Feed lines column distillation

Figure 5.6. State of the feed for a distillation column. The bold line is the operating line for a saturated liquid feed.

Exploitation of Boundary Curvature A second approach to boundary crossing exploits boundary curvature in order to produce compositions in different distillation regions. When distillation boundaries exhibit extreme curvature, it may be possible to design a column such that the distillate and bottoms are on the same residue curve in one distillation region, while the feed (which is not required to he on the column-composition profile) is in another distillation region. In order for such a column to meet material-balance constraints (i.e., bottom, distillate, feed on a straight line), the feed must be located in a region where the boundary is concave. 

So, the slope of the feed line is found as q/(q - 1). If the feed is a saturated liquid, then q = 1.0, the siope of the line is infinity and the feed iine is vertical. Conversely, if the feed is a saturated vapor, the slope is zero and the feed line is horizontal. The feed can also exist as a two-phase mixture, subcooled liquid, and superheated vapor. Figure 4.6 presents a number of representative feed lines and Figure 4.7 shows different qualities of feed entering a distillation column. 

Water supply, 2 = Distilled -water apparatus, 3 = Flow meter, 4 — Constant level device, 5 = Feed line, 6 = Overflow, 7 = Column, 8 = Boiler, 9 - Product line, 10 = 1-inch copper pipe between boiler and column, 11 = Beflux cooler, 12 13 = Main condensers, 14 = Stock tank, 15 = Drain, 16 = Boiler manometer, 17 = Differential manometer, 18 - Manostat, 19 = Magnetic valve, 20 = To pump, 21—23 — Piping... 

In a packed column, the feed can be put on to any redistributor, i.e. every 4-5 m (Fig. 4.5). Modern redistributors have a large number of very small holes which are designed to handle clean distillate liquids. The feed should be filtered in the feed line to prevent pieces of rust, scale, etc., from blocking some of these holes and spoiling the distribution of reflux and feed. This is a very important factor in the performance of a packed column. In addition, the possibility of scale being formed on the redistributor should be guarded against since it would have the same effect. 

C16. Part a. In a binary distillation column with two feeds, show that the intersection of the top and bottom operating lines occurs at the feed line for fictitious feed Fj where Fj = Fj + F2, z F-p =... 

In the reflux scheme, a column temperature controller manipulates a control valve in the reflux line. The reflux drum level controller manipulates a valve in the distillate line. The column base level controller manipulates a valve in the bottoms line. The feed and reboiler steam are each on flow rate control. In some cases, there is a controller for the pressure drop across the trays that manipulates the valve in the reboiler steam line. However, it is preferred to use a steam flow rate controller and simply monitor the tower pressure drop. With this scheme the separation power base is derived from the ratio of steam/feed. The distillate/feed material balance split is maintained by the MRT point controller. 

Enriching-column distillation. Enriching towers are also sometimes used, where the feed enters the bottom of the tower as a vapor. The overhead distillate is produced in the same manner as in a complete fractionating tower and is usually quite rich in the more volatile component A. The liquid bottoms is usually comparable to the feed in composition, being slightly leaner in component A. If the feed is saturated vapor, the vapor in the tower V = F. Enriching-line equation (11.4-7) holds, as does the ij-line equation (11.4-19). 

Kettle reboilers have five connections, two on the tube side and three on the shell side. Steam or hot oil flows through the tube side and provides the heat source. Flow rate is carefully controlled and frequently linked to the bottom temperature control system. The higher the flow rate, the hotter the bottom product. The shell side has three nozzles one liquid-product feed line, one vapor-return line to the column, and one heavy-liquid-out product line. A kettle reboiler can be used to (1) control the liquid level on the bottom of the column, (2) control the temperature of the column, and (3) help control product purity in the bottom of the column. Figure 6-5 shows what a kettle reboiler looks like, and Figure 6-6 shows two thermosyphon and one kettle reboiler arrangements on a distillation column. 

The basic components of a plate distillation column include a feed line, feed tray, rectifying or enriching section, stripping section, downcomer, reflux line, energy-balance system, overhead cooling system, condenser, preheater, reboiler, accumulator, feed tank, product tanks, bottom line, top line, side stream, and an advanced instrument control system. Plate columns hold trays that may be bubble-cap, valve, or sieve. Figure 6-19 shows the basic components of a plate distillation column. 

The symbol used on a diagram for a plate column should indicate the type of tray used in the system bubble-cap, valve, or sieve. The first distillation column was invented in 1917. Today, a number of modifications allow modern process technicians to operate much more efficiently. The design, however, still includes the original still-on-top-of-a-still approach. The basic components of a plate distillation column are a feed line feed tray stripping section below the feed line enriching or rectifying section above the feed line overhead vapor outlet, side-stream outlet, and bottom outlet reboiler instrumentation for level, temperature, flow, pressure, and composition control outer shell and a top reflux line. 

We split the saturated liquid feed into two fractions having flow rates of and introduce one fraction at the top of a distillation column, vaporize the second fraction completely and introduce it at the bottom the column. Plot the q-lines for the two streams entering the column if jtij= 0.5. Develop, under conditions of constant molal overflow, the operating line for this distillation column. Plot the line for the situation where Wtfi = Wtfi. Obtain graphically the compositions of the overhead vapor product and the bottoms liquid product for two values of the number of ideal equilibrium plates in column, N =1 and N—5. 

Fig. 13.3-1. Staged distillation separating a concentrated feed. Because the feed changes the flows within the column, the operating lines above and below the feed stage differ.

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