Preparation Of Hydrophobic Silica Gel By Two-Step Acid-Base Catalysis And The Application In Oil Vapor Recovery

Choosing suitable adsorbents can improve the efficiency of oil vapor recovery with adsorption method. Ideal adsorbents usually have some obvious advantages of high oil vapor adsorption capacity, hydrophobic performance, easy regeneration capacity and high thermal stability. Nippon Oil Corporation spoke highly of silica gel adsorption method which can achieve the optimum efficiency of oil vapor recovery. Taking sodium silicate as silica source, the modified silica gel is prepared by two-step acid-base catalysis. Propyltriethoxysilane is used as modifying agent. The main research contents are as follows:1. To prepare hydrophobic silica gel using sodium silicate as silica source, the research mainly discussed the effects of pH values of two-step acid-base catalytic process, varieties of acid catalyst, electrolyte concentration, aging conditions, ethanol dosage, dosage of different modifying agents, and modifying temperature on properties of hydrophobic silica gel.24wt%sodium silicate solution was well mixed with2wt%sodium chloride solution. The mixture was first sulfuric acid catalyzed to pH value of3. Silica sol was then base catalyzed to pH value of7with sodium hydroxide solution. The prepared silica hydro gel was aged at50℃for2h. The silica wet gel was then immersed in propyltriethoxysilane/ethanol/n-hexane mixture with volume ratio of1:1.3:1at20℃for12h.The synthesized hydrophobic silica gel which can float on the surface of water for a long time has n-hexane adsorption capacity of546.10mg/g, hydrophobic index of87.69%, pore size of4.63nm, surface area of414.2m/g, cumulative pore volume of1.078cm3/g, and thermally stable up to453℃. The surface of modified silica gel has-CH2-CH2-CH3groups, and the number of-OH groups decreases.2. The adsorption isotherms of n-hexane and93#gasoline on hydrophobic silica gel are close to IV type adsorption isotherm. The hydrophobic silica gel has n-hexane adsorption capacity of546.10mg/g and93#gasoline adsorption capacity of502.97mg/g, which are both slightly lower than that of the mesoporous activated carbon. But the regeneration capacity of the hydrophobic silica is superior to the mesoporous activated carbon. The adsorption capacity of n-hexane and93#gasoline on hydrophobic silica gel can keep basically unchanged with the increase of adsorption-desorption test times, while the adsorption capacity of n-hexane and93#gasoline on activated carbon decreases obviously. When compared with the mesoporous activated carbon, the hydrophobic silica gel has better vacuum-desorption capacity for n-hexane and oil vapor.3. The breakthrough points respectly appear at171min and235min on the n-hexane breakthrough curves of hydrophobic silica gel and mesoporous activated carbon. Under the same loading volume, mesoporous activated carbon has a better performance in adsorption capacity of n-hexane than the hydrophobic silica gel. But the hydrophobic silica gel adsorption bed has a shorter mass transfer zone than the mesoporous activated carbon bed, so the hydrophobic silica gel adsorption bed has higher efficiency. The adsorbate outlet concentration of modified silica gel adsorption bed is greater than0after67times of hexane adsorption-desorption experiments, while the adsorbate outlet concentration of mesoporous activated carbon adsorption bed is greater than0after23times of hexane adsorption-desorption experiments. The adsorption stability of the modified silica gel is better than that of the mesoporous activated carbon.The breakthrough points respectly appear at165min,260min and210min on the93#gasoline breakthrough curves of hydrophobic silica gel, mesoporous activated carbon and hydrophobic silica gel-mesoporous activated carbon composite adsorbent. The adsorbate outlet concentration of composite adsorbent bed is greater than0after22times of oil vapor adsorption-desorption experiments, while that of the modified silica gel adsorption bed is greater than0after29times of oil vapor adsorption-desorption experiments and the adsorbate outlet concentration of mesoporous activated carbon adsorption bed is greater than0after9times of oil vapor adsorption-desorption experiments. The composite adsorbent has a better performance in93#gasoline adsorption capacity than that of the hydrophobic silica gel. Meanwhile adsorption stability of the composite adsorbent is better than that of the mesoporous activated carbon. Thus, the composite adsorbent has a good application prospect in oil vapor recovery with adsorption method.