With that caveat, here’re some broad guidelines:
• Consider a DWC when the middle or B component is at least 20% of the total stream flow. Although it’s possible to utilize a DWC at values below this, there may be better methods of arranging the separation.
• Don’t use a DWC if the pressure difference between the conventional sequence is high. A DWC operates as a single tower, so it runs at a single pressure. If the condenser temperature is selected to fit the available utility, then the reboiler temperature is, by definition, set by the pressure drop and the composition of the bottom stream. This may mean that a higher level of heating medium or a lower level of condensing medium is required to meet restrictions on the top or bottom temperature.
• Carefully evaluate energy use — this is an important point. It’s been shown that the fully integrated arrangement or the DWC consumes the lowest energy, but that doesn’t account for the level of energy being used.
Figure 5. Dividing wall column -- This
Because a DWC operates as a single unit and at a single pressure, the reboiler must run at a temperature consistent with relatively pure C. In an analogous way, the condenser must run at a temperature consistent with the lightest component. Looking at Figure 2, it’s obvious that this arrangement could be configured to use a lower level of heating medium on the first pre-fractionation tower. Thus, it may be that a DWC is better from a First Law perspective but not from a Second Law perspective. DWC applications are selected because of the potential to reduce capital and energy.
For the development of new DWC applications, Dow has designed and built the Midland pilot plant (Figure 6) so that it has the same configuration as a commercial unit and closely mimics industrial operations. This setup is used to demonstrate the process concept and purification capability for a particular separation. It also generates experimental data to test the validity of both steady-state and dynamic numerical simulations of DWC applications of particular interest to Dow. In addition, the pilot plant is utilized to assess proposed control schemes and to help shorten or eliminate the learning curve in the start-up and operation of commercial DWC units. This is particularly important when numerical simulations indicate potential operational challenges. For example, when feed concentrations can vary over time or when feed conditions are such that flashing can occur inside the column, experimental information is helpful in the design of control strategies.
Figure 6. Pilot plant --
The DWC pilot plant is fully automated; more than 20 control loops are manipulated and monitored by a data acquisition and control system. As partners consider a DWC installation, a key to building confidence in the proposed installation is viewing the experimental unit on robust closed-loop control. The DWC features more thermocouples than found in full-scale applications. These thermocouples are located along the entire length of the DWC to allow for easy selection of the most responsive temperature (MRT) control locations for different applications.