Liquid-liquid extraction apparatus and process

A relatively large volume horizontal liquid separation chamber is provided as an intermediate part of the downcomers used to transfer the denser liquid phase between contiguous trays of a countercurrent liquid-liquid extraction column. The downcomers also include two smaller vertical conduits which carry the liquid into and out of the opposing ends of the chamber. The downward flowing denser liquid phase is thereby subjected to a separation step within the downcomers to produce a stream of the less dense liquid released above the upper tray.

FIELD OF THE INVENTION 
The invention relates to an apparatus for the countercurrent contacting of 
two immiscible liquid streams to perform a purification or separation such 
as described in the processes found in Classes 208-311 to 208-337 or 
210-21 and 210-22. These processes include the removal of mercaptans from 
hydrocarbon feed streams through the use of aqueous alkaline or amine 
solutions and the separation of aromatic hydrocarbons from non-aromatic 
hydrocarbons using selective solvents. The invention is specifically 
directed to a vertical liquid-liquid extraction column having horizontal 
trays and used for countercurrent operations in which the downward flow of 
the denser liquid phase is directed through downcomers. References 
concerned with similar apparatus may be found in Classes 23-267R to 
23-271. 
PRIOR ART 
Liquid-liquid extraction is a well established process used commercially in 
the chemical, petroleum and petrochemical industries. It is often utilized 
when distillatory separation is more expensive or is impractical. A very 
extensive review of the art is contained in a number of articles published 
as pages 49-104 of Chemical Engineering Progress, (Vol. 62, No. 9) Sept., 
1966. Instruction in the design of extraction processes and the selection 
of suitable equipment is provided in standard reference materials such as 
The Chemical Engineers'Handbook, 4th Ed., McGraw-Hill Publishing Co. and 
Treybal, Mass Transfer Operations, 2d Ed., McGraw-Hill Publishing Co. 
Processes for the removal of sulfur compounds, such as mercaptans, from a 
hydrocarbon feed stream by liquid-liquid extraction with an aqueous 
alkaline solution which is subsequently regenerated are described in U.S. 
Pat. Nos. 2,921,020 (Cl. 208-205); 2,988,500 (Cl. 208-206); 3,108,081 (Cl. 
254-428); 3,260,665 (Cl. 208-206); 3,409,543 (Cl. 208-234); 3,574,093; 
3,923,645 and 4,040,947 (Cl. 208-235). 
One of the commonly used types of extraction apparatus comprises a vertical 
cylindrical vessel containing a number of horizontal liquid support trays. 
This type of apparatus often includes either downcomers through which the 
descending liquid flows or guides for the rising liquid phase. Examples of 
this column-type extraction apparatus are contained in U.S. Pat. Nos. 
2,610,108 (Cl. 13-270.5); 2,623,813; 2,699,505 and 2,895,809. 
BRIEF SUMMARY OF THE INVENTION 
The invention provides a column-type multi-stage liquid-liquid extraction 
apparatus in which the separation of a less dense liquid from the downward 
flowing denser liquid is improved at each tray by a novel downcomer. The 
invention may be broadly characterized as a liquid-liquid extraction 
apparatus which comprises a cylindrical outer vessel having a vertical 
major axis; a plurality of horizontal perforated liquid support trays 
extending across the internal volume of the vessel at vertically spaced 
apart intervals; and a plurality of liquid downcomers connected to the 
liquid support trays. Each downcomer comprises a first vertical conduit 
which extends downward from a liquid support tray and which is operably 
connected to a first end of a horizontal liquid separation chamber located 
under the tray. This chamber has a larger cross-sectional area than the 
first vertical conduit and extends across the internal volume of the outer 
vessel to provide a large volume to allow entrained globules of the less 
dense liquid to separate from the main liquid stream. A second relatively 
small diameter vertical conduit is connected to the second end of the 
liquid separation chamber and extends downward to a point a short distance 
above the tray immediately below. Preferably a third small diameter 
conduit extends upward from the liquid separation chamber to a point above 
the liquid support tray to discharge the accumulated less dense liquid. 
Another embodiment of the invention comprises a liquid-liquid extraction 
process wherein an upward flowing hydrocarbon feed stream is subjected to 
multi-stage countercurrent contacting with a descending solvent stream in 
a trayed vertical extraction column. The solvent stream is transferred 
downward from tray to tray in downcomers having an enclosed quiescent 
substantially horizontal liquid phase separation zone, and entrained 
globules of liquid hydrocarbons which enter the extraction column as part 
of the feed stream are separated from the portion of the solvent stream 
flowing through the downcomer within the separation zone. The 
thus-separated liquid hydrocarbons are vented from the phase separation 
zone to a point above the tray immediately above the phase separation zone 
through a vertical conduit extending through the tray.

The structures shown in the drawings have been simplified for brevity and 
clarity of presentation of the inventive concept. Accordingly, various 
accouterments and normally utilized subsystems are not shown. These 
include support brackets, weep holes, manways, liquid transfer lines and 
distributors, valves, control systems, etc., which may be of a type 
customarily utilized in the art. 
Referring now to FIG. 1, the central portion of a hollow cylindrical 
extraction vessel 1 incorporating the preferred embodiment of the 
invention is illustrated. This vertical vessel has a cylindrical internal 
volume which is divided into a number of countercurrent contacting stages 
by the vertically spaced horizontal liquid support trays 2. These trays 
have a large plurality of evenly spaced perforations 3 spread across a 
centrally located liquid-liquid contacting area. A continuous stream of 
the denser liquid phase descends through the column. This liquid stream 
enters the section of the apparatus shown in the Drawing from above 
through a vertical conduit 8 which is part of a downcomer. As illustrated, 
the lower open end of this conduit is preferably located at a point below 
the upper horizontal edge of an imperforate vertical wall 4 used as an 
inlet weir. The liquid stream is distributed across the upper surface of 
the support tray as it flows over the wall 4. At the same time, rapidly 
flowing less dense liquid passes upward through the perforations 3 and 
intimately contacts the stream of denser liquid and the extraction 
operation is thereby effected. The thus-contacted denser liquid leaves the 
upper tray by passing into the upper open end of the vertical conduit 6 
through a relatively large opening in the upper surface of the tray. A 
second imperforate vertical wall 5 separates the contacting area of the 
tray from this downcomer inlet opening. 
The ascending less dense liquid should pass through the perforations at a 
velocity sufficient to produce substantial agitation and admixing of the 
liquid streams since this promotes efficient extraction. An undesired 
result of this agitation is the entrainment of globules of the less dense 
liquid into the stream of denser liquid entering the downcomer. The 
descending two-phase downcomer liquid stream in conduit 6 flows into a 
horizontal liquid separation chamber 7 in which the two liquid phases may 
separate. The lighter material which was entrained accumulates at the top 
of the tubular separation chamber as a separate liquid phase, if present 
in a sufficient quantity, and rises through a vertical outlet conduit 9. 
The upper open end of this conduit is preferably elevated above the normal 
level of the denser liquid on the tray immediately above. A third vertical 
conduit 8 is connected to the other end of the liquid separation chamber 
at the bottom of liquid separation chamber. The denser liquid phase exits 
the downcomer through this conduit and begins a pass across the next lower 
tray. 
FIG. 2 is the view seen looking downward at the upper liquid separation 
chamber 7 of FIG. 1. The manner in which this chamber extends from one 
lateral side of the vessel to the other may be seen in this view. The 
chamber is perpendicular to the vertical walls 4 and 5, which abut the 
inner surface of the wall of the outer vessel 1. A minimal number of 
representative perforations 3 are distributed across the lower tray 2. The 
vertical conduits 6, 8 and 9 all communicate with the internal volume of 
the liquid separation chamber. 
In FIG. 3, a side view of the preferred embodiment is presented. This view 
is seen looking along the horizontal major axis of the liquid separation 
chamber 7. The two vertical conduits 6 and 9 extend upward from this 
chamber, with conduit 9 rising above the upper surface of the tray 2 and 
the horizontal upper edge of the outlet weir or wall 5. This wall and the 
inlet weir not shown are preferably attached to the cylindrical inner 
surface of the outer vessel 1 and to the upper surface of the tray. 
DETAILED DESCRIPTION 
Multi-stage countercurrnet liquid-liquid extraction is widely applied to 
perform purifications or separations in the food, chemical, petroleum and 
petrochemical industries. In these multi-stage operations, the two basic 
steps of the extraction process, contacting and separation, are each 
repeated several times in sequence. This may be done in a wide variety of 
equipment. One type that is often used is the vertical extraction column. 
The denser liquid stream is fed into an upper portion of the column, often 
at the top, and travels downward. The less dense liquid is fed into the 
bottom of the column and travels upward. Either liquid may be the feed 
stream or the solvent stream. These columns may be designed to have the 
denser liquid phase pass downward through a large plurality of 
perforations in the contacting area of the horizontal trays which extend 
across the internal volume of the column. In this arrangement, the less 
dense phase flows across the bottom surface of the trays and into conduits 
which guide it to the next tray above until it eventually reaches the top 
of the column. However, in the subject invention, the denser liquid phase 
is retained upon the upper surface of the liquid support trays and flows 
through the downcomers while the less dense liquid flows upward through 
the perforations in the trays. The subject invention is directed to this 
specific type of countercurrent multi-stage extraction wherein the denser 
liquid stream flows through downcomers. 
One of the basic considerations in the design of a liquid-liquid extraction 
column is the adequacy of the contacting performed at each stage or tray 
in the column. Certain well established guidelines or experimental data 
are therefore utilized to design a column having a free area or porosity 
which provides which has been found to be an effective liquid velocity 
through the perforations on the tray. However, the resultant agitation of 
the liquid phases also causes small particles or globules of each liquid 
phase to become dispersed into the other liquid phase at the contacting 
point. It is not desirable to carry these smaller particles along with 
main liquid flow. It is therefore desired that the contacted liquid phases 
be maintained at quiescent conditions conducive to the separation of the 
two liquid phases for a time sufficient to allow them to completely 
separate before the phases are passed on to their next respective 
contacting stage. This separation step begins while the denser liquid 
stream is traveling from the contacting area to the inlet of the 
downcomer. 
The time required for the adequate separation of any two liquids will 
depend to some extent on the identity of the two liquids. For instance, an 
aqueous solution of an alkaline metal hydroxide such as sodium hydroxide, 
commonly referred to as caustic, and a light hydrocarbon will normally 
settle from a vigorous mixing in about seven seconds. In comparison, it 
requires about 70 seconds to effect an adequate separation after a similar 
mixing of an amine solution and a light hydrocarbon. 
It is common practice to utilize a cylindrical downcomer to remove the 
denser liquid phase from the surface of an extraction tray. The opening at 
the top of this conduit is normally separated from the contacting area of 
the tray by a vertical weir following a chord across the circumference of 
the tray. The outlet conduit is customarily evenly spaced between the 
circular outer edge of the tray and this weir or, if no weir is used, the 
point which marks the end of the contacting area of the tray. Also by 
standard design, the space allowed on each side of the outlet conduit is 
about at least three inches and may equal the diameter of the conduit. 
Therefore a chordal imperforate zone having a width at least six inches 
greater than the diameter of the outlet conduit is normally provided in a 
chordal area on the edge of the tray. This area and an approximately equal 
area on the opposite side of the tray are excluded from the contacting 
area of the tray and therefore are not available for performing the 
contacting step of the extraction process. 
It is desired that the liquid phase entering the downcomer has a residence 
time within the downcomer conduit equal to or exceeding the time necessary 
for the two liquid phases which are being contacted to separate. 
Therefore, in the case of contacting an amine solution and a light 
hydrocarbon, it is desired that a residence time in excess of 70 seconds 
is provided for the denser material in the downcomer. An exceedingly long 
residence time such as this dictates the use of very large diameter 
conduits to accommodate a large volume of liquid. The result is that a 
very large area of the tray surrounding the mouth of the downcomer is not 
available for utilization in the extraction step. For example, if a 
12-inch vertical conduit is required to provide an adequate residence 
time, then the perforations in the tray would be located no closer than 
11/2 feet from the edge of the tray on the outlet side of the tray. 
It is an objective of the subject invention to provide an apparatus for the 
countercurrent multi-stage contacting of two immiscible liquids. It is 
another objective of the invention to provide a downcomer structure for 
use in a multi-stage liquid extraction column. It is a further objective 
of the invention to minimize the inactive or dead areas required on the 
surface of trays used in liquid-liquid extraction columns while 
simultaneously effecting adequate separation of the two liquid phases at 
each stage in the contacting operation. A further objective is to provide 
a multi-stage countercurrent liquid-liquid extraction process provided 
good separation of the liquid phases between contacting steps. 
The subject apparatus is contained within a cylindrical outer vessel 
constructed in accordance with the applicable standards or codes for 
vessel design. The outer vessel and the components located inside it are 
preferably constructed of a suitable metal, such as carbon steel or 
stainless steel. Other metals or reinforced plastics may also be used. The 
cylinder is closed at the top and the bottom and is liquid-tight with the 
exception of the required liquid transfer lines. A plurality of 
substantially horizontal liquid support trays are vertically spaced apart 
within the internal volume of the vessel. The design and vertical spacing 
of the trays is determined for each particular service in accordance with 
well known design procedures. For example, for the extraction of 
mercaptans from liquid hydrocarbons, the trays are normally spaced about 4 
to 7 feet apart, which smaller distances down to about 18 inches being 
usable in other applications. Each tray has a "free area" equal to the 
total open area of all the perforations located in the contacting area of 
the tray. The perforations should be from about 1/8to 3/8-inch in 
diameter, and should be spread across the tray in a manner which assures 
uniform contact of the rising liquid phase with the liquid phase 
traversing the tray. Preferably the perforations are located in three or 
more rows perpendicular to the flow of this liquid phase. From three to 
about twenty or more tray may be utilized in the apparatus. At least two 
liquid transfer conduits communicate with the internal volume of the 
vessel at points above and below the trays to provide inlets for the feed 
and solvent streams and outlets for the extract and raffinate streams. 
The number and total cross-sectional area of the perforations in the 
contacting area of the tray are probably the most important considerations 
in obtaining an efficient extraction since they determine the velocity 
through each individual perforation. Recommended velocities are available 
in the literature for a large number of systems. If no published value is 
available, an optimum velocity should be determined experimentally. The 
trays are preferably a perforated or sieve-type tray rather than a bubble 
cap tray. They should be substantially horizontal and extend across the 
internal volume of the column. As used herein, the term "substantially 
horizontal" is intended to indicate the relevant surface or member has an 
inclination less than 5.degree. from horizontal. 
The vessel and the trays may be characterized as having two lateral halves 
or sides, with the inlet and outlet ends of the downcomers being on 
opposite sides of any specific tray. These two lateral halves are divided 
from each other by a vertical plane which passes through the vertical 
central axis of the outer vessel, and therefore through the middle of each 
tray, and which is perpendicular to a horizontal line drawn between this 
central axis and the middle of the upper opening of the downcomer removing 
liquid from the relevant tray. The vertical plane is parallel to the inlet 
and outlet weirs located on the trays. Preferably, the outlets of all the 
downcomers within the vessel are vertically aligned with each other. The 
denser liquid therefore preferably flows into all the downcomers in the 
first lateral half of a vessel and is transported to the second lateral 
side or half of the vessel by the downcomers before being discharged upon 
the surface of the lower tray. 
According to the inventive concept, each downcomer is constructed with an 
inlet or first vertical conduit which is attached to the downcomer inlet 
of the tray and which carries the descending liquid phase into a 
substantially horizontal liquid separation zone or chamber. This first 
vertical conduit is preferably relatively short and attached to the upper 
surface of the liquid separation chamber at or near the end of a liquid 
separation chamber. The liquid separation chamber is preferably an 
enclosed tubular conduit having a cross-sectional area at least twice as 
large as the cross-sectional area of the vertical conduit which removes 
the entering liquid from the tray above. More preferably, the 
cross-sectional area of the liquid separation chamber is at least five 
times greater than the cross-sectional area of the vertical conduit. 
The first or inlet vertical conduit is attached at or near a first end of 
the liquid separation chamber, and a longer second or outlet vertical 
conduit is attached to the opposing or opposite second end of the liquid 
separation chamber. The two vertical conduits are preferably connected to 
the liquid separation chamber at these distant points to maximize the time 
which liquid must reside within the chamber before leaving it, thereby 
increasing the separation efficiency of the chamber. One or both of the 
vertical conduits may be located inward from the end of the liquid 
separation chamber, but they are to be on opposite lateral halves of the 
vessel. The second vertical conduit preferably communicates with the 
internal volume of the chamber through an opening in the bottom of the 
chamber to thereby facilitate the removal of only the denser liquid phase 
contained within the chamber. 
Preferably, the chamber is substantially horizontal. It may, however, be 
tilted slightly such that the inlet end at which the first vertical 
conduit is connected is at a slightly higher elevation. It is also 
preferred that the liquid separation chamber is located closer to the 
upper of the two trays which it is between. This minimizes interference 
with the extraction and separation steps being performed at the tray 
immediately below. It should, however, also not interfere with flow 
through the tray immediately above and is preferably located at least 20 
cm. below this tray. The three conduits forming the downcomer may be 
assembled in the manner illustrated in the drawing and may be fabricated 
from standard size components. For instance, if the internal diameter of 
the outer vessel 1 is about 3.5 feet, then the vertical conduits 6 and 8 
could be fabricated from 3-inch pipe and the liquid separation chamber 7 
could be fabricated from 8-inch pipe. It is preferred that the internal 
length of the liquid separation chamber, as measured along its horizontal 
major axis, is greater than three-quarters of the diameter of the 
cylindrical internal volume of the outer vessel of the column. 
Preferably the liquid separation chamber is straight and has an internal 
length to diameter ratio above 3:1. However, if the outer vertical vessel 
has an inner diameter less than about 1.3 meters, the chamber should be 
curved to allow its passage through the manways which have to be centrally 
located in columns of such small diameter. Flow stabilizing vanes and 
coalescing means such as mesh blankets or screens may be provided near the 
inlet end of the chamber. 
The second vertical conduit, which carries the descending downcomer liquid 
stream out of the liquid separation chamber, preferably ends at a point 
which is a short distance above the upper surface of the next lower tray 
and which is a discreet distance inward from the inner surface of the 
outer wall of the outer vessel. This is shown in the drawing. It is 
preferred that the lower open end of this conduit is located below the 
upper horizontal edge of a vertical wall which is attached to the surface 
of the tray to form a liquid receiving compartment on the inlet lateral 
half of the tray. The lower end of the conduit is therefore at a greater 
radial distance from the vertical central axis of the column than the 
wall. 
The less dense liquid which separates out in the liquid separation chamber 
must be allowed to exit the chamber. This lighter phase may travel upward 
through the inlet vertical conduit if this conduit is of sufficient size 
and if the downward flow of liquid through the conduit is not excessive. 
However, according to the inventive concept, an alternative easily 
traveled flow path is provided for this lighter phase. This alternative 
flow path preferably comprises a third vertical straight conduit which 
communicates with the upper portion of the internal volume of the liquid 
separation chamber and extends upward through the liquid support tray 
located immediately above. This third vertical conduit may have a smaller 
cross-sectional area than the other vertical conduits. It should have an 
upper open end which is located at a point above the expected maximum 
normal level of the denser liquid phase on the liquid support tray but 
below the second tray above the separation chamber. The upper end of this 
conduit is therefore preferably above the horizontal edge of the vertical 
wall on the inlet side of the tray. 
Those skilled in the art will recognize that the embodiments of the 
invention shown in the drawing and heretofore described are subject to 
variation in several ways. For instance, the first and second vertical 
conduits may have a different shape, such as rectangular or chordal, and 
the liquid separation chamber may have a square or rectangular 
cross-section. The third vertical conduit utilized to allow the escape of 
the separated lighter material may extend to a higher point than shown and 
may be located near the outlet or second end of the liquid separation 
chamber. All three vertical conduits preferably have a vertical central 
axis, but they may be inclined or slightly curved if desired. Although the 
use of simple perforations to form a sieve-type contacting tray is 
preferred, other structures designed to promote or increase the 
effectiveness of the contacting on the surface of the tray may be utilized 
instead. These include various adjustable or self-adjusting variable 
opening devices and bubble-type contactors. The structure of the two 
vertical walls 4 and 5 may also differ from that shown in the Drawing. For 
instance, they may be perforated or have vanes attached to them to more 
evenly distribute the flow of the denser liquid across the upper surface 
of the tray. It is also possible to delete these two vertical walls 
entirely. However, it is preferred that they be present on the tray in 
approximately the same form as is shown in the Drawing. 
The preferred embodiment of the invention may be characterized as a 
multi-stage liquid-liquid extraction column having a plurality of liquid 
downcomers operably connected to the lower surface of horizontal liquid 
support trays which extend across the internal volume of the column at 
vertically spaced intervals, with the individual downcomers comprising a 
first vertical conduit having a first cross-sectional area and connected 
to an opening through a liquid support tray located in a first lateral 
half of the extraction column, the first vertical conduit extending 
downward and terminating at a lower end located below the tray; a 
cylindrical liquid separation chamber having a first and a second end, an 
internal volume, a horizontal major axis and a cross-sectional area at 
least five times as large as the cross-sectional area of the first 
vertical conduit, the liquid separation chamber extending across the 
internal volume of the extraction column with the lower end of the first 
vertical conduit being connected to the first end of the liquid separation 
chamber, and with the first end of the liquid separation chamber being 
located in the first lateral half of the extraction column and the second 
end of the liquid separation chamber being located in the second lateral 
half of the extraction column; a second vertical conduit having an upper 
end connected to the second end of said liquid separation chamber and an 
open lower end located at a point in the second lateral half of the 
extraction column and above a vertically contiguous lower second tray, the 
second vertical conduit having a vertical major axis and a cross-sectional 
area which is smaller than the cross-sectional area of the liquid 
separation chamber; and a third vertical conduit communicating with the 
internal volume of the liquid separation chamber through an opening in an 
upper surface of the liquid separation chamber and extending upward 
through the first liquid support tray to an upper open end located at a 
point above this tray. 
One of the more widely used extraction processes to which the present 
invention may be applied is the separation of aromatic hydrocarbons and 
non-aromatic hydrocarbons such as naphthenes and paraffins. This may be 
for the purpose of obtaining relatively pure portions of either class of 
hydrocarbon,. This operation is often found in conjunction with 
fractionation and extractive distillation step. An example of this is the 
process described in U.S. Pat. No. 3,844,902. The feed stream will 
preferably have a limited boiling point range which limits the 
hydrocarbons to those having from about 6 to 20 carbon atoms per molecule 
and more preferably from about 6 to 12 carbon atoms. Suitable feed streams 
include a debutanized reactor effluent from a catalytic reforming unit and 
a liquid by-product from a pyrolysis gasoline unit which has been 
hydrotreated for the saturation of olefins and diolefins. 
At the heart of the extraction process, is the use of a solvent which is 
selective for the preselected chemical compound which is to be removed 
from the feed stream. Besides having this property of selectivity, solvent 
material must be substantialy immiscible with the feed stream and also 
differ in density. There are available and known to those skilled in the 
art a wide variety of materials which meet these general requirements. For 
instance, aromatic hydrocarbons may be extracted with diglycol amine, 
diethylene glycol, dipropylene glycol, tetraethylene glycol or n-formyl 
morphaline, etc. These chemicals are usually mixed with water to form the 
actual solvent solution. More detailed information on these solvents is 
contained in the articles appearing at page 91 of the March, 1973 edition 
of Hydrocarbon Processing and at page 141 of the April, 1972 edition. 
A specifically preferred solvent for the separation of aromatics and 
non-aromatics is one of the sulfolane-type as described in U.S. Pat. No. 
3,652,452. 
A sulfolane-type solvent may be characterized as having a five-membered 
ring structure containing one sulfur atom and four carbon atoms with two 
oxygen atoms bonded to the sulfur atom. Preferably, two hydrogen atoms are 
bonded to each carbon atom. A specific example of a sulfolane-type solvent 
is tetrahydrothiophene 1,1,dioxide. It is specifically preferred that the 
solvent contains about 0.5 to 5.0 wt.% water. The closely related solvents 
2-sulfolene and 3-sulfolene may also be used. Yet another family of 
suitable compounds are the sultones described in U.S. Pat. No. 3,723,303. 
The feed streams which are contacted with these solvents may contain 
aromatic and non-aromatic hydrocarbons having from six to nine or more 
carbon atoms per molecule. 
Adequate extraction is obtained through the use of multi-stage 
countercurrent contacting performed at suitable extraction conditions. 
When utilizing a sulfolane-type solvent, these conditions include a 
pressure from atmospheric to about 500 psig., preferably 50 to 150 psig., 
and a temperature of from about 25.degree. C. to about 200.degree. C., 
preferably about 80.degree. C. to about 150.degree. C. These conditions 
are often set by very practical considerations. For instance, the pressure 
must be sufficient to prevent either liquid phase from vaporizing and is 
often determined by an upstream or downstream unit on which the pressure 
in the extraction zone is allowed to "float". Elevated temperatures 
normally increase the extraction capacity of the solvent but decrease the 
selectivity such that these effects must be balanced. Solvent quantities 
should be sufficient to dissolve substantially all the aromatic 
hydrocarbons present in the extraction zone feed stream. Preferred are 
solvent to feed ratios, by volume, of about 2:1 to about 10:1. These 
factors are well developed in the art and are dependent on particular 
situations. 
The process embodiment of the invention may be characterized as a 
liquid-liquid extraction process which comprises the steps of passing a 
feed stream comprising normally liquid hydrocarbons into the bottom of a 
vertical trayed extraction column operated at extraction conditions and 
upward through the extraction column, with the feed stream passing upward 
through a plurality of passageways distributed across the surface of the 
trays located within the extraction column; passing a solvent stream into 
an upper portion of the extraction column and downward through the 
extraction column, with the solvent stream flowing across the upper 
surface of trays located within the extraction column and being directed 
downward from tray to tray through a plurality of downcomers, including a 
first downcomer, as a downcomer liquid stream and effecting the contacting 
of the solvent stream and the feed stream; passing the downcomer liquid 
stream flowing through the first downcomer through a quiescent 
substantially horizontal enclosed liquid phase separation zone which forms 
a portion of the first downcomer and is located within the extraction 
column, effecting the phase separation of entrained liquid hydrocarbons 
which enter the extraction column as part of the feed stream from the 
downcomer liquid stream within the liquid phase separation zone, venting 
the thus-separated liquid hydrocarbons upward through the tray located 
immediately above the first downcomer by passage through a vertical 
conduit extending through said tray immediately above the first downcomer, 
and thereby forming a purified downcomer stream which is discharged 
downward from the liquid-phase separation zone to a tray located 
immediately below the first downcomer; withdrawing a reaffinate stream 
from the top of the extraction column; and withdrawing an extract stream 
from the bottom of the column. 
The raffinate stream comprises the unextracted or remaining portion of the 
feed stream, and the extract stream comprises the solvent stream plus the 
extracted portion of the feed stream. The raffinate stream may have a 
small amount of the solvent dissolved in it. In some processes, it is 
customary to refer to the extract stream as the rich solvent stream. 
The subject invention may also be used for the extraction of mercaptans 
from a hydrocarbon feed stream with an alkaline solution as is widely 
practiced in the petroleum industry. This alkaline solution is then 
effectively regenerated by the catalytically promoted oxidation of the 
extractive mercaptans to disulfides which are separated by decantation. 
The process may be performed with any alkaline reagent which is capable of 
extracting mercaptans from the feed stream at practical operating 
conditions and which may be regenerated in the manner described. A 
preferred reagent comprises an aqueous solution of an alkaline metal 
hydroxide, such as sodium hydroxide or potassium hydroxide. Sodium 
hydroxide may be used in concentrations of from 1-50 wt.%, with a 
preferred concentration range being from about 5 to about 25 wt.%. 
Optionally, there may be added an agent to increase the solubility of 
mercaptans in the solution, typically methanol or ethanol, although others 
such as a phenol, cresol or butyric acid may be used. 
Hydrocarbons which may be treated for mercaptan removal in this manner vary 
from propane-butane mixtures to the middle distillates. Included in this 
grouping of feed streams are streams derived from fluidized catalytic 
cracking plant gas concentration units, natural or cracked gasolines, jet 
fuels, fuel oils and kerosenes and blends of these. This process may also 
be used to remove mercaptans from many solvents, alcohols, aldehydes, etc. 
With the exception of some light C.sub.3 or C.sub.4 compounds, these 
materials may be classified as being normally liquid hydrocarbonaceous 
compounds having boiling points under about 345.degree. C. as determined 
by the standard ASTM distillation methods. As used herein, the term 
"normally liquid" is intended to specify a substance which is a liquid at 
standard conditions (60.degree. F. and 1 atm absolute). The extraction 
conditions employed for removing mercaptans may vary greatly depending on 
such factors as the nature of the hydrocarbon stream being treated and its 
mercaptan content. In general, the extraction may be performed at an 
ambient temperature and a pressure sufficient to insure liquid state 
operation. The pressure may range up to 68 atm gauge or more, but a 
pressure in the range from about 3.5 atm gauge to about 10.0 atm gauge is 
preferred. The temperture in the extraction zone is confined in the range 
of 16.degree. C. to about 121.degree. C., preferably from 25.degree. C. to 
50.degree. C. The ratio of volume of the alkaline solution required per 
volume of the feed stream will vary depending on the mercaptan content of 
the feed stream. The flow rate of the alkaline solution may be from about 
1 to about 100% of the flow rate of the hydrocarbon stream. Normally, the 
rate will equal about 2 to 3% of an LPG stream and up to about 20% of a 
C.sub.5 or light straight run gasoline. Optimum extraction in this liquid 
system is obtained with a velocity through sieve-type perforations of from 
about 5 to about 10 ft/sec. Further details on this process may be 
obtained in the previously referred to U.S. Pat. Nos. 2,921,020; 
2,988,500; 3,108,081; 3,260,665; 3,923,645 and 4,040,947. 
The invention may also be applied in a process for removing acid gases, 
such as hydrogen sulfide, from liquid phase hydrocarbon streams. This well 
developed process is widely used in petroleum refining, with aqueous amine 
solutions being the preferred solvents. Diglycolamine at concentrations 
ranging from about 50 to 70 wt.% or monoethanolamine at concentrations 
ranging from about 10 to about 30 wt.% may be used. It is conventional to 
limit the concentration of H.sub.2 S in the H.sub.2 S-rich 
monoethanolamine to less than about 0.35 to 0.4 moles of H.sub.2 S per 
mole of MEA. A positive pressure sufficient to maintain liquid phase 
conditions and preferably above 1 atm gauge is maintained in the 
extraction column. The use of an average temperature below 38.degree. C. 
is preferred during the extraction process, but the temperature may range 
from about 16.degree. C. to about 65.degree. C. The rich solvent is 
regenerated in a stripping column at an elevated temperature in a 
customary manner, with a temperature in the range of 115.degree. C. to 
150.degree. C. normally being sufficient.