고정 및 이동층 반응기에서의 고온건식탈황공정 모델링에 관한 연구

신 상백, 이 창민, 문 재웅, 김 성현

고려대학교 화학공학과

A Modeling Study of High Temperature Desulfurization

In the Fixed and Moving-Bed Reactor

Sang Baek Shin, Chang Min Lee, Jae Woong Moon, Sung Hyun Kim

Department of Chemical Engineering, Korea University

Introduction

A High Temperature process for the desulfurization of coal-derived gas constitutes an important step in the development of integrated gasification-combined cycle(IGCC) and MCFC advanced power generation processes. Both processes have the potential for reduction of pollutant emissions and high electrical generation efficiency compared to conventional coal-fired power plants. Traditional desulfurization processes using wet scrubbing are impractical at the temperatures and pressures of interst(750-1000K and 1000-2000kPa). A High Temperature desulfurization process based upon the reaction of an appropriate mixed-metal oxide with H2S to form the corresponding metal sulfide is under development.

In this modeling study, zinc titanate(Zn2TiO4) and zinc ferrite(ZnFe2O4) is considered as prime candidate sorbents to remove H2S component in coal-derived gas. This sorbents combine favorable thermodynamic properties with high theoretical sulfur capacity and rapid reactivity. The performance of both zinc titanate and zinc ferrite sorbents is tested in a fixed-bed reactor using simulated coal gas. H2S concentrations in the exit gas were measured as a function of time and the runs were terminated soon after H2S breakthrough, with effluent H2S concentrations in the range of 10-1000ppmv.

Development of the basic models

The Reaction between H2S and spherical pellet of zinc based sorbents is gas-solid reaction that can be encountered in many chemical processes. In this study, grain model and unreacted core model are considered to analyze high temperature desulfurization process in a fixed and moving-bed reactor.

The chemical reaction may be written

A(g) + bS(s) → gas and solid product

Mathematical model formulation is based upon the following assumptions: (1)the pellet retains its initial size and spherical shape throughout the reaction; (2)the reaction system is isothermal; (3)the chemical reaction is irreversible; (4)the intrinsic reaction rate is first order with respect to gas concentration, and (5)the pseudo-steady-state approximation is valid.

In grain model, the differential material balance on the gaseous and solid reactant in a fixed-bed reactor yields

[eqn: {dX} over {dt^* }```=```□ ``exp` left[-`int _0 ^y^* `□```d□ ~right]]                         (1)

where [eqn: y^*```=```{k`A_g } over {U_0 `F_g` V_g }``(1-□_v )(1-□_)``y]

[eqn: t^* ~=~ bk over rho_s left(A_g overF_gV_g right)C_AS cdot t]

The differential equation describing the concentrations of gaseous and solid reactant in a moving-bed reactor yields

cocurrent flow :      [eqn: y^*~ =~int from0 to X {d□} OVER {(R-~□)`beta(□)}□]                        (2)

countercurrent flow :  [eqn: L^* ~=~int from 0 to X_0 {dX} OVER {(R~-~X~+~X_0 )`beta(X)}]          (3)

where  [eqn: L^* ~=~{k`A_g `C_A0 `(1-□)`(1-□_v )} over {G_g` F_g` V_g }`L]

[eqn: R~=~{b`G_g} over {G_s}]

In unreacted core model, the differential material balance on gaseous and solid reactant in a fixed-bed reactor yields

[eqn: partialC_A* over partialz^*`=`-G(X)`C_A^*]                                (4)

[eqn: partialX over partialt^*`=`G(X)`C_A^*]                                     (5)

where [eqn: t^*`=` (tU_o`C_Ao`b)[(1- epsilon _v`)(LC_So`)]]

[eqn: G(X)`=`left[3(1- epsilon _V`)`L over{U_0`R} right]`left[1 over {k'C_S0{(1-X)^2/3}}`-`k over D_e`(1-(1-X)^-1/3`)`+`1 over k_g right]^-1]

Results and Discussion

Figure 1 is the result of calculations for the cocurrent and countercurrent case in a moving bed. As might have been expected, a shorter bed length is required to bring about a given extent of reaction of pellets or gas with pellets of smaller □ values. This parameter □ incorporates both kinetic and structural properties and is the measure of the relative magnitude of chemical reaction and diffusion rates. Comparing  cocurrent with countercurrent case reveals a significantly greater conversion for solids leaving a countercurrent reactor than a comparable cocurrent reactor, especially at high conversion.

Figure 2 is the result of calculations for the fixed bed. Eq.(1) is solved using Gauss Legendre quadrature and fourth-order Runge-Kutta procedure. As might have been anticipated from the form of eq.(1), this figure shows that after an initial period (roughly after t*=1), the plot of the extent of reaction within the bed takes on a sigmoid shape, which then travels down the bed with unvarying profile and a constant dimensionless velocity of unity. The width of the reaction zone is larger as □ value increases. As a consequence, pellets with small values of □ might be expected to react rapidly compared with pellets of large □ due to less hinderance of reaction from intrapellet diffusion in the smaller pellets.

Comparing unreacted core model with grain model through H2S breakthrough curve in a fixed-bed reactor is presented in Figure 3. Each curve shows very sharp breakthrough, indicative of fast reaction and diffusion rates. In grain model, □ is treated as an adjustable parameter whose value is chosen to provide agreement between predicted and experimental breakthrough data. Otherwise, in unreacted core model, effective diffusivity within the pellet, De is treated as an adjustable parameter.

In this figure, unreacted core model is better than grain model in matching experimental data.

Acknowledgement

The authors wish to acknowledge financial support from the R & D Management Center for Energy and Resources, Ministry of Trade, Industry and Energy.

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