In the case of energy flows in the multi-megawatt range in many plants, smaller-scale solutions (such as direct heat generation by electricity via classic – resistance – heaters) are insufficient. Simply put, a small plant that produces heat via direct electric heaters is not a viable solution for plants in the multi-megawatt class. For processes with temperatures above 250°C/482°F (for special chemical processes, cement, metal and glass producing, for example) it makes sense to use direct electrification by arc, plasma, induction, resistance or other heater types. Hydrogen, derived from electrolysis, works for most of the mentioned applications just fine, also. However, it is produced from electricity with high energy losses, and it requires greater efficiencies, or an abundance of renewable electricity, before its wide scale deployment.
While the solution for decarbonisation of the process and downstream industry may be electrification, industry has to use renewable energy in the electricity mix in a wise way: because there are many other areas of daily life that also require precious renewable electricity to decarbonise.
Nevertheless, modern daily life does provide something of a useful insight for electric alternatives to fossil fuels. Research shows that the best methods to heat domestic homes in colder climate areas are through renewable heat from heat pumps, district heating or a combination of both and other sources (biomass and solar, for example). Similarly, another compressor-based technology sits in sharp contrast to the above-mentioned direct routes of converting electricity into process heat: mechanical vapor recompression (MVR) technology and its close sister, the industrial heat pump.
While industrial, large-scale heat pumps rely on a closed loop, filled with a carefully selected working fluid (the refrigerant) to fulfil the duty of heat recycling, MVR technology utilizes the very same fluid that is used in the process to recycle process heat. Importantly, depending on the temperature levels, in most situations these two technologies – heat pumps and MVRs – offer a much higher electricity-to-heat efficiency than other technologies.
This article takes a look specifically at the role direct MVRs can play in the energy transition within the petrochemical, chemical and downstream industry. It outlines how the compressor plays a central role in the success of MVR systems, and it provides an overview of a case study in the Netherlands, which highlights MVR’s exciting potential.
The central role of the compressor
As with heat pumps, though MVR technology is proven and not new, innovations have improved its performance over many decades. Central to the success of MVRs (and, of course, in most heat pumps) is the compressor. Together with the electricity invested, the system delivers a blend of recovered and electric heat that is locally carbon free, ideally with the electricity invested from a carbon-free source. Even if driven with electric power from a fossil-fuel powerplant, the heat from an MVR always has a lower carbon intensity than the same amount of direct fossil-fuel derived heat.
Integrally geared radial turbocompressors come with a special feature that makes them a particularly good match for high-temperature MVRs. In this type of compressor, it is possible to access the gas flow between the stages, allowing the insertion of cooling gas. This in turn improves the efficiency of the cycle with a very high temperature lift (80K and higher) compared to a single-stage compressor or blower. To compensate for fluctuations in heat demand, the compressor stages can be precisely controlled via inlet guide vanes (IGVs). In most cases, the entire mechanical control system makes expensive power electronics for speed control redundant. In fact, in critical applications in large production plants, a simple mechanical power and capacity control is often the preferred option.
For the downstream industry, one of the most omnipresent auxiliaries is low-pressure steam of between 2 and 5 barg (30 to 80 PSIG), which is commonly used as heating medium for distillation and other process duties. Instead of swapping out miles of steam lines or distillation tower reboilers for electric alternatives, installing a local MVR compressor at the distillation tower manages the shift to an electricity-based process without much change to the infrastructure. Furthermore, as it takes in otherwise lost low-grade waste heating in the form of the top vapor, it reduces the thermal load on the cooling water system.
Vapor and steam-booster compressors
Modern MVR systems can provide water steam at discharge pressures up to 40 barg (580 PSIG), which opens the range to medium and high-pressure steam applications. Other fluids may be compressed to higher pressures, depending on their correlating temperature. Following the golden rule in heat recovery, it is important to “collect the heat at the highest temperatures possible.” In a distillation process, for example, the heat of an overhead condenser is usually transferred to a cooling-water loop and dissipated. Recovering the heat of the overhead vapor is directly possible by utilising MVR, which is subsequently sent to the bottom reboiler of the column. There, the vapor condenses to liquid and rejects its latent heat to the fluid inside the column, which in turn boils. The energetic cycle is closed and only fractions of the heat are dissipated.
MVR technology allows direct recompression of most widely used fluids, such as hydrocarbons, water, organic compounds, and alcohols, and which then raises the vapor pressure and the condensation temperature. As the heat transfer coefficient and temperature of the recompressed, condensing vapor is lower than the used water steam, the introduction of MVR technology often requires a new bottom reboiler. However, and in contrast to a heat pump, this is the only heat exchanger in the process, which keeps the efficiency high. Energy efficiency of machines is measured in coefficient of performance (COP), which shows the ratio between the recovered thermal power and the supplied electrical compressor power. The COP value indicates the energy efficiency of a range of machines, such as chillers, heat pumps and MVRs. Typical MVR applications can reach COP values up to 10, an exceptional level if compared to heat pumps and up to 10x times more efficient than direct electrification (and much more than hydrogen).
Case study: MVR at Terneuzen
In a pilot project in Terneuzen, in the south west of the Netherlands, a plant uses MVR to upgrade low-pressure steam and reuse it to supply energy. Central to the plant’s MVR solution is an Atlas Copco Gas and Process two-stage integral gear centrifugal compressor, which compresses superheated steam from 3 barg (43.5 PSIG) to 12.5 barg (181.3 PSIG) in two steps.
For the Terneuzen plant, the company designed the two-stage compressor with one pinion with two impellers on each end. The number of stages is defined by the pressure ratio limit for each stage, though, if required, it would have been possible to design a three-stage compressor for this purpose (with the advantage of a slightly lower power consumption of the e-motor).
The nominal mass flow of the installation is 12 metric tons per hour and the steam is cooled by water injection with a desuperheater at the inlet and between the stages. The larger droplets are caught downstream by a knockout drum and the steam enters the compressor on the suction side at 3 barg (43.5 PSI) and a temperature of 150°C–220°C (302-428°F). The steam is sent through the desuperheater and the knockout drum in case temperatures reach higher than 170°C (338°F) to avoid higher temperatures in the compressor discharge stage.
Between late 2020, and late 2021, the measurements resulted in an overall COP of 7.5. A COP of 7.5 means that for 1 MW of electricity, 7.5 MW of thermal energy was produced. In simple terms, COP shows the ratio between the recovered thermal power and the supplied electrical compressor power.
After reaching a COP high of 7.5, as in the Terneuzen case study, the expectations on the potential of steam compression are certainly high. In addition to good a COP achievement, steam recompression underpins natural gas savings and CO2 emission reductions. The result at the plant was a reduction in natural gas usage of around 10 million normal cubic meters (MMNm³) in a 12-month period and a net reduction in carbon dioxide (CO2) emissions of 17.8 kilotons (kt).
Conclusion and outlook
This article has shown that MVR systems are proven at providing an efficient form of heat generation. As the world transitions away from fossil fuels to fulfil our energy needs, industries with a high demand for thermal heat to run their processes can longer overlook the challenge of heat generation. MVR technology is not new, and it has proven its reliability over decades. Today, underpinned by modern compressor technology, MVR is now recognised as a key element in industrial decarbonisation plans.
Author - Rasmus Rubycz
rasmus.rubycz@atlascopco.com
Market Segment Manager for Atlas Copco Gas and Process