Debottlenecking Takes A Broader View

Modeling is gaining a greater role in improving throughput and energy efficiency.

By Seán Ottewell, Editor at Large

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Simulation techniques are enabling greater integration of debottlenecking projects into wider plant operations. At the same time, processes previously considered too difficult to model now are benefiting from the same techniques.

"The key issue now is getting the model to fit the plant," says Rian Reyneke, senior advisor, process consulting services, AspenTech, Burlington, Mass. "For example, you might have to choose between repacking a tower or installing a completely new one. But is the model good enough to give the correct answer? So we spend a lot of time validating simulation models because we have to have complete confidence in them. Otherwise the client will default to the more conservative solution -- a replacement tower, for example -- which is generally a more expensive approach and, in the extreme, may render the project unviable. What we are trying to help clients achieve is to squeeze as much out of their plants without big capital charges."

AspenTech's strategy is to seek more innovative and lower-cost debottlenecking solutions by looking at the system in a broader way, considering whole plant operation from a process and energy point of view as opposed to addressing each bottleneck in isolation.

"Our teams look at both the process and energy sides and consider both design and the operation. It's a very multi-faceted approach to debottlenecking. So we can offer better design while also limiting operational costs. And by better designs we mean greater process flexibility, lower energy costs and often optimal capital investments. It is ultimately an optimization solution in response to the customer's business objectives," says Sanjeev Mullick, director, industry marketing, AspenTech.

INCREASING INTEREST IN DEBOTTLENECKING
A telltale sign is the record number of papers on debottlenecking to be presented at the company's Optimize 2011 global conference in Washington, D.C., in late May.

One such case study involves a 39,000-tonne/yr Reliance Industries' acrylonitrile plant in India. Here, AspenTech's modeling tool, Aspen Plus, was used to develop a steady-state model of the total plant in an effort to address a number of processing challenges. The simulation so far has spurred a 50% reduction in hydrogen cyanide emissions, a 75% decrease in effluent color and a 15% increase in acetonitrile concentration. An ongoing study at the same site also might lead to a cut in flare losses that currently are running the equivalent of about $22,500/yr.

"The capability of Aspen Plus for simulation of large flowsheets with complex unit operations and highly non-ideal systems helped us in developing a plant model that is being utilized for process optimization, environmental protection, plant safe operation, and energy optimization," notes Reliance's abstract.

Another project spotlighted at the conference involves cryogenic unit number one at Pemex's Ciudad Pemex gas processing plant in Mexico. It had been operating at an efficiency of 76.72% for C2+, well below its originally designed capability of 81.94%. Once updated to reflect the plant's current operating conditions, the Aspen Plus model pinpointed low efficiency in a heat transfer unit. Adjusting that unit gave a production improvement worth $7.6 million/yr.

"When considering energy, it is important to remember that many of these projects get their payback via energy reduction. It's not necessarily extra production capacity that a customer is looking for. Debottlenecking projects often involve both increasing throughput and reducing energy consumption. So we always deal with them together. There is a big drive to reduce specific energy consumption and to reduce the carbon footprint. Companies want these projects to have an attractive payout in both situations of increased or lower product demand," notes Mullick.

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