Increasing the Productivity of Gas-Liquid Reactors

Fri, 02/16/2007 - 11:56am
Find out how existing reactors and plants can be improved significantly
By Michael Rosellen, Mark Lovallo, and David Houlton

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Recent advances in process technology and equipment design methods offer opportunities to "uprate" plants at relatively modest cost. Previous performance limitations, such as mass transfer, heat removal, and homogeneity issues for three-phase reactions, have been overcome. Operating issues can be eliminated and service intervals extended by assessing and optimizing the static and dynamic responses of individual components as well as the whole vessel.

Gas-liquid reactions are usually performed in agitated vessels due to the robustness and flexibility of such equipment. Common gaseous components are hydrogen, oxygen, chlorine, ethylene, and ethylene oxide. For fast reactions, the limiting step is usually mass transfer of gas into the liquid. Mass transfer is primarily a function of the properties of the gas-liquid system and the conditions at the gas-liquid interface. Other factors include the design of the feed system and recirculation of gas within the reactor. Optimization can provide benefits vis-à-vis productivity and therefore profitability.
Effective Gas Use
Reactions using pure gases, such as hydrogen, should aim for complete reaction of the gas because of the cost of raw materials and reprocessing and because of the undesirable safety and environmental consequences of discharges.

Only part of the freshly fed gas reacts as it passes up through the liquid. The rest eventually fills and pressurizes the headspace. This would cause the pressure regulator to stop the feed unless gas was released from the headspace. Therefore, it is necessary to provide means to redisperse this gas back into the liquid. Various methods have been used, but each has disadvantages. Venturi systems use external recirculation of the liquid to re-entrain gas through a nozzle. However, liquid in the loop can experience low dissolved gas concentrations with potential consequences for side-product formation and catalyst life. Alternatively, an external compressor can be used to recirculate gas from the headspace back into the feed system. This avoids risks associated with external liquid loops, but the capital investment and maintenance costs can be unattractive. Creating surface mass transfer by splashing or vortexing is a simple, cheap, and compact solution, but the low efficiency of these mechanisms is only sufficient for slower reactions.
High-Performance Reactors
New reactor systems use a high efficiency gas turbine to induce large flows from the headspace via a hollow shaft into the liquid. Unreacted gas is perpetually pumped back into the liquid until it is consumed. The amount of recirculated gas is readily calculable and can be 10 times higher than the feed rate of fresh gas. Stable performance characteristics allow mass transfer to be optimized without needing complex and costly monitoring of the agitator.

Costs are reduced, productivity is enhanced, and operation is simplified. Many older reactors in existing plants can be uprated for a modest investment. Payback time may be measured in weeks rather than years.
Reliability and Safety
The size of reactors used for modern world-scale plants have lead to a new focus on process efficiency and equipment reliability. These are important when uprating existing equipment. If the static and dynamic loads exceed the strength of components or if resonances are set up, wear or even serious damage can result. On the other hand, careful investigation of these factors can help ensure safe and reliable operation with minimum maintenance.
Vessel Vibrations
Very large thin-walled vessels with gas feeds, such as bioreactors, are particularly vulnerable to vibrations. So are vessels supported on legs or in a high structure and semi-batch vessels on load cells. All these arrangements are relatively "flexible" so that the vessel, or components within it, have a lower natural frequency that could coincide with the frequencies generated by the agitation system.

The dominant excitation frequencies are generally equal to the agitator speed, the blade passing frequency (agitator speed times number of blades) or, due to circulation, patterns set up in the liquid (so-called Karman vortices). To assess and avoid resonance, the excitation frequencies must be known and compared to the natural frequencies of the structure, vessel, internals, and critical components.
Finite Element Analysis
An important tool for assessing natural frequencies is modal analysis using finite element analysis. It can be used to model and analyze complex components. Potential weak points can be identified, and expensive repairs and modifications avoided.

The simulation must be capable of taking into account the actual operating environment and, in particular, the presence of liquid around the internal components (e.g., the feed pipe and heat exchange surfaces). Older simulation techniques did not take into account the "fluid-structure coupling" of the liquid around the component and suffered errors up to 50 percent.

Such sophisticated techniques are not required to ensure safety and reliability in all equipment designs. What is necessary is the ability to recognize when problems may occur. Also needed is access to the technology to make required detailed assessments.
Single Source of Responsibility
Ideally, the responsibility for all design aspects should rest with a single-source supplier who has all the competencies required to ensure high performance, reliability, and safety. Target process performance must be defined. Unless chemical and physical data are available, measurements may be required. Miniplants that operate at well-defined conditions are often used. Then, the reactor, gas feed, agitator, and heat transfer equipment can be designed. Additionally, dynamic and static mechanical loads and frequencies are assessed to ensure reliable and safe mechanical design.

Michael Rosellen heads the reaction and reactor technology business of Ekato RMT in Germany. He has a degree in mechanical engineering from Lippe University of Applied Sciences and has been with the company since 1988. Mark Lovallo heads the North American Technology Center of Ekato Corp. He previously worked as a research engineer at a major U.S. chemicals firm and has a doctorate in chemical engineering from the University of Massachusetts, Amherst. David Houlton is responsible for reaction consultancy and design in Ekato RMT's R&D department. He has more than 25 years of experience in process plant design and research. Ekato was founded in 1933 and has become a global leader in agitation technology. More information is available by contacting Ekato Corp., 48 Spruce St., Oakland, NJ 07436, calling 201-825-4684, sending an e-mail to info.vus@ekato.com, or visiting www.ekato.com

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