Two-step solvent gradients in simulated moving bed chromatography. Numerical study for linear equilibria

The application of gradients in simulated moving bed (SMB) chromatography has recently attracted interest as a method for further improving the performance of this continuous separation process. One possible implementation of gradients consists in setting the solvent strength in the desorbent stream higher than that in the feed stream. As a result, the components to be separated are more retained in the zones upstream of the feed position and more easily eluted in the zones downstream of the feed position. If a liquid mobile phase is used, gradients can be created by dosing different solvents into the feed and desorbent ports. In a closed-loop gradient SMB arrangement the solvent strength within the unit will depend on the two feed compositions and on the characteristic flow-rates of the process. In this work an equilibrium stage model describing a true moving bed process is used to analyze numerically the main features of a two-step gradient SMB process. The adsorption isotherms are assumed to be always linear under isocratic conditions. The relevant Henry constants depend in a nonlinear manner on the composition of the solvent. Based on numerical simulations the impact of the two inlet solvent compositions is demonstrated in terms of the size and shape of regions of applicable flow-rates. Different strategies of designing the process are discussed and compared with respect to maximizing productivities and minimizing desorbent requirements.     

Solvent gradient operation of simulated moving beds. I. Linear isotherms

The simulated moving bed (SMB) is a multi-column chromatographic separation process, which--with respect to the single-column preparative batch process--allows for a continuous separation with larger productivity and smaller solvent consumption at the same time. The benefits of this process have been shown for several different applications in fine chemistry, particularly for the separation of enantiomers. In general, SMBs are operated under isocratic conditions. However, separation performance can be further improved by applying some sort of gradient mode operation, in order to optimize the operating conditions of each individual section of the unit. This can be achieved by tuning the retention behavior of the solutes to be separated along the unit, namely by enforcing weak adsorption conditions in sections 1 and 2, and strong adsorption conditions in sections 3 and 4. This can be achieved by applying a temperature gradient (high temperature in section 1, and low temperature in section 4), a pressure gradient (e.g. in the supercritical SMB, when pressure is high in section 1, and low in section 4), or a solvent gradient, which is the aim of this work. In the solvent gradient mode the mobile phase consists of a mixture of two or more solvents. To different mobile phase compositions corresponds a different retention behavior of the solutes, i.e. different adsorption isotherms. In this work we study a closed loop SMB unit with solvent mixtures of two different compositions entering the unit at the feed and desorbent inlet ports, respectively. Thereby two different mobile phase compositions are established in sections 1 and 2, and sections 3 and 4, respectively. To optimize this process the equilibrium theory design criteria for non-linear SMBs are extended to describe this operation mode. It is shown how the region of separation is derived and how the optimal operating conditions can be found. Finally the solvent gradient mode is compared with the isocratic mode in terms of productivity and solvent consumption.     

Solvent gradient operation of simulated moving beds. 2. Langmuir isotherms

Simulated Moving Bed separations of enantiomers or fine chemicals are usually carried out in the isocratic mode, i.e. by applying the same operating conditions (temperature, pressure, mobile phase composition, pH) in the whole SMB unit. However, it has been recently recognized that by properly modulating operating conditions in the SMB sections. i.e. Sections 1-4 normally, separation performance in terms of productivity and solvent consumption can be significantly improved. In this work, we study solvent gradient SMB (SG-SMB) operation, where the concentration of a modifier in the main solvent constituting the mobile phase is adjusted along the SMB unit, so as to have weaker retention of the species to be separated in the first two sections, and stronger retention in Sections 3 and 4. Overload chromatographic conditions are considered, where the adsorption behavior is characterized by a nonlinear competitive adsorption isotherm, e.g. a binary Langmuir isotherm. Design criteria to achieve complete separation are developed in the frame of the equilibrium theory of chromatography. The theoretical findings are discussed in view of typical effects of the modifier concentration on retention times and solubility of the species to be separated, and an overall assessment of the SG-SMB technology is attempted.     

反相高效液相三元流动相台阶梯度分离条件的快速优化方法

 根据溶质在柱内的迁移规律 ,建立了一种利用线性梯度实验快速获得溶质保留值方程系数 ,然后以串行响应函数为优化指标进行多台阶梯度分离条件优化的方法。与利用等度实验获得保留值方程的方法相比 ,该法可以大大缩短优化时间。通过该方法对芳香胺和衍生化氨基酸样品进行了分离 ,获得了满意的分离度 ,表明该方法的预测精度很好。     

Optimization of azeotropic protein separations in gradient and isocratic ion-exchange simulated moving bed chromatography

The separation of dilute binary mixtures of proteins by salt aided ion-exchange simulated moving bed (SMB) chromatography is optimized with respect to throughput, desorbent consumption and salt consumption. The optimal flow-rate ratios are analytically determined via an adopted "triangle theory". Azeotropic phenomena are included in this procedure. The salt concentrations in the feed and recycled liquid are subsequently determined by numerical optimization. The azeotropic separation of bovine serum albumin and a yeast protein is used to illustrate the procedure. Gradient operation of the SMB is generally preferred over isocratic operation. A feed of azeotropic salt concentration can only be separated in a gradient SMB. Desorbent and salt consumption are always lower in gradient than in isocratic SMB chromatography.     

Separation of stereoisomers in a simulated moving bed-supercritical fluid chromatography plant

The combination of two techniques, simulated moving bed (SMB) and supercritical fluid chromatography (SFC), leads to an apparatus with unique features. Besides the known advantages of the SMB process, like reduced solvent consumption and its continuity, the use of supercritical carbon dioxide as the mobile phase offers an easy product recovery by depressurizing the supercritical fluid. Details of a SMB-SFC plant are presented for the first time. Due to the large number of process parameters a simulation of the SMB process is necessary to achieve optimal operating conditions. The most important thermodynamic information for a SMB process is the adsorption isotherms. Therefore, isotherms for two phytol isomers are measured and correlated. A fast dynamic model for the simulation of SMB is used to calculate the region of complete separation taking different column configurations and the compressibility of the mobile phase into account.     

Retention modeling and simultaneous optimization of pH value and gradient steepness in RP‐HPLC using feed‐forward neural networks

A novel approach is proposed for the simultaneous optimization of mobile phase pH and gradient steepness in RP‐HPLC using artificial neural networks. By presetting the initial and final concentration of the organic solvent, a limited number of experiments with different gradient time and pH value of mobile phase are arranged in the two‐dimensional space of mobile phase parameters. The retention behavior of each solute is modeled using an individual artificial neural network. An “early stopping” strategy is adopted to ensure the predicting capability of neural networks. The trained neural networks can be used to predict the retention time of solutes under arbitrary mobile phase conditions in the optimization region. Finally, the optimal separation conditions can be found according to a global resolution function. The effectiveness of this method is validated by optimization of separation conditions for amino acids derivatised by a new fluorescent reagent.     

Simulated moving bed process with cyclic modulation of the feed concentration

The improvement of the simulated moving bed (SMB) process based on the introduction of a cyclic modulation of the feed concentration is described. It is demonstrated that such a feed concentration gradient during the shifting cycle can improve the performance significantly. The productivity and the product concentrations can be increased while simultaneously the solvent consumption can be decreased compared to the conventional SMB process with constant feed parameters.     

Novel simulated moving-bed method for reduced solvent consumption

Simulated moving-bed (SMB) chromatography is attractive for reducing sorbent and solvent consumption relative to fixed-bed systems. In this contribution, we describe a novel and versatile method for further reducing solvent consumption in the case of reversed-phase chromatography. The method is based on the variation of the distribution coefficients of solutes to be separated upon varying the composition of a multi-component mobile phase. If the solvent strength of the desorbent is set higher than the solvent strength of the feed, the components will have smaller distribution coefficients in the extraction section of the SMB and hence will be more easily eluted. This will result in a lower desorbent flow and possibly also in a shorter desorbent zone, and, ultimately, in more concentrated products. The so-called "Triangle-method" by Storti et al. [AIChE J., 39 (1993) 471] to obtain the region of complete separation, is extended for this novel SMB method. Theoretical evaluation of the proposed methodology supports the anticipated solvent reduction relative to fixed-bed RP-HPLC for the cases of the purification of the polyketide antibiotic nystatin and the separation of bovine insulin from porcine insulin.     

Linear two-step gradient counter-current chromatography analysis based on a recursive solution of an equilibrium stage model

The implementation of gradients in continuously operated chromatographic counter-current processes has recently attracted considerable interest as a method to improve the performance of this effective separation method. If liquid mobile phases are applied it is advantageous to set the solvent strength in the desorbent stream higher than that in the feed stream. As a consequence. the components to be separated are more retained in the adsorption zones and more easily eluted in the desorption zones. Due to the additional degrees of freedom the design and the optimization of such a two-step gradient counter-current process is difficult. In this paper a steady state equilibrium stage model is used to simulate the process under linear conditions. A simple solution of the underlying model equations is presented capable to describe efficiently the unit for large stage numbers typically encountered in chromatographic columns. Due to the rapidity of the algorithm developed a broad range of operating conditions can be evaluated systematically for different types of gradients. The impact of (a) the functional dependence of the adsorption equilibrium constants on the solvent composition, (b) the number of equilibrium stages and (c) the specification of purity requirements is illustrated and discussed based on results of parametric calculations. The results achieved emphasize the potential of two-step gradient counter-current chromatography.     

The role of the mobile phase in the resolution of racemic esters of aminoacids by normal phase high-performance liquid chromatography

Summary The work in this paper demonstrates the effect of the mobile phase composition on interactions involving racemic esters of aminoacids and a chiral stationary phase derived from N-formylisoleucine. Aprotic solvents have been employed to test the resolution of these racemates. An increase in the alcohol concentration of the mobile phase results in the deterioration of selectivity. These solvents seem to favour high separation factors. The solvent 1,1,2-trichlorotri fluoroethane is a suitable mobile phase modifier for normal phase liquid chromatographic resolution of enantiomers of aminoacid esters.     

Optimal operation of simulated moving bed chromatographic processes by means of simple feedback control

In this contribution, simple methods are presented for controlling a simulated moving bed (SMB) chromatographic process with standard PI (proportional integral) controllers. The first method represents a simple and model-free inferential control scheme which was motivated from common distillation column control. The SMB unit is equipped with UV detectors. The UV signals in the four separation zones of the unit are fixed by four corresponding PI controllers calculating the ratio of liquid and solid flow in the respective separation zone. In order to be able to adjust the product purity a second, model-based control scheme is proposed. It makes use of the nonlinear wave propagation phenomena in the apparatus. The controlled chromatographic unit is automatically working with minimum solvent consumption and maximum feed throughput--without any numerical optimization calculations. This control algorithm can therefore also be applied for fast optimization of SMB processes.     

Ion-Interaction Chromatographic Separation of Free Amino Acids

Abstract

An extensive study of the HPLC separation of 20 free amino acids by the addition of alkanesulfonate salts to the mobile phase was previously reported (1). This paper describes modifications in the procedure that improves the separation and resolution of the 20 free amino acids. Mobile phase variables (type and concentration of alkanesulfonate salt, organic modifier concentration, mobile phase pH, and mobile phase ionic strength), and stationary phase variables (particle size, type of packing) which can affect amino acid separation, resolution and selectivity were studied. Two stationary phases were compared, the 5 μm Hamilton PRP-1 and Phase Separations 3 μm, ODS-2. Longer chain alkanesulfonate salts (octane and decanesulfonate salts) were evaluated as mobile phase additives. A mobile phase gradient of increasing per cent organic modifier was necessary for separating complex mixtures of polar and nonpolar-basic amino acids. It is now possible to separate 19 of 20 free amino acids with this ion-interaction chromatographic procedure.     

Model-based design of a pilot-scale simulated moving bed for purification of citric acid from fermentation broth

One of the conventional processes used for the recovery of citric acid from its fermentation broth is environmentally harmful and cost intensive. In this work an innovative benign process, which comprises simulated moving bed (SMB) technology and use of a tailor-made tertiary poly(4-vinylpyridine) (PVP) resin as a stationary phase is proposed. This paper focuses on a model-based design of the operation conditions for an existing pilot-scale SMB plant. The SMB unit is modeled on the basis of experimentally determined hydrodynamics, thermodynamics and mass transfer characteristics in a single chromatographic column. Three mathematical models are applied and validated for the prediction of the experimentally attained breakthrough and elution profiles of citric acid and the main impurity component (glucose). The transport dispersive model was selected for the SMB simulation and design studies, since it gives a satisfactory prediction of the elution profiles within acceptable computational time. The equivalent true moving bed (TMB) and SMB models give a good prediction of the experimentally attained SMB separation performances, obtained with a real clarified and concentrated fermentation broth as a feed mixture. The SMB separation requirements are set to at least 99.8% citric acid purity and 90% citric acid recovery in the extract stream. The complete regeneration in sections 1 and 4 is unnecessary. Therefore the net flow rates in all four SMB sections have been considered in the unit design. The influences of the operating conditions (the flow rate in each section, switching time and unit configuration) on the SMB performances were investigated systematically. The resulting SMB design provides 99.8% citric acid purity and 97.2% citric acid recovery in the extract. In addition the citric acid concentration in the extract is a half of its concentration in the pretreated fermentation broth (feed).     

Ion-pair reversed-phase high-performance liquid chromatography analysis of oligonucleotides: retention prediction

An ion-pair reversed-phase HPLC method was evaluated for the separation of synthetic oligonucleotides. Mass transfer in the stationary phase was found to be a major factor contributing to peak broadening on porous C18 stationary phases. A small sorbent particle size (2.5 microm), elevated temperature and a relatively slow flow-rate were utilized to enhance mass transfer. A short 50 mm column allows for an efficient separation up to 30mer oligonucleotides. The separation strategy consists of a shallow linear gradient of organic modifier, optimal initial gradient strength, and the use of an ion-pairing buffer. The triethylammonium acetate ion-pairing mobile phases have been traditionally used for oligonucleotide separations with good result. However, the oligonucleotide retention is affected by its nucleotide composition. We developed a mathematical model for the prediction of oligonucleotide retention from sequence and length. We used the model successfully to select the optimal initial gradient strength for fast HPLC purification of synthetic oligonucleotides. We also utilized ion-pairing mobile phases comprised of triethylamine (TEA) buffered by hexafluoroisopropanol (HFIP). The TEA-HFIP aqueous buffers are useful for a highly efficient and less sequence-dependent separation of heterooligonucleotides.     

On-line monitoring of enantiomer concentration in chiral simulated moving bed chromatography

Simulated moving bed chromatography is a key technology for the pilot and production scale separation of enantiomers of chiral chemical species. Product quality control is probably the most important issue in this kind of separation at both scales, and for this it is clear that on-line monitoring of absolute enantiomer concentrations plays a major role. In this work, an on-line system consisting of a UV detector and a polarimeter in series is used to monitor the composition of the extract and raffinate streams of a laboratory SMB unit. The model system adopted is the separation of the enantiomers of the Tr?ger's base on microcrystalline cellulose triacetate (CTA) using ethanol as mobile phase. The technique is effective and accurate, thus providing promising perspectives for SMB process control and dynamic optimization.     

Implementation of an automated on-line high-performance liquid chromatography monitoring system for ‘cycle to cycle’ control of simulated moving beds

In continuous chromatography simulated moving bed (SMB) is a firmly established powerful technique for the separation of fine chemicals and enantiomers. The use of a controller could improve the operation conditions and increase the productivity of an SMB unit. However, the performance of any controller is greatly affected by the reliability and the quality of the feedback information from the plant. Therefore, to overcome the limitations of optical detectors, such as UV and polarimeter, an automated on-line HPLC monitoring system was developed and installed to monitor the product streams. The performance of the system is tested experimentally separating a mixture of guaifenesin enantiomers on Chiralcel OD columns with ethanol as mobile phase in our laboratory SMB unit under both linear and nonlinear chromatographic conditions. The results show that the new monitoring system provides precise and accurate data about the concentration of the components in the two product streams. Moreover, they prove that despite disturbances a combination of the controller and the new on-line monitoring system allows to fulfill the product specifications and to improve the performance of the process in terms of feed throughput and solvent consumption.     

Analysis of phenolic acids in fruits by HPLC with monolithic columns

Reversed-phase HPLC was optimised for simultaneous determination of several derivatives of benzoic and hydroxycinnamic acids (so-called phenolic acids) in plums using a commercially available monolithic column. Mobile phase pH and concentration of organic modifier (methanol and acetonitrile) were tested in order to obtain the best resolution. Satisfactory separation was achieved in gradient mode with a mobile phase consisting of 50 mM phosphate buffer at pH 2.2 (solvent A) and acetonitrile (solvent B). The limits of detection for a UV detector ranged between 0.098 and 2.04 microg/mL for vanillic acid and p-hydroxyphenylacetic acid, respectively. The developed method was used for monitoring the content of polyphenolic acids in plums during their ripening process. The presence of these constituents was confirmed by checking their MS spectra.     

Automation of pH Optimization Experiments During LC Development

The selection of an optimal mobile phase pH under solvent gradient conditions is experimentally challenging. Although quaternary pumps are widely available, they are often used in binary mode to run simple solvent gradients with one pH-adjusted buffer at a time. A more effective use of quaternary pumps is to deliver two different aqueous buffer components (A and B) in a constant proportion to simulate a single, premixed buffer component, while simultaneously producing a solvent gradient by increasing the organic solvent component (S). This approach largely automates the pH optimization experiments. A more detailed investigation of pH effects becomes possible with less time and effort. Once a suitable pH has been identified, the same separation can be reproduced by a simpler binary gradient method which is more suitable for routine work. This study demonstrates the feasibility of this approach both theoretically and through actual examples.     

Optimization of a hybrid chromatography-crystallization process for the separation of Tröger's base enantiomers

This paper presents an analysis of a hybrid process consisting of simulated moving bed (SMB) chromatography and crystallization and studies its performance for the separation of the Tr?ger's base enantiomers. The SMB is simulated using a detailed model including column efficiency, thus, implying a proper evaluation of the effect of column size on column efficiency and separation performance. The crystallization operations are accounted for through material balances, assuming equilibrium between enantiopure crystals and mother liquor. A genetic algorithm is used to optimize the combined process, using proper definitions of objective functions. Multi-objective optimization of this hybrid process for productivity and evaporation cost in terms of operating parameters, column length, and SMB feed concentration shows an optimum SMB purity value as a trade off between increased SMB performance and recycle of the mother liquor.