US3197396A - Method of preventing deposit formation - Google Patents

Description

United States Patent 3 197 396 METHOD or PREVENTING DEPOSIT FonMArroN Russell F. Stedman, Des Plaines, Ill., assignor to Universal Oil Products Company, Des Plaines, 11]., a corporation of Delaware No Drawing. Filed Nov. 24, 1961, Ser. No. 154,892 2 Claims. (Cl. 208-48) The free sulfur is a strong oxidizing catalyst and, upon subsequent heating of the hydrocarbon oil, results in the formation of deposits. 0

In accordance with the present invention, deposit formation upon heating of the hydrocarbon fraction is retarded by adding a mercaptan to the hydrocarbon fraction and thereby converting the free sulfur to hydrogen sulfide and the mercaptan to a disulfide according to the following reaction:

The oxidation of the mercaptan to the disulfide by the free sulfur occurs prior to and preferentially to the reactions which cause deposit formation. The mercaptan is added to the hydrogen fraction prior to heating of the hydrocarbon fraction. It is general practice at the present time to heat a hydrocarbon fraction by passing the same in indirect contact with a hot fluid. This heating is effected in indirect heat exchangers and heretofore difficulty has been experienced in the fouling of the heat exchanger by deposits formed in the exchanger. At first the deposits reduce the efficiency of the heat exchange and eventually plug the heat exchanger. This, in turn means that operation of the heat exchanger must be discontinued and the heat exchanger must be either cleaned or replaced. Because the heat exchange is an integral and important part of a refining process, misoperation or non-operation of the heat exchanger may, in turn, result in misoperation or the necessity of shutdown of the refining process.

The heating of a hydrocarbon fraction is illustrated, for example, in a process wherein the hydrocarbon charge is heated to an elevated temperature for effecting the desired conversion, and the heated effluent products from such conversion are cooled for subsequent separation into the desired fractions. At least a portion of the heating of the charge is economically obtained by heat exchange with hot products from within the process. Furthermore, at least a portion of the cooling of the hot efiiuent products likewise is economically obtained by heat exchange with the charge to the process. These economies also are obtainable in systems in which a charge is heated and then subjected to distillation or fractionation in order to separate the charge into different fractions. In such distillation or fractionation systems, the lower portion of the distillation or fractionation zone is heated in order to vaporize and remove the lighter components as overhead and/or side streams. The bottoms from the distillation or fractionation zone are withdrawn and generally are cooled before being sent to storage or subsequent treatment and, here again, economics are effected by transferring at least a portion of the heat from the hot bottoms product to the charge. While the present invention is particularly useful in systems in which the charge to the process is passed in heat exchange with hot efiiuent products of the process, it is understood that the present invention also may be used in any heat exchange system in which undesirable deposit formation is encountered.

An example of a process in which the charge is passed in heat exchange with hot etliuent products is a hydrotreating process in which oil is subjected to hydrogen treating in the presence of a catalyst comprising aluminamolybdenum oxide-cobalt oxide or alumina-molybdenum sulfide-cobalt sulfide. The oil may comprise gasoline, kerosene, gas oil or mixtures thereof and is treated to remove impurities. The treating is effected at a temperature within the range of from about 500 to about 800 F. or more at hydrogen pressures of from about to about 1000 pounds per square inch or more. The oil charged to the process generally is introduced at a temperature of from ambient to 200 F. and is passed in heat exchange with products withdrawn from the reactor at a temperature of from about 500 to about 800 F. During this heat exchange the charge is heated to a temperature of from about 300 to about 600 F. and then may be heated further in a furnace or otherwise to the temperature desired for effecting the treating. At the same time the hot reactor efiiuent products are cooled to a temperature of from about 300 to about 600 F. but below that at which they are withdrawn from the reactor. Generally the partly cooled reactor effluent products are cooled further by heat exchange with water or otherwise and then are passed into a separator, wherefrom gases and liquids are each separately Withdrawn. Another illustrative example of a process in which the charge is passed in heat exchange with reactor effluent products is a reforming process in which gasoline is contacted with hydrogen in the presence of a platinum-containing catalyst at a temperature of from about 700 to about 1000 F.

An example in which oil is subjected to fractionation and the charge is passed in heat exchange with the hot effluent products is in a crude column. In this column, crude oil is subjected to distillation at a temperature of from about 600 to about 700 F. in order to remove lighter components as overhead and/or side streams. In some cases the charge first is passed in heat exchange with the overhead and/or side streams from this column and then is passed in heat exchange with the hotter products withdrawn from the bottoms of the crude column, In this way the charge is progressively heated and the hotter products are cooled.

The above examples are illustrative of typical uses of heat exchange to effect economies in the process. However, difficulty is experienced in the heat exchange due to deposit formation, with the consequent necessity of interrupting plant operation as hereinbefore set forth. In accordance with the present invention, deposit formation in heat exchanger is reduced to an extent that normal plant operation need not be interrupted for this reason.

It is understood that the advantages of the present invention may be obtained in any suitable heat exchange equipment. In general, this equipment comprises a series of tubes or a tube coil positioned within a shell. One of the fluids is passed through the tubes, while the other fluid is passed through the shell. The heat exchange equipment generally is positioned externally to a fractionator or reactor. However, in some cases, the heat exchanger takes the form of a reboiler or condenser, and

either a tube coil or a shell containing tubes is positioned within the lower or upper portion of the fractionator or reactor.

In one embodiment the present invent-ion relates to a method of retarding deposit formation upon heating to a temperature above about 300 F. of a hydrocarbon fraction containing free sulfur, which comprises, prior to said heating, adding a mercaptan to said hydrocarbon fraction to convert said mercaptan to a disulfide and said free sulfur to hydrogen sulfide, and thereafter heating said hydrocarbon fraction to a temperature above about 300 F.

In a specific embodiment the present invention relates to a method of retarding deposit formation in a heat exchanger through which a hydrocarbon oil containing free sulfur is to be passed and heated therein to a temperature above about 300 F., which comprises adding a mercaptan to said oil prior to passing said oil through said heat exchanger, said mercaptan being added in a ratio of at least two mole proportions of mercaptan per one mole proportion of free sulfur, whereby said mercaptan is converted to a disulfide and said free sulfur is converted to hydrogen sulfide.

It Will be noted that deposit formation is retarded in accordance with the present invention by adding a mercaptan to the hydrocarbon fraction. The addition of a mercaptan to a hydrocarbon fraction generally is considered objectionable because, in most cases, it is desired to remove sulfur compounds from hydrocarbon fractions. However, the addition of the mercaptan in accordance with the present invention serves the important func tion of retarding deposit formation upon heating of the oil, and also converts the mercaptan into a harmless disulfide. While generally the addition of a sulfur compound to an oil would be frowned upon, the present invention provides a novel method of utilizing such an addition to obtain important advantages.

Any suitable mercaptan may be used in accordance with the present invention. Preferred mercaptans include methyl mercaptan, ethyl mercaptan, propyl mercaptan, butyl mercaptan, amyl mercaptan, hexyl mercaptan, etc., although higher boiling mercaptans may be used including, for example, heptyl mercaptan, octyl mercaptan, nonyl mercaptan, decyl mercaptan, undecyl mercaptan, dodecyl mercaptan, etc. In another embodiment an aryl mercaptan may be used, including thiophenol and alkylated derivatives thereof, benzyl mercaptan and alkylated derivatives thereof, the alkyl substitutions being generally selected from methyl, ethyl, propyl, hexyl, heptyl, octyl, etc. When desired, a mixture of mercaptans may be used as this eliminates the time and expense of separating individual fractions. A primary mercaptan is preferred because of its more ready oxidation to disulfide although, in some cases, a secondary mercaptan may be employed.

In a preferred embodiment of the present invention, the mercaptan is added in a ratio of two mole proportions of mercaptan per one mole proportion of free sulfur in order to obtain complete reaction as per Equation 2 above. In order to assure sufiicient mercaptans, in some cases an excess of mercaptan is used, but generally this should not be too great an excess because of the general objection of introducing sulfur compounds into hydrocarbon oils.

The mercaptan may be added to the hydrocarbon fraction in any suitable manner. Generally, it is preferred to form a solution of the mercaptan in a suitable solvent and add the mercaptan in this manner into the hydrocarbon fraction. Any suitable solvent may be used, with hydrocarbons being particularly preferred, although other solvents, such as alcohols, ketones, etc. may be employed. Preferably the mercaptan is intimately mixed with the hydrocarbon fraction in order to assure reaction with the free sulfur in the hydrocarbon fraction. This may be accomplished in any suitable manner, including the use of mixing blades, recirculation of the oil by pumping out of and back into the storage tank, pumping the liquid through orifice mixers, etc.

The mercaptan may be added to the hydrocarbon fraction at any suitable temperature which may range from atmospheric up to 300 F. At least partial reaction will occur at the lower temperature, but any incomplete reaction will be effected when the hydrocarbon fraction is heated to a temperature above 300 F. As stated hereinbefore, the oxidation of the mercaptan to disulfide is a fast reaction and will occur prior to and preferentially to the reactions which cause deposit formation.

While the novel features of the present invention are particularly applicable to the treatment of hydrocarbon oils, it is understood that the invention may be used for the treatment of gaseous hydrocarbon fractions con-taining hydrogen sulfide which, as described by Equation 1, react to form free sulfur. Saturated gaseous hydrocarbon fractions comprise methane, ethane, propane and/or butane.

The following examples are introduced to illustrate further the novelty and utility of the present invention but not with the intention of unduly limiting the same.

Example I Straight run naphtha containing 17 parts per million of hydrogen sulfide is placed in tank storage. The straight run naphtha. is to be subject to reforming in the presence of a platinum-alumina-combined halogen catalyst at a temperature of 890 F. The naphtha will be passed into heat exchange in a series of steps in which the first heat exchange increases the temperature of the naphtha to about 400 F. and the second heat exchange increases the temperature of the naphtha to about 600 F. The naphtha then is passed through tubes in a furnace and the naphtha is heated therein to the desired reaction temperature of 890 F.

During passage of the naphtha through the heat exchangers, deposit formation occurs, gradually decreasing the efficiency of the heat exchange and eventually plugging the heat exchanger. This necessitates removing and cleaning the heat exchangers, with the concomitant interruption of plant operation.

In accordance with the present invention, ethyl mercaptan is added to the storage tank in a concentration of 35 parts per million (a ratio of about two mole proportions of mercaptan per one mole proportion of free sulfur). This serves to convert the free sulfur contained in the naphtha into hydrogen sulfide and to convert the mercaptan into diethyl disulfide. The naphtha then is passed into heat exchange in the manner described above. The prior addition of the mercaptan serves to retard deposit formation in the heat exchangers and thereby to extend the continuous operation of the plant.

Example 11 A heptane-octane fraction is to be heated at an elevated temperature for further processing. However, it has been found that heat exchanger deposits were formed during the preheating of the heptane-octane fraction, which caused frequent interruptions of the plant operation. A careful analysis of the heptane-octane fraction disclosed the presence of hydrogen sulfide in a concentration of about 10 parts per million. Twenty parts per million of propyl mercaptan is added to the heptane-octane fraction prior to passing the same through the heat exchangers.

I claim as my invention:

1. A method of retarding deposit formation in a heat exchanger upon heating therein to a temperature above about 300 F. of a hydrocarbon fraction containing free sulfur, which comprises, prior to said heating, adding a mercaptan to said hydrocarbon fraction and converting said mercaptan to a disulfide and said free sulfur to hydrogen sulfide, and thereafter heating the resultant mixture 0s ailgl heat exchanger to a temperature above about 2. A method of retarding deposit formation in a heat exchanger through which a hydrocarbon oil containing free sulfur is to be passed and heated therein to a temperature above about 300 R, which comprises adding a mercaptan to said oil prior to passing said oil through said heat exchanger, said mercaptan being added in a ratio of at least two mole proportions of mercaptan per one mole proportion of free sulfur converting said mercaptan to a disulfide and said free sulfur to hydrogen sulfide prior to said heating, "and thereafter heating the resultant mixture to above about 300 F. in said heat exchanger.

References Cited by the Examiner UNITED STATES PATENTS 1,668,225 5/28 Wendt 208-200 OTHER REFERENCES Kalichevsky and Stagner, Chemical Refining of Petroleum, The Chemical Catalog Company, Inc., 330 W. 42nd 10 St, New York, NY. (1933), page 137.

Canapry, R. C., HOW To Control Refinery Fouling, Oil and Gas Journal, 59, number 41, pages 114-118 (October 1961).

1 ALBERT T. MEYERS, Primary Examiner.

JULIUS GREENWALD, Examiner.

Claims (1)

1. A METHOD OF RETARDING DEPOSIT FORMATION IN A HEAT EXCHANGER UPON HEATING THEREIN TO A TEMPERATURE ABOVE ABOUT 300*F. OF A HYDROCARBON FRACTION CONTAINING FREE SULFUR, WHICH COMPRISES, PRIOR TO SAID HEATING, ADDING A MERCAPTAN TO SAID HYDROCARBON FRACTION AND CONVERTING SAID MERCAPTAN TO A DISULFIDE AND SAID FREE SULFUR TO HYDROGEN SULFIDE, AND THEREAFTER HEATING THE RESULTANT MIXTURE IN SAID HEAT EXCHANGER TO A TEMPERATURE ABOVE ABOUT 300*F.