Column pressure, selection distillation

Selecting an appropriate column pressure for distillation is an inportant decision that is usually done early in the design. As discussed in Section 2.1. in order to have a liquid phase, the condenser pressure must be below the critical pressure of the distillate mixture. In addition, if possible we would like to meet the following heuristics (Biegler et al.. 19971 ... 

Vacuum distillation varies with the system tmd particularly with the absolute pressure required at the bottom of the column normally select low pressure drop in the range of 0.1 to 0.2 in. water/ft of packing. For in vacuum service of 75 mm Hg and lower, the pressure drop obtained from the GPDC,... 

Extractive distillation the separation of different components of mixtures which have similar vapor pressures by flowing a relatively high-boiling solvent, which is selective for one of the components in the feed, down a distillation column as the distillation proceeds the selective solvent scrubs the soluble component from the vapor. 

To initiate the calculational procedure for the determination of the product distribution for specified reflux and distillate rates, a number of plates between the two pinches is selected. (As discussed in a subsequent section, too few plates but not too many plates may be selected.) Next L/V and temperature profiles for the plates between and including the two pinches as well as the distillate and bottoms temperatures are selected. Next the components of the feed are classified according to the above criteria. Since it is supposed that the complete definition of the feed, the reflux and distillate rates, as well as the column pressure and type of condenser are specified, the component-material balances can be solved for the component-flow rates throughout the column. The component-material balances may be simplified by taking advantage of the unique characteristics of the three classes of components, the distributed components, the separated lights, and the separated heavies. 

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. 

Constraint control is another type of selector or override that is intended to keep the controlled variable near a constraining or limiting value. Chapter 19 discusses how constraints influence the selection of operating conditions and why it is necessary in many cases to operate near a constraint boundary. Riggs (1998) has described a constraint control application for distillation columns with dual composition control, where reboiler duty Qr controls bottoms composition of xb and reflux flow R controls overhead composition The reboiler becomes constrained at its upper limit when the steam flow control valve is completely open. Several abnormal situations can result (1) the column pressure increases, (2) heat transfer surfaces become fouled, or (3) the column feed rate increases. When the reboiler duty reaches the upper limit, it is no longer able to control bottoms composition, so constraint control forces one composition (the more valuable product) to be controlled with the reflux ratio while the other product composition is left uncontrolled (allowed to float ). Computer control logic must be added to determine when the column has returned to normal operation, and thus the constraint control should be made inactive. 

One of the most important design parameters for reactive distillation is column pressure. Pressure effects are much more pronounced in reactive distillation than in conventional distillation. In normal distillation, the column pressure is selected so that the separation is made easier (higher relative volatilities). In most systems this corresponds to low pressure. However, low pressure implies a low reflux-dmm temperature and low-temperature coolant. The typical column pressure is set to give a reflux-drum temperature high enough (49 °C, 120 °F) to be able to use inexpensive cooling water in the condenser and not require the use of much more expensive refrigeration. 

In conventional distillation, column pressure is usually selected to be as low as possible while still being able to use cooling water in the condenser. This is because relative volatilities increase with decreasing temperamre in many chemical systems. The other situation... 

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]. 

Once packing heights are determined in other sections from HETP (distillation) or Koa (absorption), the height allowances for the internals (from Figure 1) can be added to determine the overall column height. Column diameter is determined in sections on capacity and pressure drop for the selected packing (random dumped or structured). 

Select design pressure drop for operations. Suggested values of below 1.0 in. water/ft. Low-pressure, atmospheric, and pressure columns usually require 0.5 to 0.7 in. water/ft, with absorbers and strippers around 0.2-0.6 in. water/ft. For vacuum distillation low values of 0.05-0.6 in. water/ft are often necessary, usually depending on the required boiling point of the bottoms. 

Liu (12) had reported that III could be conveniently separated from IV by selective thioketalization to V followed by purification by column chromatography. Accordingly, a mixture of approximately 70% III and 30% IV (60.9 g) was treated with 1,2-ethanedithlol (50 ml) and boron trifluoride etherate (6 ml). The crude product was distilled under reduced pressure to yield 45.1 g (54.4%, 77.6% based upon III as starting material) of crude V, sufficiently pure for the next step. Column chromatography of lower boiling impure fractions furnished the dithioketal VI (10% yield), unreacted IV, and additional V. ... 

A reboiler at the bottom of a distillation column contains an organic liquid at 1 atm and 320°F, at which its density is 0.7 g/cm3, its viscosity is 0.5cP, and its vapor pressure is 800 mmHg. The liquid must be pumped to another column at a rate of 200 gpm and discharges at 1 atm at a point 30 ft higher than the reboiler. You must select the best pump from those represented by the curves in Appendix H and determine where the pump is to be installed. The suction line of the pump will include 20 ft of 2 in. sch 40 pipe, eight elbows, four gate... 

In catalytic distillation the temperature also varies with position in the column, and this will change the reaction rates and selectivities as well as the equilibrium compositions. Temperature variations between stages and vapor pressures of reactants and products can be exploited in designing for multiple-reaction processes to achieve a high selectivity to a desired product with essentially no unwanted products. 

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