveryEvaluate PA heat duty distribution in atmospheric tower

(vacuum configuration constant)

Results – Case Study 1

BASE 16,20,0 16,14,6

Results – Case Study 1

Case 16,14,6:

– Bottom PA: 6x106 kcal/h; Tout = 338°C; Treturn = 303°C

– Pinch: HVGO; Tpinch = 312°C– Bottom PA: Above the Pinch = 338 – 312 = 26°C (74,3%)

6 x 0,743 = 4,45x106 kcal/h ~ 4,27x106 kcal/h (4.05 + 0.22)

- 4.05

60.4

5.74

16,14,6

64.766.1Cold Utility

+ 0.220� Hot Utility

(Base)

10.019.79Hot Utility

16,20,0Base

Duties in 106 kcal/hAtmospheric Tower

Results – Case Study 1

12.43213.14014.520Overall Cost(106 US$/yr)

2.0881.3800Savings

(106 US$/yr)

6.6447.0148.504Utility Cost(106 US$/yr)

5.7896.1266.016Capital Cost(106 US$/yr)

19.214.926.2�T optimum

(ºC)

16,14,616,20,0Base

* For Case 16,20,0 at �T =19.2 ºC – Capital Cost = 5.637x106 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

Atmospheric Tower

Case Study 2

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?

– 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)

LVGO

HVGO

LVGO

HVGO

MVGO

Vacuum Tower – MVGO Draw

Results – Case Study 2

16,14,6 MVGO

Results – Case Study 2

0.165.74Hot Utility at pinch

(106 kcal/h)

54.160.4Cold Utility at pinch (106

kcal/h)

-9.63- 4.05� Hot Utility (Base)

(106 kcal/h)

1.400

13.120

8.555

4.565

13.2

MVGO (pinch – Mid PA)

12.432Overall Cost(106 US$/yr)

2.088Savings

(106 US$/yr)

6.644Utility Cost(106 US$/yr)

5.789Capital Cost (106 US$/yr)

19.2�T optimum (ºC)

16,14,6

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).

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 processrecovery 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)

Results – Case Study 2

16,14,6 MVGO MVGO 285

Results – Case Study 2

5.420.165.74Hot Utility at pinch

(106 kcal/h)

59.454.160.4Cold Utility at pinch

(106 kcal/h)

- 4.37-9.63- 4.05� Hot Utility (Base)

(106 kcal/h)

1.400

13.120

8.555

4.565

13.2

MVGO (pinch – Mid PA)

3.008

11.512

5.728

5.784

13.3

MVGO 285 (pinch – HVGO)

12.432Overall Cost(106 US$/yr)

2.088Savings

(106 US$/yr)

6.644Utility Cost(106 US$/yr)

5.789Capital Cost (106 US$/yr)

19.2�T optimum (ºC)

16,14,6

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.

Case Study 3

Pinch Stream Pumparound �T

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?

Results – Case Study 3

285260230200T HVGO PA return (ºC)

314321324326T HVGO pan (ºC)

57.8

3.78

MVGO 200

58.5

4.50

MVGO 260

59.458.1Cold Utility (106 kcal/h)

5.424.07Hot Utility (106 kcal/h)

MVGO 285MVGO 230

350ºC

326ºC

314ºC

�

�o

pinchendcold

55)arctan(

TT

UtilityHot)arctan(

��

���

Pinch Stream Pumparound �T

GrandCompositveCurve

Hot utility

Heat moving across pinch

Results – Case Study 3

MVGO 260 MVGO 285

Results – Case Study 3

2.881

11.639

5.954

5.685

14.1

MVGO 260

2.549

11.971

5.977

5.994

17.2

MVGO 230

11.51212.209Overall Cost(106 US$/yr)

3.0082.311Savings

(106 US$/yr)

5.7846.181Utility Cost(106 US$/yr)

5.7286.028Capital Cost(106 US$/yr)

13.319.0�T optimum

(ºC)

MVGO 285MVGO 200

As HVGO flow rate increases, the HEN approach becomes higher, resulting less Capital Cost, allowing more heat integration.

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)

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).

Conclusion

– Appropriate variation of process streamsobserving thermodynamics may result in high process integration (grass root orrevamp)

– Optimization taking into account theseinsights could improve the design.

Thank you very much!

Antonio V. S. de Castro, antonio.castro@petrobras.com.br

Claudio L. M. Kuboski

Carlos Ney da Fonseca

Silvia Waintraub

Washington de O. Geraldelli