Side draw columns design

Problem A structured packing vacuum tower had too much heavy key in a vapor side-draw between the feed and the column bottom. The side-draw heavy key concentration was several times the design value. 

A column is designed for/feeds, a partial condenser with only a vapor product, a reboiler, a bottoms product, i side draws, and h pumparound heaters/coolers. Assuming the feeds are of fixed rates, compositions, and thermal conditions, determine the number of independent variables required to uniquely define the column during design, and its operation after construction. 

A distillation column is designed for the separation of a mixture of benzene, toluene, and biphenyl into a distillate (mostly benzene), a side draw (mostly toluene), and a bottoms product (mostly biphenyl). The column will operate at a pressure of about 175 kPa and a temperature ranging from about 90°C in the condenser to about 240°C in the reboiler. The three products define two column sections, and in each section a A -value for biphenyl, designated as the reference component, and relative volatilities for the other components are estimated based on the average temperature and pressure in the section. The feed stream component flow rates, relative volatilities, reference K-values, and product rates are given below ... 

Description GTC s GT-BenZap process features a reliable traditional design paired with a proven active hydrogenation catalyst. The process consists of hydrotreating a narrow-cut Cg fraction, which is separated from the full-range reformate to saturate the benzene component into cyclohexane. The reformate is first fed to a reformate splitter, where the Cg heart cut is separated as a side-draw fraction while the C/ cut and the Cg- light fraction are removed as bottom and top products of the column. 

The expander discharge stream enters the bottom of the upper (rectification) section of the deethanizer column. The upper section contains packed bed and lower section contains trays. The SCORE process include two side draw loop on the deethanizer that result in to significant improved NGL recovery compared to conventional processes. Heat supplied to deethanizer column through reboiler and reboiler is design to operate using LP and MP steams depending on the mode of SCORE process either TT or Turbo expander. 

The side draw can be very pure when a DWG (Dividing Wall Column) is used (Figure 5.1). In that case, there is another design variable, which is the ratio of liquid to the feed side of the wall divided by the liquid to the product side of the wall. 

There are other process flowsheets for the HDA process. Some of the other alternatives are recycling of diphenyl to eliminate losses of benzene to the waste diphenyl by way of the reverse reaction in reaction 2 combine the stabilizer and product columns into a single column with the benzene product as a side draw stream, rind the use of a membrane system to reduce hydrogen losses in the purge. These alternative designs are not examined in this work, rather, this work considers the control of the benchcase design discussed in the literature by [1,7,19]. 

A more positive approach is to measure internal reflux, not only to the column base, but also to each feed tray, and fi-om each tray where there is a side draw. A possible way to accomplish this is to design the downcomer bottom reservoir so that its outlet weir is a Sutro weir. The outflow from such... 

To illustrate better how the control strategy design method described is put into practice, consider the case of a liquid side draw benzene column. 

In the second part of this problem, we will examine the possibility to separate the ternary mixture benzene/toluene/diphenyl in a complex column. The relative volatility of the components is approximately 2.5/1/0.3. For the composition from Table 7.28 we obtain 5/<1.6, and A<0.72, B <0.26, C-0.02. Consequently, a side-stream draw of toluene below the feed position is recommended. This design ensures high purity of benzene, but only moderate purity of the toluene to be recycled, which in addition may tolerate small amounts of diphenyl. 

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