Process to compress air and its use in an air separation process and systems using said processes

This application claims the benefit of European Application No. 07111919.2 filed Jul. 6, 2007 and U.S. Provisional Application No. 60/948,649 filed Jul. 9, 2007.

The present invention is directed to a process to compress air and the use of such a process as part of an air separation process. The invention is also directed to systems using said processes to respectively compress air and to separate oxygen from air.

Air separation in an Air Separation Unit (ASU) is frequently applied to obtain a substantially pure or enriched stream of air for use in a combustion process. This combustion may be a complete combustion as applied in the so-called oxy-fuel processes. The combustion may also be a partial combustion of a carbonaceous fuel to obtain a mixture of hydrogen and carbon monoxide. The latter gas mixture is also referred to as synthesis gas and can be used as feedstock in a so-called Fischer Tropsch synthesis to prepare paraffins. Such a route is used in the so-called gas (GTL) or coal to liquid (CTL) process. Such a process is economically interesting when applied at a large scale. The synthesis gas can also be used as fuel in an Integrated Combined Cycle process or in direct ore reduction process.

The following two articles are directed to air separation and GTL:

Ramdohr, M., “Optimize air compressor performance for GTL plants new mechanical advances enable compression solutions for evolving gas-to-liquids processes”, Hydrocarbon Processing, Gulf Publishing Co. Houston, US, vol. 83, no. 1 Jan. 2004 (2004-1) pages 49-51 and Scharle W J et al., “Oxygen facilities for synthetic fuel projects”, November 1981 (1981-11), transactions of the American Society of Mechanical Engineers, series B: Journal of Engineering for Industry, ASME. New York, US, pages 409-417.

EP-A-757217 is directed to an air separation process. In this process air is compressed in a main air compressor (MAC) to a pressure between 5 and 6 bar. The compressed air is further increased in pressure in a series of so-called booster compressors (BAC) to a pressure of 49 bar. Part of the compressed air is let down in pressure and the cold is used to cool the remaining part of the compressed air. The cooled and compressed air is subsequently distilled and oxygen is separated from the other air components.

EP-A-1197717 describes a process to compress air in an air separation unit wherein the main air compressor (MAC) and the booster air compressor (BAC) are driven by a common steam turbine. Between the steam turbine and the booster compressor a gear system is present.

As discussed above there is a desire to increase the scale GTL and CTL processes. This means that the capacity of the air separation unit increases. In a large scale GTL or CTL application an ASU having a capacity of 3600 t/d oxygen is feasible with a combined MAC and BAC driven by a common steam turbine.

When desiring an even larger capacity, e.g. about 5000 t/d oxygen the MAC becomes difficult to design.

The present invention provides a high capacity process to compress air in a single compressor train.

In one embodiment, the invention provides a process to compress air by performing the following steps,

(i) compressing air in a compressor,
(ii) cooling the compressed air as obtained in step (i)
(iii) further compressing the air of step (ii) in a compressor,
(iv) further compressing at least part of the compressed air as obtained in step (iii) in a booster compressor, wherein the compressor in step (i) is an axial compressor, the compressor of step (i) and (iii) are driven by a common first driver and the booster compressor is driven by a separate second driver and wherein the compressors of steps (i) and (iii) are split into two individual casings.

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