The Interaction Between Fluid Flow And Chemical Reaction In Reactive Porous Media

The fluid flow and chemical reaction in reactive porous media are very common in everyday life, engineering and technological applications. While the fluid reactant flowing through the pore channel, the heterogeneous reaction, between reactive fluid and solid which consists of the pore wall, will take place, and give rise to the change of physical and chemical properties of the fluid and solid. The difference volume for solid reactant and product will lead to the changes of pore structure; also for fluid reactant and product will lead to the change of fluid mixture pressure. On the one hand, under the driving of pressure gradient, the fluid mixture flow through pore channel and bring fluid reactant to reactive zone, carry fluid product out of reactive zone simultaneously, which makes the reaction proceed continuously. On the other hand, the change of pore structure due to chemical reaction will give rise to the change of permeability of porous media, which will affect the fluid flow in pore channel immediately. So the interaction between fluid flow and chemical reaction in reactive porous media is obvious.Using the research method and fundamental knowledge of mechanics of fluid through porous media and physical-chemistry in this paper, the basic equations, which represent the fluid flow and chemical reaction in reactive porous media, are submitted, also the numerical method for the basic equations. Three models are given, which reflect the interaction between compressible gas seepage and gas-solid reaction, for one-dimensional flow in reactive porous media, which is formed by packing pellets. Using the three models, the mass and heat interaction for the synthesis and metathesis gas-solid reaction are surveyed. An approach for researching solid-liquid phase transition by experiment was set forth. Using the kinetic model, combined with kinetic experiment, the kinetic parameters are obtained by optimal solution for overdetennined nonlinear equations. The reactive porous cylinder samples are made in lab, and conducted seepage with constant hydraulic head on triaxial shearing seepage apparatus. The mathematical model and experiment results fit very well.Followed by these researches, we find that:(1)、Under the isothemal conditions, for the synthetic gas-solid reaction, such as aA(g)+bB(s)=dD(s), the effects that chemical reaction has on the fluid flow involve two aspects: the changes of mass and density. For the seepage of gas mixture, if we overlook the density effect that gas reactant having on the gas mixture, an extra term, which denotes the gas source or sink, will turn out. (MI-MA)RA Its sign is dependent upon the proportion of molar mass of reactant and inert gas. The more the discrepancy between inert and reactant gas molar mass is, the more the effect of the extra term is. Although the molar masses of reactant and inert gas have no effects on the pressure of gas mixture, velocity and gas concentration, they have some effects on gas density in deed. The density of reactive fluid is determined by both pressure and component molar mass. The chemical reaction will hamper the chemical wave propagation evidently. The more rapid the reaction is, the more slowly the wave front propagates. Before the concentration wave front of gas reactant arrive the outlet of the reactor, the profile of gas mixture velocity may be non-monotone. After the wave front reaches the exit, the velocity profile becomes monotone decreasing. For the coupled modelⅠ, the velocity profiles are dependent on time, however, for the non-coupled model, the velocity profiles are monotonic increasing all the time. With the increase of reactor length, the dimensionless distance for wave front propagation decrease, because Thiele Number is proportional to the square of reactor length and Peclet Number is proportional to the unit power of reactor length. This concludes that the reactor length itself is an important factor which can affect the reactor properties. The influence of gas-solid reaction on the gas mixture seepage is obvious, so in the non-active region, the mode of gas velocity is similar to non-coupled model. With the propagation of active region, the profile of gas mixture may behave complicated.(2)、Under the isothermal conditions, for the metathesis gas-solid reaction, such as aA(g)+bB(s)=cC(g)+dD(s), the effects that reaction has on the fluid pressure only depend on stoichiometric proportion c/a. The model, that taking into account both fluid mass and density changeover due to chemical reaction, can express the physical fact exclusively. While c/a>1, the gas-solid reaction will enhance gas pressure. While c/a<1, it will decrease gas pressure. While c/a=1, it has no effect on gas pressure. The flow velocity is dependent on both chemical reaction and average pressure drop along the reactor. The former produce nonlinear velocity profile, the latter increases velocity linearly. The ultimate velocity profile depends on the overall effect of the two factors. While a≠c, with the change of pellet radius, the velocity profile will alter considerably, not only in quantification, but also in qualification. Also the gas concentration and solid conversion are affected by pellet radius significantly. The coupled modelⅡ, which neglects the effect that chemical reaction has on the gas density, but takes into account the mass change due to chemical reaction, will produce an extra term [(?)(Mc-Mx)+(MI-MA)]RA The sign of this term is dependent on a, c, MA, Mc, MI. It will become unaccountable almost all, even more far away from physical truth than non-coupled model. This shows that the coupled model is not always superior to non-coupled model. The density of gas mixture depends on both system characters and molar mass of gas components. With a longer reactor, the chemical reaction has more effect on gas seepage, even modify its trend.(3)、Considering the thermal effect of the chemical reaction, only for exothermic reaction, the profiles of temperature, pressure, velocity, concentration and density are astable strongly. The heat of reaction affects the compressible gas flow manifestly. With the proceeding of chemical reaction, the pressure profile change quantitatively, and the velocity profile change quantitatively and qualitatively. The release of reaction heat leads to a significant change for gas density. With the variation of productive rate for heat, the density profile will alter quantitatively and qualitatively. When the reaction system is non-isothermal, the higher conversion region not always locates at the inlet of the reactor, and sometimes it stands in the interior of reactor. The transport action of convection for mass and heat is outstanding. For the mass transport action for gas product, its concentration profile shows increasing along the reactor. Also the heat transport makes the outlet temperature higher than the inlet temperature almost all the time.(4)、Considering the thermal effect of the chemical reaction, only for endothermic reaction, the effects that chemical reaction and heat transfer have on the gas convection should not be overlooked, when the system is in local thermal non-equilibrium. The two factors, chemical reaction and heat transfer, will affect the velocity profile quantitatively and qualitatively. Ignoring the mass action, that reaction has on the fluid flow, is equivalent to throwing off a term that denotes a pressure source or sink, which will affect the gas convection and chemical reaction proceeding notably. The main cause for system’s local thermal non-equilibrium is the heat consumption due to endothermic reaction. The interaction among compressible flow, endothermic reaction and heat transfer should be payed more attention on.(5)、By means of measuring buoyed weight continuously, we have some researches on kinetics of phase transition for solid to liquid, using shrinking core model to describe the proceeding of solid pellet solution. Combined the date getting from experiments and numerical solution of mathematic model, the kinetics parameters was obtained by method of least squares for nonlinear overdetermined equations.(6)、The solid pellets, which consist of the reactive porous media, dissolve into solution, which make the pore structure changing obviously, porosity and permeability increase. So the resistance of porous media decreases and the fluid flow is accelerated. Under the action of pressure gradient, the fluid reactant flows through the pore channel, and phase change takes place on the interface between reactive solid and liquid. The liquid product was carried out by convection. For the phase change initiates from the zone near the inlet, the concentration profile of liquid product is ascending along the sample. The solution for upstream reactive solid is promoted, the downstream is contrary.