1. Introduction
Distillation or fractionation of crude oils is the first step into separate fractions of hydrocarbon groups in the petroleum refining. Most of the resultant products are further converted into more valuable products by changing their properties through different conversion processes such as cracking, reforming and other. To produce finished products, various separation and treatment processes, such as extraction, hydrotreating, sweetening and adsorption are consequently treated of the products .
Clays are a naturally occurring mineral, and considered very efficient sorbents, inexpensive and environmental friendly according to several physical and chemical properties such as high surface area, mechanical stability and thermally inert. Clays are played an important part in petroleum refining industry. They used in several applications as active components, adsorbents, binders, catalysts and ion-exchanger. Several types of clay such as bentonite, attapulgite, hectorite, kaolin and sepiolite are obtainable and used in different applications.
Adsorption process is to be one of the simplest and efficient separation methods, because it can be achieved at ambient pressure, temperature and without use of any expensive materials. Additionally, it seems to be very promising and economical methods and saves energy consumption. Adsorption process is used to treat crude oil and petroleum fractions. It plays an important role in the treatment and finishing of petroleum fractions through desulfurization, deasphalting, bleaching and other.
Clay percolation treatment was the original method to treat a petroleum fraction through a coarse clay pellets tower. Clay activity lowered through absorbing impurities from the petroleum fraction. To restore the clay activity by remove the used clay from the tower periodically and burnt it under controlled conditions so as not to sinter the clay. Clay percolation treatment was broadly used for lubricating oils, but clay contacting has been largely replaced.
Several types of clay are obtainable and applied in adsorption to treat the petroleum fractions. Attapulgite clay can be used to separate crude oils into saturates, aromatics and resins through a chromatography column packed. Bentonite is considered very good adsorbent clay and specifically commonly used as a sorbent according to its layered structure and high surface area (~ 800 m2/g). Several types of bentonite can be absorbed water up to 100% and oil up to 80% of their dry weight. Another type of clay is sepiolite. It has a surface area lower than bentonite (~ 300 m2/g), according to its high porosity it has a several applications for paints, cosmetics, bleaching agent, filter support to an industrial adsorbent. Oil adsorption by sepiolite have been reported by a few studies.
A finishing step by clay treatment is still used in the production of lubricating oils and waxes. Removing traces of asphaltic materials and other compounds by clay that give lubricating oils and waxes undesirable colors and odors. Often clay treated cracked naphtha to prevent gums formation in gasoline by removing diolefins.
The aim of this review is to summarize the applications of the different types of clay through adsorption process in the petroleum refining industry, such as the removal of sulfur compounds through desulfurization, base oil finishing, the recovery of waste oils, the removal of asphaltenes and heavy metals, bleaching of petroleum fractions, the reduction of corrosion.
2. Desulfurization
Sulfur compounds removing from petroleum fractions are essential to produce clean burning fuels can be used environmentally. Sulfur compounds in petroleum fractions can cause several problems such as air pollution, acid rain, poison emission control catalysts and corrosion of parts in engines. Adsorption plays a significant role in petroleum fractions and crude oil desulfurization. The selective adsorption of sulfur compounds in crude oil and petroleum fractions, such as kerosene and diesel oil can be accorded through several clays used as adsorbents including kaolinite, montmorollinte, palygorskite and vermiculite.
Different low-cost adsorbents can be used for desulfurization of diesel fuel for removal of sulfur compounds. Bentonite was used to remove sulfur compounds from diesel fuel. Kaoilinte showed the maximum desulfurization yield of about 60%, 76% and 64% after 6 hrs adsorption contact time with crude oil, kerosene and diesel oil, respectively. Younis and Simo compared the desulphurization efficiency of Tawke diesel fuel by adsorption through Na-Y zeolite, local clay and activated charcoal. Desulfurization by activated charcoal is about 20%, it is more efficient than by zeolite and clay. Baia et al., studied the kinetic and isothermal of the adsorption performance of different commercial clays such as attapulgite clay and bentonite clays to remove sulfur and nitrogen compounds from a stream of real diesel. The highest adsorption capacity to remove sulfur and nitrogen compounds was achieved using bentonite clays, probably according to the presence of Brönsted acid sites. However, attapulgite clay was more selective to remove nitrogen compounds. On the other hand, Ahmad et al. concluded that fuller’s earth and calcium oxide were ineffective in desulfurization for the treatment of oils found during the pyrolysis of used tires.
Several studies compared the desulfurization ability of different clays with their modifications. Montmorollonite (MMT) clay is locally available and can be efficiently used for desulfurization. Modification of MMT clay by metals impregnation increases its adsorption characteristics. Desulfurization by adsorption of commercial kerosene and diesel oil was carried out through different metals impregnated acid modified MMT clay. Metals were wet impregnated on MMT included Fe, Cr, Ni, Co, Mn, Pb, Zn and Ag. Results show that, the highest desulfurization of kerosene and diesel oil of about 76% and 77% was achieved with adsorption through Zn-MMT, respectively. The increasing of the surface area, pore size and pore volume and additionally, the improvement of the surface morphology of the MMT was occurred with Zn impregnation. Desulfurization by adsorption of mercaptans from model oil using bentonite impregnated with Cu+2, Cu+1, Fe+3 and MnO4-1 was investigated. Authors concluded that the highest capacity of desulfurization of bentonite impregnated with Fe+3 and MnO4-1 can be according to the oxidation of marcaptans. Ishaq et al., studied the removal of dibenzothiophene from model oil using untreated bentonite, acid activated bentonite and magnetite impregnated bentonite as the adsorbents. Magnetite impregnated bentonite exhibits the best performance and the highest capacity in the desulfurization of fuel. It can be according to the catalytic activity of magnetite for the co-conversion or destruction of sulfur compounds. Additionally, the recovery of the magnetite can be easily by using the magnetic separation in the application. Yi et al., also, investigated the sulfur content reduction in model oil as liquid hydrocarbon fuels by adsorption for removing dimethyl sulfide and propylmercaptan using copper (II) impregnated bentonite as adsorbent. Results show that the highest sulfur adsorption capacity was achieved at a Cu (II) loading of 15 wt%, at 150°C of the optimum calcination temperature.
Dynamic and static evaluation of the capacity and efficiency of the sorbents for alkyl dibenzothiophenes removal was studied using silver modified bentonite and raw activated bentonite. Results show that silver ions loaded on bentonite had a higher adsorption of alkyl dibenzothiophenes than the activated bentonite. It can be attributed to silver ions could have complex reactions with alkyl dibenzothiophenes. Additionally, the adsorption capacities of alkyl dibenzothiophenes increased gradually with an increasing of silver ions loading and with a decreasing temperature.
Acid-activated kaolinite and bentonite clays, charcoal, petroleum coke and cement kiln dust were selected to adsorb of dimethyl disulfide sulfur compound from petroleum fraction. Acid-activated bentonite showed higher efficiency towards sulfur adsorption, may be according to the structure of bentonite that, the silicate-silicate structure (which produced from the water molecule dissociation in between silicate sheets) possesses Brönsted acid sites. Also, the surface of clay after acid modification would possess positive hydrogen sites. Hence, the bentonite-clay lattice can be disturbing the charge equilibrium by increasing in acid sites, that can be interact more selectively with the sulfur compound. Wan et al., studied a batch adsorption of oxidized sulfur compounds in diesel oil using activated clay at different process parameters. Results show that good adsorption performance in real diesel oil using activated clay batch adsorption. The adsorption parameters as pH, adsorbent dosage and temperature showed an important effect on the sulfones adsorption in diesel oil while contact time and agitation speed were insignificant.
Acid treated attapulgite clay was effective under microwave thermal activation and ultrasonic vibration and its performance was evaluated the desulfurization of simulated gasoline as the feed. The results show that the removal rate of thiophene increased with an increasing amount of hydrochloric acid during modification of attapulgite clay under microwave.
Miller and Bruno studied the effect of clay and organo-clay on the adsorption enthalpies and interaction of common odorants add in the fuel gas industry by using gas chromatography with wall-coated open-tubular column. Results show that, sulfide odorants have larger enthalpies than thiol odorants on surfaces of clay and organo-clay. In addition, the results explain the Lewis acid–base chemistry has significantly difference in enthalpy magnitudes on clay surfaces between the sulfide and thiol odorants.
"to be continued in the next part"