Membranes on the move for chemical-free degassing3 March 2020
The range of mobile water treatment services available to power plants has steadily widened over the years to including membrane degassing units, which avoid the need to use chemicals (such as hydrazine) on site.
Much has changed since mobile water services were first introduced into the market. Originally, they were simply ion exchange resins mounted inside trailers, which could be transported to a site to provide a temporary supply of high purity water.
However, needs have diversified, generating demand for a broader range of available physical and chemical technologies, including degasification, pre-treatment by clarification and filtration, reverse osmosis, absorption and ultrafiltration – with modularisation allowing a variety of process configurations to be combined.
No chemicals needed
Veolia offers mobile membrane degassing units, called REMOX, which can remove oxygen and carbon dioxide from process water – taking oxygen down to <10 ppb and carbon dioxide down to <1 ppm – in a flexible, fast and efficient way, without the use of chemicals.
The containerised membrane system is compact, efficient and maintenance free and the portability of the REMOX assets enables them to be positioned to make the best use of the available space, eliminating or reducing the need for building infrastructure to house the equipment.
Any number of assets can be operated in parallel or in series to provide the required flow rate. In addition, these modular units can be easily exchanged over time for the latest, updated technology, ensuring that a facility’s water treatment systems remain at the cutting edge.
REMOX employs 3MTM Liqui-CelTM membrane contactors, which use microporous hollow fibre membranes to remove gases from water. The hollow fibre is knitted into an array and wrapped around a centre tube in a housing.
The water to be treated flows over the outside of the hollow fibre array while a vacuum and sweep gas (nitrogen) is applied to the inside of the fibres. The membrane is hydrophobic, allowing direct contact between gas and water without dispersion.
A higher pressure is applied to the water stream than the gas stream. This creates a driving force for the dissolved gas in the water to pass through the membrane pores. The gas is then carried away by the vacuum pump and the nitrogen sweep gas.
The REMOX unit, which can treat up to 100 m3/h, includes onboard services such as nitrogen generation, vacuum and compressed air. Allowable inlet pressures range from 0.8 to 2 bar, while the allowable temperature range is 5 to 25°C.
One recent application of REMOX was provision of temporary water treatment for a power company in the Netherlands supplying water to three district heating networks. Temporary water treatment services were required when it became necessary to carry out long term maintenance on the existing water treatment plant.
Maintaining continuity of water treatment is of particular importance for district heating systems and avoidance of corrosion in the pipework.
A REMOX 50C system (50 m3 of water per hour) was used to fill the district heating system’s ‘heat buffers’, with special measures required to ensure that oxygen did not get into the water post-treatment, including use of special pumps and fixed piping instead of hoses.
Another recent example of REMOX employed in a heat buffer project was in Germany, where Veolia worked with an engineering, procurement and construction contractor responsible for building a new circa 40 000 m3 hot water storage tank at a power plant site.
The heat buffer storage tank, which can provide up to about 70 hours of district heating hot water supply, allows the power plant to run more flexibly and economically – enabling it to operate at low power or even shut down, depending on electricity demand.
The heat buffer employs >100°C water and needed to be filled with deoxygenated water. For this project Veolia Mobile Water Services provided a MORO 4x25T reverse osmosis system running in double pass configuration feeding a REMOX 100C to produce 45m3/h, 24/7 deoxygenated demineralised water.
The conductivity required was <40 μS/cm and O2 <50 ppb. Additional supply of a feed water buffer and two MOPS 170i pump skids, assured constant filling of the heat buffer.
An important benefit here was the chemical-free production of deoxygenated water.
Reliability and security
Water treatment asset rental offers an attractive solution to support and complement existing facilities, says Veolia, ensuring the provision of treated water at all times, including in the event of emergencies and during planned interruptions due to maintenance or refurbishment.