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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At the end of the day, he adds, the better the model, the better the insight into potential debottlenecking opportunities.

As an example of this in action, Mullick cites a recent project carried out on a Dow plant in the U.S. The plant uses four large thermosiphon reboilers to supply heat to bottoms that are recycled back into a main distillation column where an organic product is separated from an organic peroxide. However, the emergence of significant flow oscillations in the column raised fears that the peroxide could be nearing its decomoposition point -- a potential explosion hazard. So, production was cut back.

To understand what had changed in the process, Aspen Plus was used to simulate the operation of the thermosiphons -- replicating what was happening in the field by increasing the pressure on the hot side of the heat exchanger model.

The modeling led to two potential solutions, with the one finally chosen being somewhat controversial. It involved placing a valve at the outlet to the high vapor compartment on the process side of the thermosiphon to suppress vaporization on the thermosiphon side and allow some vaporization on the column side of the valve. This only works because the compartments are completely sealed from one another for safety reasons.

The result was elimination of column flow oscillations, a 13% increase in column capacity and lowering of peroxide composition to a safe operating level -- overall, a debottleneck worth $65 million/yr in extra capacity.

Meanwhile, Honeywell Process Solutions, London, Ont., notes its UniSim simulation solution is in demand in the oil and gas, air separation and plant construction sectors. "Within these there are a number of large segments, for example, oil-and-gas production and treatment, petrochemicals and end products. So debottlenecking projects can include, for example, greenfield design, process optimization and flare analysis," says Peter F. de Jonge, a Calgary, Alta.-based simulation business consultant at the firm.

The company has just completed a typical debottlenecking project at Borealis' main integrated petrochemical complex in Porvoo, Finland. The site includes a cracker for the production of olefins (ethylene, propylene and butadiene), a phenol and aromatics plant that produces phenol, acetone, benzene and cumene, two polyethylene plants and one polypropylene plant (Figure 1).

UniSim helped Borealis debottleneck a plant expansion. Specific benefits include: a record-fast startup, documented as the best in the history of the Porvoo cracker: ethylene product was on-spec in an unprecendented three days, ten hours from feed in -- two days earlier than planned; ramp-up was the best ever recorded, with the new production capacity target exceeded in just 21 days; average normalized production rate versus boilerplate capacity 45 days after feed in was at 92%, compared with 67% and 57% for previous turnarounds; flaring losses were 2,000 tonnes less than ever before. Return on investment was significant, with the total value of the startup improvements estimated to be over five times the cost of the simulator.

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