Mechanical Vapor Recompression key to steam energy upgrade within a polyolefin plant

The task of reducing energy costs and creating more sustainable local energy sources is a familiar challenge across many industry sectors. Polyolefin process plants require a considerable amount of energy, often in the form of steam. Given the challenge of coming up with a more sustainable solution that would reduce its energy use, engineers focused on the untapped potential of low-pressure steam.

In an ongoing pilot project, this particular polyolefin plant, located in the Dutch municipality of Terneuzen, uses mechanical vapor recompression (MVR) to upgrade low-pressure steam so it can be reused to supply energy. Central to this MVR solution is an Atlas Copco Gas and Process two-stage centrifugal compressor that compresses superheated steam from 3 barg to 12.5 barg in two steps. The result has been a significant reduction of natural gas usage and a net reduction in CO2 emissions.

The story goes back to 2014 and the Dutch government’s “Power to Products” project, in which numerous companies from the process industry, energy suppliers, network operators and technology partners came together to address three interrelated central points: how, at what cost and revenues, and under what conditions the process industry could make its electricity demand more flexible. The aim was to allow the process industry to use cheap renewable wind and solar electricity while at the same time contributing to grid stability.  

The process industry can achieve this in two ways: increased electrification and greater flexibility. The first, electrification, simply means that the industry will use electricity instead of fossil fuels, such as natural gas. The second, flexibility, means that the process industry can offer control of power requirements, which it does in two possible ways: One is by storing energy during those cheaper moments (in chemical products and intermediates, such as hydrogen, as pressure, or heat or cold thermal storage via charging and discharging) and then using this later, during the more expensive periods. The second way is to reduce production and power demand temporarily at expensive times, and then catch up later when the price is reduced.

By doing this, industry helps in reducing the demand for fossil fuels and reducing CO2 emissions. The result is to make the overall energy supply more sustainable and cost efficient.

For its part in the Power to Products project, our customer chose mechanical vapor recompression. MVR, or in this case steam recompression, is a good example of the electrification of industrial energy demand. It is also a highly energy-efficient solution because residual heat is upgraded to high-quality heat.

MVR technology was appealing in this case because of the availability of low-pressure steam at the Terneuzen polyethylene processing plant, even though the supply is not constant (especially during winter, when it is needed to heat buildings and for steam tracing). In contrast, when the plant is running steam turbines, or during summer when buildings are not heated, there is excess. Usually, excess low-pressure steam is condensed and the water sent to the boiler to create high-pressure steam.

 A feasibility study for steam recompression, with the aim of using MVR to convert heat from condensation to efficient energy, was undertaken in 2015. The polyethylene plant uses water for reactor cooling. At the end of the process, the cooling water becomes low-pressure steam. Especially during the summer, the demand for heating is lower, which in some periods means that part of the low-pressure steam from the polyethylene plant cannot be used and must be sent to the atmosphere or is condensed. During the initial research phase, it became clear that MVR had the potential to be a good solution to avoid this waste of energy by uprating the steam to 12.5 barg or 35 barg.

The result of the 2015-2016 feasibility study showed that the MVR project was economically less feasible for the smaller flowrates, at 10 t/h. The most feasible case study from an economic viewpoint back then were those with 50t/h. However, Dow wanted to test the MVR technology on a small scale, with the prospect of including the technology in possible future polyethylene expansion projects (and other projects where this would be an option).

Key to the decision on which of the tens of case studies to use for the MVR pilot project, therefore, was selection based on the preferred steam pressure level rather than a more attractive flow rate. In this case it meant using the low-pressure steam (from the reactor cooling water) that supplies the 3.5 barg grid. This is used mostly for process heating purposes and upgrading it to the lowest pressure level would result in less firing of the auxiliary boilers and a reduction of fossil-fuel usage. For this reason, even though 3.5 barg (10 t/h) was economically less attractive, this lower pressure provided the best conditions for the pilot’s primary aim: simply testing the technology. The MVR pilot project got underway in 2018.

By making use of the untapped potential of low-pressure steam via mechanical vapor recompression, plant operators successfully found a more energy efficient and sustainable supply of energy. The project additionally highlighted that MVR has the potential to be used anywhere where there is low-pressure steam. The key component at this particular plant was an Atlas Copco two-stage centrifugal compressor, which compresses superheated steam from 3 barg to 12.5 barg in two steps.

The compressor now operates without significant problems, and it has only been briefly out of service for maintenance of the steam network. In a 12-month period in 2020/2021, approximately 10 million Nm³ of natural gas was saved and a CO2 reduction of 17.8 kton was achieved.