MVR compressor upgrade at Terneuzen polyolefin plant
In an ongoing pilot project, a chemical process plant in the Netherlands uses mechanical vapor recompression (MVR) to upgrade low-pressure steam and reuse it to supply energy. MVR, or in this case steam recompression, is a good example of the electrification of industrial energy demand. Central to the plant’s MVR solution is a two-stage integrally-geared compressor, which compresses superheated steam from 3 barg to 12.5 barg in two steps. The result has been a reduction in natural gas usage and a net reduction in carbon dioxide (CO2) emissions.
Cross-section of an integrally geared compressor core
In 2015, the plant collaborated on a feasibility study focused on using MVR to convert heat from surplus low-pressure steam to valuable medium-pressure steam. The plant then contacted several compressor manufacturers to explore the potential of supplying the requisite equipment. The plant chose Atlas Copco for its proposed solution of an integrally geared two-stage centrifugal compressor.
The overhung arrangement of the machine’s impellers on the pinion shaft ends means separate compressor bearings are not necessary. The bearings are identical to those of the compressor rotor, and the rotor shafts are made from a single heat-treated forged alloy steel. Shaft seals can be labyrinths, dry gas seals or floating carbon rings. Its integral setup makes it possible to have an intercooling/desuperheater after each compression stage, which results in increased efficiency.
Exterior of an integrally geared compressor core
Research on the compressor’s coefficient of performance (COP) was very positive. In simple terms, COP shows the ratio between the recovered thermal power and the supplied electrical compressor power. Between November 2020 and November 2021, the MVR system achieved an overall COP of 7.5. This means that for 1 MW of electricity, 7.5 MW of thermal energy was produced, which represents a high performance (partly attributable to the inherent advantage of an open-heat pump directly compressing steam, compared to a standard compression heat pump with refrigerants).
This highly energy-efficient process upgrades residual heat to high-quality heat. Using the untapped potential of low-pressure steam via MVR, the project shows how plant operators have successfully found a more energy-efficient and sustainable energy supply. It also highlights that MVR can be used anywhere where there is low-pressure steam or waste heat in excess of 80°C/180°F.
In addition to the good COP achievement, steam recompression underpins natural gas savings and CO2 emissions reductions. In the last 12 months, approximately 100 000 MWh (10 MMNm³) of low caloric natural gas was saved, and a CO2 reduction of 178 000 tons was achieved.
