Reflux distributor

Example 9.1 A packed column is used to achieve the separation described in Example 2.1, Sec. 2.2.4. Due to fouling, some holes In the reflux distributor are... 

The liquid reflux distributors above each packed zone are often arrays of solid-cone sprays preferably the nozzles should be V4 iiich or larger in orifice, free passage, and pipe sizes. The spray liquid needs to be screened at all times. The sprays are not as good distributors as the drip-pan type, but they are economical and contribute to heat transfer without adding substantial pressure drop. The nozzles should be designed to operate at a flow rate that gives a good spray pattern without excessively small droplet sizes. 

Cause As was suspected, the reflux distributor was tilted allowing all the liquid to flow down one side of the column. The distributor had not been securely attached to its support ring because the installation drawings didn t specify a method of attachment. A clever troubleshooter confirmed the hypothesis prior to shutdown by measuring vessel wall temperatures with a contact pyrometer stuck through the insulation. One side of the column was, of course, colder than the other for several feet below the reflux distiibutor. 

Feed inlet distributors are recommended for large-diameter singlepass trays (207, 354). In multipass trays, feed and reflux distributors are essential to ensure uniform distribution. The only exceptions are... 

The main consideration for introducing reflux or intermediate feed into a packed tower is adequately distributing the incoming stream to the packing. Unlike most tray columns, packed towers are sensitive to distribution. Maldistribution is detrimental to packing efficiency and turndown. The main devices that set the quality of distribution in a packed column are the top (or reflux) distributor, the intermediate feed distributor, the redistributor, and sometimes the vapor distributor. Adequate hydraulics in the inlet area is also important failure to achieve this can affect distributor performance and can also cause premature flooding. 

Washing may pick up debris resting in the column piping and deposit them inside the column or distributors. In one case (203), a welding rag lodged in a reflux distributor led to excessive liquid carryover from a column. The rag was transported by water from the column piping into the reflux distributor, where it remained stuck. 

Olefins Water-wash tower A welding rag left in the column reflux line found its wt to and partially blocked the reflux distributor to two-pass trays. EhGoessive entrainment resulted. Inspect for debris at points where lines leadingto the column were modified. 

So this was a design error which caused reflux maldistribution," summarized the plant manager. "Your peripheral external temperature survey above the top reflux distributor supports your theory. While I agree with your conclusion," he continued, "how can we improve fractionation without an extended FCCU shutdown "... 

In high purity distillate production, any vapor bypassing results in increased heavy-key concentration in the distillate. To avoid excessive vapor wall flow, the packing size should be selected in accordance with Table 7-8. In such operations, it is important that the reflux distributor wet the top of the packed bed all the way to the column wall. Special measures must be used to ensure that the support ring for the liquid distributor or bed limiter does not interfere with the liquid irrigation pattern imposed on the packed bed top surface. 

FK . 14-63 Efficiency of beds of 51 mm Pali rings with two different distributors. Column diameter =1.2 m, cyclohexane/n-heptane system at 1.65 bar and total reflux. [Shariat and Kunesh, Ind. Eng. Chem. Res., 34 1273 (1995).] Reproduced with permission. Copyright 1995, American Chemical Society. 

FIG. 14-74 HETP values for Max-Pak structured packing,. 35 kPa (5 psia), two column diameters. Cyclohexane/n-heptane system, total reflux. For 0.4.3 m (1.4 ft) column perforated pipe distributor, 400 streams/m2, 3.05 m (10 ft) bed height. For 1.2 m (4.0 ft) column tubed drip pan distributor, 100 streams/m ,. 3.7 m (12 ft) bed height. Smaller column data. University of Texas/Austin Larger column data. Fractionation Research, Inc. To convert (ft/s)(lb/ft ) to (m/s)(kg/m ) , multiply by 1.2199. (Couiiesy Jaeger Troducts, Inc., Housion, Texas.)... 

For columns that are wider than several centimeters, reflux and feed distributors should be used, particularly for wet foam [Haas and Johnson, Am. Jn.st. Chem. Fng. J., 11, 319 (1965)]. Liquid content within the foam can be monitored conduc tometricaUy [Chang and Lemhch, J. Colloid Jnteiface Sci., 73, 224 (1980)]. See Fig. 22-44. TheoreticaUy, as the limit =K = 0is veiy closely approached, 2] = 3K [Lemhch,y. Colloid Jnteiface Sci, 64, 107 (1978)]. 

Distributor switched from feed to reflux. (Drawing wrong) Everything looked good except separation. Liquid maldistribution problem. Design error. 

Feintuch [221] pre.sents calculations for a pipe distributor with tray and downcomers to disperse the reflux or feed liquid uniformly across the tray (which should not be counted as a working tray, but a distribtuion device) and... 

Reflux nozzles must be arranged to enter the tray with proper designed internal pipe to the tray downcomer or distributor. 

For distributors of any design, including the PAN, it is important to filter the feed or reflux liquid entering tlie distributor to reduce the possibilities of plugging of tlic orifice holes. Otherwise, the landom plugging will cause non-uniform distribution onto the packing below. It is important to avoid leakage around the risers because this can destroy the liquid distiibution pattern [131]. 

Turndown on a PAN type tray should be limited to 2 1 (ratio of high to low flow rates), which results in a reasonable design. Bonilla [131] points out that it is cosdy to design for short-term high turndown rates such as start-up, shutdown, or for other short term periods, because it is better to increase reflux ratio to increase internal loads for such periods rather than design the distributor for large turndown. 

Unfortunately, packing does not redistribute liquid, or internal reflux. Unless the initial reflux distribution is good, the liquid flow distribution through the entire packed bed will be poor. Figure 7.1 shows a common orifice plate liquid distributor. Vapor flows up through the large chimneys, and liquid drains through the smaller distribution holes in the tray deck. 

Let s further assume that the orifice plate distributor is 1 in out-of-level. This could easily happen in a 14-ft 0-in-ID tower. Figure 7.2 shows the results. The flow of internal reflux or liquid through the higher portion of the tray deck falls to zero. Worse yet, vapor starts to... 

Packed beds also seem to have a better turndown capability than valve or sieve trays, at low vapor flows. On the other hand, many packed fractionators seem quite intolerant of reduced liquid or reflux flow rates. This is typically a sign of an improperly designed distributor in the packed fractionator. 

Flashing Feed and Vapor Distributors When the feed or reflux is a flashing feed, the vapor must be separated out of the liquid before the liquid enters a liquid distributor. At low velocities (only), this can be achieved by a bare nozzle (Fig. 14-70a). A V baffle (Fig. 14-70h) is sometimes installed as a primitive flashing feed or vapor distributor. 

At first, the distillation still was charged with methanol—the low-boiling reactant—and heated under total reflux until steady-state conditions were achieved. At that moment, acetic acid—the high-boiling reactant—was fed above the reaction zone to the second distributor. After 30 min the reflux ratio was changed from infinity to 2 and the product withdrawal at the top of the column began. During the... 

Lab-scale trials indicated that elimination of reflux and radiation losses combined with an optimization of the steam distributor could reduce the necessary steam consumption significantly (6). As a result, a final overall vaporization efficiency of 0.93 could be achieved (7). Industrial deodorizers, however, have a lower efficiency, as they usually operate under conditions that are less ideal than those of a laboratory deodorizer. 

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