PRIOR ART Sulfides suitable for use as sulfiding agents are generally exemplified by dimethyldisulfide, ethyl mercaptan, dimethylsulfide, tetrahydrothiophene, thiophene, diethylsulfide, n-butylmercaptan, and hydrogen sulfide (H2S). On a catalyst such as CoMo or NiMo, sulfiding agents are decomposed into hydrogen sulfide and adsorbed to allow the catalytic metals to be of reactive activity. In an ethylene cracking process, sulfide agents are converted into hydrogen sulfide through a thermal cracking mechanism and adsorbed into nickel, which is an active metal for catalytic coking on the surface of the cracking tube, thereby deactivating the coking reaction and lowering the production of carbon monoxide.

It is required that sulfiding agents should be decomposed into hydrogen sulfide at temperatures as low as possible and be easy to handle safely and to store. Dimethyldisulfide has been prevalent over other

sulfiding agents since it best satisfies these requirements (see: Hydrocarbon Engineering Jan./Feb. p. 26,1987).

Produced as by-products during a LPG desulfurizing process in an oil refinery, disulfide mixtures are different in composition from one another in accordance with the type of oil to be treated and process conditions. In general, the disulfide mixtures comprise dimetyldisulfide, diethyldisulfide and ethylmethyldisulfide : these three compounds comprise 98 % or higher of the disulfide compounds produced. Among them, dimethyldisulfide has been demonstrated to be an excellent sulfiding agent, and has been used extensively in practice, while there have as yet been no demonstrations of the utility of diethyldisulfide and ethylmethyldisulfide. In particular, when being discharged outside, the disulfide mixtures which are produced as by- products during an LPG desulfurizing process of oil refineries cause serious pollution of the environment and thus, stand in great need of being utilized.

Further, the disulfide mixtures which are produced as by-products during LPG desulfurizing process contain caustic and sodium compounds invariably. When being used as they are, the disulfide mixtures containing caustic and sodium compounds exhibit extremely deteriorated catalytic activity. Thus, there remains a need to effectively remove or isolate the undesirable compounds from the disulfide mixtures which are to be used as sulfiding agents.

DISCLOSURE OF THE INVENTION As a result of intensive and extensive research on sulfiding agents, the present inventors developed a method. of effectively removing caustic

and sodium compounds serving as catalyst poisons from the disulfide mixtures which are produced as by-products during the LPG desulfurizing process of oil refineries and demonstrated through various experimental assay and practical process applications that the remaining disulfide mixtures act as sulfiding agents with similar activity to that of dimethylsulfide.

Therefore, it is an object of the present invention to provide a method for preparing a pure disulfide mixture from a by-product produced in the LPG desulfurizing process of the oil refinery, by which caustic and sodium compounds can be effectively removed.

It is another object of the present invention to provide use of such a pure disulfide mixture as a sulfiding agent applicable for a catalytic desulfurizing processes and ethylene cracking processes.

In accordance with an embodiment of the present invention, there is provided a method for preparing a sulfiding agent suitable for use in a free sulfiding-required catalytic process comprising the removal step of caustic and sodium compounds from a by-product produced in an LPG desulfurizing process of an oil refinery by washing the by-product with water to give a disulfide mixture and separating the disulfide mixture from the water contained therein.

In accordance with another embodiment of the present invention, there is provided use of a disulfide mixture as a sulfiding agent suitable in reducing a coking reaction within a cracking tube for an ethylene cracker, wherein the disulfide mixture is prepared by the removal of caustic and sodium compounds from a by-product produced in an LPG desulfurizing process of an oil refinery to give a disulfide mixture.

BEST MODES FOR CARRYING OUT THE INVENTION In an LPG desulfurizing process, a disulfide mixture is produced, together with a large quantity of caustic and sodium compounds, as a by- product. Hence, where the disulfide mixture is used as a sulfiding agent, the catalyst is greatly deteriorated in the performance thereof owing to the caustic and sodium compounds.

In the present invention, a sulfiding agent is prepared from an impurity-free disulfide mixture recovered through a process of washing a disulfide mixture produced in an LPG desulfurizing process with copious water to completely remove caustic and sodium compounds which act as catalyst poisons; and a process of completely removing from the disulfide mixture the water which is present in trace amounts. In the recovered impurity-free disulfide mixture, disulfide compounds preferably amount to at least 98 % by weight.

With reference to Fig. 1, there is a process flow illustrating the separation of a disulfide mixture from the by-products produced in an LPG desulfurizing process, in accordance with the present invention.

A crude LPG is fed into a mercaptan extracting column 1 where sulfur is removed and from which desulfurized LPG is recovered via a separate stream. The by-products produced in this process, including a disulfide mixture and caustic and sodium compounds, are transferred through a separator 3 into a washer 4. Preferably, the washer 4 is structured to be a water wash column. Transferred from the washer 3, the

by-products comprising the disulfide mixture and the caustic and sodium compounds are introduced into a lower position of the washer 4. In this connection, washing water is together introduced to effectively conduct the washing of the by-products. After being separated and discharged from an upper position of the washer 4, the washed disulfide mixture is transferred into a secondary separator 5 from which the water used in the washing is discharged through the lower position of the secondary separator 5. In the secondary separator 5, the disulfide mixture is separated from the water contained therein by taking advantage of the difference in their specific gravities. The separation can be performed by sufficiently settling the disulfide mixture. The longer the residence time period in the secondary separator 5 is, the better the separation is. However, a period of several hours to one day is sufficient for the separation.

The separation may be conducted in a continuous process or a batch process.

Larger amounts of water introduced into the washer 4 bring about greater effects in the washing, but suffer from the problem of requiring a larger apparatus. It is preferable that the ratio of disulfide mixture to water is in the range of about 6: 1 to 1: 2.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a process flow illustrating the separation of a disulfide mixture from the by-products produced in an LPG desulfurizing process, in accordance with the present invention.

A better understanding of the present invention may be obtained in light of the following examples which are set forth to illustrate, but are not to be construed to limit the present invention.

In the following examples, the disulfide mixtures which were purified of the caustic and sodium compounds in accordance with the separation process illustrated in Fig. 1, were analyzed for physical properties, composition, thermal cracking properties, and catalyst-free sulfiding and reacting performance.

EXAMPLE I Impurities such as caustic and sodium compounds were separated from the by-products of various LPG desulfurizing processes to give disulfide mixtures which were analyzed for their compositions. The results are given in Table 1, below.

TABLE 1 Compounds Process 1 Process 2 Process 3 Dimethyldisulfide (wt%) 12.5 3. 6 7. 6 Ethylmethyldisulfide (wt%) 47.1 29.5 38. 2 Diethyldisulfide (wt%) 39.1 65. 5 52.5 Total 98. 7 98.6 98. 3 Sodium (ppb) ND ND ND Note: ND not detected

As is apparent from the results in Table 1, the disulfide mixtures were shown to be greatly different in composition from one another in accordance with LPG desulfurizing processes, but after being subjected to the removal of caustic and sodium compounds therefrom, the disulfide mixtures were shown to have 98% or higher purity. Also, sodium levels were reduced to an amount lower than the detection limit. Thus, these mixtures could be applied in practical processes without causing problems associated with the presence of sodium, which is causative of corrosion during the processes.

EXAMPLE II To evaluate the utilization of disulfide mixtures as sulfiding agents for the cracking tubes of the ethylene cracker, ethylmethyldisulfide and diethyldisulfide were used for thermal cracking at 470 °C and 530 °C respectively, for the residence time of 2.5 seconds using a micro reactor.

The results are given in Table 2, below.

TABLE 2 Reaction Temp. % Cracking (°C) Dimethyldisulfide Ethyhnethyldisulfide Diethyldisulfide 470 62. 8 70. 4 92. 4 530 98. 3 95. 3 96. 6

It is apparent from the results of Table 2 that both ethylmethyl disulfide and diethyldisulfide are not problematic in being used, showing cracking efficiencies as great as that of dimethyldisulfide, which is most extensively used now.

EXAMPLE III The disulfide mixture was tested for free-sulfiding and desulfurizing performance as a free-sulfiding agent. In this regard, a CoMo/alumina catalyst, used in a naptha hydrotreating process, was employed along with a catatester. The temperature condition for free sulfiding was set at 350 °C for the disulfide mixture and the dimethyldisulfide, respectively. After completion of the free sulfiding, the reacting performance of the disulfide mixture was examined over about 70 hours under the same operational conditions as those of a practical process, having a temperature of 300 °C, a reaction pressure of 22 kg/cm2, and a sulfide content in reactants of 417 wtppm. The results are given in Table 3, below.

TABLE 3 Reaction Sulfur Content in Reaction Conversion Suliiding Agent Time (Hr) Product (wtppm) Rate (%) 9 0.09 99. 98 Disulfide Mixture 0. 1 99.98 57 0.1 99.98 8 0. 12 99. 97 Dimethyl 32 32 0. 05 99.99 disulfide 48 48 0. 1 99.98

As shown in Table 3, all of the reaction products had sulfur contents of 0.5 wtppm or less and showed reaction performances almost identical to that of dimethyldisulfide.

EXAMPLE IV This example was also to assay the disulfide mixture for its utility as a free sulfiding agent for a catalytic process. For this, its free sulfiding and desulfurizing reaction performance was evaluated in a pilot plant in the presence of a CoMo/alumina catalyst used for a diesel hydrotreating process.

The temperature condition for free sulfiding was set at 280 °C for the disulfide mixture and the dimethyldisulfide, respectively. After completion of the free sulfiding, the performance of the disulfide mixture was examined over about 70 hours under the same operational conditions as those of a practical process, having temperatures of 340 °C and 360 °C, a reaction

pressure of 40.5 kg/cm2, and a sulfide content in reactants of 1.4 wt%. The results are given in Table 4, below.

TABLE 4 Sulfiding Agent Reaction Temp. (°C) Desulfurizing (%) 340 86 Disulfide Mixture 360 95 340 84 Dimethyldisulfide 360 94

As shown by the data in Table 4, the disulfide mixture was demonstrated to have the same desulfurizing performance as dimethyldisulfide.

EXAMPLE V The disulfide mixture was used as a free sulfiding agent for a kerosene hydrotreating process in the presence of a CoMo/alumina catalyst.

The results are given in Table 5, below.

TABLE 5 Sulfiding Sulfur Content in Sulfur Content in Desulfurized Agent Reactant (ppm) Kerosene (ppm) Disulfide Mixture 2200 183 Dimethyldisulfide 2100 196

After completion of the free sulfiding by use of the disulfide mixture, the sulfur content of kerosene obtained from the kerosene hydrotreating process was shown to have decreased from 2,200 ppm to 200 ppm or less, which was a similar level to that obtained when dimethyl sulfide was used. From these results, the disulfide mixture was shown to be superior in sulfiding performance.

INDUSTRIAL APPLICABILITY As described hereinbefore, the present invention provides a method with which caustic and sodium compounds can be removed from the by- products produced in an LPG desulfurizing process to prepare a disulfide mixture which is highly suitable for use as a sulfiding agent and applicable for all of the sulfiding processes where conventional sulfiding agents are used. In addition, the present invention has many advantages in that the by-products of oil refineries can be converted into a high value-added material through a relatively simple separation operation and the conversion of the by-products is favorable for the environment because the by-products produce pollutants of the air.

The present invention has been described in an illustrative manner, and it is to be understood that the terminology used is intended to be in the nature of description rather than of limitation. Many modifications and variations of the present invention are possible in light of the above teachings. Therefore, it is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.