Reactive Dividing-Wall Columns – Towards Process Intensification
Special Symposium - EPIC-1: European Process Intensification Conference - 1
EPIC-1: Multifunctionality (MF-1)
Keywords: Reactive DWC, equilibrium limitations, energy savings
Authors: Anton A. Kiss, Hans Pragt, Cornald van Strien
Affiliation: Akzo Nobel Chemicals Research, Arnhem, The Netherlands
Due to its many advantages, distillation is the major separation process used in the chemical processing industry. However, one key drawback is its significant energy requirements. Dividing-wall distillation column (DWC) offers an alternative to conventional distillation towers or multicolumn arrangements, with potential significant cost savings. [1]
The direct sequence of two distillation columns evolved via the Petlyuk column to the concept of dividing-wall column. DWC is a very attractive design alternative as it saves the cost of building two columns and cuts operating costs by using a single condenser and reboiler. Compared to the conventional design arrangement, the novel design using DWC can save up to 30% in capital and energy savings. [2] However, using DWC requires a match between the operating conditions (T, P) of the two stand-alone columns. In addition, the main weakness of DWC is its inflexibility if there is a change in the nature of the feed.
Integrating reaction and separation into one unit, leads to reactive distillation. What about integrating a reactor and two distillation columns into one unit? The answer is the reactive dividing-wall column (RDWC). However, this is not a trivial task, especially considering the multi-component mixtures used in the chemical industry.
This work presents an industrial case study within Akzo Nobel Chemicals. One of our processes involves a relatively complex fast equilibrium of 10 species (A-J, listed in descending order of relative volatility). After the reactor, the reaction mixture is separated in a series of distillation columns. Most of the streams are recycled back to the reactor while component H is purified (98.5%) and sold as main product. Due to the market demand changes and the price increase of product C, the focus shifts from main product H to component C. However, the current plant design is not suitable for producing C. So what is the least expensive design alternative available?
1. A + J ↔ C + H
2. B + H ↔ C + E
3. D + H ↔ C + I
4. B + E ↔ A + F
5. F + J ↔ 2G
To solve this problem we propose a conventional design that uses a reactive distillation column, followed by a simple distillation column. In addition we consider a more integrated design that combines reaction and separation into one RDWC. The reactions take place only on the feed side of the column, where the light components are separated from the heavy ones. The formation of heavy components F and G (useless by-products) is avoided by adding an additional feed of light component A, in the bottom of the column.
Compared to the conventional case using two columns, the RDWC design allows 35% savings in capital and 15% savings in energy costs, respectively. In the lexture we will show the benefits of this integrated design and discuss the practical aspects of implementing this in industry.
References
1. H. Becker, S. Godorr, H. Kreis, Partitioned distillation columns - why, when & how, J. Chem. Eng., 2001, January, 68-74.
2. M. A. Schultz, D. G. Stewart, J. M. Harris, S. P. Rosenblum, M. S. Shakur, D. E. O’Brien, Reduce Costs with Dividing-Wall Columns, Chem. Eng. Prog., 2002, May, 64-71.
Presented Thursday 20, 09:30 to 09:50, in session EPIC-1: Multifunctionality (MF-1).
