U.S. patent application number 12/144305 was filed with the patent office on 2009-02-26 for process and apparatus for steam cracking hydrocarbon feedstocks.
Invention is credited to Arthur R. Di Nicolantonio, James M. Frye, James N. McCoy, David B. Spicer, Richard C. Stell, Robert D. Strack.
Application Number | 20090050530 12/144305 |
Document ID | / |
Family ID | 40381168 |
Filed Date | 2009-02-26 |
United States Patent
Application |
20090050530 |
Kind Code |
A1 |
Spicer; David B. ; et
al. |
February 26, 2009 |
Process and Apparatus for Steam Cracking Hydrocarbon Feedstocks
Abstract
The present disclosure provides a process for treating a
hydrocarbon feedstock comprising: (a) feeding the hydrocarbon
feedstock at a linear velocity equal to or less than 0.9 m/s to a
first preheating zone in the convection section of a steam cracking
furnace; (b) preheating the hydrocarbon feedstock in the first
preheating zone to vaporize less than 99 wt. % of the hydrocarbon
feedstock to form a vapor-liquid mixture; (c) separating at least a
portion of the vapor-liquid mixture to form a vapor fraction and a
liquid fraction; and (d) feeding at least a portion of the vapor
fraction to the steam cracking furnace.
Inventors: |
Spicer; David B.; (Houston,
TX) ; Di Nicolantonio; Arthur R.; (Seabrook, TX)
; Frye; James M.; (Tokyo, JP) ; Stell; Richard
C.; (Houston, TX) ; McCoy; James N.; (Houston,
TX) ; Strack; Robert D.; (Houston, TX) |
Correspondence
Address: |
EXXONMOBIL CHEMICAL COMPANY
5200 BAYWAY DRIVE, P.O. BOX 2149
BAYTOWN
TX
77522-2149
US
|
Family ID: |
40381168 |
Appl. No.: |
12/144305 |
Filed: |
June 23, 2008 |
Current U.S.
Class: |
208/130 ;
422/600 |
Current CPC
Class: |
C10G 9/20 20130101; C10G
2400/20 20130101 |
Class at
Publication: |
208/130 ;
422/194 |
International
Class: |
C10G 9/36 20060101
C10G009/36 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 21, 2007 |
US |
PCT/US2007/018486 |
Claims
1. A process for treating a hydrocarbon feedstock comprising: a.
feeding said hydrocarbon feedstock at a linear velocity equal to or
less than 0.9 m/s to a first preheating zone in the convection
section of a steam cracking furnace; b. preheating said hydrocarbon
feedstock in said first preheating zone to vaporize less than 99
wt. % of said hydrocarbon feedstock to form a vapor-liquid mixture;
c. separating at least a portion of said vapor-liquid mixture to
form a vapor fraction and a liquid fraction; and d. feeding at
least a portion of said vapor fraction to said steam cracking
furnace.
2. The process of claim 1, wherein said hydrocarbon feedstock is
fed in substantially liquid phase.
3. The process of claim 1, wherein said first convection section
comprises multiple banks of heat exchange tubes and said
hydrocarbon feedstock flows inside said tubes.
4. The process of claim 1, wherein said linear velocity is in the
range of 0.05 to 0.85 m/s.
5. The process of claim 1, wherein said linear velocity is in the
range of 0.1 to 0.80 m/s.
6. The process of claim 1, wherein said first preheating zone
comprises a first preheating section and a second preheating
section, wherein said hydrocarbon feedstock is supplied to said
first preheating section at a pressure in the range of 790 to 1480
kPa-a and a temperature in the range of 25 to 250.degree. C. to
form a preheated hydrocarbon product exiting said first preheating
section at a temperature in the range of about 100 to 350.degree.
C., and then at least a portion of said preheated hydrocarbon
product is supplied with a first diluent stream to said second
preheating section to form said vapor-liquid mixture exiting said
first preheating section at a temperature in the range of 350 to
500.degree. C., at least 1 wt. % said vapor-liquid mixture based on
the total weight of hydrocarbons in said vapor-liquid mixture is in
liquid phase, and at least 60 wt. % said vapor-liquid mixture based
on the total weight of hydrocarbons in said vapor-liquid mixture is
in vapor phase.
7. The process of claim 1, wherein 10 to 99.99 wt. % of said
hydrocarbon feedstock boils below 590.degree. C. measured according
to ASTM D-2887.
8. The process of claim 1, wherein said preheated hydrocarbon
product is in substantially liquid phase.
9. The process of claim 1, wherein said vapor-liquid mixture has a
temperature in the range of 400 to 500.degree. C. and comprises at
least 2 wt. % liquid based on the total weight of hydrocarbons in
said vapor-liquid mixture and wherein 50 to 99.99 wt. % of said
hydrocarbon feedstock boils below 590.degree. C. measured according
to ASTM D-2887.
10. The process of claim 1, wherein said vapor-liquid mixture has a
temperature in the range of 450 to 500.degree. C. and comprises at
least 5 wt. % liquid based on the total weight of hydrocarbons in
said vapor-liquid mixture and wherein 10 to 99.99 wt. % of said
hydrocarbon feedstock boils below 590.degree. C. measured according
to ASTM D-2887.
11. The process of claim 1, further comprising mixing a secondary
diluent stream with said vapor-liquid mixture prior to said step
(c).
12. The process of claim 11, wherein a secondary diluent stream
comprises steam.
13. A process for cracking a hydrocarbon feedstock in a steam
cracking furnace having a convection section, said convection
section comprises a first bank, a second bank and a third bank of
heat exchange tubes, said process comprising: a. feeding said
hydrocarbon feedstock having at least 1 wt. % low volatile
components and at least 0.1 wt. % coke precursors to said first
bank of heat exchange tubes provided in said convection section,
with an inlet linear velocity equal to or less than 0.9 m/s; b.
preheating said hydrocarbon feedstock to form a preheated
hydrocarbon product having a temperature below 350.degree. C.; c.
supplying at least a portion of said preheated hydrocarbon product
with a first diluent stream to said second bank of heat exchange
tubes to vaporize at least a portion of said hydrocarbon feedstock
to form a vapor-liquid mixture having a temperature in the range of
350 to 500.degree. C. and comprising at least 1 wt. % liquid based
on the total weight of hydrocarbons in said vapor-liquid mixture;
d. separating at least a portion of said vapor-liquid mixture from
step (c) to form a vapor fraction and a liquid fraction; and e.
feeding at least a portion of said vapor fraction from step (d) to
said third bank of heat exchange tubes.
14. The process of claim 13, further comprising mixing a second
diluent stream to said vapor-liquid mixture prior to step (d).
15. The process of claim 13, further comprising mixing said vapor
fraction with a third diluent stream comprising steam prior to step
(e).
16. The process of claim 13, wherein 10 to 95 percent of said
hydrocarbon feedstock boils below 590.degree. C. measured according
to ASTM D-2887.
17. The process of claim 13, wherein said hydrocarbon feedstock
comprises one or more of steam cracked gas oil and residues, gas
oils, heating oil, jet fuel, diesel, kerosene, gasoline, coker
naphtha, steam cracked naphtha, catalytically cracked naphtha,
hydrocrackate, reformate, raffinate reformate, Fischer-Tropsch
liquids, Fischer-Tropsch gases, Fischer-Tropsch waxes, distillate,
crude oil, atmospheric pipestill bottoms, vacuum pipestill streams
including bottoms, vacuum gas oils, heavy gas oil, naphtha
contaminated with crude, atmospheric resid, heavy resid,
C.sub.4's/residue admixture, naphtha residue admixture, and low
sulfur waxy resid.
18. A process for cracking a hydrocarbon feedstock to light olefins
in a steam cracking furnace having radiant section burners and a
convection section, said convection section comprises a first bank,
a second bank and a third bank of heat exchange tubes, said process
comprising: a. feeding said hydrocarbon feedstock at a pressure in
the range of 790 to 1480 kPa-a and a temperature in the range of 25
to 250.degree. C. to said first bank of heat exchange tubes
provided in said convection section, with a linear velocity equal
to or less than 0.9 m/s; b. preheating said hydrocarbon feedstock
to form a preheated hydrocarbon product having a temperature less
than 350.degree. C.; c. supplying at least a portion of said
preheated hydrocarbon product with a first diluent stream to said
second bank of heat exchange tubes to vaporize at least a portion
of said hydrocarbon feedstock to form a vapor-liquid mixture having
a temperature in the range of 350 to 500.degree. C. and comprising
at least 1 wt. % liquid based on the total weight of hydrocarbons
in said vapor-liquid mixture; d. separating at least a portion of
said vapor-liquid mixture from step (c) to form a vapor fraction
and a liquid fraction; and e. feeding at least a portion of said
vapor fraction from step (d) to said third bank of heat exchange
tubes.
19. The process of claim 18, wherein said vapor-liquid mixture has
a temperature in the range of 400 to 500.degree. C. and comprises
at least 2 wt. % liquid based on the total weight of hydrocarbons
in said vapor-liquid mixture and wherein 50 to 99.99 wt. % of said
hydrocarbon feedstock boils below 590.degree. C. measured according
to ASTM D-2887.
20. The process of claim 18, wherein said vapor-liquid mixture has
a temperature in the range of 425 to 500.degree. C. and comprises
at least 3 wt. % liquid based on the total weight of hydrocarbons
in said vapor-liquid mixture and wherein 40 to 99.99 wt. % of said
hydrocarbon feedstock boils below 590.degree. C. measured according
to ASTM D-2887.
21. The process of claim 18, wherein said vapor-liquid mixture has
a temperature in the range of 450 to 500.degree. C. and comprises
at least 5 wt. % liquid based on the total weight of hydrocarbons
in said vapor-liquid mixture and wherein 10 to 99.99 wt. % of said
hydrocarbon feedstock boils below 590.degree. C. measured according
to ASTM D-2887.
22. The process of claim 18, wherein the total pressure drop for
said first bank of heat exchange tubes is less than 100 kPa
calculated by subtracting pressure at the inlet point of said first
bank of heat exchange tubes from the exit point of said first bank
of heat exchange tubes.
23. The process of claim 18, wherein the total pressure drop for
said second bank of heat exchange tubes is less than 500 kPa
calculated by subtracting pressure at the inlet point of said
second bank of heat exchange tubes from the exit point of said
second bank of heat exchange tubes.
24. Apparatus adapted for steam cracking a hydrocarbon feedstock to
light olefins, comprising: a. a steam cracking furnace comprising
radiant section burners adapted to provide radiant heat and hot
flue gas and a convection section having a first bank, a second
bank and a third bank of heat exchange tubes; b. means for feeding
said hydrocarbon feedstock to said first bank of heat exchange
tubes with a linear velocity equal to or less than 0.9 m/s; c.
means for maintaining said hydrocarbon feedstock exiting said first
bank of heat exchange tubes at a temperature less than 350.degree.
C.; d. means for supplying a first diluent stream together with at
least a portion of the preheated hydrocarbon product from (c) to
said second bank of heat exchange tubes adapted to vaporize less
than 99 wt. % of said hydrocarbon feedstock to form a vapor-liquid
mixture; e. means for maintaining said vapor-liquid mixture exiting
said second bank of heat exchange tubes at a temperature in the
range of 350 to 500.degree. C.; f. a vessel adapted to separate at
least a portion of said vapor-liquid mixture from step (e) in to
form a vapor fraction and a liquid fraction; and g. means for
feeding at least a portion of said vapor fraction from step (f) to
said third bank of heat exchange tubes.
25. The apparatus of claim 24, further comprising means for mixing
a second diluent stream to said vapor-liquid mixture prior to (f).
Description
RELATIONSHIP TO OTHER APPLICATIONS
[0001] This application claims benefit of and priority to
International Patent Application Serial Number PCT/US2007/018486,
filed Aug. 21, 2007.
FIELD OF THIS DISCLOSURE
[0002] This disclosure pertains to a method for the manufacture of
light olefins in a steam cracking furnace or a pyrolysis furnace,
more particularly to a process of steam cracking a hydrocarbon
feedstock containing at least 0.01 wt. % low-volatile
compounds.
BACKGROUND OF THIS DISCLOSURE
[0003] Steam cracking, also referred to as pyrolysis, has long been
used to crack various hydrocarbon feedstocks into olefins,
preferably light olefins such as ethylene, propylene, and butenes.
Conventional steam cracking utilizes a steam cracking furnace which
has two main sections: a convection section and a radiant section.
The hydrocarbon feedstock typically enters the convection section
of the furnace as a liquid (except for light feedstocks which enter
as a vapor) wherein it is typically heated and vaporized by
indirect contact with hot flue gas from the radiant section and by
direct contact with steam. The vaporized feedstock and steam
mixture is then introduced into the radiant section where the
cracking takes place. The resulting products, including olefins,
leave the steam cracking furnace for further downstream
processing.
[0004] Conventional steam cracking systems have been effective for
cracking high-quality feedstocks such as natural gas liquids,
(NGL's), gas oil and naphtha. However, steam cracking economics
sometimes favor cracking low cost heavy feedstock such as, by way
of non-limiting examples, condensates, which is an associated oil
occurring in small quantities with the production of gas from gas
fields, crude oils, atmospheric resids, also known as atmospheric
pipestill bottoms, and vacuum gas oils, crude oil, vacuum gas oil
and atmospheric resid contain high molecular weight, low-volatile
components with boiling points in excess of 590.degree. C. and/or
sometimes coke precursors with boiling points in excess of
760.degree. C. The low-volatile components and/or coke precursors
of these feedstocks would lay down as coke in the convection
section of conventional steam cracking furnaces as the lighter
components were vaporized. Only very low levels of low-volatile
components and coke precursors can be tolerated in the convection
section downstream of the point where the lighter components have
fully vaporized because the coke deposition normally fouls tubes in
convection section which lowers the heat transfer efficiency and
increase the pressure drop in the tubes.
[0005] Additionally, some naphthas are contaminated with heavy
crude oil containing low-volatile components and coke precursors.
Conventional steam cracking furnaces do not have the flexibility to
process residues, crude oils, or many residue- or
crude-contaminated gas oils or naphthas which are contaminated with
low-volatile components and coke precursors.
[0006] To address coking problems, U.S. Pat. No. 3,617,493,
incorporated herein by reference, discloses the use of an external
vaporization drum for the crude oil feed and discloses the use of a
first flash to remove naphtha as vapor and a second flash to remove
vapors with a boiling point between 230 and 590.degree. C. The
vapors are cracked in the steam cracking furnace into olefins, and
the separated liquids from the second flash tank are removed,
stripped with steam, and used as fuel.
[0007] U.S. Pat. No. 3,718,709, incorporated herein by reference,
discloses a process to minimize coke deposition. It describes
preheating of heavy feedstock inside or outside a pyrolysis furnace
to vaporize 50% of the heavy feedstock with superheated steam and
the removal of the residual, separated liquid. The vaporized
hydrocarbons, which contain mostly light volatile hydrocarbons, are
subjected to cracking.
[0008] U.S. Pat. No. 5,190,634, incorporated herein by reference,
discloses a process for inhibiting coke formation in a furnace by
preheating the feedstock in the presence of a small, critical
amount of hydrogen in the convection section. The presence of
hydrogen in the convection section inhibits the polymerization
reaction of the hydrocarbons thereby inhibiting coke formation.
[0009] U.S. Pat. No. 5,580,443, incorporated herein by reference,
discloses a process wherein the feedstock is first preheated and
then withdrawn from a preheater in the convection section of the
pyrolysis furnace. This preheated feedstock is then mixed with a
predetermined amount of steam (the dilution steam) and is then
introduced into a vapor-liquid separator to separate and remove a
required proportion of the low-volatile components and coke
precursors as liquid from the separator. The separated vapor from
the vapor-liquid separator is returned to the pyrolysis furnace for
heating and cracking.
[0010] U.S. Pat. No. 6,632,351, incorporated herein by reference,
discloses a process for pyrolyzing a crude oil feedstock or crude
oil fractions containing pitch feedstock, and a pyrolysis furnace,
comprising feeding the crude oil or crude oil fractions containing
pitch feedstock to a first stage preheater within a convection
zone, wherein said crude oil or crude oil fractions containing
pitch feedstock is heated within the first stage preheater to an
exit temperature of at least 375.degree. C. to produce a heated
vapor-liquid mixture, withdrawing from first stage preheater the
vapor-liquid mixture to a vapor-liquid separator, separating and
removing the gas from the liquid in the vapor-liquid separator, and
feeding the removed gas to a second preheater provided in the
convection zone, further heating the temperature of said gas to a
temperature above the temperature of the gas exiting the
vapor-liquid separator, introducing the preheated gas into a
radiant zone within the pyrolysis furnace, and pyrolyzing the gas
to olefins and associated by-products.
[0011] U.S. Pat. No. 7,097,758, incorporated herein by reference,
discloses a process to increase the non-volatile removal efficiency
in a flash drum in the steam cracking system. The gas flow from the
convection section is converted from mist flow to annular flow
before entering the flash drum to increase the removal efficiency.
The conversion of gas flow from mist flow to annular flow is
accomplished by subjecting the gas flow first to at least one
expander and then to bends of various degrees and forcing the flow
to change directions at least once. The change of gas flow from
mist to annular helps coalesce fine liquid droplets and thus
increases the efficiency with which they are removed from the vapor
phase.
[0012] U.S. Pat. No. 7,138,047, incorporated herein by reference,
discloses a process for feeding or cracking hydrocarbon feedstock
containing non-volatile hydrocarbons comprising: heating the
hydrocarbon feedstock, mixing the hydrocarbon feedstock with a
fluid and/or a primary dilution steam stream to form a mixture,
flashing the mixture to form a vapor phase and a liquid phase, and
varying the amount of the fluid and/or the primary dilution steam
stream mixed with the hydrocarbon feedstock in accordance with at
least one selected operating parameter of the process, such as the
temperature of the flash stream before entering the flash drum.
[0013] U.S. patent application Ser. No. 11/068,615, filed Feb. 28,
2005, incorporated herein by reference, describes a process for
cracking hydrocarbon feedstock which mixes hydrocarbon feedstock
with a fluid, e.g., hydrocarbon or water, to form a mixture stream
which is flashed to form a vapor phase and a liquid phase, the
vapor phase being subsequently cracked to provide olefins, and the
product effluent cooled in a transfer line exchanger, wherein the
amount of fluid mixed with the feedstock is varied in accordance
with a selected operating parameter of the process, e.g.,
temperature of the mixture stream before the mixture stream is
flashed.
[0014] U.S. application Ser. No. 10/851,434, filed May 21, 2004,
incorporated herein by reference, and U.S. Provisional Application
Ser. No. 60/573,474, filed May 21, 2004, incorporated herein by
reference, describe a process to increase the non-volatile removal
efficiency in a flash drum used in a steam cracking system, the
flash drum having a lower boot comprising an inlet for introducing
stripping steam, a ring distributor for recycle quench oil,
anti-swirl baffles, and a grate.
[0015] There is therefore a need for a novel and energy efficient
process for steam cracking hydrocarbon feedstocks with low level of
coke formation. The present inventors surprisingly find that the
coke formation in the first preheating section of the first
preheating zone is negligible when the feedstock is fed at an inlet
linear velocity below a threshold value and the feedstock is
preheated to a temperature below a threshold value. Furthermore,
the coke formation in the first preheating zone is minimized so
long as at least 1 wt. % of the hydrocarbon feedstock exits the
first preheating zone in liquid phase. This disclosure therefore
offers a steam cracking process capable of processing hydrocarbon
feedstock with minimum coke formation in the preheating zone and
low pressure drop for the feedstock to flow through the convection
section by optimizing the linear velocity of the feedstock entering
the preheating section of a steam cracking furnace.
SUMMARY OF THIS DISCLOSURE
[0016] In some embodiments, the present disclosure provides a
process for treating a hydrocarbon feedstock comprising: [0017] (a)
feeding the hydrocarbon feedstock at a linear velocity equal to or
less than 0.9 m/s to a first preheating zone in the convection
section of a steam cracking furnace; [0018] (b) preheating the
hydrocarbon feedstock in the first preheating zone to vaporize
equal to or less than 99 wt. % of the hydrocarbon feedstock to form
a vapor-liquid mixture; [0019] (c) separating at least a portion of
the vapor-liquid mixture to form a vapor fraction and a liquid
fraction; and [0020] (d) feeding at least a portion of the vapor
fraction to the steam cracking furnace.
[0021] According to one embodiment, the convection section
comprises multiple banks of heat exchange tubes and the hydrocarbon
feedstock flows inside the tubes.
[0022] In a preferred embodiment, the hydrocarbon feedstock is fed
to the first preheating zone at a linear velocity in the range of
0.05 to 0.85 m/s, preferably from 0.1 to 0.8 m/s, and more
preferably from 0.1 to 0.75 m/s.
[0023] In one preferred embodiment of this disclosure, the first
preheating zone comprises a first preheating section and a second
preheating section, wherein the hydrocarbon feedstock is supplied
to the first preheating section at a pressure in the range of 790
to 1825 kPa-a (kilopascal absolute), preferably in the range of
790-1450 kPa-a, more preferably in the range of 790-1400 kPa-a,
even more preferably in the range of 790-1200 kPa-a, and most
preferably in the range of 790-1100 kPa-a, and a temperature in the
range of 25 to 250.degree. C. to form a preheated hydrocarbon
product exiting the first preheating section at a temperature in
the range of about 100 to 350.degree. C., and then at least a
portion of the preheated hydrocarbon product is supplied together
with a first diluent stream to the second preheating section to
form the vapor-liquid mixture exiting the first preheating section
at a temperature in the range of 350 to 500.degree. C. and
comprising at least 1 wt. % liquid phase based on the total weight
of the hydrocarbons in the vapor-liquid mixture.
[0024] In some aspects, the hydrocarbon feedstock comprises one or
more of steam cracked gas oil and residues, gas oils, coker
naphtha, steam cracked naphtha, catalytically cracked naphtha,
hydrocrackate, reformate, raffinate reformate, virgin naphtha,
crude oil, atmospheric pipestill bottoms, vacuum pipestill streams
including bottoms, vacuum gas oils, heavy gas oil, naphtha
contaminated with crude, atmospheric resid, heavy resid,
C.sub.4's/residue admixture, naphtha/residue admixture,
Fischer-Tropsch liquids, Fischer-Tropsch gases, Fischer-Tropsch
waxes, and low sulfur waxy resid. In one embodiment, about 10 to
99.99 wt. % of the hydrocarbon feedstock boils below 590.degree. C.
measured according to ASTM D-2887. In another embodiment, about 10
to 95 wt. % of the hydrocarbon feedstock boils below 590.degree. C.
measured according to ASTM D-2887.
[0025] In one embodiment, the present disclosure also provides a
process for cracking a hydrocarbon feedstock to light olefins in a
steam cracking furnace having radiant section burners and a
convection section, the convection section comprises a first bank,
a second bank and a third bank of heat exchange tubes, the process
comprising: [0026] (a) feeding the hydrocarbon feedstock in at
least 99 wt. % liquid phase at a pressure in the range of 790 to
1825 kPa-a, preferably in the range of 790-1450 kPa-a, more
preferably in the range of 790-1400 kPa-a, even more preferably in
the range of 790-1200 kPa-a, and most preferably in the range of
790-1100 kPa-a, and a temperature in the range of 25 to 250.degree.
C. to the first bank of heat exchange tubes provided in the
convection section, with a linear velocity in the range of 0.1 to
0.9 m/s; [0027] (b) preheating the hydrocarbon feedstock to form a
preheated hydrocarbon product having a temperature less than
350.degree. C.; [0028] (c) supplying at least a portion of the
preheated hydrocarbon product with a first diluent stream to the
second bank of heat exchange tubes to vaporize at least a portion
of the hydrocarbon feedstock to form a vapor-liquid mixture having
a temperature in the range of 350 to 500.degree. C. and comprising
at least 1 wt. % liquid based on the total weight of the
hydrocarbons in the vapor-liquid mixture; [0029] (d) separating at
least a portion of the vapor-liquid mixture from step (c) to form a
vapor fraction and a liquid fraction; [0030] (e) feeding at least a
portion of the vapor fraction from step (d) to the third bank of
heat exchange tubes and further to radiant section of the steam
cracking furnace to from a product comprising the light olefins,
wherein the hydrocarbon feedstock comprises one or more of steam
cracked gas oil and residues, gas oils, coker naphtha, steam
cracked naphtha, catalytically cracked naphtha, hydrocrackate,
reformate, raffinate reformate, distillate, virgin naphtha, crude
oil, atmospheric pipestill bottoms, vacuum pipestill streams
including bottoms, vacuum gas oils, heavy gas oil, naphtha
contaminated with crude, atmospheric resid, heavy resid,
C4's/residue admixture, naphtha residue admixture, and low sulfur
waxy resid.
[0031] In some embodiments, the vapor-liquid mixture has a
temperature in the range of 400 to 500.degree. C. and comprises at
least 2 wt. % liquid based on the total weight of the hydrocarbons
in the vapor-liquid mixture and wherein 50 to 99.99 wt. % of the
hydrocarbon feedstock boils below 590.degree. C. measured according
to ASTM D-2887. In other embodiments, the vapor-liquid mixture has
a temperature in the range of 425 to 500.degree. C. and comprises
at least 3 wt. % liquid based on the total weight of the
hydrocarbons in the vapor-liquid mixture and wherein 40 to 99.99
wt. % of the hydrocarbon feedstock boils below 590.degree. C.
measured according to ASTM D-2887. In yet other embodiments, the
vapor-liquid mixture has a temperature in the range of 435 to
500.degree. C. and comprises at least 4 wt. % liquid based on the
total weight of the hydrocarbons in the vapor-liquid mixture and
wherein 30 to 99.99 wt. % of the hydrocarbon feedstock boils below
590.degree. C. measured according to ASTM D-2887. In still yet
other embodiments, the vapor-liquid mixture has a temperature in
the range of 450 to 500.degree. C. and comprises at least 5 wt. %
liquid based on the total weight of the hydrocarbons in the
vapor-liquid mixture and wherein 10 to 99.99 wt. % of the
hydrocarbon feedstock boils below 590.degree. C. measured according
to ASTM D-2887.
[0032] There is now provided apparatus adapted for steam cracking a
hydrocarbon feedstock to light olefins, wherein 10 to 99.99 wt. %
of the hydrocarbon feedstock boils below 590.degree. C. measured
according to ASTM D-2887, the apparatus comprises: [0033] (a) a
steam cracking furnace comprising radiant section burners adapted
to provide radiant heat and hot flue gas and a convection section
having a first bank, a second bank and a third bank of heat
exchange tubes; [0034] (b) means for feeding the hydrocarbon
feedstock to the first bank of heat exchange tubes with a linear
velocity in the range of 0.1 to 0.9 m/s; [0035] (c) means for
maintaining the hydrocarbon feedstock exiting the first bank of
heat exchange at a temperature less than 350.degree. C.; [0036] (d)
means for supplying a first diluent stream together with at least a
portion of the preheated hydrocarbon product from (c) to the second
bank of heat exchange tubes adapted to vaporize less than 99 wt. %
of the hydrocarbon feedstock to form a vapor-liquid mixture; [0037]
(e) means for maintaining the vapor-liquid mixture exiting the
second bank of heat exchange tubes at a temperature in the range of
350 to 500.degree. C.; [0038] (f) a vessel adapted to separate at
least a portion of the vapor-liquid mixture from step (e) in to
form a vapor fraction and a liquid fraction; and [0039] (g) means
for feeding at least a portion of the vapor fraction from step (f)
to the third bank of heat exchange tubes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIG. 1 is a schematic process flow diagram of a steam
cracking furnace of this disclosure.
DETAILED DESCRIPTION OF THIS DISCLOSURE
[0041] The present disclosure relates to a process for heating and
steam cracking a hydrocarbon feedstock to produce light olefins,
e.g., ethylene and/or propylene. Typical products of a steam
cracking furnace include, but are not limited to, ethylene,
propylene, butenes, butadiene, benzene, hydrogen, methane, and
other associated olefinic, paraffinic, and aromatic products.
Ethylene is the predominant product, typically in the range of 15
to 30 wt. %, based on the weight and composition of the vaporized
feedstock. The process of this disclosure comprises preheating a
hydrocarbon, mixing the preheated hydrocarbon with a diluent stream
comprising at least one of steam, water, N.sub.2, H.sub.2, and
hydrocarbon(s) to form a mixture, further preheating the mixture to
form a vapor-liquid mixture, separating at least a portion of the
vapor-liquid mixture in a vessel to form a vapor fraction and a
liquid fraction, and feeding at least a portion of the vapor
fraction to the steam cracking furnace for further heating and
cracking.
[0042] Unless otherwise stated in this disclosure, all percentages,
parts, ratios, etc., are by weight. A reference to a compound or
component includes the compound or component by itself, as well as
in combination with other compounds or components, such as mixtures
of compounds. Further, when an amount, concentration, or other
value or parameter is given as a list of upper preferable values
and lower preferable values, this is to be understood as
specifically disclosing all ranges formed from any pair of an upper
preferred value and a lower preferred value, regardless whether
ranges are separately disclosed.
[0043] As used herein, "low-volatile components", sometimes
referred as non-volatile components or resids, are the fraction of
the hydrocarbon feed with a nominal boiling point above 590.degree.
C. as measured according to ASTM D-2887. This disclosure works well
with a hydrocarbon feedstock containing 0.01 to 90 wt. %
low-volatile components. As used herein, "coke precursors", are the
fraction of the hydrocarbon feed with a nominal boiling point above
760.degree. C. as measured according to ASTM D-2887. This
disclosure works well with a hydrocarbon feedstock containing 0.01
to 90 wt. % coke precursors. The boiling point distribution of the
hydrocarbon feed is measured by Gas Chromatograph Distillation
(GCD) according to ASTM D-2887.
[0044] The term "substantially liquid phase" as used herein means
at least 99 wt. %, preferably at least 99.5 wt. %, even more
preferably at least 99.9 wt. %, and most preferably at least 99.99
wt. %, liquid phase. For example, a stream in substantially liquid
phase means that at least 99 wt. %, preferably at least 99.5 wt. %,
even more preferably at least 99.9 wt. %, and most preferably at
least 99.99 wt. %, of the stream is in liquid phase.
[0045] The term "vapor fraction" as used herein means a fraction
predominately, preferably at least 75 wt. %, more preferably at
least 90 wt. %, even more preferably at least 95 wt. %, in vapor
phase. The term "liquid fraction" as used herein means a fraction
is predominate, preferably at least 75 wt. %, more preferably at
least 90 wt. %, even more preferably at least 95 wt. %, in liquid
phase.
[0046] The term "predominately" or "predominate" as used herein
means more than 50 wt. %. For example, a diluent stream comprises
predominately steam means that the diluent stream comprises more
than 50 wt. % steam.
Hydrocarbon Feedstock
[0047] The hydrocarbon feedstock can comprise at least a portion,
such as in the range of 0.01 to 90 wt. %, 1 to 90 wt. %, or 5 to 90
wt. %, of low-volatile components and coke precursors. Such
feedstock could comprise, by way of non-limiting examples, one or
more of steam cracked gas oil and residues, gas oils, heating oil,
jet fuel, diesel, kerosene, gasoline, coker naphtha, steam cracked
naphtha, catalytically cracked naphtha, hydrocrackate, reformate,
raffinate reformate, Fischer-Tropsch liquids, Fischer-Tropsch gas
oils, Fischer-Tropsch waxes, natural gasoline, distillate, virgin
naphtha, atmospheric pipestill bottoms, vacuum pipestill streams
including bottoms, wide boiling range naphtha to gas oil
condensates, heavy non-virgin hydrocarbon streams from refineries,
vacuum gas oils, heavy gas oil, naphtha contaminated with crude,
atmospheric residue, heavy residue, C.sub.4's/residue admixtures,
naphtha/residue admixtures, hydrocarbon gas/residue admixtures,
hydrogen/residue admixtures, gas oil/residue admixtures, crude oil,
and low sulfur waxy resid.
[0048] The hydrocarbon feedstock can have a nominal end boiling
point of at least 315.degree. C., generally greater than
510.degree. C., typically greater than 590.degree. C., for example
greater than 760.degree. C. The economically preferred feedstocks
are generally low sulfur waxy residues, atmospheric residues,
naphthas contaminated with crude oil, various residue admixtures,
and crude oils.
[0049] The gas to liquid (GTL) technologies, such as SMDS, AGC-21
and SSPD processes for production of middle distillates show a
great potential for fuel alternatives and higher value products.
The product of any Fischer-Tropsch gas to liquid process may
further be subjected to, optionally hydrotreating, fractionating to
Fischer-Tropsch liquids (also called Fischer-Tropsch naphtha),
Fischer-Tropsch gas oils (also called Fischer-Tropsch gases), and
Fischer-Tropsch waxes. Fischer-Tropsch naphtha, Fischer-Tropsch gas
oils and Fischer-Tropsch waxes produced by these GTL processes are
attractive for steam cracking applications because of their high
concentration of normal paraffin components. The high paraffinic
content of the Fischer-Tropsch liquids and the Fischer-Tropsch
gases allows them to be cracked at very high severities not
normally seen for conventional feedstocks.
[0050] In some embodiments, the process of this disclosure finds to
be useful to process a feedstock comprise at least 1 wt. % of at
least one of Fischer-Tropsch liquids, Fischer-Tropsch gases,
Fischer-Tropsch waxes, crude oils, crude oils fraction. In other
embodiments, the process of this disclosure finds to be useful to
process a feedstock comprise at least 1 wt. % of at least one of
Fischer-Tropsch liquids resids, Fischer-Tropsch gases resids,
fraction of Fischer-Tropsch liquids, and fraction of
Fischer-Tropsch gases.
Process Description
[0051] This disclosure is described below while referring to FIG. 1
as an illustration of one of many embodiments of this disclosure.
It is to be understood that the scope of the disclosure may include
any number and types of process steps between each described
process step or between a described source and destination within a
process step.
[0052] The steam cracking furnace may be any type of conventional
olefins steam cracking furnace operated for production of lower
molecular weight olefins, especially including a tubular steam
cracking furnace. The tubes within the convection zone of the steam
cracking furnace may be arranged as a bank of heat exchange tubes
in parallel, or the tubes may be arranged for a single pass or
multiple passes of the feedstock through the convection zone. At
the inlet, the feedstock may be split among multiple single pass
tubes, or may be fed to one single pass tube through which all the
feedstock flows from the inlet to the outlet of the tubes, and more
preferably through the whole of the convection zone. Preferably,
the first preheating zone comprises at least one single pass bank
of heat exchange tubes disposed in the convection zone of the steam
cracking furnace. In a preferred embodiment, the convection zone
comprises less than 20 passes tube having two or more banks through
which the hydrocarbon feedstock flows. Within each bank, the tubes
may be arranged in a coil or serpentine type arrangement within one
row, and each bank may have several rows of tubes.
[0053] The number of pass(es) of heat exchange tubes disposed in
the convection zone of the steam cracking furnace useful for this
disclosure is in the range of 1 to 20. In some embodiments, the
number of pass(es) of heat exchange tubes disposed in the
convection zone of the steam cracking furnace useful for this
disclosure is 2, 4, 6, 8, 10, 12, 14, 16, 18, or 20. In other
embodiments, the number of pass(es) of heat exchange tubes disposed
in the convection zone of the steam cracking furnace useful for
this disclosure is 1, 3, 5, 7, 9, 11, 13, 15, 17, or 19.
[0054] In some embodiments, the steam cracking furnace 1 useful for
this disclosure, as illustrated in FIG. 1, comprises a convection
section 3 and a radiant section 13. The radiant section 13
comprises radiant burners which provide radiant heat and hot flue
gas 12. The convection section 3 of the steam cracking furnace 1
comprises a first preheating zone 5 and a second preheating zone
11. The first preheating zone 5 comprises a first preheating
section 7 and a second preheating section 9. The first preheating
zone and the second preheating zone comprise multiple banks of heat
exchange tubes. In one embodiment, the first preheating section 7
comprises a first bank of heat exchange tubes 15, the second
preheating section 9 comprises a second bank of heat exchange tubes
17, and the second preheating zone 11 comprises a third bank of
heat exchange tubes 19. The steam cracking furnace 1 also comprises
a vessel 53. It is to be understood that the steam cracking furnace
1 may include any number of process equipments, such as, pump(s),
valve(s), injection point(s), meter(s), gauge(s), and controlling
device(s).
[0055] A hydrocarbon feedstock 31 comprising at least a portion,
such as 0.01 to 90 wt. %, 1 to 90 wt. %, or 5 to 90 wt. %, of
low-volatile components and coke precursors is supplied to and
preheated in the first preheating section 7 of the first preheating
zone 5 in the convection section 3 of a steam cracking furnace 1.
The heating of the hydrocarbon feedstock can take any form known by
those of ordinary skill in the art. However, it is preferred that
the heating comprises indirect contact of the hydrocarbon feedstock
in the first preheating section 7 with hot flue gases 12 from the
radiant section 13 of the furnace. This can be accomplished, by way
of non-limiting example, by passing the hydrocarbon feedstock
through the first bank of heat exchange tubes 15 located within the
first preheating section 7.
[0056] The pressure at which the hydrocarbon feedstock is fed to
the inlet of the first preheating section in the convection zone is
maintained to ensure a pressure less than 1825 kPa-a, preferably
less than 1480 kPa-a, more preferably less than 1400 kPa-a, and
most preferably less than 1200 kPa-a. In some embodiments, the
pressure and temperature at which the hydrocarbon feedstock is fed
to the inlet of the first preheating section in the convection zone
is maintained to ensure a pressure in the range of between 790-1825
kPa-a, more preferably from 790-1480 kPa-a, yet more preferably in
the range of 790-1450 kPa-a, even more preferably in the range of
790-1400 kPa-a, yet even more preferably in the range of 790-1200
kPa-a, and most preferably in the range of 790-1100 kPa-a, and a
temperature in a range from 25 to 250.degree. C., typically from
50.degree. C.-200.degree. C. The feeding rate at which the
hydrocarbon feedstock is fed to the inlet of the first preheating
section in the convection zone is controlled to maintain an inlet
linear velocity of the hydrocarbon feedstock less than 1.1 m/s,
preferably less than 1 m/s, more preferably less than 0.9, yet more
preferably from 0.05 to 0.9 m/s, still yet more preferably from 0.1
to 0.9 m/s, and even more preferably from 0.2 to 0.8 m/s.
[0057] In preferred embodiments of this disclosure, the inlet
linear velocity of the hydrocarbon feedstock is less than 0.9 m/s.
In other embodiments, the inlet linear velocity of the hydrocarbon
feedstock is in the range of 0.05 to 0.9 m/s. The following inlet
linear velocities of the hydrocarbon feedstock are useful lower
inlet linear velocity limits: 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6,
0.7 and 0.8. The following inlet linear velocities of the
hydrocarbon feedstock are useful upper inlet linear velocities
limits: 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2 and 0.1. The inlet
linear velocity of the hydrocarbon feedstock ideally falls in a
range between any one of the above-mentioned lower limits and any
one of the above-mentioned upper limits, so long as the lower limit
is less than or equal to the upper limit. The inlet linear velocity
of the hydrocarbon feedstock may be present in an amount in the
range of 0.05 to 1 in one embodiment, alternatively 0.1 to 0.5,
alternatively from 0.4 to 0.9, alternatively 0.5 to 0.85,
alternatively 0.2 to 0.5, alternatively and from 0.5 to 0.6 in
another embodiment.
[0058] We surprisingly find that the coke formation in the first
preheating section of the first preheating zone is negligible.
Furthermore, the coke formation in the first preheating zone is
minimized so long as at least 1 wt. % of the hydrocarbon feedstock
exits the first preheating zone in liquid phase. Therefore, the
inlet linear velocity of the hydrocarbon feedstock may be selected
to maintain optimum heat transfer efficiency and low pressure drop.
An appropriate linear velocity for a particular feedstock improves
both heat transfer efficiency and reduces pressure drop downstream
of the first preheating section.
[0059] The preheated hydrocarbon feedstock 33 exits the first
preheating section 7 and then is optionally mixed with a fluid 35.
The fluid can be a liquid hydrocarbon, water, steam, or mixture
thereof. The preferred fluid is water. The temperature of the fluid
can be below, equal to or above the temperature of the preheated
feedstock. The mixing of the preheated hydrocarbon feedstock and
the fluid can occur inside or outside the steam cracking furnace 1,
but preferably it occurs outside the furnace. The mixing can be
accomplished using any mixing device known within the art.
[0060] The preheated feedstock exits the first bank of heat
exchange tubes 15 at a temperature in the range of 100 to
350.degree. C., preferably in the range of 150 to 325.degree. C.,
more preferably in the range of 160 to 300.degree. C., and most
preferably in the range of 170 to 300.degree. C. In one preferred
embodiment, the preheated hydrocarbon feedstock 33 exits the first
preheating section 7 in substantially liquid phase.
[0061] In a preferred embodiment in accordance with the present
disclosure, a first diluent stream 37 is mixed with the preheated
hydrocarbon feedstock. In some embodiments, the first diluent
stream comprises at least one of steam, water, nitrogen, hydrogen,
and hydrocarbons. Preferably the first diluent stream comprises at
least one of steam and water. The first diluent stream can be
preferably injected into the preheated hydrocarbon feedstock before
the resulting stream mixture enters the second preheating section 9
of the first preheating zone 5 in the convection section 3 of a
steam cracking furnace 1 for additional heating by radiant section
flue gas.
[0062] The first diluent stream can have a temperature greater,
lower or the same as the preheated hydrocarbon feedstock but
preferably greater than that of the preheated hydrocarbon feedstock
and serves to partially vaporize the preheated hydrocarbon
feedstock. Alternatively, the first diluent stream is superheated
before being injected into the preheated hydrocarbon feedstock.
[0063] The mixture of the preheated hydrocarbon feedstock, the
first diluent stream, and optionally the fluid, is further heated
in the second preheating zone 9 in the convection section 3 of a
steam cracking furnace 1 to produce a vapor-liquid mixture. The
heating can be accomplished, by way of non-limiting example, by
passing the feedstock mixture through the second bank of heat
exchange tubes 17 located within the second preheating zone 9 and
thus heated by the hot flue gas from the radiant section of the
furnace. The thus-heated mixture 39 leaves the convection section
as a mixture stream.
[0064] The vapor-liquid mixture stream 39 temperature is limited by
highest recovery/vaporization of volatiles in the feedstock while
avoiding coking in the furnace tubes or coking in piping and
vessels conveying the mixture from the vessel to the furnace. The
selection of the vapor-liquid stream 39 temperature is also
determined by the composition of the feedstock materials. When the
feedstock contains higher amounts of lighter hydrocarbons, the
temperature of the mixture stream 39 can be lower. When the
feedstock contains a higher amount of low-volatile hydrocarbons,
the temperature of the vapor-liquid mixture stream 39 should be
higher. By carefully selecting a mixture stream temperature, the
present disclosure can find applications in a wide variety of
feedstock materials.
[0065] Typically, the temperature of the vapor-liquid mixture
stream 39 is set and controlled at between 315 and 510.degree. C.,
preferably between 370 and 490.degree. C., more preferably between
400 and 480.degree. C., and most preferably between 430 and
475.degree. C. These values will change with the boiling curve and
the concentrating volatiles in the feedstock.
[0066] The amount of liquid phase in the vapor-liquid mixture
stream 39 is calculated based on the total weight of the
hydrocarbons in the vapor-liquid mixture stream 39. The
vapor-liquid mixture stream 39 comprises at least 1 wt. % liquid.
The amount of liquid phase in the vapor-liquid mixture stream 39 is
limited by highest recovery/vaporization of volatiles in the
feedstock while avoiding coking in the furnace tubes or coking in
piping and vessels conveying the mixture from the vessel to the
furnace. The selection of the vapor-liquid stream 39 liquid content
is also determined by the composition of the feedstock materials.
When the feedstock contains higher amounts of lighter,
hydrocarbons, the liquid content of the mixture stream 39 can be
set lower. When the feedstock contains a higher amount of
low-volatile hydrocarbons, the liquid content of the vapor-liquid
mixture stream 39 should be set higher. By carefully selecting the
liquid content of the mixture stream, the present disclosure can
find applications in a wide variety of feedstock materials.
[0067] In some embodiments, the liquid content of the vapor-liquid
mixture stream is in the range of from 1 wt. % to 99 wt. %. In
other embodiments, the liquid content of the vapor-liquid mixture
stream is in the range of from 2 wt. % to 60 wt. %. In yet other
embodiments, the liquid content of the vapor-liquid mixture stream
is in the range of from 5 wt. % to 30 wt. %. The following liquid
contents of the vapor-liquid mixture stream are useful lower liquid
contents limits: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 and 15. The
following liquid contents of the vapor-liquid mixture stream are
useful upper liquid contents limits: 99, 90, 80, 70, 60, 50, 40,
30, 25, 20 and 15. The liquid content of the vapor-liquid mixture
stream ideally falls in a range between any one of the
above-mentioned lower limits and any one of the above-mentioned
upper limits, so long as the lower limit is less than or equal to
the upper limit.
[0068] In a preferred embodiment, a diluent stream 41 comprising at
least one of steam, water, nitrogen, hydrogen, and hydrocarbons,
preferably predominately steam and/or water, is first heated in a
bank of heat exchange tubes 43 to a desire temperature, preferably
superheated. The resulting diluent stream 45 is withdrawn from the
convection section 3 and optionally, splits into a second diluent
stream 47 which is mixed with the vapor-liquid mixture 39 withdrawn
from the second preheating section 9 before the vessel 53 and a
bypass diluent stream 49 which bypasses the vessel and, instead is
mixed with the vapor fraction from the vessel before the vapor
fraction is cracked in the radiant section of the furnace. In one
embodiment, the present disclosure can operate with all diluent
stream 45 used as flash second diluent stream 47 with no bypass
stream 49. Alternatively, the present disclosure can be operated
with all diluent stream 45 directed to bypass stream 49 with no
second diluent stream 47. In a preferred embodiment in accordance
with the present disclosure, the ratio of the second diluent stream
47 to bypass stream 49 should be preferably 1:20 to 20:1, and most
preferably 1:2 to 2:1. The second diluent stream 47 is mixed with
the vapor-liquid mixture stream 45 to form a flash stream 51 before
flashing in the vessel 53.
[0069] Preferably, the secondary diluent stream is superheated in a
superheating section 43 in the furnace convection before splitting
and mixing with the vapor-liquid mixture. The addition of the flash
stream 47 to the vapor-liquid mixture stream 39 ensures the
vaporization of nearly all volatile components of the mixture
before entering the vessel 53.
[0070] The mixture 51 of the vapor-liquid mixture and second
diluent stream is then introduced into a vessel 53 for separation
into two fractions: a vapor fraction comprising predominantly
volatile hydrocarbons and a liquid fraction comprising
predominantly low-volatile hydrocarbons. The vapor fraction is
preferably removed from the vessel 53 as an overhead vapor stream
55. The vapor stream 55, preferably, is fed back to the second
preheating zone 11 of the convection section 3 of the steam
cracking furnace 1 for optional heating and further supplying
through crossover pipes 59 to the radiant section of the steam
cracking furnace for cracking. The liquid fraction of the
separation is removed from the vessel 53 as a bottoms stream
57.
[0071] The flash is conducted in at least one vessel. Preferably,
the flash is a one-stage process with or without reflux. The vessel
53 is normally operated at 275 to 1400 kPa-a pressure and its
temperature is usually the same or slightly lower than the
temperature of the mixture 51 before entering the vessel 53.
Typically, the pressure of the vessel 53 is 275 to 1400 kPa-a and
the temperature is 310 to 510.degree. C. Preferably, the pressure
of the vessel 53 is 600 to 1100 kPa-a and the temperature is 370 to
490.degree. C. More preferably, the pressure of the vessel 53 is
700 to 1000 kPa-a and the temperature is 400 to 480.degree. C. Most
preferably, the pressure of the vessel 53 is 700 to 760 kPa-a and
the temperature is 430 to 480.degree. C. Depending on the
temperature of the flash stream, usually 50 to 95% of the mixture
entering the vessel 53 is vaporized to the upper portion of the
vessel 53, preferably 60 to 95%, more preferably 65 to 95%, and
most preferably 70 to 95%.
[0072] In the vessel 53, the vapor fraction 55 usually contains
less than 400 ppm of coke precursors, preferably less than 100 ppm,
more preferably less than 80 ppm, and most preferably less than 50
ppm. The vapor fraction is very rich in volatile hydrocarbons (for
example, 55-70 vol. %) and steam (for example, 30-45 vol. %). The
boiling end point of the vapor phase is normally below 760.degree.
C.
[0073] The vapor fraction stream 55 continuously removed from the
vessel 53 is preferably superheated in the steam cracking furnace
lower convection section 11 to a temperature in the range of, for
example, 430 to 650.degree. C. by the flue gas 12 from the radiant
section of the furnace. The vapor fraction is then introduced to
the radiant section of the steam cracking furnace to be
cracked.
[0074] The vapor fraction stream 55 removed from the vessel 53 can
optionally be mixed with a bypass steam stream 49 before being
introduced into the furnace lower convection section 11.
APPARATUS EMBODIMENTS
[0075] There is now provided apparatus adapted for steam cracking a
hydrocarbon feedstock to light olefins, comprising [0076] (a) a
steam cracking furnace comprising radiant section burners adapted
to provide radiant heat and hot flue gas and a convection section
comprised of a first bank, a second bank and a third bank of heat
exchange tubes; [0077] (b) means for feeding the hydrocarbon
feedstock to the first bank of heat exchange tubes with a linear
velocity in the range of 0.1 to 0.9 m/s; [0078] (c) means for
maintaining the hydrocarbon feedstock exiting the first bank of
heat exchange at a temperature less than 350.degree. C.; [0079] (d)
means for supplying a first diluent stream together with at least a
portion of the preheated hydrocarbon product from (c) to the second
bank of heat exchange tubes adapted to vaporize less than 99 wt. %
of the hydrocarbon feedstock to form a vapor-liquid mixture; [0080]
(e) means for maintaining the vapor-liquid mixture exiting the
second bank of heat exchange tubes at a temperature in the range of
350 to 500.degree. C.; [0081] (f) a vessel adapted to separate at
least a portion of the vapor-liquid mixture from step (e) in to
form a vapor fraction and a liquid fraction; and [0082] (g) means
for feeding at least a portion of the vapor fraction from step (f)
to the third bank of heat exchange tubes and further to the radiant
section of the steam cracking furnace.
[0083] The means for feeding in steps (b) and (g) can be any
conventional pumping mechanism, or piping for transporting
materials. The means for maintaining in steps (c) and (e) can be
any conventional mechanism for controlling temperature, pressure,
flowrate, feedback control, and/or control valve(s). One mechanism
for maintaining the vapor-liquid mixture exiting the second bank of
heat exchange tubes at a temperature in the range of 350 to
500.degree. C. is the injection of a fluid, such as water, to the
preheated hydrocarbon product from (c) prior to (d). The vessel in
step (f) can be any type of container, tank, or drum capable of
separating the vapor-liquid mixture from step (e) in to form a
vapor fraction and a liquid fraction. In one embodiment, the vessel
in step (f) is a flash drum. In another embodiment of this
disclosure, the vessel in step (f) is at least one of column, pipe,
distillation tower, flash tower, and tank.
EXAMPLES
[0084] The following examples illustrate some of the embodiments of
this disclosure and are not intended to limit the scope of the
disclosure. Comparative examples 1, 2, 3, 4 and examples 1, 2, 3,
and 4 are prophetic examples which were simulated using the
modeling program Simulated Sciences ProVision Version 6.0 and 7.1,
wherein the ProVision Version 7.1 was used for hydraulics
simulation. Example 2A and example 4A were results obtained in a
plant facility.
[0085] The following feedstocks, A, B1, C, and D1 were used for the
simulation as shown in Table 1. The feedstocks B2 and D2 were
tested in examples 2A and 4A. These feedstocks were characterized
using 1) ASTM D 86 (A Standard Test Method for Distillation of
Petroleum Products at Atmospheric Pressure) for liquid volume
percentage boiling point curve; and/or 2) ASTM D 2887 (A Standard
Test Method for Boiling Range Distribution of Petroleum Fractions
by Gas Chromatography) method for weight percentage boiling point
curve, which is a graph of temperature versus mass-percent
distilled curve corresponding to a laboratory technique which is
defined at 15/5 (15 theoretical plate, 5:1 reflux ratio) or TBP.
All molecular weight values are weight average molecular
weights.
TABLE-US-00001 TABLE 1 Feed: A B1 B2 C D1 D2 Specific Gravity
(g/ml) 0.8769 0.821 0.8302 0.8566 0.9082 0.8787 D86 IBP (0.5 vol
%), (.degree. C.) 63 62 122 236 309 277 D86 5 vol %, (.degree. C.)
143 315 D86 10 vol %, (.degree. C.) 131 99 172 290 362 346 D86 20
vol %, (.degree. C.) 219 377 D86 30 vol %, (.degree. C.) 225 159
257 319 403 398 D86 40 vol %, (.degree. C.) 288 413 D86 50 vol %,
(.degree. C.) 307 240 318 342 434 431 D86 60 vol %, (.degree. C.)
347 455 D86 70 vol %, (.degree. C.) 400 316 375 364 466 490 D86 80
vol %, (.degree. C.) 405 553 D86 90 vol %, (.degree. C.) 535 472
456 394 508 711 D86 95 vol %, (.degree. C.) 515 815 D86 EP(99.5 vol
%), (.degree. C.) 662 626 643 440 546 871 Molecular Weight, 210 163
250 293 422 479 TBP (15/5) IBP (0.5 wt %), (.degree. C.) -1 -11 79
196 251 208 TBP (15/5) 5 wt %, (.degree. C.) 79 35 119 232 335 307
TBP (15/5) 10 wt %, (.degree. C.) 119 73 154 287 360 346 TBP (15/5)
20 wt %, (.degree. C.) 186 118 219 316 396 388 TBP (15/5) 30 wt %,
(.degree. C.) 238 157 264 324 421 416 TBP (15/5) 40 wt %, (.degree.
C.) 285 221 301 333 442 437 TBP (15/5) 50 wt %, (.degree. C.) 333
251 336 350 462 457 TBP (15/5) 60 wt %, (.degree. C.) 384 289 368
366 482 485 TBP (15/5) 70 wt %, (.degree. C.) 438 350 401 378 503
534 TBP (15/5) 80 wt %, (.degree. C.) 500 425 438 389 529 626 TBP
(15/5) 90 wt %, (.degree. C.) 606 535 503 413 558 847 TBP (15/5) 95
wt %, (.degree. C.) 685 630 590 440 580 950 TBP (15/5) EP (99.5 wt
%), (.degree. C.) 799 777 959 465 621 1032 Viscosity @ 49.degree.
C., 896 kPa-a, (CP) 4.1564 1.708 3.21 5.0996 37.479 42.21
Comparative Example 1
[0086] Feed A, a crude oil feedstock, having the properties listed
in Table 1 above, is used as the hydrocarbon feedstock for this
example. This crude oil feedstock A which has a specific gravity
0.8769 ml/g, and an average molecular weight of 210, is fed at a
temperature of 127.degree. C., a pressure of 2413 kPa-a and a rate
of 111.8 tons/hr to the entrance of the first bank of heat exchange
tubes 15 in the convection section 3. The feedstock A, being all
liquid at this point, is routed through the first bank of heat
exchange tubes 15 having eight rows of tubes. The feedstock A is
fed to the entrance of the first bank of convection section heat
exchange tubes 15 at a linear velocity of 1.28 m/s. The feedstock A
is heated to a temperature of 181.degree. C. and exits at a
pressure of 2393 kPa-a in all liquid phase. The pressure drop
across the first bank of heat exchange tubes 15 in the convection
section is about 21 kPa.
[0087] The heated feedstock A exits the first bank of heat exchange
tubes 15 in liquid phase and is mixed with a flow of 30 tons/hr of
steam. After mixing with steam, a portion of the hydrocarbon
feedstock is vaporized to form a vapor-liquid mixture having 71 wt.
% liquid phase based on the total weight of the combined stream of
hydrocarbon feedstock and steam.
[0088] The vapor-liquid mixture is subsequently fed to a second
bank of heat exchange tubes 17 with a tube diameter about 13%
bigger than the tube diameter of the first bank of heat exchange
tubes 15. The vapor-liquid mixture is fed to the second bank of
heat exchange tubes 17 at a linear velocity of 12 m/s, wherein the
vapor-liquid mixture is further heated to a temperature of
458.degree. C., and exits the second bank of heat exchange tubes 17
at that temperature and at a pressure of about 952 kPa-a. At the
exit of the second bank of heat exchange tubes 17, the liquid
weight percentage exiting the second bank of heat exchange tubes 17
is now reduced down to 10 wt. % of the entire stream. The pressure
drop across the second bank of heat exchange tubes 17 in the
convection section is about 1448 kPa. The combined pressure drop
across both the first bank of heat exchange tubes 15 and the second
bank of heat exchange tubes 17 in the convection section is 1469
kPa.
[0089] The vapor-liquid mixture exits the second bank of heat
exchange tubes 17 in the convection section of the steam cracking
furnace at a linear velocity of about 35 m/s and mixes with a flow
of about 2.7 tons/hr of steam superheated to 482.degree. C. at a
pressure of 952 kPa-a. The resulting vapor-liquid mixture flows to
a vapor-liquid separator 53 at a temperature of 458.degree. C. and
a pressure of 811.7 kPa-a and having a liquid weight percentage of
7 wt. % of the entire stream due to the addition of superheated
steam.
Example 1
[0090] Feed A, a crude oil feedstock, having the properties listed
in Table 1 above, is used as the hydrocarbon feedstock for this
example. This crude oil feedstock A which has a specific gravity
0.8769 ml/g, and an average molecular weight of 210, is fed at a
temperature of 127.degree. C., a pressure of 958 kPa-a and a rate
of 111.8 tons/hr to the entrance of the first bank of heat exchange
tubes 15 in the convection section 3. The feedstock A, being all
liquid at this point, is routed through the first bank of heat
exchange tubes 15 having eight parallel passes of tubes. The
feedstock A is fed to the entrance of the first bank of convection
section heat exchange tubes 15 at a linear velocity of 0.55 m/s.
The feedstock A is heated to a temperature of 181.degree. C. and
exits at a pressure of 967 kPa-a in all liquid. The pressure drop
across the first bank of heat exchange tubes 15 in the convection
section is about -9 kPa (the negative pressure drop is partially
due to gravity).
[0091] The heated feedstock A exits the first bank of heat exchange
tubes 15 in liquid phase and is mixed with a flow of 30.5 tons/hr
of steam at 1142 kPa-a and 211.degree. C. After mixing with steam,
a portion of the hydrocarbon feedstock is vaporized to form a
vapor-liquid mixture having 70.6 wt. % liquid phase based on the
total weight of the combined stream of hydrocarbon feedstock and
steam. The vapor-liquid mixture is subsequently fed to a second
bank of heat exchange tubes 17. The vapor-liquid mixture is fed to
the second bank of heat exchange tubes 17 at a linear velocity of
11.9 m/s, wherein the vapor-liquid mixture is further heated to a
temperature of 458.degree. C., and exits the second bank of heat
exchange tubes 17 at that temperature and a pressure of about 819
kPa-a. At the exit of the second bank of heat exchange tubes 17,
the liquid weight percentage of the hydrocarbon feedstock exiting
the second bank of heat exchange tubes 17 is now reduced down to 10
wt. % of the entire stream. The pressure drop across the second
bank of heat exchange tubes 17 in the convection section is about
145 kPa. The combined pressure drop across both the first bank of
heat exchange tubes 15 and the second bank of heat exchange tubes
17 in the convection section is 136 kPa.
[0092] The vapor-liquid mixture exits the second bank of heat
exchange tubes 17 in the convection section of the steam cracking
furnace at a linear velocity of about 34.7 m/s and mixes with a
flow of about 2.7 tons/hr of steam superheated to 482.degree. C. at
a pressure of 819 kPa-a. The resulting vapor-liquid mixture flows
to a vapor-liquid separator 53 at a temperature of 458.degree. C.
and a pressure of 811.7 kPa-a and having a liquid weight percentage
of 7 wt. % of the entire stream due to the addition of superheated
steam.
Comparative Example 2
[0093] Feed B1, a light crude oil feedstock, having the properties
listed in Table 1 above, is used as the hydrocarbon feedstock for
this example. This crude oil feedstock B1 which has a specific
gravity 0.821 ml/g, and an average molecular weight of 163, is fed
at a temperature of 88.degree. C., a pressure of 1896 kPa-a and a
rate of 93.4 tons/hr to the entrance of the first bank of heat
exchange tubes 15 in the convection section 3. The feedstock B1,
being all liquid at this point, is routed through the first bank of
heat exchange tubes 15 having eight parallel passes of tubes. The
feedstock B1 is fed to the entrance of the first bank of convection
section heat exchange tubes 15 at a linear velocity of 1.23 m/s.
The feedstock B1 is heated to a temperature of 144.degree. C. and
exits at a pressure of 1875 kPa-a in all liquid. The pressure drop
across the first bank of heat exchange tubes 15 in the convection
section is about 21 kPa.
[0094] The heated feedstock B1 exits the first bank of heat
exchange tubes 15 in liquid phase and is mixed with a flow of 27
tons/hr of steam. After mixing with steam, a portion of the
hydrocarbon feedstock is vaporized to form a vapor-liquid mixture
having 63 wt. % liquid phase based on the total weight of the
combined stream of hydrocarbon feedstock and steam.
[0095] The vapor-liquid mixture is subsequently fed to a second
bank of heat exchange tubes 17 with a tube diameter about 19.4%
bigger than the tube diameter of the first bank of heat exchange
tubes 15. The vapor-liquid mixture is fed to the second bank of
heat exchange tubes 17 at a linear velocity of 10 m/s, wherein the
vapor-liquid mixture is further heated to a temperature of
446.degree. C., and exits the second bank of heat exchange tubes 17
at that temperature and at a pressure of about 855 kPa-a. At the
exit of the second bank of heat exchange tubes 17, the liquid
weight percentage exiting the second bank of heat exchange tubes 17
is now reduced down to 5 wt. % of the entire stream. The pressure
drop across the second bank of heat exchange tubes 17 in the
convection section is about 1027 kPa. The combined pressure drop
across both the first bank of heat exchange tubes 15 and the second
bank of heat exchange tubes 17 in the convection section is 1048
kPa.
[0096] The vapor-liquid mixture exits the second bank of heat
exchange tubes 17 in the convection section of the steam cracking
furnace at a linear velocity of about 26 m/s and mixes with a flow
of about 5.5 tons/hr of steam superheated to 473.degree. C. at a
pressure of 855 kPa-a. The resulting vapor-liquid mixture flows to
a vapor-liquid separator 53 at a temperature of 446.degree. C. and
a pressure of 889.5 kPa-a and having a liquid weight percentage of
4 wt. % of the entire stream due to the addition of superheated
steam.
Example 2
[0097] Feed B1, a light crude oil feedstock, having the properties
listed in Table 1 above, is used as the hydrocarbon feedstock for
this example. This crude oil feedstock B1 which has a specific
gravity 0.821 ml/g, and an average molecular weight of 163, is fed
at a temperature of 88.degree. C., a pressure of 979 kPa-a and a
rate of 93.4 tons/hr to the entrance of the first bank of heat
exchange tubes 15 in the convection section 3. The feedstock B1,
being all liquid at this point, is routed through the first bank of
heat exchange tubes 15 having eight parallel passes of tubes. The
feedstock B1 is fed to the entrance of the first bank of convection
section heat exchange tubes 15 at a linear velocity of 0.49 m/s.
The feedstock B1 is heated to a temperature of 144.degree. C. and
exits at a pressure of 989 kPa-a in all liquid. The pressure drop
across the first bank of heat exchange tubes 15 in the convection
section is about -10 kPa (the negative pressure drop is partially
due to gravity).
[0098] The heated feedstock B1 exits the first bank of heat
exchange tubes 15 in liquid phase and is mixed with a flow of 26.6
tons/hr of steam at 1142 kPa-a and 211.degree. C. After mixing with
steam, a portion of the hydrocarbon feedstock is vaporized to form
a vapor-liquid mixture having 63 wt. % liquid phase based on the
total weight of the combined stream of hydrocarbon feedstock and
steam.
[0099] The vapor-liquid mixture is subsequently fed to a second
bank of heat exchange tubes 17 with a tube diameter about 44%
bigger than the tube diameter of the first bank of heat exchange
tubes 15. The vapor-liquid mixture is fed to the second bank of
heat exchange tubes 17 at a linear velocity of 10.5 m/s, wherein
the vapor-liquid mixture is further heated to a temperature of
446.degree. C., and exits the second bank of heat exchange tubes 17
at that temperature and at a pressure of about 896 kPa-a. At the
exit of the second bank of heat exchange tubes 17, the liquid
weight percentage exiting the second bank of heat exchange tubes 17
is now reduced down to 5 wt. % of the entire stream. The pressure
drop across the second bank of heat exchange tubes 17 in the
convection section is about 117 kPa. The combined pressure drop
across both the first bank of heat exchange tubes 15 and the second
bank of heat exchange tubes 17 in the convection section is 107
kPa.
[0100] The vapor-liquid mixture exits the second bank of heat
exchange tubes 17 in the convection section of the steam cracking
furnace at a linear velocity of about 26.4 m/s and mixes with a
flow of about 5.5 tons/hr of steam superheated to 473.degree. C. at
a pressure of 896 kPa-a. The resulting vapor-liquid mixture flows
to a vapor-liquid separator 53 at a temperature of 446.degree. C.
and a pressure of 889.5 kPa-a and having a liquid weight percentage
of 4 wt. % of the entire stream due to the addition of superheated
steam.
Example 2A
[0101] Feed B2, a light crude oil feedstock, having the properties
listed in Table 1 above, was used as the hydrocarbon feedstock for
this example. This light crude oil feedstock B2 which has a
specific gravity 0.8302 ml/g was fed at a temperature of
115.degree. C., a pressure of about 1355 kPa-a and a rate of 61.5
tons/hr to the entrance of the first bank of heat exchange tubes 15
in the convection section 3. The feedstock B2, being all liquid at
this point, was routed through the first bank of heat exchange
tubes 15 having eight parallel passes of tubes. The feedstock B2
was fed to the entrance of the first bank of convection section
heat exchange tubes 15 at a linear velocity of 0.36 m/s. The
feedstock B2 was heated in the first bank of heat exchange tubes 15
in the convection section 3 and exited at an estimated 96 wt. %
liquid phase.
[0102] The heated feedstock B2 exiting the first bank of heat
exchange tubes 15 was mixed with a flow of 11.6 tons/hr of water at
2999 kPa-a and 138.degree. C. and a flow of 2.4 ton/hr steam at
1138 kPa-a and 191.degree. C. After mixing with water and steam, a
portion of the hydrocarbon feedstock was vaporized to form a
vapor-liquid mixture having an estimated 77 wt. % liquid phase
based on the total weight of the combined stream of hydrocarbon
feedstock and steam.
[0103] The vapor-liquid mixture was subsequently fed to a second
bank of heat exchange tubes 17. The vapor-liquid mixture was fed to
the second bank of heat exchange tubes 17 at an estimated linear
velocity of about 1.07 m/s, wherein the vapor-liquid mixture was
further heated to a temperature of 421.degree. C., and exited the
second bank of heat exchange tubes 17 at that temperature and at a
pressure of about 834 kPa-a. At the exit of the second bank of heat
exchange tubes 17, the liquid weight percentage exiting the second
bank of heat exchange tubes 17 was now reduced down to an estimated
8 wt. % of the entire stream. The pressure drop across both the
first bank of heat exchange tubes 15 and the second bank of heat
exchange tubes 17 in the convection section was about 521 kPa.
Comparative Example 3
[0104] Feed C, a heavy atmospheric gas oil (HAGO) feedstock, having
the properties listed in Table 1 above, is used as the hydrocarbon
feedstock for this example. This feedstock C which has a specific
gravity 0.8566 ml/g, and an average molecular weight of 293, is fed
at a temperature of 99.degree. C., a pressure of 910 kPa-a and a
rate of 95 tons/hr to the entrance of the first bank of heat
exchange tubes 15 in the convection section 3. The feedstock C,
being all liquid at this point, is routed through the first bank of
heat exchange tubes 15 having eight parallel passes of tubes. The
feedstock C is fed to the entrance of the first bank of convection
section heat exchange tubes 15 at a linear velocity of 1.33 m/s.
The feedstock C is heated to a temperature of 256.degree. C. and
exits at a pressure of 862 kPa-a in all liquid. The pressure drop
across the first bank of heat exchange tubes 15 in the convection
section is about 48 kPa.
[0105] The heated feedstock C exits the first bank of heat exchange
tubes 15 in liquid phase and having a linear velocity of 32
m/s.
Example 3
[0106] Feed C, a heavy atmospheric gas oil (HAGO) feedstock, having
the properties listed in Table 1 above, is used as the hydrocarbon
feedstock for this example. This feedstock C which has a specific
gravity 0.8566 ml/g, and an average molecular weight of 293, is fed
at a temperature of 99.degree. C., a pressure of 876 kPa-a and a
rate of 95 tons/hr to the entrance of the first bank of heat
exchange tubes 15 in the convection section 3. The feedstock C,
being all liquid at this point, is routed through the first bank of
heat exchange tubes 15 having eight parallel passes of tubes. The
feedstock C is fed to the entrance of the first bank of convection
section heat exchange tubes 15 at a linear velocity of 0.82 m/s.
The feedstock C is heated to a temperature of 256.degree. C. and
exits at a pressure of 862 kPa-a in all liquid. The pressure drop
across the first bank of heat exchange tubes 15 in the convection
section is about 14 kPa.
[0107] The heated feedstock C exits the first bank of heat exchange
tubes 15 in liquid phase and having a linear velocity of 31.7
m/s.
Comparative Example 4
[0108] Feed D1, a low sulfur vacuum gas oil (LSVGO) feedstock,
having the properties listed in Table 1 above, is used as the
hydrocarbon feedstock for this example. This feedstock D1 which has
a specific gravity 0.9082 ml/g, and an average molecular weight of
422, is fed at a temperature of 110.degree. C., a pressure of 724
kPa-a and a rate of 68 tons/hr to the entrance of the first bank of
heat exchange tubes 15 in the convection section 3. The feedstock
D1, being all liquid at this point, is routed through the first
bank of heat exchange tubes 15 having eight parallel passes of
tubes. The feedstock D1 is fed to the entrance of the first bank of
convection section heat exchange tubes 15 at a linear velocity of
1.31 m/s. The feedstock D1 is heated to a temperature of
292.degree. C. and exits at a pressure of 683 kPa-a in all liquid
phase. The pressure drop across the first bank of heat exchange
tubes 15 in the convection section is about 217 kPa.
[0109] The heated feedstock D1 exits the first bank of heat
exchange tubes 15 in liquid phase and having a linear velocity of
17 m/s.
Example 4
[0110] Feed D1, a low sulfur vacuum gas oil (LSVGO), having the
properties listed in Table 1 above, is used as the hydrocarbon
feedstock for this example. This feedstock D1 which has a specific
gravity 0.9082 ml/g, and an average molecular weight of 422, is fed
at a temperature of 110.degree. C., a pressure of 730 kPa-a and a
rate of 68 tons/hr to the entrance of the first bank of heat
exchange tubes 15 in the convection section 3. The feedstock D1,
being all liquid at this point, is routed through the first bank of
heat exchange tubes 15 having eight parallel passes of tubes. The
feedstock D1 is fed to the entrance of the first bank of convection
section heat exchange tubes 15 at a linear velocity of 0.3 m/s. The
feedstock D1 is heated to a temperature of 292.degree. C. and exits
at a pressure of 758 kPa-a in all liquid. The pressure drop across
the first bank of heat exchange tubes 15 in the convection section
is about -28 kPa (the negative pressure drop is partially due to
gravity).
[0111] The heated feedstock D1 exits the first bank of heat
exchange tubes 15 in liquid phase and having a linear velocity of
17.4 m/s.
Example 4A
[0112] Feed D2, a low sulfur waxy resid (LSWR) feedstock, having
the properties listed in Table 1 above, was used as the hydrocarbon
feedstock for this example. This light crude oil feedstock D2 which
has a specific gravity 0.8787 ml/g was fed at a temperature of
93.degree. C., a pressure of about 925 kPa-a and a rate of 65
tons/hr to the entrance of the first bank of heat exchange tubes 15
in the convection section 3. The feedstock D2, being all liquid at
this point, was routed through the first bank of heat exchange
tubes 15 having eight parallel passes of tubes. The feedstock D2
was fed to the entrance of the first bank of convection section
heat exchange tubes 15 at a linear velocity of 0.44 m/s. The
feedstock D2 was heated in the first bank of heat exchange tubes 15
in the convection section 3 and exited at an estimated 100 wt. %
liquid phase.
[0113] The heated feedstock D2 exiting the first bank of heat
exchange tubes 15 was mixed with a flow of 2.6 tons/hr of water at
1100 kPa-a and 120.degree. C. and a flow of 15.6 ton/hr steam at
925 kPa-a and 210.degree. C. After mixing with water and steam, a
portion of the hydrocarbon feedstock was vaporized to form a
vapor-liquid mixture having an estimated 94.6 wt. % liquid phase
based on the total weight of the combined stream of hydrocarbon
feedstock and steam.
[0114] The vapor-liquid mixture was subsequently fed to a second
bank of heat exchange tubes 17. The vapor-liquid mixture was fed to
the second bank of heat exchange tubes 17 at an estimated linear
velocity of about 23.75 m/s, wherein the vapor-liquid mixture was
further heated to a temperature of 455.degree. C., and exited the
second bank of heat exchange tubes 17 at that temperature and at a
pressure of about 827 kPa-a. At the exit of the second bank of heat
exchange tubes 17, the liquid weight percentage exiting the second
bank of heat exchange tubes 17 was now reduced down to an estimated
32 wt. % based on the total weight of hydrocarbon in the entire
stream (estimated 25 wt. % liquid phase based on the total weight
of the entire stream). The pressure drop across both the first bank
of heat exchange tubes 15 and the second bank of heat exchange
tubes 17 in the convection section was about 98 kPa.
[0115] The following table (Table 2) lists all pressure drops for
comparative examples 1-4 and examples 1-4. In summary, by supplying
feedstock to the first bank of heat exchanger at a linear velocity
less than 1.1 m/s, lower pressure drop across both the first bank
and especially the second bank of heat exchanger can be achieved.
The pressure drop for the second bank of heat exchange tubes of the
Examples 1 and 2 are about 9 times less than the pressure drop for
the second bank of heat exchange tubes of the Comparative Examples
1 and 2. Because of the low pressure drop, the process of this
disclosure has the advantage of supplying hydrocarbon feedstock at
a lower inlet pressure which saves energy required for the steam
cracking process. Furthermore, the lower inlet pressure results in
lower outlet pressures at the exits of the first bank and second
bank of the heat exchange tubes, which has the advantage of using
first and second diluent streams with lower pressures. By lowering
the pressure required for the first and the second diluent streams,
the process of this disclosure offers the advantage of energy
saving and steam cracking efficiency.
TABLE-US-00002 TABLE 2 Pressure drop for Pressure drop for Combined
pressure the 1.sup.st bank of the 2.sup.nd bank of drop for both
1.sup.stand heat exchange heat exchange 2.sup.nd bank of heat tubes
(kPa) tubes (kPa) exchange tubes (kPa) Comparative Example 1 21
1448 1469 Example 1 -9 145 136 Comparative Example 2 21 1027 1048
Example2 -10 117 107 Example 2A 521 Comparative Example 3 48 NA NA
Example 3 14 NA NA Comparative Example 4 41 NA NA Example 4 -28 NA
NA Example 4A 98
[0116] From the foregoing description, one skilled in the art can
easily ascertain the essential characteristics of this disclosure,
and without departing from the spirit and scope thereof, can make
various changes and modifications of this disclosure to adapt it to
various usages and conditions.
[0117] While the present invention has been described and
illustrated by reference to particular embodiments, those of
ordinary skill in the art will appreciate that this disclosure
lends itself to variations not necessarily illustrated herein. For
this reason, then, reference should be made solely to the appended
claims for purposes of determining the true scope of the present
invention.
* * * * *