An energy-saving approach to to recycling steam

Paul Boughton

Tanya van Essen explores the potential of mechanical vapour recompression (MVR) to recycle steam and reduce energy wastage 

Steam systems are a part of almost every major industrial process today. Steam is used for various heating processes, as a heat carrier for evaporating solvents in distillation columns, in drying processes, reactor columns, etc. Roughly a quarter of the oil and gas consumed in the process industry is used to generate steam. As steam production is in most cases one of the largest energy users and plant arrangements are often outdated, improvements here can lead to considerable energy savings. Besides saving costs, enterprises are taking more responsibility nowadays for their impact on the environment and are therefore focused on improving energy efficiency. Mechanical vapour recompression (MVR) can be a solution for greatly reducing energy costs and carbon footprint.

The basic principle of MVR is recycling unused low-value steam, mechanically converted into high-value steam while preserving its latent energy, reaching coefficient of performance (COP) values up to 10.

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 vapour. Contrary to the compression heat pump with its separate circulating fluid (closed system), MVR operates as an open system. In an open heat pump system the process fluid (in this case steam) also acts as the circulating fluid. Due to the elimination of the evaporator, condenser and separate circulating fluid, high COPs can be obtained.

A new solution

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. However, each compressor type has its limitations on pressure ratio, volume flow and operating flexibility. Bronswerk Heat Transfer has developed a new compressor that eliminates various limitations known from conventional compressors. The Radiax compressor’s specially designed inlet rotor ensures no-stall characteristics, which results in a smooth axial pressure rise. The divergent design of the rotor contains a large number of blades. The rotor is capable of reaching tip speeds close to the speed of sound while converting flow speed in dynamic pressure. The rotor is directly driven by the electric motor. 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 3D designed channels, ensuring  a perfect transition in axial direction. The rotor, electric motor and stator-diffusor are integrated in a single casing, which minimises the amount of components and leads to a compact design.

Bronswerk believes that there are several advantages of the Radiax compressors compared to conventional compressors. Firstly, they are completely variable with regard to pressure and flow without compromising on the overall compressor efficiency and offering maximum flexibility. Next, the high pressure ratio up to 2.0 per stage for steam, maximum two stages per Radiax compressor, is a key selling point. The compressor is also able to handle two-phase fluids. This enables BFW injection upstream to temper the temperature of the compressed superheated steam without damaging the rotor. The solution is free of oil and oil seals due to the integrated electromotor in the compressor unit, eliminating the risk of contamination of steam caused by leakages. Finally, this new compressor is approximately 60% smaller in size and weight compared to conventional MVR compressors, thus saving plot space.

Case study

To illustrate the advantages of MVR using Radiax technology, a case study is described. A chemical plant derives its product from a reactor column that requires 7,000 kg/hr of steam at 4 bar(a). Steam is derived from a steam boiler and directly injected in the reactor column. During this process 300,000 kg/hr of condensate is generated. For this case energy prices are €25 per ton of steam and €0.06 per kWh for electricity. Assuming this plant runs for 8,600 hour per year the total energy expenses are € 1,505,000 per year.

This system can easily be optimised by adding a flash tank and thermal vapour recompression (TVR) unit. By releasing the pressure of the 300,000 kg/h of condensate from 4 to 3.4 bar, 4,500 kg/hr of condensate flashes. This low-pressure steam will be upgraded by a steam ejector (acting as a thermal heat pump in this situation) where 2,500 kg/hr of high-pressure steam is necessary as driving steam. This configuration reduces the expenses on energy to €537,5000 per year.

Applying MVR using the Radiax technology reduces the energy expenses further to only €103,200 per year. In this scenario, pressure of the 300,000 kg/h condensate is released to 2.85 bar, flashing 7,000 kg/h 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 electric energy. This relatively simple modification saves  €1,401,800 on energy compared to the conventional system.

MVR using Radiax technology is a simple and effective solution for reducing energy costs and carbon footprint. The favourable characteristics of the compressor create opportunities for a wide range of applications. Minor modifications are required, using only a small area of the available plot space. This solution will amortise within a short period of approximately two years, despite the current fossil fuel prices.

Tanya van Essen is with Bronswerk

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