Solvent Feeding Mode

Mujtaba (1999) considered the conventional configuration of BED processes for the separation of binary close boiling and azeotropic mixtures. Dynamic optimisation technique was used for quantitative assessment of the effectiveness of BED processes. Two distinct solvent feeding modes were considered and their implications on the optimisation problem formulation, solution and on the performance of BED processes were discussed. A general Multiperiod Dynamic Optimisation (MDO) problem formulation was presented to obtain optimal separation of all the components in the feed mixture and the recovery of solvent while maximising the overall profitability of the operation. 

First, two examples using both problems OP1 and OP2 are presented to explain the effects of different solvent feeding modes and path constraint on the operation. In these examples only Task 1 of Figure 10.6 is carried out where component 1 is recovered at a given purity. Then, example 3 using Multiperiod Optimisation Problem (OP) is presented, where all three Tasks of Figure 10.6 are carried out. 

Effect oiF - Semi-Continuous Solvent Feeding Mode (Full Charge)... 

Here, the same mixture used for example 1 is considered. Semi-continuous solvent feeding mode with full charge strategy is opted in this example. The objective is to maximise the productivity of Task 1 of the STN shown in Figure 10.6. The specification on the distillate composition is 0.95 molefraction in Heptane. The optimisation problem (OP1) is considered and both the reflux ratio and solvent rate profiles are optimised. Again two time intervals are used for the entire operation period (Task 1). In each interval, constant reflux ratio and solvent feed rate are used, the values of which are optimised. The input data are the same as those in Table 10.1 except that the maximum reboiler capacity is 25 kmol. The solvent is introduced in plate 6 (Nf). 

Figure 8.1-7. Comparison between compressor- and pump-mode operation for fish oil extraction with different solvent/feed ratios.

Mujtaba (1999) discussed two modes of solvent feeding for conventional BED processes. 

Tran and Mujtaba (1997), Mujtaba et al. (1997) and Mujtaba (1999) have used an extension of the Type IV- CMH model described in Chapter 4 and in Mujtaba and Macchietto (1998) in which few extra equations related to the solvent feed plate are added. The model accounts for detailed mass and energy balances with rigorous thermophysical properties calculations and results to a system of Differential and Algebraic Equations (DAEs). For the solution of the optimisation problem the method outlined in Chapter 5 is used which uses CVP techniques. Mujtaba (1999) used both reflux ratio and solvent feed rate (in semi-continuous feeding mode) as the optimisation variables. Piecewise constant values of these variables over the time intervals concerned are assumed. Both the values of these variables and the interval switching times (including the final time) are optimised in all the SDO problems mentioned in the previous section. 

Note that the path constraint does not appear in the optimisation problem for batch mode solvent feeding. 

Campaign Mode Operation with Batch Solvent Feeding... 

Table 10.2. Summary of the Results using Batch Mode Solvent Feeding. [Adopted from Mujtaba, 1999]...

Since the column is operating in full charge mode the constraints given by Equations 10.2 and 10.5 must be satisfied for the first and second intervals respectively. A number of cases were studied using different maximum allowable values for the solvent feed rate (E ) in the first interval. These give values for for the first interval. For each case, Table 10.8 shows the optimal reflux ratio, the solvent feed rate, the lengths of the time intervals and the productivity. The case 1 represents the base case where no solvent was introduced into the column. 

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... 

Pressure Temperature Solvent/Feed Ratio Superficial Velocity Recycle Effects Entrainers and Cosolvents Extractor Size, Shape and Configuration Processing Modes (e.g. Sub/Supercritical CO2 or Dry/Wet CO2 Extraction)... 

Table III shows the results of operating the SRT unit in the hydrogen donor mode (catalytically hydrogenated solvent) with and without the addition of Light SRC to the distillate solvent Batch I solvent was used in Run 9 A blend of Batch VI solvent and Light SRC, 70/30 weight ratio, were catalytically hydrogenated as the feed to Runs 1 and 3 The hydrogen donor capability of the solvents were measured by the Equilibrium microautoclave tests These bench-scale SRT results are rather extraordinary in respect to increased distillate yields and improvement in unit operability with addition of Light SRC In Table III the integrated yields refer to the combination of liquefaction, CSD, and catalytic hydrogenation of the solvent ...

The condensation of an aromatic nitro compound with a second reactant should have been performed in an aqueous solution with DMSO in the semi-batch mode. The nitro-compound is initially charged into the reactor with water and DMSO as solvent. Before the progressive addition of the second reactant had been started, the initial mixture was heated to the process temperatures of 60-70 °C. Then a failure of the cooling water system of the plant occurred. It was decided to interrupt the process at this stage and to maintain the mixture under stirring until the failure had been repaired. The feed of the second reactant was postponed and the jacket of the reactor had been emptied. 

Evaporative-cooling crystallizers are fed with a liquor whose temperature is such that solvent flashes upon feed entry to the crystallizer. They typically are operated under vacuum, and flashing of solvent increases the solute concentration in the remaining liquor while simultaneously reducing the temperature of the magma. The mode of this operation can be reduced to that of a simple cooling crystallizer by returning condensed solvent to the crystallizer body. 

In this mode the solvent is charged in the reboiler with the feed mixture at the beginning of the process. As the reboiler has a limited capacity, it, therefore, limits the amount of feed mixture that can be processed. This increases the number of batches to be processed in a campaign mode operation. In this mode, finding the optimum feed charge to solvent ratio is an important factor to maximise the productivity. 

In this mode the solvent is fed to the column in a semi-continuous fashion at some point of the column (Figure 10.3). Mujtaba (1999) noted two strategies in this mode of operation as far as charging of the initial feed mixture is concerned. 

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