Abstract:
Specified sulfur compounds such as N,N-diethylthiourea, N,N-dibutylthiourea, tetramethylthiuram monosulfide, tetrabutylthiuram monosulfide, tetraethylthiuram disulfide, tetrabutylthiuram disulfide are supplied as a coking inhibitor for preventing coking in equipment used in hydrocarbon treatment processes wherein a hydrocarbon is treated at a temperature of from about 250° C. to 950° C. to produce petroleum products, petrochemical products and/or their intermediate products.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method for preventing coking in equipment used in hydrocarbon treatment processes wherein a hydrocarbon is treated at a temperature of from about 250° to about 950° C. to produce petroleum products, petrochemical products and/or their intermediate products. More particularly, the present invention relates to a method for preventing coking in equipment such as heat exchangers, heating furnaces, reactors, connecting pipes and the like wherein a hydrocarbon is subjected to a process for refining, separation, extraction, desulfurization, reforming, polymerization, thermal cracking, catalytic cracking, synthetic reaction or a combination of at least two of them at a temperature of from about 250° to about 950° C. to produce petroleum products, petrochemical products and/or their intermediate products, the method comprising supplying a coking inhibitor to said equipment to prevent coking in the equipment. 
     2. Description of Prior Art 
     In many hydrocarbon treatment processes employed in petroleum and petrochemical industries, a hydrocarbon feed is heated to high temperatures in equipment such as heat exchangers, heating furnaces, reactors, ethylene cracking furnaces and the like to achieve an intended purpose. It is known that, in such case, coke-like substances (coke-like fouling) accumulate in thick deposits inside the equipment, particularly at the high temperature portions of the equipment, causing various disadvantages such as reduction in heat conduction efficiency and hindrance of the material flow inside the equipment. In order to remove these disadvantages, operators of such equipment generally conduct decoking or cleaning for the fouled equipment at frequent intervals. This solution is very undesirable when viewed from the aspect of energy saving or process rationalization. 
     As examples of the hydrocarbon treatment process according to the present invention, mention can be made of an atmospheric distillation column, a vacuum distillation column, a hydrodesulfurization column, a visbreaker, a coker, an aromatic hydrocarbon extraction column, etc. In these processes, a hydrocarbon feed liable to cause polymerization and coking with relative ease is used and relatively high temperatures are further employed; therefore, the coking that occurs has been big problem and countermeasures for it have been sought. 
     Coking is basically brought about in the following manner. A heavy fraction in feed hydrocarbon or a heavy fraction produced by partial polymerization of feed hydrocarbon adheres to the inside walls of process equipment, is exposed to high temperatures for a long period of time and grows into a coke-like substance. As methods for suppressing the formation and accumulation of such a coke-like substance, there have been proposed, for example, a method of adding a phosphoric acid derivative (U.S. Pat. No. 4,105,540), and so forth. However, all of the hitherto proposed methods are far from satisfactory and further improvements are desired. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a method for preventing coking in hydrocarbon treatment process equipment by using an excellent coking inhibitor for process equipment which exhibits an inhibitory action for coking even at high temperatures of from about 250° to about 950° C., is stable, is soluble in hydrocarbons and easy to handle, and is suited for use in a wide range of hydrocarbon treatment processes. 
     Other objects and advantages of the present invention may become apparent to those skilled in the art from the following description and disclosure. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     According to the present invention, the coking that occurs in heat exchangers (e.g., about 250°-600° C.), heating furnaces (e.g., about 300°-650° C.), reactors (e.g., about 250°-560° C.), ethylene cracking furnaces (e.g., about 260°-927° C.), and the like, used in hydrocarbon treatment processes can be advantageously suppressed by adding to a feed hydrocarbon used in said processes 10 to 5,000 ppm, desirably 10 to 1,000 ppm, of a compound represented by the following general formula I and/or II: ##STR1## where R 1 , R 2 , R 3  and R 4  are each a hydrogen atom, an alkyl group of 1 to 30 carbon atoms, a cycloalkyl group, an aryl group, an alkanol group, an aralkyl group or an alkenyl group and can be the same or different, and x is an integer of 1 to 6. In the present invention, the compound represented by the general formula I and/or II is added to a feed hydrocarbon in an amount of 10 to 5,000 ppm, desirably 10 to 1,000 ppm. This is based on the idea that addition of too small an amount will not give sufficient effect and addition of an amount exceeding 5,000 ppm is not economical although sufficient effect would still be obtained. In the present invention, use of at least one compound represented by the formula I and/or II is proposed. However, mixed use of two or more of these compounds, as well as combined use of such a compound and another fouling inhibitor or coking inhibitor is possible. The present invention has no restriction on such mixed or combined use. As specific examples of the compounds represented by the formulas I and II of the present invention, there are mentioned N,N-diethylthiourea, N,N-dibutylthiourea, tetramethylthiuram monosulfide, tetrabutylthiuram monosulfide, tetraethylthiuram disulfide, tetrabutylthiuram disulfide, etc. 
     In the most usual way of carrying out the present invention method industrially, a compound represented by the formula I and/or II is dissolved in an organic solvent such as a hydrocarbon solvent, an oxygen-containing hydrocarbon solvent such as alcohol, a sulfur-containing solvent such as dimethyl sulfoxide or the like and then is injected into a feed hydrocarbon. In order to conduct suppresion of coking effectively, it is necessary that a coking inhibitor be uniformly mixed with the feed hydrocarbon between the time of inhibitor addition to feed hydrocarbon and the time of inhibitor arrival at heat exchanger, heating furnace, reactor, etc., by making the distance between the inhibitor addition point and the fouling sites as long as possible or by providing flections in the course of that distance. 
    
    
     Next, the present invention method will be explained specifically by way of Examples. However, the present invention is in no way restricted to these Examples which follow. 
     EXAMPLE 1 
     Into a test tube (material: SUS 316; inside diameter: 2.2 mm; length: 40 mm) heated at 450° C. was fed 1.5 ml/min of a mixture consisting of 90% by volume of kerosene and 10% by volume of toluene. A predetermined amount of the coking inhibitors shown in Table 1 are dissolved in said mixture of kerosene and toluene and are provided for tests. After the lapse of a predetermined time, the test tube was taken out. From the tube weights before and after the test, the amount of coke adhered to the tube was calculated. The same tests were repeated except that the coking inhibitors were absent. The results are given in the following Table 1. 
     
                       TABLE 1______________________________________      Other test conditions                    Test results        Addition   Test time                            Amount of cokeCompounds used        amount (ppm)                   (hr.)    adhered (mg)______________________________________Blank        --          5       10        --         10       15ExamplesN,N--diethylthiourea          500       5       5&#34;              500      10       7&#34;            1,000      10       5Tetramethylthiuram          500      10       6disulfideTetramethylthiuram        1,000      10       4disulfideTetrabutylthiuram          500      10       7disulfideTetrabutylthiuram        1,000      10       4disulfideTetramethylthiuram        1,000      10       6monosulfide______________________________________ 
    
     EXAMPLE 2 
     Tests were repeated in the same manner as in Example 1, except that a mixture consisting of 50% by volume of naphtha and 50% by volume of water was fed into the test tube heated at 650° C. and 850° C., and when blocking up of the test tube occurred due to the hard coking of the naphtha during the test, the tests were stopped to allow calculation of the amount of coke adhered to the tube. The results are given in the following Tables 2 and 3. 
     
                       TABLE 2______________________________________Tests at 650° C.      Other test                Test results      conditions                Amount   Amount of        Addition Test   of coke                               coke adhered        amount   time   adhered                               per hr.Compounds used        (ppm)    (hr)   (mg)   (mg/hr)______________________________________Blank        --       10.0   44.0   4.4ExamplesN,N--diethylthiourea        100      10.0   31.2   3.1&#34;            500      10.0   19.3   1.9N,N--dibutylthiourea        500      10.0   17.0   1.7Tetrabutylthiuram        100      10.0   28.7   2.9disulfideTetrabutylthiuram        500      10.0   14.9   1.5disulfideTetramethylthiuram        500      10.0   16.2   1.6monosulfideComp. ExamplesDi(2-ethylhexyl)        500      10.0   52.0   5.2phosphateTris(2-ethylhexyl)        500      10.0   49.5   5.0phosphiteHydrogen sulfide        100      10.0   43.2   4.3______________________________________ 
    
     
                       TABLE 3______________________________________Tests at 850° C.      Other test  Test results      conditions           Amount        Addi-             Amount of coke        tion    Test      of coke                                 adhered        amount  time      adhered                                 per hr.Compounds used        (ppm)   (hr)      (mg)   (mg/hr)______________________________________Blank        --      3.0       126.5  42.2                (blocking)ExamplesN,N--diethylthiourea        100     4.5       144.9  32.2                (blocking)&#34;            500     7.0       140.0  20.0                (blocking)&#34;            1,000   10.0      160.0  16.0                (blocking)N,N--dibutylthiourea        500     6.5       137.8  21.2                (blocking)&#34;            1,000   9.0       157.5  17.5                (blocking)N,N--dilaurylthiourea        100     4.5       153.0  34.0                (blocking)&#34;            500     6.2       147.6  23.8                (blocking)Tetrabutylthiuram        100     5.0       147.5  29.5disulfide            (blocking)Tetrabutylthiuram        500     7.2       159.1  22.1disulfide            (blocking)Tetrabutylthiuram        1,000   10.0      180.0  18.0disulfideTetraethylthiuram        100     5.0       155.0  31.0disulfide            (blocking)Tetraethylthiuram        500     7.2       174.2  24.2disulfide            (blocking)Tetramethylthiuram        500     7.0       164.5  23.5disulfide            (blocking)Tetramethylthiuram        500     6.5       158.0  24.3monosulfide          (blocking)Composition of        100     7.0       155.2  22.2tetrabutylthiuram        +       (blocking)disulfide + di(2-         20ethylhexyl)phosphateComposition of        500     8.0       161.6  20.2tetrabutylthiuram        +       (blocking)disulfide + tris(2-         50ethylhexyl)phosphiteComp. ExamplesDi(2-ethylhexyl)        500     1.5        78.0  52.0phosphate            (blocking)Di(2-ethylhexyl)        1,000   1.0        88.0  88.0phosphate            (blocking)Tris(2-ethylhexyl)        1,000   1.1        80.0  72.7phosphite            (blocking)Hydrogen sulfide        100     3.0       118.5  39.5                (blocking)______________________________________ 
    
     According to the present invention, coking in hydrocarbon treatment process equipment is suppressed substantially; reduction in heat conduction and heat efficiency as well as hindrance of fluid transfer is alleviated; and the need for frequent suspension of operation and cleaning is reduced. Thus, the present invention has significant effects on energy saving, production rationalization and facility maintenance.