1 Operation of Energy-Efficient Divided Wall (Petlyuk) Column Speaker : Ambari Khanam Course : Specialization Project-tkp4550 Department of Chemical Engineering Date : December 17 th, 2012 Project-Supervisor : Sigurd Skogestad

2  Introduction to Divided Wall Column (DWC) −Background and description  The project work is related to most likely situations in industrial practice. - The column is designed for optimal operation - Optimal Energy/close to Vmin for the given product specifications - Actual avialable energy less (therefore non-optimal R V ) - Effect on product purities in all three streams - Minimize the impurities (the objective function)  Conclusion and further work

3 DWC (Dividing-Wall column) was introduced by Wright in However, lack of reliable design method and concerns about the operation and control of DWC have prevented the widespread application. People started to pay much attention to DWC after the Energy Crisis (1980). In 1985, BASF built the first commercial DWC. There are now more than 100 columns installed worldwide. Energy consumption in distillation and greenhouse gas emissions are strongly related Three products Petlyuk Arrangement can save upto 20-30% energy and 30% capital cost.

4 (a) Implementation with three separate columns (b) DWC implementation with a side-product Figure. Thermodynamically equivalent implementations of three-product Petlyuk column

5 Side-draw section prefractionator

6 Mode 1 Fixed products specifications Minimize Energy Mode 2 Given Energy Minimize impurities in product Fixed specifications

7

8 List of Tasks Task 1 Spec.M.V. Task 2 Spec.M.V. M.V Spec. Task 3 Spec.M.V. Spec. Task 4 Spec.M.V. Spec. Task 5 Spec.M.V. Spec.M.V. Task 6 Spec.M.V. Spec.M.V. Spec.

9 The Composition Profiles for Given energy with Variable R V,R L,L and S

10,% of V 0 L E-01 RL E-01 RV E-01 S E-01 X B D E-03 X A S E-03 X C S E-03 X B B E-03 -J E % % % % % % % % % % % of V 0 L E-01 RL E-01 RV E-01 S E-01 X B D E-03 X A S E-03 X C S E-03 X B B E-03 -J E % % % % % % % % % % Variable R V Fixed R V

11 % of V L E-01 RL E-01 RV E-01 S E-01 X B D E-03 X A S E-03 X C S E-03 X B B E-03 -J E % % % % % % % % % % Variable R V and fixed side stream purity % of V L E-01 RL E-01 RV E-01 S E-01 X B D E-03 X A S E-03 X C S E-03 X B B E-03 J E % % % % % Variable R V and fixed Top and side stream purities

12 The composition profiles for given energy with fixed R V and variable R L,L and S

13 Compositions Profiles at 80% Energy for Variable R V and Fixed R V Respectively

14 Variable R V Fixed R V

15 Variable R V fixed side stream purity Variable R V fixed side stream and top purities

16 Product specifications can not be maintained at energy lower than optimal because of non-optimal R V. The optimal in terms of minimizing the impurities sum of all three products tends to increase the impurity in the side stream. The results obtained are useful when product specifications are not given and one gets paid for the purity in products. The other case could be that the side stream has no value and one get paid for the purity in both top product and bottom product. When side stream purity is fixed then bottom product tends to be least pure.

17  The results can be further verified by doing the simulations for following cases: –Change in feed composition –Change in relative volatility of components.  A simple and robust control structure can be designed for operation at lower energy when the product is valuable based upon its purity.

18

19 Products purities versus energy for fixed D/F Cost function for Binary Product Column

20 Bottom product purity versus energy for fixed top product purity Top product purity versus energy for fixed bottom product purity

21 By operating the column at lower energy than minimum energy we can no longer stick to product specifications When D/F is fixed then both products tend to be equally impure with decreasing energy. when one of the products is valuable and the other one has no value then the column can be operated at lower value of energy. and will still get one desired product at specifications.