MAN Diesel & Turbo's AIRTRAIN™ and AIRMAX air separation trains cover a broad spectrum. With oxygen production volumes of 500 to over 7000 tpd (tons per day), they are particularly in demand for the production of liquid hydrocarbons (XtL = X-to-Liquid). With the latest order for three AIRMAX machine trains, exactly 102 integrally geared centrifugal compressors for air separation trains have been ordered since 2001.
"China, in particular, is a booming market for air separation plants," states Tamer Bayri, Vice President Sales & Contracts at MAN Diesel & Turbo in Berlin. "They are installed there to produce large volumes of oxygen and are required primarily for the processes of coal liquefaction and gasification." These industrially produced liquid hydrocarbons from coal (known as CtL – Coal-to-Liquid) are helping the country to become more independent. They can be used to manufacture commodity chemicals and fuels, thus reducing reliance on expensive oil imports.
Each train consists of three MAN turbomachinery units – a main air compressor, a centrally located steam turbine drive unit and a downstream air compressor. An integrally geared centrifugal compressor (RG) always serves as the downstream air compressor. It increases the pressure of the pre-pressurized air with the help of up to six compressor stages that are arranged around a single casing with integral gear unit. The benefit is that a single drive is sufficient to operate all stages. The compressed air is then fed to the rectification process. Rectification is a thermal separation process that separates the air into its main components (oxygen, nitrogen and various noble gases).
"The industrial plants for this process are getting bigger and bigger," states Bayri. "This means for one thing that the size of our machinery is growing. And secondly that more trains are needed." As a result the MAN Diesel & Turbo Berlin works, which manufactures the integrally geared centrifugal compressors, has not only had to build ever bigger casings over the past years, it has also had to manufacture an increasing number of machines per order. This has posed new challenges both for the design and production departments.
Dr. Harald Stricker, Vice President Engineering in Berlin, explains: "13 years ago we designed the first integrally geared centrifugal compressors for air separation trains for a power output of 10 to 15 megawatts – today this has risen to 30 megawatts." This has consequences according to Stricker: "You can't just upscale an existing RG. We have had to constantly adapt and develop each component for each individual order." This is the case for all components of an RG – compressor stages, inlet guide vanes and integral gear units. In this process MAN Diesel & Turbo has succeeded in constantly improving efficiency levels.