PTQ Q1 2023 Issue

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Bottom zone

Middle zone

Top zone

Days on stream

Figure 2 Temperature profile operating data of a typical refinery over days

involving SO 2 , which are also Co/Mo catalysed. These reac- tions are parallel to the conventional ones and address compositionally dependent observations. Whereas stoichi- ometry for the reaction is well known, reaction order and kinetics must be determined from reaction rate data and generally these do not follow stoichiometric coefficients. The following steps were carried out in the development of the reaction kinetics model:  Collection of published experimental data  Postulation of a reaction set  Determination of reaction order from published data  Assessment of equilibrium influences, if >10% outlet, including reverse reaction  Determination of kinetic rate temperature coefficients for Arrhenius expression from data  Identification and regression of compositional resis - tances to reaction rates  Refinement of the reaction set and rates with experi - mental and operational observations. The kinetic rate model was fitted to published experi - mental data. 1,2 An apparent kinetics approach was adopted, which lumps the intrinsic active-site-based kinetics, k int , with effectiveness factor, η , for resistances to reaction rate that result from mass transfer and diffusional effects within the pores of the catalyst pellet. This decision was made by considering

first or second order, so there is concern of confounding kinetics if mass transfer presents controlling resistance. The approach provided a good representation of temperature- dependent kinetics using Arrhenius-based rate coefficients determined from the experimental data. The operational domain for fluid dynamics in commercial applications has sufficient velocity that stagnation/backflow areas do not have a significant influence on bed fluid dynamics and reac - tor efficiency. The effect of adsorption resistances and compositional interaction with the reaction rate were modelled using either Langmuir-Hinshelwood type factors or a fractional- order rate expression. An example of this application is the analysis of the interaction between hydrogen, water, and carbon dioxide in the water-gas shift reaction: CO + H 2 O ⇌ CO 2 + H 2 Water is usually present in substantial excess (normally the reaction kinetics would be addressed as pseudo-first order with water as zero order), but competitive water adsorption on active sites influences the reaction rate. Additionally, at lower water concentrations, the reac- tion rate declines, exhibiting second-order behaviour. The Langmuir/Hinshelwood relationship provides an effective form to represent this relationship:

k app = η k int and k app = A exp(– E a /RT) Arrhenius

– [CO] * k eff ] * K w ] * [H 2 O]

rate =

(1 + K w [H 2 O]

aw + K

c [CO 2 ]

ac + K

h [H 2 ]

ah ) d

the magnitudes of Thiele modulus and effectiveness fac- tors. Experimentally determined effectiveness factors were 0.7 or greater (although this excludes sulphur dioxide and elemental sulphur reactions). These reactions are generally

Hydrogen is a product of the reaction, yet it has an influ - ence beyond an equilibrium limitation expression because it is strongly adsorbed and, as a leaving group, becomes


PTQ Q1 2023

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