U.S. patent application number 14/494854 was filed with the patent office on 2015-01-08 for systems and processes for improving distillate yield and quality.
The applicant listed for this patent is ThioSolv, LLC. Invention is credited to Mark C. Anderson, George R. Winter.
Application Number | 20150008160 14/494854 |
Document ID | / |
Family ID | 44646348 |
Filed Date | 2015-01-08 |
United States Patent
Application |
20150008160 |
Kind Code |
A1 |
Anderson; Mark C. ; et
al. |
January 8, 2015 |
Systems and Processes for Improving Distillate Yield and
Quality
Abstract
Systems and processes for improving quality and yield of one or
more distillate products generated in a distillation column are
disclosed. The system comprises a feed inlet distributor that
reduces the amount of liquid entrained in vapor rising from the
feed zone of the distillation column, a wash zone collection
apparatus having an improved design for collecting slop wax falling
from a liquid/vapor contacting structure provided in the wash zone,
a recirculation subsystem for recirculating at least a portion of
the collected slop wax to the top of the wash zone for distribution
as wash oil, and a control subsystem. The feed inlet distributor
ensures a horizontal fluid flow path free of transverse surfaces
thereby minimizing atomization of liquid droplets entrained in
vapor in the feed stream.
Inventors: |
Anderson; Mark C.; (Spring,
TX) ; Winter; George R.; (Fond du Lac, WI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ThioSolv, LLC |
Spring Branch |
TX |
US |
|
|
Family ID: |
44646348 |
Appl. No.: |
14/494854 |
Filed: |
September 24, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13050521 |
Mar 17, 2011 |
8864951 |
|
|
14494854 |
|
|
|
|
61340576 |
Mar 19, 2010 |
|
|
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Current U.S.
Class: |
208/313 |
Current CPC
Class: |
C10G 7/12 20130101; B01D
3/32 20130101; C10G 7/08 20130101; B01D 3/42 20130101 |
Class at
Publication: |
208/313 |
International
Class: |
C10G 7/08 20060101
C10G007/08 |
Claims
1. A process for improving quality and yield of one or more
distillate products drawn from a distillation column whose feed
stream is a mixture of vapor and a liquid containing one or more
components that are substantially non-volatile at column
conditions, the distillation column comprising a feed zone, a wash
zone located above the feed zone having disposed therein a
liquid/vapor contacting structure and a wash oil distributor for
distributing wash oil to the upper surface of the contacting
structure, and means above the wash zone to remove the vapor
product or liquid condensed therefrom, the process comprising:
collecting a liquid ("slop wax") falling from the wash zone
contacting structure by means of a collection apparatus disposed
below the wash zone and above the feed zone; withdrawing the slop
wax from the collection apparatus to a slop wax pump; conducting at
least a portion of a slop wax pump discharge liquid to the wash oil
distributor at a rate sufficient to assure thorough wetting of the
wash oil liquid/vapor contacting structure and to substantially
eliminate entrained liquid or solid particles from vapor rising
from the wash zone; and adding to the slop wax stream conducted to
the wash oil distributor only enough of a heaviest distillate
product as makeup to maintain a liquid level on the slop wax
collection apparatus.
2. The process of claim 1, wherein the wash zone irrigated only by
the slop wax and the makeup distillate, if any, comprises the sole
means of removing entrained liquid or solids from the vapor rising
from the feed zone, thereby minimizing the amount of distillate
product downgraded by use as wash oil, and the vapor rising from
the wash zone is substantially free of entrained non-volatile
material.
3. The process of claim 1, wherein enough heat is removed from the
wash oil stream to condense in the wash zone enough of the feed
vapor to maintain the level on the slop wax collection apparatus
without using any distillate as makeup to the wash zone.
4. The process of claim 1, wherein a liquid collection apparatus is
provided above the wash zone to collect downflowing liquid and an
insulating material is applied to the liquid collection apparatus
above the wash zone to reduce condensation on the lower surface of
the collection apparatus of feed vapor rising from the wash zone,
thereby increasing the fraction of feed vapor that passes through
the collection apparatus to be condensed and yielded as distillate
product.
5. The process of claim 4, wherein the insulating material is
applied to an upper surface of a floor of the liquid collection
apparatus.
6. A process for improving quality and yield of one or more
distillate products drawn from a distillation column whose feed
stream is a mixture of vapor and a liquid containing one or more
components that are substantially non-volatile at column
conditions, the distillation column comprising a feed zone, a wash
zone located above the feed zone having disposed therein a
liquid/vapor contacting structure and above it a wash oil
distributor for distributing wash oil to the upper surface of the
contacting structure, and means above the wash zone to remove vapor
product or liquid condensed therefrom, the process comprising:
collecting liquid slop wax falling from the wash zone contacting
structure by means of a collection apparatus disposed below the
wash zone and above the feed zone; withdrawing the collected slop
wax from the collection apparatus to a slop wax pump; conducting at
least a portion of the withdrawn slop wax to the wash oil
distributor at a rate sufficient to assure thorough wetting of the
wash oil zone liquid/vapor contacting structure and to
substantially eliminate entrained liquid or solid particles from
the vapor rising from the wash zone; and removing enough heat from
the recirculated slop wax to condense in the wash zone enough of
the feed vapor to maintain a liquid level on the slop wax
collection apparatus.
7. The process of claim 6, wherein the vapor product rising from
the wash zone is substantially free of entrained non-volatile
material and the wash zone irrigated by wash oil comprising only
recirculated slop wax plus the liquid condensed in the wash zone
from the feed vapor comprises the sole means of removing entrained
liquid or solids from the vapor rising from the feed zone.
8. The process of claim 6, wherein a liquid collection apparatus is
provided above the wash zone to collect downflowing liquid and an
insulating material is applied to the collection apparatus to
reduce condensation on the lower surface of the collection
apparatus of the vapor product rising from the wash zone, thereby
increasing the fraction of feed vapor that passes through said
collection apparatus to be yielded as distillate product
essentially free of entrained contaminants.
9. The process of claim 8, wherein the insulating material is
applied to an upper surface of a floor of the collection
apparatus.
10. A process for reducing the amount of clean wash liquid required
to effectively eliminate from a vapor stream entrained liquid or
solid material and the amount of contaminated wash liquid produced
from a columnar liquid/vapor contacting device whose feed stream is
a mixture of vapor and a liquid or solid containing one or more
components that are substantially non-volatile at column conditions
and which would be deleterious to the vapor product or liquid
condensed therefrom, where at least one of the objectives of the
column is to remove the entrained phase from the vapor rising from
the feed zone, the column comprising a feed zone, a wash zone
located above the feed zone having disposed therein a liquid/vapor
contacting structure and above it a wash liquid distributor for
distributing wash liquid to the upper surface of the contacting
structure, and above the wash liquid distributor, a means for
removing the vapor or liquid condensed from same, the process
comprising: collecting liquid falling from the wash zone contacting
structure, containing material previously entrained in the rising
vapor, by means of a liquid collection apparatus disposed below the
wash zone and above the feed zone; withdrawing the wash zone liquid
from said collection apparatus to a wash liquid pump; conducting at
least a portion of the wash liquid pump discharge liquid to the
wash liquid distributor at a rate sufficient to substantially
eliminate entrained liquid or solid particles from the vapor rising
from the wash zone, wherein substantially all entrained
non-volatile material is effectively eliminated from the rising
vapor and concentrated in the wash zone withdrawn liquid, and
wherein the wash zone irrigated by wash liquid comprising
recirculated wash zone outlet liquid comprises the sole means of
removing entrained liquid or solids from the vapor rising from the
feed zone.
11. The process of claim 10, wherein a portion of the withdrawn
wash zone liquid is removed from the columnar liquid/vapor
contacting device to carry the captured entrained material to an
external destination.
12. The process of claim 10, wherein at least a portion of the
recirculated wash zone outlet liquid is filtered to remove solids
and the filtrate is passed to the wash liquid distributor.
Description
[0001] CROSS-REFERENCE TO RELATED APPLICATION(S)
[0002] This application claims the benefit, and is a continuation
application of prior U.S. application Ser. No. 13/050,521 filed
Mar. 17, 2011 entitled "Systems and Processes for Improving
Distillate Yield and Quality," which claims priority under 35
U.S.C. .sctn.119(e) to U.S. Provisional Patent Application Ser. No.
61/340,576 filed on Mar. 19, 2010, which is incorporated herein by
reference in its entirety.
BACKGROUND
[0003] Various technologies and techniques exist for separating
mixtures of liquids into their individual components.
Distillation--specifically fractional distillation--is the most
common form of separation technology used in the chemical process
industries and is a critical part of petroleum refining,
petrochemical and chemical production, and natural gas processing.
Oil refineries, for example, employ fractional distillation to
separate crude oil into useful components comprised of different
hydrocarbons with different boiling points.
SUMMARY
[0004] Systems and processes for improving the yield and quality of
distillate obtained from fractionation of black oil streams are
disclosed. Embodiments of the invention relate to structural and
design improvements to distillation columns as well as distillation
process improvements that generate significant advantages over
conventional systems and processes.
[0005] Throughout this entire disclosure, including the detailed
description and the claims, "a portion of" a substance may refer to
an entire amount of the substance or any lesser amount thereof.
[0006] According to an embodiment of the invention, a system for
improving yield and quality of one or more distillate products
generated in a distillation column is disclosed. The distillation
column comprises a feed zone, a wash zone located above the feed
zone, and a fractionation zone located above the wash zone. The
wash zone has disposed therein a liquid/vapor contacting structure
a wash oil distributor for distributing wash oil to the contacting
structure.
[0007] The system according to the above-described embodiment of
the invention comprises a feed inlet distributor that reduces an
amount of liquid entrained in a vapor rising from the feed zone, a
wash zone collection apparatus having an improved design for
collecting slop wax falling from the wash zone, a recirculation
subsystem for recirculating a portion of the slop wax withdrawn
from the wash zone collection apparatus, and a control subsystem
for controlling other subsystems.
[0008] The recirculation subsystem comprises a pump, a first
conduit for conducting collected slop wax from the wash zone
collection apparatus to the pump, a second conduit for
recirculating a portion of the pumped slop wax to the wash oil
distributor for distribution as wash oil to the contacting
structure, and a third conduit for conducting a portion of the
pumped slop wax to equipment located externally to the distillation
column.
[0009] The control subsystem may comprise means for controlling a
flow rate of the recirculated portion of the pumped slop wax and
means for controlling a flow rate of the portion of the pumped slop
wax conducted to the external equipment.
[0010] In accordance with one or more embodiments of the invention,
the recirculation subsystem may further comprise a means for
combining a portion of a distillate product collected by a
distillate product collector disposed in the fractionation zone
with the recirculated portion of the slop wax to form a combined
stream that is conducted to the wash oil distributor for
distribution as wash oil to the contacting structure. The slop wax
recirculation rate in embodiments of the invention is several times
higher than the wash oil rate in conventional apparatuses, and as
such, the rate at which distillate product, if any, is conducted to
the distributor is a small fraction of the rate required in
conventional apparatuses to remove liquid entrained in vapor
passing through the wash zone.
[0011] In certain embodiments of the invention, the system may
further comprise an insulating apparatus comprising one or more
insulating materials applied to the distillate product collector to
reduce condensation of vapor on a bottom surface thereof.
[0012] According to one or more embodiments of the invention, the
wash zone collection apparatus comprises one or more transverse
collector troughs inclined towards an opening that provides for
fluid communication with the recirculation subsystem and a
plurality of lateral troughs disposed in two or more layers and
inclined towards the one or more transverse collector troughs,
wherein the lateral troughs of each layer are staggered with
respect to the lateral troughs of an adjacent layer.
[0013] According to one or more embodiments of the invention in
which the distillation column includes a tangential feed entry
nozzle, a tangential feed inlet distributor is provided that
comprises a horizontally disposed annular ring extending around an
entire circumference of the circumferential wall of the
distillation column, and a cylindrical skirt connected to an inner
edge of the annular ring and extending downward therefrom. The
annular ring, the cylindrical skirt, and the circumferential wall
of the distillation column together define an open-bottomed tunnel
within the distillation column into which the fluid feed stream is
injected or channeled via the tangential feed inlet. The tunnel is
free of any surface transverse to the substantially tangential flow
path of the fluid feed stream thereby reducing atomization of
entrained liquid in the fluid feed stream.
[0014] In alternative embodiments of the invention in which the
feed entry nozzle is radial to the distillation column, a radial
feed inlet distributor is provided that comprises a roof plate that
extends substantially across an entire diameter of the distillation
column and a skirt comprising opposing walls laterally spaced from
each other about a distance equal to a diameter of the feed inlet
and extending downwards from the roof plate about a distance equal
to the diameter of the feed inlet. The opposing walls of the skirt
may terminate prior to reaching a distal end of the circumferential
wall of the distillation column. The roof plate and the opposing
walls of the skirt together define an open-bottomed tunnel within
the distillation column into which the fluid feed stream is
injected or channeled via the radial feed inlet.
[0015] The radial feed inlet distributor further comprises a feed
splitter provided in proximity to where the opposing walls of the
skirt terminate, the feed splitter splitting the fluid feed stream
bilaterally into two smaller fluid feed streams directed in
opposing horizontal directions and along flow paths substantially
tangential to the circumferential wall of the distillation column.
The tunnel is free of any surface transverse to the initial radial
flow path of the fluid feed stream or the horizontal tangential
flow paths of the smaller fluid feed streams thereby reducing
atomization of entrained liquid in the fluid feed stream.
[0016] According to an embodiment of the invention, a process for
improving quality and yield of one or more distillate products
generated in a distillation column is disclosed. The process
comprises: providing a tangential or radial feed inlet distributor
within the distillation column in dependence on whether the feed
inlet is tangential or radial to the column, collecting slop wax
falling from the contacting structure using a wash zone collection
apparatus disposed in the wash zone, and recirculating a portion of
the slop wax to the wash oil distributor for distribution as the
wash oil to the contacting structure. In a more specific
embodiment, the process further comprises conducting collected slop
wax from the wash zone collection apparatus to a pump,
recirculating a portion of the pumped slop wax to the wash oil
distributor, and conducting a portion of the pumped slop wax to
equipment located externally to the distillation column.
[0017] The process may further comprise combining a portion of
distillate product collected by a distillate product collector
disposed in the fractionation zone with the recirculated portion of
the slop wax to form a combined stream, conducting the combined
stream to the wash oil distributor, and distributing the combined
stream as the wash oil to the contacting structure. In certain
embodiments, the process may additionally comprise applying one or
more insulating materials to the distillate product collector to
reduce condensation of vapor on a bottom surface thereof.
[0018] These and other embodiments of the invention are described
in further detail through reference to the following drawings in
the detailed description that follows.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a schematic representation of a conventional
distillation system.
[0020] FIG. 2 is a schematic representation of a system in
accordance with one or more embodiments of the invention.
[0021] FIGS. 3A-3C depict various plan and/or sectional views of a
wash zone collection apparatus in accordance with one or more
embodiments of the invention.
[0022] FIGS. 4A and 4B depict plan and sectional views,
respectively, of a tangential feed inlet distributor in accordance
with one or more embodiments of the invention.
[0023] FIGS. 5A and 5B depict plan and elevation views,
respectively, of a radial feed inlet distributor in accordance with
one or more alternate embodiments of the invention.
[0024] FIG. 6 is a photograph of two samples of HVGO product
obtained by the experimental testing.
DETAILED DESCRIPTION
[0025] Systems and processes disclosed herein are applicable to a
wide range of petroleum refining processes (e.g. atmospheric
distillation of crude oil, vacuum distillation of reduced crude,
fractionation of effluent from pyrolysis furnaces), and may be
employed, for example, in the main columns of Delayed Coking, Fluid
Catalytic Cracking and Residue Hydrotreating Units. Moreover,
systems and processes according to embodiments of the invention may
also be used in connection with numerous other industry recovery
processes such as, for example, tar sands bitumen and coke oven
distillate recovery.
[0026] Although the invention will be described primarily through
reference to embodiments involving the processing of reduced crude,
it should be noted that the invention is not limited to such
embodiments, and, as noted above, systems and processes of the
invention are applicable to any distillation process in which the
column feed is at least partially vaporized and comprises
non-volatile components which, if allowed to contaminate the
distillate, are deleterious to its subsequent processing. More
specifically, embodiments of the invention are applicable to
distillation columns that process feed streams that include
components that are not volatile under process conditions (e.g.
asphaltenes and organometallic compounds), but which, if allowed to
contaminate the distillate by entrainment of liquid droplets in the
vapor rising from the feed inlet point, discolor the distillate and
produce significant adverse effects on the activity and selectivity
of catalysts used in connection with further processing of the
distillate.
[0027] One of the first process units that crude oil enters for
processing is the Atmospheric Crude Fractionating Column
(hereinafter "Crude Unit"). The reduced crude obtained from this
process is typically further distilled in a fractionating column
operating under sub-atmospheric pressure (a Vacuum Crude
Fractionating Unit or more commonly known as a "Vacuum Unit") to
separate additional distillate from the heavier fraction that
contains essentially all metals and asphaltenes present in the
crude oil. The vacuum distillate recovered from the Vacuum Crude
Fractionating Unit is generally converted to more valuable products
by downstream processes such as Fluid Catalytic Cracking,
Hydrotreating, and Hydrocracking. Those processes employ catalysts
whose activity and selectivity are reduced by the presence of
metals and asphaltenes in the vacuum distillate. Accordingly, a
strong economic incentive exists for minimizing the entrainment of
liquid containing those contaminants in the distillate product.
[0028] Although embodiments of the invention are discussed
primarily in connection with the processing of a feed stream fed to
a Vacuum Unit in which the operating pressure is low, the feed
inlet velocities and vapor upflow are high, and the liquid
entrainment in the vapor stream is especially severe, the operating
principles and advantages of embodiments of the invention are
applicable to any distillation system and process in which it is
desirable to prevent liquid entrainment in the vapor phase rising
from the feed point.
[0029] FIG. 1 depicts a sectional view of a portion of a
conventional Vacuum Unit distillation column and accompanying
elements disposed externally to the column. The column 100 includes
a packed Heavy Vacuum Gas Oil (HVGO) section 101 where HVGO
product, typically boiling between 650.degree. F. and 1050.degree.
F., is condensed from the vapor rising through the column. Located
below the HVGO section 101 is a draw tray 102 for collecting the
condensed HVGO product. An HVGO pump 110 recirculates at least a
portion of the collected HVGO product back to the HVGO section 101
through conduit 110a. Flow controller 108 is provided for
controlling a flow rate of the recirculated HVGO product through
conduit 110a via valve 107. Pump 110 also pumps at least a portion
of the collected HVGO product through conduit 110b for further
processing, for example, in a FCC Unit or a Hydrocracking Unit.
Level controller 109 is provided for controlling a flow rate of
distillate through conduit 110b via valve 111. Further, in
conventional distillation apparatuses such as that shown in FIG. 1,
the HVGO pump 110 additionally pumps a significant portion of the
collected HVGO product through conduit 110c for use as wash oil in
wash zone 100b.
[0030] The portion of the distillation column 100 depicted in FIG.
1 can be segmented into three zones. A fractionation zone 100a
encompasses the HVGO section 101 and the draw tray 102. The wash
zone 100b encompasses a region of the column extending between the
bottom of draw tray 102 and the bottom of a collection tray 105.
The feed zone (or flash zone) 100c encompasses a region of the
column between the bottom of collection tray 105 and a position in
relative proximity to the feed inlet 106 to the column.
[0031] A fluid feed stream 106a generally enters the vacuum column
100 from a fired heater and comprises mostly vapor by weight and
almost all vapor by volume. As a result of the high feed stream
velocities in the feed conduit from the heater to the column, a
mist flow is generated with liquid entrained as small diameter
droplets within the vapor. In the feed zone 100c, a portion of the
liquid phase in the feed stream 106a falls out of the rising vapor
of the stream by gravimetric separation alone. In the wash zone
100b, entrained liquid droplets are removed from the rising vapor
of the feed stream 106a by scrubbing the vapor with a liquid (i.e.
wash oil) while using trays or packing to enhance liquid/vapor
contact.
[0032] In conventional apparatuses, a portion of the heaviest
distillate product--obtained by condensation in the fractionation
zone located just above the wash zone--serves as the sole source of
the wash oil used to remove entrained liquid droplets from the
vapor rising through the wash zone. For example, in a conventional
vacuum column such as that depicted in FIG. 1, a significant
portion of the heaviest distillate product (i.e. the condensed HVGO
product collected by draw tray 102) is pumped by HVGO pump 110 to
the wash zone 100b via conduit 110c for use as wash oil. Valve 113
controls the flow rate of the wash oil in response to a signal
communicated by flow controller 112. The wash oil is supplied to
one or more liquid distributors 103 provided at a top portion of
packing 104 in the wash zone 100b. As vapor in the fluid feed
stream 106a rises upwards in the column 100 through the packing
104, the vapor is scrubbed with the wash oil leading to removal of
a portion of the liquid entrained in the vapor.
[0033] A collection tray 105 is provided at the bottom of the wash
zone 100b to collect liquid falling from the packing 104. This
liquid, also known in the art as slop wax, is then conducted
through conduit 117 to pump 115. A level controller 114 is provided
to determine an amount of slop wax that has accumulated in the
collection tray 105 and communicate a signal to valve 116 which
controls the flow of slop wax through to fluid line 118. The slop
wax is carried through fluid line 118 to further blending or
recycling treatment systems. However, certain conventional
distillation apparatuses do not include a slop wax collection tray
or the associated pump and instead allow the slop wax to fall past
the feed zone to combine with the bottoms product from the
column.
[0034] There are several drawbacks to conventional distillation
apparatuses that utilize a significant portion of the heaviest
distillate product as wash oil for removing liquid droplets
entrained in vapor rising through the wash zone. First, the use of
distillate product as wash oil clearly decreases the yield of
distillate, which is substantially more valuable than the bottoms
product. The significant economic costs associated with the use of
distillate as wash oil creates a strong incentive to minimize the
amount of the distillate used. This is typically accomplished by
minimizing the rate at which distillate is provided as wash oil to
the wash zone.
[0035] However, minimization of the wash oil rate is subject to the
constraint that the rate must be high enough to provide wetting of
the packing sufficient to limit coke formation to an economically
acceptable rate. In order to prevent the development of unwetted
areas in the packing 104 where coke can accumulate, the flow rate
of wash oil distributed to the top of the wash zone 100b in
conventional apparatuses must be sufficient to maintain the packing
104 in a thoroughly wetted condition, particularly, a bottom
portion of the packing which is especially prone to coke
accumulation. The need to maintain a sufficiently high wash oil
flow rate so as to both capture entrained liquid and
reduce/minimize coking runs counter to the strong financial
incentive to minimize the wash oil rate to obtain greater
distillate yields.
[0036] Experimental and operational data has shown that lower
liquid flow rates over the wash zone packing as well as poorer
liquid distribution across the packing leads to an increase in coke
deposition and accumulation rates. Moreover, the accumulation of
coke deposits on the packing reduces the efficiency of entrained
liquid removal from the vapor rising through the packing. In fact,
there exists a threshold level of coke accumulation at which the
efficiency of entrained liquid removal is reduced to such an extent
that the distillation unit must be shut down and the packing
disassembled and cleaned of the coke. The resultant downtime caused
by this coke removal maintenance operation comes at an enormous
economic cost to the refinery, particularly when the process unit
that is being shut down affects the operation of valuable
downstream units.
[0037] To balance the competing objectives of capturing entrained
liquid from the vapor rising through the column, minimizing loss of
distillate yield, and maintaining wash zone packing in a
sufficiently wetted condition so as to prevent excess accumulation
and deposition of coke, those of ordinary skill in the art have
chosen to direct their efforts at modifying the mechanical design
of the packing and the mechanism by which wash oil is distributed
onto the packing. Efforts have also been focused on modifying the
geometry of internal structures within the feed zone with the
avowed aim of achieving a uniform velocity of the rising vapor
across the cross section of the column prior to reaching the wash
zone.
[0038] To assist in these efforts, those skilled in the art have
employed sophisticated Computational Fluid Dynamics ("CFD")
programs to design feed inlet devices that attempt to uniformly
distribute the vapor flowing up through the column. In addition,
efforts have also focused on designing beds of structured packing
and distributors that attempt to minimize the amount of liquid wash
necessary to remove entrained liquid from the vapor while still
preventing excess accumulation of coke. These efforts, however,
have resulted in an only marginal reduction in the amount of
distillate that must be supplied to the wash zone as wash oil in
order to maintain the sufficiently high liquid flow rates required
to remove entrained droplets from the vapor and to prevent excess
accumulation of coke in the wash zone packing.
[0039] Systems and processes according to embodiments of the
invention eliminate the tension present in the operation of
conventional apparatuses between preventing excess coke
accumulation in the packing and minimizing loss of distillate
yield. Embodiments of the invention are directed to novel systems
and processes that contradict conventional wisdom in the art and
essentially eliminate both the loss of distillate yield and the
excess accumulation of coke in the wash zone packing that occurs in
conventional systems while achieving essentially complete removal
of entrained liquid from the vapor rising through the wash
zone.
[0040] According to one or more embodiments of the invention, a
system for improving quality and yield of one or more distillate
products generated in a distillation column is disclosed. The
distillation column comprises a feed zone, a wash zone located
above the feed zone, and a fractionation zone located above the
wash zone.
[0041] The wash zone has disposed therein a liquid/vapor contacting
structure and a wash oil distributor that distributes a liquid
stream onto the liquid/vapor contracting structure without
producing fine droplets that can become entrained in the vapor
rising through the distillate collector into the fractionation
zone. The contacting structure may comprise trays or packing that
enhance liquid/vapor contact to reduce an amount of liquid
entrained in vapor rising through the wash zone. The system further
comprises a feed inlet distributor that reduces an amount of liquid
entrained in a vapor stream rising through the wash zone, a wash
zone collection apparatus having an improved design for collecting
slop wax falling from the contacting structure, a recirculation
subsystem, and a control subsystem. The recirculation and control
subsystems will be described in more detail through reference to
FIG. 2, the wash zone collection apparatus through reference to
FIGS. 3A-3C, and the feed inlet distributor through reference to
FIGS. 4A-5B.
[0042] A key feature of the recirculation subsystem is the
recirculation or recycling of at least a portion of the slop wax
collected by the wash zone collection apparatus back to the top of
the wash zone for use as wash oil. Because of its high metals and
asphaltene content, those skilled in the art have not previously
considered the benefits of recycling collected slop wax to the top
of the wash zone for use as wash oil. In fact, conventional wisdom
has repeatedly taught away from recycling the black, viscous slop
wax to the top of the wash zone for use as wash oil based on the
belief that use of the slop wax would result in poor liquid
distribution throughout the packing and the development of dry
sections vulnerable to coke accumulation. In conventional systems,
this collected liquid is either combined with the bottoms product
or recycled to the charge heater.
[0043] Applicants recognized--contrary to the pervasive teachings
of the prior art--that slop wax recirculation in accordance with
embodiments of the invention in fact minimizes coke accumulation by
allowing for high wash oil rates that are not economically feasible
when distillate alone is used as the wash oil. By virtue of the
recirculation of slop wax to the top of the wash zone for use as
wash oil in accordance with embodiments of the invention,
distillate is no longer used as a primary source for the wash oil,
if at all, and significantly higher distillate yields are obtained.
Moreover, as will be described in greater detail hereinafter,
minimizing the wash oil flow rate is no longer a consideration in
embodiments of the invention, and as a result, greater entrained
liquid removal rates and lower coke accumulation rates are observed
as compared to conventional apparatuses.
[0044] FIG. 2 depicts a sectional view of a portion of a
distillation column 200 and elements external thereto in accordance
with one or more embodiments of the invention. Any elements shown
in FIG. 2 that are not separately described through reference to
FIG. 2 perform functions similar to corresponding elements
described through reference to FIG. 1. The distillation column
comprises a fractionation zone 200a, a wash zone 200b located below
the fractionation zone 200a, and a feed zone 200c located below the
wash zone 200b. The wash zone has disposed therein a liquid/vapor
contacting structure 204 and a wash oil distributor 203 for
distributing wash oil to the contacting structure 204.
[0045] Still referring to FIG. 2, the system according to
embodiments of the invention includes a feed inlet distributor (not
shown), a wash zone collection apparatus 205 disposed in the wash
zone 200b for collecting slop wax falling from the contacting
structure 204, a recirculation subsystem, and a control
subsystem.
[0046] The recirculation subsystem comprises conduit 217 for
conducting collected slop wax from the wash zone collection
apparatus 205 to pump 215. The recirculation subsystem further
comprises conduit 218 for recirculating a portion of the slop wax
pumped from pump 215 to the wash oil distributor 203 for
distribution as wash oil to contacting structure 204, and conduit
219 for conducting a portion of the pumped slop wax (i.e. that
portion not recirculated to the top of the wash zone) to external
equipment for further treatment (e.g. the inlet of a charge heater)
or for commingling with the heavy liquid bottoms product.
[0047] The portion of the slop wax recirculated through conduit 218
and the portion of the slop wax conducted through conduit 219
together constitute a total amount of slop wax pumped from the wash
zone collection apparatus 205. In one or more embodiments of the
invention, the portion of the slop wax recirculated through conduit
218 represents a majority of the slop wax pumped from the wash zone
collection apparatus 205. Corrosion inhibitors and/or antifoulants
may in certain embodiments be added to the recirculated portion of
the slop wax or to other system streams.
[0048] The control subsystem according to embodiments of the
invention may comprise a level controller 214 that determines an
amount of slop wax that has accumulated in the wash zone collection
apparatus 205 and communicates a signal to valve 216 to control the
flow of that portion of the slop wax conducted through conduit 219
to further blending or recycling treatment systems. The control
subsystem may further comprise flow controller 220 that
communicates a signal to valve 221 for controlling a flow of the
recirculated portion of the slop wax through conduit 218. In
alternate embodiments of the invention, the flow of that portion of
the slop wax conducted through conduit 219 may be controlled by a
flow controller.
[0049] It should be noted that the invention is not limited to the
particular arrangement shown in FIG. 2 and that numerous other
configurations and arrangements are within the scope of the
invention. For example, in accordance with one or more additional
embodiments of the invention, the wash zone 200b may comprise one
or more wash zone sections and the slop wax that is recycled to the
top of the wash zone 200b for use as wash oil may be obtained from
anywhere in the wash zone 200b below the wash column inlet through
which wash oil is supplied to the wash zone 200b. Each wash zone
section may comprise a contacting structure, a distributor provided
above the contacting structure for distributing wash oil to the
contacting structure, a wash zone collection apparatus provided
below the contacting structure for collecting slop fax falling from
the contacting structure, and a means for recirculating a portion
of the collected slop wax to the distributor in the same wash zone
section for distribution as wash oil.
[0050] In contrast to the streams of HVGO and Light Vacuum Gas Oil
(LVGO) product that are typically recirculated back to the HVGO and
LVGO sections of a vacuum column, respectively, the removal of heat
from that portion of the slop wax that is recirculated from the
wash zone collection apparatus 205 to the top of the wash zone 200b
is neither necessary nor desired in embodiments of the invention.
Because no thermal energy is removed from the recirculated slop
wax, condensation of vapor flowing up through the wash zone 200b
from the feed zone 200c does not occur.
[0051] The contacting structure 204 may comprise one or more types
of packing material including one or more trays, one or more beds
of loose packing, and/or one or more layers of structured packing.
As previously noted, the goal of packing designers has heretofore
been to design packing that minimizes the wash oil rate necessary
to obtain desired entrained liquid removal rates while maintaining
the packing in a sufficiently wetted condition, because increased
use of wash oil in conventional distillation systems equates to an
increase in the loss of valuable feed to downstream processes.
[0052] In contrast, according to embodiments of the invention, the
contacting structure 204 can be subjected to significantly higher
liquid flow rates because the wash oil comprises primarily, if not
solely, recirculated slop wax rather than expensive distillate, and
is thus much less sensitive to the performance of the wash oil
distributor. Moreover, due to the high liquid flow rate, the
contacting structure 204 is more resistant to the channeling of
vapor that conventional designs are prone to and which has been
known to produce dry areas vulnerable to coke accumulation. The
contacting structure 204 according to embodiments of the invention
is liberally flushed with liquid that captures--as it falls through
the contacting structure--liquid entrained in vapor rising through
the wash zone. Thus, the contacting structure 204 remains clean and
functional for longer periods of time than in conventional
systems.
[0053] The distributor in conventional systems is designed and
sized so as to function with only a small flow rate of wash oil to
the packing. In embodiments of the invention, however, it is
economically feasible to maintain wash oil flow rates that are
several times higher than in conventional systems, and thus the
ability of the wash oil distributor to uniformly distribute wash
oil to the contacting structure is not a significant concern. As
such, the distributor 203 according to embodiments of the invention
need not be designed for maintaining a uniform distribution of a
low flow density of liquid, but instead may be designed for
complete wash oil cross-sectional coverage of the contacting
structure 204 by a relatively high wash oil flow rate. For example,
the wash oil distributor 205 may comprise robust spray nozzles or
even overflow troughs capable of high liquid flow rates. Applicants
have recognized that modern spray nozzles with large openings and
modern trough distributors may advantageously be employed as they
are able to handle liquids containing considerable suspended
solids. Further, according to embodiments of the invention, the
liquid flow density through the contacting structure 204 is
sufficient to carry out any suspended solids. Moreover, the
distributor 204 is specifically designed to distribute a liquid
stream onto the liquid/vapor contracting structure without
producing fine droplets that can be entrained in the vapor rising
through the distillate collector into the fractionation zone.
[0054] Applicants have determined that an amount of liquid that
remains entrained in the upward flowing vapor after scrubbing with
wash oil is inversely related to the wash oil flow rate per unit of
cross-sectional area of the packing. Experimental testing has
confirmed that the increased wash oil flow rates capable of being
maintained in embodiments of the invention are accompanied by only
a negligible (if any) loss of distillate and result in considerably
less entrained heavy material appearing in the distillate product
than in conventional systems. Because embodiments of the invention
decouple the wash oil flow rate from the loss of valuable
distillate, almost all of the material condensed in the HVGO
section 201 can be recovered as HVGO product essentially free of
asphaltenes and non-volatile organometallic compounds. A
prohibitively expensive amount of distillate yield would need to be
sacrificed in conventional systems in order to obtain theoretically
high enough wash oil flow rates to produce entrained liquid removal
rates comparable to those obtained in embodiments of the
invention.
[0055] In order to, for example, limit the concentration of
entrained liquid in the portion of the slop wax that is
recirculated back to the top of the wash zone for use as wash oil,
in certain embodiments of the invention, the recirculation
subsystem may further comprise a means for combining a portion of a
distillate product collected by distillate product collector 202
with the portion of the slop wax recirculated through conduit 218.
More specifically, a portion of the distillate pumped from pump 210
may be diverted through conduit 210c and combined with the
recirculated portion of the slop wax via a juncture between conduit
218 and conduit 210c. Flow of the distillate product through
conduit 210c may be controlled by flow controller 212 which
communicates a signal to valve 213. In alternate embodiments of the
invention, flow of the distillate through conduit 210c may be
controlled by level controller 214. In those embodiments of the
invention in which a portion of the collected distillate is
combined with the recirculated portion of the slop wax, the amount
of distillate added is miniscule compared to conventional designs.
For example, in one or more embodiments of the invention, the
amount of distillate added to the wash oil may constitute about ten
percent or less of the wash oil by weight.
[0056] During experimentation, Applicants unexpectedly observed
that even when the addition rate of collected distillate to the
wash oil was reduced to zero, slop wax continued to be pumped from
the wash zone collection apparatus 205 and the portion of the slop
wax recirculated back to the top of the wash zone 200b for use as
wash oil was more dilute than necessary to ensure complete capture
of the entrained liquid in the vapor rising through the wash zone
200b.
[0057] Applicants reasoned that because there is no significant
change in temperature across the wash zone 200b in embodiments of
the invention, the temperature at the top of the wash zone 200b was
higher than the temperature of the distillate product collected in
the distillate product collector 202. Applicants further reasoned
that this temperature differential was causing hot vapor rising
past the wash zone 200b to condense on the bottom of the relatively
cool distillate product collector 202 (e.g. HVGO draw tray) thereby
diluting the recirculated slop wax more than necessary to ensure
capture of essentially all liquid entrained in the rising vapor.
That is, the small amount of heat transfer across the distillate
product collector 202 is more than sufficient to provide the small
amount of liquid that may be necessary to dilute the wash oil in
order to remove all entrained material in the vapor rising through
the wash zone.
[0058] Accordingly, based on these observations, the system
according to embodiments of the invention may further comprise an
insulation apparatus comprising one or more insulating materials
applied to a surface of the distillate product collector 202. The
one or more insulating materials may comprise a castable refractory
applied to an upper surface of a floor of the distillate product
collector 202 in order to reduce condensation of vapor on a bottom
surface of thereof. This reduction in vapor condensation may in
turn require the addition of a very small amount of collected
distillate to the recirculated slop wax in order to keep the
concentration of captured entrained liquid in the wash oil low
enough for the wash oil to continue to be effective in removing
essentially all of the entrained liquid from the rising vapor.
[0059] Of particular advantage is that the addition of one or more
insulating materials to the distillate product tray coupled with
the addition of a small amount of the collected distillate to the
wash oil may in certain embodiments produce a greater distillate
product yield than if the distillate product collector is not
insulated and no collected distillate is added to the wash oil.
That is, because the flow of make-up distillate necessary to
sustain the performance of the wash zone generally, and the wash
oil in particular, may be less than the amount of distillate
condensed on a bottom surface of the distillate product collector,
the distillate yield may in fact be maximized by insulating the
distillate product collector 202 immediately above the wash zone
200b. Reducing the amount of condensation on the underside of the
distillate product collector 202 reduces the uncontrolled loss of
potential distillate to the recirculated wash oil and transfers
control of the dilution of the wash oil with collected distillate
product to flow controller 212.
[0060] In one or more exemplary embodiments of the invention, an
operating ratio of 19:1 recycled wash oil to fresh distillate
results in effective removal of entrained liquid from the rising
vapor while providing ease of control. According to an exemplary
process of the invention, initially fresh distillate alone is
provided as wash oil to the wash zone. Subsequently, the amount of
distillate added to the wash oil is progressively reduced while the
rate at which collected slop wax is recycled to the wash zone is
concomitantly increased. The addition rate of distillate to the
wash oil may be adjusted based on: (1) the rate of dilution needed
to keep the collected slop wax pumpable, and/or (2) the amount of
fresh distillate required to maintain essentially complete capture
of the entrained liquid from the rising vapor.
[0061] Applicants conducted an experiment that compared the
distillate yields obtained by a conventional distillation system
and a system in accordance with an embodiment of the invention. The
95% distillation point for the conventional distillation system
based on the industry standard ASTM Method D-1160 distillation of
HVGO product was 1050.degree. F. However, after modifications were
made to the conventional system to produce a system in accordance
with an embodiment of the invention, the 95% distillation point of
the HVGO product increased to 1250.degree. F. and the distillate
obtained was almost completely free of contamination by entrained
feed liquid.
[0062] It is known in the art that conventional vacuum distillation
units are incapable of yielding an HVGO product with the 95%
distillation point higher than 1050.degree. F. without increasing
the charge heater outlet temperature to such an extent that
excessive coking of the charge heater and cracking of the feed
stream results. Applicants unexpectedly determined that the
200.degree. F. increase in the 95% distillation point obtained by
the system of the invention resulted in an increase in the HVGO
product yield by about 5% of the fluid feed stream to the column.
If the distillate yield is supplied as feed to an FCC unit, for
example, it is estimated that this increase in the distillate yield
can potentially increase refinery profit by US$110 per incremental
barrel of FCC feed. To provide context for this estimation, the
spot market price of West Texas Intermediate crude oil was
approximately US$90/barrel at the time the estimate was made.
[0063] FIG. 6 is a photograph showing two samples of HVGO product
obtained by the experimental testing described above. The sample on
the left is one obtained using a conventional apparatus and the
sample on the right is one obtained in the same vacuum column using
a system in accordance with an embodiment of the invention. As is
evident, the sample on the right is almost completely free of
darkening by the black entrained feed liquid. In addition, the
sample on the right has a much higher end point (about 1250.degree.
F.) than the sample on the left (1050.degree. F.) corresponding to
about a 5% increase in distillate yield.
[0064] In addition to the recycling of slop wax for use as wash
oil, another novel aspect of systems according to embodiments of
the invention is the design and structure of the wash zone
collection apparatus.
[0065] FIGS. 3A-3C depict various plan and/or sectional views of a
wash zone collection apparatus in accordance with one or more
embodiments of the invention. The wash zone collection apparatus so
depicted minimizes turbulence and thus pressure drop in the rising
vapor in three primary ways: by minimizing the deflection of vapor
as it rises through the collection apparatus, by occluding less
than 50% of the cross-sectional area of the column at any layer of
the apparatus, and by not introducing any drag-inducing sharp edges
in the vapor path.
[0066] FIG. 3A shows a plan view of the wash zone collection
apparatus. The apparatus 300 includes one or more center channels
or troughs 301. One or more layers of lateral troughs may be
provided so as to be inclined towards the center trough(s) 301. A
single layer comprising lateral troughs 303A is shown in the plan
view of FIG. 3A.
[0067] FIG. 3B depicts a sectional view of the wash zone collection
apparatus taken along Section 3B-3B in FIG. 3A. Two layers 302, 303
of lateral troughs are provided. Layer 302 comprises lateral
troughs 302A and layer 303 comprises lateral troughs 303A. The
lateral troughs 302A of layer 302 are positioned so as to be
staggered laterally with respect to the lateral troughs 303A of
layer 303. The plurality of lateral troughs in each layer may be
substantially equal in width and each lateral trough in each layer
may be spaced laterally from an adjacent trough in the same layer
by a distance of about 1.0 to about 1.2 times the width of each
lateral trough. Moreover, each layer of lateral troughs may be
spaced vertically from an adjacent layer by a distance about equal
to a distance between adjacent lateral troughs in the same
layer.
[0068] The staggering of the lateral troughs of one layer with
respect to the lateral troughs of another layer such that each
lateral trough in each layer is spaced laterally from an adjacent
trough in the same layer by a distance of about 1.0 to about 1.2
times the width of each lateral trough produces significant
advantages over conventional slop wax collectors. In particular,
the design described above causes vapor flowing upward through the
gaps between adjacent lateral troughs in a lower layer to split on
the bottom surface of a lateral trough in an immediately adjacent
upper layer. This in turn directs liquid that has passed through
the contacting structure toward a center line of the gap between
adjacent troughs in an upper layer and thus onto the center axis of
a lateral trough in the lower layer. Accordingly, more effective
slop wax collection is achieved.
[0069] To further minimize pressure drop across the wash zone
collection apparatus, the center channel or trough 301 as well as
the lateral troughs 302A, 303A may comprise rounded or V-shaped
bottoms. In an exemplary embodiment of the invention, at least the
upper edges of each lateral trough may be curved with a radius of
about 1 cm to about 2 cm about 60 degrees toward the center axis of
the lateral trough. This design not only provides adequate
stiffness but also produces the unexpected additional advantage of
reducing pressure drop across the apparatus and reducing
re-entrainment of liquid droplets deposited on the lower surfaces
of the lateral troughs and forced upwards by the rising vapor. The
rounding of the upper edges of the lateral troughs inward out of
the vapor path induces the rising vapor to direct droplets falling
out of the wash zone packing toward the center axis of the trough,
which in turn allows a cross-sectional area occluded by each layer
of lateral troughs to be less than about 50% of a cross-sectional
area of the distillation column and the total plan area of the
troughs to be less than 100% of the cross-sectional area of the
column. Additionally, liquid residence time in the collection
apparatus 300 is minimized by the narrowed V-bottomed surfaces and
strong incline of the lateral troughs toward the central trough 301
as well as the incline of the central trough 301 toward an opening
that provides for fluid communication with the recirculation
subsystem. The rounded V-shape of the bottom surfaces of the
lateral troughs also serves to minimize vapor drag.
[0070] As previously mentioned, a system according to one or more
embodiments of the invention additionally comprises a feed inlet
distributor that reduces or minimizes an amount of liquid entrained
in a fluid feed stream fed to the distillation column. The gas
velocity that will cause entrainment of a liquid droplet is
proportional to the square of the droplet's diameter. The amount of
liquid entrained in the vapor rising through the wash zone is
dependent on the velocity of the vapor, which is a function of the
vapor mass flow rate and specific volume, and the distribution of
droplet sizes in the feed liquid. In general, the more finely
atomized the feed liquid, the greater the level of entrainment.
[0071] FIGS. 4A and 4B depict plan and sectional views,
respectively, of a tangential feed inlet distributor 400 in
accordance with one or more embodiments of the invention. The
tangential feed inlet distributor 400 shown in FIGS. 4A and 4B
distributes a fluid feed stream that enters the column through a
tangential feed inlet 401 along a flow path substantially
tangential to a circumferential wall 402 of the column. The feed
distributor 400 comprises a horizontally disposed annular ring 403
extending around an entire circumference of the circumferential
wall 402 of the distillation column, and a cylindrical skirt 404
connected to an inner edge of the annular ring 403 and extending
downward therefrom. In certain embodiments of the invention, the
annular ring 403 may extend from the circumferential wall 402 of
the distillation column by a distance about equal to a diameter of
the feed inlet 401 and the cylindrical skirt 404 may extend
downward a distance equal to about 1.25 times the diameter of the
feed inlet 401.
[0072] The horizontal annular ring 403, the cylindrical skirt 404,
and the circumferential wall 402 of the distillation column
together define an open-bottomed tunnel within the distillation
column into which the fluid feed stream is injected or channeled
via feed inlet 401. The tunnel temporarily segregates the incoming
fluid from the vapor rising past the tunnel to allow the fluid to
decelerate and the entrained liquid to separate. The tangential
flow path of the fluid feed stream exploits centrifugal force and
gravity to enhance gravimetric separation of liquid from the
vapor.
[0073] FIGS. 5A and 5B depict various views of a radial feed inlet
distributor 500 in accordance with one or more alternate
embodiments of the invention. In this embodiment, the feed inlet
nozzle 501 is radial to the vessel rather than tangential. The
radial feed inlet distributor 500 comprises a roof plate 504 that
extends substantially across an entire diameter of the distillation
column and a skirt comprising opposing walls 503A, 503B laterally
spaced from each other about a distance equal to a diameter of the
feed inlet 501 and extending downwards from the roof plate 504
about a distance equal to the diameter of the feed inlet 501. The
opposing walls 503A, 503B of the skirt terminate prior to reaching
a distal end of the circumferential wall 502 of the distillation
column. For example, the opposing walls 503A, 503B may terminate a
distance from the circumferential wall of the distillation column
equal to about one half of the diameter of the feed inlet 501. A
feed splitter or deflector 505 is provided in proximity to where
the opposing walls 503A, 503B of the skirt terminate, the feed
splitter 505 splitting the fluid feed stream bilaterally into two
smaller fluid feed streams directed in opposing horizontal
directions and along flow paths substantially tangential to the
circumferential wall 502 of the distillation column.
[0074] The fluid feed stream is injected or channeled via radial
feed inlet 501 into an open-bottomed tunnel defined by the roof
plate 504 and the opposing walls 503A, 503B of the skirt. As such,
the high velocity liquid in the vapor stream is constrained from
spraying upward or laterally and a bulk portion of the liquid falls
out of the vapor stream through gravimetric separation. The feed
splitter 505 converts the radial flow path of the fluid feed stream
into bilateral horizontal tangential fluid flow paths, thereby
exploiting the vapor velocity to enhance liquid separation by
centrifugal force.
[0075] In an embodiment of the invention, the feed splitter 505 may
comprise a sharp-edged vertical bar 505A connected at an upper end
to the roof plate 504 and extending downward therefrom a distance
about 1.5 times a diameter of the feed inlet nozzle 501. The feed
splitter 505 may further comprise symmetrically disposed deflector
plates 505B, each plate being rolled to form a quarter of the
circumference of a vertical cylinder having a height equal to that
of the vertical bar 505A. One vertical edge of each deflector plate
may be attached to the vertical bar 505A while the other vertical
edge lies along the circumferential wall 502 of the column.
[0076] Conventional inlet distributors comprise various types of
vanes and deflectors in the fluid feed stream path that exacerbate
atomization of entrained liquid in the feed stream. Further,
panels, plates or other devices provided in the path of the
high-velocity vapor produce eddy currents in the flowing fluid. For
every eddy in the downward direction, the velocity of the vapor
elsewhere in the same horizontal plane must increase, thereby
increasing the amount of entrained liquid. Moreover, structures
disposed within the feed stream flow path convert the horizontal
velocity of the feed into a vertical velocity component that can
exacerbate entrainment of liquid in the rising vapor.
[0077] Feed inlet distributors according to embodiments of the
invention are free of surfaces transverse to the feed fluid
stream's tangential flow path and thereby avoid the atomization of
entrained liquid droplets and the formation of eddy currents
observed in conventional inlet distributors. The feed is allowed to
travel tangentially along the circumferential wall of the column
unobstructed by any surface that could shatter liquid droplets or
deflect the vapor vertically and exacerbate liquid entrainment in
the rising vapor. Feed inlet distributors according to embodiments
of the invention are thus able to exploit the horizontal velocity
of the vapor stream to separate liquid gravimetrically as it flows
tangentially along the wall of the column.
[0078] Additionally, in conventional systems, the increase in the
amount of entrained liquid in the vapor feed stream caused by
impingement of the feed stream against surfaces in the fluid flow
path often necessitates increased washing action in the wash zone,
which in turn requires an increase in the amount of distillate that
must be added to the wash oil circulation to maintain its
effectiveness. However, feed inlet distributors according to
embodiments of the invention eliminate this additional disadvantage
of conventional systems
[0079] Conventional wisdom in the art has heretofore advocated the
installation of vanes of various shapes and orientations transverse
to the fluid feed path with the stated objective being to establish
uniformity of vertical velocity of the rising vapor below the wash
oil collector. CFD modeling has been used to design inlet
distributors and associated impingement surfaces that attempt to
achieve a uniformity of distribution of the rising vapor. A
significant drawback of such modeling is that it has focused only
on the vapor phase of the fluid feed stream without regard to the
liquid phase.
[0080] Applicants have determined that uniformity of the vapor flow
prior to reaching the wash zone is irrelevant to wash zone
performance for several reasons. First, the velocity profile of the
vapor as it rises through the wash zone is determined more by the
design of the wash oil collector than by the vapor velocity pattern
below the collector. Second, with the high wash oil flow rate
achieved by embodiments of the invention, minor variations in the
velocity of vapor entering the wash zone packing are made more
uniform by flow through the packing alone. Third, given the high
liquid flow rates in the wash zone, the rising vapor is effectively
washed despite any variations in vapor velocity below the
packing.
[0081] Even assuming arguendo that feed inlet distribution
subsystems according to embodiments of the invention are not
capable of achieving as uniform a distribution of vapor velocities
across the cross section of the column above the feed zone as is
achieved in conventional designs, embodiments of the invention are
still capable of providing essentially complete removal of
entrained liquid. Applicants have determined that the uniformity of
vapor distribution below the wash zone is inconsequential to
removal of entrained liquid in the wash zone because the path of
vapor flow through the wash oil collector has a far greater
influence on the distribution of vapor into the wash zone packing
than the velocity profile below the collector. Moreover, any
potential adverse effect of non-uniform distribution of vapor
leaving the feed zone is negated by the high wash oil flow rates
capable of being maintained in embodiments of the invention, which
ensure complete wetting of the wash zone packing and essentially
complete removal of entrained material.
[0082] It should be noted that regardless of how well feed inlet
distributors according to embodiments of the invention are capable
of performing, some entrainment of liquid in the vapor rising from
the feed zone is likely to occur. The recirculation subsystem
according to embodiments of the invention recirculates the majority
of the slop wax that has fallen from the wash zone back to a top of
the wash zone for use as wash oil. Only a miniscule portion of the
distillate may be combined with the recirculated slop wax prior to
distribution to the top of the wash zone. As such, because
distillate addition to the wash oil is negligible, significantly
higher wash oil flow rates can be maintained as compared to
conventional systems. Moreover, because the wash oil is at a higher
temperature than the distilled liquid higher in the column, there
is less condensation of the up-flowing vapor, and thus less loss of
distillate yield. The end result is that the distillate product so
obtained (e.g. HVGO product) contains negligible contamination by
entrained material and is actually increased in yield. As an
additional benefit, the modifications that must be made to
conventional systems to arrive at embodiments of the invention do
not require significant expense.
[0083] While the invention has been described with respect to a
particular number of embodiments, those having ordinary skill in
the art will understand that numerous other embodiments involving
variations or modifications to the systems and processes described
are also within the scope of the invention.
* * * * *