Fouling fractionating column

The mixture is cooled and noncondensable gases are scmbbed with water. Some of the resultant gas stream, mainly hydrogen, may be recycled to control catalyst fouling. The liquids are fractionally distilled, taking acetone overhead and a mixture of isopropyl alcohol and water as bottoms. A caustic treatment may be used to remove minor aldehyde contaminants prior to this distillation (29). In another fractionating column, the aqueous isopropyl alcohol is concentrated to about 88% for recycle to the reactor. 

Only trace amounts of side-chain chlorinated products are formed with suitably active catalysts. It is usually desirable to remove reactive chlorides prior to fractionation in order to niinimi2e the risk of equipment corrosion. The separation of o- and -chlorotoluenes by fractionation requires a high efficiency, isomer-separation column. The small amount of y -chlorotoluene formed in the chlorination cannot be separated by fractionation and remains in the -isomer fraction. The toluene feed should be essentially free of paraffinic impurities that may produce high boiling residues that foul heat-transfer surfaces. Trace water contamination has no effect on product composition. Steel can be used as constmction material for catalyst systems containing iron. However, glass-lined equipment is usually preferred and must be used with other catalyst systems. 

Figure 3.14. The lower ends of fractionators, (a) Kettle reboiler. The heat source may be on TC of either of the two locations shown or on flow control, or on difference of pressure between key locations in the tower. Because of the built-in weir, no LC is needed. Less head room is needed than with the thermosiphon reboiler, (b) Thermosiphon reboiler. Compared with the kettle, the heat transfer coefficient is greater, the shorter residence time may prevent overheating of thermally sensitive materials, surface fouling will be less, and the smaller holdup of hot liquid is a safety precaution, (c) Forced circulation reboiler. High rate of heat transfer and a short residence time which is desirable with thermally sensitive materials are achieved, (d) Rate of supply of heat transfer medium is controlled by the difference in pressure between two key locations in the tower, (e) With the control valve in the condensate line, the rate of heat transfer is controlled by the amount of unflooded heat transfer surface present at any time, (f) Withdrawal on TC ensures that the product has the correct boiling point and presumably the correct composition. The LC on the steam supply ensures that the specified heat input is being maintained, (g) Cascade control The set point of the FC on the steam supply is adjusted by the TC to ensure constant temperature in the column, (h) Steam flow rate is controlled to ensure specified composition of the PF effluent. The composition may be measured directly or indirectly by measurement of some physical property such as vapor pressure, (i) The three-way valve in the hot oil heating supply prevents buildup of excessive pressure in case the flow to the reboiier is throttled substantially, (j) The three-way valve of case (i) is replaced by a two-way valve and a differential pressure controller. This method is more expensive but avoids use of the possibly troublesome three-way valve.

In a similar vein, Parliment (33) used reverse phase C18 adsorbents to concentrate and fractionate low levels of volatile organic compounds from dilute aqueous streams. The aqueous phase must be particulate free to prevent fouling of the adsorbent, but soluble solids are not necessarily a problem. For example, a commercial cola beverage containing caramel color, caffeine, phosphoric acid and sweetener was passed over a reverse phase column. Desorption with acetone produced an aroma concentrate which could be analyzed by gas chromatography. 

Association is such an important characteristic of asphalts, believed by many to be an indicator of asphalt performance, that attempts have been made to use poorer solvents to emphasize this feature. Unfortunately, poorer solvents lead to column fouling and bad tailing of the adsorbed material. Figure 15 is an extracted core asphalt and its Corbett fractions run in toluene and is similar to... 

In the Coastal Refinery in Aruba, we used sieve trays with one-half-inch holes, which seemed to work fine. Avoid packed towers. They are subject to vapor-liquid channeling and poor fractionation efficiency due to sloppy installation, fouling, or poor liquid feed distribution. When calculating the required hole area for the trays, don t forget that the vapor loads and hence the required tray hole area will substantially diminish as the vapor flows up the column from the reboiler to the feed tray. This will normally require a reduction in the tray deck hole area in proportion to the reduced vapor flow rate. 

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