Vacuum operation

Both vacuum operation and the use of refrigeration incur capital and operating cost penalties and increase the complexity of the design. They should be avoided if possible. For a first pass through... 

The design of these distillation systems and the operating conditions used depend on the physical properties of the alkylphenols involved and on the product requirements. Essentially all alkylphenol distillation systems operate under vacuum, but the actual pressures maintained vary considerably. Vacuum operation allows reasonable reboder temperatures (200—350°C) so that thermal dealkylation reactions of the alkylphenols are slow. 

Packed vs Plate Columns. Relative to plate towers, packed towers are more useful for multipurpose distillations, usually in small (under 0.5 m) towers or for the following specific appHcations severe corrosion environment where some corrosion-resistant materials, such as plastics, ceramics, and certain metaUics, can easily be fabricated into packing but may be difficult to fabricate into plates vacuum operation where a low pressure drop per theoretical plate is a critical requirement high (eg, above 49,000 kg/(hm ) (- 10, 000 lb/(hft )) Hquid rates foaming systems or debottlenecking plate towers having plate spacings that are relatively close, under 0.3 m. 

The vacuum plate drwer is provided as pari of a closed system. The vacuum dryer has a cylindrical housing and is rated for fiill-vacuum operation (typical pressure range 3-27 kPa absolute). The exhaust vapor is evacuated try a vacuum pump and is passed through a condenser for solvent recovery. There is no purge-gas system required for operation under vacuum. Of special note in the vacuum-drying system... 

FIG. 12-78 Indirect-heated continuous plate dryer for atmospheric, gastight, or fuU-vacuum operation. (Krauss Maffei.)... 

Comparison Data—Plate Dryers Comparative studies have been done on products under both atmospheric and vacuum drying conditions. See Fig. 12-79. These curves demonstrate (1) the improvement in drying achieved with elevated temperature and (2) the impact to the drying process obtained with vacuum operation. Note that cui ve 4 at 90°C, pressure at 6.7 kPa absolute, is comparable to the atmospheric cui ve at 150°C. Also, the comparative atmospheric cui ve at 90°C requires 90 percent more diying time than the vacuum condition. The dramatic improvement with the use of vacuum is important to note for heat-sensitive materials. 

Temperature-sensitive mixtures are to be separated. To avoid decomposition and/or polymerization, vacuum operation may then be necessary. The smaller liqmd holdup and pressure drop theoretical stage of a packed column may be particularly desirable. 

The actual liquid-to-gas ratio (solvent-circulation rate) normally will be greater than the minimum by as much as 25 to 100 percent and may be arrived at by economic considerations as well as by judgment and experience. For example, in some packed-tower applications involving veiy soluble gases or vacuum operation, the minimum quantity of solvent needed to dissolve the solute may be insufficient to keep the packing surface thoroughly wet, leading to poor distribution of the liquid stream. 

FIG. 14-72 HETP values for four sizes of metal pall rings, vacuum operation. Cyclobexane/n-beptane system, total reflux,. 35 kPa (5.0 psia). Column diameter = 1.2 m (4.0 ft). Bed height = 3.7 m (12 ft). Distributor = tubed drip pan, 100 streams/nv. [Adapted from Shafiat and Kunesh, Ind. Eng. Cbem. Res., 34,1273 (1.9.9.5),] Reproduced with permission. Copyright 1995 American Chemical Society. 

Continuous Precoat Filters These filters may be operated as either pressure or vacuum filters, although vacuum operation is the prevailing one. The filters are really not continuous but have an extremely long batch cycle (1 to 10 days). Apphcations are for continuous clarification of liqmds from slurries containing 50 to 5000 ppm of solids when only very thin unacceptable cakes would form on other filters and where perfect clarity is required. 

Modules Eveiy module design used in other membrane operations has been tried in peivaporation. One unique requirement is for low hydraulic resistance on the permeate side, since permeate pressure is veiy low (O.I-I Pa). The rule for near-vacuum operation is the bigger the channel, the better the transport. Another unique need is for neat input. The heat of evaporation comes from the liquid, and intermediate heating is usually necessary. Of course economy is always a factor. Plate-and-frame construc tion was the first to be used in large installations, and it continues to be quite important. Some smaller plants use spiral-wound modules, and some membranes can be made as capiUaiy bundles. The capillaiy device with the feed on... 

This continuous process is to be compared with a batch process, such as the Belgian retort process. In this, zinc oxide, free of lead or iron is reduced with carbon to produce zinc vapour, which is condensed in the cold section of the retort. The oxygen potential in this system is very much lower dran in the blast furnace, approximately at the C/CO equilibrium value. A vacuum-operated variant of dris level of reduction is caiTied out to produce zinc vapour which is subsequently converted to zinc oxide before condensation of the metal could take place. 

The majority of industrial chemical and petrochemical plants vacuum operations are in the range of 100 microns to 760 torn This is practically speaking the rough vacuum range noted above. For reference ... 

As a guide, the follotving is a suggested procedure for rating and selecting an ejector system for vacuum operation. 

The overall temperature difference is 10-18°F, depending on fouling. Vacuum operation on the steam side is indicated. 

Similar considerations apply to the selection of pressure drops where there is freedom of choice, although a full economic analysis is justified only in the case of very expensive units. For liquids, typical values in optimised units are 35 kN/m2 where the viscosity is less than 1 mN s/m2 and 50-70 kN/m2 where the viscosity is 1-10 mN /m2 for gases, 0.4-0.8 kN/m2 for high vacuum operation, 50 per cent of the system pressure at 100- 200 kN/m2, and 0 per cent of the system pressure above 1000 kN/m2. Whatever pressure drop is used, it is important that erosion and flow-induced tube vibration caused by high velocity fluids are avoided. 

For vacuum operation, design pressures are 15 psig and fuii vacuum. 

Rotary drum filters (usually vacuum operation) (Figure 10.13)... 

The height of the weir determines the volume of liquid on the plate and is an important factor in determining the plate efficiency (see Section 11.10.4). A high weir will increase the plate efficiency but at the expense of a higher plate pressure drop. For columns operating above atmospheric pressure the weir heights will normally be between 40 mm to 90 mm (1.5 to 3.5 in.) 40 to 50 mm is recommended. For vacuum operation lower weir heights are used to reduce the pressure drop 6 to 12 mm ( to in.) is recommended. 

Thermosyphon reboilers are the most economical type for most applications, but are not suitable for high viscosity fluids or high vacuum operation. They would not normally be specified for pressures below 0.3 bar. A disadvantage of this type is that the column base must be elevated to provide the hydrostatic head required for the thermosyphon effect. This will increase the cost of the column supporting-structure. Horizontal reboilers require less headroom than vertical, but have more complex pipework. Horizontal exchangers are more easily maintained than vertical, as tube bundle can be more easily withdrawn. 

Kettle reboilers have lower heat-transfer coefficients than the other types, as there is no liquid circulation. They are not suitable for fouling materials, and have a high residence time. They will generally be more expensive than an equivalent thermosyphon type as a larger shell is needed, but if the duty is such that the bundle can be installed in the column base, the cost will be competitive with the other types. They are often used as vaporisers, as a separate vapour-liquid disengagement vessel is not needed. They are suitable for vacuum operation, and for high rates of vaporisation, up to 80 per cent of the feed. 

Thermosyphon reboilers are usually cheapest but not suitable for high-viscosity liquids or vacuum operation. Further discussion of reboilers will be deferred until Chapter 15. In the preliminary phases of a design, it is best to assume that enough stages are available in the column itself to achieve the required separation. 

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