Development of sorbents for desulfurization of liquefied petroleum gas (LPG)

Liquefied petroleum gas (LPG) is often a mixture of saturated C3 and C4 hydrocarbon that are mainly contained in butane (n-butane and i-butane) and propane. LPG gases can all be compressed into liquid at relatively low pressures. LPG is generally accumulated, as a liquid, in steel vessels ranging from small gas bottles to larger gas cylinders and LPG storage tanks. LPG is carried to storage terminals by large LPG transporters, pipelines or by rail. These storage terminals then act as centre for local distribution. LPG has been applied in many areas especially as domestic, transport, agriculture, refinery and chemical. LPG is frequently used for fuel in heating, cooking, hot water and vehicle fuel, as well as for refrigerants, aerosol propellants and petrochemical feedstock. Its use as a propellant is especially widespread in chemical industry. LPG has a importantly lower cost when hold a candle to other propellants such as dimethylether (DME) and chlorofluorocarbons like 134a and 152a. Moreover, LPG is non-corrosive and non-toxic gas. In addition, LPG is a miscellaneous and effective propellant. LPG has two bases: approximately %60 is recovered during the extraction of oil and natural gas, and the remaining %40 is manufactured at the same time as the refining of crude oil. LPG is thus a naturally consisting of by-product. As a petroleum product, LPG can contains contaminations like sulphur compunds, nitrogen and water. Moreover, LPG can also be added to mercaptan derivatives as an environmental stimulant to notice LPG leakage. Most of these compounds are hydrogen sulphide, carbonyl sulphide, ethyl mercaptan, methyl mercaptan, dimethyl sulphide, dimethyldisulphide, thiophene derivatives. Because of worldwide environmental legislations, refiners are facing the challenge of producing progressively sweet fuels. Furthermore, the smell formed by the sulfur compounds is a major problem in terms of the use of LPG as aerosol gas, especially in the cosmetics industry and other chemical industries. In 1998, the European Union first legislation new sulfur specifications for compulsively minimized levels that started to be phased in from the year 2000. Similar arrangements were enacted in the U.S. and elsewhere soon after. The EPA Tier II arrangements need reductions of sulfur in highway diesel from the current average of 500 ppmw to 15 ppmw by June 2006, and that in gasoline from 350 ppmw to 30 ppmw by January 2005. However, refinery pipelines would probably need even lower sulfur content in their fuels, below 10 ppmw, to balance for possible impurity from the higher sulfur concentration of some products and the characteristic evaluation inequality. Future fuel cells will also need free sulfur fuels, if at all possible free sulfur. For example, methanol-based fuels for on-board fuel cell applications enforced the use of a fuel with sulfur content <1 ppmw in order to abstain poisoning and deactivation of the reformer catalyst.Desulfurization methods from liquid fuels are classified as hydrodesulphurization, adsorption, merox and selective catalytic oxidation process. For the desulphurization of mercaptans in LPG, the most largely used technology is Merox process , which was improved by Universal Oil Products Company . The Merox process transforms mercaptans to disulfides in alkaline solution using air as the oxidant in the presence of Merox catalyst. One of the other methods of desulphurization is HDS. It is a typical and very efficient desulfurization process. However, HDS is processed at high pressure and temperature and need addition of hydrogen. Furthermore, during the removal sulphur process, olefin may be lost importantly due to the conversion of olefin to alkane. Another desulphurization process, SCO has the major advantage that it does not use hydrogen, but it has the disadvantage that is not converted nitrogen and oxygen from the C4 hydrocarbon should be eliminated in an extra separation unit. When compared to the above methods, the adsorptive desulfurization has the many advantages that it does not only can be processed at relative low pressure and temperature, but also need no hydrogen. Therfore, the adsorptive desulfurization is accepted one of the most appropriate and hopeful methods. Adsorption is a highly efficient method by using several adsorbents for removal sulphure. Adsorbents generally used in desulfurization by adsorption are zeolites, activated carbons, meso porous materials, clay and metal organic frameworks. One of the most important of these adsorbents is zeolites. Zeolites are found as to be more effective adsorbent for removal sulphur from LPG than active carbons.The use of zeolites is widespread due to their yield and especially their cost. Adsorbents for desulfurization by adsorption should have the properties like selectivity, high sulfur capacity, regenerable and low cost. Adsorbents can perform sulfur compounds in liquid fuel using different adsorption mechanisms. These are π- complexation, van der Waals and electrostatic interaction, reactive adsorption and chemical adsorption. The effectiveness of inorganic adsorbents to adsorb organic sulfur compounds in the liquid-liquid phase has been strongly influenced by the structural and physical properties of the adsorbent which is crystal structure, cage and crystal defects, surface area, pore volume, element forming the main structure and quantities of charged ions. The commercially important adsorbents are highly porous and have high surface areas per gram. According to investigations for desulfurization by adsorption from liquid fuels on zeolites, sulphur capacity of zeolites increase by ion exchange method on zeolite surface. Zeolites are crystalline aluminosilicates of alkali or alkali earth elements, such as sodium, potassium, and calcium. At least 40 different types of naturally occurring zeolites have been found, beginning with the discovery of stilbite (STI) by the Swedish mineralogist Cronstedt in 1756, who also coined the term "zeolite." The principal natural zeolites are chabazite, gmelinite, mordenite, heulandite, clinoptilolite, levynite, and faujasite. More than 150 types of zeolites have been synthesized and are designated by a letter or group of letters (Type A, Type X, Type Y, Type ZSM, etc.). The ratio of Si/Al in Type A zeolite is normally one, while those in types X and Y are typically one to five. Thermal stability of zeolites varies over a large temperature range. The decomposition temperature for Low-silica zeolites is 700 °C, whereas completely siliceous zeolite, such as silicalite, is stable up to 1300 °C. Low-silica zeolites are hydrophilic, whereas high-silica zeolites are hydrophobic. Cation concentration, siting, and exchange selectivity vary significantly with Si/Al ratios and play an important role in adsorption, catalysis, and ion-exchange applications.