3M Espe retrofits membrane degassing module

Seefeld, Germany-  3M Espe AG in Seefeld, near Munich, is a leader in the development of dental products, its production facility in Seefeld manufacturing more than 2,000 products for use in dentistry. Pharmaceutical products for local anaesthesia are produced in the department "local anaesthetics" (LOK), where an Osmotron 1500 select/soft select produces the water used for cleaning the equipment and the water which acts as the basis for production of WFI (Water for Injections). The WFI is used for cleaning the product containers and for the production of solutions.

The system from Christ GmbH was put into service in 2005 and consists of a water softener, a reverse-osmosis stage and an electro-deionisation (EDI) stage with patented Christ Septron modules. "Since the beginning of 2006, our production has increased by about 30%, which means that we also need more purified water," says Andreas Stellwag, technical business administrator and team leader in the local anaesthetics department.

"In the winter of that year, we suddenly had problems with the water treatment system: the Septron modules occasionally ceased to operate optimally," he says. "Initially, we assumed that was due to the heavier load on the system which resulted from the longer daily operating hours." And Stellwag also remembers that "the outdoor temperatures that November were unusually low," said Stellwag.

After consultation with Christ, we detected an excessive concentration of carbon dioxide in the incoming raw water, the team leader continued.

CO2, which is dissolved in water, can pass through the membrane of the reverse osmosis stage. When its concentration in the water leaving this stage reaches a certain level, it places a heavy load on the Septron modules in the EDI stage, causing the resins to become exhausted very quickly. The apparent conductivity caused by the carbon dioxide thus has a negative effect on the quality of the diluate and causes the permissible limit value to be exceeded.

3M Espe urgently needed help in returning the Osmotron to continuous, problem-free operation in order to avoid long down times of the system. As a first, one-time measure, Christ implemented the addition of caustic soda to the water in order to reduce the concentration of carbon dioxide, allowing the resins in the Septron modules to "recover".

As a long-term solution, Christ offered 3M Espe a method of degassing the water with air - the so-called membrane degassing method - or, alternatively, the installation of a permanent caustic- soda dosing system. "We decided to use the membrane degassing method, which requires no chemicals, since it is easier to use," said Stellwag. "Degassing needs much less monitoring and it is not necessary to rinse out any chemicals."

The membrane degassing method is a membrane diffusion process which makes use of the physical effect that semipermeable membranes permit only certain substances to pass. The degassing unit uses hollow-fibre membrane modules (so-called contactors) which contain bundles of hollow fibres and have a very large transfer surface area. The fibres, made from polypropylene, have an external diameter of 0.3 mm and an internal diameter of 0.2 mm. The hollow-fibre membranes are hydrophobic, which means that only gases can pass through them and liquids are blocked.

The method makes use of a stripping gas (control air). The water to be degassed flows through the contactors and the stripping air flows in the opposite direction through the bundles of hydrophobic hollow fibres. Due to the resulting partial pressure gradient, the gas diffuses from the water into the stripping air and is removed. A vacuum can be used instead of the stripping air, or a combination of the two can be used.

Once the membrane degassing unit had been retrofitted, the malfunction of the Osmotron was a thing of the past. The module was installed between the reverse-osmosis and electro-deionisation (EDI) stages, where it removes the carbon dioxide from the water and protects the Septron modules.

The degassing unit was planned, designed and delivered by Christ, who also made the necessary modifications to the soft- and hardware. The unit was installed by an external partner of 3M Espe. After installation, the degassing unit was set to the design values in cooperation with Christ and put into service. Chemical analyses and monitoring showed an immediate reduction in the amount of CO2 in the water. Stellwag, who supervised the project from start to finish, can report positive results: Since the conversion, the system has run without problems and without the need for maintenance – even in the cold winter months."