This invention relates to a process for removing carbonyl sulfide from hydrocarbon fluids, and to a specific reagent therefor. More specifically, this invention relates to the treatment of hydrocarbons including the so-called normally gaseous hydrocarbons from any source for the selective removal of carbonyl sulfide associated with said hydrocarbons.
Hydrocarbon fluids such as those obtained from crude petroleum oils and other sources usually contain varying amounts of deleterious sulfur compounds as impurities. The kinds and amounts of sulfur compounds occurring in any hydrocarbon fluid vary with the source material and with the method of manufacturing and processing said fluid. For example, thermal cracking operations have a tendency to convert hydrogen sulfide and open-chain sulfur compounds into cyclic compounds and to cause the combination of hydrogen sulfide with carbon compounds to form organic sulfur compounds including carbon sulfides.
Many of the sulfur compounds present in hydrocarbon fluids are detrimental to the processing or marketing of said fluids or of products derivable therefrom. Thus, there are conventional methods for removing hydrogen sulfide from hydrocarbon fluids and for converting mercaptans to less obnoxious form. Further, there are means known to the art for extracting mercaptans as such. However, carbonyl sulfide, a sulfur compound occurring in the lower-boiling products from the thermal processing of hydrocarbon oils does not belong in the classifications mentioned, and being relatively inert is not satisfactorily removed by conventional treating processes employed by the industry for the removal of hydrogen sulfide, mercaptans, and the like.
Carbonyl sulfide is presumably formed by reaction of hydrogen sulfide with oxides of carbon under the conditions of heat and pressure and exposed metal surfaces encountered in thermal cracking and reforming operations. The pure compound has a boiling point slightly lower than that of propane, although we have found its apparent boiling point is somewhat higher in hydrocarbon mixtures. Thus, the fractionation of cracking still gases to segregate a propane-butane fraction results in the inclusion of substantially all the carbonyl sulfide present within that fraction. Likewise a butane and heavier fraction containing only minor percentages of propane may contain appreciable amounts of carbonyl sulfide.
The necessity for selectively removing carbonyl sulfide arises when a hydrocarbon fluid, e, g., a C4 fraction from refinery gases, is to be substantially completely desulfurized prior to processing to effect polymerization, alkylation or the like. An object of this invention, then, is to provide a more complete desulfurization of said hydrocarbons after conventional methods for the removal of hydrogen sulfide and mercaptans have been applied.
Carbonyl sulfide is relatively stable toward acidic reagents, and is only slowly affected by strongly alkaline treating reagents such as solutions of caustic soda and the like. The slow reaction with alkaline reagents is apparently based on the hydrolysis of the compound to form hydrogen sulfide which reacts with the alkaline medium. In view of the relatively slow rate of the hydrolysis reaction, incomplete removal of carbonyl sulfide results in a continuous-type treating system wherein the time of intimate contact of hydrocarbon with treating reagent is relatively short. For example, in washing a propane-butane mixture with a solution of caustic soda to remove hydrogen sulfide, we have found that with caustic solutions of normal strength-say 10-20 per cent by weight of sodium hydroxide, only 20-30 per cent of the carbonyl sulfide is hydrolyzed and extracted even with multi-stage contacting.
We have now discovered a method of treatment and a reagent which effects complete removal of carbonyl sulfide from hydrocarbon fluids of the type described. Since our reagent effects direct combination with the carbonyl sulfide, no intermediate hydrolysis is involved and removal is substantially complete on contact.
We have found that when hydrocarbon fluids containing carbonyl sulfide are brought into contact with adsorbent reagents impregnated with monoethanolamine or with alkylene polyamines, or in general with organic bases having at least one primary amine group a reaction takes place by which carbonyl sulfide is removed from the hydrocarbon in the form of an oil-insoluble compound. This compound is retained by the adsorbent reagent.
We presume that the reaction mechanism involves the formation of a monothiocarbonic acid or the amine salt thereof, although we do not limit the present process to such a mechanism.
We have further discovered that the end product of the reaction may be quite different due to the tendency of the initial products to further react or rearrange when monoethanolamine is used for the reagent, The exact nature of the reactions Involved in the rearrangement of the reaction product of monoethanolamine and a carbonyl sulfide are not known but we think it likely that a compound of the alkyl isothiocyanate type is formed which under certain conditions tends to split out hydrogen sulfide to produce alkyl isocyanates which in turn are further convertible by hydrolysis into the carbonic acid salt of the original amine. Such a chain of reactions makes possible at least a partial regeneration of the amine for further reaction with carbonyl sulfide under the conditions of our process. We have therefore discovered that a superior reagent for the removal of carbonyl sulfide results when wee use monoethanolamine in combination with a material which combines with nascent hydrogen sulfide according to the projected reaction mechanism described above, and thereby promotes the successive reactions. For this added material we prefer to use a lead salt such as sodium plumbite although other metal salts which form insoluble metal sulfides are operative under chosen conditions.
A further advantage of our reagent is that the formation of a metal sulfide by the added metal salt prevents any possible oxidation of hydrogen sulfide or other resultant sulfur compounds as formed to produce elemental sulfur which would be carried away by the hydrocarbon fluid. Such oxidative reactions are apparently possible in our process due to the treating conditions and to the presence of oxidizing agents in the adsorbent carrier materials.
We have noted that our process is much more efficient in the removal of carbonyl sulfide because of certain advantages obtained by the use of a solid-type reagent. We not only obtain a more complete removal of carbonyl sulfide, but also a more efficient utilization of the active ingredients of our reagent. This latter effect may be due in part to the great amount of reagent exposed on the surfaces of the carrier material.
Thus, while a portion of the carbonyl sulfide present in a hydrocarbon fluid might be extracted by an aqueous solution of the reagents disclosed, the capacity of such a solution for carbonyl sulfide is far below the theoretical capacity during the relatively short period of complete removal. An additional advantage of our solid-type reagent-is that ideal counter-current treating conditions are obtained. Our reagent is gradually and uniformly spent in the direction of hydrocarbon flow, a condition that is not obtainable 5c in contacting hydrocarbons with an aqueous treating solution the entire volume of which is spent to the same degree. In tree n ating with our solid reagent, the section of the reagent bed adjacent to the hydrocarbon exit port remains 6( in the most active condition to effect the removal of the last traces of carbonyl sulfide.
Further, in our method of passing the hydrocarbon stream over a bed of solid reagent we have the advantage of prolonged contact time 65 to aid in the completion of the removal reaction.
Thus, the time of contact of our reagent with the hydrocarbon fluid may be controlled as desired and ranges from 12 minutes to two hours at preferred liquid flow rates. This is in con- 70 trast to contact times of ordinarily less than three minutes in a treating process utilizing aqueous solutions.
Since our reagent combines with hydrogen sulfirl. nnrl - - u process to treat hydrocarbon fluids which have already been given treatment for the removal of hydrogen sulfide and mercaptans. Thus we avoid the uneconomic spending of our reagent with these impurities which are ordinarily present In quantities far exceeding the quantity of carbonyl sulfide in hydrocarbon fluids. By this sequence of operations only the very minor amounts of hydrogen sulfide and/or mercaptans remaining after conventional methods of removal could be present in a hydrocarbon fluid treated by our process.
Our reagent may be prepared by first impregnating an adsorbent carrier material with a lead 153 salt solution such as sodium plumbite solution prepared by dissolving lead monoxide in a solution of sodium hydroxide of suitable strength.
This solution is then sprayed onto the absorbent carrier. Other means of Impregnating the carrier with a lead salt will be obvious to those skilled in the art. Since the solubility of lead monoxide in sodium hydroxide is limited, the weight per cent of lead salt on the reagent may be increased by adding powdered lead monoxide directly to the moist particles of the carrier immediately after the initial spraying. In such a case, we prefer to use a reagent consisting of about 1-3 per cent by weight of lead monoxide in solution as sodium plumbite, and 1-3 pecentr cermore by jweight of the dry powdered material. The powder adheres to the dampened particles of the carrier and a satisfactory reagent results.
After the application of the lead salt to the reagent, the adsorbent material is impregnated o3 with monoethanolamine by the addition of from about 1 to 15 per cent by weight of this liquid, depending on the adsorptive capacity of the carrier. Instead of the amine itself, a concentrated aqueous amine solution may be used, but to we prefer to use as little water as possible at this stage in order that the finished reagent may appear dry and not ball or crumble during handling.
As adsorbent carriers for the reagent, we may [5 use fuller's earth of suitable size and hardness.
A grade of earth which has resistance to crumbling is preferable. Other suitable carriers are highly adsorbent materials such as activated alumina and the like, depending upon cost and 0 availability.
Other lead salts which may be used are those which are soluble in water or in alkaline media, for it is highly desirable for a substantial proportion of the lead salt to be in the adsorbed solution 3 at all times during the use of the reagent. As the lead from the sodium plumbite is precipitated by the action of the sulfur compounds, the solid lead monoxide may either go into solution in the adsorbed caustic solution or it may react directly ) given sufficient contact time.
Other metal salts which may replace the lead salt in my reagent are those forming water and oil-insoluble sulfides which have no deleterious effect on the hydrocarbons being treated. Suitable examples are salts of cadmium, preferably the sulfates or chlorides.
The following example will serve to illustrate one method of practicing our invention: Example Fuller's earth of 8-20 mesh size was impregnated with a solution of sodium plumbite in 15 per cent sodium hydroxide solution. The solution was saturated while hot with lead monoxide and the quantity of sodium plumbite thus applied c pans We contempl 5 to the fuller's earth corresponded to 1.2 per cent t by weight of the reagent. While the earth was still damp, an additional 2 per cent by weight of dry powdered lead monoxide was mixed in with substantially all being bound to the earth particles. The reagent was then sprayed with monoethanolamine; the quantity added was 5 per cent by weight of the weight of the fuller's earth before impregnation. This reagent was loaded into towers without drying. A propane-butane fraction from refinery cracked gas was passed over the reagent described above, after preliminary treatment for the removal of hydrogen sulfide and mercaptans.
The hydrocarbon liquid contained 0.002 per cent by weight of carbonyl sulfide before contact with the reagent, and less than 0.0001 per cent by weight after treatment. The flow rate was maintained between 1 and 2 volumes of liquid hydrocarbon per hour per volume of reagent, Complete removal of carbonyl sulfide was accomplished throughout the life of the reagent which was considered spent when the theoretical quantity of sulfur had been extracted.
The temperatures of treatment using our process are ordinary atmospheric temperatures between 300 and 1100 F. The pressures at which we prefer to operate may be low super-atmospheric pressures between 50 and 500 pounds gage.
The operating pressure may depend on the fluid being treated; for example, in treating propane and/or butane in liquid phase, sufficient pressure is used to avoid vaporization.
We usually prefer to treat in liquid phase since the volume of reagent required for nominal flow rates, say 0.5 to 5 volumes per hour per volume of reagent is not excessive. However, gas or vapor phase treating of normally gaseous hydrocarbons is satisfactory if provision is made in the size of the reagent bed to allow contact times corresponding to linear vapor velocities under five feet per minute.
We claim: 1. The process for the removal of carbonyl sulfide from hydrocarbon fluids subsequent to treatment for removal of hydrogen sulfide and mercaptans which comprises contacting said fluids with a reagent comprising an adsorbent carrier impregnated with a solution of a lead salt and monoethanolamine.
2. The process for the removal of carbonyl sulfide from hydrocarbon fluids subsequent to treatment for removal of hydrogen sulfide and mercaptans which comprises contacting said fluids with a reagent comprising an adsorbent carrier impregnated with a lead salt and monoethanolamine.
3. The process for the removal of carbonyl sulfide from hydrocarbon fluids subsequent to treatment for removal of hydrogen sulfide and mercaptans which comprises contacting said fluids with a reagent comprising fuller's earth impregnated with a solution of sodium plumbite, lead monoxide and monoethanolamine.
4. The process for the removal of carbonyl sulfide from hydrocarbon liquids containing the same which comprises contacting said liquids with a reagent comprising an adsorbent carrier impregnated with a solution of a lead ;alt and monoethanolamine.
5. In a process for the desulfurization of lowboiling hydrocarbon liquids which comprises treating said liquids in successive stages to remove hydrogen sulfide mercaptans and other sulfur compounds, the step of contacting said liquids subsequent to the removal of hydrogen sulfide and mercaptans with a reagent comprising an adsorbent carrier impregnated with a solution of a lead salt and monoethanolamine.
6. A process for the removal of carbonyl sulfide from liquefied petroleum gases which comprises treating said liquefied gases subsequent to treatment for removal of hydrogen sulfide and mercaptans and at atmospheric temperature over a reagent comprising a solid adsorbent carrier impregnated with sodium plumbite solution, lead monoxide and monoethanolamine.
WALTER A. SCHULZE.
GRAHAM H. SHORT.