Design of Unsteady‐State Fixed‐Bed Processes for Heat Regeneration, Ad‐/Desorption, and Gas‐Solid Reactions

Abstract

This review covers all three unsteady-state fixed-bed processes important in chemical engineering, namely, heat transfer from a hot gas to a cold fixed bed (heat regenerator), ad-and desorption of a gas for a Langmuir and a linear isotherm, and regeneration of a coked catalyst by coke burn-off as an important example of a transient gas-solid process. Albeit of inherent differences, the processes have strong analogies. For example, a moving heat or mass transfer zone, spreading or with constant pattern behavior, is formed. For each case, numerical and suitable analytical solutions for process design either selected from literature or developed in this work are derived and compared. Experimental data, if available, are also considered.

Design and Technoeconomic Analysis of Fischer–Tropsch Fixed‐Bed Synthesis with Cobalt‐Based Catalysts

“…Details on FT kinetics with Co catalysts, modeling of FT fixedbed reactors, and on FTS, in general, are outlined in previous publications [1][2][3] and the literature cited therein.…”

“…[1,2] η pore is then less than one above 180 °C, and reaches a value around 0.2 for 240 °C. [1][2][3] This leads to a higher molar H 2 -to-CO ratio in the particles compared to the bulk phase with a ratio of typically two, and increases unwanted CH 4 formation relative to C 2þ -HCs, as the diffusion coefficient of H 2 in liquid HCs is 2 times higher as of CO. This effect is strong above 240 °C, and the CH 4 selectivity S CH 4 then may exceed 20% compared to 10% without diffusion limitations.…”

“…The inhibition by steam is here also considered, as a high partial pressure (p H 2 O ) may be reached at least in the rear part of the tubes. The evaluation of our experiments [4,8] yielded: [3]…”

“…[9][10][11] The decrease of the reaction rate of FTS for cobalt-based catalysts by the formation of steam was recently also implemented in the model. [3] Finally, the influence of the changing molar flow rate by the FT reaction and of the pressure drop (momentum balance) on the reactor performance was also inspected and considered, [3] as these aspects should be included in FT fixed-bed reactor models but are often disregarded. [9,11] This advanced model, which is, to our best knowledge, the rare case or even the first 2D FT-reactor model that covers all of the aforesaid aspects including the radial and axial variation of the catalyst effectiveness (intraparticle mass transfer resistance), was now used to determine favorable design parameters of a technical FT reactor, presented in this article.…”

“…In three recent publications, [1][2][3] we presented a model of a multitubular FT reactor with a cobalt-based catalyst without/with gas recycle. [1,2] The intrinsic rates of formation of methane and of C 2þ -hydrocarbons (HCs) as well as internal diffusion limitations induced by the low rate of diffusion of dissolved H 2 and CO in the liquid-filled pores of the catalyst are considered by an effectiveness factor.…”