Operation of the SMB Unit

Nevertheless, near the feed point, there is a difference between TMB and all SMB cases due to the fact that the internal flow rates in the TMB are smaller than in the SMB, leading to a small dilution of the feed stream. As a consequence, near the feed inlet, TMB concentrations will be higher than in the SMB operation. The raffinate and extract purities in SMB units with four (95.2 % and 89.5 %), eight (98.7 % and 95.9%) and 12 columns (99.1 % and 96.8%) are increasing towards the one obtained in the equivalent TMB unit (99.3 % and 97.7 %). The optimum degree of subdivision of the SMB unit will depend of the difficulty of the separation and the product purity requirements. Typically, systems for the pharmaceutical industry have six to 16 columns. 

Let us assume a standard four-zone SMB unit, in which the complete separation of a binary mixture, constituted of the more retained component A and the less retained component B is to be achieved. In the framework of the Equilibrium Theory, the key operating parameters through which the performance of the SMB can be controlled are the flow rate ratios, rrij, /=l,...,4,in the four sections of the SMB unit, according to ... 

When a specific feed composition is given, the constraints on m1 and m4 as well as the complete separation region in the (m2, m3) plane can be determined,since these depend only on the parameters of the adsorption equilibrium isotherms and the feed composition itself. Based on these values an operating point can be selected, i. e. a set of four values of = 1,..., 4 fulfilling the complete separation requirements. Since the flow rate ratios are dimensionless groups combining column volumes, flow rates and switching intervals, the constraints on the flow rate ratios are independent of the size and productivity of the SMB unit. 

The successful operation of an SMB process relies on the successful fulfillment of a number of separation conditions. To achieve a good separation, the flow rates in all four sections must be chosen in such a way that, at the end of each cycle, the fronts and rears of the bands of the two components of the feed are located within specific ranges in the proper column (or section, in a more complex, multi-column xmit). As we discussed earlier, an SMB separation can be studied as an equivalent TMB process, the solid-phase flow rate of which is related to the switching time of the SMB unit by... 

Gradient elution chromatography is a separation method that exploits the effect of the fluid phase composition on the retention behavior of the feed components. It is widely used, especially for analytical separations in the areas of the life sciences, in biochemistry, and in the biotechnologies e.g., separation of complex mixtures of proteins or peptides), hi its conventional implementations, SMB units are operated under isocratic conditions. The composition of the fluid phase, e.g., the organic modifier concentration, the pH, or the buffer concentration remain constant in all the sections of the SMB unit. However, it has recently been shown that SMB units can also be operated under solvent gradient mode (SG-SMB). Then, the feed and desorbent streams introduced have a different composition. The fluid phase composition is different in each section. It is chosen independently, in order to... 

The effect of the particle size was also considered with the multiobjective optimization, using a genetic algorithm [81]. The particle size has a dual effect on the operation of an SMB unit. Larger particles allow a lower back pressure but lead to a poorer column efficiency. The effect of the particle size in the case of a 5-column SMB process is illustrated in Figure 18.43. The largest particle size considered in the study dp — 40jflow rate and the bad column efficiency limits the productivity. The use of an in-... 

In the design of SMB experiments, one is mostly concerned with the projection of the four-dimensional space, (j=l,...4), onto (m2,) plane, i.e., the plane in the operating parameter space spanned by the flow rate ratios of the two key seetions of the SMB unit. From adsorption isotherm determined previously and the feed eoncentration, complete separation regions for propranolol hydroehloride separation was constructed in the (m2, m3) plane, as shown in Figure 5. It is worth noting that for proper operation of SMB to obtain desired complete separation, adsorbent and fluid should be regenerated in section 1 and 4 respectively. 

The conditions for a complete separation of a binary mixture can be defined in terms of the Yj model parameters, which are directly related with the TMB (SMB) operating variables (fluid and solid velocities in the four sections of the TMB unit). From the constraints presented, those related to sections II and III play the crucial role on the separation performance of the TMB. It is in these central zones that the separation between the two species takes place. The role of the adjacent sections (I and IV) is to prevent cross-contamination and to allow the improvement of the continuous operation of the system by regenerating the solid and liquid phases. Taking into account these considerations, a region of complete separation in a Ym-Yn plane can be defined. Considering that the constraints concerning sections I and IV are fulfilled, the YnrYn plot is 311 important tool in the choice of best operating conditions. 

The operating point of an SMB unit is characterized by the flow rates of the liquid phase in the four sections, Vt, as well as the switching time, Another degree of freedom is the concentration of the solute in the feed stream - but according to Char-ton (1995) the feed concentration should be at its maximum to achieve best productivity. For the following considerations feed concentrations as well as temperatures are kept constant. 

In the early 1970s, Union Oil developed and patented a chromatographic system based on the principle of a simulated moving bed (SMB) [6-8]. A schematic of a SMB unit is shown in Figure 1.4. Streams of the mobile phase (the desorbent ) and of the feed to separate are continuously injected into the column while streams of the less retained (the raffinate ) and the more retained components (the extract ) are continuously withdrawn, all at constant flow rates. The rotary valves switch periodically the positions in the columns where these streams enter or exit. The operation of SMB imits is discussed in detail in Chapter 17. Manufacturing facilities have been built and are operated for the fractionation of various petroleiun distillates, for example, the selective separation of p-xylene, o-xylene and ethylbenzene from the C7-C8 aromatic fraction of light petroleum reformates, the separation of olefins from paraffins in feed mixtures of hydrocarbons having 10 to 14... 

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