Catalytic distillation (CD) is a novel reactor technology that combines a heterogeneous catalytic reaction and separation in a single reactor. The heterogeneous catalyst acts as a distillation packing as well as a catalyst for the reaction. The use of a heterogeneous catalyst distinguishes CD from reactive distillation where a homogeneous catalyst is used. Although the concept of carrying out the reaction and separation in a single reactor is not new, the problem of high pressure drop when catalyst pellets are placed in a distillation tower has delayed the actual commercial utilization of this technology. A breakthrough came in 1980 when Smith  in Texas patented a method of placing catalyst particles in fibreglass bags which subsequently are rolled in bundles with demister wire in between to provide the void space for vapour flow. This form of catalytic distillation packing is also known as "Texas teabags". Smith has a number of patents on the application of CD including the production of methyl-tert-butyl-ether (MTBE) . Chemical Research and Licensing (CR&L) was formed to license and market the CD technology. The first commercial application of CD was the production of MTBE by Charter Oil at their Houston, TX, refinery in 1981. The success of the CD technology for the production of MTBE has led to great interest in using CD as a more general reaction technique.
Advantages and Applications of the CD Technology
There are a number of advantages of the CD technology due to the combinations of reactions and distillation in a single column. Indeed, CD is deemed to play a major role for the chemical and petroleum industry in the 21st century . Some of the major benefits for CD are as follows:
* Reduction in capital cost
* Increased conversion for equilibrium limited reactions due to the continual removal of products via distillation
* Improved product selectivity
* Improved catalyst lifetime due to the reduction of hot spots and removal of fouling substances from the catalyst
* Reduction in energy cost due to the utilization of reaction heat for vaporization and distillation
However, not all catalytic reactions are suitable for carrying out in the CD mode. Some of the key requirements to be satisfied are as follows:
* Distillation must be a practical method of separating the reactants and products
* The reaction must proceed at a reasonable rate at the temperature equivalent to the boiling point of the liquid mixture in the column
* The reaction cannot be overly endothermic
MTBE Production and other Applications for CD
The production of MTBE and other oxygenates such as ethyl tert-butyl ether (ETBE) and tert-amyl methyl ether (TAME) has been the main stay for CD due to the reformulated gasoline program (RFG) introduced by the US Government to amend the Clean Air Act in 1990 in an effort to reduce emissions. A key component of RFG is oxygen in the form of oxygenates. This fueled the interest in the CD technology for the production of MTBE and other ethers as oxygenates. Currently there are more than 60 MTBE units and 10 TAME units world wide. Recently, MTBE has been found in ground waters due to the leakage of gasoline from storage tanks. MTBE is polar, and, hence, it is easily transported by ground water. This sparked concern on the use of MTBE in gasoline. In fact, California has proposed the phase out of MTBE in gasoline by 2002. The fate of MTBE in RFG in the other US states or the rest of the world is uncertain at the moment. However, this does not mean the demise of CD. On the contrary, CD has been reported to be applicable in many other processes such as hydrogenation, alkylation, oligomerization of olefins, aldol condensation, esterification, hydration of olefins, dehydration of alcohols, amination and desulfurization.
A comprehensive review of the various uses for CD is available in CHEMTECH  which summarizes the literature prior to the end of 1996. A recent literature search revealed that a large number of patents were granted in the last few years on the use of CD for a variety, of chemical processes, although fundamental studies on CD are still rather limited. These new CD processes are at various stages of development. Of particular interest is the recent announcement of the CDHDS process by CDTech (a partnership between CR & L and ABB Lummus Global Inc.) in the 1998 NPRA annual meeting in San Francisco, CA for the improved sulfur removal from fluid catalytic cracking (FCC) gasoline via CD . At the 1999 Hart World Fuel Conference in San Antonio, TX, the CDHDS process won the Brian Davis Refining Technology Award for the best new technology. The CDHDS process is scheduled for commercialization by early 2000 .
CDHDS and CDHYRDO
The current gasoline pool contains about 330 ppm of sulfur. Sulfur is the largest contributor to N[O.sub.x], thus sulfur reduction is the key issue for the gasoline producers. Indeed the US Environmental Protection Agency pushes to get the sulfur level down to 30 ppm. Besides the US, Canada and Europe have also shown interest in limiting the sulfur content of gasoline. Nearly all the sulfur comes from FCC gasoline, including the gasoline pool in China. Therefore, the announcement of the CDHDS process has attracted world-wide attention. The distribution of sulfur in FCC gasoline is a function of the boiling point, i.e. the sulfur is more concentrated in the higher boiling fraction.
In the conventional approach of hydrodesulfurization (HDS) to remove sulfur as [H.sub.2]S in a fixed bed reactor at moderate pressure, significant amounts of olefins are also hydrogenated which result in the reduction of the octane number of gasoline. Conducting HDS in a CD tower offers the unique advantage to attain high sulfur conversion with minimal octane loss. This is due to the fact that the light olefins are concentrated at the top of the distillation column where the temperature is lower than the bottom of the distillation tower. This reduces the saturation of the light olefins and at the same time, the light sulfur components could still be desulfurized at the top of the column. The higher molecular weight sulfur containing compounds undergo HDS at the bottom of the distillation column where the higher temperature facilitates the HDS process.
Another interesting application of CD is the CD Hydro process which combines fractionation together with hydrogenation. This is primarily used to pretreat feed for the production of MTBE, TAME and the C4 and C5 alkylation plants. Besides the hydrogenation of dienes, ethyl and propyl mercaptans are also removed via thioetherification where the diolefins react with the mercaptans to form olefinic sulfides which are higher boiling and concentrate at the bottom of the tower. These olefinic sulfides are thermally stable and do not decompose in the reboiler. The CDHYDRO process has been reported to remove mercaptans to less than 1 ppm. It was also reported that the CDHydro process operates at substantially lower pressure than conventional hydrotreating processes due to the enhanced mass transfer between the vapour and liquid phases in the distillation process which enables hydrogenation at high rates even at low hydrogen pressures. Therefore, the CD process has an added safety feature compared with the conventional hydrotreating process due to the lower operating pressure, utilization of the reaction heat for distillation and the reduction of the number of reactors.
CD Research in Canada
Although CD has many commercial applications as seen in the patent literature, there are very few literature reports on the fundamental aspects or new applications of catalytic distillation. Our group has been involved in CD for the past ten years. There are a number of other groups working on various aspects of CD in Canada, for example, K. Chuang, FCIC at the University of Alberta; T. Harris, FCIC and B.W. Jackson at Queen's University; and P. Douglas at the University of Waterloo. It is of interest to note that Canada was a key player in the licensing of CD technology since CR&L was first bought by Polysar in Sarnia in the late 1980s which later was sold to NOVA. In 1998, CRI, a subsidiary of Shell, acquired CR&L.
Our group has been involved in both the experimental and theoretical aspects of CD research. We have a CD pilot plant in our laboratory. Our focus has been on the development of new processes that could benefit from CD and also carry out fundamental research on the elucidation of the key parameters that govern enhanced product yield and selectivity.
The two processes that we have developed are:
1. the aldol condensation of acetone to diacetone alcohol which undergoes subsequent dehydration to mesityl oxide, and
2. the oligomerization of butenes to octenes and higher oligomers of butenes.
We are particularly interested in elucidating the requirements for enhanced product selectivity in the CD process. According to some literature reports, due to the fast removal of intermediates from the catalyst zone via distillation, the yield of the intermediate product could be enhanced.
However, we found that when the catalyst was packed in fibre glass bags, the selectivity to the intermediate products was less than when the reaction was carried out in a conventional reactor. based on a detailed kinetic study on the aldol condensation of acetone  and also the mass transfer parameters determined for the packing geometry in our distillation tower , we developed two rate based models to predict yield and selectivity for CD under steady state conditions [8, 9]. Excellent agreement between model predictions and experimental data were obtained. We also concluded that the mass transfer characteristics of the catalytic reaction zone have a major effect on the product selectivity. In order to improve mass transfer, we have developed new catalyst packings whereby enhanced C8 selectivity and improved catalyst lifetime was achieved. This result is quite significant since it is generally recognized that olefins are precursors to coke formation in many processes.
CD offers new opportunities for chemical and environmental engineering in a variety of research areas such as catalysis, separation, reaction engineering, process design, and optimization. Product selectivity is the key for "green" process development since it will maximize the desired product yield and minimize the waste stream; CD offers potential product selectivity.
A distinct feature of a CD reactor is the temperature gradient in the distillation tower which is quite different from operations in a conventional reactor. Therefore, one could achieve a number of catalytic processes within the CD tower by placing different catalysts in different sections of the tower to achieve a specific goal. One could also take advantage of the temperature gradient to treat a complex feedstream as seen in the descriptions of the CDHDS process. Another feature of the CD reactor is that it functions like an internal recycle reactor. Hence, at a low conversion per pass, high selectivity could be achieved if the mass transfer between the catalyst and the bulk liquid could be facilitated. The major challenge for CD lies in the development and improvement of catalytic distillation packings with catalytic activity in the temperature range of the boiling points of the reactants and/or the products, long lifetime, and high separation efficiency. Due to the high flow rates normally encountered in a distillation column and the requirements for low pressure drop, opportunities exist for catalyst researchers to make robust catalysts with large void space that can withstand attrition while maintaining high stability and appropriate activity for a variety of process applications.
1. Smith, L.A. Jr., US Patent 4 232 177 (1980), US Patent 4 307 254 (1981), Can. Patent 1 125 728 (1982), US Patent 4 336 407 (1980).
2. Rock, K., G.R. Gildert, and T. McGuirk, 'Catalytic Distillation extends its Reach', Chemical Engineering, 104(7):78, 1997.
3. Podrebarac, G., F.T.T. Ng, and G.L. Rempel, 'More Uses for Catalytic Distillation', CHEMTECH, 27(5): 37, 1997.
4. Rock, K.L., R. Foley, and Putnam, 'Improvements in FCC Gasoline Desulphurization via Catalytic Distillation', 1998 NPRA Annual meeting, March 15-17, 1998, San Francisco, CA, AM-98-37.
5. World Refining, p. 30, May/June 1999.
6. Podrebarac, G., F.T.T. Ng, G.L. and Rempel, 'A kinetic study of the aldol condensation of acetone using an anion exchange resin catalyst', Chem. Eng. Sci., 52(17):2991, 1997.
7. Huang, C., G. Podrebarac, F.T.T. Ng, and G.L. Rempel, 'A Study of Mass Transfer Behaviour in a Catalytic Distillation Column', Can. J. Chem. Eng, 76(2):323, 1998.
8. Podrebarac, G., F.T.T. Ng, and G.L. Rempel, 'The Production of Diacetone Alcohol with Catalytic Distillation, Part I. Catalytic Distillation Experiments, Part II. A Rate-based Catalytic Distillation Model for the Reaction Zone', Chem Eng. Sci, 53(5):1067, 1077, 1998.
9. Huang, C., L. Yang, F.T.T. Ng, and G.L. Rempel, 'Application of catalytic distillation for the aldol condensation of acetone: a rate-based model in simulating the catalytic distillation performance under steady-state operations', Chem Eng. Sci, 53(19):3498, 1998.
Flora T.T. Ng, MCIC and Garry L. Rempel, FCIC, FRSC are professors at the department of chemical engineering, University of Waterloo, Waterloo, ON. In addition to CD research, Ng's research includes the generation of in situ hydrogen for desulfurization and upgrading heavy oil/bitumen emulsions, solid acid catalysis, and recycling of plastics. Rempel's other major research interest lies in the area of chemical modification of polymers and separations via novel polymer sorbents.
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|Title Annotation:||advantages of combining catalytic reaction and separation in the same reactor|
|Comment:||Catalytic distillation.(advantages of combining catalytic reaction and separation in the same reactor)|
|Author:||Ng, Flora T.T.; Rempel, Garry L.|
|Publication:||Canadian Chemical News|
|Date:||Jul 1, 1999|
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