A converter for explosive gas mixtures

The gases generated by electrolysis and other electro membrane processes contain a mixture of hydrogen and oxygen. Technical installations require safe removal of this potentially explosive gas mixture. We take a look at an integrated equipment concept for degassing and the direct combustion of gases from electrolytical processes.

Introduction
Electro-membrane and electrolysis processes are widely used in industrial water treatment. Today electro-deionisation (EDI) is used instead of mixed-bed ion exchangers as the polishing stage in the production of ultra-pure water. A further example is the electrolytic generation of disinfectants such as ozone and chlorine for the disinfection of water for industrial and domestic use. In both of the above cases, the use of electricity makes it possible to dispense with chemicals. In the first example, regeneration in an electric field replaces the acid and caustic normally needed for regeneration of a mixed bed. In the second case, disinfection chemicals are produced at the point of use.

This is particularly advantageous for small systems and decentralised applications, since it eliminates the need for purchase and storage of chemicals and avoids handling of toxic, corrosive and environmentally dangerous substances. In many cases no additional equipment for dosing of the chemicals is required.Furthermore, there is no need for auxiliary equipment such as neutralisation tanks and there is no risk of contamination of the process water by additives.

Formation of gases in electrolytic processes
In the pharmaceuticals industry, in particular, compact water treatment systems are installed close to the various production processes. These systems are frequently integrated into existing buildings. It is thus advantageous to minimise the number of interfaces to the infrastructure. However, one aspect which is often neglected is the treatment of the exhaust gases. Electrochemical processes generate gases such as hydrogen and oxygen at their electrodes, and these gases do not dissolve well in water .

Accumulations of gases form insulating layers on the surfaces of the electrodes and cause a considerable reduction in the performance of the electrochemical process. For this reason, the electrode chambers are continuously flushed with water. In some cases, such as in the generation of ozone or chlorine, the gas which forms at the anode is the desired product of the electrochemical reaction. In many cases, such as in electrodeionisation, this gas is simply oxygen, and this is simply released into the surrounding air.

In small systems, the hydrogen which forms at the cathode is not utilised, but simply vented into the atmosphere. Gases which are dispersed in water are not explosive regardless of their composition. However, there is a risk of oxyhydrogen explosions wherever pockets of gas containing hydrogen are formed.

The catalytic waste-gas converter Hydrokat protects electrochemical processes against oxyhydrogen reactions.The heart of the system is a catalytic burner with automatic oxygen supply and integrated temperature regulation.

In addition to the standard version, there is now a version Hydrokat XL for larger volume-flow rates.

Possible solutions for waste gas
Many manufacturers of EDI modules and electrolytic gas generators still do not have convincing solutions for removal of the resulting gas. The proposed measures, such as discarding the rinsing water into a drain, are rather impractical. One solution which is frequently used is to dilute the electrode gases and to vent them into the open air. For this, the hydrogen concentration must be reduced to < 4 % by volume. If, for example, the system is located in a cellar, in a separate part of the building or in a clean room, the installation of the necessary ventilation systems can be very expensive.

Another possible way of removing the oxyhydrogen is to oxidise the hydrogen with the aid of noble metal catalytic converters. On some noble metals, hydrogen ignites at room temperature and can then be burnt in a controlled manner. A combustion zone is formed within the catalyst block and is stabilised by the high activity and thermal capacity of the catalyst. This process can be used to implement a stand-alone waste-gas treatment system.

Converting potentially explosive gas mixtures
With the Hydrokat, Christ has now developed a waste-gas converter specially designed to meet the requirements of electrochemical processes. The technical design of the Hydrokat is adapted to the standard systems Osmotron and Loopo. The Osmotron series [2] comprises compact systems with outputs of 0.5 m3/h to 10 m3/h for the production of deionised water of the quality levels “Purified Water” and “Highly-Purified Water”. The Septron EDI module is used for the subsequent demineralisation of the water. The electrode chambers in the Septron are flushed with the entire flow of concentrate for optimal removal of the electrode gases.

The concentrate leaving the module contains a dispersed mixture of hydrogen and oxygen. The Loopo series [3] consists of compact systems for the disinfection and distribution of pure water in accordance with the requirements of the pharmaceuticals industry. It has electrolytic zone generators with outputs of 4 gO3/h to 8 gO3/h. Ozone is generated by anodic oxidation and injected directly into the water stream. At the same time, hydrogen is formed at the cathode and escapes through a vent hole.

The Hydrokat consists of a gas separator and a catalytic burner.The gas separator is used when it is necessary to separate the waste gas from the rinsing water.It must always be filled with water during operation. In order to ensure this, the drain line is designed as a riser tube. Phase separation is carried out in a settling section. Optionally the efficiency of the gas separator can be improved by means of additional components.

The waste gas then flows through a drop separator, which also acts as a flame trap, into the catalytic burner. Here the hydrogen is burnt. The combustion tube is heated by an electric heater which is regulated by an integrated temperature switch. This ensures ignition of the hydrogen and also prevents condensation of water vapour from

Formation of gases in electro-membrane processes.

The process combination of reverse osmosis and electro-deionisation (EDI) commonly used today for the production of ultra-pure water in the pharmaceuticals industry is supplemented by the Hydrokat, which can be retrofitted easily in existing systems.

combustion. Heating of the off-gas has another effect: In combination with the cover of the unit, it generates a chimney effect. This cools the cover and ensures that sufficient fresh air is sucked in to provide the necessary excess air for the catalytic combustion. The specific utilisation of these mechanisms makes it possible to treat waste gas containing almost no oxygen. The catalytic burner can thus be used to treat hydrogen flows of up to 20 lN/h (corresponding to an electrode current of about 50 A). This is the maximum amount of hydrogen produced by the systems of the Osmotron and Loopo series.

Integration of the Hydrokat into an existing system is extremely easy: the unit only needs to be connected hydraulically and electrically and then runs independently; the catalyst is permanently pre-heated and the combustion reaction starts as soon as there is hydrogen in the waste gas. A temperature monitor in the heating block serves to detect the burner temperature as the result of a fault. The resulting signal can be processed in the control system.

Large volume-flow rates
In addition to the standard version, the version Hydrokat XL is now available for larger volumeflow rates (Figure 2). This version has the same principle of operation, but the gas separator and the burner are modified in order to handle the larger volume of hydrogen. The waste gas entering the unit is split between several combustion tubes, each of which is identical to that in the standard version. This ensures the same operational reliability as in the standard version. The burners are mounted in a common heating block. The version Hydrokat XL is suitable for treatment of waste gases containing up to 200 lN/h of hydrogen.

Summary
The Hydrokat is a standardised waste-gas treatment system for water-treatment and electrochemical processes such as electrolysis, electrodialysis or electro-deionisation. It permits the catalytic combustion of hydrogen and offers advantages over conventional venting systems. The location for a water treatment system can now be selected without taking into account special ventilation systems. The lack of moving parts in the Hydrokat ensures safety and reliability in the handling of oxyhydrogen gases. Since the Hydrokat is designed as a stand-alone system, its installation requires no modification of the piping or control system of the water treatment system. The Hydrokat can be retrofitted in existing systems without expensive modifications.