Upgrading the Bottom of the Barrel

Abstract

Crude oils usually contain substantial yields of heavy hydrocarbons boiling above 600–800 °F. These are referred to as atmospheric and vacuum residual oils, residua, or resids. Resids were primarily used for many years as heavy fuel oils (bunker oil) or various tar or asphalt products. Modern refinery economics and environmental regulations make the processing of residua to light oils and feedstocks for other units desirable and, indeed, necessary in many areas. Several process approaches are available for resid conversion. In this chapter, we explore several of the resid conversion processes, including thermal cracking, visbreaking, delayed coking, Flexicoking™, deep oil FCC, and residuum hydrocracking. A detailed design example of thermal cracker key equipment is provided.

David S. J. Jones: deceased.

Appendix 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 Sassan 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 through 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:

• The convection section with a heat flux of 12,000 Btu/h sq.ft.

• The radiant heater section with a heat flux of 15,000 Btu/h sq.ft.

• The soaker section with a heat flux of 10,000 Btu/h sq.ft.

The heater section and the soaker section are divided by a fire wall. A predicted temperature profile is given in Fig. 17.

Fig. 17

Temperature profile across the thermal cracker heater

The salient temperature and pressure conditions are as follows:

$$ \begin{array}{c}\mathrm{Convection}\ \mathrm{inlet}=500{}^{\circ}\mathrm{F} \mathrm{at} 330 \mathrm{psig}\ \mathrm{pressure}\\ {}\mathrm{Heater}\ \mathrm{inlet}=700{}^{\circ}\mathrm{F} \mathrm{at} 320 \mathrm{psig}\\ {}\mathrm{Soaker}\ \mathrm{inlet}=820{}^{\circ}\mathrm{F} \mathrm{at} 290 \mathrm{psig}\\ {}\mathrm{Soaker}\ \mathrm{outlet}=920{}^{\circ}\mathrm{F} \mathrm{at} 250 \mathrm{psig}\end{array} $$

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 Fig. 18):

Fig. 18

The TBP curve for the Sassan crude residue feed

$$ \begin{array}{c}\mathrm{F}\mathrm{eed}\ \mathrm{gravity}=18{}^{\circ}\mathrm{A}\mathrm{P}\mathrm{I}\ \left(7.88 \mathrm{l}\mathrm{b}\mathrm{s}/\mathrm{gal}\right).\\ {}\mathrm{F}\mathrm{eed}\ \mathrm{rate}=25,500 \mathrm{BPSD} = 351,645\ \mathrm{lb}/\mathrm{h}.\\ {}\mathrm{Volume}\ \mathrm{o}\mathrm{f}\ \mathrm{waxy}\ \mathrm{distillate}\ \mathrm{in}\ \mathrm{t}\mathrm{he}\ \mathrm{f}\mathrm{eed}\ \mathrm{t}\mathrm{o} 1,022{}^{\circ}\mathrm{F} \mathrm{cut}\ \mathrm{point}=68 \mathrm{vol}.\%\\ {}=17,340 \mathrm{BPSD}\end{array} $$

$$ \begin{array}{c}\mathrm{Gravity}\ \mathrm{of}\ \mathrm{the}\ \mathrm{distillate}=26{}^{\circ}\mathrm{A}\mathrm{P}\mathrm{I}\ \left(7.48 \mathrm{l}\mathrm{b}\mathrm{s}/\mathrm{gal}\right).\\ {}\mathrm{Weight}\ \mathrm{of}\ \mathrm{distillate}=17,340\times 7.48\times 1.75=226,981\ \mathrm{l}\mathrm{b}\mathrm{s}/\mathrm{h}.\\ {}\mathrm{P}\mathrm{ercent}\ \mathrm{weight}\ \mathrm{distillate}\ \mathrm{on}\ \mathrm{feed}=65\%\end{array} $$

Referring to Fig. 7 in this chapter, the soaking volume factor (SVF) corresponding to a 9 % conversion with a distillate content of 65 % is 0.135.

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:

• Section 1

  Pressure = 365 psia

  	V/L	°F	°API	lbs/h	Btu/lb	MMBtu/h
  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

  $$ \mathrm{S}\mathrm{q}.\mathrm{ft}.\ \mathrm{of}\ \mathrm{coil} = \frac{47,824,000}{12,000}=3,985.3\ \mathrm{sq.}\;\mathrm{ft.} $$

• Section 2

  Pressure = 335 psia

  Six hundred psig steam is introduced into this section of the furnace.

  	V/L	°F	°API	lbs/h	Btu/lb	MMBtu/h
  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

• Section 3

  Pressure = 315 psia

  	V/L	°F	°API	lbs/h	Btu/lb	MMBtu/h
  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 Fig. 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 (Figs. 8 and 9)

Section 4 temp 840 ° F

	Gas		Naphtha		Gas oil		Residue
	wt%	lbs/h	wt%	lbs/h	wt%	lbs/h	wt%	lbs/h
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

Section 5 temp 860 °F

		Gas	Naphtha		Gas oil		Residue
	wt%	lbs/h	wt%	lbs/h	wt%	lbs/h	wt%	lbs/h
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/h	wt%	lbs/h	wt%	lbs/h	wt%	lbs/h
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/h	wt%	lbs/h	wt%	lbs/h	wt%	lbs/h
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 temp 920 ° F

	Gas		Naphtha		Gas oil		Residue
	wt%	lbs/h	wt%	lbs/h	wt%	lbs/h	wt%	lbs/h
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		236,473

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.

Section 4

	V/L	°F	°API	lbs/h	Btu/lb	MMBtu/h
In
From Section 3		820		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
Residue	L	840	17	300,989	476	143.271
Steam	V	840		35,165	1,471	51.728
Heat of cracking					547a	10.223
Total out				386,810		235.879

1. ^aHeat of cracking is 547 Btu/lb of the converted gas and naphtha

$$ \mathrm{Heater}\ \mathrm{coil}\ \mathrm{duty} = 12,274,500\ \mathrm{Btu}/\mathrm{h} $$

Heat flux is 10,000 Btu/sq.ft.:

$$ \mathrm{S}\mathrm{q}.\mathrm{ft}.\ \mathrm{of}\ \mathrm{coil}\frac{12,274,500}{10,000}=1,227.5\ \mathrm{sq.}\;\mathrm{ft.} $$

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 are based on coils constructed using 4″ schedule 80 steel pipes. The ratio of area to volume is 0.11 cuft/sq.ft.

Coil section	Exit temp °F	Duty MMBtu/h	Sq.ft. of coil	Volume of coil cuft	Cumulative volume cuft	KT/K800
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 versus volume of coil is plotted over each coil section and is given in Fig. 19. The plot of coil volume above 800 °F versus the K_T/K₈₀₀ ratio is also plotted in Fig. 20. The area under the curve developed in Fig. 20 is calculated and then divided by the throughput in terms of BPSD gives the SVF for the conversion. Thus,

Fig. 19

Coil temperature versus coil volume

Fig. 20

Coil volume above 800 °F versus the K _t /K _800°f ratio

$$ \begin{array}{c}\mathrm{Area}\ \mathrm{under}\ \mathrm{the}\ \mathrm{curve}\ \mathrm{o}\mathrm{f}\ \mathrm{F}\mathrm{ig}. 20={\displaystyle {\int}_{800}^{920}{K}_T/K{}_{800}\times \mathrm{coil}\ \mathrm{vol}}\\ {}=3,867\\ {}\mathrm{allow}\ 10\%\ \mathrm{f}\mathrm{o}\mathrm{r}\ \mathrm{the}\ \mathrm{steam}=3,\ 480\\ {}\mathrm{the}\mathrm{n}\ \mathrm{calculated}\ \mathrm{S}\mathrm{V}\mathrm{F}=\frac{3,\ 480}{25,500}\\ {}=0.136\end{array} $$

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 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.