Heat recovery analysis of an existing crude distillation unit

Heat Recover2,"Systems Vol. 6, No. 5, pp. 361-367, 1986

0198-7593/86$3.00+ .00 PergamonJournals Ltd

Printed in Great Britain

HEAT RECOVERY ANALYSIS OF AN EXISTING CRUDE DISTILLATION UNIT* A. A. BALLUT Department of Chemical Engineering, Al-Fateh University, PO Box 13317, Tripoli, Libya (Received 19 February 1986)

Abstract--The cost of energy has become a significant factor in the design of new refineriesas well as in retrofitting existing designs. Process heat accounts for about 65% of an average refinery energy consumption. As such, reducingheater duties represents the singlemost important energysavingmethod. One way of reducing heater duty is the recovery of more process heat and thus increase heater inlet temperature. Greater energy pick up, off-course,is subject to both thermodynamicas well as economic constraints.

INTRODUCTION In view of their large energy consumption and the energy bill they have to pay for it, refineries have become more aware of energy recovery and conservation. Typically, refineries consume between five and ten percent of the energy equivalent of the crude processed as energy in the form of fuel for process heat, steam and electrical power with process heat accounting for about 65% of total energy requirements [1]. As a result, considerable emphasis is usually given to energy conservation in the area of process heat. One area which has received particular interest is energy conservation in crude distillation units. Reducing crude heater duty represents the single most important method o f saving energy in these units. One way of reducing crude heater duty is the recovery of more process heat in the crude preheat train to increase crude inlet temperature to the heater. In this paper, the feasibility of greater heat recovery in an existing 60000 bpsd crude distillation unit is analysed both thermodynamically and economically. First, the maximum recoverable energy is determined and energy targets are established. Heat exchanger networks are then systematically synthesized for different levels o f heat recovery and their economics assessed.

THE CRUDE DISTILLATION UNIT Figure 1 is a schematic diagram of the crude distillation unit studied in this paper. The five streams exchanging heat with the crude are: the residuum (RES), heavy gas oil (HGO), light gas oil (LGO), bottom pump-around (BPA) and top pump-around (TPA). The kerosine product is not exchanged with the crude because it is used to preheat the crude desalter water. The overhead product is simply cooled using air and water. Further utility cooling is required to bring the process streams involved in crude preheating to their final rundown temperatures. The heat loads of these cooling utilities is approximately 55 MM Btu/h (16.1 MW), a sizeable quantity of heat that simply gets rejected to the atmosphere. Recovering part of this wasted heat into the process to increase the crude preheat temperature is highly desirable as was previously stated. This, however, must be done carefully and after analyzing the important factors that affect the design of heat recovery systems such as the temperatures at which heat is available, the cost of fuel saved in the heater and the cost of additional equipment required.

*Paper presented at 38th Annual Session of Indian Institute of Chemical Engineers, Calcutta, India, 17-20 Dec., 1985. H.a.S6/5--1~

361

362

A, A. BALLUT

CW

Overhead

Crude

.~77 = ~

59

RES

131

O

177 •

7j

"~

L60

=

~

Kerose~ l

product

I 24o = ~ . , ~

131

~ ~ 6

I

66~ "F~-J

13,

~

:

°F

~ BPA500 327

1 363

H60 :

Temp

~----"~2 ( ~ 530H60

V

609

433

Fig. I. Schematic diagram of the crude unit.

MAXIMUM RECOVERABLE HEAT To avoid trying fruitlessly to recover energy that simply cannot be recovered and to set from the start feasible energy targets, a heat availability analysis is carded out. The analysis is limited to heat available from the five process streams currently used to preheat the crude. The overhead and kerosine products are excluded due to their relatively low t~mperatures. The analysis method used [2] calls for the construction of the so-called heat availability diagram, which is simply a temperature-cnthalpy diagram, using individual cooling and heating curves of the process streams involved.

f

640

600

RE.S/ ..G.O

480

o w

440 40O 360 32O

.G,

28O

24O 200 160

120 80 4

0

8

0

~

2 0 EnthoLpymmBtu /

e

1624324048566472808896

hr

Fig. 2. Heating and cooling curves.

Heat recovery analysis of an existing crude distillation unit

363

600 550 500 = - - M o x i m u m

crude o u t l e t t e m p e r o t u r e - - j ~

450

•- 4O0 _ v E

Existing crude o u t l e t . . . . . . t

7"

•

. . . . . .

,"

f

- -~

//,;o

•

rye

350 30o

250 20O t50 tO0 /

/

i

/j

Minimum cooling

-

50 0

I 20

I 40

I 60

I 80

I I00

I 120

I 140

EnthoWy

I 160 mm

I I I I 180 200 220 240

I 260

I 280

I 300

B t u / hr

Fig. 3. Heat availability diagram.

Figure 2 shows the cooling and heating curves of the six process streams involved. The solid lines on the cooling curves represent existing heat exchange with the crude while the dotted lines indicate further cooling requirements. In Fig. 3, the so-called 'super-cooling' and 'super-heating' curves of the heat availability diagram are constructed. Curve A represents the super-heating curve and it is simply the heating curve for the crude; the only cold stream to which heat is added. Curve B is the super-cooling curve formed by merging the cooling curves of all the hot process streams. Moving these two curves arbitrarily so that curve A is above curve B and just touches it represents the maximum recoverable heat (or the minimum utility requirements) for the problem as illustrated by curve C. At this point then: The minimum cooling utility is approximately 24 MM Btuh-~ (7.9 MW). The maximum crude preheat temperature is about 495°F. The minimum temperature difference (ATm=) is equal to zero.

OPTIMUM HEAT RECOVERY The minimum utility requirement indicated by the heat availability analysis represents the theoretical limit for the maximum recoverable heat. This limit can only be approximated in practice because of the infinite amount of heat surface required. In practice, heat exchanger networks are designed away from this theoretical minimum by specifying ATtar, at economical levels. If, for example, ATmi, thought to be most economic is 20°F (10°C), then the maximum recoverable heat will be reduced and the cooling utility requirement increased. Heat exchanger networks designed to meet the reduced target are optimal from an energy point of view. The cost of the additional heat exchange surface, however, must be justified economically. Thus it is essential to evaluate various levels of heat recovery in order to determine the most feasible energy recovery system. This objective is sought in this study by evaluating the economics of heat recovery for two crude preheat target temperatures: 469°F (243°C); case A, and 489°F (257°C); case B, relative to the existing level or target of 433°F (223°C); base case, subject to ATmi, = 20°F. The evaluation is done in two steps. First, optimal heat exchanger networks are synthesized for the above cases. Second, the additional heat transfer areas required are determined using suitable overall heat transfer coefficients and their economics are assessed relative to the savings which result.

364

A.A.

SYNTHESIS

OF

BALLUT

OPTIMAL

NETWORKS

The task of synthesizing optimal heat exchanger networks for a set of hot and cold streams can be quite formidable due to innumerable combinatorial possibilities. As a result, considerable attention has been given to finding a satisfactory solution to this problem and several methods have been proposed [3]. The particular method used in this study is the 'Temperature Interval' or TI method of Linnhoff and Flower [4]. According to the TI method, the case problem is first partitioned by temperature intervals subject to the prescribed AT,.~. Next, the minimum utility bounds are determined using what is referred to as the problem table. Finally, network synthesis is carried out to solve each case with maximum energy recovery as determined by the problem table. Table I. Stream data for base case (T = 433°F, ATm~ = 20<'F) Stream* number

T (°F)

Tr (~'F)

H or C

Cr (Btu h- I °F -1)

I 2 3 4 5 6

59 242.6 609.8 419 509 530.6

433.4 131 177.8 131 3632 131

C H H H H H

374578.7 657617.1 145061.5 57245.1 172174.7 44014.1

Heat load ( B t u h -I) - 140242275 73390079 62666588 16486599 25103082 17588060 54992134

Table 2. Problem table for base case (T = 433°F, AT,,,, = 20°F)

S

2

Streams & temperatures Hot °F 4 5 6 609 589

1

530

510 489 469 433 399

2 3 4

489 453

5

41_._99

6 7 8

36____33 266 242

9 I 10

34___L_3 246 222 157 111 59

177 131 79

11

Cold I

Deficit (Btu h - t )

Accumulated Input Output

Maximum permissible Input Output

-- I 1 4 8 8 8 7 4

0

11 4 8 8 8 7 4

0

--4084035 --7225009 - 12860517 +458491 -2450562 + 12466665 + 3001234 - 34302483 - 17985135 + 19478093

11488874 15572910 22797919 35652437 35199945 37650508 25 183 842 22182608 56485092 74470227

15572910 22797919 35658437 35199945 37650508 25103842 22182608 56485092 74470227 34992134

11488874 15572910 22797919 36658437 35199945 37650508 25183842 22182608 56485092 74470227

11 488874 15572910 22797919 35658437 35199945 37650508 25183842 22182608 56485092 74470227 54992134

*l--crude, 2 - - T P A , 3----RES, 4---LGO, 5---BPA, 6---HGO. Table 3, Stream data for case A (T = 469°F, A T e = T

Tr

C

Stream

(°F)

CF)

H or C

(Btu h - f ° F ~)

1 2 3 4 5 6

59 242.6 609.8 419 509 530.6

469 131 177.8 131 363.2 131

C H H H H H

374 578.7 6576t7.1 145061.5 57245. I 172 174.7 44014.1

20°F) Heat load (Btuh -I) - 153 579278 73390079 62666588 16486599 25103 082 17588060 41657131

Table 4. Probicrn table for case A (T = 469°F. AT,m = 20°F)

S 1

2

Streams & temperatures Hot °F 4 5 6 609 589 53O

2 3 4 5 6 7

510 489 469 433

8

9 10 11

-gi 79

i--iii-m

Cold 1

Deficit (Btu h - ' )

Accumulated Input Output

M s permissible Input Output

- 11488874

0

I 1488874

0

-4084035 -7225009 +474485 +458491 -2450562 +12466665 +3001234 -34302483 - 1785136 + 19478093

11488874 15572910 22797919 22323434 21864943 24315506 11848840 8847605 431 50089 61 135225

15572910 22797929 22323434 21864943 24315506 118,t8840 8847605 43150089 61 135225 41657131

11488874 15572910 22797919 22 323434 21964943

11488874 15 572910 22797919 22323434 21864943

24315506

34315506 ii8o84o I 1848840 8847605 43150089 61 135225

8847605 43150089 6I 135225 41657131

H e a t recovery analysis o f an existing crude distillation unit

365

Table 5. Stream data for case B ( T = 489°F, A T u : 20°F) Stream number

T

Tr

(°F)

(°F)

H or C

(Btuh -I °F -~)

1 2 3 4 5 6

59 242.6 609.8 419 509 530

489 131 177.8 131 363.2 131

C H H H H H

374578 657617 145061 57245 172174 44014

Heat load (Btuh -I)

Cp

- 161068853 73390079 62666588 16486599 25103082 17588060 34165556

Table 6. Problem table for case B (T = 489°F, AT,M = 20°F)

S

2

Streams & temperatures Hot °F 3 4 5 6 609 589

1 2 3 4 5 6 7 8 9 10 11

530- - ~N

I T T T T T T

Cold 1

510 489

--T6-

2--i/Vi-/ii--

Deficit (Btuh-*)

Maximum permissible Input Output

Accumulated Input Output

-- 11488874 0 --4084035 11488874 +266564 15572910 +474485 15306345 +458491 148318594 -2450562 14373368 + 12466665 1 6 8 2 3 9 3 1 + 3001234 4357265 --34302483 1356031 -179851 35658515 +19478093 53643650

I 1488874 0 15572910 11488874 15306345 15572910 14831859 15306345 14373368 14831859 16823931 1 4 3 7 3 3 6 8 4357265 16823931 1 3 5 6 0 3 1 4357265 35658515 1356031 53643650 35658515 34165556 53643650

11488 874 15572910 15306345 14831859 14373363 16823931 4357265 1356031 35658515 53643650 34165556

Thus, the partitioning procedure as illustrated by Linhoff and Flower [4] is carried out using the streams data for the three cases given in Tables 1, 3 and 5. The minimum utility bounds for these cases are established for their respective problem tables which are presented in Tables 2, 4 and 6. The bottom figure in Column 5 gives the minimum cooling load for each case. The synthesized heat exchanger networks that solve the three cases with maximum heat recovery are shown in Figs 4-6. The synthesis task is carried out in a manner that insures: Satisfying the target temperature of stream no. 5 without resorting to utility cooling; Realizing an intermediate crude temperature of about 266°F (130°C) (desalter temperature). NETWORK ECONOMICS With the heat exchanger networks for the base case, case A and case B established, the exchangers are sized and preliminary cost estimates for the networks are prepared. Exchanger sizing 242

242

419

509

530

Crude 5 9 ° F ~

609

• 4 3 3 :F

'~ 2 L 4 ~'~ 3 ,

5 6 I

Fig. 4. H e a t e x c h a n g e n e t w o r k for base case.

366

A, A, BALLUT 242

TPA I Crude 5

31~0

250

RESI

9

HGO 1

° v

16:3

F v

230

419 LGO I

310

~ 3281

530

509

609

RE S

266

419

~F 3281

~

T

~

~

~

469 °F

~,'~I

©

2

,)

l

T

T

•

4

=

3

'- 5

,

©

"- 6 1

Fig. 5. Heat exchanger network for case A.

242

242

3i8

276

363

419

318 |

530

509

Crude 59°F

609

,~5OF

149

298

2.52

363

3B5

298

385

Fig, 6. Heat exchanger network for case B. 9000(:

AT.,.: 20 "F (-)I0"I,

eooo¢

)esign U

U = over - oLL heot tronsfer

7oooc

)IO/.

A 6oooc

%..

~,ooc

4ooo¢

3oooc

Jo

[ 440

1 4eo

I, 46o

1 4~'o

l 4o0

Temp ("F) Fig. 7. Heat exchange area requirements.

t 490

Heat recovery analysis of an existing crude distillation unit

17455O0_

~T.,=

=

20

367

°F

r/'28~00 1635000 141~SOOO

~~/(*110%

.,.. 12~ 00¢ fl2S~

615~

I

I

I

I

1

I

i

Temp (°F)

Fig. 8. Network costs. Table

Case A B

7.

Payback period (months)

assessment

Fuel cost SMMBtu -~ 3.0 3.5 4.0 18 15 12 41 28 26

is carried out on the basis of overall heat transfer coefficients used in the design of the existing preheat train with a variation of + 10%. The results are presented in Figs 7 and 8. Quite evident from these two figures that higher degrees of heat recovery are rather costly particularly beyond a target temperature of about 470°F. The economic incentives for investing in additional heat recovery equipment depends largely on the value of the fuel saved in the crude heater. Three levels of fuel cost are used to determine the feasibility of implementing cases A and B, namely, $3.0, $3.5 and $,4.0 per MM Btu. The expected payback periods for the cases are summarized in Table 7. The payback periods range from 12-41 months. Thus, depending on what payback period is considered reasonable, the degree of heat recovery can be determined. If it is determined that 12 months is an acceptable payback period, then case A can be implemented provided that the cost of fuel is $4.0 per MM Btu or higher. CONCLUSIONS The upper bound on heat recovery for crude preheating as determined by the heat availability analysis is limited to about 50 percent of the current heat losses put at 55 MM Btu h- ~. Of this, however, only about 14 MM Btuh-~ may be recovered practically and economically. The resulting savings could pay for the cost of additional heat recovery equipment in about one year. Acknowledgement--The assistance o f Mr. Emhemed Arafa is gratefully acknowledged.

REFERENCES I. 2. 3. 4.

R. E. N. B.

V. Elshout, Retrofitting for energy savings, Hydrocarbon Processing 109, 0982). C. Hohmann, Optimum networks for heat exchange, Ph.D. Thesis, University of Southern California (1971). Nishida et al., A review of process synthesis, AIChE J. 27, 321 (1981). Linnhoff and J. R. Flower, Synthesis o f heat exchange networks, AIChE J. 24, 633 (1978).