US3518166A

US3518166A - Hydrogen sulfide and ammonia recovery by degassing and plural distillation with ammonia recycle

Info

Publication number: US3518166A
Application number: US762701A
Authority: US
Inventors: Robert J Klett
Original assignee: Chevron Research and Technology Co
Current assignee: Chevron USA Inc
Priority date: 1968-09-26
Filing date: 1968-09-26
Publication date: 1970-06-30
Anticipated expiration: 1987-06-30
Also published as: DE1948427B2; DE1948427A1

Description

June 30, 1970 R. J. KLETT 3,518,166

HYDROGEN SULFIDE AND AMMONIA RECOVERY BY DEGASSING AND PLURAL DISTILLATION WITH AMMONIA RECYCLE Filed Sept. 26, 1968 HIGH .3 PRESSURE NH3 DEGASSER 22 24\ lo a Hg$ 23 LOW 15 7 PRESSURE 2/ DEGASSER 20 Z5 12 m l 71 35 w 27 l3 0. g

(I II la 0) FEED SURGE E" TANK i z INVENTOR ROBERT J. KLETT ATTORNEYS United States Patent US. Cl. 203-78 6 Claims ABSTRACT OF THE DISCLOSURE Operation of a process to recover H 8 and NH from an aqueous solution of H 8, NH and light hydrocarbons under superatmospheric pressure, wherein the H 8 and NH are recovered separately from an H 8 Stripper and an NH stripper, respectively, is improved from a control and stability standpoint, and aqueous feed solutions of high H S content are more advantageously handled by (l) combining an NH -rich, H S-lean condensate stream generated in the overhead system of the NH stripper with the aqueous solution of NH H 5, and light hydrocarbons; then (2) removing light hydrocarbons as gases by reducing the pressure on the combined solution; (3) providing residence time for the combined solution; and then (4) feeding the combined aqueous feed stream to the H 8 stripper.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to methods for recovering H 8 and NH from aqueous streams containing the same. More particularly, this invention relates to processes for separately recovering H 8 and NH from aqueous streams.

Description of the prior art containing nitrogen compounds and sulfur compounds and recycle hydrogen-rich gas and makeup hydrogen are passed through a reaction zone, usually containing a catalyst, at elevated temperature and pressure at which at least a portion of the hydrocarbons are vaporized; and

there is obtained as a reaction zone effluent a mixture of vaporized hydrocarbons, hydrogen, H 5 and NH:. The efiluent may also contain heavier hydrocarbons which are liquid at the reaction conditions. The reaction effluent is cooled to condense vaporized hydrocarbons, whereby the liquid hydrocarbons can be separated from hydrogenrich recycle gas, which is then reused in the process.

When the reaction effiuent contains both H 8 and NH it has been found that on cooling to temperatures below about 300 F. the H 8 and NH may react to form salts which sometimes cause clogging problems in the heat exchangers and the lines. Injection of water into the reaction efiiuent upstream of the heat exchangers has been used to wash out such deposits and/or to prevent their forming. This water injection can provide a means of removing much of the NH formed, if rather large amounts of water are injected sufficient to dissolve the NH 3,518,166 Patented June 30, 1970 In a typical hydroconversion process, such as hydrotreating or hydrocracking, there are considerable amounts of light hydrocarbons and hydrogen present in the reactron efiiuent. Upon washing H S and NH out of the rethe Water phase, particularly if the Washing is at a high pressure. In many hydroconversion processes, the Washing is done at a pressure of 500 to 5,000 p.s.i.g., more well as a small portion of the hydrogen, will dissolve in action efiluent, a portion of these light hydrocarbons, as typically 1,000 to 3,000 p.s.i.g. For example, in hydrotreating gas oil, the effiuent from the hydrotreater typically is cooled by heat exchange to a temperature between about and F. at a pressure of 2,000 p.s.i.g. Be cause the water is mixed with the gaseous effluent from the hydrotreater reactor at high pressure and before the effluent is cooled to 100 to 150 F., significant amounts of light hydrocarbons, such as methane, ethane, propane, butane, etc., will dissolve in the water.

These light hydrocarbons, as Well as dissolved hydrogen, may be removed from the aqueous phase prior to treatment to remove H 8 and NH, by reducing the pressure on the aqueous solution. It is desirable to retain H 8 in the aqueous solution when the pressure is reduced because (1) the H 8 is an impurity in the light hydrocarbon vapors; and (2) the H 8 is preferably recovered in a single concentrated stream as a valuable byproduct in a subsequent H 8 and NH recovery process. However, in many instances, the content of H 3 in the aqueous solution which is formede.g., in the overhead system of strippers or fractionators associated with hydroconversion processes-is too high for the H 5 to be substantially completely retained in the solution when the pressure is reduced.

Another problem that exists according to previous methods to recover H 8 and NH separately from foul Water containing the same resides largely in obtaining steady and stable control of the strippers used to recover H 5 and NH respectively. In the process described in US. Pat. 3,335,071 issued to W. M. Bollen et a1. and assigned to Chevron Research Company, an aqueous solution of H 8 and NH may be treated according to the following steps:

(a) Stripping the NH H 8 aqueous solution in a first stripping distillation column to obtain as an overhead an H S-rich vapor and as bottoms an NH -rich aqueous bottoms solution;

(b) Stripping the NH -rich aqueous bottoms in a second distillation column to obtain an NH -rich vapor overhead;

(c) Partially condensing the NH' -rich vapor overhead to obtain an NH rich vapor and an NHgI'lCh aqueous overhead condensate; and

(d) Recycling a portion of the NH -rich aqueuous overhead condensate to the first distillation column (the H 8 stripper).

The disclosure of US. Pat. 3,335,071 is incorporated by reference into the present specification.

Recycling the Nl-l -rich overhead condensate to the H S stripper is highly desirable for some purposes, but fluctuations in the NH content in this stream tend to cause periodic upsets in the H 8 stripper operation. This is thought to be partly due to the considerable amounts of NH contained in the recycle relative to the amount of NH in the feed to the H 8 stripper. Although it is not completely understood why, in some instances when the NH content of the NH stripper overhead condensate increases, a large bubble of NH would travel up the H 8 stripper reducing the purity of the H 8 overhead and requiring adjustment in the overhead control in the H 8 stripper. Also, increases in NH content of the recycle NH stripper overhead condensate result in higher pres- 3 sures being required at the bottom of the H 8 stripper when operating at a given temperature. Because these changes would occur relatively rapidly, steady and stable control of the H 8 stripper and NH stripper was not always attainable.

In summary, according to practices employed under the prior art, considerable amounts of H S may be lost from the aqueous solution desired to be treated for separate recovery of H 8 and NH when the pressure is reduced on the aqueous solution so as to remove light hydrocarbons and/or hydrogen dissolved in the aqueous solution of H 8 and NH Also, according to previously employed methods for separately recovering H 5 and NH difficulties are incurred in attempting to obtain steady and stable operation of the H 5 and NH strippers. In the process of the present invention, these problems are substantially overcome.

SUMMARY OF THE INVENTION According to the present invention, in a process for separately recovering H 8 and NH from a first aqueous solution comprised of H 8, NH and light hydrocarbons under superatmospheric pressure according to the steps:

(a) Degassing the first aqueous solution by reducing the pressure, thereby removing light hydrocarbons and obtaining a second aqueous solution comprised of H 8 and 3;

(b) Stripping H S out of the second aqueous solution in a first distillation column to obtain an H S-rich overhead stream and an aqueous bottoms stream of reduced H 8 content;

(c) Stripping NH out of the aqueous bottoms stream in a second distillation column to obtain an NH -rich vapor overhead; and

(d) Partially condensing the NH -rich vapor overhead to obtain an NH -rich vapor and an NH -rich overhead condensate.

The improvement is made which comprises:

(e) Combining at least a portion of the NH -rich overhead condensate with the first aqueous solution to obtain a combined aqueous solution; then (f) Removing at least a portion of the hydrocarbons contained in the first aqueous solution in a degassing zone as vapors by reducing the pressure on the combined aqueous solution to obtain said second aqueous solution, and then (g) Passing the second aqueous solution to the first distillation column.

Operating in this manner avoids direct recycle of the NI-I stripper overhead condensate to the H28 stripper; and improved control and stability of the H 5 stripper is obtained. In addition, the recycle of the NH -rich overhead condensate from the NH stripper to the degassing zone serves to retain H 8 in the aqueous phase while degassing light hydrocarbons and/or hydrogen from the aqueous feed containing the H S-NH The improved stability of the H 8 stripper is thought to be largely a result of avoiding direct recycle of NH rich condensate. According to the present invention, the NH -rich condensate is recycled to the degassing zone where it achieves or closely approaches equilibrium with the net feed to the H 8 stripper; and fluctuations in the feed composition are dampened out due to the extra residence time. A residence time of at least five minutes after combining the aqueous streams and prior to introduction to the H 5 stripper is desirable. Much more preferable, a residence time of about one to three hours is provided for the combined gross feed streams of recycle NH -rich condensate from the NH stripper and net H 8- NH aqueous feed solution. Still more preferable, a residence time of between about 3 and 24 hours or longer is provided for the combined, that is gross, feed streams. Provision for residence time for the recycle NH -rich condensate prior to introduction to the H 8 stripper is in contrast to previous operation wherein the recycle of NH -rich condensate directly interconnected the H28 stripper and the NH, stripper so that upsets in either of these strippers could often affect the other stripper.

Also, it has been found that, if a residence time of about 24 hours or longer is used, then oil may be essentially completely separated from the foul water feed streams so that the strippers are kept cleaner. Additionally, when the feed streams include foul water from fluid catalytic cracking units or the like, the residence time of about 24 hours or longer will allow hydrocyanide acids which are likely to be present in such foul water streams to be converted to thiocyanate. Conversion of the hydrocyanide to thiocyanate helps minimize corrosion problems in the strippers.

It has been found that with the recycle of the NH overhead condensate large amounts of H 8 in the aqueous feed streams to the present process may be dealt with without excessive losses of H S in the light hydrocarbons and/ or hydrogen from the degassing step. Under most conditions, the H 8 content of the gases from the degassing step is very low. The degasser off-gases may be used as refinery fuel gas as only a very small amount of S0 will be produced by burning the gases. Thus air pollution is substantially reduced.

A number of streams containing H 8 and/ or NH;; may be treated in the process of the present invention, but it is preferable that there be a vessel or some means to provide residence time and allow mixing of the NH rich condensate and the net feed to the present process. For example, there may be provided simply a surge vessel with no removal of light hydrocarbons or a degasser to remove light hydrocarbons from at least one of the feed streams to the present process. In the more usual case, it is necessary to provide a degassing step which in many instances results in loss of H 8 and/or extra expense to remove H S from the light hydrocarbons when not using the process of the present invention. As indicated above, the H 8 contents of the net feed streams may be relatively high in the present process but still dealt with without large H 8 losses or H 8 impurities in the degasser Zone off-gases. The recycle of the NH stripper overhead condensate to the degasser zone serves to retain H 8 in the aqueous phase.

In the process of the present invention, it has been found that it is advantageous to use two stages of degassing. Use of two stages of degassing serves to further minimize the H 8 losses in the light hydrocarbon streams which are removed from the aqueous solution when the pressure is reduced on the aqueous solution in the degassing zone. The first stage of degassing is a high pressure degassing stage wherein the pressure is maintained between 50 and 500 p.s.i.g. Preferably the pressure is maintained at about 70 to 200 p.s.i.g. The liquid phase aqueous solution from the high pressure degassing stage is then passed to a low pressure degassing stage.

Aqueous streams containing H 5 and/or NH together with small amounts of light hydrocarbons, which light hydrocarbons are dissolved in the aqueous solution due to relatively low pressures, for example, 10 p.s.i.g. to p.s.i.g., are advantageously introduced to the process of the present invention by combining such streams with the aqueous solution fed to the low pressure degasser. The low pressure degasser is maintained at a pressure between 0 p.s.i.g. and 50 p.s.i.g., preferably between 1 p.s.i.g. and 10 p.s.i.g

BRIEF DESCRIPTION OF THE DRAWING The drawing schematically illustrates a preferred embodiment of the present process for separately recovering H S and NH wherein two degassing stages are used.

DETAILED DESCRIPTION OF THE DRAWING AND THE INVENTION Referring now in more detail to the drawing, an aqueous feed stream containing H S and NH is introduced to the process via line 1. In a preferred embodiment of the present invention, this aqueous feed stream is obtained by commingling or contacting the efliuent from a hydrocracking reactor at a pressure of about 2,000 p.s.i.g. with water. As indicated in the discussion under Background of the Invention, this contacting is carried out to remove ammonia and H S from the hydrocracker reactor efiluent. Because the hydrocracker reactor eifluent contains substantial amounts of hydrogen and light hydrocarbons, the aqueous solution which is formed is comprised of hydrogen and light hydrocarbons in addition to H S and NH In the process of the present invention, this aqueous solution is combined with the recycle NH -rich aqueous stream obtained from the overhead of the NH stripper, as will be described in more detail hereinbelow. The NH -rich aqueous solution is recycled via line 26.

A stream rich in H S obtained from the overhead of one of the stripping distillation columns used to remove light hydrocarbons from the hydrocracker eflluent product is introduced to the process via line 2. Among the many H S-rich streams that may be treated in the process of the present invention are those streams derived from steam stripping liquid hydrocarbon etiluents from hydrotreating or hydrofining processes. These liquid hydrocarbon eflluents contain H 8 and light hydrocarbons which are removed by stripping or distillation. The stripper or distillation column overhead vapors which result from the stripping operation contain appreciable amounts of H 8 which dissolve to a significant extent in the water formed when the overhead is partially condensed. Frequently the stripping is carried out at low pressures, for example, 5 to 50 p.s.i.g., in the overhead accumulator. In this instance, these overhead condensate streams may be introduced to the process of the present invention via line 7. It is particularly important to remove light hydrocarbons from the overhead condensate streams when the H 8, which is removed from the H 8 stripper via line 15, is desired in a highly purified form. For example, when the H 8 is to be used as feed to a Claus process for manufacture of sulfur, it is desirable that the H 8 stream contain less than 0.1 volume percent hydrocarbons.

In some instances, the hydrocarbon effluent from the hydrotreating or hydrocracking process will be stripped or fractionated to remove H 8 and light hydrocarbons at a pressure above 50 p.s.i.g. For example, in US. Pat. 3,356,608, a process is described wherein gas oil and hydrogen are contacted with a sulfactive hydrogenation catalyst and the eflluent hydrocarbon stream, after separation of recycle hydrogen, is steam stripped at pressures above 150 p.s.i.g. Upon condensing the overhead from the stripper, an aqueous phase is formed which may be very rich in H S compared to aqueous solutions formed in the presence of H 8 at lower H 8 partial pressures.

Referring again to the drawing, the combined

streams

1, 2 and 26 are introduced via line 3 to the high pressure degasser 4. In order to achieve low H 8 contents in the off-gases, the high pressure degasser is preferably maintained at a pressure of about 185 p.s.i.g. and a temperature of about 80 F. Lower pressures and higher temperatures will result in increased H 8 contents in the off-gas. Light hydrocarbons and hydrogen are removed via line 5 from the top of the high pressure degassing vessel. When operating at about 100 to 200 p.s.i.g. and 80 to 100 F., the H 8 content of stream 5 is generally less than 3 volume percent. When operating at high pressure and low temperature for the high pressure degasser in accordance with the present invention, the H 8 content may be maintained between about 0.1 to 2.0 volume percent. Thus stream 5 has a very low H 8 content and is generally suitable as refinery fuel gas. The partially degassed aqueous solution is withdrawn from the bottom of the high pressure degasser via line 6.

An aqueous solution of H 8, NH and small amounts of dissolved hydrocarbons, which aqueous solution is obtained as overhead condensate from a hydrocarbon stripper operating at an overhead pressure of about 50 p.s.i.g., is introduced via line 7. The combined aqueous streams in lines 6 and 7 are passed via line 8 to low pressure degasser 9. The low pressure degasser is preferably maintained at a pressure of about 2 p.s.i.g. Light hydrocarbons are withdrawn in line 10 from the low pressure degasser, and an aqueous solution of H 8 and NH is withdrawn from the bottom of the degasser via line '11. The H 8 content of stream 10 is generally less than about 4 volume percent when operating in accordance with the present invention. The percent of H 8 in the low pressure degasser off-gases may be reduced further, for example, to the range for the high pressure degasser, by increasing the amount of NH -rich condensate and by lowering the temperature and raising the pressure. In most instances, the major portion of the off-gases, which are mostly hydrogen and methane, are released in the high pressure degasser. Usually about to volume percent of the dissolved gases flash off in the high pressure degasser. Therefore, there is only a relatively small quantity of H 8 carried off with the off-gases from the low pressure degasser. Thus, in accordance with the present invention, nearly all the H 8 is left in the aqueous phase so that it may be recovered as one overhead stream from the H 8 stripper.

The aqueous solution from the low pressure degasser is introduced to feed surge tank 12 wherein a residence time preferably between 3 and 24 hours is provided. The feed surge tank 12 should be a floating roof tank or inert gas blanketed. If air is allowed to come in contact with the aqueous solution, hydrogen sulfide will be oxiodized to form free sulfur.

Aqueous solution is withdrawn from the feed surge tank via line 13 and introduced to H S stripper 14. Due to heat input in the bottom of the H 8 stripper, hot upflowing vapors are generated which serve to strip H S out of the aqueous solution. A cool stream of water is introduced via line 16 to the upper part of the H 5 stripper so as to generate a downward flowing aqueous stream which serves to fractionate the NH from the H 8. A relatively pure stream of H 8 is withdrawn via line 15 from the top of the H 5 stripper. The NH content in this H 8 stream is usually less than 2 Weight percent, commonly as low as a few tenths of a percent; and preferably the H 8 stripper conditions are maintained so as to result in an N'H content of less than l00 p.p.m., for example, 10 to 30 p.p.m.

An aqueous solution which is rich in NH but still containing substantial amounts of H 8 is withdrawn via line 17 from the bottom of the H 8 stripper. This stream is fed to NH;; stripper 18. Hot upflowing vapors are generated by heat input into the bottom of the NH;, stripper. These hot upflowing vapors serve to strip NH and H 8 out of the aqueous solution fed to the NH stripper. A purified Water stream is withdrawn from the bottom of the NH stripper via line 19.

A vapor stream comprised of H 0, NH and H 8 is withdrawn from the top of the NH, stripper via line 20. The operating conditions in the NH stripper are controlled so as to regulate the amount of H 0 in the vapor stream in line 20 so that, when this vapor stream is partially condensed in condenser 21, the liquid condensate formed and withdrawn from the condenser in line 22 will contain a very large fraction of the H 8 present in the overhead system. Thus the NH withdrawn as a vapor in line 24 is substantially freed of H S. The H 5 content in the NH vapor is generally less than about one to two percent and may be reduced to as low as 0.1 to 0.5 percent H 8 by volume. The condensate which is formed by cooling the NH stripper overhead vapors in condenser 21 is rich in NH Typically, the ratio of =NI-I to 'H S will be between 10:1 and 2:1 in the condensate withdrawn from reflux drum 23 via line 25. Preferably, the ratio of NH to H 3 is between 321 to 6:1 on a molar basis. A portion of this NH -rich condensate is refluxed to the NH; stripper via line 27 to provide cool, downward flowing liquid in the upper part of the NH stripper. According to the process of the present invention, a second portion of the NH -rich condensate is recycled via line 26 to be combined with the net feed introduced to the process via

lines

1 and 2.

In the present invention it is preferred to control the amount of NH -rich condensate which is recycled, as well as the ratio of NH to H 5 in the recycle, so that the ratio of NH to H S of the combined streams fed to the high pressure degasser is at least 1.1:1.0 on a molar basis. For streams which contain more than a percent or two of dissolved NH and H 8 it is preferable to use more recycle NH -rich condensate, so that the ratio of NH to H S (calculated as separate species) is at least 1.2 to 1.0, and in many instances it is preferred to have as much as one and one-half to about five times as much NH as H28.

EXAMPLE This example illustrates the advantages obtained using the process of the present invention for treating aqueous streams which contain large amounts of H 8 relative to NH in addition to light hydrocarbons and/or hydrogen dissolved in the aqueous stream due to high pressure.

A solution comprised of about 984 pounds of H 8, 516 pounds of NH and about 760 standard cubic feet of hydrogen plus light hydrocarbons dissolved in 27,820 pounds of H is obtained by water washing a hydrocracker reactor eflluent. The hydrocracker reactor eflluent stream which is water washed is at about 250 F. and 1,250 p.s.i.g. The aqueous stream thus obtained is introduced via line 1 to the process schematically illustrated by the drawing, except that there is no recycle NH -rich condensate to the high pressure degasser. Part of the NH rich overhead from the -NH stripper is, however, recycled I directly to the H 8 stripper.

An aqueous solution comprised of 28,000 pounds of H 0, 436 pounds of H 3 and only trace amounts of NH is obtained as overhead condensate from a fractionating column in the fractionation section of the hydrocracking unit. This stream is introduced via line 2 to the process. In this first instance, the NH -rich condensate from the overhead of the NH stripper is recycled directly to the H 5 stripper. The above streams and the resulting streams corresponding to the numbered streams shown in the drawing are summarized in the table below:

TABLE I Stream No HgO,1b./h1. HgS, lb./l1r. N11 lb./hr.

The combined streams 5 and 10 contain about 760 s.c.f. of hydrogen plus light hydrocarbons.

Using the process of the present invention,

identical streams

1 and 2 are fed to the process schematically illustrated in the drawing. In addition, a portion of the condensed overhead from the NH stripper 18 is recycled via line 26 to the high pressure degasser. This recycle stream is rich in NH, relative to H 8. The moles NH in the recycle stream are 26.2; the moles H S total 8.7, thus resulting in a molar ratio of NH to H 5 of about 3.01. There is suflicient NH in the NH recycle via line 26 so that the re sulting combined streams in line 3 have a ratio of NH to H 8 on a molar basis greater than 1.0. Preferably, the ratio of NH to H 5 in stream 3 is maintained above 1.1.

8 I In this example, the ratio of NH to H 5 is 1.22. The feed streams and resulting streams are summarized below:

The combined streams 5 and 10 contain about 7 60 s.c.f. of hydrogen plus light hydrocarbon gases.

As can be seen from the comparison of streams 5 and 10 in Table 'II to streams 5 and 10 in Table I, the H 8 losses are drastically reduced. Using the process of the present invention, the H S losses are reduced from 400 pounds per hour (Table I) to 4 pounds per hour (Table II). Thus the process of the present invention results in reducing the H S present in the flashed 0ft gases by a factor of 100 in this example. Thus recycling NH -rich overhead condensate from the NH stripper is of particular advantage when feeding streams which have relatively large amounts of H 8 compared to NH When the H 5 concentrations in the net feed streams are lower, the advantage is correspondingly reduced, but generally the H 8 loss is reduced by a factor of at least five. Also, the recycle of the NH -rich condensate will still serve to some degree to help retain H S in the aqueous phase so that it may be recovered via line 15 as a unitary product stream. Furthermore, the recycling of NH -rich condensate to the degassing section instead of directly to the H 5 stripper has the very important advantage of improving control stability of the H 8 stripper and the NH stripper.

Although various specific embodiments of the invention have been described and shown, it is to be understood they are meant to be illustrative only and not limiting. Certain features may be changed without departing from the spirit or essence of the invention. It is apparent that the invention has broad application to the recovery of separate streams of H 8 and NH from aqueous solutions of the same. Accordingly, the invention is not to be construed as limited to the specific embodiments illustrated but only as defined in the following claims.

I claim:

1. A process for separately recovering an H S-rich stream and an NH -rich stream from a first aqueous solution comprising H O, H S, NH and dissolved methane under superatmospheric pressure according to the steps:

(a) combining at least a portion of an NH -rich overhead condensate with the first aqueous solution to obtain a combined aqueous solution having a molar ratio of NH to H 8 of at least 1.1 moles NH to 1.0 moles H 8;

(b) removing from the combined aqueous solution at least volume percent of the dissolved methane as a gas by reducing the pressure on the combined aqueous solution to a pressure at least below 200 p.s.i.g. to thereby flash off methane;

(c) providing at least one hour residence time for the combined aqueous solution; and then (d) passing the combined aqueous solution to a first distillation column;

(e) stripping H 5 out of the second aqueous solution in the first distillation column to obtain an H S-rich overhead stream and an aqueous bottoms stream of reduced H S content;

(f) stripping NH out of the aqueous bottoms stream of reduced H S content in a second distillation column to obtain an NH -rich vapor overhead; and

(g) partially condensing the NH -rich vapor overhead to obtain an NH -rich vapor stream and said NH rich overhead condensate.

2. A process according to claim 1, wherein between about 3 and 24 hours residence time is provided for the combined aqueous solution before passing the combined aqueous solution to the first distillation column.

3. A process according to claim 1, wherein at least 24 hours of residence time is provided for the combined aqueous solution before passing the combined aqueous solution to the first distillation column.

4. A process according to claim 1, wherein the combined aqueous solution is degassed first in a high pressure degasser at a pressure between 50 and 500 p.s.i.g. and then is degassed in a low pressure degasser at a pressure between and 50 p.s.i.g.

5. A process according to claim 4, wherein the combined aqueous solution is degassed first in a high pressure degasser at a pressure of between 70 and 200 p.s.i.g. and then is degassed in a low pressure degasser at a pressure between 1 and 10 p.s.i.g.

'6. A process according to claim 4, wherein the gross feed to the high pressure degasser is comprised of:

(a) an aqueous solution of H 5, NH light hydrocarbons, and hydrogen (stream a) obtained by contacing with water the eifiuent from a hydroconversion proces at a pressure of at least 500 p'.s.i.g.,

(b) an aqueous solution comprised of H 5 and light hydrocarbons (stream b), wherein the H S content is such that the combined streams (a) and (b) have an NH to H S molar ratio less than 1.1210, and (c) the NH -rich condensate stream (stream c) from the overhead of the NH stripper, and wherein the amound of NH in the NH -rich condensate recycled from the overhead of the NH;,, stripper is sufiicient so that the combined streams (a), (b) and (c) have an NH to H 8 molar ratio of at least 1.1:1.0.

References Cited UNITED STATES PATENTS WILBUR L. BASCOMB, 111., Primary Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,518,166 June 30, 1970 Robert J. Klett It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, lines 5 thru 9 should be deleted and the following inserted instead therefor action effluent, a portion of these light hydrocarbons, as well as a small portion of the hydrogen, will dissolve in the water phase, particularly if the washing is at a high pressure. In many hydroconversion processes, the washing is done at a pressure of 500 to 5,000 psig, more Signed and sealed this 20th day of October 1970.

'(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

1 Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents