Gas desulphuring plants applying Liquid Phase Oxidation process
The process belongs to recoverable methods of gas mixture desulphurization. It is based on hydrogen sulphide oxidation to elemental sulphur with water solutions of iron chelate compounds with their subsequent regeneration with the air.
Standard LPO process diagram
Brief description and basic technical specifications
CHEMICAL REACTIONS DURING THE PURIFICATION PROCESS
Overall reaction of H2S catalytic oxidation:
2H2S + O2 = 2S + 2H2O (1)
The process of oxidation has 2 stages.

On the first stage the feed gas flows through the water solution of the catalyst based on iron chelates, with preset concentration of iron. The hydrogen sulphide interacts with ferrous ions (the degree of oxidation +3), with generation of finely dispersed elemental sulphur and a solution with ferrous ions (the degree of oxidation +2):
2Fe3+ + H2S = S + 2Fe2+ + 2H+ (2)
There is practically no interaction between the ferrous ions and other APG components, since the catalyst selectively acts on the hydrogen sulphide. Elemental sulphur is filtered from the solution.
4Fe2+ + O2 + 4H+ = 4Fe3+ + 2H2O (3)
The regenerated solution flows back to the first stage.

Despite of the resisting agents, in course of the regeneration process according to the formula (3) the chelating agent gradually degrades due to the action of hydroxyl radicals. Besides some chemical reagents are removed from the process along with the elemental sulphur, thus reducing Fe2+ and Fe3+ content in the solution and decrease of redox potential of the solution. Therefore the system should be regularly replenished with fresh catalyst solution.
The exhausted solution flows to the second stage of oxidation, in course of which the ferrous ions Fe 2+ are regenerated by the atmospheric oxygen:
Functional operation range of this technology corresponds to gas consumption parameters 30 to 110% of the rated capacity. The top level of hydrogen sulphide content in the purified gas is unlimited and enables treatment of gases with hydrogen sulphide concentration up to 100%. However, in the context of economic feasibility and advantages over alternative technologies, this method is limited by the maximum output of the product sulphur about 12 t/day.

Liquid phase oxidation of hydrogen sulphide ensures APG purification to hydrogen sulphide concentration not more than 3 mg/m3 due to high rate and irreversibility of hydrogen sulphide redox reaction with Fe3+ ions regardless of process environment and gas blend composition.
The LPO process has a number of advantages:
Ensures reduction of hydrogen sulphide to requirements of the Company Standard СТО Газпром 089−2010 and lower regardless of operation pressure of the feed gas, which is one of the major factors giving the edge over competing technologies, in particular, amine scrubbing or alkali purification;
No concurrent side reactions of the absorbent (the catalyst solution) with other, sulphur - free hydrocarbon gas traces, including СО2;
Provides for high efficiency of hydrogen sulphide stripping due to irreversibility and high rate of hydrogen sulphide redox reaction with chelate iron compound.
Ecological safety of the catalyst solution;
Since this technology is unlimited in terms of minimum pressure of the purified gas, when a transportation line for the gas flow after its treatment is provided to its destination point, so it can be applied for onsite desulphurization of gases from the final stage of oil separation with further utilization of the gases for auxiliary needs (when necessary, supplemented with pressure boost and additional treatment).
Lower power consumption is ensured for pumping equipment applied for absorbent compression after its regeneration, from atmospheric to operation pressure due to efficient performance of the technology under a lower gas pressure.

Minimization of thermal energy ensured due to heatless regeneration of the catalyst (primary heat supply is needed only at the startup of the station).
Combination of technological features within the hydrogen sulphide LPO process provides for the higher energy efficiency for the facilities compared to the facilities where amine scrubbing is applied:
As considerable advantages of the LPO technology, it is important to note its high reliability, robustness to variations in gas flows and composition blend of the feed gas. An additional advantage of plants with bubble-type absorbers is their temporary operational capability at external power supply tripping.
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