Z.P. Lu

Importance of intraparticle convection in the performance of chromatographic processes

Abstract Large-pore materials are used in separation engineering as high-performance liquid chromatographic packings and adsorbents; however, they find also many applications in reaction engineering as catalyst supports and ceramic membrane reactors and in biotechnology as supports for mammalian cell cultures or biomass growth. In these large-pore materials, mass transport by intraparticle forced convection should be considered in process analysis. A brief historical survey of research work concerned with intraparticle forced convection is given, showing that the concept to be retained is that of effective diffusivity augmented by convection. The behaviour of the height equivalent to a theoretical plate as a function of bed superficial velocity is explained on the basis of the above concept. Analogies between slab and spherical particle geometries are discussed.

Peak resolution in linear chromatography : effects of intraparticle convection

Abstract The quantitative relationship between separation performance and intraparticle convection in large-pore, permeable supports for chromatography is examined. A model for linear chromatography is used in the analysis for particles that contain both throughpores where diffusive and convective transport occur and purely diffusive micropores. The key concept is that of a convection-augmented intraparticle diffusivity, which depends only on the intraparticle Peclet number, λ. The separation performance is expressed in terms of both resolution and peak profiles at the column outlet. It is shown that, for given operating conditions, the separation enhancement obtained for intraparticle convection is dependent on the relative importance of throughpore and micropore diffusion rates. In the absence of micropore resistances, the resolution is always increased by intraparticle convection. The latter, however, is shown to have no effect on the separation performance when micropore diffusional resistances are dominant. Relationships necessary to assess the importance of these effects for linear chromatography conditions are provided.

Influence of adsorption-desorption kinetics on the performance of chromatographic processes using large-pore supports

Abstract Large-pore packings are being widely used for bioseparations, e.g. high-pressure liquid chromatography and also in some gas-phase applications. Most studies are based on the assumption of instantaneous equilibrium at the pore-solid interface. This paper shows how adsorption-desorption kinetics influences the performance of chromatographic processes measured by the height equivalent to a theoretical plate (HETP). A criterion is derived allowing us to predict when the assumption of instantaneous equilibrium at the pore-solid interface is valid.

Pressurization and blowdown of adsorption beds. II: Effect of the momentum and equilibrium relations on isothermal operation

Abstract Pressurization and blowdown of adsorption beds with a binary mixture of inert and adsorbable gas are studied. For pressurization, the stoichiometric penetration distance for the feed is predicted using a simplified analysis. An equilibrium model is then developed involving material and momentum balance equations to describe the bed dynamics. The pressure drop is given Erguns or Darcys equations, and adsorption equilibrium is described by linear or Langmuir isotherms. The effects of pressure drop equation, isotherm shape, adsorbent capacity, pressure ratio and feed composition on penetration distance, pressurization time and pressurization amount are assessed. For blowdown, the equilibrium model is used to evaluate the bed dynamics. Also, plateau concentrations for blowdown, estimated using a simplified analysis, are compared with predictions of the equilibrium model.

Separation of enantiomers of 1a,2,7,7a-tetrahydro-3-methoxynaphtha-(2,3b)-oxirane by liquid chromatography: laboratory-scale elution chromatography and modelling of simulated moving bed

Abstract The separation of enantiomers of 1 a ,2,7,7 a -tetrahydro-3-methoxynaphtha-(2,3 b )-oxirane (Sandoz epoxide) on cellulose triacetate HPLC columns was investigated on the laboratory scale. The performance of the columns was calculated by HEPT measurements and the slopes of the adsorption equilibrium isotherms and effective diffusivities were calculated from elution chromatographic experiments. Multi-component adsorption equilibrium isotherms were calculated from single isotherms by using the ideal adsorbed solution (IAS) model. Simulation of continuous chromatographic separation of the racemic mixture of Sandoz epoxide in a simulated moving bed was carried out and the effect of mass transfer coefficient on process performance was analysed.

Dynamics of pressurization and blowdown of an adiabatic bed: III

Abstract The importance of heat effects in an adiabatic bed during the pressurization and blowdown steps of pressure swing adsorption (PSA) processes is assessed. The compression energy is included in the energy balance. The effect of the temperature fluctuations on the separation results is much more important than the effect of the adsorption capacity. It is very important and useful to improve the bed dynamics by using high-heat-capacity adsorbent particles or heat exchange in order to reduce the temperature fluctuations in PSA processes as suggested by R.T. Yang and P. Cen ( Ind Eng Chem Des Dev (1986) 25 54–59) 1 .

Dynamics of pressurization and blowdown of an adiabatic adsorption bed: 4. Intraparticle diffusion/convection models

Abstract The importance of intraparticle mass transfer during pressurization and blowdown steps of PSA processes in an adiabatic adsorption bed was assessed by comparing intraparticle diffusion/ convection and intraparticle diffusion models. Film mass/heat transfer resistances are also considered in the model. The film heat transfer resistance is more important than the heat transfer resistance inside the particle; it can be assumed to be negligible in PSA processes when the temperature variation is not very large, otherwise it leads to serious errors when the adsorption capacity of adsorbents is high and the heat capacity of the system is not high. Intraparticle convection improves the mass transfer inside the particle and leads to faster heat releases into and out of the adsorbents.

The effect of intraparticle convection on conversion in heterogeneous isothermal fixed-bed reactors with large-pore catalysts for first-order reactions

Abstract Residence time distributions have been largely used in chemical engineering to make diagnosis of the ill-functioning of chemical reactors and to predict conversion in homogeneous isothermal reactors once the kinetic rate of reaction is known. The prediction is unambiguous for first-order reactions. In this paper, the impulse tracer response of isothermal heterogeneous fixed-bed reactors packed with large-pore catalysts, in which intraparticle convection occurs, is derived. From the impulse tracer response, the residence time distribution of the outer phase is calculated and used to predict the steady state conversion in the reactor for first-order reactions. The effect of intraparticle convection, measured by the intraparticle Peclet number λ for a given reaction-catalyst system, i.e. for a set of Damkholer, Thiele, Biot and Peclet numbers (Da, o, Bi m and Pe respectively) is to drive the conversion between the diffusion-controlled and the kinetic-controlled limits. The problem of scaling from batch to continuous packed beds is also addressed.

Modeling and Operation of a Simulated Moving Bed for the Separation of Optical Isomers

A model for predicting the steady state behavior of the simulated moving bed (SMB) is developed. SMB performance is characterized by purity, recovery, solvent consumption and productivity. The effect of switching time (rotation period), extract flowrate and section length on the SMB performance is discussed. A pilot plant for the separation of optical isomers is operated and tested with two systems: “Sandoz epoxide” and “bi-naphtol”.

Intraparticle Diffusion/Convection Models for Pressurization and Blowdown of Adsorption Beds with Langmuir Isotherm

Abstract Intraparticle diffusion/convection and equilibrium models are used to simulate the bed dynamics of pressurization and blowdown steps of PSA processes with binary mixtures of inert and adsorbable species. The effect of the nature of the equilibrium isotherm, i.e., linear and Langmuir isotherms, is discussed. The improvement of mass transfer inside the adsorbent by increasing particle permeability or decreasing particle size is addressed. Simulation results show that using “large-pore” adsorbents, i.e., increasing permeability to cause a high intraparticle convective flow instead of decreasing particle size to reduce intraparticle mass transfer resistances, is a good choice in PSA processes.