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Compact Reactors Boost Productivity

By Chemical Engineering |

Velocys Inc. (Plain City, Ohio; www.velocys.com), a part of the Oxford Catalyst Group (Abingdon, U.K.; www.oxfordcatalysts.com), has been awarded the Kirkpatrick Award for Chemical Engineering Achievement from Chemical Engineering. The company’s winning technology features microchannel reactors for producing synthetic fuels. The award was announced at a reception September 12 in Houston. The reception kicked-off the ChemInnovations 2011 Conference and Expo, held at the George R. Brown Convention Center. Velocys commercial director Jeff McDaniel accepted the award on the company’s behalf. “It’s an honor to receive the award,” said McDaniel, explaining that a great deal of work by many people had gone into the technology, which allows faster processing in a range of reactions.

The Velocys technology was selected from a group of four award finalists, who were also honored at the reception. With its win, Velocys joins a distinguished group of companies to have received the biennial award, which was first handed out by the magazine in 1933. Past winners include BOC Group’s low-temperature NOx absorption out of fluegases (2001), Amoco Chemical’s anaerobic treatment of process wastewater (1991), Tennessee Eastman Co.’s coal-based acetic anhydride (1985), DuPont’s hollow-fiber reverse osmosis (1971), Dow Corning’s silicone products (1955), Dow Chemical’s magnesium from seawater (1941) and Carbide & Carbon Chemical’s petrochemical syntheses (1933).

The Kirkpatrick Award aims to honor the most noteworthy chemical engineering technology commercialized anywhere in the world during 2009 or 2010. Award nominations are collected and validated by the Kirkpatrick Award Committee Secretary, Rebekkah Marshall, who is Chemical Engineering’s editor-in-chief. Once it is verified that the nominee’s technologies have actually been commercialized during the appropriate time period, they are submitted to a board of judges. The Kirkpatrick Award judges are all senior professors and department heads at accredited university chemical engineering departments in the U.S. and Europe, who are recruited to evaluate and rank the nominees based on a set of criteria that includes the technology’s novelty, the difficulty of the chemical engineering problems solved, and the overall engineering excellence. The secretary tabulates the judges’ scores to arrive at the winning technology.

 Figure 1. Velocys commercial director Jeff McDaniel (middle) accepted the 2011 Kirkpatrick Award from CE managing editor Dorothy Lozowski (left) and CE editor-in-chief Rebekkah Marshall (right) at a September 12 reception in Houston
Photo: Scott Jenkins

 

Microchannel reactors

The estimated ten trillion cubic feet of associated natural gas from oil wells that is either flared or reinjected yearly offers a vast market opportunity. Velocys and its parent company Oxford Catalysts has applied its microchannel-technology patent portfolio to that opportunity by developing a method for converting natural gas to liquid synthetic fuels. The microchannel technology could also be applied to biomass-to-liquids (BTL) and coal-to-liquids (CTL) technology. The reactors are designed for steam-methane reforming (SMR) of natural gas to form synthesis gas, followed by Fischer-Tropsch (F-T) synthesis to generate liquid fuels. The microchannel reactors are loaded with catalysts specially designed for microchannel use by Oxford.

Reactors using microchannel technology have parallel arrays of channels with sizes in the range of 0.1 to 5.0 mm in width. The smaller dimensions allow processes to accelerate by a factor of 10 to 1,000 by reducing the distances required for heat and mass transfer, thus decreasing the transfer resistance between process fluids and channel walls. As a result, system volumes can be reduced ten-fold or more compared to conventional hardware, such as fixed-bed or slurry-bed reactors.

A major advantage to microchannel reactors revolves around the greatly enhanced productivities compared to traditional plants that are enabled by the microchannels. The ability to boost productivity through distributed production — production carried out in small-scale plants located near the source of feedstocks, as well as near markets has been made economically and practically feasible, Velocys says.

Because of their heat management properties, microchannels are ideally suited for carrying out catalytic reactions that are either highly endothermic (such as SMR) or highly exothermic (such as F-T synthesis). Velocys microchannel reactors “exploit rapid reaction rates and intensify processes by minimizing heat and mass transport limitations,” the company says. Additional benefits result from the process efficiency afforded by the microchannels, as well as their smaller size, which saves on capital costs and space. The Velocys GTL system can scale the SMR and F-T processes to match associated and stranded natural gas resources at a particular site, both onshore and offshore. Possible products from the Velocys GTL system include diesel, jet fuel and naptha, as well as feedstocks for synthetic lubricants and waxes for specialty chemicals.

FIGURE 2. The small dimensions of the microchannels promote favorable heat- and mass-transfer properties

Aside from SMR and F-T chemistry, microchannel process technology could be applied in a wide variety of areas, Velocys says. These include thermal processing, such as ethane cracking and fuel processing; chemical production, such as ethylene oxidation, as well as separations, mixing and emulsification; gas processing; biological processes and multiphase systems.

Microchannel technology has experienced skepticism from industry over concern about plugging or fouling of the narrow channels. Velocys acknowledges that fouling is a concern, but has experimented with two interrelated strategies that help to mitigate the risk of plugging: high wall shear and good flow distribution. The ability of microchannel devices to handle solids is attributed to high wall shear that sweeps particles out instead of allowing them to build up, the company says. The Velocys team found that for devices with good flow distribution, no pressure-drop increases were observed, even when the feed water was doped intentionally with low levels of dissolved solids. Higher levels did cause fouling, similar to the case for plate-and-frame heat exchangers.

Engineering and manufacturing

Among the major engineering challenges required to bring the microchannel technology into operation was to develop a reactor design capable of efficient and effective heat management in both exothermic and endothermic reactions.

In the case of the endothermic SMR reaction — in which natural gas is mixed with steam and passed over a catalyst to produce synthesis gas — Velocys scientists constructed a network in which the SMR process occurs in channels adjacent to those for heat-generating combustion of hydrogen gas. Heat-transfer properties of the microchannels increase the efficiency of the process. The M-shaped pattern of channels allows the reactor to have a cold end and hot end, which simplifies installation — all supports and manifolding are attached to the cold end, and the hot end is allowed to expand freely.

For the F-T reactor, the same heat-transfer properties are key, as the synthesis gas is converted to parafinnic hydrocarbons over a cobalt catalyst. Thousands of process channels filled with catalyst are interleaved with coolant channels, resulting in exceptionally high heat flux, Velocys says.

Another engineering challenge revolved around the manufacturing for the reactors. Velocys developed a new manufacturing process, known as laminate construction, in which photochemically machined shims are precisely stacked and joined by diffusion bonding, brazing or welding. The parallel microchannels are formed by interleaving thin sheets of formed material (shims) with solid sheets (walls).

Specially designed catalysts had to be designed as well, in order to maximize the benefits of the microchannels. Using Oxford’s OMX method, a highly selective catalyst was developed with activities an order of magnitude higher than conventional catalysts. The proprietary OMX method uses an organic component in the calcination procedure to modify the catalyst properties such that the metal crystallites are of the optimum size to maximize activity, selectivity and stability for a given application. In demonstrations of the catalyst productivity, the Oxford catalyst achieved 1,500 kg/m3/h, compared to productivities of 100 and 200 kg/m3/h for conventional fixed-bed and slurry-bed reactors, respectively. 

Commercial activity

After successful demonstration of its GTL process at a facility in Güssing, Austria, Velocys is working on fulfilling commercial orders. One order comes from the Portugese company SGC Energia for a microchannel BTL facility. Another order comes from an integrated GTL facility in Brazil that includes both microchannel F-T and microchannel SMR. The Brazil plant is slated to start operation in Fall 2011.

Additional Award Honorees

The three other Kirkpatrick award finalists were also honored at the September 12 reception in Houston. 
 
Environ International Corp. (Arlington, Va.; www.environcorp.com) was recognized for its system for biological treatment of volatile organic compounds (VOCs). The system utilizes existing biological wastewater-treatment facilities for destruction of biodegradable VOCs and other organic, hazardous air pollutants (HAPs). Environ’s technology has been demonstrated at three U.S. petroleum-refining and chemical facilities, and the company has plans to extend the U.S. patent-pending treatment approach to eight additional facilities in coming months.
Environ developed the treatment method, known as VOC BioTreat, as an alternative to incineration or to systems involving activated-carbon VOC treatment. The VOC BioTreat protocol has demonstrated the ability to meet VOC and HAP handling requirements in U.S. state and federal emissions regulations.
VOC BioTreat works by piping VOC offgases into an existing wastewater-treatment tank that contains activated sludge at depths of greater than 18 ft. Microbes in the tank break down VOCs as they bubble up through the tank. VOC BioTreat can be retrofitted into existing wastewater treatment facilities for somewhat lower capital costs than those associated with installing thermal oxidizers or activated-carbon VOC-treatment systems, but the annual operating costs are less than 10% of those for conventional systems.
In addition to the VOC BioTreat technology, Environ has developed a test method to confirm the performance of the proprietary technology within a plant setting. The ability to reliably test for VOCs is critical for acceptance from the regulatory authorities, the company says.
The VOC BioTreat technology recently received the grand prize for research excellence in the American Academy of Environmental Engineers’ E3 competition.
NSR Technologies Inc. NSR (Decatur, Ill.; www.nsr-tech.com) has developed a “green” chemical pathway to potassium hydroxide (KOH) solution using membrane separations technology and ion-exchange chromatography. The manufacturing process, which yields 45–50% KOH solutions and 7% hydrochloric acid, is the first environmentally friendly, cost-effective alternative to electrolysis (chlor-alkali) in decades, NSR says. The process generates high-purity products free of mercury and oxidizing species. Also, it does not produce chlorine gas.
The strong base KOH is used for the manufacture of potassium-containing products, such as the food additive potassium citrate and the water-treatment agent potassium permanganate. It is also a key ingredient in soap and detergent processes, as well as agricultural fertilizers and pharmaceuticals.
NSR’s process uses a multipass design that reduces the fluid recirculation requirements, allowing a smaller plant size and lower costs. The company also designed filter cells that minimized internal leaks and shunt/stray losses. Finally, NSR’s chromatographic purification process removes 95–99% of the salt from cell-stack KOH product.
NSR’s process consumes 40% less energy than a conventional process per unit of product manufactured.
Invensys Operations Management. Along with ConocoPhillips (Houston; www.conocophillips.com), Invensys Operations Management (Plano, Tex.; www.invensys.com) has developed a method for online monitoring of hydrofluoric acid (HF) catalyst in the production of octane. The non-spectroscopic method, called ACA.HF Alkylation Measurement Solution, lowers the cost of online HF monitoring while simplifying the measurement and reducing risk to plant workers.
To measure HF levels, the Invensys approach involves analyzing differential responses from online sensors. The system takes readings of water concentration from electrode-free, non-contacting conductivity sensors, as well as simultaneously measuring density and mass-flow levels with a Coriolis flowmeter. The company has developed specialized software to calculate HF levels from the readings.
HF alkylation is a widely used process to produce isooctane for blending into gasoline. In the process, HF catalyzes the reaction between isobutane and four-carbon olefins to form octane. There are three main components in the alkylation catalyst stream, Invensys explains: HF (usually around 90%); water (about 1%); and acid-soluble organic molecules (ASO; which make up the rest). “Tight control of these constituent concentrations, which can save millions of dollars per year, requires accurate monitoring of the levels of all three components,” the company says. Using the formula %HF + %water + %ASO = 100%, the Invensys system can correct for temperature effects and for second-order influences of interactions between ASO and water.
Early approaches to HF monitoring involved manual samples and laboratory analysis, which offers limited accuracy and can expose laboratory workers to toxic substances. More recent Fourier-transform near infrared (FTNIR) techniques are very accurate, but their adaptation for realtime online monitoring is complex and costly, Invensys says.
The new HF monitoring system costs about half as much as an FTNIR system, and it requires minimal maintenance because its core components are built from rugged materials long-proven in industrial HF applications. The sampling system amounts to a continuously flowing sample from a slipstream of the process, Invensys says. “Based on established industry methods, estimated mean time between failures of the technology exceeds 29 years,” the company adds. Additional advantages include minimized potential for corrosion and greatly reduced potential for plant and laboratory workers to be exposed to the sample.
The system’s hardware is a sampling panel, located in the hazardous area, that contains all fluid-handling components, sensors and signal transmitters. Data are transmitted from the panel to a distributed control system (DCS), with which users interact via a human-machine interface in the nonhazardous area.

 

Scott Jenkins

 

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