Solvent flowrate

This is shown in Figure 10.1 to be a straight line with slope L/V. If it is assumed that the gas flowrate is fixed, the solvent flowrate can be varied to obtain the minimum solvent flowrate, as shown in Figure 10.2. 

In Figure 10.6a, the operating line for minimum solvent flowrate touches the equilibrium line at xR = 0.1576 and xR = 0.05166. Thus ... 

S Entropy (kJ-K-1, kJkg-1-K-1, kJkmol-1-K-1), or number of streams in a heat exchanger network (-), or reactor selectivity (-), or reboil ratio for distillation (-), or selectivity of a reaction (-), or slack variable in optimization (units depend on application), or solvent flowrate (kg s-1, kmol-s-1), or stripping factor in absorption (-)... 

Study the effect of the extraction stage on reactor performance by varying the magnitudes of the the mass transfer coefficient Ka, the equilibrium distribution ratio m, the recycle ratio R, the relative reactor and extraction volumes and solvent flowrate. 

In this example of a five stage extraction column with backmixing, proportional plus integral control of the exit raffinate concentration is to be achieved by regulating solvent flowrate. 

The detector should be optimized according to furnace temperature, reactant gas flowrate, solvent flowrate, composition and surface area of the nickel catalyst, and cell voltage. 

The ratio U /Mf is inversely proportional to residence time of the eluent in the extractor tr (see equation 1), and this ratio will have to be conserved, especially for extraction limited by internal diffusion. These extractions are almost not dependent on the solvent flowrate, but the "contacting" time of the feed with the solvent is the determinant factor of plant design. Therefore, it will be necessary to use very large extractors or to use several extractors in series in order to maximize the contacting time of the solvent with the feed. On the other hand, it is possible to minimize the solvent flowrate and the energy consumption of the plant. 

The control system shown holds the temperature profile in each column by manipulating heat inputs. Enough reflux is used on both columns to keep the product purities above specification. The solvent flowrate is ratioed to the fresh feed flowrate. 

We use rigorous simulation to determine feasible separations using water as a solvent. For a theoretical ten-stage liquid/liquid extraction process, we find that rather little water is needed to recover virtually all methanol from the pentane. At higher solvent flowrates the water-rich extract contains more and more acetone, but it cannot produce a complete separation of acetone and pentane. Thus, we select the solvent flow at which the methanol-pentane separation is sufficiently sharp. Figure 35 gives the separation selected. 

For case (a), since the solvents are immiscible, the rate of solvent in the feed stream (F ) is the same as the rate of feed solvent in the raffinate stream (/ ) Also, the rate of extraction solvent S ) entering the unit is the same as the extraction solvent leaving the unit in the extract phase ( ) However, the total flowrates entering and leaving the unit will be different since the extraction solvent is removing solute from the feed. Thus, the ratio of extraction-solvent to feed-solvent flowrates S /F ) is equivalent to E /R ). 

Although the procedure in Example 4 works well (provided solvent flowrate is above the minimum required), the coordinates of the mixing points Mj, M2 etc. have to be calculated manually. This tedious step can be avoided by observing that all the operating lines (i.e. L0V2, L1V3 etc.) have a common point called the difference point. 

The y-axis can be estimated from physical properties and the corresponding x-axis found. Then, the required dispersed or continuous solvent phase velocity can be found as a function of a specified diluent flowrate. If the diluent is the dispersed phase, the continuous solvent flowrate is found and likewise if the diluent is the continuous phase, the dispersed solvent flowrate is determined. Obviously, there will be some variation with column diameter and the best design is most easily determined with a spreadsheet... 

Figure 5.18 Minimum solvent flowrate calculation water-acetic acid-isopropyl ether, Example 5.3.

Find the minimum solvent flowrate for Example 5.2. Solution ... 

Since On+i = 1000 kg/hr, Vbmin = 1600 kg/hr. The actual solvent flowrate in the previous example was 2500 kg/hr, so Vo/ Vbmin = 1.6 for that extraction process. Using even more solvent would decrease the required number of equilibrium stages. 

The concentration of solute (A) in the outlet extract stream is limited to a relatively low value because the outlet extract stream is a passing stream with feed On+i. So the maximum concentration of A in Vn will occur when the column is operated at the minimum solvent flowrate. As with distillation, this limitation can be overcome by using... 

Minimum reflux corresponds to the overlap of an operating line and a tie-line (infinite stages at a pinch point). This concept is similar to minimum solvent flowrate for an extraction process without reflux. Total reflux corresponds to the minimum number of stages. Remember that total reflux means that no streams are going into or out of the column, so that F, B, and D are zero, and A = A. ... 

Next, a mass balance gives us S, the solvent flowrate ... 

We need to calculate Aia, the exiting wax concentration in the solvent prior to Stage 1. Since we assume no separation occurs in Stage 1, we can estimate it as a function of solvent flowrates and wax concentration leaving in the extract... 

For Example 5.3, illustrating minimum solvent flowrate, show that increasing the solvent flowrate will decrease the required number of equilibrium stages. 

Another Important consideration Is the compatibility of the flowcell with the normal range of operating conditions In liquid chromatography. First, the flowcell should be compatible with a wide variety of solvents for both normal- and reversed-phase separations. Second, It should readily accommodate variations In solvent flowrate to permit both hlgh-efflclency and high-speed... 

Insufficient absorption or off-specification for exit scrubbed gas feed gas concentration off spec/feed gas temperature or pressure outside operating window for amine absorbers > 50 °C for H2S and < 24 °C for C02/feed gas pressure has de-creased/[solvent flowrate too low] for glycol dehydration 12.5 to 25 L TEG per kg water removed/[solvent incorrect] /incorrect feed tray location/[column operation faulty] /absorber operating conditions differ from design/[absorber malfunction]. ... 

Solvent flowrate too low] flowmeter or sensor error/absorber pressure > design/ plugged strainer, lines or filters/low liquid level in pump feed tank/[cavitation] / air locked pump and see Section 2.3 for trouble shooting pumps. 

This flowsheet differs from that given in the Knapp-Doherty design in one important feature the solvent to feed ratio. Knapp and Doherty use a ratio of unity. For a feed flowrate of 540 kmol/h, this corresponds to a solvent flowrate to the extractive column of 540kmol/h. However, as Figure 11.3 shows, a solvent flowrate of 1100 kmol/h is used... 

The other difference between this design and the Knapp and Doherty design is the reflux ratios in the two columns. They report 2.76 and 1.06, while the present design values are 3.44 and 1.61. This difference is probably due to our higher solvent flowrate. 

Controller faceplates arc shown at the bottom of Figure 11.8. Notice that the eontroUer FCtot is on easeade beeause it reeeives its remote setpoint from the multiplier ratio so that the solvent flowrate is ratioed to the feed flowrate. 

The optimum design presented by Kossack et al. provides the number of stages in each column and feed locations. Figure 11.19 shows the flowsheet developed based on these columns. The acetone product and the methanol product are specified to be 99.5 mol% pure. Figure 11.20 shows that this methanol purity can be achieved in the extractive column distillate if the solvent flowrate is greater than 1800 kmol/h. These results are generated holding a bottoms composition in the extractive column of 0.01 mol% methanol. 

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