DWCU

The second type of DWC is DWCU. The subscript means the wall is in the upper section of column. The DWCU has one reboiler and two condensers. This construction is evolved from direct sequence (DS) (see Fig. 1.2-2)

The function of first column is the separation between A and B. Product A goes

Fig. 1.2-2 The evolution of DWCU.

If we do the thermal coupling the reboiler in the first column is removed. The vapor is from a certain tray in the second column, and the reflux is sent to a certain tray in the second column. This structure is called the side rectifier sequence. The Side Rectifier sequence can be divided into three parts. The vapor from the lower section flows into the left section and right section. There is a condenser in each left and right section. Liquid reflux from the left and right section meet at the lower section. The column with wall in the upper section has the same effect with the side rectifier sequence. And DWCU is thermally equivalent to the side rectifier sequence.

1.2.3 DWCM

The third type of DWC is DWCM. The subscript means the wall is in the middle section of column. The DWCM needs only one reboiler and one condenser. This construction is evolved from prefractionator sequence (PF) (see Fig. 1.2-3).

Fig. 1.2-3 The evolution of DWCM.

Compared with IS and DS, the difference for PF is the function of first column.

The first column separates A and C. Most of species A goes into the second column near the top of the first column. Most of species C goes into the second column near the bottom of the second column. Part of the B goes into the second column near the

in the lower section of second column. Finally, A goes out from the top of second column, B goes out from the sidedraw and C goes out from the bottom of second column.

If we do the thermal coupling, the reboiler and condenser in the first column are removed. The vapor is from a certain tray in the second column. And the reflux is sent to a certain tray in the second column. The reflux is from a certain tray in the second column. And the vapor is sent to a certain tray in the second column. This structure is called Petlyuk Column. It is also called fully thermally coupled column.

A Petlyuk column can be divided into four parts. The vapor from the lower section flows into the left section and right section and meets at the upper section.

Liquid reflux from the upper section flows into the left section and right section and meets at the lower section. The column with wall in the middle section has the same effect with Petlyuk column. And DWCM is thermally equivalent to Petlyuk column.

1.3 Literature survey

A literature study reveals that a variety of controllers are used for distillation columns. For the study of control of DWC systems, Wolff and Skogestad [2] proposed a control strategy for the ethanol/propanol/butanol system. They demonstrated that at some operating conditions the ‘holes’ phenomenon in the steady state feasibility space made the energy control structure infeasible. They used the M type of DWC systems.

Abdul Mutalib et al. [3] proposed a control strategy for methanol/2-propanol/butanol system. Their performance is just for feed flow rate disturbance and they didn’t use the other manipulated variables to minimize energy consumption. In the same year they proposed a temperature control strategy for the same system [4]. Their simulation results showed that the control structure which controls two temperatures and fixes the side stream flow does not provide effective control. They used the M type of DWC systems.

Several authors studied the design phase of the dividing-wall column in order to improve the energy efficiency [5] [6]. The design stage of a DWC is very important as

going out the top of the prefractionator. The second one is keeping the lightest component from going out the bottom of the prefractionator. The optimal solution surface of the minimal boilup is given as a function of the control variable liquid split and the design variable vapor split. They used response surfaces to describe the relationship between liquid split and energy consumption. They suggested that the control of temperature differentials is a good policy to infer composition. The system of DWC that they used was M type.

A more practical approach is suggested by Serra et al. [8]. A linearized model is used to design PI feedback controller. They proposed both PID control and DMC control strategies for three different systems. The authors used several linear analysis tools – Morari resiliency index (MRI), condition number (CN), relative gain array (RGA) to select variable pairings for three compositions control. They demonstrated that PID control gave better load disturbance than DMC control. In 2003 they concluded their previous work and gave two observations [9]. The first one is that DWC has better controllability for mixtures with ESI close to 1. The second one is that the DWC controllability at the optimal operating point is worse than the non-optimal one. The system of DWC that they used was M type.

A more advanced approach for DWC control is model predictive control (MPC) as discussed by Adrian et al. [10]. They proposed PID and MPC control strategies for butanol/petanol/hexanol system. In the PID control, the reboiler heat input was not used to control compositions, but it was in MPC control. The MPC controller outperforms a single PI loop. Three temperatures are controlled by the reflux ratios, the liquid split, and the sidedraw flow rate, respectively. The disturbance variable in this case was the feed flow rate. They used the M type of DWC systems.

Wang and Wong [11] proposed a control policy for the ethanol/1-propanol/

1-butanol system. There were large product purity deviations for feed composition disturbances. The authors suggested using a temperature-composition cascade control structure to solve the problem. The performances in dynamic simulation were good.

The system of DWC that they used was M type. Cho et al. [12] proposed a control strategy for the benzene/toluene/ p-xylene system. They proposed a profile position control scheme for the control of a DWC with vapor side draw. Relative gain array (RGA) and singular value decomposition (SVD) analysis were used to determine the optimal control configuration. Dynamic simulation showed that the profile

manipulated variables to minimize energy consumption. The system of DWC that they used was M type. Ling and Luyben [13] proposed a control strategy for the benzene/toluene/o-xylene system. They concluded that the composition of the heavy component at the top of the prefractionator is an implicit and practical way to minimize energy consumption in the presence of feed disturbances. This specific composition was controlled by the liquid split variable. They also used the M type of DWC systems. Nowadays more and more research groups focus on the controllability of divided-wall column [14] [15].

Huang et al. [16] published a report on the development of DWC systems in industry. There are also many applications of DWC in industry that have been reported. In 1980 BASF built the first commercial DWC. Presently they have twenty-eight DWC columns in operation. BASF is in the minority of chemical companies which that have used DWC systems for over ten years. They focus on the application of DWC in petrochemical field.