Elemental sulfur recovery from oil and gas refined acid gases including tail gas cleanup
- 1. Direct Claus process including treatment of acid gas with increased ammonia content.
- 2. Process of direct hydrogen sulphide oxidation into elementary sulfur on catalyst layer.
- 3. Sulfur recovery using «1/3 – 2/3» method.
- 4. Sulphur recovery process with partial burning of the recovered sulphur.
- 5. Sulfur recovery process with partial process gas recycling.
- 6. Intensification of Claus process by air enrichment with oxygen.
- 7. Cleanup of tail gas from sulfur recovery unit based on further adsorption catalytic processing using Claus reaction.
- 8. Cleanup of tail gas from sulfur recovery unit based on sulfur compounds hydrogenation to hydrogen sulphide.
- 9. Cleanup of tail gas from sulfur recovery unit based on sulfur compounds oxidation to sulfur dioxide.
- 10. Processes of sulfur treatment and production of market products
Up to last century sixties the basic sources of industrial sulfur was production of native sulfur and sulfur recovered in metal smelting from sulphide ores. Oil and gas with increased sulfur content have started to be actively involved in the treatment process since the middle of last century. Bypass product of hydrocarbon feed cleaning was acid gas which was routed for sulfur recovery. Now, over 90% of sulfur is recovered using this method.
It was possible to recover up to 80 – 90% of hydrogen sulphide in the first sulfur recovery units. Remaining hydrogen sulphide was burnt in the furnaces and vented into atmosphere in the form of sulfur dioxide.
Emissions environmental requirements have greatly increased since that time. Various hydrocarbon feed including the one with increased sulfur content is subjected to treatment owing to requirements of industry. Therefore, acid gas consumption and its hydrogen sulphide content vary over a huge range. As a consequence, a large variety of gas sulfur recovery and tail gas cleanup processes has been developed.
Main parameters governing the structural scheme of sulfur recovery unit are as follows: content of hydrogen sulphide and consumption of acid gas, presence and nature of its impurities, required conversion rate of hydrogen sulphide to sulfur. At the present time, all the accumulated experience allows designing of sulfur recovery unit that satisfies the most stringent environmental requirements in respect of actually any acid gas.
1. Direct Claus process including treatment of acid gas with increased ammonia content.
НаверхThis method of sulfur recovery is based on partial oxidation of acid gas hydrogen sulphide through its burning in the air whose amount is not sufficient for complete combustion. In this process, up to 75% of sulfur contained in the initial hydrogen sulphide is capable to be recovered in the thermal reactor firebox. Further sulfur recovery is carried out on catalyst at catalytic stages and when required, in tail gas cleanup unit. The process is applied if hydrogen sulphide concentration in acid gas is 45 – 100% mol.
Presence of ammonia in acid gas can adversely affect the unit performance. Special measures are therefore taken for its burning to nitrogen in thermal stage. When using this process, the rate of hydrogen sulphide conversion to sulfur depends on hydrogen sulphide concentration in acid gas and is around 95 – 96% for the scheme with two catalytic stages and around 97 – 98% for three-stage process scheme.
2. Process of direct hydrogen sulphide oxidation into elementary sulfur on catalyst layer.
НаверхThe process consists of hydrogen sulphide reaction with air oxygen on special catalyst. The process is applied when hydrogen sulphide concentration in acid gas ranges between 0.1 – 0.5 and 9% mol. As for low concentrations of hydrogen sulphide – up to 3% mol – the process is implemented as a single stage, if the concentration rises the number of stages is increased. Generally, conversion rate of hydrogen sulphide to sulfur does not exceed 85% in direct oxidation unit without tail gas cleanup.
3. Sulfur recovery using «1/3 – 2/3» method.
НаверхThe process is applied if hydrogen sulphide concentration in acid gas is 20 – 45% mol. The process consists in partial acid gas (up to 2/3 of the total amount) bypassing thermal reactor directly into catalytic reactor. The remaining acid gas is burnt in thermal rectors in conditions ensuring complete combustion of hydrogen sulphide. Therefore, the sulfur is only recovered in catalytic stages, and these may be a few ones. The process has technologic disadvantages and thus its utilization is limited. The total conversion rate of hydrogen sulphide to sulfur for this process depends upon a number of catalytic stages and composition of acid gas fed for treatment.
4. Sulphur recovery process with partial burning of the recovered sulphur.
НаверхThe process is applied if hydrogen sulphide concentration in acid gas is lower 20% mol. The process consists in formation of sulfur dioxide required for Claus reaction by burning the part of recovered sulfur. This technology has the same disadvantages as «1/3 – 2/3» process but in some cases this process is a preferred one. The total conversion rate of hydrogen sulphide to sulfur in this process depends upon a number of catalytic stages and can reach 98%.
5. Sulfur recovery process with partial process gas recycling.
НаверхThe process is applied art low content of hydrogen sulphide in acid gas. In this process a part of reacted gas is fed to catalytic reactor inlet in order to reduce excessive heating of this reactor in the course of sulfur recovery, Special catalyst and gas blower are used in this process. Hydrogen sulfide conversion rate is determined by number of catalytic stages.
6. Intensification of Claus process by air enrichment with oxygen.
НаверхUse of oxygen and oxygenated air in Claus process results in increase of gas burning temperature in ceramic reactor and reduction of combustion products volume. This governs the oxygen utilization in this process: 1) to intensify burning of “weak” acid gas, i.e. gas with low hydrogen sulphide content, and 2) to reduce capital expenditures in designing the new unit or increasing the efficiency of existing one. Oxygen utilization has a number of special features that shall be taken into consideration in the unit design. Economic feasibility of oxygen utilization shall be calculated for each particular case based on specific conditions of each plant.
7. Cleanup of tail gas from sulfur recovery unit based on further adsorption catalytic processing using Claus reaction.
НаверхThis group of processes includes continuation of Claus reaction on catalyst at low temperature with simultaneous condensation of recovered sulfur vapors. Since the sulfur is adsorbed by catalyst, special requirements are imposed to it by contrast with normal catalyst in Claus process. The process is batchwise – as soon as some amount of sulfur is adsorbed the catalyst is subjected to regeneration by stripping sulfur with hot gas. 2 or 3 catalytic reactors operating within a single cycle are installed thus providing continuous process run: one – in regeneration phase, and another one (ones) – in adsorption phase. The total conversion rate of hydrogen sulphide using this process considerably depends on process accuracy. Theoretical conversion rate is around 99.6 – 99.7%, actually it does not exceed 99.5%.
8. Cleanup of tail gas from sulfur recovery unit based on sulfur compounds hydrogenation to hydrogen sulphide.
НаверхIn this group of processes the tail gas of Claus process is treated on special catalyst in reducing medium. All sulfur compounds (SO2, COS, CS2, and sulfur – vapor and drop) are reduced to hydrogen sulphide. Then, excessive water is removed from the process gas and after that differential adsorption of hydrogen sulphide is taken place in amine absorber. In the course of amine solution regeneration, the hydrogen sulphide is removed and fed into thermal reactor of Claus process. Total degree of hydrogen sulphide recovery can reach 99.9% and up under this process. Centralized regeneration of amine solution within the refinery OSBL facilities significantly improves the process economic performance.
9. Cleanup of tail gas from sulfur recovery unit based on sulfur compounds oxidation to sulfur dioxide.
НаверхIn the course of this group of processed gaseous sulfur compounds (H2S, COS, CS2, and sulfur – vapor and drop) are oxidized to sulfur dioxide. Hereafter, sulfur dioxide in the presence of special catalyst is oxidized to sulfuric anhydride which reacts with water forming diluted sulfuric acid. The acid can be consumed within the refinery or may be routed to firebox of Claus thermal reactor for burning. Total degree of hydrogen sulphide recovery can reach 99.9% and up under this process.
10. Processes of sulfur treatment and production of market products
НаверхSulfur produced under the Claus process leaves the unit in melted form at above 125оС. Generally it contains dissolved hydrogen sulphide in amounts up to 200 – 300 ppm (by weight). Before now such sulfur was discharged on specially prepared sites and after solidification it was loaded on motor or railroad vehicles using bulldozer. Field experience has shown that such a treatment of produced sulfur has given rise to fires, explosions, poisoning of service personnel and environmental contamination. Nowadays, nearly all produced sulfur is subjected to additional treatment prior to the transportation to the consumer.
10.1. Liquid sulfur degassing.
НаверхThe purpose of this process is to reduce hydrogen sulphide content in molten sulfur down to 10 ppm (by weight) and lower. But according to applicable regulations only this sulfur could be transported in liquid state by motor or railroad vehicles. Besides, sulfur degassing is an obligatory stage preceding its treatment at granulation unit.
There are several industrially proven sulfur degassing processes, which differ in technological implementation, catalyst availability, stripping gas, equipment, etc.
10.2. Sulfur granulation
НаверхGranulated sulfur is sulfur composed of homogenous particles with diameter from 1 to 5 mm. Presence of lower size particles and sulfur dust is inacceptable. Granulated sulfur is convenient for the consumer and transportation; essentially it does not generate dust during handling operations.
There are several sulfur granulation processes existing. Among them the most prevailing ones are as follows:
- belt cooling – sulfur is fed by metered drops on steel water-cooled conveyor-type belt;
- drum type – sulfur granules are enlarged by contact of cold sulfur particles with dispersed liquid sulfur in rotating drum;
- tower type – sulfur granules are crystallized during counterflow movement of liquid sulfur drops and air;
- wet granulation – sulfur granules are formed under contact of liquid sulfur drops with water.
Selection of granulation unit type mainly depends on sulfur production capacity, requirements to granules quality based on technical and economic parameters of process.
10.3. Organization of stations for liquid sulfur loading into motor and railroad vehicles.
НаверхIn many cases it is profitable for consumers to use sulfur in liquid state. In this case, the supplying plant loads the produced sulfur in specially equipped tank-cars and delivers them by railroad or motor vehicles. A number of important characteristics shall be taken into account when arranging liquid sulfur loading stations. Liquid sulfur temperature shall be maintained within 130 – 150оС. The pumping sulfur is capable of being electrified and accumulate static electricity thus causing its ignition under certain conditions. The area of liquid sulfur loading station shall be equipped with hydrogen sulphide gas detectors. Since the sulfur is supplied to the third party consumers its recording shall be organized.
The cost of liquid sulfur loading station is calculated on the basis of its production capacity and loading procedure automation degree.
10.4. Intraplant and long-distance transportation of liquid sulfur
НаверхIntraplant transportation of liquid sulfur is carried out via pipelines by gravity and using special pumps. The pipelines shall be equipped with steam or electric tracing. Design of liquid sulfur pipelines has a number of special features that are a compilation of operating experience of numerous sulfur production units at oil and gas processing plants.
The long-distance sulfur transportation possesses its own distinctive features compared to intraplant transportation. It is effective in case of long-term cooperation between the supplying plants and sulfur manufactures. There are only three such pipelines in the world. Specialists of OJSC “Giprogazoochistka” are sufficiently experienced in design of liquid sulfur pipelines for long-distance transportation.
10.5. Organization of sulfur long-term storage.
10.6. Production of construction and road materials based on sulfur compositions
Use of sulfur in industrial and road construction is comparatively new field of its utilization. On the one side, it is connected with existing excess of sulfur production compared to its consumption. Moreover, construction sulfur-added composite materials take on unique properties, which are more advantageous than the common ones. At present, manufacturing techniques of different sulfur containing construction compositions have been proven, and a complex of pilot tests has been fulfilled.