Vapor Sidestream Column with Rectifier

If the sidestream is a vapor, a rectifier can be added that removes some of the heavy impurity in the vapor sidestream coming from the main tower. To Ulustrate this configuration, we take the same DME/MeOH/water system studied in Section 10.3 but remove a vapor sidestream below the feed stage. 

The rectifier must operate at a lower pressure than the main colmnn so that the vapor sidestream can flow down hill. The main column operates at 11 atm. The pressure in the 

The feed composition is 35mol% DME, 35mol% MeOH, and 30mol% H2O. The distillate from the main column D1 is the DME product and has an impurity specification of 1 mol% MeOH. The bottoms from the main column B is the water product and has an impurity specification of 2 mol% MeOH. The distillate from the rectifier D2 is the MeOH product and has an impurity specification of 1 mol% water. The DME concentration in this stream is negligible because there is very little DME in the main column below the feed tray. The feed is fed on Stage 11 near the top. The vapor sidestream is withdrawn down at Stage 31. 

The steady-state design of this two-column system with recycle was achieved by a sequential approach. First, the flow rates of the distillates from the two columns were set equal to the molar flow rates of DME and MeOH in the feed. The RR in the main column was set at 2. The flow rate of the vapor sidestream 51 was set at three times the MeOH product rate, which gives an RR in the rectifier of 2. Note that there is only one degree of freedom in the rectifier, so setting the distillate flow rate completely specifies the column. 

The next step was to adjust the various degrees of freedom to attain the desired purities of the three products. Three Design SpecA ary functions were used in a sequential manner  

---

Sidestream column with rectifier. Figure 6.236 shows a process where a vapor sidestream is fed into a rectifying column to remove some of the C impurity in the vapor stream. A 4 X 4 multivariable control strategy... 

A simple model for side-rectifiers suitable for shortcut calculation is shown in Figure 11.12. The side-rectifier can be modeled as two columns in the thermally coupled direct sequence. The first column is a conventional column with a condenser and partial reboiler. The second column is modeled as a sidestream column, with a vapor sidestream one stage below the feed stage4. The liquid entering the reboiler and vapor leaving can be calculated from vapor-liquid equilibrium (see Chapter 4). The vapor and liquid streams at the bottom of the first column can then be matched with the feed and sidestream of the second column to allow the calculations for the second column to be carried out. 

Sidestream columns come in several flavors. Both liquid and vapor sidestreams are used. Sometimes the sidestream is a final product. Because the purity attainable in a sidestream is limited, the sidestream from the main tower is sometimes fed to a second column (usually a stripper or a rectifier) for further purification with a recycle stream back to the main column. Several examples are studied in this chapter. 

---