Process for Removing Residual Mercury From Treated Natural Gas

This invention relates to a method for purifying and removing trace amounts of mercury from natural gas.

Trace quantities of mercury are known to exist in natural gases but the significance of these trace quantities has not been recognized until recently. The mercury detected in the produced gas is now known not to result from well drilling or well completion operations and does not result by accident in the gas stream. The mercury is produced in association with the gas and is thought to originate from geologic deposits in which the natural gas occurs. Even in trace quantities however, mercury is an undesirable component of natural gas. The processing of natural gas in LNG plants requires contact of mercury-laden natural gas with some equipment made primarily of aluminum. This is particularly true after the steps of treating the gas to remove carbon dioxide and hydrogen sulfide when the gas is chilled or cooled in aluminum-constructed heat exchangers. Aluminum heat exchangers represent a capital investment of several million dollars. Damage to these exchangers is to be avoided if at all possible. Although the concentration of mercury as it amalgamates with the aluminum. The result can be damage to the system, particularly the heat exchangers, such as corrosion cracking leading to equipment failure, fires and similar catastrophe. Repair is correspondingly difficult because of damage to the welded seams of the aluminum. Replacement of the heat exchangers in an LNG plant represents a large expenditure. The problem of mercury in natural gas is discussed further in U.S. Pat. No. 4,094,777 and French Pat. No. 2,310,795, both of which are incorporated herein by reference.

Several methods have been proposed for absorbing mercury from natural gas. For example, J.E. Leeper in Hydrocarbon Processing, Volume 59, November, 1980, pages 237-240, describes a procedure wherein natural gas is contacted with a fixed bed of copper sulfide on an alumina-silica support to remove the mercury present. Another commercial process is based on contacting the mercury-contaminated gas with sulfur supported on activated carbon. According to the Leeper article, the sulfur impregnated activated charcoal process is regarded as the best system for treating a gas stream, particularly one free of heavy hydrocarbons. The reference, Hydrocarbon Processing, Volume 59, November, 1980, pages 237-240, is incorporated herein by reference.

A primary object of this invention is to provide an improved process for removing trace quantities of mercury present in a gas, particulary a natural gas.

Briefly stated, this invention comprises reducing the concentration of mercury in a gas by first contacting it with an aqueous solution of an alkali polysulfide such as sodium polysulfide (Na.sub.2 S.sub.x) and subsequently contacting the gas with a cobalt salt supported on a suitable support. Therafter a gas substantially depleted of mercury and of hydrogen sulfide is recovered for subsequent processing.

As stated above, the initial step in treating the gas containing mercury is to contact it with an aqueous solution of a water soluble alkali polysulfide. Preferably the water soluble alkali polysulfide is sodium polysulfide and the concentration of sulfur in the polysulfide solution, is between a 5 and 20%. The gas to be treated is contacted with the aqueous polysulfide solution in several different ways. A preferred method is to introduce the gas into the bottom of a contact tower such as a bubble tower or packed tower and to inject the aqueous solution of polysulfide near the top of the tower in countercurrent flow so that the gas thoroughly contacts the aqueous polysulfide solution. The effluent gas is then transferred to a contact bed where the gas contacts a cobalt salt supported on a non-reactive carrier material such as alumina, calcium sulfate, or a silica. Any soluble cobalt salts can be used. These include chloride, bromide, sulfate, and nitrate sulfide forms. The concentration of cobalt salt on the non-reactive carrier preferably is between about 1.0 and 20.0 percent by weight.

The gas subsequently is withdrawn and treated to remove any moisture or other undesirable materials present in the gas. The temperature at which this process is carried out can be ambient, i.e., about 70.degree. F. and the pressure can be at or near atmospheric pressure. Temperature and pressure conditions can also be at the existing process conditions of the existing process plant that utilizes the elements of this treating process, i.e. polysulfide washing and cobalt salt treating.

Gases to be treated according to this invention ordinarily will comprise methane although other gases such as nitrogen or other nonreactive gases contaminated with mercury may also be considered for this treatment.

Nitrogen gas containing 0.03 ppb of mercury and 0.3% of carbon dioxide was treated by bubbling it through a solution of sodium polysulfide containing 4.4% by weight of sodium polysulfide. The effluent gas was determined to have a mercury content of less than 0.01 ppb.

Nitrogen gas containing 0.03 ppb of mercury and 0.3% of carbon dioxide was flowed through a bed containing about 50 grams of calcium sulfate (CaSO.sub.4) which has been impregnated with about 1.5 grams of cobalt sulfide. Nitrogen gas leaving the cobalt sulfide bed was determined to have a mercury content of 0.013 ppb. This test demonstrates that cobalt sulfide can remove mercury from a mercury-contaminated gas.

Nitrogen gas containing 0.03 ppb of mercury and 0.3% of CO.sub.2 was treated by contacting it first with an aqueous solution of sodium polysulfide containing 4.4 weight percent of polysulfide. The effluent gas was subsequently passed through a pack of about 10 grams of calcium sulfate impregnated with about 0.3 grams of cobalt chloride. The effuent gas was determined to have a mercury content of less than 0.01 ppb. It was noted that the gas upon contacting the calcium sulfate which was impregnate with cobalt chloride reacted and yielded a black colored salt (cobalt sulfide). The presence of hydrogen sulfide in the gas leaving the polysulfide treater was determined by noting the reaction of the gas with paper impregnated with lead acetate and by the distinct odor of rotten eggs.

The solium polysulfide solution is prepared from:

and elemental sulfur, by heating a solution of Na.sub.2 S.9H.sub.2 O in water with the desired amount of sulfur to provide a solution containing Na.sub.2 S.sub.x. Typically such aqueous solutions contain between 15 and 25% sulfur.