By: Editorial team
Reading time: 4 Minutes
◤ ENVIRONMENTAL PROTECTION
Methyl chloride recycling processes
Legal regulations limit the release of the base chemical methyl chloride to the atmosphere during production, transport, storage or use. Two major companies use cryogenic nitrogen to condense and recover methyl chloride vapor. This involves several different processes.
How can efficient production and logistics be combined with reliable environmental protection? This question arises regularly in the chemical industry. Take the processing of chloromethane (CH₃Cl), for example – which is the formal chemical name for a substance more widely known as methyl chloride.
This chlorinated hydrocarbon, which is gaseous at room temperature, is harmful to health in the long term and highly flammable. It is however required in large quantities to produce, among other things, surfactants, water treatment agents, silicones and butyl rubber, which are used for numerous useful everyday products.
Heat exchanger plus absorber
Nobian, a European market leader in the production of salt, essential chemicals and energy for the industrial sector, is one of the largest suppliers of this base material. The company produces methyl chloride at the Industrial Park Höchst in Frankfurt, Germany. From there, the methyl chloride, liquefied under pressure, is shipped by semi-trailer or rail tank car to customers. To speed up the unloading process into the customers’ storage tanks, nitrogen is fed at high pressure into the tankers. Once empty, the freight containers are returned to Nobian containing a residual mixture of the two gases.
Before they can be refilled, this mixture must be removed – without releasing any methyl chloride to the atmosphere. One of the means of preventing such a release is using cryogenic liquid nitrogen. The waste gas is sent through a heat exchanger.
This cools down the liquid nitrogen, in a special condenser, to exactly -95°C, which is precisely 1°C above the melting point of the methyl chloride. Most of the methyl chloride condenses there and can be reused immediately. A molecular sieve absorbs the remaining fraction that cannot be captured by this method. It also takes nitrogen to operate this absorber. There, it serves two purposes:
- liquid nitrogen is used to maintain a constant low operating temperature.
- gaseous nitrogen is used for regeneration.
This processing combination was developed especially for Nobian, thus ensuring continuous compliance with the regulatory limits and preventing the release of methyl chloride into ambient air.
Methyl chloride recycling unit at Nobian in Höchst, Germany
Purely cryogenic process
As with the development and implementation of that process in Höchst, Messer also played a lead role in developing another, somewhat different recovery method, the one that Evonik uses in Steinau, Germany. The specialty chemicals manufacturer needs methyl chloride to produce surfactants, the key component of most cleaning agents. The CH₃Cl does not react completely with the other components here, however: a residual amount remains dissolved in the product and must be removed. This is accomplished by applying vacuum pressure inside the reactor to vaporize the excess methyl chloride.
The next step is similar to the process used at Nobian: the vapor is pumped through a nitrogen-cooled heat exchanger that forms part of a heat transfer medium circuit. The methyl chloride condenses there while simultaneously generating the required vacuum pressure to empty the reactors. The solvent then returns to the production process as a subcooled liquid. To recover the uncondensed fractions as well, instead of an absorber Evonik uses an additional freeze-out step, which also draws its cooling from the nitrogen. That makes this recycling process a purely cryogenic one. It can be used here because in surfactant production, the share of inert gas in the process gas is much smaller than in the residual gas left over after emptying the tank cars.
These cryogenic processes used for methyl chloride condensation are particularly cost-effective because the liquid nitrogen that provides the cooling, once vaporized by the equipment, is then recuperated as a gas by the nitrogen supply network of the respective plant. The nitrogen gets used twice as a result: for cooling and for inerting.