Abstract:
A method for treating mercury-contaminated aluminum surfaces to render aluminum-mercury amalgam and liquid mercury harmless to aluminum is provided. The method involves converting the mercury to a compound which is inert to aluminum, such as mercuric sulfide, at temperatures below the freezing point of water. This is accomplished by contacting the aluminum surface with a condensable solvent such as carbon disulfide, carbonyl sulfide, or tetrahydrofuran having gaseous hydrogen sulfide dissolved therein, and then dissolving a reactive gas such as sulfur dioxide within the solvent. The reaction of the two dissolved gases within the solvent generates the sulfur which reacts with the mercury to form mercuric sulfide.

Description:
NATURE OF INVENTION 
     This invention relates to a method for rendering nonreactive and noncorrosive the mercury and mercury amalgams which accumulate in gas processing equipment. More importantly this invention relates to a method for restoring the ductility of aluminum bodies, particularly aluminum tubing whose metallurgical properties have been damaged by exposure to mercury. 
     BACKGROUND OF THE INVENTION 
     The material of choice for heat exchangers in LNG and LPG plants is aluminum. Aluminum is chosen primarily because of its high thermal conductivity, ease of fabrication, reasonable cost and availability. A difficulty arising from the use of aluminum to construct heat exchangers for LNG plants is the fact that all natural gas contains measureable amounts of mercury. Mercury is hazardous to aluminum equipment because the mercury promotes rapid deterioration of the aluminum once the mercury penetrates the coating of aluminum oxide usually present on aluminum surfaces. The aluminum becomes embrittled and is subject to failure when tensile stresses are applied. 
     Attempts to cope with the problem of mercury contamination in natural gas have been based on either replacing the equipment from time to time, flaring the gas, or diluting it with an inert gas. None of these approaches has been particularly successfully either from a technical or economic view point. 
     Plants processing gases of relatively high mercury concentration, for instance greater than one part per billion, usually treat incoming feed gas to reduce the mercury concentration to less than 0.1 parts per billion. Nevertheless, over a number of years of operation mercury accumulates on the aluminum surfaces in sufficient quantity to become potentially hazardous and engender failure. 
     Ordinarily the incoming feed gas is passed through beds of activated charcoal containing elemental sulfur deposited thereon at moderately elevated temperatures. The elevated temperature is required to promote the direct reaction of mercury vapor and sulfur to form mercuric sulfide. As noted previously, however, some mercury escapes from this treatment system and in time permeates throughout the rest of the plant, particularly in areas where the equipment is made from aluminum or aluminum alloys. A method to convert the aluminum-mercury amalgam formed and the accumulated liquid mercury is a highly desirable objective. 
     Thus, a primary object of this invention is to provide a physical-chemical treatment for aluminum heat exchangers and other aluminum equipment that has become contaminated with mercury. Another object of the invention is to restore the ductility of aluminum components, particularly aluminum tubing that has been exposed to mercury and thereby damaged. Still, another object of this invention is to decontaminate the aluminum surface during the time when the equipment is not in use, that is during the normal derime time for gas liquefaction equipment. Another object is to accomplish this decontamination without diminishing either mechanical properties or heat transfer properties of the equipment. 
     SUMMARY OF THE INVENTION 
     Briefly stated this invention comprises a process for treating a mercury contaminated surface or a surface on which mercury amalgam is present. The process comprises the steps of: (1) contacting the surface of the equipment with a solvent which is nonreactive with aluminum and mercury but which will dissolve a sulfur-containing reactive material such as hydrogen sulfide; (2) contacting the liquid coated surface with hydrogen sulfide; and (3) contacting the liquid wet surface with anhydrous ammonia or sulfur dioxide. All the preceding operations are carried out preferably at temperatures below the freezing point of water (0° C.) and above a temperature of -40° C. Alternatively steps (1) and (2) can be combined by bubbling the hydrogen sulfide through the solvent and condensing a gas saturated solvent on the surface of the aluminum surface. Also, if desirable the hydrogen sulfide can be diluted by the inclusion of a non-reactive gas such as nitrogen. 
     DETAILED DESCRIPTION OF THE INVENTION 
     As indicated above the equipment contemplated for the treatment described herein includes aluminum heat exchangers in which expansive surfaces are exposed to contact with mercury-contaminated gas. It is recognized that there may be other instances however, when it will be desirable to treat an aluminum surface which has been contaminated with mercury. 
     The solvent used to contact the aluminum surface can be any solvent which will dissolve hydrogen sulfide. Such solvents include carbon disulfide, carbonyl sulfide, tetrahydrofuran or any liquid which remains liquid at these low temperatures and is a solvent for gases such as hydrogen sulfide, sulfur dioxide and ammonia. The solvent is applied to the surface by either spraying it with solvent or by vaporizing the solvent and allowing it to condense on the chilled surface. The preferred range of temperature for this operation is 0° C. to -40° C. Following the coating of the aluminum surface with the solvent, the hydrogen sulfide is applied to the surface. Sufficient time should be allowed to enable the surface to absorb the hydrogen sulfide, after which time any excess hydrogen sulfide can be removed by purging with a gas such as nitrogen, over the surface. As indicated previously the application of solvent and introduction of hydrogen sulfide gas can be combined in a single step wherein the solvent is vaporized by spraying with the hydrogen sulfide and flowing the mixture over the mercury contaminated surface. The hydrogen sulfide can be diluted with a carrier gas such as nitrogen. 
     Following this operation anhydrous ammonia or sulfur dioxide gas is then flowed over the surface and allowed to contact and react with the solvent and hydrogen sulfide present therein. The result of this combination of reactants is that the mercury present in droplet form, as well as the mercury amalgam, is coated on the exterior with a mercury oxide or a mercuric sulfide coating. Following this operation, the system can be again purged of ammonia or sulfur dioxide gas and is then returned to service or is further subjected to deriming procedures as may be appropriate. 
    
    
     EXAMPLES 
     Example 1 
     To demonstrate the effectiveness of this invention the exterior surfaces of five (5) aluminum tubes were first exposed to or contacted with mercury vapor providing an opportunity for the mercury to form an amalgam on the aluminum surface and to otherwise contaminate it. Four of the tubes were then further treated by condensing vaporized carbon disulfide on to the tubes. Three (3) of the tubes with the carbon disulfide wet surface were exposed to an atmosphere of hydrogen sulfide for a period of 60 minutes. No visible reaction occurred during this period. The three tubes were then contacted briefly with nitrogen to purge any hydrogen sulfide gas present and were then contacted with ammonia. Immediately a yellow compound, ammonium sulfide, formed on the surface of the tubes and subsequently, a black material formed on the mercury surfaces present. The tubes were then allowed to warm from the temperature of -4° F. to room temperature and were tested in an ATS machine. The ATS machine determines tensile strength, and the number for reduction in area provided in the following table is one of several mechanical properties that demonstrates embrittlement or the lack of embrittlement. A higher number (over 36) indicates less embrittlement and restoration of lost ductility. Results of these test runs are shown in the following Table. 
     
                       TABLE 1______________________________________Run No.   Treatment       Reduction in Area______________________________________130T      (Control) No induced                     41.8%     mercury contamination131T      (Control) No induced                     41.9%     mercury contamination151T      Liquid mercury  24.4%     in crevice153T      Liquid mercury. 38.8%     in crevice; H.sub.2 S in     CS.sub.2 followed by NH.sub.3154T      Liquid mercury in                     43.0%     crevice; H.sub.2 S in     CS.sub.2 followed by NH.sub.3______________________________________ 
    
     Example 2 
     A similar test was made in which sulfur dioxide was used in place of the ammonia in the previous example. Sulfur dioxide is preferred over ammonia. The test procedure was the same as that in the previous example except that after the solvent-contacted surface had been saturated with hydrogen sulfide, the hydrogen sulfide was vented and the carbon disulfide-hydrogen sulfide wet surface was contacted with sulfur dioxide The SO 2  reacts with the hydrogen sulfide. After sufficient contact time with the sulfur dioxide, the tubes were again vented of sulfur dioxide and tested for tensile strength. Results listed in the following Table 2, show that this treatment also resulted in restoration of ductility. The reaction between hydrogen sulfide and sulfur dioxide is surprising, since it has been formerly thought the two would react only at elevated temperatures and in the presence of a catalyst. The stoichiometry of the reaction is believed to be 2H 2  S(CS 2 )+SO 2  (→3S+2H 2  O. 
     
                       TABLE 2______________________________________Run No. Treatment     Reduction in Area                              Results______________________________________   None (control)                 41.3%    (3)*  DuctileV44     Liquid mercury,                 34.4     (8)   Embrittled   vapor depositedV45     H.sub.2 S through CS.sub.2                 41.6     (5)   Ductility   plus SO.sub.2 for 15         restored   minutes at -53° C.V48     H.sub.2 S through CS.sub.2                 39.1     (4)   Ducti1ity   plus SO.sub.2 for 118        restored   minutes at -25° C.V48A    Same as V48, except                 35.1     (4)   Embrittled   only for 56 minutes______________________________________ 
    
     EXAMPLE 3 
     A third series of tests similar to those described previously were performed under the conditions outlined in Table 3. These tests were made to help in determining the optimum conditions for carrying out the process of this invention. 
     
                                           TABLE 3__________________________________________________________________________Run Number    V45            V53               V52                  V54                     V56                        V49                           V55                              V50                                 V57__________________________________________________________________________Temp. °C. of Al         -55            -35               -35                  -55                     -35                        -55                           -55-                              -35                                 -62tubing during treatmentTemp. °C. of CS.sub.2 source         -15            -5 -15                  -5 -15                        -5 -15                              -5 -13.5during treatmentGas bubbled through CS.sub.2         H.sub.2 S            H.sub.2 S               SO.sub.2                  SO.sub.2                     H.sub.2 S                        H.sub.2 S                           SO.sub.2                              SO.sub.2                                 H.sub.2 SMolar ratio of H.sub.2 S to         1:1            1:1               1:1                  1:1                     4:1                        4:1                           4:1                              4:1                                 3.8:1SO.sub.2 flowed through systemRate, Ft.sup.3 /hr of N.sub.2         0.1            0.9               0.9                  0.1                     0.1                        0.9                           0.9                              0.1                                 0.1carrier through systemTime, minutes allowed for         15 105               15 105                     105                        15 105                              15 37.5(H.sub.2 S, SO.sub.2, CS.sub.2)to flow through systemDrying time before         8  8  40 40 40 40 8  8  4.8tensile testing, hoursReduction in cross         41.64            24.98               21.02                  33.14                     38.70                        30.46                           29.58                              36.96                                 40.3section at tensilefailure, percent__________________________________________________________________________ 
    
     The test designated V-45 shows what is deemed to be an example of the optimum conditions. In Runs V-53, -52, -49, and -55 the rate of carrier gas injection was too great so that the H 2  S and SO 2  did not have time to react. Also in V-52, and -55 the temperature of the CS 2  source through which the SO 2  was bubbled was less than the liquefaction temperature of SO 2  (-10° C.) so that much of it remained liquid and did not react. V-57 represents the optimum conditions for performing the test. 
     EXAMPLE 4 
     This example illustrates the use of tetrahydrofuran (butylene oxide) as a solvent for H 2  S and SO 2 . A test (V59) was conducted under conditions the same as those of test V57, Table 3 using tetrahydrofuran as the solvent. However, the temperature of the tetrahydrofuran was increased to 4° C., to raise its vapor pressure to a value equivalent to that of the carbon disulfide in use in run number V-57. Five specimens were tested and showed an average of 40.0 percent reduction in cross section when tested for tensile strength. This value demonstrates a remarkable restoration of tensile strength.