1 Introduction
By virtue of its nature, continuous multistage distillation involves handling of liquid and vapor mixtures, with corresponding bubble and dew point lines enclosing the distillation space (a coexisting saturated vapor and liquid region). The bubble and dew point temperatures tend to increase (or decrease) with increasing (or decreasing) pressure, while the distance between the dew and boiling-point lines corresponds approximately to the difference in the boiling-point temperatures of two components (in the case of binary mixtures) or two key components (in the case of multicomponent mixtures).
Figure 1 shows a schematic drawing of a standard, one feed-two products, tray distillation column, including all ancillary equipment (i.e., a total condenser, a partial kettle-type reboiler, and a reflux accumulator drum, as well as two pumps needed to feed the reboiler and transport the reflux to the top of the column, respectively). Upon startup, the initial amount of the liquid phase is generated by condensing the saturated overhead vapor and returning the required quantity of the liquid (reflux) into the top of the column. The vapor phase (boil-up), which is introduced into the column at the bottom, is obtained by evaporation of the required quantity of the boiling-point liquid leaving the column at the bottom. An additional amount of liquid and/or vapor is introduced as the feed along the column. Usually it is a single feed, as shown schematically in Figure 1, but distillation columns can have a number of feed points receiving liquid and/or vapor. Most frequently, a single feed is a saturated liquid, but also it can be a saturated vapor or a two-phase mixture, and in some cases also a subcooled liquid or an overheated vapor. Distillation columns can have one or more side product draw-offs, mainly as saturated liquid, but also it is possible to arrange a saturated vapor draw-off.
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| FIGURE 1 Schematic Representation of a Tray Distillation Column with Ancillary Equipment |
Inside a column, from the top to the bottom, there is a continuous transfer of less volatile components from saturated vapor to saturated liquid by condensation and more volatile components from the saturated liquid into the saturated vapor phase by evaporation. If the components of the mixture do not significantly differ in their molar enthalpy of vaporization, the amounts of liquid and vapor generated at each stage will be equivalent (constant molar overflow or equimolar mass transfer). In case of unequal molar enthalpy of vaporization, as is usually the case when distilling aqueous mixtures, the internal liquid and vapor traffic will be different and needs to be accounted for by including enthalpy balance into stage requirement calculations to arrive at a proper design. The lowest pressure and temperature are established at the reflux accumulator placed below the condenser. In contrast, the reboiler is the place with the highest pressure and temperature for the given system. The difference in top and bottom temperatures may go from 10 K for a close boiling system to hundreds of K as encountered in crude oil-distilling columns.
The overhead vapor temperature needs to comply with the availability and temperature of the cooling media, with water and air being the preferred choices. This will set the condenser temperature, and the top of the column temperature is per definition the dew point temperature of the overhead vapor corresponding to the top of the column pressure. This top pressure is the reflux accumulator pressure enlarged by the pressure drop generated by the condenser and the vapor line between the top of the column and the condenser. The top of the column pressure enlarged by the pressure drop of column internals under operation yields the bottom pressure, and the corresponding temperature is the bubble point of the liquid in the column sump.
The bottom temperature should generally be below that causing thermal degradation of the material distilled. If the bottom temperature is too high, the operating pressure needs to be reduced accordingly. Therefore, dealing with high-boiling components implies generally an operation under vacuum conditions. The heavier the components are, the deeper the vacuum will be. This means less and less tolerance for pressure drop. In critical cases, an effective measure is to place the condenser inside the top of the column. Indeed, the amount of tolerable pressure drop is an important design consideration and affects the equipment choice and internal configuration of a column.
While the pressure drop is just a concern in dealing with above-atmospheric applications, it is a design and operating parameter of primary importance for vacuum applications. When considering a vacuum application, trays are the least preferred choice, followed by random packings. Owing to their relatively lowest pressure drop per stage, structured packings are the preferred choice for distillation under vacuum. Being most effective in this respect, well-established wire gauze packings are found in multistage distillation columns operating with top pressures of around 0.1 bar or 100 mbar (10 kPa). Below this pressure, distillation is performed using wetted-wall (falling film type) columns. For applications at pressures as low as 1 mbar 100 Pa, a “thin-film” or “wiped-wall” column is used. This is a wetted-wall column employing a rotating blade that keeps the film thickness controlled. Under highest vacuum (i.e., at an absolute pressure of a few Pascal), special, molecular, or short-path distillation equipment is used. The present chapter focuses on conventional multistage distillation applications. It addresses and discusses criteria for the selection of the operating pressure, the effects of pressure on separation by distillation, and the overall performance of vapor-liquid contacting equipment used in distillation.
2 Operating pressure ranges and selection criteria
Design of a distillation column starts with setting the design pressure. In general, a distinction is made between columns designed to operate at above atmospheric pressures and the columns designed to operate under vacuum. The present section addresses the design pressure from a process design standpoint, while distillation columns (cylindrical pressure vessels) operated at vacuum and above-atmospheric pressure conditions have to comply with related mechanical design requirements. This also implies considerations related to the maximum allowable temperature, to guarantee the preservation of mechanical integrity of the chosen construction material under all operating conditions and anticipated extreme situations. One should note that building in large safety margins is expensive and that mechanical engineers also strive for economic designs.
The constraints in this respect for industrial distillation applications and a thorough analysis of unfavorable and favorable pressure effects as well as related optimization considerations can be found in books by Kister [1] and by Stichlmair and Fair [2]. Specific features of high-pressure distillation are addressed in detail by Brierley [3].
Atmospheric pressure is a natural choice, but it may be approached in practice to the extent depending on the boiling range of the system to be distilled in conjunction with the availability and costs of cooling and heating media, as well as the thermal degradation sensitivity of the bottom product. Practically, the overhead product temperature should be set high enough to perform condensing in an economic way using the cheapest available cooling media. Water, if readily available, is a standard choice, and the overhead vapor temperature should be 10 to 20 K above the anticipated highest (worst-case) water temperature for the given location or source. If water is scarce or too expensive, the air is used as a cooling medium. The air temperature is subjected to more extreme fluctuations; therefore, a higher temperature approach needs to be considered. If refrigeration is inevitable, then a much smaller temperature approach is chosen. This is done in conjunction with special compact heat exchangers exhibiting a very large heat transfer area per volume. The most sophisticated devices of this kind are employed in the cryogenic distillation of air. On the other side, steam is the most common source of heat used in reboilers. Low-pressure (lowest temperature and cost) steam is a preferred choice. Medium- and high-pressure steam is available at sites with integrated power generation, and it comes at correspondingly higher prices. Hot oil and fired heaters are more expensive options. Where appropriate, hot process fluids are used. A common heat integration practice is to use bottom product to preheat the feed stream.
Table 1 shows the atmospheric pressure (normal) boiling points of a number of commonly distilled inorganic and organic chemicals. At atmospheric pressure, several important inorganic and light hydrocarbon components have a boiling point well below 0°C (273.15 K). For instance, to get the propylene to the level of temperature suitable for water to be used as cooling medium, an absolute pressure of about 19 bar (19x10^5 Pa) is required at the top of the column.
Table 9.2 shows typical operating pressures of water- or air-cooled distillation columns. Approximately 35 bar (35.10^5 Pa) is the upper limit, which is not exceeded in practice because it would require the adoption of unaffordable shell thickness as set by high-pressure vessel design codes. For this reason, demethanizers and deethanizers usually operate at the highest reasonable pressure and use an appropriate refrigerant as cooling medium. In these and other so-called cold-box separations, special refrigeration systems are used, which increases the costs of the separation significantly. The coldest application is the cryogenic distillation of air (temperatures close to 90 K or 183°C), which, depending on the products used (argon, nitrogen, and/or oxygen) and the purity requirements, is carried out at atmospheric and/or at elevated pressures (up to 10 bar or 10.10^5 Pa).
Thermodynamically, the upper limit for separation of a component from a mixture by distillation is the critical pressure (i.e., critical temperature). There is no thermodynamic or mechanical limitation on the lower end (columns subject to vacuum usually are designed for full vacuum), where the amount of tolerable pressure drop is a critical design consideration.
A typical example of the difficulties encountered with the choice of the operating (vacuum) pressure is a separation of ethylbenzene and styrene, with a polymer-grade styrene monomer as the bottom product. Styrene is a highly reactive molecule that tends to polymerize strongly at temperatures above 363 K. In the past, sieve or valve trays have been used for styrene distillation. Relying on water as cooling medium, the typical pressure at the top of the column was not exceeding 70 mbar (0.07 bar or 7 kPa). To keep the pressure drop low enough, a typical single-shell column usually contained 70 trays and was designed at around 50% of flooding to reduce total pressure drop to an acceptable level. Even with a pressure drop as low as 3 mbar (300 Pa) per tray, the bottom pressure was still around 300 mbar (0.3 bar or 30 kPa), resulting in a bottom temperature of about 378-383 K. At this temperature, the polymerization was a real threat and the only practical remedy was to use a costly polymerization inhibitor, which had to be removed in a subsequent makeup distillation operation.
To reduce related costs from adopting the same column design (i.e., pressure drop), an option was to reduce the column’s top pressure by avoiding the pressure drop associated with transport of the vapor from the top of the column to the condenser. Indeed, this was achieved by installing an air-cooled condenser at the top of the column, allowing a lower column top pressure (i.e., a gain of some 25 mbar or 2.5 kPa with respect to conventional water-cooled condensers placed outside the column). It should be realized that such a “small” pressure drop reduction was so significant in this case that a different design approach was chosen involving additional complexities on the mechanical-engineering (construction) side and related costs. It should also be noted that placing condensers, both air and water cooled, at or in the top of the column is an effective engineering solution for demanding vacuum distillations. Regarding the ethylbenzene recovery column mentioned here, due to large vapor density differences as imposed in trayed columns by a pressure drop of about 250 mbar (25 kPa), the rectification section diameter was considerably larger than the stripping section diameter. For example, with feed rates above 30 t/h and high reflux ratios (6-8), the diameter of the rectification section of the trayed columns with top pressures of about 40 mbar (0.04 bar or 4 kPa) has been close to 10 m [4].
A real technology breakthrough in this respect occurred upon implementation of structured packings in these applications (in the late 1970s and early 1980s), which resulted in such a large reduction in the column pressure drop that the bottom temperature dropped below that causing polymerization. Now, there is no more need for using and recovering the antipolymerization additive. This is a specific, highly rewarding additional advantage of the application of corrugated-sheet structured packings in this particular application.
A common benefit of a reduction in bottom temperatures is the increase of relative volatility, which often allows a reduction in the operating reflux ratio and consequently leads to reduced investment and/or operating costs. However, the vapor density at the bottom conditions is practically halved, which means dealing with much larger volumetric flows of vapor. This appeared to be a limiting factor in revamps, but in new designs it allows the design of single-diameter packed columns. To avoid extremely large diameters in these applications, two options are considered: increasing the number of installed stages to reduce the operating reflux ratio, and/or increasing the operating pressure accordingly. Both options are possible because of a rather low total pressure drop involved.
Certainly, the operating pressure and column pressure drop are two important design considerations because they directly influence the separation via vapor-liquid equilibrium and the performance of the chosen vapor-liquid contacting device.
3. Pressure effects
3.1 Stage and reflux requirement
Relative volatility is a direct measure of the difficulty of separation of a mixture by distillation. Practically, it is expressed as the ratio of equilibrium constants (K-values), which in the case of an ideal mixture reduces to the ratio of saturated vapor pressures of more and less volatile components. For multicomponent mixtures, this is the ratio of light to heavy key components. Figure 2 shows atmospheric pressure equilibrium curves for some common test systems. Methanol-water (M/W) is a nonideal system, with relative volatility increasing from 2 at the upper end to nearly 7 at the lower end. Chlorobenzenee-ethylbenzene (CB/EB) is an ideal system with a constant relative volatility
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