Alternative feed point

Feed composition variation. One way of allowing for design uncertainties and feedstock variation is by installing alternate feed points. In packed towers, every alternate feed point requires expensive distribution equipment. 

Ol ns Depropanizer Instability and fluctuations oocuned because of operation below the dumping loads in a section of the cohinm just below the feed. Ensure adequate hydraulics above rmd below alternate feed points. 

P IDs (piping and instrumentation diagrams) should identify instruments, sample locations, the presence of sample valves, nozzle blinding, and control points. Of particular importance are the bypasses and alternate feed locations. The isolation valves in these hues may leak and can distort the interpretation of the measurements. 

The relation is solved for Sr/Sj. The results are not exact, because the feed tray composition is very seldom the same as the feed which is the assumption in this relation. Actually, the feed point or correct location for the feed may be off by two or three theoretical trays. This will vary with the system. It does mean, however, that when this approach is used for feed plate location, alternate feed nozzles should be installed on the column to allow for experimental location of the best feed point. These extra nozzles are usually placed on alternate trays (or more) both above and below the calculated location. A minimum of three alternate nozzles should be available. 

When the feed point is located by a match from tray-bytray calculation, the correct point can be established with greater confidence, but still alternate nozzles are suggested since even these detailed calculations can be off to a certain extent. 

A limitation of the Erbar-Maddox, and similar empirical methods, is that they do not give the feed-point location. An estimate can be made by using the Fenske equation to calculate the number of stages in the rectifying and stripping sections separately, but this requires an estimate of the feed-point temperature. An alternative approach is to use the empirical equation given by Kirkbride (1944) ... 

Some distillation columns must handle two or more feed streams simultaneously. Furthermore, alternative feed nozzles are often provided to allow the actual feed-point locations to be altered. By optimizing the feed-point locations, energy consumption in the reboiler can often be minimized. 

Alternatively, results from a computer simulation can be plotted to determine the optimum feed stage. Simulation runs ere performed at several different feed points, keeping the material balance, reflux ratio, and total number of stages constant. Ksy component concentrations in the product streams are plotted against the feed stage number (Fig. 3.7). The minimum is at the optimum feed stage. 

It since appears that the frequency and size of the turbulent velocity fluctuations are more compatible with phenomena on a molecular micron size scale In a mixing vessel. For example, in a publication Paul Treybal (4) shows the yield of a given reaction which had several alternate paths was determined by the RMS velocity fluctuations at the feed point. Paul used data from Schwartzberg Treybal (5) and Cutter to calculate RMS velocity at the feed point. 

Branches to allow feed to be put into the column and product to be taken as a liquid side draw. If a new column is being fabricated it should be borne in mind that branches may be used as feed points, side draws, sample points, pressure tappings, and temperature pocket points. As an emergency expedient they can also be used to by-pass a blocked downcomer. For this multitude of reasons a branch on alternate trays is not excessive. 

It should be noted that separate material and energy balances have to be set up for the bottom and top of the column and around the feed points. Alternatively, corresponding terras could be included in the basic equations and set to zero at all other places in the column. 

The idea of the simpler single-temperamre loop-control strategy is to use an aqueous reflux flowrate to hold some tray temperature inside the column and to ratio the other two manipulated variables (reboiler duty and entrainer makeup) to the feed flowrate. From the earlier sensitivity analysis in Figure 9.9a, the most sensitive control point inside the column is Stage 6, which is close to the top of the column. Since the main acetic acid product is drawn out from the bottom of the column, an alternative control point is to select the second most sensitive control point, which is closer to the bottom of the column. This alternative control point will be Stage 16. Thus two single-loop-control stmctuies will be evaluated using closed-loop dynamic tests. They are ... 

Coal is fed to the dense bed by a small number of simple slide chutes fed by rotary valves and located along one wall of the unit. A single coal feed point may service 9-30 m of bed cross section (Brereton, 1997). Alternatively, coal may be fed at a single point into the solids recirculation leg. Limestone (less than 6 mm in diameter) is normally fed separately above the bed. 

The feed solution passes through the prechillers and into the warm solution drum. Up to this point the propane dewaxing process is a continuous process. The warm solution drum alternates feeding one of two chillers and the process now becomes a batch process. 

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