Enginuity 2011

Process modeling

Computer simulations help industries run better

Computers are an important part of everyday life. They are especially important to chemical engineers because of their use in process engineering, which focuses on the design, operation, control and optimization of chemical, physical and biological processes with the help of computer-based methods. It is often considered a branch of chemical engineering and is used in a wide array of industries, including the petrochemical industry.

Kuyen Li and Qiang XuKuyen Li, professor, and Qiang Xu, associate professor, are researchers in the Dan F. Smith Department of Chemical Engineering. Xu is the director of the Laboratory of Integrated Systems Engineering in the Center of Process & Information Technology. They have been working with process modeling, a core part of process engineering.

They’ve studied refrigerant, flare and industrial cracking furnace systems for chemical plants and have discovered more efficient ways to run the systems, saving time and money. Optimizing operations also decreases energy use and pollution. It’s a win-win situation for the chemical industry and the environment.

How long does it take to run a simulation? “It depends on the model’s complexity and size,” Xu said. “Normally, it may take several minutes to several hours.”

The team’s recent work entails modeling and operational optimization for a mixed-refrigerant system. According to Li and Xu, the refrigerant system is one of the most important and critical operating systems in chemical-process industries. A refrigerant system generally works by removing heat from low-temperature streams and transferring it to higher-temperature streams through vapor-compression cycles. Since a refrigerant system can cool down a process stream far below the ambient temperature, it is critical to cooling and separation operations in many chemical processes, including the production of ethylene, oxygen, nitrogen and liquefied natural gas.

A refrigerant system can use a single compound as the refrigerant as long as it is environmentally safe, thermodynamically desirable and operationally feasible. A multi-component mixture can also be used as the refrigerant. From a thermodynamic viewpoint, the mixed-refrigerant system provides refrigeration at a continuum of temperatures, with smaller temperature differences at the lower temperatures. The mixed-refrigerant system has several advantages over the traditional single-component refrigerant system, which has led to the application of the mixed-refrigerant system in some new chemical processes.

For example, an ethylene plant needs to process various streams with temperatures ranging from positive 40 degrees Celsius to negative 140 degrees Celsius. With the conventional refrigeration method, this broad temperature range is accomplished by a cascade-refrigerant system, where three single-component refrigerant subsystems are integrated together. Each refrigerant subsystem employs a compressor and many other types of auxiliary equipment. To reduce capital costs and the operational complexity of the refrigerant system, an ethylene plant can employ a single refrigerant system with mixed refrigerants to accomplish the same refrigeration task. This decreases the number of compressors from three to one, eliminating the need for about 25 pieces of related operating equipment. By introducing a mixed-refrigerant system, the capital cost of the whole ethylene plant can be reduced by 7 percent.

The team studied the operating performance of a mixed-refrigerant system used in an ethylene plant through rigorous simulation. Based on the simulation model, optimization strategies for improving the mixed-refrigerant system operation under the disturbance of cooling-water temperature change were developed. One major insight is between the total compressor work and the compressor output pressure. Although lower pressure will reduce compressor work consumption and operational cost, it will require lower cooling-water temperature.

Another industry project involves cyclic scheduling for the best profitability of industrial cracking furnace systems. According to Li and Xu, ethylene is the most important product with the largest bulk productivity in the petrochemical industry. Its derivatives include ethylene oxide, vinyl acetate, linear alcohols, and ethylene dichloride, and they are used extensively in other chemical industries.

An ethylene plant runs multiple cracking furnaces at the same time to convert various hydrocarbon feed stocks to smaller hydrocarbon molecules, mostly ethylene and propylene. The continuous operational performance of cracking furnaces gradually decays because of coke (the residue of coal and other materials such as petroleum) formation in the reaction coils, which requires each furnace to be periodically shut down for decoking. Given multiple feeds and different cracking furnaces, as well as various product prices and manufacturing costs, the operational scheduling for the entire furnace system should be optimized to achieve the best economic performance.

flowchartThe team developed a mixed-integer nonlinear programming model to obtain cyclic scheduling strategies for cracking furnace systems. Compared to previous studies, the researchers found, the new model has more capabilities to address operation profitability of multiple feeds cracked in multiple furnaces. And it also avoids impractical conditions such as the simultaneous shutdown of multiple furnaces. The team’s case studies demonstrate the efficacy of the developed model and its significant economic benefits.

Like other researchers in the department, Li (who was awarded the title of University Professor in 2009) and Xu are also looking at chemical plant flares. Flaring is crucial to chemical plant safety, but excessive flaring, especially the intensive flaring during the chemical plant start-up operation, emits huge amounts of volatile organic compounds and highly reactive volatile organic compounds. It also results in tremendous material consumption and energy loss. Because of this, flare emissions should be minimized.

The research team's article, "Flare Minimization Strategy for Ethylene Plants" published in Chemical & Engineering Technology, has been selected as one of the Hottest Articles in Green & Sustainable Chemistry (http://www.wiley-vch.de/newsletter/Hottest_Articles/Green_spring_2011.html). Another three papers from the team have won American Institute of Chemical Engineers (AIChE) best paper awards in 2010: "Dynamic Simulation and Optimization for the Startup Operation of An Ethylene Oxide Plant," Process Development Division; "Thermodynamic Analysis-based Design and Operation for Boil-off Gas Flare Minimization at LNG Receiving Terminals," Environmental Division; and "Emission Source Characterization for Proactive Flare Minimization during Ethylene Plant Start-ups," Sustainability Forum.

The team came up with a general methodology on flare minimization for chemical plant start-up operations with a plant-wide dynamic simulation. The methodology starts with the setup and validation of plant-wide steady-state and dynamic simulation models. The validated dynamic model is then run virtually to check the plant start-up procedures. Any infeasible or risky scenarios are fed back to plant engineers for operation improvement. The plant-wide dynamic simulation provides an insight into process dynamic behaviors, which is crucial for the plant to minimize the flaring while maintaining operational feasibility and safety. The team’s methodology was proven effective in a real start-up test at BASF-Total in 2007, and many area chemical plants, including Huntsman Corp. and LyondellBasell Industries, have adopted the flare-minimization method.