Solutia Inc.'s Trenton, Mich., plant makes polyvinyl butyral, a plastic interlayer used to make laminated glass for automotive and architectural applications. The plant handles corrosive process media and operates 24 hours per day, seven days per week. The plant's original gasketed plate heat exchanger, which incorporated Hastelloy C-276 plates and EPDM gaskets, was installed for heat recovery and used hot bottoms to preheat feed to one of the distillation columns - a duty that involved a deep temperature cross. Unfortunately, the process conditions proved to be more than the gasketed plate heat exchanger could handle. The plant was constantly dealing with mechanical and operational problems.
The average temperature of the column's bottoms stream exceeded the EPDM gaskets' design temperature, and extended periods of operation above this temperature drastically reduced their lifespan. As a result, Solutia had to replace the gaskets every six months. Furthermore, a change in the process stream from a reducing acid solution to an oxidizing acid solution caused significant crevice corrosion on the back of the gasket groove, and the Hastelloy C-276 plates also had to be replaced every six months. The plant's total annual maintenance and downtime costs were estimated to be in excess of $120,000.
In addition to these mechanical issues, the plate heat exchanger was experiencing operational issues. After the heat exchanger was installed, the process conditions were modified, and the feed stream to the column was increased. However, the unit was unable to reach the targeted column feed temperature at the increased flow rates, so Solutia had to use additional steam to preheat the column feed. This change resulted in an unanticipated utility cost increase of $20,000 per year.
By 2001, the plant was fed up with these problems. The company turned to Alfa Laval, Richmond, Va., for a heat exchanger that could fit in a small footprint and handle the plant's demanding conditions, as well as a unit that could be mechanically cleaned on both sides.
An All-Welded SolutionAlfa Laval recommended the Compabloc, a fully welded and accessible plate heat exchanger that consists of a stack of corrugated heat-transfer plates, alternately welded to form the channels. “The hot and cold process streams flow in the alternately welded channels between the corrugated plates,” explains Tracey Lee, an applications engineer for Alfa Laval. “All four cover panels can be easily removed for mechanical cleaning.”
In 2001, a Compabloc with 250 plates and a 726 ft2 heat transfer area was installed in the Trenton plant on a 10 x 12' concrete slab during a scheduled maintenance shutdown.
“The elimination of gaskets between the plates, and the choice of Hastelloy C-2000 to replace Hastelloy C-276 as the wetted material of construction eliminated the failures experienced with the plate heat exchanger,“ Lee notes.
After four years of maintenance-free operation, Solutia's plant engineers decided to open the heat exchanger during a shutdown to investigate the heat transfer core. A thin brown film was observed on the bottom surface of the plates on the hot side; however, this film had not affected the performance of the heat exchanger. The unit was power washed in less than 30 min, and was reassembled and tested during one shift.
“The heat exchanger has now operated successfully since installation with no signs of corrosion, leakage or a decrease in performance,” Lee says. The fully welded construction effectively eliminated all previous crevice corrosion problems and has provided an internal rate of return of approximately 43 percent. Upgrading from Hastelloy C-276 to C-2000 extended the life expectancy of the plate pack and saved Solutia considerable time and money.
“As a result of the high wall shear stress in the heat exchanger, the heat exchanger's operation has remained constant, thereby eliminating the need for frequent maintenance and downtime,” Lee adds.
In addition to these mechanical benefits, the fully welded heat exchanger also supplied an improved heat transfer coefficient, which has allowed Solutia to recover 70,000 BTU/hr in energy consumption.
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