Jones D.S.J., Pujado P.R. Handbook of Petroleum Processing
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472 CHAPTER 11
Table 11.7. Conditions and yield data on hydrocracking M.E. residue
SOR EOR
Reactor conditions
Pressure, psia 3,000 3,000
Inlet temperature,
◦
F 700 750
Outlet temperature,
◦
F 750 800
Space velocity, V/V/hr 0.5 0.4
Feed
Vacuum residue +1,025
◦
F TBP cut point
BPSD 13,000 13,000
lbs/hr 190,266 190,266
Make up hydrogen
Scf/Bbl 1,290 1,875
lbs/hr 4,010 5,606
Products
Ammonia
lbs/hr 209 203
Mwt 17
H
2
S
lbs/hr 6,964 6,680
Mwt 34
Net separator gas
lbs/hr Nil 2,405
M wt 4.3
Cold flash gas
lbs/hr 3,772 5,569
M wt 9.6
Cold flash liquid
lbs/hr 22,586 39,886
lBPSD 1,852 3,321
Hot flash liquid
lbs/hr 160,744 141,129
BPSD 11,672 10,280
Conclusion
Catalytic cracking of residues described above is a poor competitor to the other direct
processes for residue upgrading. The process itself is costly to build and has relatively
high operating and maintenance costs. The products however are good quality, and
require very little downstream treatment. The naphtha produced is (as in the case
of distillate hydrocracking) high in naphthene content ideal for gasoline production
or as feed to an aromatic production process. The middle distillates also are of high
quality with the kerosene fraction having very low smoke point and the gas oil fraction
having a good cetane number. In the process illustration given as Figure 11.15 the
light distillate liquid from the cold flash drum and the associated vapor are routed
to a distillate hydrocracker recovery side for further processing and fractionation.
UPGRADING RESIDUES 473
Figure 11.16. A typical TBP curve for ‘hot flash’liquid.
These recovery units consist of a main fractionation tower, and alight ends unit which
includes an absorber column.
Appendix 11.1: Sizing a thermal cracker heater/reactor
In this example it is required to define a thermal cracker in terms of coil volume
and temperature profile in processing an atmospheric residue from Sasson Crude.
The 25,500 BPSD of the +600
◦
F residue is preheated to 500
◦
F by heat exchange
with the cracker’s products and reflux stream before entering the convection side of
the cracker’s heater/ reactor. It is required to produce a conversion (based on gas to
naphtha of 260
◦
F cut point) of 9 wt% and the heater will be fitted with 4
schedule
80 tubes throughout. It will be designed to have three sections which are:
r
The convection section with a heat flux of 12,000 Btu/hr sqft.
r
The radiant heater section with a heat flux of 15,000 Btu/hr sqft.
r
The soaker section with a heat flux of 10,000 Btu/hr sqft.
The heater section and the soaker section are divided by a fire wall. A predicted
temperature profile is given in the diagram below as Figure 11.17.
The salient temperature and pressure conditions are as follows:
474 CHAPTER 11
Section 1
Section 4
Section 7
Section 3
Section 5
Section 6
Section 8
Section 2
700°F
820°F
840°F
900°F
860°F
880°F
920°F
Heater Side
500°F
Feed In
600 psig Steam
Convection Side
760°F
Cracked
Effluent
Out
Soaker Side
Figure 11.17. Temperature profile across the thermal cracker heater.
Convection inlet = 500
◦
F at 330 psig pressure
Heater inlet = 700
◦
F at 320 psig
Soaker inlet = 820
◦
F at 290 psig
Soaker outlet = 920
◦
F at 250 psig
A 600 psig steam is introduced into the heater inlet coil. This will be at 10 wt% of
the residue feed. Details of the residue feed are as follows (a TBP curve of the feed
is given in Figure 11.18):
Feed gravity = 18
◦
API (7.88 lbs/gal).
Feed rate = 25,500 BPSD = 351,645 lb/h.
Volume of waxy distillate in the feed to 1,022
◦
F cut point = 68 vol%
= 17,340 BPSD
UPGRADING RESIDUES 475
% Volume Distilled and Mid Volume %
API
Temperature
°
F
2000
1800
1600
1400
1200
1000
800
600
100
806040200
0
10
20
30
40
TBP Curve
API Curve versus Mid V01%
Figure 11.18. The TBP curve for the sassan crude residue feed.
Gravity of the distillate = 26
◦
API (7.48 lbs/gal)
Weight of distillate = 17,340 × 7.48 × 1.75 = 226,981 lbs/hr
Percent weight distillate on feed = 65%
Referring to Figure 11.7 in this chapter the soaking volume factor (SVF) correspond-
ing to a 9% conversion with a distillate content of 65% is 0.135.
476 CHAPTER 11
The heater coil is divided into the following sections:
Temp in,
◦
F Temp out,
◦
F
Section 1 Convection side 500 700
2 Heater side 700 760
3
”
760 820
4 Soaker side 820 840
5
”
840 860
6
”
860 880
7
”
880 900
8
”
900 920
Heat balance over the convection side is as follows:
1st Section
Pressure = 365 psia
V/L
◦
F
◦
API lbs/hr Btu/lb mmBtu/hr
IN
Feed L 500 15 351,645 250 87.911
Heater duty By diff 47.824
Total In 135.735
OUT
Feed L 700 15 351,645 386 135.735
Total out 135.735
Sqft of coil =
47,824,000
12,000
= 3,985.3
2nd Section
Pressure = 335 psia
600 psig steam is introduced into this section of the furnace.
V/L
◦
F
◦
API lbs/hr Btu/lb mmBtu/hr
IN
Feed L 700 15 351,645 386 135.735
Steam V 700 35,165 1,383 48.633
Heater duty By diff 16.652
Total in 201.020
OUT
Steam V 35,165 1,417 49.829
Feed L 760 15 351,645 432 151.191
Total out 201.020
UPGRADING RESIDUES 477
3rd Section
Pressure = 315 psia
V/L
◦
F
◦
API lbs/hr Btu/lb mmBtu/hr
IN
Feed L 760 15 351,645 386 151.191
Steam V 760 35,165 1,417 49.829
Heater duty By diff 22.584
Total in 223.604
OUT
Liq feed L 820 20 344,612 482 165.103
Vap V 820 73 7,033 425 2.989
Steam V 820 35,165 1,440 50.638
Ht of crack 547 3.874
Total out 386,810 223.604
Cracking begins at a temperature of 800
◦
F and the oil feed and steam enters the
soaking section at 820
◦
. The purpose of the soaking section is to provide a space for
the cracking function to occur at a moderate increase in temperature. To calculate the
required coil volume the first step is to assign the degree of cracking that occurs at
the end of each section. This is a trial and error process and provides an SVF value to
each section which in turn is used to calculate the amount of cracked products leaving
each section. Using these amounts the heat balances for each section of the soaker
coil are calculated. Thus:
Final trial
Percent crack at section outlet
% Conversion SVF (from Figure 11.7)
Section 4 3.0 0.045
Section 5 4.0 0.059
Section 6 6.0 0.090
Section 7 8.0 0.120
Section 8 10.0 0.150
Material compositions (Figures 11.8 and 11.9).
Section 4 temp 840
◦
F
Gas Naphtha Gas Oil Residue
wt% lbs/hr wt% lbs/hr wt% lbs/hr wt% lbs/hr
Distillate 0.9 2,043 2.5 5,675 5.9 13,392 – 205,871
Residue 2.2 2,743 6.6 8,228 14.9 18,575 – 95,118
Total 4,786 13,903 31,967 300,989
478 CHAPTER 11
Section 5 temp 860
◦
F
Gas Naphtha Gas Oil Residue
%wt lbs/hr wt% lbs/hr wt% lbs/hr wt% lbs/hr
Distillate 1.1 2,497 3.3 7,490 7.3 16,570 – 200,424
Residue 5.9 3,740 8.6 10,721 17.2 21,442 88,761
Total 6,237 18,211 38,012 289,185
Section 6 temp 880
◦
F
Gas Naphtha Gas Oil Residue
%wt lbs/hr %wt lbs/hr %wt lbs/hr wt% lbs/hr
Distillate 1.7 3,859 5.0 11,349 9.9 22,471 – 189,302
Residue 4.4 5,785 23.2 16,456 21.2 26,429 75,994
Total 9,644 27,805 48,900 265,296
Section 7 temp 900
◦
F
Gas Naphtha Gas Oil Residue
wt% lbs/hr wt% lbs/hr wt% lbs/hr wt% lbs/hr
Distillate 2.1 4,767 6.4 14,527 11.5 26,103 – 181,584
Residue 5.8 7,231 16.8 20,944 23.6 29,421 – 67,068
Total 11,998 35,471 55,524 248,652
Section 8.0 temp 920
◦
F
Gas Naphtha Gas Oil Residue
wt% lbs/hr wt% lbs/hr wt% lbs/hr wt% lbs/hr
Distillate 2.6 5,902 7.3 16,570 12.5 28,373 – 176,136
Residue 7.2 8,976 19.0 23,686 25.4 31,665 – 60,337
Total 14,878 40,256 60,038 2,36473
Heat balances over the soaker section
The heat balances for Sections 4, 5, 6, 7, and 8 can now be developed to establish
the heat surface area for each of these coil sections. Only the balance for Section 4 is
shown here in detail. The remaining coil sections are given in the summary table that
follows.
UPGRADING RESIDUES 479
The heat balances for each section of the soaker coil can now be written. Only the
first section heat balance is shown here:
Section 4
V/L
◦
F
◦
API lbs/hr Btu/lb mmBtu/hr
IN
From sec 3 820 3,386,810 223.604
Heater duty By diff 12.275
Total in 235.879
OUT
Gas V 840 73 4,786 622 2.978
Naphtha V 840 65 13,903 616 8.564
Gas oil V 840 38 31,967 598 19.116
Res L 840 17 300,989 476 143.271
Steam V 840 35,165 1,471 51.728
Ht of crack 547* 10.223
Total out 386,810 235.879
*Heat of cracking is 547 Btu per lb of the converted gas & naphtha.
Heater coil duty = 12,274,500
heat flux is 10,000 Btu/sqft
Sqft of coil =
12,274,500
10,000
= 1,227.5
A summary of coil section exit temperature, surface areas and coil volumes is given
in the following table:
The volume data in the table below is based on coils constructed using 4
schedule
80 steel pipe. The ratio of area to volume is 0.11 cuft/sqft
Duty Volume of Cumulative
Coil section Exit temp
◦
F mmBtu/hr Sqft of coil coil cuft volume cuft K
T
/K
800
1 700 47.824 3,985 438.4 438.4 –
2 760 48.633 1,110 122.1 560.5 –
3 820 22.584 1,506 165.7 726.2 1.55
4 840 12.275 1,227 135.0 861.2 3.02
5 860 5.682 568 62.5 923.7 5.0
6 880 21.739 2,174 239.1 1,162.8 7.3
7 900 13.855 1,386 152.5 1,315.3 9.0
8 920 11.634 1,163 127.9 1,443.2 10.2
Total 184.226 1,443.2
Temperature vs. volume of coil is plotted over each coil section, and is given as Figure
11.19 below. The plot of coil volume above 800
◦
FV’s the K
T
/K
800
ratio is also plotted
480 CHAPTER 11
Temperature °F
Coil Volume Cu ft
500 600 700 800 900 1000
200
400
600
800
1000
1200
1400
1600
Total Volume = 1443 Cu ft
Coil Outlet
Figure 11.19. Coil temperature versus coil volume.
as Figure 11.20. The area under the curve developed in Figure 11.20 is calculated and
then divided by the throughput in terms of BPSD gives the SVF for the conversion.
Thus:
Area under the curve of Figure 11.20 =
920
800
K
T
/K
800
× Coil vol
= 3,867
allow 10% for the steam = 3,480
then calculated SVF =
3,480
25,500
= 0.136
which compares well with the estimate for a 9% conversion originally used.
The duty specification for the heater can now be developed with the coil profile and
other data to meet the required conversion. The final material composition can also
be used now to develop the syncrude composition for the design of the recovery side
UPGRADING RESIDUES 481
Shaded Area = KT /KT800 . Coil Volume
920
800
= 3867
1.0 3.0 5.0 7.0 9.0 11.0
K
T/ K800 °F Ratio
600
800
1000
1200
1400
Coil Volume Cu ft
Coil Outlet
Figure 11.20. Coil volume above 800
◦
F versus the K
T
/K
800
◦
F
ratio.
which will probably consist of a main fractionator with possibly a vacuum distillation
unit for the cracked residue. The main fractionator usually produces a ‘Wild’full
range naphtha which is routed to the naphtha product stream leaving the Atmospheric
Crude Unit. A full range gas oil would be blended with the straight run atmospheric
light gas oil to be hydrotreated and routed to the diesel pool.