Electrochemical oxidation of hydrogen sulfide

Details Electrochemical oxidation of hydrogen sulfide Electrochemical oxidation of hydrogen sulfide

1999-05-19

2001-06-05

Valentine, Donald R. (Department: 1741)

Electrolysis: processes, compositions used therein, and methods

Electrolytic synthesis

Preparing nonmetal element

C205S639000

Reexamination Certificate

active

06241871

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to electrochemical H
2
S spontaneous conversion to sulfur and water with the concurrent production of electrical energy or to sulfur and hydrogen.
DESCRIPTION OF THE PRIOR ART
Hydrogen sulfide (H
2
S) is a corrosive and extremely toxic gas that is present in natural gas wells at concentrations ranging from a few ppm to 50% or even higher. Hydrodesulfurization of heavy oil and bitumen and coal gasification also produce gas streams containing hydrogen sulfide as an undesirable by-product. At present the approach to removal of H
2
S has been to destroy it by oxidation to steam and sulfur, and not to utilize H
2
S as a hydrogen resource.
A number of processes are available for the removal of H
2
S from natural gas and process gas streams, and for converting it into useful or at least harmless products. Most of these methods are multistage processes that begin with chemical or physical absorption of H
2
S. In practice, H
2
S is usually removed by contacting the process gas with a thin film of a basic organic solvent. The solvent is regenerated by heating in a second unit, and the H
2
S evolved is destroyed using the well-established Claus process. In this process part of the H
2
S is oxidized yielding SO
2
and H
2
O. The SO
2
then reacts with further stoichiometric amounts of H
2
S over an alumina based catalyst to produce elemental sulfur, water and heat. The overall chemical reaction occurs at 525-700° C., and can be summarized as follows:
⅓ H
2
S+½ O
2
→⅓ SO
2
+⅓ H
2
O  (1)
⅔ H
2
S+⅓ SO
2
→S+⅔ H
2
O  (2)
Net reaction:
H
2
S+½ O
2
→S+H
2
O+
Q
  (3)
Although the Claus process is exothermic and generates thermal energy, the heat is generally not utilized and therefore has no economic value. Electrolysis of H
2
S solutions has been considered to be an attractive alternative strategy, due to more favourable thermodynamics compared to water electrolysis. Neither this, nor other approaches recovering hydrogen using thermal catalytic decomposition and membranes for separation, has been commercialized yet, which is partly due to an overall net energy input being required. A far more desirable strategy is to directly electrochemically oxidize hydrogen originating from the H
2
S decomposition reaction. In such a manner, a fuel cell using H
2
S as the feed would generate electrical energy, leaving only sulfur and water as environmentally acceptable product.
There is very little literature information pertaining to electrochemical oxidation of gas phase H
2
S in a fuel cell. A fuel cell using a fuel containing H
2
S is described in Pohl et al., U.S. Pat. No. 3,874,930. The electrolyte was a mineral acid, and the anode comprised MoS
2
or WS
2
admixed with a conductive material. Work has recently been reported in which both yttria- and calcia-stabilized zirconia were used as a solid oxygen ion-conducting electrolyte operated at 900° C. Practical cell voltages were below 0.9 V at current densities of only a few mA. The problem of producing undesirable by-product SO
2
has not been eliminated.
The potential applications of solid state, oxygen ion-conducting membranes for oxidation of H
2
S to sulfur has been described in Sammells, U.S. Pat. No. 4,920,015. The same group has investigated the use of mixed (oxygen-anion and proton) solid conductors in an “electrochemical Claus process”. The findings indicate the possibility that there exists a reforming mechanism to give hydrogen, which subsequently reacts as fuel at the anode. There was a significant decrease in cell voltage when the H
2
S content in inert gas was increased. This finding suggests that elemental sulfur covers the electrocatalytic sites and limits diffusion currents for hydrogen oxidation.
Venkatesan et al., U.S. Pat. No. 4,544,461, aluminosilicate materials (zeolites) were used both as proton conductors and catalytic materials in a H
2
S—O
2
fuel cell. Cell temperatures were <370° C., which appears to be of crucial importance for zeolite conductivity. It was stated that aluminosilicates can be activated to a satisfactory conductivity by partial removal of water. The maximum electromotive force obtained was 0.35 V. The disadvantage of the design seems to be that the porous zeolite structure cannot ensure both high enough conductivity and gas impermeability. In a related system, Li
2
SO
4
was tested as a proton-conducting electrolyte in a 700° C. H
2
S—O
2
fuel cell.
From the above description of the prior art, it can be seen that dissociation of H
2
S exclusively to its elements has not previously been achieved with high efficiency. Thus, no economically viable system has heretofore existed for the electrochemical oxidation of H
2
S exclusively to sulfur and steam with generation of electrical power.
SUMMARY OF THE INVENTION
One embodiment of the present invention relates to a process for gas phase electrochemical oxidation of H
2
S to sulfur and water or steam using an electrolysis cell having an anode chamber on one side of a solid proton conducting membrane and a cathode chamber on the other side of the membrane. The process comprises the steps of passing H
2
S-containing gas through the anode chamber to contact a catalytic anode, where it reacts to produce elemental sulfur, protons and electrons. The protons pass through the membrane from the anode chamber to the cathode chamber. An oxygen-containing gas is passed through the cathode chamber to contact the catalytic cathode, where it reacts with protons and electrons to produce water or steam. During the process, both the anode chamber and cathode chamber are maintained at a temperature of at least 120° C. and an elevated pressure sufficient to keep the membrane moist. Sulfur is obtained in liquid or vapour form and is removed from the anode chamber while water or steam is removed from the cathode chamber. An electric current can be withdrawn from the anode and cathode.
The solid proton conducting membrane may be made from a variety of materials, such as perfluorosulfonic acid or polybenzimidazole. A particularly effective proton conducting membrane is the perfluorosulfonic acid product sold under the trade mark Nafion®.
The anode and cathode may be formed from a variety of different materials, such as carbon products and electrodes made of compressed carbon powder have been found to be particularly effective. These are loaded with a metal catalyst, which may be selected from a large variety of metals, such as Mo, Co, Pt, Pd, Cu, Cr, W, Ni, Fe, Mn, etc. Preferably, the catalyst except Pt and Pd is in the sulfide form. The body of the electrolysis cell may also be formed from a variety of materials such as Teflon, carbon block, metal, etc. Preferably the body of the electrolysis cell is metal for operation at high temperature and pressure.
A preferred anode catalyst according to the invention is a metal sulfide prepared by the sol-gel technique (S. T. Srinivasan, P. Kanta Rao, “Synthesis, Characterization and Activity Studies of Carbon Supported Platinum Alloy Catalysts”,
Journal of Catalysis,
179 (1998) 1-17). Using the sol-gel technique, the metal sulfide is deposited on carbon in a very highly dispersed state. Thus, the particles of active material are each small and well dispersed over all the surface of the carbon. The result of this well dispersed array of very small particles is an increased surface of active catalyst. Moreover, each particle is intimately in contact with the support rather than simply admixed with the carbon. This affects both the chemistry of the particles and the capability to transfer electrons and protons within the anode.
The basic fuel cell according to the invention has the following configuration:
H
2
S/anode/solid electrolyte/cathode/O
2
(g)
The essential components of the reaction mechanism are as follows:
Anode H
2
S→S+2H
+
+2e
−
Cathode ½ O
2
+2H
+
+2e
−
→H
2
O
Cell H
2
S+½ O
2
&ra

Affiliated with

Also associated with

Rating

Electrochemical oxidation of hydrogen sulfide does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Electrochemical oxidation of hydrogen sulfide, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Electrochemical oxidation of hydrogen sulfide will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2447159