Column Operable Ranges

A column operator can control the column performance by manipulating the reboiler and condenser duties. Consider starting up a column with a mixed-phase feed introduced at some intermediate tray between the condenser and reboiler. With no condenser or reboiler duties, the liquid flows down the column and out as bottoms, and the vapor flows up the column and out as overhead. The column thus acts as a flash drum. 

If the reboiler is turned off and the condenser is activated, the column operates as a rectifier. The condensed liquid is returned to the top of the column as reflux and fractionation takes place on the trays between the overhead and the feed, with no fractionation occurring below the feed. In this mode of operation, higher reflux results in higher concentration of the lighter components in the distillate, but the bottoms cannot be enriched in the heavier components as long as the reboiler is inactive. 

The column operable range is determined in part by the requirement that no tray be allowed to dry up, that is, liquid and vapor must exist on each tray to maintain phase equilibrium. This range may be defined by the limits over which the condenser and reboiler duties may vary. As such, the condenser and reboiler duties are considered the two independent variables required to define the column performance (Sections 3.2.3 and 5.2.1). Alternatively, other pairs of variables may be chosen as the independent variables defining the column performance and each set can vary 

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Fig. 2-72. Packed column operation range (a) (representation according to Molzahn and Schmidt (2.92]) and schematic representation of the separation effiency as a function of the vapor load (b). System Methanol/Methanol-vapor, p = 1 bar, d = 64°C, metal Pallrings, packing diameter ds = 1-5 m...

Distillation capital costs. The classic optimization in distillation is to tradeoff capital cost of the column against energy cost for the distillation, as shown in Fig. 3.7. This wpuld be carried out with distillation columns operating on utilities and not integrated with the rest of the process. Typically, the optimal ratio of actual to minimum reflux ratio lies in the range 1.05 to 1.1. Practical considerations often prevent a ratio of less than 1.1 being used, as discussed in Chap. 3. 

The term in equation 42 is called a Souders-Brown capacity parameter and is based on the tendency of the upflowing vapor to entrain Hquid with it to the plate above. The term E in equation 43 is called an E-factor. and E to be meaningful the cross-sectional area to which they apply must be specified. The capacity parameter is usually based on the total column cross section minus the area blocked for vapor flow by the downcomer(s). Eor the E-factor, typical operating ranges for sieve plate columns are... 

However, any given column operated at a specific flow rate will exhibit a range of efficiencies depending on the nature and capacity ratio of the solute that is chosen for efficiency measurement. Consequently, under exceptional circumstances, the predicted conditions for the separation of the critical pair may not be suitable for another pair, and the complete resolution of all solutes may still not be obtained. 

A countercurrent moving-bed adsorption column is used to remove benzene from a gaseous emission. Activated carbon is employed as the adsorbent. The flowrate of the gas is 1.2 kg/s and it contains 0.027 wt/wt% of benzene. It is desired to recover 99% of this pollutant. The activated carbon entering the column has 2 X 10 wt/wt% of benzene. Over the operating range, the adsorption isotherm (Yaws et al., 1995) is linearized to... 

Exclusion limits and practical operating ranges specified for PLgel individual pore size columns are summarized in Table 12.1. Table 12.1 illustrates the versatility of this family of columns. Pore volume can also be expressed as... 

TABLE 12.1 Operating Ranges for PLgel Individual Pore Size Columns... 

Column type Exclusion limit (polystyrene equivalent) Operating range (polystyrene equivalent)... 

Each of the PLgel individual pore sizes is produced hy suspension polymerization, which yields a fairly diverse range of particle sizes. For optimum performance in a chromatographic column the particle size distribution of the beads should be narrow this is achieved by air classification after the cross-linked beads have been washed and dried thoroughly. Similarly, for consistent column performance, the particle size distribution is critical and is another quality control aspect where both the median particle size and the width of the distribution are specified. The efficiency of the packed column is extremely sensitive to the median particle size, as predicted by the van Deemter equation (4), whereas the width of the particle size distribution can affect column operating pressure and packed bed stability. 

The resolving capability is the same over the full operating range of the column. This offers an ideal situation for the analysis of very polydisperse polymers that may also contain lower molecular weight additives. 

TABLE 12.2 Operating Ranges for PLgel Mixed Gel Columns... 

The use of mixed gel technology has become widespread since the late 1980s. If the column is well designed it should not he necessary to supplement resolution in certain areas of the operating range (particularly the extremes) hy the addition of individual pore size columns. This practice is not necessary or recommended for PLgel mixed gel columns, although it is a common practice for other commercial products. 

Column type Operating range (PEO/PEG equivalent) Guaranteed minimum efficiency (plates/meter)... 

It follows that, at least for SEC, column temperature control can be important. An example of a commercially available column oven is shown in figure 17. The available temperature range varies a little from instrument to instrument but the model shown above has an operational range from 10°C to 99°C. One of the problems associated with the temperature control of ovens is the high thermal capacity of... 

The pump must provide stable flow rates from between 10 ttlmin and 2 mlmin with the LC-MS requirement dependent upon the interface being used and the diameter of the HPLC column. For example, the electrospray interface, when used with a microbore HPLC column, operates at the bottom end of this range, while with a conventional 4.6 mm column such an interface usually operates towards the top end of the range, as does the atmospheric-pressure chemical ionization (APCI) interface. The flow rate requirements of the different interfaces are discussed in the appropriate section of Chapter 4. 

Use of 10 pm LiChrosorb RP18 column and binary eluent of methanol and aqueous 0.1 M phosphate buffer (pH 4.0) according to suitable gradient elution program in less than 20-min run time with satisfactory precision sensitivity of spectrophotometric detection optimized, achieving for all additives considered detection limits ranging from 0.1 to 3.0 mg/1, below maximum permitted levels Simultaneous separation (20 min) of 14 synthetic colors using uncoated fused silica capillary column operated at 25 kV and elution with 18% acetonitrile and 82% 0.05 M sodium deoxycholate in borate-phosphate buffer (pH 7.8), recovery of all colors better than 82%... 

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