Eluent specific consumption

For a preparahve applicahon, focus is on recovering the targeted products while ophmizing produchon costs. The object of the separation is to reach the purity and recovery yield required for one or more specific components of the feed mixture. To maximize produchon, injechons are made as often as possible. The amount of stahonary phase used is set in order to minimize the costs of the product, equipment, and eluent consumption. Figure 12.2 presents the preparahve chromatogram of the same compounds as in Figure 12.1, where the injected amount is maximized in order to ophmize the process [5]. 

In all cases, using 5 colinnns, the optimal column configurations are found to be 1/2/1/1 and 1/1/1/2—I/I/2/1-1/1/2/1—1/2/1/1 for the SMB and 4 subinterval Varicol unit, respectively. It can be observed that for fixed purity specifications, both the SMB and the Varicol processes require to increase the eluent consumption in order to increase the feed flow rate. Secondly, the Varicol process consumes less eluent, D than the SMB process for the same feed flow rate, F or equivalently for the same eluent consumption, D, the Varicol process can treat more feed, F. However, the extent of improvement depends on the purity specifications. The more stringent the purity requirement, the larger the improvement achieved by Varicol over SMB. For example, at D = 5.6 ml/min, the improvement in production rate, F of Varicol over SMB is 10%, 25% and 127% for a purity requirement both in the extract and in the raffinate streams of 90%, 95% and 99%, respectively. Finally, it is seen from Figure 3 (for the case of purity requirement of 95%)... 

In many cases the cost of eluent (i.e. solvent) is not negligible and in some cases it even represents the greatest contribution to the total separation cost. Therefore, it is advisable in these cases to observe the eluent consumption, or better still, the efficiency of eluent-usage during the chromatographic separation. This can be characterized by specific eluent consumption EQ, which means the amount of eluent required to purify a certain amount of product i. 

Yield, productivity, eluent consumption and purity are clear defined objective functions, while total separation cost and other objective functions related to economics depend on the individual company and are based on site-related parameters. Therefore, the calculation of total separation cost is complex due to various influencing parameters and cost structures. This chapter proposes the following cost functions, which can be easily adapted to specific conditions ... 

Since values that characterize column performance (i.e. yield, eluent consumption and productivity) at the optimal point are known, the required column diameter for a desired production rate can be easily determined using volume-specific productivity at the optimal point (Eq. 7.9). 

Figure 7.10 illustrates the inverted values of specific eluent consumption in dependency of dimensionless parameters. Higher values are therefore the more favorable values in Fig. 7.10 as they indicate a small eluent consumption. The dependency of... 

Dependency of specific eluent consumption on number of stages and loading factor. 

Needless to say that, beside the operating parameters, the design of an SMB plant has a significant influence on the process performance in terms of various objective functions like the volume and cross section specific productivity (VSP mb and ASPi>SMB) as well as the eluent consumption EQjSmb. Important design parameters for SMB processes are... 

According to Tab. 7.11 a maximum VSP is reached for a total number of plates of approximately 65 and the corresponding flow rate ratios m,. This is more than a twofold increase over the productivity at a plate number of 231. But, in addition to the development of productivity, the specific eluent consumption (EC) has increased. As reported by Jupke (2002) analogue correlations can be found for different particle diameter. With decreasing particle diameter an increase in terms of productivity can be observed, while the total number of stages remains nearly constant. 

Figure 7.28 Influence of the plate-number on the specific productivity and specific eluent consumption.

Figure 7.31 Comparison of specific eluent consumption at 99.9% purity.

Figure 7.33 Comparison of specific eluent consumption (EC) between two batch modes.

Figure 18.45 Influence of the plate number on the specific productivity, specific eluent consumption and specific separation costs. Reproduced with permission from A. Jupke et ah, ]. Chromatogr., 944 (2002) 93 (Fig. 17).

Jupke et al. also studied the economics of the SMB separation. They determined the specific productivity, the specific eluent consumption and the specific separation costs as functions of the column plate numbers. The specific costs were determined for two different production rates 1000 and 5000 kg/year. They could calculate the optimum plate number for maximum value of the specific productivity. The eluent consumption and the specific separation costs continuously decrease with increasing column plate number. Both are relatively high at the maximum specific productivity. These results are similar to those derived in the case of batch chromatography. The economic optimum is a compromise between the effects of conflicting experimental parameters. 

---