Find out how existing reactors and plants can be improved significantlyBy Michael Rosellen, Mark Lovallo, and David Houlton
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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 UseReactions 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.
High-Performance ReactorsNew 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 SafetyThe 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.
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 AnalysisAn 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
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 [email protected], or visiting www.ekato.com