Bottoms Stripping Section

The objective of the bottoms stripper is to strip distillate from the flash zone liquid, revaporize residual distillate that may be in the spent wash, and correction of bottoms flash. The bottom stripper typically has a design of 4 to 6 bubble cap trays or sieve trays. Some of the newer designs are using packing. Steam is used to reduce the hydrocarbon partial pressure to vaporize lighter molecules. A quench recycle is used to cool the stripped bottoms below 360°C (680°F) to reduce coking and cracking of the hydrocarbons. 

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But for the bottom stripping section trays, a reduction in reboiler duty will directly reduce the vapor flow from the reboiler to the bottom tray. This statement is approximately valid for all the trays in the stripping section of the tower. 

A high tower bottoms level will cause liquid to flood the bottom stripping section. This backs up resid into the flash zone. When the frothy liquid level rises to the flash zone inlet nozzle, entrainment of bottoms will carry up into the wash oil section, which will also flood. 

Set the value for overflash. This liquid returns to the flash zone from the tray immediately above it and mixes with the feed flash liquid. The sum of these two liquids is the feed to the top tray of the bottoms stripping section. 

In another alternative, shown in Fig. 13-3 7, the rectifying section may be operated at a pressure sufficiently higher than that of the stripping section such that heat can be transferred between any desired pairs of stages of the two sections. This technique, described by Mah et al. (op. cit.) and referred to as SRV (secondary reflux and vaporization) distillation, can result in a significant reduction in utihty requirements for the overhead condenser and bottoms reboiler. 

The equilibrium relationship for component in the stripping section can be expressed in terms of component flow rate in the bottoms, h = Bxg, and = KyjL as... 

The rate-based model gave a distillate with 0.023 mol % ethylbenzene and 0.0003 mol % styrene, and a bottoms product with essentially no methanol and 0.008 mol % toluene. Miirphree tray efficiencies for toluene, styrene, and ethylbenzene varied somewhat from tray to tray, but were confined mainly between 86 and 93 percent. Methanol tray efficiencies varied widely, mainly from 19 to 105 percent, with high values in the rectifying section and low values in the stripping section. Temperature differences between vapor and liquid phases leaving a tray were not larger than 5 F. 

Our example system has a flow-controlled feed, and the reboiler heat is controlled by cascade from a stripping section tray temperature. Steam is the heating medium, with the condensate pumped to condensate recovery. Bottom product is pumped to storage on column level control overhead pressure is controlled by varying level in the overhead condenser the balancing line assures sufficient receiver pressure at all times overhead product is pumped to storage on receiver level control and reflux is on flow control. 

Steam used in the sidestream strippers and in the stripping section of the main column is condensed in the overhead condenser. This water settles to the bottom of the distillate dmm and is drawn off through a small water pot in the bottom. In most installations, ammonia gas is injected into the overhead line to raise the pH of this water and reduce corrosion. 

Now substitute this value x = 0.88 into the equation and calculate the vapor coming up from the first plate below the top (t - 1). Thus, if x = top plate, yn + i = vapor from plate below top. Now, read equilibrium curve aty(j i) and get X(n +1) or Xt 1 which is liquid on plate below top. Then using Xf 1, calculate yt- 2 (second plate below top, etc.). Then, read equilibrium curve to get corresponding liquid X( 2- Continue until feed plate composition is reached, then switch to equation of stripping section and continue as before until desired bottoms composition is reached. Operating line of stripping section ... 

A benzene-toluene mixture is to be separated in a tower packed with 1-in. fieri saddles. The feed is 55.2 mol% (liquid feed, saturated), and an overhead of 90 mol% benzene, and bottoms of not more than 24 mol% benzene is desired. Using the data of Ref. 51 plotted in Figure 9-98, determine the number of transfer units in the rectifying and stripping sections using a reflux ratio (reflux to product, L/B) = 1.35. 

The excess benzene is distilled over a column and used as recycled benzene in the alkylation. In the bottom of the stripping section of the column the raw alkylates, consisting of LAB, heavy alkylate, and excess paraffin, are separated. This mixture is fed to a second column in which the excess paraffin is separated off. The actual purification of the LAB follows in a third column. The bottom product, heavy alkylate, consisting mainly of dialkylbenzene is also separated. Heavy alkylates are used in various applications. Both the paraffin and the LAB column are operated under vacuum. 

The mass balance relationships for the feed plate, the plates in the stripping section, of the column and for the reboiler must, however, be modified, owing to the continuous feed to the column and the continuous withdrawal of bottom product from the reboiler. The feed is defined by its mass flow rate, F, its composition xp and the thermal quality or q-factor, q. The column bottom product is defined by its mass flow rate, W, and composition, xw and is controlled to maintain constant liquid level in the reboiler. 

In the section below the feed, the more volatile components are stripped from the liquid and this is known as the stripping section. Above the feed, the concentration of the more volatile components is increased and this is called the enrichment, or more commonly, the rectifying section. Figure 11.1a shows a column producing two product streams, referred to as tops and bottoms, from a single feed. Columns are occasionally used with more than one feed, and with side streams withdrawn at points up the column, Figure ll.lt . This does not alter the basic operation, but complicates the analysis of the process, to some extent. 

The corresponding mass balance can also be carried out around the stripping section of a column, as illustrated in Figure 12.16. A mass balance around the bottom of the column for Component i gives ... 

Equation 12.22 relates the compositions of vapor and liquid streams passing each other in the stripping section of a column. Equation 12.22 can be used together with vapor-liquid equilibrium calculations to calculate a composition profile in the stripping section of the column, similar to that of the rectifying section of the column as described above. The calculation is started with an assumed bottoms composition and Equation 12.22 applied repeatedly with vapor-liquid equilibrium calculations working up the column. 

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