Selection of column pressure

The specification of the feed pressure takes a little thought. We will discuss the selection of column pressure in more detail later in this chapter. We know that the distillate product is propane. We will want to use cooling water in the condenser because it is an inexpensive heat sink compared with refrigeration. Cooling water is typically available at about 305 K. A reasonable temperature difference for heat transfer in the condenser is 20 K. Therefore, reflux drum temperature will be about 325 K. The vapor pressure of propane at 325 K is about 14 atm (206psia). Therefore, the column will have a pressure at the feed tray of something a little higher than 14 atm. 

The selection of column characteristics is determined by solvent resistance, the need to visually inspect the bed, the pressure rating of the system, and the dimensions [column inner diameter (i.d.) and length (L)] required from productivity considerations. Productivity considerations will vary if the requirement is based on the amount of information per unit time (analytical gel filtration) or the amount of substance per unit time (preparative gel filtration). 

Selection of columns and mobile phases is determined after consideration of the chemistry of the analytes. In HPLC, the mobile phase is a liquid, while the stationary phase can be a solid or a liquid immobilised on a solid. A stationary phase may have chemical functional groups or compounds physically or chemically bonded to its surface. Resolution and efficiency of HPLC are closely associated with the active surface area of the materials used as stationary phase. Generally, the efficiency of a column increases with decreasing particle size, but back-pressure and mobile phase viscosity increase simultaneously. Selection of the stationary phase material is generally not difficult when the retention mechanism of the intended separation is understood. The fundamental behaviour of stationary phase materials is related to their solubility-interaction... 

There are a number of limitations on the use of extremes of temperature in HPLC. Clicq et al. [91] note that instrumental issues become increasingly limiting as one goes to very high temperatures and flow rates. They suggest that most separations will occur below 90°C where there are less instrumental constraints. As detailed below, column bleed can limit the selection of columns. Highspeed separations require a faster detector response than many systems allow and constrain extra column volume. This is especially true for narrow bore columns and sub-2 jam particles. In many cases, the additional speed gained above the temperature limits of commercial HPLC ovens will not be worth the additional expense and complexity required. For macromolecules, the effect of extreme pressure can also impact retention time as noted by Szabelski et al. [92]. 

In Table 2-17, important criteria for the selection of column internals are listed. Technical considerations include separation efficiency, maximum loading and pressure drop. To date, it has not been possible to... 

For the final selection of column internals, mixture behavior, operating conditions, characteristic performance figures, costs, pressure drop, and other criteria are important. Substantial criteria are listed in Table 2-31. 

Different evaluation criteria prove to have an opposite influence. For example, the requirement of good separation efficiency leads to a higher pressure drop and higher costs. The selection of column inter-... 

Figure 3 shows the fraction of N2 removed from the column (ratio of the amount of N2 removed to the total amount of N2 initially present in the column as adsorbed and void gases) as a function of column pressure (P) during the evacuation process. It is much easier to remove the N2 from the column by evacuation when it is held less tightly (e.g. NaX). A much deeper vacuum is necessary to remove a substantial quantity of N2 from the zeolites when its selectivity of adsorption over O2 is large (e.g. CaX, CaLSX). 

Fig. 4 illustrates the time-dependence of the length of top s water column in conical capillary of the dimensions R = 15 pm and lo =310 pm at temperature T = 22°C. Experimental data for the top s column are approximated by the formula (11). The value of A is selected under the requirement to ensure optimum correlation between experimental and theoretical data. It gives Ae =3,810 J. One can see that there is satisfactory correlation between experimental and theoretical dependencies. Moreover, the value Ae has the same order of magnitude as Hamaker constant Ah. But just Ah describes one of the main components of disjoining pressure IT [13]. It confirms the rightness of our physical arguments, described above, to explain the mechanism of two-side liquid penetration into dead-end capillaries. 

Selection of solvents. The choice of solvent will naturally depend in the first place upon the solubility relations of the substance. If this is already in solution, for example, as an extract, it is usually evaporated to dryness under reduced pressure and then dissolved in a suitable medium the solution must be dilute since crystallisation in the column must be avoided. The solvents generally employed possess boiling points between 40° and 85°. The most widely used medium is light petroleum (b.p. not above 80°) others are cycZohexane, carbon disulphide, benzene, chloroform, carbon tetrachloride, methylene chloride, ethyl acetate, ethyl alcohol, acetone, ether and acetic acid. 

Examination of possible systems for boron isotope separation resulted in the selection of the multistage exchange-distillation of boron trifluoride—dimethyl ether complex, BF3 -0(CH3 )2, as a method for B production (21,22). Isotope fractionation in this process is achieved by the distillation of the complex at reduced pressure, ie, 20 kPa (150 torr), in a tapered cascade of multiplate columns. Although the process involves reflux by evaporation and condensation, the isotope separation is a result of exchange between the Hquid and gaseous phases. 

The absolute pressure may have a significant effect on the vapor—Hquid equiHbrium. Generally, the lower the absolute pressure the more favorable the equiHbrium. This effect has been discussed for the styrene—ethylbenzene system (30). In a given column, increasing the pressure can increase the column capacity by increasing the capacity parameter (see eqs. 42 and 43). Selection of the economic pressure can be faciHtated by guidelines (89) that take into consideration the pressure effects on capacity and relative volatiHty. Low pressures are required for distillation involving heat-sensitive material. 

While process design and equipment specification are usually performed prior to the implementation of the process, optimization of operating conditions is carried out monthly, weekly, daily, hourly, or even eveiy minute. Optimization of plant operations determines the set points for each unit at the temperatures, pressures, and flow rates that are the best in some sense. For example, the selection of the percentage of excess air in a process heater is quite critical and involves a balance on the fuel-air ratio to assure complete combustion and at the same time make the maximum use of the Heating potential of the fuel. Typical day-to-day optimization in a plant minimizes steam consumption or cooling water consumption, optimizes the reflux ratio in a distillation column, or allocates raw materials on an economic basis [Latour, Hydro Proc., 58(6), 73, 1979, and Hydro. Proc., 58(7), 219, 1979]. 

Selection of Equipment Packed columns usually are chosen for very corrosive materials, for liquids that foam badly, for either small-or large-diameter towers involving veiy low allowable pressure drops, and for small-scale operations requiring diameters of less than 0.6 m (2 ft). The type of packing is selected on the basis of resistance to corrosion, mechanical strength, capacity for handling the required flows, mass-transfer efficiency, and cost. Economic factors are discussed later in this sec tion. 

Several compromises are involved in the selection of the correct particle size. On one hand, one desires the highest possible resolution in the shortest amount of time. Therefore, the smallest particle size should be chosen that still gives resolution of the polymer without causing excessive column back pressure. On the other hand, there are constraints on both the strength of the particle and the strength of the polymer. This section discusses the selection of the best particle size. 

To determine the column (with trays) diameter, an approach [130] is to (1) assume 0 hours (2) solve for V, Ib/hr vapor up the column at selected, calculated, or assumed temperature and pressure (3) calculate column diameter using an assumed reasonable vapor velocity for the type of column internals (see section in this volume on Mechanical Designs for Tray Performance ). 

As packing factor, F, becomes larger by selection of smaller sized packing gas capacity for the column is reduced and pressure drop will increase for a fixed gas flow. 

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