True Moving Bed Model Research Articles

Bayesian optimization for quick determination of operating variables of simulated moving bed chromatography

The Simulated Moving Bed (SMB) is a continuous chromatographic separation process that operates on the principle of counter-current movement between the solid and liquid phases. Due to periodic switching of feed and product ports across numerous connected columns, adjusting SMB operating variables such as feed and product flow rates and switching time to achieve desired separations is challenging. While equilibrium theory can help narrow the search space, obtaining essential information such as accurate adsorption isotherms is crucial. This requirement, combined with often highly stringent production specifications, makes it challenging to identify even a feasible operating condition, let alone an optimal one. Trial-and-error-based approaches are often impractical as reaching cyclic steady state can be time-consuming, and any waste produced during this period can lead to significant economic losses. While rigorous dynamic models are available, they are computationally intensive and often do not accurately mirror actual process behavior. To address these challenges, the use of Bayesian Optimization (BO) is proposed to sequentially approach optimal SMB operation. Furthermore, it is suggested to employ the simpler True Moving Bed (TMB) model as a prior for the BO, which significantly accelerates convergence. This approach is demonstrated on an SMB process for cresol separation. Initially, the effectiveness of the BO using the TMB model is examined to gain insights into its behavior. Subsequently, we apply BO to the rigorous SMB model, informed by prior knowledge from the TMB model. Our results show that the developed BO framework rapidly converges to the optimal operating parameters that satisfy the purity constraints. We examine the efficiency improvements over various search algorithms and highlight the advantages of using the TMB model as a prior.

Model-based cost optimization of a reaction–separation integrated process for the enzymatic production of the rare sugar d-psicose at elevated temperatures

The integration of biocatalysis with continuous separation of product and substrate constitutes an attractive option for overcoming the intrinsic yield limitation in enzymatic reactions that suffer from an unfavourable position of the equilibrium. One example is the recently established continuous process combining enzymatic epimerization in an enzyme membrane reactor (EMR), simulated moving bed (SMB) chromatography and nanofiltration for the high-yield production of the rare sugar d-psicose from its bulk epimer d-fructose with recycling of the unused epimer to the EMR. In this work, we present a first, comprehensive analysis of this novel reaction-separation process concept on the basis of an integrated process model. Therefore, a process model consisting of a continuous stirred tank reactor model with reversible Michaelis–Menten kinetics for representation of EMR operation, a transport-dispersive true moving bed model for representation of SMB operation and an ideal filtration model was constructed. As enzyme costs are particularly difficult to estimate due to their strong dependence on the reactor temperature, we further established a model-based procedure for characterizing the biocatalyst in terms of operational stability and activation as a function of the reactor operating temperature. The integrated process model then enabled the identification of optimum operating points for a fully integrated process operation. First, the established design procedure was used to implement a highly productive run on our existing lab-scale plant (i.e. 2.3kg of d-psicose per L of SMB column volume and day and 4.5kg d-psicose per g enzyme and day), where Dpsicose was produced both at high purity (≈ 98%) and at remarkable yield (96%). Next, the established process model was used for the optimization of the overall process economics. Therefore, a comprehensive analysis of the process in terms of suitable reactor temperatures was performed based on a multi-objective function addressing important cost contributions in the integrated process (i.e. enzyme, desorbent, solid phase, substrate). The presented integrated process scheme can be easily adapted to a number of similar isomerization and epimerization reactions enabling economic production of (almost) the complete set of C6 sugars.

Intensification of Paraxylene Production using a Simulated Moving Bed Reactor

Multifunctional reactors, which combine a reaction step and a separation step in one single unit, constitute an important advance in design of sustainable processes to save energy and reduce environmental impact. They allow reductions of recycle flows and size units in order to have more safety and less expansive processes. This paper deals with separation by adsorption and reaction coupled in a Simulated Moving Bed reactor (SMBR) for paraxylene (PX) production. In the current industrial process, the major part of the separation step comes from a recycle flow where the C<sub>8<sub/> aromatics are isomerized. The SMBR, by decreasing this recycle stream, may reduce the energy needed to treat and convert the raffinate into a rich PX stream. As separation takes place in the liquid phase, the first part of this paper establishes the feasibility of liquid phase isomerization of xylene. Tests in a fixed bed reactor validate the use of a HZSM-5 zeolite catalyst. Paradiethylbenzene (paraDEB), the classical desorbent used in xylene separation, isomerizes into orthodiethylbenzene and metadiethylbenzene so it is replaced by toluene. Experimental data permit one to estimate the parameters used in a simple analytical model implemented in a classical True Moving Bed model. This TMBR model permits to find the various operating regimes of such a SMBR. The conditions found allow a 40% reduction of the recycle flow without any productivity loss. With this lower recycle flow, a reduction of investment and operating costs is expected on the global PX production process thanks to the SMBR process.

A theoretical study of continuous counter-current chromatography for adsorption isotherms with inflection points

Continuous counter-current chromatographic processes have been increasingly applied in the last years. For chromatographic systems characterized by linear or Langmuir adsorption equilibria, there are nowadays reliable design rules available to decide whether a separation is feasible and which operating parameters should be used. For more complex equilibrium functions, theoretical methods are less developed. More realistic adsorption isotherms are frequently characterized by inflection points in their courses. A flexible model capable to quantify single solute and competitive adsorption isotherms is provided by statistical thermodynamics. In this work, second-order truncations of a statistical model involving inflection points are investigated. The classical scanning technique is used to identify the region of applicable operating parameters. Then an alternative approach is suggested, which verifies the existence and shape of the suitable parameter region by infeasibility certificates. Using the True Moving Bed model, both approaches are compared for different feed concentrations, purity requirements and column efficiencies.

Performance of simulated moving bed with conventional and monolith columns

In this article the performances of different bed structures, i.e., conventional adsorbent beads (with different diameters) and monolithic columns, in the separation of chiral species by the simulated moving bed (SMB) technology are compared. In order to assess these different morphologies the equivalent particle size for monolithic structure was calculated based on equivalent permeabilities. The performance of the SMB units was compared for the conditions quantified by maximum productivity. The configuration of the SMB unit and the total system volume were not varied. The maximum operating pressure drop of 20 bar and the purity in both product streams over 99% were set as constraints in all computations. The true moving bed model was used as an analogy to simulated moving bed in order to reduce computation time. The productivity of the SMB unit using particle diameter of 27 μm can be slightly higher than of the SMB with particle diameter ≥50 μm but at the cost of higher eluent consumption. Moreover, there is no need to operate an SMB unit at its maximum pressure drop to get the best performance values, since in the presence of mass-transfer resistances, the contact time becomes the unit major limitation.

Operation of an Industrial SMB Unit for p-xylene Separation Accounting for Adsorbent Ageing Problems

The industrial separation of p-xylene was considered by means of a standard SMB (Simulated Moving Bed) system as well as the Varicol mode of operation. Both modes of operation were simulated accounting for adsorbent capacity decline using the TMB (True Moving Bed) approach in the case of standard SMB and by a TMB with a non-integer number of columns per section in the Varicol case. Additionally, the TMB model takes into account variable fluid velocity due to the adsorption/desorption rate of p-xylene and its isomers. Dynamic results were used to study the influence of the adsorbent capacity decline on the SMB unit performances and two different compensating measures were tested for a 10 years period: (i) switching time decrease (solid flowrate increase) and (ii) columns redistribution. Both strategies kept the initial purity requirements and reached higher productivity values than the respective unit working without any compensation measure. The switching time compensating strategy proved to be more efficient than the second one, achieving, for the same purity requirements, higher productivity values.

On simplified modelling approaches to SMB processes

The simulated moving bed (SMB) technology is important in various fields, from sugar to enantiomer separation, and operating conditions must be carefully selected and regulated, based on an appropriate process model. Basically, two modelling approaches exist, i.e., true moving bed (TMB) and SMB models. Both approaches show advantages and drawbacks. In this work, an attempt is made to develop a model which retains the simplicity of the original TMB model and its modest computational load, while capturing the essential features of the cyclic steady state reproduced by the SMB model.

An optimization strategy for nonlinear simulated moving bed chromatography: Multi-level optimization procedure (MLOP)

This paper presents a multi-level optimization strategy to obtain optimum operating conditions (four flow-rates and cycle time) of nonlinear simulated moving bed chromatography. The multi-level optimization procedure (MLOP) approaches systematically from initialization to optimization with two objective functions (productivity and desorbent consumption), employing the standing wave analysis, the true moving bed (TMB) model and the simulated moving bed (SMB) model. The procedure is constructed on a non-worse solution property advancing level by level and its solution does not mean a global optimum. That is, the lower desorbent consumption under the higher productivity is successively obtained on the basis of the SMB model, as the two SMB-model optimizations are repeated by using a standard SQP (successive quadratic programming) algorithm. This approach takes advantage of the TMB model as well as surmounts shortcomings of the TMB model in the general case of any nonlinear adsorption isotherm using the SMB model. The MLOP is evaluated on two nonlinear SMB cases characterized by i) quasi-linear/non-equilibrium and ii) nonlinear/nonequilibrium model. For the two cases, the MLOP yields a satisfactory solution for high productivity and low desorbent consumption within required purities.

On simplified modelling approaches to SMB processes

The Simulated Moving Bed (SMB) technology is important in various fields, from sugar to enantiomer separation, and operating conditions must be carefully selected and regulated, based on an appropriate process model. Basically, two modelling approaches exist, i.e. True Moving Bed (TMB) and 5MB models. Both approaches show advantages and drawbacks. In this work, an attempt is made to develop time-varying-velocity TMB models, which retain the simplicity of the original TMB model and its modest computational load, while capturing the essential features of the cyclic steady state reproduced by the SMB model. Alternative modelling approaches are compared and their use is discussed.

Continuous Separation of Enantiomers by Simulated Moving Bed Chromatography: Effect of Operating Parameters on the System Performance

The separation of 1,1’-bi-2-naphthol racemic mixtures by the simulated moving bed (SMB) method with chiral columns of Pirkle D-Phenylglycine was investigated in this study. First, a steady-state true moving bed (TMB) model, considering non-linear equilibrium, lumped kinetic and axial dispersion effects, was developed for predicting the cyclic steady-state behavior of the SMB system. The “triangle theory” was used to provide guidelines for the selection of feasible operating parameters of the SMB system, including liquid flow rates, switch time, feed concentration and zone length. Then, the effect of various operating parameters on the SMB performance, characterized by product purity, recovery, solvent consumption, and productivity, and the exact good separation region were studied through simulation of the TMB model. The package is an important tool to select the optimal operating conditions for the SMB operation.

Mass‐transfer effects during separation of proteins in SMB by size exclusion

AbstractThe chromatographic fractionation of proteins by size‐exclusion chromatography in a simulated moving bed (SMB) is studied. During experimental fractionation of a mixture of bovine serum albumin (BSA) and myoglobin on Sepharose Big Beads, mass‐transfer effects are shown to limit the performance of the SMB. The internal profiles, as well as the extract and raffinate compositions, are described well by a steady‐state equivalent true moving‐bed (TMB) model that incorporates mass‐transfer effects. The selection of the particle size in SMB is a trade‐off between productivity and mass transfer. Based on the equivalent TMB model, the optimum particle size and configuration of the SMB can be selected, at which preset performance criteria (purity, recovery) are met at specified flow‐rate ratios, total column length, and pressure drop. For the current feed and apparatus, an optimal particle size of approximately 145 μm is calculated for achievement of purities and overall recoveries of 95%.

Modeling and Simulation of a Simulated Moving Bed for the Separation of p-Xylene

The industrial-scale adsorptive separation of p-xylene from a mixture of C8 aromatics in a four-section simulated moving bed (SMB) unit is analyzed through simulation. In the order to describe the behavior of the SMB unit by means of a mathematical model, two approaches were used: the true moving bed (TMB) approach and the SMB approach. Both approaches assume a constant selectivity nonstoichiometric Langmuir isotherm, an axial dispersion for the fluid flow, and a linear driving force for the intraparticle mass transfer. The TMB and SMB model predictions of steady-state performance of the SMB unit are very close. Therefore, the TMB model was selected to study the effect of the switching time period, adsorbent deactivation, and mass-transfer resistances on the process performance. The adsorbent deactivation which occurs during its lifetime will influence the SMB performance; to keep the p-xylene purity and acceptable recovery, the switching time should be decreased. The TMB package is a useful tool for fast visualization of the SMB process behavior under particular operating conditions. The novel “separation volume” methodology is used to find the operating conditions of the SMB unit in the presence of mass-transfer resistances.

Modeling and simulation of a SMB chromatographic process designed for enantioseparation

Abstract This paper focuses on modeling of a simulated moving bed process (SMB) dedicated to the separation of racemic mixtures. In the first approach, a true moving bed model is derived, which assumes an equivalent counter-current movement of the solid phase. The good agreement between the model and the real system is demonstrated with experimental results. Then, a more rigorous approach is developed, which considers the system as an arrangement of static chromatographic columns and takes into account periodic switching. Attention is focused on model formulation and numerical solution techniques in order to develop efficient dynamic simulation programs.

Modeling and Simulation of a SMB Chromatographic Process Designed for Enantioseparation

This paper focuses on modeling of a simulated moving bed process dedicated to the separation of racemic mixtures. In a first approach, a true moving bed model is derived, which assumes an equivalent counter-current movement of the solid phase. The good agreement between the model and the real system is demonstrated with experimental results. Then, a more rigorous approach is developed, which considers the system as an arrangement of static chromatographic columns and takes into account periodic switching. Attention is focused on model formulation and numerical solution techniques in order to develop an efficient dynamic simulation program.

Automatic control of the simulated moving bed process for C8 aromatics separation using asymptotically exact input/output-linearization

Abstract An approach to automatic control of the simulated moving bed process (SMB) applied to the separation of C 8 aromatics is presented. The principle of asymptotically exact input/output-linearization is used. The controller is based on a nonlinear state estimator using the true moving bed model (TMB). The estimator receives measurement data from four spectroscopic measurement cells. The problem of moving measurement positions with respect to the TMB model is addressed. An exactly linearizing feedback of the estimated states is designed using the nonlinear TMB model equations. The performance of the controller is shown in simulations using a detailed SMB model as a representative of the real process.