Patent Publication Number: US-2006011518-A1

Title: Process for reducing the level of elemental sulfur in hydrocarbon streams

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application claims benefit of U.S. Provisional Patent Application Ser. No. 60/587,916 dated Jul. 14, 2004. 
    
    
     FIELD OF THE INVENTION  
      This invention relates to a process for reducing the level of elemental sulfur and organic sulfur pick-up by refined hydrocarbon streams such as gasoline, diesel, jet fuel, kerosene or fuel additives such as ethers or iso-octane that are transported through a pipeline used to transport various sulfur-containing petroleum streams. The oxygen level of the hydrocarbon stream of interest to be pipelined as well as the oxygen level in at least the first hydrocarbon stream sequenced immediately ahead of the hydrocarbon stream of interest is reduced.  
     BACKGROUND OF THE INVENTION  
      It is well known that elemental sulfur in hydrocarbon streams, such as petroleum streams, is corrosive and damaging to metal equipment, particularly to copper and copper alloys. Sulfur and sulfur compounds may be present in varying concentrations in refined petroleum streams, such as in gasoline and distillate boiling range streams. Additional contamination will typically take place as a consequence of transporting the refined stream through pipelines that contain sulfur contaminants remaining in the pipeline from the transportation of sour hydrocarbon streams, such as petroleum crudes. The sulfur has a particularly corrosive effect on equipment such as brass valves, gauges and in-tank fuel pump copper commutators.  
      The total sulfur in gasoline after 2005 will be limited to less than 30 wppm while the total sulfur in diesel after 2006 will be limited to a maximum of 15 wppm. Elemental and organic sulfur contaminants that are picked-up in the pipeline by gasoline and diesel products will adversely affect their ability to meet the ultra low sulfur specifications. Organic sulfur pick-up is any non-elemental sulfur component in the hydrocarbon stream that was not present in the hydrocarbon product stream prior to injecting it into the pipeline.  
      Various techniques have been reported for removing elemental sulfur from petroleum products. For example, U.S. Pat. No. 4,149,966 discloses a method for removing elemental sulfur from refined hydrocarbon fuels by adding an organo-mercaptan compound plus a copper compound capable of forming a soluble complex with the mercaptan and sulfur and contacting the fuel with an adsorbent material to remove the resulting copper complex and substantially all the elemental sulfur.  
      U.S. Pat. No. 4,011,882 discloses a method for reducing sulfur contamination of refined hydrocarbon fluids transported in a pipeline for the transportation of sweet and sour hydrocarbon fluids by washing the pipeline with a wash solution containing a mixture of light and heavy amines, a corrosion inhibitor, a surfactant and an alkanol containing from 1 to 6 carbon atoms.  
      U.S. Pat. No. 5,618,408 teaches a method for reducing the amount of sulfur and other sulfur contaminants picked-up by refined hydrocarbon products, such as gasoline and distillate fuels, which are pipelined in a pipeline used to transport heavier sour hydrocarbon streams. The method involves controlling the level of dissolved oxygen in the refined hydrocarbon stream that is to be pipelined.  
      The removal of elemental sulfur from pipelined fuels is also addressed in U.S. Pat. No. 5,250,181 which teaches the use of an aqueous solution containing a caustic, an aliphatic mercaptan, and optionally a sulfide to produce an aqueous layer containing metal polysulfides and a clear fluid layer having a reduced elemental sulfur level. U.S. Pat. No. 5,199,978 teaches the use of an inorganic caustic material, an alkyl alcohol, and an organo mercaptan, or sulfide compound, capable of reacting with sulfur to form a fluid-insoluble polysulfide salt reaction product at ambient temperatures.  
      While such methods have varying degrees of success, there still exists a need in the art for reducing elemental and organic sulfur pick-up by hydrocarbon products when transported in pipelines. Reducing the elemental and organic sulfur pick-up in products transported in the pipelines reduces the treating requirements as taught in U.S. Pat. No. 5,250,181, which is incorporated herein by reference. It may even alleviate these requirements if the elemental and organic sulfur pick-up is sufficiently low.  
     SUMMARY OF THE INVENTION  
      In accordance with the present invention, there is provided a process for reducing the level of sulfur picked-up in a hydrocarbon stream of interest being transported in a pipeline that is used to transport various sulfur-containing petroleum streams, which process comprises: (i) reducing the level of oxygen to less than about 30 wppm in the hydrocarbon stream of interest to be transported through a pipeline; and (ii) reducing the level of oxygen to less than about 30 wppm in at least the first hydrocarbon stream transported in the pipeline immediately ahead of the transport of said hydrocarbon stream of interest.  
      In a preferred embodiment of the present invention the oxygen level in both the hydrocarbon stream of interest and at least the first hydrocarbon stream transported immediately ahead of the said hydrocarbon stream of interest is reduced to less than about 10 wppm and a most preferably to less than 1 wppm.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Hydrocarbon streams that are treated in accordance with the present invention are any petroleum or chemical streams that can be transported via a pipeline used to transport various sulfur-containing petroleum streams. Such streams will generally contain from about 10 wppm to about 50 wppm sulfur and they will typically pick-up additional quantities of elemental sulfur as high as 1000 mg sulfur per liter, typically from about 10 to about 100 mg per liter, more typically from about 10 to 60 mg per liter, and most typically from about 10 to 30 mg per liter. In addition to elemental sulfur such streams will also typically pick-up organic sulfur as high as 100 mg sulfur per liter, typically from 1 to 50 mg per liter and more typically from 1 to 20 mg per liter. These streams can be effectively treated by any conventional sulfur-reduction techniques to reduce the elemental and organic sulfur contamination to less than about 10 mg per liter, preferably to less than about 1 mg sulfur per liter, or lower. Hydrocarbon streams transported through pipelines are those streams that have become contaminated with elemental sulfur and/or organic sulfur contaminants as a result of being transported in a pipeline that was previously used to transport sour hydrocarbon streams, such as sour petroleum crudes, sour condensates or products containing high sulfur levels. Residual H 2 S and organic sulfur from the sour crudes will contaminate the pipeline. Non-limiting examples of such hydrocarbon streams that can be treated in accordance with the present invention include gasoline, jet fuel, diesel fuel, kerosene and dialkyl ethers. Alkyl ethers and iso-octane are typically used to improve the octane rating of gasoline. These ethers are typically dialkyl ethers having 1 to 7 carbon atoms in each alkyl group. Illustrative ethers are methyl tertiary-butyl ether, methyl tertiary-amyl ether, methyl tertiary-hexyl ether, ethyl tertiary-butyl ether, n-propyl tertiary-butyl ether, and isopropyl tertiary-amyl ether. Mixtures of these ethers and hydrocarbon streams may also be treated in accordance with this invention.  
      Preferred are refined hydrocarbon streams, particularly those wherein the elemental and organic sulfur picked-up is detrimental to the performance of the intended use of the hydrocarbon stream. The preferred streams to be treated in accordance with the present invention are naphtha boiling range streams that are also referred to as gasoline boiling range streams and distillate boiling range streams that are also referred to as diesel boiling range streams. Naphtha boiling range streams can comprise any one or more refinery streams boiling in the range from about 10° C. to about 230° C., at atmospheric pressure. The naphtha stream generally contains cracked naphtha that typically comprises fluid catalytic cracking unit naphtha (FCC catalytic naphtha, or cat cracked naphtha), coker naphtha, hydrocracker naphtha, resid hydrotreater naphtha, debutanized natural gasoline (DNG), and gasoline blending components from other sources from which a naphtha boiling range stream can be produced. FCC catalytic naphtha and coker naphtha are generally more olefinic naphthas since they are products of catalytic and/or thermal cracking reactions. Hydrocarbon streams boiling in the distillate range include diesel fuels, jet fuels, heating oils, and lubes. Such streams typically have a boiling range from about 150° C. to about 600° C., preferably from about 175° C. to about 400° C.  
      As previously mentioned, such hydrocarbon streams are often transported great distances via a network of pipelines. These pipelines can often transport several hundred thousand barrels of different types of petroleum fluids each day. Each type of petroleum fluid is generally transported simultaneously with one or more other types of petroleum fluids, either upstream, downstream, or both through the pipeline. Because a variety of petroleum fluids (hydrocarbon streams) having various ranges of sulfur are transported through a given pipeline each day, an undesirable amount of elemental and organic sulfur is picked-up by a less sulfur tolerant petroleum fluid, such as gasoline or diesel, from an earlier transported dirtier fluid, such as a crude oil, condensate or high sulfur product.  
      It has been found that reducing the oxygen level in the hydrocarbon stream of interest will result in a lower level of sulfur pick-up when that fluid of interest is transported in a pipeline. There still remains a problem because merely reducing the oxygen level in the hydrocarbon stream of interest may not result in a desired reduction of sulfur pick-up. In accordance with the present invention it has been found by the inventor hereof that not only must the oxygen level be reduced in the hydrocarbon stream of interest before being transported via pipeline, but that is preferred that at least the first and up to the first to third hydrocarbon stream immediately ahead of the hydrocarbon stream of interest should also have the oxygen content reduced as well. It is most preferred that the three, more preferably the two, and most preferably the one hydrocarbon stream immediately transported ahead of the hydrocarbon stream of interest have its oxygen level reduced. By reduced oxygen level we mean that the oxygen level in both the hydrocarbon stream of interest and the hydrocarbon streams ahead of the said hydrocarbon stream of interest, will be reduced to less than 30 wppm oxygen, preferably less than about 10 wppm oxygen, and more preferably to less than about 1 wppm oxygen. By oxygen we mean dissolved molecular oxygen and not atomic oxygen that are part of chemical compounds.  
      Any suitable method can be used to reduce the oxygen level of the hydrocarbon streams. Non-limiting examples of such methods include the use of mercaptans, nitrogen purging, pulling a vacuum on the petroleum fluid, and limiting the exposure of the petroleum fluid of interest to air.  
      It is preferred that a mercaptan be used and that it be mixed with the hydrocarbon stream of interest. It is also preferred that the mercaptan bean organo mercaptan that includes a relatively wide variety of compounds having the general formula RSH, where R represents an organic radical that may be alkyl, alkenyl, cycloalkyl, cycloalkenyl, aryl or arylalkyl having from 1 to about 16 carbon atoms. Thus, the radical may be, for example methyl, ethyl, n-propyl, I-propyl, n-butyl, I-butyl, sec-butyl, t-butyl, amy, n-octyl, decyl, dodecyl, octadecyl, phenyl, benzyl, and the like. Most preferably, RSH is an alkyl mercaptan containing 2 to 5 carbon atoms. The concentration of mercaptan that is added to the stream to be treated will be an effective amount. That is, an amount that will be capable of reducing the level of elemental sulfur by a predetermined amount, preferably by at least about 80 wt. %, more preferably by at least about 90 wt. %, and most preferably by at least about 95 wt. %, of the elemental contaminants. This amount of mercaptan compound will typically range from about 1 wppm to about 1000 wppm, preferably from about 10 wppm to about 100 wppm. In terms of mole ratios, the amount of mercaptan compound will range from about 0.2 to about 20 moles of mercaptan per mole of elemental sulfur in the refined hydrocarbon stream, or the estimated moles of elemental sulfur that will be picked-up by the stream during its transport through a pipeline.  
      In general, the process of the present invention preferably involves the addition of an effective amount of mercaptan to the hydrocarbon stream to be treated. The mercaptan can be added at any time, such as prior to, during, or after the hydrocarbon stream has been transported through a pipeline. It is preferred to mix the mercaptan with the stream prior to its being transported through a pipeline. Conditions at which the stream is treated with the mercaptan will be relatively mild conventional conditions. That is, the mercaptan is added when either or both the product stream and mercaptan are at a temperature from about ambient temperature (about 22° C.) to about 100° C., or higher. Substantially atmospheric pressures are suitable, although pressures may, for example, range up to about 1000 psig. The contact time will be an effective contact time. Contact times may vary widely depending on the particular hydrocarbon product stream to be treated, the amount of elemental sulfur present, and the particularly mercaptan used. The contact time will be chosen to affect the desired degree of mixing and subsequent elemental sulfur reduction. In most cases, contact times ranging from about a few hours to a few days will be adequate. The reaction proceeds faster with aliphatic mercaptans than with aromatic mercaptans. Lower carbon number mercaptans will react faster than the higher carbon number mercaptans.  
      It is preferred that the mercaptan be adequately mixed with the hydrocarbon product stream to be treated. For example, if the mercaptan is added prior to the product stream being pipelined, the transportation of the product stream through the pipeline will provide adequate mixing of the mercaptan with the hydrocarbon product stream. If the mercaptan is added to the product stream after it is pipelined, then it is preferred to use a suitable mixing devise, such as a static mixer wherein the mercaptan is injected into a moving flow of hydrocarbon product stream prior to entry into the static mixer.  
      The following examples are illustrative of the invention and are not to be taken as limiting in any way.  
     EXAMPLE 1  
      A 5% H 2 S in N 2  mixture was bubbled through four liters of crude at 4 ft 3 /hr for approximately one hour in order to replace any H 2 S that may have evolved from the original crude sample. The H 2 S/crude mixture was then recycled overnight at 10 cc/min through a 0.75″ OD×18″ long column of iron filings (˜40 mesh) to simulate the crude cycle in a pipeline. The packed bed of iron filings was used to simulate the pipeline wall effects. The column was then subjected to a series of refined products to simulate the product cycle in a pipeline. The following product cycle was tested: (1) 300 cc of ultra low sulfur diesel (ULSD) initially containing 0.5 mg/l total sulfur and 0 mg/l of elemental sulfur (S°); (2) 1 liter of MTBE initially containing 9.3 mg/l total sulfur and 0 mg/l of elemental sulfur (S°); (3) four liters of ULSD initially containing 0.5 mg/l total sulfur and 0 mg/l of S°; and (4) four liters of regular unleaded gasoline (RUL) initially containing 1.7 mg/l total sulfur and 0 mg/l of S°. All products were first N 2 -purged to displace oxygen and then pumped through the column of iron filings on a once-through basis at 50 cc/min. Following the product cycle the column of iron filings was then flushed with air-purged toluene to remove residual sulfur compounds from the iron filings. The total sulfur content in the total sample of RUL exiting the column was determined by ASTM D5453. The S° content in the total sample of RUL exiting the column was determined by HPLC. The total sulfur pick-up in the RUL was determined by the subtracting the total sulfur content in the RUL exiting the column minus the total sulfur content in the original RUL. The S° pick-up was determined in a similar fashion. The organic sulfur pick-up in RUL was determined by subtracting the total sulfur pick-up minus the S° pick-up. The sulfur pick-up in ULSD was determined by the same methodology described above for RUL.  
     EXAMPLE 2  
      A 5% H 2 S in N 2  mixture was bubbled through four liters of crude at 4 ft 3 /hr for approximately one hour in order to replace any H 2 S that may have evolved from the original crude sample. The H 2 S/crude mixture was then recycled overnight at 10 cc/min through a 0.75″ OD×18″ long column of iron filings (˜40 mesh) to simulate the crude cycle in a pipeline. The packed bed of iron filings was used to simulate the pipeline wall effects. The column was then subjected to a series of refined products to simulate the product cycle in a pipeline. The following product cycle was tested: (1) 300 cc of ultra low sulfur diesel (ULSD) initially containing 0.5 mg/l total sulfur and 0 mg/l of elemental sulfur (S°); (2) 1 liter of MTBE initially containing 9.3 mg/l total sulfur and 0 mg/l of S°; (3) four liters of ULSD initially containing 0.5 mg/l total sulfur and 0 mg/l of S°; and (4) four liters of regular unleaded gasoline (RUL) initially containing 1.7 mg/l total sulfur and 0 mg/l of S°. The ULSD and RUL products were first N 2 -purged to displace oxygen and then pumped through the column of iron filings on a once-through basis at 50 cc/min. The MTBE was first air-purged to saturate the product with oxygen and then pumped through the column of iron filings on a once-through basis at 50 cc/min. Following the product cycle the column of iron filings was then flushed with air-purged toluene to remove residual sulfur compounds from the iron filings. The total sulfur content in the total sample of RUL exiting the column was determined by ASTM D5453. The S° content in the total sample of RUL exiting the column was determined by HPLC. The total sulfur pick-up in the RUL was determined by the subtracting the total sulfur content in the RUL exiting the column minus the total sulfur content in the original RUL. The S° pick-up in RUL was determined in a similar fashion. The organic sulfur pick-up in RUL was determined by subtracting the total sulfur pick-up minus the S° pick-up. The sulfur pick-up in ULSD was determined by the same methodology described above for RUL.  
               TABLE 1                          Effect of Nitrogen-Purging Products on Sulfur Pick-up                             Example 1   Example 2                                             MTBE Purge   N 2     Air           ULSD Sulfur Pick-up           Total Sulfur Pick-up, wppm   4   7           S° Pick-up, wppm   1   2           Organic Sulfur Pick-up, wppm   3   5           RUL Sulfur Pick-up           Total Sulfur Pick-up, wppm   5   9           S° Pick-up, wppm   1   2           Organic Sulfur Pick-up, wppm   4   7                      
 
      Table 1 shows that very low sulfur pick-up values were found in ULSD and RUL when both the products and the MTBE transported ahead of the products were purged with nitrogen. However, a significant increase in the total sulfur pick-up (˜80%) was observed in both the ULSD as well as the RUL when the MTBE ahead of these products contained oxygen (i.e., air-purged). As shown in Table 1, both the elemental and organic sulfur pick-up increased when the MTBE ahead of these products contained oxygen.