The Distillation Column

FIGURE 6.13. Effect of conversion on (a) the capital cost, Cjj and (b) the production costs Pjj of the electrochemical reactor, (current density = 2000 A/m ). 

The conditions around a simple distillation column are shown in Fig. 6.15. Products D and W leave the top and bottom of the column respectively and feed F is introduced at some intermediate point. A liquid stream L is returned to the top of the column and a vapor stream V to the bottom. The stream L is the reflux and LjD is the reflux ratio R. The number of plates or stages in the column is N. Heating and cooling services are provided by a reboiler and condenser. 

If F and D are fixed, the only variable open to choice is R its optimum value is determined, as usual, by comparing capital investment with production costs. Since R is a noninteractive variable, the comparison is confined to costs associated with the column. 

Capital investment is the installed costs of the column, its internals, the condenser, and the reboiler. Production costs are the cost of cooling water, steam, and losses of material due to imperfect separation. R can vary from a minimum to infinity. Capital investment is infinite and production 

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Consider again the simple process shown in Fig. 4.4d in which FEED is reacted to PRODUCT. If the process usbs a distillation column as separator, there is a tradeofi" between refiux ratio and the number of plates if the feed and products to the distillation column are fixed, as discussed in Chap. 3 (Fig. 3.7). This, of course, assumes that the reboiler and/or condenser are not heat integrated. If the reboiler and/or condenser are heat integrated, the, tradeoff is quite different from that shown in Fig. 3.7, but we shall return to this point later in Chap. 14. The important thing to note for now is that if the reboiler and condenser are using external utilities, then the tradeoff between reflux ratio and the number of plates does not affect other operations in the flowsheet. It is a local tradeoff. 

The consequences of placing distillation columns in different locations relative to the pinch will now be explored. There are two possible ways in which the distillation column can be integrated. The reboiler and condenser can be integrated either across the pinch or not across the pinch. 

Let us now consider a few examples for the use of this simple representation. A grand composite curve is shown in Fig. 14.2. The distillation column reboiler and condenser duties are shown separately and are matched against it. Neither of the distillation columns in Fig. 14.2 fits. The column in Fig. 14.2a is clearly across the pinch. The distillation column in Fig. 14.26 does not fit, despite the fact that both reboiler and condenser temperatures are above the pinch. Strictly speaking, it is not appropriately placed, and yet some energy can be saved. By contrast, the distillation shown in Fig. 14.3a fits. The reboiler duty can be supplied by the hot utility. The condenser duty must be integrated with the rest of the process. Another example is shown in Fig. 14.36. This distillation also fits. The reboiler duty must be supplied by integration with the process. Part of the condenser duty must be integrated, but the remainder of the condenser duty can be rejected to the cold utility. 

If the distillation column will not fit either above or below the pinch, then other design options can be considered. One possibility is... 

Establish the heat integration potential of simple columns. Introduce heat recovery between reboilers, intermediate reboilers, condensers, intermediate condensers, and other process streams. Shift the distillation column pressures to allow integration, where possible, using the grand composite curve to assess the heat integration potential. 

The pressure in distillation column 1 has been increased to allow feed vaporization by heat recovery (from the distillation column condenser). Inspection of the new curves in Fig. 14.9a raises further possibilities. With the proposed modification, the overheads from the... 

Acryflc acid, alcohol, and the catalyst, eg, sulfuric acid, together with the recycle streams are fed to the glass-lined ester reactor fitted with an external reboiler and a distillation column. Acrylate ester, excess alcohol, and water of esterification are taken overhead from the distillation column. The process is operated to give only traces of acryflc acid in the distillate. The bulk of the organic distillate is sent to the wash column for removal of alcohol and acryflc acid a portion is returned to the top of the distillation column. If required, some base may be added during the washing operation to remove traces of acryflc acid. 

Conventional polymeri2ation inhibitors are fed to each of the distillation columns. The columns are operated under reduced pressure to give low bottom temperatures and minimi2e polymeri2ation. 

In France, Compagnie Europnene du Zirconium (CEZUS) now owned jointly by Pechiney, Eramatome, and Cogema, uses a separation (14) based on the extractive distillation of zirconium—hafnium tetrachlorides in a molten potassium chloride—aluminum trichloride solvent at atmospheric pressure at 350°C. Eor feed, the impure zirconium—hafnium tetrachlorides from the zircon chlorination are first purified by sublimation. The purified tetrachlorides are again sublimed to vapor feed the distillation column containing the solvent salt. Hafnium tetrachloride is recovered in an enriched overhead fraction which is accumulated and reprocessed to pure hafnium tetrachloride. 

Fig. 42. Integrated distillation/pervaporation plant for ethanol recovery from fermentors. The distillation columns concentrate the ethanol—water mixture from 5 to 80%. The pervaporation membrane produces a 99.5% ethanol product stream and a 40—50% ethanol stream that is sent back to the distillation...

A number of processes have been devised for purifying thionyl chloride. A recommended laboratory method involves distillation from quinoline and boiled linseed oil. Commercial processes involve adding various high boiling olefins such as styrene (qv) to react with the sulfur chlorides to form adducts that remain in the distillation residue when the thionyl chloride is redistilled (179). Alternatively, sulfur can be fed into the top of the distillation column to react with the sulfur dichloride (180). Commercial thionyl chloride has a purity of 98—99.6% minimum, having sulfur dioxide, sulfur chlorides, and sulfuryl chloride as possible impurities. These can be determined by gas chromatography (181). 

The carbon monoxide-rich, Hquid condensate from the primary separator is expanded and exchanged against the incoming feed and is then sent to a distillation column where the carbon monoxide is purified. The bottoms Hquor from the methane wash column is expanded, heat-exchanged, and sent to the bottom section of the distillation column for methane rectification and carbon monoxide recovery. The methane bottom stream is recompressed and recycled to the top of the wash column after subcooling. A sidestream of methane is withdrawn to avoid a buildup of impurities in the system. 

Observed problem Increased pressure drop in the distillation column... 

Calcium oxide. (Preheated to 700-900° before use.) Suitable for alcohols and amines (but does not dry them completely). Need not be removed before distillation, but in that case the head of the distillation column should be packed with glass wool to trap any calcium oxide powder that might be carried over. Unsuitable for acidic compounds and esters. Suitable for drying gaseous amines and ammonia. 

After actual theoretical trays are determined (see Actual reflux and theoretical stages) one needs to estimate the actual physical number of trays required in the distillation column. This is usually done by dividing the actual theoretical trays by the overall average fractional tray efficiency. Then a few extra trays are normally added for offload conditions, such as a change in feed composition. 

The wastewater stream of the plant is composed of the off-gas condensate, aqueous layer of the decanter, bottom product of the distillation column and the... 

Furthermore, for quality and operability objectives the plant does not wish to recycle the AN product stream (top of distillation column), the feed to the distillation column and the feed to the decanter. 

In one incident, a man was draining water, through a 2-in.-diameter line, from a small distillation column rundown tank containing benzene. He left the water running for a few minutes to attend to other jobs. Either there was less water than usual or he was away longer than expected. He returned to find benzene running out of the drain line. Before he could close it, the benzene was ignited by the furnace which heated the distillation column. The operator was badly burned and died from his injuries. 

In a total condenser all of the overhead vapor is condensed to the liquid state. When the heat load or duty on the condenser is exactly equal to the latent heat of the saturated or dew point of the overhead vapor from the distillation column, the condensed liquid will be a saturated bubble point liquid. The condenser and accumulator... 

Multicomponent distillations are more complicated than binary systems due primarily to the actual or potential involvement or interaction of one or more components of the multicomponent system on other components of the mixture. These interactions may be in the form of vapor-liquid equilibriums such as azeotrope formation, or chemical reaction, etc., any of which may affect the activity relations, and hence deviations from ideal relationships. For example, some systems are known to have two azeotrope combinations in the distillation column. Sometimes these, one or all, can be broken or changed in the vapor pressure relationships by addition of a third chemical or hydrocarbon. 

The entrance of a liqmd-flashing vapor mixture into the distillation column feed location requires a specially designed distribution tray to separate the vapors from the liquid, w hich must drop onto the packing bed for that section in a uniform pattern and rate. 

This unit is usually used as the reboiler for the distillation column and, in this service, operates by the thermosiphon action of the difference in static head in the column and in the vapor-liquid phase leaving the reboiler. When tied into the bottom chamber, the liquid is usually recirculated many times, vaporizing only 10-25% of the reboiler feed per pass however, when used as a draw-off from the bottom tray seal pan, the feed to the reboiler is not recirculated flow. The basic operation is the same, however. 

This represents the fraction of the total available head between points A and B, which represents the sensible heating zone. This neglects liquid friction in the sensible zone and assumes the liquid level in the distillation column is maintained even with the top of the tubesheet. Equation 10-170 then gives the fractional tube length devoted to sensible heating. Refer to Figure 10-110 and note that ... 

The best designs provide for the percentage vaporization per pass to have been completed by the time the fluid mixture reaches the upper end of the tube and the mixture is leaving to enter the bottom chamber of the distillation column. In order to assist in accomplishing this, the initial reboiler elevation should be set to have the top tubesheet at the same level as the liquid in the column bottom section. A liquid-level control adjustment capability to raise or lower this bottoms level must exist to optimize the recirculation. Sometimes, the level in the bottom of the column may need to be 25-30% of the reboiler tube length above the elevation of the tubesheet. Therefore, the vapor nozzle return from the reboiler must enter at sufficient elevation to allow for this possibility. 

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