Nonisothermal Pellet Research Articles

Optimization of Liquid Organic Hydrogen Carrier (LOHC) dehydrogenation system

In this paper, the perhydrodibenzyltoluene dehydrogenation flowsheet has been simulated. Modelling of the dehydrogenation reactor has been performed using the 1-D model. External and internal mass transfer resistances are also considered. Non-isothermal pellet condition has been considered for simulating the dehydrogenation reactor. The flowsheet simulation has been carried out in DW-Sim v 6.5.2 integrated with the reactor model coded in Python. NET. The dehydrogenation reactor is operated at a feed temperature between 523 K −613 K, a wall temperature of 623 K and 653 K, and a reactor pressure maintained at 1.2 atm. The amount of catalyst required for the perhydrodibenzyltoluene (PDBT) dehydrogenation reactor is evaluated such that the conversion reaches 99%. The process flowsheet has been simulated to produce 10 Nm3/hr of industrial-grade hydrogen. The effects of feed temperature, wall temperature, and hydrogen burner efficiency on various system requirements, including catalyst weight, energy supplied to the dehydrogenation reactor, areas of the heat exchanger, and hydrogen production from the reactor, have been discussed. Preliminary cost optimization based on the heat exchangers and catalyst at various feed temperatures, reactor wall temperature, and hydrogen burner efficiency has been carried out.

Heterogeneous modeling of an autothermal membrane reactor coupling dehydrogenation of ethylbenzene to styrene with hydrogenation of nitrobenzene to aniline: Fickian diffusion model

Coupling of reactions in catalytic membrane reactors provides a route to process intensification. Dehydrogenation of ethylbenzene and hydrogenation of nitrobenzene form a promising pair of processes to be coupled in a membrane reactor. The heat released from the hydrogenation side is utilized to break the endothermality on the dehydrogenation side, while hydrogen produced on the dehydrogenation side permeates through the hydrogen-selective membranes, enhances the equilibrium conversion of ethylbenzene and reacts with nitrobenzene on the permeate side to produce aniline. Mathematical reactor models are excellent tools to evaluate the extent of improvement before experiments are set up. However, a careful selection of phenomena considered by the reactor model is needed in order to obtain accurate model predictions.To investigate the effect of the intraparticle resistances on the performance of the cocurrent configuration of the coupling reactor, a heterogeneous fixed bed reactor model is developed with Fickian diffusion inside the catalyst pellets. For the condition of interest, the styrene yield is found to be 82% by the homogenous model, 73% by the heterogeneous model for isothermal pellets, and 69% by the heterogeneous model with non-isothermal pellets. Hence, the homogeneous model overestimates the yield by 5–15% of their actual values.

Effect of Pore Size Distribution and Temperature on the Catalyst Tortuosity

Effect of temperature gradients, for an exothermic reaction, inside a catalyst pellet and the pore size distribution on the value of tortuosity factor is studied. The structure of the catalyst pellet is simulated using a two dimensional pore network. Material and energy balances for the pore network are written and discretised using a finite difference scheme. Tortuosity factors obtained for the isothermal and non-isothermal pellets are compared. Significant variation of the estimated tortuosity factor with internal particle temperature gradient and pore size is obtained. DOI: http://dx.doi.org/10.3329/cerb.v16i1.13191 Chemical Engineering Research Bulletin 16(2013) 61-72

Comparison of diffusion models in the modeling of a catalytic membrane fixed bed reactor coupling dehydrogenation of ethylbenzene with hydrogenation of nitrobenzene

Coupling of dehydrogenation of ethylbezene with hydrogenation of nitrobenzene in a catalytic membrane reactor can lead to a significant improvement in the conversion of ethylbenzene and production of styrene. In this work, the homogeneous reactor model for a cocurrent flow configuration is compared to two heterogeneous models based on the Fickian diffusion model and the dusty gas model for both isothermal and non-isothermal pellets. It is observed that both heterogeneous models predict a significant drop in yield and conversion compared to the homogeneous model, indicating the importance of heterogeneity. This drop is generally less severe for the dusty gas model than for the Fickian diffusion model. The assumption of isothermality causes larger deviations than the assumption of Fickian diffusion. The deviations in the predictions of the homogenous model and the heterogeneous models from those of the dusty gas model for non-isothermal pellets are ∼6% and ∼11%, respectively.

Optimal catalyst activity distribution for effectiveness factor, productivity and selectivity maximization in generalized nonisothermal reacting systems with arbitrary kinetics

Abstract Optimal catalyst activity distribution for effectiveness factor, productivity and selectivity maximization in nonisothermal pellets has been determined for generalized reacting systems with arbitrary kinetics. It has been shown that optimal catalyst distribution is a Diracδ(x) function in all cases.

OPTIMUM DILUTION PROFILES OF COMPOSITE ZEOLITES IN PACKED BEDS

Abstract The paper examines theoretically the conversion of methanol over composite zeolite spherical catalytic particles in a fixed bed reactor. The composite zeolite contains small zeolite particles uniformly distributed in an amorphous silica-alumina matrix. Two problems are considered. The first is concerned with the effect of dilution of the catalyst with amorphous silica-alumina on the concentrations of species and temperature along the bed. Isothermal and nonisothermal pellets are considered. The second is concerned with the calculation of the optimum dilution profile which maximizes the rate of consumption of methanol.

Optimal catalyst activity distribution in pellets for selectivity maximization in triangular nonisothermal reaction systems: Application to cases of light olefin epoxidation

The solution to the general problem of determining the optimal catalyst activity distribution for maximization of global selectivity in flat, cylindrical, or spherical catalyst pellets for parallel, consecutive, or triangular reaction networks in presented. The analysis considers both isothermal and nonisothermal pellets with no external gradients and is applicable to any type of kinetics with arbitrary numbers of chemical species participating in each reaction step. It is shown that the optimal catalyst distribution is an appropriately chosen Dirac δ function. Physically, this implies that the active catalyst must be deposited in a thin zone at a specific distance from the center of the pellet. Analytical expressions are derived for the optimal catalyst location in terms of the pertinent physicochemical and operating parameters. The analysis is applied to C 2H 4 and C 3H 6 epoxidation on Ag. It is shown that pellets with optimal catalyst distribution result in global selectivities that are substantially higher than those obtained with uniformly activated pellets.

Optimal catalyst distribution for selectivity maximization in nonisothermal pellets: the case of consecutive reactions

The problem of determining the optimal catalyst activity profile for the maximization global selectivity in nonisothermal flat, cylindrical or spherical pellets is analyzed and solved for the case of consecutive reactions with arbitrary kinetics. It is proven that the optimal catalyst distribution is an appropriately chosen Dirac ▪-function, i.e. all the active catalyst must be deposited at a specific distance from the center of the pellet. Analytical expressions are derived for the optimal catalyst location in terms of the pertinent physicochemical and operating parameters. It is shown that pellets with optimal catalyst distribution result in global selectivities which can be dramatically higher than those obtained with uniformly activated pellets.

An experimental investigation of optimal active catalyst distribution in nonisothermal pellets

Cas de l'hydrogenation de l'ethylene sur Pd/Al 2 O 3 et de la methanation de CO sur Ni/Al 2 O 3

Optimal catalyst distribution for selectivity maximization in nonisothermal pellets: The case of parallel reactions

The problem of determining the optimal catalyst activity distribution for selectivity maximization in nonisothermal flat, cylindrical or spherical pellets is solved for the case of parallel reactions with arbitrary kinetics. It is shown that the optimal catalyst distribution is an appropriately chosen Dirac δ-function. Simple analytical expressions are derived for the optimal catalyst location in terms of the pertinent physicochemical parameters. Global pellet selectivities obtained with optimal catalyst profiles are shown to be substantially higher than those obtained with uniformly activated pellets.

Uniqueness conditions for steady solutions in the case of m-th order reactions—non-isothermal pellets with variable transport coefficients

Etude theorique du cas d'une reaction exothermique entre des particules spheriques par deux methodes de calcul differentes

Uniqueness of the steady state for an mth order reaction in a non-isothermal pellet with variable transport coefficients

Conditions analytiques d'unicite pour des reactions exothermiques et endothermiques, lorsque les parametres de transfert varient avec la temperature

Non-isothermal effects in a single pellet diffusion reactor

A theoretical analysis of an adiabatic non-isothermal single-pellet diffusion reactor shows that non-isothermal effects can alter the value of the centerplane concentration. Deviation from isothermal behavior is significant under exothermic conditions where multiplicity of solutions can lead to reactor instabilities or falsification of reaction parameters. Conversely, it is shown that the internal isothermal model is a good approximation to detect non-isothermal pellet behavior from centerplane concentration measurements.

3 Intraparticle diffusion effects in the methanation reaction

Though much of the theoretical literature on reactions in porous catalyst pellets is concerned with special cases, such as binary mixtures at the Knudsen diffusion limit, in situations of industrial interest the reaction mixture usually contains several substances. There is often more than one independent chemical reaction, and pore sizes may correspond to a diffusion law anywhere between the extremes of Knudsen diffusion and bulk diffusion. A method of calculating the behavior of porous catalyst pellets in these more difficult but realistic circumstances has recently been developed, and its practicability has been demonstrated by applying it to an isothermal catalyst pellet. In the present paper this method is extended to deal with non-isothermal pellets and is applied to the industrially important SNG-process (low temperature, high pressure methanation on a shift-type catalyst). The computed results are compared with those obtained when a simple Knudsen type diffusion model is used.

Effects of temperature and pressure gradients on catalyst pellet effectiveness factors—I

The dusty gas model is used to establish the effects of temperature and pressure gradients on catalyst pellet effectiveness factors for reaction systems in which species molecular weights and transport coefficients are indistinguishable and Σν i = 0. For this class of reactions, the total molar flux of species i is shown to be expressible simply as N i = − cD ∇ϰ i in terms of the molar concentration, the Bosanguet diffusivity, and the mole fraction gradient. The effects of temperature and pressure gradients are reflected only in variations in molar concentration and diffusivity. Furthermore, the temperature-pressure relationship is shown to be given by the thermal transpiration equation for a pure gas. Typical numerical results are reported for first order reactions in spherical pellets under diffusive conditions ranging from the Knudsen through the bulk diffusion regimes. The variation in diffusion regime is shown to be controlled by an additional parameter α, the Knudsen to bulk diffusion ratio. Comparisons are made with the classical Weisz-Hicks nonisothermal pellet solutions based on N i = − D eff∇ c i. For highly exothermic reactions, effectiveness factors are 18% lower in the Knudsen regime and 30% higher in the bulk diffusion regime than are the Weisz—Hicks values. For highly endothermic reactions with a significant diffusion limitation, the effectiveness factors are 30% lower than the Weisz—Hicks values. The classical Damköehler relationship for pellet temperature rise is shown to apply in the Knudsen regime, with the maximum dimensionless center temperature given by (1 + β), where β is the heat of reaction parameter. This temperature is accompanied by a maximum dimensionless center pressure of (1 + β) 1 2 . In the bulk diffusion regime, the maximum center temperature is shown to be increased by the additional term β 2/4. This additional temperature rise accounts for the 42% increased bulk diffusion effectiveness for highly exothermic reactions.

An optimal catalyst activation policy for poisoning problems

Pontryagins minimum principle is used to calculate the optimum distribution of active material throughout a single pellet that is uniformly experiencing activity decay. The definition of optimality is based upon balancing catalyst costs against the net return from reactant conversion. The optimal policy is full activation to a fractional depth with an inert core, the depth depending upon the Thiele parameter, poisoning time constant, operating time, and an economic parameter. An isothermal, first order, irreversible reaction is considered. Auxiliary calculations of the effectiveness factor are given for the reaction rate in the nonisothermal pellet with the catalyst distribution found to be optimal in the isothermal case. The results of the pellet calculations are utilized in numerically calculating a uniform activation policy that is optimal for a homogeneously poisoned bed. The definition of optimality is on the same basis as that for the single pellet and the results depend upon the same physical parameters in addition to the time constant for convection in the tube relative to that for diffusion in the pellets. Four regimes of behavior arise depending upon catalyst costs: (i) the reactor cannot be operated economically, (ii) only partial activation policies are economical, a single one being optimal, (iii) both partial and full activation schemes are economical, with one of the former being optimal, and (iv) full activation is optimal but the reactor can be operated economically with partially activated pellets. The results for both the single pellet and the packed tube are viewed as the homogeneous poisoning limit of many practical problems and are believed to reflect the major characteristics of more complicated poisoning mechanisms.

Modeling of chemical reactors—XX heat and mass transfer in porous catalyst the particle in a non-uniform external field

A problem of simultaneous heat and mass transfer in a porous catalyst in a nonuniform external field is solved. The behaviour of such systems for a sphere as well as an infinite slab is investigated. A relaxation procedure for solving the non-linear elliptic equations describing heat and mass transfer inside a catalyst is presented. On the basis of the computed results the shape of the concentration fields inside as well as the effect of an external field on the effectiveness factor are discussed. For an isothermal catalyst the effect of an external field may be neglected. For a nonisothermal pellet the maximum decrease of 10 per cent in effectiveness factor can be expected.