Effects of zeolite molecular sieve on the hydrocarbon adsorbent and diffusion performance of gasoline engine during cold start

In this work, adsorption characteristic of the MOR, MFI, and CON zeolites used in hydrocarbon catcher and diffusion properties of hydrocarbon (HC) molecules on them were studied for hydrocarbon emission control during cold start. To find adsorbents with promising adsorption performance for hydrocarbon catcher, the numerical simulation was carried out by using Material studio software with benzene (C6H6) and ethylene (C2H4) as the probes, the Grand Canonical Monte Carlo (MC) method was used in the adsorption simulation, and Molecular Dynamics (MD) method was used to simulate the diffusion of hydrocarbon molecules. The isotherms, adsorption sites, heats of adsorption, and interaction energy distribution of hydrocarbon single-component and multi-component molecules on three different zeolites were studied at various low temperatures and pressures. Based on the simulation results, the most suitable type of molecular sieve was found from the three molecular sieves when the type and quantity of components were different. Results of adsorption simulation show that CON molecular sieve has batter adsorption performance for benzene component. The adsorption amount is 65/unite cell (UC) benzene molecules per cell, which is about twice of MFI molecular sieve. MFI molecular sieve has batter adsorption performance for ethylene component. The adsorption amount is 130/UC ethylene molecules per cell, which is about 2 and 1.5 times of MOR and CON, respectively of MOR molecular sieve. The adsorption simulation results of mixed hydrocarbon components show that the adsorption capacity of each component on the three molecular sieves was ranked as follows: C6H6 > C3H6 > C2H4 > C3H8 > H2O > CH4, collectively. The results of diffusion simulation show that the single component of ethylene molecule has relatively small diffusion coefficient on MFI molecular sieve about 2.2×10−10m/s and good adsorption performance at low temperature, and the largest diffusion coefficient on CON molecular sieve about 3.4×10 −10m/s. The single component of benzene molecules has the smallest diffusion coefficient on the CON molecular sieve 5.9×10−11m/s about and the largest diffusion coefficient on the MOR molecular sieve about 1.76×10−10m/s. In multicomponent diffusion. In multicomponent diffusion. The diffusion degree of each component molecule in the MFI and CON molecular sieve from strong to weak is about: H2O, CH4, C2H4, C3H8, C3H6 and C6H6. The diffusion pattern within the MOR molecular sieve from strong to weak approximately: C3H8, C6H6, C3H6, C2H4, CH4 and H2O. The results show that the high temperature had an obvious effect on the adsorption capacity of zeolite. As temperature rises the molecular diffusion coefficient increases and the molecular motion becomes more intense. After about 500 K, the adsorption capacity is at a very low level.