Compression solutions for air separation

Siemens has been a preferred supplier of pre-engineered and engineered-to-order equipment for large air separation customers around the world for more than 70 years.

While it’s true that air itself is free, producing industrial gases from air requires abundant energy (hence money) so configuring an efficient air compression system is critical to the success of air separation plants.

Centrifugal compressors play a key role in the air separation process. They “suck in” filtered air at atmospheric pressure and compress it to the necessary pressure before it is cooled and separated into nitrogen and oxygen and other inert gases that can then be compressed and fed into a pipeline network for further use. About 78% of air is nitrogen, and 21% is oxygen; the remaining 1% consists of gases like argon, methane and carbon dioxide.

A variety of applications require large volumes of purified oxygen in their production processes. Some of these include chemical and petrochemical production, synthetic liquid fuel production (gas-to-liquids and coal-to-liquids), integrated gasification combined cycle (IGCC) power production, methanol production, steel (and other metals) production and liquefaction of nitrogen for distribution, to name a handful of examples.

Typical uses of nitrogen include enhanced oil recovery (EOR) and food packaging, while argon is important in incandescent lighting and thermal insulation in windows.

Cryogenic air separation

Cryogenic air separation produces highly purified gases or liquids and is the most common process used to extract one or more of these main constituents of ambient air. The process works by taking large volumes of ambient air, then compressing, cooling and liquefying it. The air is then separated into its major components via distillation.

While there are several variations in process details, all large air separation plants use the Joule-Thompson process that requires compressed air. High-powered compressor trains consisting of a main air compressor (MAC), a booster air compressor (BAC) and a steam turbine or electrical motor drive not only provide the necessary gas pressures and flows throughout the reaction, but are one of the most important components in producing oxygen in air separation facilities. These systems must be reliable and efficient to keep customers’ operating costs low.

MACs compress filtered air to the necessary pressure before it is cooled in large cold boxes (pressure vessels that hold a gas or liquid at a very low temperature) and separated into oxygen, nitrogen and argon. While the MAC volume flow determines the output per plant, the BAC outlet pressure defines the purity of the oxygen.

Siemens integrally-geared, vertically-split STC-GV turbocompressor

Siemens has supplied compression equipment to large air separation customers around the world for more than 70 years, beginning with the development of the integrally geared turbocompressors (STC-GV) patented in the 1940s that later became the industry standard for all original equipment manufacturers (OEM).

The tailor-made STC-GV integrally-geared compressor is ideal for both air separation and gas compression because of its high efficiency, reliability and flexibility in process optimization. Last spring, Siemens received an order to provide the largest geared turbo units it has ever built, with a volume flow that exceeds 750,000 Nm³/h, for a gas-toliquids (GTL) facility.

Siemens is the only compressor manufacturer that develops and supplies proprietary gear technology and developed a specific design concept that integrates the MAC and BAC onto one gear, called a combined machine, to operate in smalland meduim-sized ASU plants. Based on this gear technology, these gear-type compressors are also used for process air, CO2, natural gas and NH3 compression in ammonia plants, or for process gases and CO2 or recycling applications in chemical, petrochemical and oil and gas installations.

Two petrochemical customers based in China recently placed orders for air separation unit equipment. One order included three trains consisting of a main and booster air compressor driven by a Siemens steam turbine and the other order was for four trains, each consisting of a MAC, BAC and steam turbine. The engineered-to-order design of our product portfolio, combined with state-of-the-art components and testing were key factors in being selected to ensure reliable production while keeping operating costs low, all while providing an easy-to-maintain compressor solution for anticipated commercial production in 2018.

Design studies indicate that future ASU plants will require even higher volume flows. Today, the STC-GV compressor stands ready to handle projects twice the volume flow of those referenced above.

Also, Siemens can combine the main air and booster compressor in one casing to help customers reduce their overall capital expenditures.

Siemens is capable of manufacturing air separation equipment at several of its facilities around the globe, including Germany and China, and can provide complete solutions for the ASU market, including drive systems, E-houses and modules, and digitalization software.

Tags: Air Separation, STC Compressors

other articles from insights issue 12.2017