Separation column, pressure

Whether heat integration is restricted to the separation system or allowed with the rest of the process, integration always benefits from colder reboiler streams and hotter condenser streams. This point is dealt with in more general terms in Chap. 12. In addition, when column pressures are allowed to vary, columns with smaller temperature differences are easier to integrate, since smaller changes in pressure are required to achieve suitable integration. This second point is explained in more detail in Chap. 14. 

Due to possible environmental problems with acetone, new technologies are being developed for the production of deoiled lecithins involving treatment of Hpid mixtures with supercritical gases or supercritical gas mixtures (10—12). In this process highly viscous cmde lecithin is fed into a separation column at several levels. The supercritical extraction solvent flows through the column upward at a pressure of 8 MPa (80 bar) and temperature between 40 and 55°C. The soy oil dissolves together with a small amount of lecithin. 

Example 8 Calculation of Rate-Based Distillation The separation of 655 lb mol/h of a bubble-point mixture of 16 mol % toluene, 9.5 mol % methanol, 53.3 mol % styrene, and 21.2 mol % ethylbenzene is to be earned out in a 9.84-ft diameter sieve-tray column having 40 sieve trays with 2-inch high weirs and on 24-inch tray spacing. The column is equipped with a total condenser and a partial reboiler. The feed wiU enter the column on the 21st tray from the top, where the column pressure will be 93 kPa, The bottom-tray pressure is 101 kPa and the top-tray pressure is 86 kPa. The distillate rate wiU be set at 167 lb mol/h in an attempt to obtain a sharp separation between toluene-methanol, which will tend to accumulate in the distillate, and styrene and ethylbenzene. A reflux ratio of 4.8 wiU be used. Plug flow of vapor and complete mixing of liquid wiU be assumed on each tray. K values will be computed from the UNIFAC activity-coefficient method and the Chan-Fair correlation will be used to estimate mass-transfer coefficients. Predict, with a rate-based model, the separation that will be achieved and back-calciilate from the computed tray compositions, the component vapor-phase Miirphree-tray efficiencies. 

The fix for the erratic reflux drum pressure problem was to provide for separate pressure control of the fractionator column and the reflux drum. A new pressure control valve was installed upstream of the condenser and the old condenser outlet control valve was removed. A hot gas bypass, designed for 20% vapor flow, was installed around the pressure control valve and condenser. A control valve was installed in the hot gas bypass line. The column pressure was then maintained by throttling the new control valve upstream of the condenser. The reflux drum pressure w as controlled by the hot gas bypass control valve and the psv saver working in split range. The new system is shown in the figure below. 

Since the boiling point properties of the components in the mixture being separated are so critical to the distillation process, the vapor-liquid equilibrium (VLE) relationship is of importance. Specifically, it is the VLE data for a mixture which establishes the required height of a column for a desired degree of separation. Constant pressure VLE data is derived from boiling point diagrams, from which a VLE curve can be constructed like the one illustrated in Figure 9 for a binary mixture. The VLE plot shown expresses the bubble-point and the dew-point of a binary mixture at constant pressure. The curve is called the equilibrium line, and it describes the compositions of the liquid and vapor in equilibrium at a constant pressure condition. 

Adsorbers, distillation colunuis, and packed lowers are more complicated vessels and as a result, the potential exists for more serious hazards. These vessels are subject to tlie same potential haz. uds discussed previously in relation to leaks, corrosion, and stress. However, llicse separation columns contain a wide variety of internals or separation devices. Adsorbers or strippers usually contain packing, packing supports, liquid distributors, hold-down plates, and weirs. Depending on tlie physical and chemical properties of the fluids being passed tlirough tlie tower, potential liazards may result if incompatible materials are used for llie internals. Reactivity with llie metals used may cause undesirable reactions, which may lead to elevated temperatures and pressures and, ullinialely, to vessel rupture. Distillation columns may contain internals such as sieve trays, bubble caps, and valve plates, wliicli are also in conlacl with tlie... 

Figure 2.6 Gas cluotnatograni of a 10 ml test sample containing C I4 C26 alkanes in -hexane (about 1 ppb each) the earner gas (H2) inlet pressure was 2.5 bar for a 22 m X 0.32 mm id separation column coupled with a 2 m X 0.32 mm id uncoated precolumn (no vapour exit). Reprinted from Journal of High Resolution Chromatography, 9, K. Grob et al., Concunent solvent evaporation for on-line coupled HPLC-HRGC , pp. 95-101, 1986, with peimission from Wiley-VCH.

Figure 13.5 Schematic presentation of the procedure involved in coupled-column RPLC AS, autosampler C-1 and C-2, first and second separation columns, respectively M-1 and M-2, mobile phases S-1 and S2, interferences A, target analytes HV, high-pressure valve D, detector. Reprinted from Journal of Chromatography, A 703, E. A. Hogendoom and R van Zoonen, Coupled-column reversed-phase liquid cliromatography in environmental analysis , pp. 149-166, copyright 1995, with permission from Elsevier Science.

Figure 8-12. Algorithm for establishing distillation column pressure and type condenser. Used by permission, Heniey, E. J. and Seader, J. D., Equilibrium Stage Separation Operations in Chemical Engineering, John Wiiey, (1981), p. 43, aii rights reserved.

Where a large number of stages is required, it may be necessary to split a column into two separate columns to reduce the height of the column, even though the required separation could, theoretically, have been obtained in a single column. This may also be done in vacuum distillations, to reduce the column pressure drop and limit the bottom temperatures. 

The upper layer which contains, in addition to acrylonitrile, hydrogen cyanide, acrolein, acetonitrile, and small quantities of other impurities, passes to a second reactor (E) where, at a suitable pH, all the acrolein is converted to its cyanohydrin. (Cyanohydrins are sometimes known as cyanhydrins.) The product from the reactor (E) is fed to a cyanohydrin separation column (F), operating at reduced temperature and pressure, in which acrolein cyanohydrin is separated as the bottom product and returned to the ammox-idation reactor (A) where it is quantitatively converted to acrylonitrile and hydrogen cyanide. 

Solution Although the relative volatilities are recalculated for each column, Table 11.7 shows the relative volatilities of the feed mixture to the sequence at a pressure of 1 atm. This shows clearly that the ethyl benzene/xylenes separation is by far the most difficult with relative volatilities for the xylenes close to unity. The volatilities of the components are such that all separations can be carried out at atmospheric pressure and at the same time allow the use of cooling water in the condensers. Thus, column pressures are fixed to atmospheric pressure with the relative volatilities... 

For a simple distillation column separating a ternary system, once the feed composition has been fixed, three-product component compositions can be specified, with at least one for each product. The remaining compositions will be determined by colinearity in the ternary diagram. For a binary distillation only two product compositions can be specified independently, one in each product. Once the mass balance has been specified, the column pressure, reflux (or reboil ratio) and feed condition must also be specified. 

It is desired to separate 1 kg/s of an ammonia solution containing 30 per cent NH3 by mass into 99.5 per cent liquid NH3 and a residual weak solution containing 10 per cent NH3. Assuming the feed to be at its boiling point, a column pressure of 1013 kN/m2, a plate efficiency of 60 per cent and that an 8 per cent excess over the minimum reflux requirements is used, how many plates must be used in the column and how much heat is removed in the condenser and added in the boiler ... 

UOP in a joint venture with ChevronTexaco developed an additive technology named Alkad . The additive is based on HF salts of amines, which form liquid onium polyhydrogen fluoride complexes with HF, reducing the vapor pressure of the catalyst 65% to more than 80% aerosol reduction is claimed with this additive. As in the ReVap technology, additional separation columns have to be installed. Both additives are claimed to increase the product octane number, especially when propene, isobutylene, and pentenes are employed in the feedstock. 

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