Reboiled Stripper

The computations could start by initially solving the column section with an assumed vapor feed, to calculate and The reboiler is then solved with feed stream Lf, and one specification (reboiler duty, or the bottoms rate or temperature, etc.). The column section is solved again with the updated stream, V v+j. The calculations are repeated until the product streams stabilize within acceptable tolerance. 

The vapor stream Vg from the reboiler (stage 8) to the bottom of the column section must be estimated to start the calculations. A rough initial estimate can be obtained by assuming the liquid stream from the column section to the reboiler, iy, to be the same as the liquid feed Lq. This stream is flashed at the reboiler conditions (using the reboiler K-values) to generate estimates for the vapor and liquid streams from the reboiler, Vg and fg. 

The column section is solved with the liquid feed Lq and the estimated vapor feed Vg, using Equations 12.32, 12.33, and 12.34. The flow rates ig and Vg can be used for V and L in Equation 12.32 to calculate the stripping factors. The calculated liquid product iy is flashed at the reboiler conditions to update Vg, and the calculations are repeated until convergence, that is, until stream Vg stabilizes. The results are as follows  

FIGURE 12.7 Distillation column with a partial condenser. 

With fixed feeds, heat duties, pressure profiles, and number of stages, all the modules except unit 6 have zero degrees of freedom. Unit 6 is a splitter with two products and, therefore, has one degree of freedom. Hence, the entire column as defined has one degree of freedom and requires one more specification to be completely defined. If the parameters such as the condenser and reboiler duties are allowed to vary, the number of specifications must increase to match the number of variables. 

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Bottoms and three side-cut strippers remove light ends from products and may utilize steam or reboilers. In Fig. 13-92 a reboiled stripper is utilized on the light distillate, which is the largest side cut withdrawn. Steam-stripping rates in side-cut strippers and at the bottom of the atmospheric column may vary from 0.45 to 4.5 kg (1 to 10 lb) of steam per barrel of stripped liquid, depending on the fraction of stripper feea hquid that is vaporizea. 

Many water strippers are initially designed with steam reboilers, rather than with open stripping steam. The amount of steam required is the same in either case. The great advantage of the reboiled stripper is that the steam condensate is recovered, and recycled back to the boilers. When open stripping steam is used, the steam condensate is added to the stripped water, thus increasing the plant s water effluent. Hence, the use of open stripping steam is environmentally unfriendly. 

A pressure of 14 bar gives a good compromise between the above aspects. The RD column is simulated as reboiled stripper with reactive stages. Although the highly exothermic reaction should make unnecessary the use of a heat source, we consider just a small reboiler to prevent residual propylene entrained in the bottom. For this reason, few reactive stages below the low feed of propylene are useful. 

The feasibility of the above setup can be evaluated by simulation with Aspen Plus [19]. The RD column is built-up as a reboiled stripper followed by a condenser and a three-phase flash, with organic phase refluxed to column. The result is that only 3 to 5 reactive equilibrium stages are necessary to achieve over 99% conversion. The stripping zone may be limited at 2-3 stages, while the rectification zone has 1-2 stages. 

Example 6 ReboUed Stripper Compute stage temperatures and interstage vapor and liquid flow rates and compositions and reboiler heat duty for the reboiled stripper shown in Fig. 13-48. Thermodynamic properties may be estimated by using the Grayson-Streed modification of the Chao-Seader method. 

FIG. 13 48 Specifications and calculated product stream flows and reboiler heat duty for a reboiled stripper. Flows are in pound-moles per hour. 

FIG. 13-49 (a) Temperature, (b) flow, and (c) vapor mole fraction profiles in reboiled stripper in Fig. 13-48. 

FIG. 13-53 Specifications for the calculation of a reboiled stripper by the Naphtali-Sandholm method. 

The discussion in this chapter centers around multi-component distillation with conventional columns, that is, columns with a single feed and overhead and bottoms products. In general, such columns have a condenser and a reboiler, although the special cases of rectifiers and reboiled strippers are also considered. 

A reboiled stripper is the opposite of a rectifier in that it has no rectifying section. The feed is sent either directly to the condenser or, if sufficiently cold, to the top tray, requiring no condenser. The liquid portion of the feed acts as reflux. The main purpose of this type of column is to produce a bottoms product that is enriched in heavier components and stripped of the lighter ones. With no rectifying trays, a reboiled stripper could result in some heavies being lost in the overhead. 

Note that in reboiled strippers without a condenser or in rectifiers without a reboiler, there is only one degree of freedom or one independent variable. In a rectifier, for instance, varying the condenser duty directly affects both the reflux ratio and the overhead rate. These two variables can no longer be varied independently and, therefore, only one performance specification may be made. 

A reboiled stripper column is to be designed to separate a feed stream to produce an overhead and a bottoms product. The feed is of fixed rate, composition, and thermal conditions and will be sent to the top of the column. The column will have a reboiler, but no condenser. Determine the number of degrees of freedom. 

FIGURE 9.3 Steam stripper and reboiled stripper (Example 9.3). 

A liquid stream of hydrocarbons with a flow rate of 500 kmol/h is sent to the top of a reboiled stripper column. The column has the equivalent of seven equilibrium stages and a reboiler in a configuration similar to Figure 12.6. The column and reboiler pressure may be assumed uniform at 1000 kPa. The feed temperature to the column is controlled such that the average column temperature is 95°C. The heat duty to the reboiler is controlled such as to maintain the reboiler temperature at 120°C. The feed stream is given below, along with average component K-values in the column and reboiler, evaluated at the pressure and temperatures provided. Determine by the modular column section method the rates and compositions of the stripper column products. 

A reboiled stripper is to be designed for the task shown below. Determine ... 

Liquid air is fed to the top of a perforated tray reboiled stripper operated at... 

A feed at 2I.1°C, 101 kPa (70°F, 1 atm) containing 50.0 mass% ethanol in water is to be stripped in a reboiled stripper to produce a bottoms product containing 1.0 mass% ethanol. Overhead vapors are withdrawn as a top product. 

Figure 12.18. Various coupling schemes for absorbent recovery, (u) Use of steam or inert gas stripper. (i ) Use of reboiled stripper, (c) Use of distillation.

Edmister applied the group method to complex separators where cascades are coupled to condensers, reboilers, and/or other cascades. Some of the possible combinations, as shown in Fig. 12.24, are fractionators (distillation columns), reboiled strippers, reboiled absorbers, and refluxed inert gas strippers. In Fig. 12.24, five separation zones are delineated (1) partial condensation, (2) absorption cascade, (3) feed stage flash, (4) stripping cascade, and (5) partial reboiling. 

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