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Removal sulfur from crude oil ‎

Removal sulfur from crude oil 

Refining of crude oil to final products requires desulfurization of the oil. Fuel specifications that govern transportation fuels have over the years become increasingly stringent with respect to sulfur content. Many petrochemical products are likewise produced to be almost sulfur-free. 

Sulfur in crude oil comes generally from the decomposition of organic matter, and with the passage of time and of gradual settling into strata, the sulfur segregates from crude oil in the form of hydrogen sulfide that appears in the associated gas, some portion of sulfur stays with the liquid. Another theory behind origin of sulfur compounds is the reduction of sulfates by hydrogen by bacterial action of the type desulforibrio desulfuricans:

4H2 + SO4 =(bacteria)→H2S + 2OH- + 2H2O

Hydrogen comes from the reservoir fluid and the sulfate ions are kept in the reservoir rock, as a result hydrogen sulfide is generated. The H2S formed can react with the sulfate or rock to form sulfur that remains in composition of crude as in the case of oil from Moreover, under the conditions of pressure, temperature and period of formation of the reservoir H2S can react with the hydrocarbons to give sulfur compounds:

3H2S + SO4 → 4S + 2OH- + 2H2O

Sulfur compounds are among the most important non-hydrocarbon heteroatomic constituents of petroleum. There are significant amount of sulfur species found in crude oil and sulfur compounds of one type or another are present in all crude oils. Furthermore, only preferred type of sulfur exist in any particular crude oil, and this is dictated by the prevailing conditions during the formation, maturation, and even in situ alteration. In general, the higher the density of the crude oil, the lower the API gravity of the crude and the higher the sulfur content. The total sulfur in crude oil can vary from 0.04% w/w for light crude oil to about 5% w/w for heavy crude oil and tar sand bitumen. Nevertheless, the sulfur content of crude oils which is produced from different locations varies with time, depending on the chemical composition of newly discovered fields, especially those in different geological environments

Crude oil is a unique mixture of a great number of individual chemical compounds. Each crude oil has a compound which is not matched exactly in composition or in properties by any other sample of crude oil. Chemical and physical composition of crude oil can vary not only with the location and age of the oil field, but also with the depth of the individual well. More than that, two neighboring wells may produce hydrocarbons with considerably different characteristics.

In order to understand the nature of sulfur compounds in crude oil the basic knowledge of general crude composition is needed. The main constituents present in crude oils are hydrocarbons. The hydrocarbon content may be as high as 97% by weight in light paraffinic oils or as low as 50% by weight in heavy crude and bitumen. Other nonhydrocarbon constituents include small amount of organic compounds containing sulfur, oxygen, and nitrogen, as well as compounds containing metallic elements, such as vanadium, nickel, iron, and copper.

A process for removing hydrogen sulfide from crude petroleum, which comprises contacting the crude petroleum to be freed from hydrogen sulfide with a stripping gas for removing hydrogen sulfide in a hydrogen sulfide stripping column, supplying the spent stripping gas now containing the thus stripped hydrogen sulfide to a hydrogen sulfide absorbing tower, separating the stripping gas from hydrogen sulfide in said absorbing tower by contacting the gas with a hydrogen sulfide absorbing agent to absorb hydrogen sulfide therein and returning the so refreshed hydrogen sulfide stripping gas now freed from hydrogen sulfide to said hydrogen sulfide stripping column under compression at a superatmospheric pressure in order to effect recirculation of the gas.

Heavy oil usually contains percentage levels of sulfur. Most of the sulfur in heavy oil is found in bulky thiophenic structures. Thiophenic sulfur is difficult to remove by catalytic hydro desulfurization, but it can readily be oxidized. The sulfoxides and sulfones produced from sulfur oxidation can be solvent extracted from the heavy oil as a result of their increased. Strategies for heavy oil desulfurization were evaluated by reviewing desulfurization literature and critically assessing the viability of the various methods for heavy oil. The desulfurization methods including variations thereon that are discussed include hydrodesulfurization, extractive desulfurization, oxidative desulfurization, biodesulfurization and desulfurization through alkylation, chlorinolysis, and by using supercritical water. Few of these methods are viable and/or efficient for the desulfurization of heavy oil. This is mainly due to the properties of the heavy oil, such as high sulfur content, high viscosity, high boiling point, and refractory nature of the sulfur compounds. The approach with the best chance of leading to a breakthrough in desulfurization of heavy oil is autoxidation followed by thermal decomposition of the oxidized heavy oil. There is also scope for synergistically employing autoxidation in combination with biodesulfurization and hydrodesulfurization.