Novel reactor raises yields, lower costs

Menlo Park, Calif.-A proprietary, novel chemical-reactor technology has demonstrated improved ratios of desired product to waste product by a factor of 20 over comparable current technology, according to its developers. In addition to reducing or eliminating waste disposal and increasing product quality, the technology also increases yields and thus decreases raw material costs, they claim.

Patented by R&D consulting organization SRI International, the new reactor design appears to have broad application in the chemical process industries. The technology is applicable to both grassroots and retrofit applications. Chemical reactors are at the heart of chemical manufacturing and determine raw materials, products, by-products, energy consumption, and capital and operating costs.

"One of the major technical challenges facing chemical companies is the minimization of waste production during highly exothermic, or heatproducing, reactions," said William J. Asher, principal chemical engineer in SRI's Chemical Engineering Development Center. "To identify prospective solutions to this challenge, SRI assembled a multidisciplinary group of chemical engineers, chemists, process economic analysts, and business consultants. The resulting novel reactor invented by SRI has the potential to improve product yields and minimize waste generation in a broad range of applications."

The technology was demonstrated with collaborative support from five chemical companies-Air Products and Chemicals, Eastman Chemical Co., Monsanto Co., Olin Corp., and Rhone Poulenc-brought together with SRI by AIChE's Center for Waste Reduction Technologies (CWRT) along with matching funds from the U.S. Department of Energy.

The proprietary SRI reactor is simple, small, and low cost. It is essentially a heat exchanger with commercially available porous metal tubes. The permeability of the tubes is reduced in situ by filtration of a fine particle slurry prior to a given reaction, and the interiors of the tubes are filled with a packing to enhance mixing.

As shown in Figure 1, one reactant is passed through the inside of the tube, and the other (permeating) reactant enters the tube from the shell side through the porous wall. The product leaves from the tube side. Uniform mixing in the entire reactor volume enhances heat and mass transfer and eliminates high local concentration gradients and hot spots. Together with controlled feed of key reactants, this mixing also provides highly uniform radial temperature and concentration profiles, improved yields, and minimized waste generation via side reactions. This uniform mixing contrasts to that in stirred tanks, which have intense mixing at the turbine tips and minimal mixing in the bulk of the volume. The mixing elements can be spheres, any packing material, commercial static mixing elements, or catalyst particles.

The SRI process was examined using spheres as the mixing elements. In the liquid phase, where flows are laminar, the spheres changed the mass transport from molecular diffusion to convective diffusion, increasing it by a factor of 105. In the gas phase, transport was increased by a factor of 102. These order-of-magnitude transport improvements will result in less waste products and increased yields.

The patent covering the reactor process was issued in December 1996. The process is applicable to noncatalyzed and many catalyzed reactions and can be used in liquid and gas phases and in batch and continuous operations. Candidate reactions include alkylation, carbonylation, carbamylation, chlorination, direct oxidation, ethoxylation, hydroformylation, hydrogenation, nitration, and sulfonation.

For the CWRT collaborative project, the liquid sulfur trioxide sulfonation of methyl laurate was selected as the model system. Computer simulations were performed to determine the appropriate permeability for the porous tubes. These showed that the permeability of the tube walls needed to be at least an order of magnitude lower than the lowest permeability available for sintered porous metal. Filtration of a fine particle slurry produced the desired permeability.

Three types of single-tube reactors were built: a state-of-the-art reactor with nonpermeable walls, one with porous walls without permeability reduction, and one with porous walls with reduced permeability (the novel SRI reactor). A bench-scale reactor was built and operated to obtain data and evaluate performance. When measured against comparable technology, demonstration showed the SRI reactor improved the ratio of desired product to waste product by a factor of 20.

Commercial implementation will require many tubes in shell and tube configurations similar to a heat exchanger. Figure 2 shows a possible practical configuration. SRI studies indicate that the most economic method for removal of the heat of reaction is adiabatic reactor operation for incremental conversion, followed by heat removal in a conventional heat exchanger.

The CWRT project collaborators have not only the rights to use technology from this project but also a co-exclusive option to obtain individual exclusive licenses for application to specific products. It will be possible for additional collaborators, both members and nonmembers of CWRT, to join the project in its second phase early in 1997.

Phase two will provide a firm foundation for application to specific products through the development of a flexible computer model, testing of permeability, demonstration of recycle and staging operating capabilities, evaluation of mixing elements, and demonstration of prolonged operation, as well as other experiments to be proposed by the collaborators. -John A. Hill

Figure 2

Copyright Instrument Society of America Apr 1997
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