Pressure-optimized column

Instances of a task are replicas of the task operating under different conditions. The concept is used to optimize the operating conditions, such as the column pressure, and assumes the development of an operating range and a discretization scheme. Feasible ranges of pressure are identified by the physical properties (e.g., critical pressure) of the key components (upper limit) and the available utility levels (lower limit). The discretization scheme may be either uniform or based on the available utilities. The modeler can use a small or large number of discrete levels to capture associated trade-offs. 

The first subsystem to be optimized is the column. Given a set of specified product purities and column pressure the reflux ratio remains as the only column variable, that is, Ap is a function of reflux ratio. Using values for X and As obtained from the analysis of the working design, Ap is computed from Equation 35 for several values of reflux ratio (Table III). The optimal reflux ratio is obtained via a search of these values. 

The next component to be optimized is the reboiler. The reboiler area is fixed once the values for steam temperature, Tst, and process stream temperature, Ij, are fixed (assuming a constant heat transfer conductance). The process stream temperature is fixed by the column pressure and the product purities. Thus only Tst remains as a variable. The unit cost of the process stream,... 

Column pressure has a strong effect. Increasing the pressure from 0.2 to 0.5 bar will increase the reaction temperature by about 10 °C, sufficient for doubling the reaction rate. Since the reaction is slightly exothermal, the reboiler duty is only needed to compensate the differences in the enthalpy of components. Consequently, the reboiler duty is small and has no effect on optimization. 

Column process design specifies the separation, and sets column pressure, reflux, stages, and feed point. These in turn yield internal flows and reboiler and condenser duties. This chapter addresses the main column process design considerations. The column is optimized during the process design, and many times later during operation. Computer control continuously optimizes the column on-line. Both design and on-line optimization are also addressed in this chapter. 

Since column efficiency is usually expressed by the number of stages, the flow rate through the column can be increased without changing the column efficiency, thus increasing the production rate of the column. The optimal flow rate for such chromatographic systems is therefore limited by the maximum allowed pressure drop. 

The temperature profile in a CD column is controlled by column pressure and is not isothermal. The kinetics of the reaction should be known prior to the CD experiments so that the CD packing could be located in the section of the column where the temperature is optimal for production of a specified product. 

Martin et al. published a paper on the theoretical limits of HPLC which is well worth reading.They used relatively simple mathematics to calculate pressure-optimized columns for which the length L, particle size and flow rate u of the mobile phase were selected such that a minimum pressure Ap is required to solve a separation problem. It has been shown that these optimized colunms are operated at their van Deemter curve minima. Some astonishing facts have emerged from the study, provided that the chromatography is performed on well packed columns (reduced plate height h = 2-3 see Section 8.5). 

A pressure-optimized column has L = 2.2 cm, dp = 6.9 )Xm and A/ = 2.3 bar Shorter columns are preferred for simple problems with a separation factor of ca. 1.2. Both time and solvent are saved by using columns 3-5 cm in length (also available commercially), but the injection and extra-column volumes and the detector time constant must be kept small in order not to deteriorate the separation performance. 

The relative volatility of propylene/propane at column pressure and average column temperature is a,2 = 1.11 (0.22 = 1.00). The feed stream enters the column as saturated liquid. In order to optimize the column design, a number of options will be considered, each using a different number of stages. Determine the reflux ratio required to meet the above specifications for N = 100, N = 125, and N = 150 stages. 

If the catalyst is a solid, it must create neither excessive pressure drop nor an excessively large column. An optimal arrangement of solid catalysts in the column is needed. 

The calculational procedures are presented first for conventional distillation columns and then for complex distillation columns. The conventional distillation column is completely determined by fixing the following variables (1) the complete definition of the feed (total flow rate, composition, and thermal condition), (2) the column pressure (or the pressure at one point in the column, say in the accumulator), (3) the type of condenser, (4) ku the number of plates above and including the feed plate, (5) /c2, the total number of plates, and (6) two other specifications which are usually taken to be the reflux ratio and the distillate rate LJD, D or two product specifications such as bjdh bh/d,, XDh xBh > Td, 7, or combinations of these. The subscript / is used to denote the light key and the subscript h is used to denote the heavy key. In all of the optimization problems considered herein, the variables listed in items (1), (2), and (3) are always fixed. For convenience these variables are referred to collectively as the usual specifications. The remaining four variables, ku /c2, and two other specifications such as LJD and D are called additional specifications. ... 

Consider first the case where the specifications bt /dt and bh /dh are made for a conventional distillation column. In addition, the column pressure, the type of condenser, and the complete definition of the feed are specified. This type of problem may be solved by use of a combination of the procedure used to find the product distribution for a column with infinitely many plates and an optimization procedure such as the one described in Chap. 9. 

As reference we consider the direct separation scheme. The design of columns was done in Aspen Plus by means of shortcut methods followed by rigorous simulation. The energetic consumption depends on the reflux ratio. We assume that the optimal R/Rmi is 1.3, and column pressures of 2 and 1 bar with 0.2 bar pressure drop. Table 11.2 presents the results. Note that the initial feed temperature is 298 K, and therefore the reboiler duty of the first column includes feed preheating. 

Series coupling of normal-length columns (30 cm) to increase efficiency or to optimize the pore-size range for the separation is common practice. Series coupling of columns is facilitated by the low optimum mobile phase velocity and thus lower column pressure drop per unit column length, typical for SEC separations. 

Series coupling of columns containing the same stationary phase is used to enhance efficiency and with different stationary phases to fine tune selectivity [8]. Series coupling of packed columns became popular after it was demonstrated that the column pressure drop did not limit the total column efficiency to the extent that had been predicted (section 7.4.2). Serial coupling of 2 or 3 standard columns is practical for routine applications and provides a total plate count in excess of 50,000. There is no theory for selectivity optimization for coupled packed columns but suitable conditions often can be estimated from separations on the individual columns. Effective selectivity changes require that the coupled columns have different retention properties. A number of practical examples for the separation of polymer additives, polycyclic aromatic hydrocarbons, phytic acid impurities and enatiomers have been described [137,195,196]. Series coupling of open tubular columns with different stationary phases is less common, but changes in selectivity are predictable, at least when the pressure drop is low [197]. 

The bottoms is essentially a binary methanol/water (23.5 mol% methanol), which is fed to a 32-stage column operating at atmospheric pressure. The number of trays in the second column was optimized by determining the total annual cost of columns over a range of tray numbers. Reboiler heat input and condenser heat removal are 8.89 and 9.53MW, respectively. The column diameter is 2.24 m. 

A pressure-optimized column has L = 10 cm and dp = 6.3 rm. The recommended pressure is 11.8 bar (for a mobile phase viscosity of 0.4 mPa, which is a typical value in adsorption chromatography the viscosity may be up to 4.5 times greater in reversed-phase chromatography, increasing the pressure required to around 50 bar). This pressure is much lower than usual in most separations. 

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