Low-pressure column

Figure 3.8a shows the temperature-composition diagram for a minimum-boiling azeotrope that is sensitive to changes in pressure. This azeotrope can be separated using two columns operating at different pressures, as shown in Fig. 3.86. Feed with mole fraction of A Ufa)) of, say, 0.3 is fed to the high-pressure column. The bottom product from this high-pressure column is relatively pure B, whereas the overhead is an azeotrope with jcda = 0-8, jcdb = 0.2. This azeotrope is fed to the low-pressure column, which produces relatively pure A in the bottom and in the overhead an azeotrope with jcda = 0.6, jcdb = 0.4. This azeotrope is added to the feed of the high-pressure column.

Purifications are made simpler with Toyopearl HW media because there is no risk of leached polysaccharides to contaminating eluted fractions. Saccharide derivatives have been known to leach from conventional low-pressure column packings, such as dextran or agarose gels. 

Other chromatographic techniques that have been applied to the tetracyclines, including CTC, involve low pressure column chromatography. Ascione et al. (64) developed a semiautomated system whereby sample solutions are automatically injected onto a column of diatomaceous earth mixed with... 

The condensation rate in the condenser L, changes as the pressure in the condenser varies since the condensing temperature depends on pressure. Thus depends on column pressure, overhead vapor composition, and the temperature of the coolant in the condenser. Equation (S.33) assumes ideal gas behavior, which is usually adequate in these low-pressure columns where pressure changes are significant. 

Back pressure column, BDS Hypersil Cl 8 Low Pressure Column (Keystone Scientific, PA, USA 053-46-1). 

Low-pressure column chromatography and pH gradient extraction were used to separate fangchinoline and tetrandrine from S. tetrandra root extracts (601). A quantitative method for determination of thalicarpine in pharmaceutical preparations using TLC-UV... 

The modified flowsheet is presented in Figure 6.10 the low-pressure column (C-2A) operates at 3 bar, while the high-pressure column (C-2B) operates at 8 bar. Note that the bottom stream of (C-2B) may contain an important amount of benzene. The examination of the profiles shows an imbalance in vapor flow in the lower part of the column. In consequence, the duty of the reboiler of (C-2B) may be ensured to a large extent by a side-stream heater. In this way, the heat can be transferred at a more convenient temperature level and higher driving force. In... 

Low-pressure-drop applications. By virtue of their low pressure drop compared to trays (above), packings are favored in any application where it is economical to minimize pressure drop. A typical example is an atmospheric or low-pressure column whose overheads are compressed. Every pound per square inch of pressure drop here translates into greater compression ratio requirement and higher compressor capital and energy costs. 

The feed split is approximately 50/50. but slightly more feed goes to the low-pressure column if the separation is easier at lower pressure. The system as pictured runs neat. i.e., all the heat available from condensing the vapor from the high-pressure column is used to reboil the low-pressure column. In some systems auxiliary reboilers and/or condensers are used to balance the heat loads both at steady state and dynamically. 

The system represents a 4 X 4 interacting control problem since there are four product compositions to be controlled at each end of both columns. Reflux flowrrates control the distillate purities in each column. Bottoms purity in the high-pressure column is controlled by manipulating the heat input to the reboiler. Bottoms purity in the low-pressure column is controlled by manipulating the fraction of the feed that is fed into the low pressure column. 

Split feed eonfiguration, in which the feed is split between the high-pressure and the low-pressure columns... 

Sloppy first bottom split, first column pressurized, in which the bottoms from the first, high-pressure column is further separated in the low-pressure column... 

Low pressure column chromatography, or more simply liquid chromatography, is carried out either using hydrostatic pressure or with the aid of a peristaltic pump... 

In one parametric study, six different designs (Fig. 22) were compared (Blakely, 1984). Design 1 was the single-column reference case. Design 2 consisted of a high-pressure and a low-pressure column. The feed was split and fed to both columns in parallel. Both columns performed the same sharp separation, producing pure products. The condenser of the high-pressure column was the reboiler of the low-pressure column. 

In Design 6, the feed was sent to the high-pressure column, which produced a pure low-boiler distillate but a mixed underflow (again, a sloppy separation). The mixed bottoms was then completely separated in the low-pressure column. 

The condenser of the high-pressure column was the reboiler of the low-pressure column (heat integration in the same direction as flow). 

The last obvious combination, with the feed sent to the low-pressure column producing a pure high-boiler bottoms product, with a mixed overhead completely separated in the high-pressure column, was not considered because it always requires a higher temperature utility than the other schemes.)... 

In the study, the pressure in the low-pressure column and the boiling point of the more volatile component were fixed. The composition of the mixed stream was chosen so that the heat duty of the high-pressure condenser was equal to that of the low-pressure reboiler. A parametric study on feed flowrate, feed composition, and relative volatility between the binary pair was conducted with a detailed process simulator. For each set of conditions, the NPC of each design was compared to that of the single-column reference case. 

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