Bronswerk Heat Transfer explores the potential of mechanical vapour recompression (MVR) using Radiax® technology.
Steam systems are a part of almost every major industrial process today. For various heating processes, steam is used as a heat carrier for evaporating solvents in distillation columns, drying and reactor columns. Roughly a quarter of the oil and gas consumed in the process industry is used to generate steam.
As one of the largest sources of energy demand, and given the frequency of outdated plant arrangements, improvements can lead to significant energy and cost savings. As more companies also look to reduce emissions and improve efficiency, steam production processes are a frequent target.
Mechanical vapour recompression (MVR) is one solution. The process works by recycling unused low-value steam, mechanically converting it to high-value steam while preserving its latent energy. It can reach coefficient of performance (COP) values of up to 10, and when used in conjunction with Bronswerk Heat Transfer’s advanced Radiax® compressor can offer even greater advantages.
The Principle of MVR
During MVR, a mechanically driven compressor increases the pressure of a steam flow. The compressor operates as a heat pump by adding energy to the water vapour. But contrary to the compression heat pump, which uses separate circulating fluid (a closed system), MVR works as an open system.
In an open heat pump system the process fluid – in this case steam – also acts as the circulating fluid. With the elimination of the evaporator, condenser and separate circulating fluid, the system can maintain high COPs of up to 10.
In Europe, steam prices can be between 20-35 euros (US$22-39) per tonne. With thousands of tonnes being used per hour, savings made here can be considerable. Bronswerk business developer Geert ten Brink notes that the technology is gaining traction, primarily because “declining electricity prices in Western Europe in contrary to gas prices making MVR more favourable, even though the technology has been around for 15 years or so.”
An example MVR cycle is illustrated in the log(p)-h diagram of water (Figure 1) where flashed vapour is upgraded from 2 bar(a) to 4 bar(a) of superheated steam in a single-stage Radiax® compressor. The mechanical work delivered by the compressor is depicted as dH. Steam leaving the compressors is slightly superheated as a result of the dissipated thermal and mechanical losses of the compressor. Additionally, the superheated steam can be tempered or de-superheated to its saturation point by injecting boiler feed water (BFW) in order to attain the required process conditions.
Different types of compressors such as centrifugal fans, turbo-compressors and rotary root blowers are suitable as mechanical vapour compressors, if operating according to the principle of continuous flow machines. But each compressor type has its limitations on pressure ratio, volume flow and operating flexibility.
Bronswerk Heat Transfer has developed a new compressor which eliminates various limitations known from conventional systems. Based on new work in fluid dynamics, the unit is a compressor with an operating range rather than a single operating point. The unit provides continuously variable speed, flow and pressure, from a very compact size, many times lighter and smaller than conventional machinery.
Figure 2 illustrates a cross section of the Radiax®. The specially designed inlet rotor – directly driven by an electric motor – provides no-stall characteristics, meaning a smooth axial pressure rise. The divergent rotor design also contains a large number of blades reaching tip speeds close to the speed of sound, while converting flow speed in dynamic pressure.
Vapour leaving the rotor enters the stator-diffusor, where dynamic pressure is converted into static pressure. Vapour enters the stator-diffusor tangential and is guided by 3-D channels which ensure a perfect transition in axial direction. The rotor, electric motor and stator-diffusor are all integrated in a single casing, minimising the amount of components and producing a far more compact design.
The Radiax® offers highly variable inputs in terms of pressure and flow, meaning maximum flexibility without compromising on overall efficiency. It is also around 60% smaller in size and weight compared to conventional MVR compressors, saving on transport, deployment and plot space.
It also features a high pressure ratio of up to 2.0 per stage for steam, with a maximum of two stages per compressor. It is capable of handling two-phase fluids, enabling boiler feed water (BFW) injection upstream to temper the temperature of the compressed superheated steam.
This ability to offer a range of pressure change, rather than a fixed step, also means users can adjust compression according to the anticipated demand for steam, again building greater overall efficiencies and potential cost savings.
Because of the integrated electromotor, the compressor is free of oil and oil seals, meaning there is no risk of leakages which could contaminate the steam.
One recent case study used a Radiax® compressor as part of the MVR process. In this project, a chemical plant derived its product from a reactor column that requires 7,000 kg/hr of steam at 4 bar. Steam is sourced from a steam boiler and directly injected in the reactor column, to produce 300,000 kg/hr of condensate.
For this case, energy prices are 25 euros (US$28) per tonne of steam and 0.06 euros (US$0.067) per kWh for electricity. Assuming that the plant runs for 8,600 hours per year, total energy expenses are 1.505 million euros (US$1.68 million) per year.
This system can be easily optimised by adding a flash tank and a thermal vapour recompression (TVR) unit. By relaxing the pressure of the 300,000 kg/h of condensate from 4 to 3.4 bar, 4,500 kg/hr of condensate is flashed. This low-pressure steam will be upgraded by a steam ejector (acting as a thermal heat pump in this situation) which only requires 2,500 kg/hr of high-pressure steam to drive. This configuration reduces the expenses on energy to 537,500 euros (US$600,000) per year.
Applying MVR using Radiax® technology can cut energy expenses further, to only 103,200 euros (US$115,000) per year. In this scenario, the pressure of the 300,000 kg/hr condensate is relaxed to 2.85 bar, flashing 7,000 kg/hr of condensate to low-pressure steam. Steam derived from the flash tank is compressed in a single compression stage to 4 bar, absorbing less than 200 kW of electric energy.
The MVR configuration using a Radiax® compressor is illustrated in figure 3. This relatively simple modification can save around 1.4 million euros (US$1.5 million) on energy compared to the conventional system.
The study demonstrates that MVR using Radiax® technology is a simple and effective solution for reducing both a plant’s energy usage and its carbon footprint. With maximised flexibility and minimised complexity, the unit is suitable for use in a wide range of applications, from factory processes to oil and gas production, transport and storage.
Additionally, only minor modifications are required to fit the Radiax®, using only a small area of the available plot space. The savings offered also mean that initial capex of the compressor is returned within 2 years.
Building on the success of the Radiax®, Brink says that the company is actively “looking for more cases in which to participate,” and with new Radiax® compressors now in development, 2016 is set to be an exciting and busy year. “More and more industries are now getting interested in MVR,” he concludes. “There are still a lot of processes that can be optimised and places where users can significantly reduce their costs.”