Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2002-03-11
2003-07-29
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S656000, C062S913000, C062S915000
Reexamination Certificate
active
06598424
ABSTRACT:
The invention relates in general to a process and apparatus for separating a gas mixture for example, cryogenically in a separating plant, in which process feed gas is compressed to a first pressure and then introduced into a separation system in order to separate a first product stream.
Whereas the separating plant usually operates in the steady state, the invention is directed to conditions wherein the plant is beset with at least a partial loss of compressor function.
In such separating plants, various compressor systems can be used to compress the feed gas:
a single compressor, for example having a plurality of sequential stages
two or more serially-connected separate compressors
two or more compressors connected in parallel
comprehensive gas compression system which supplies not only the separation plant, but other large consumers (for example pressurized gas system, in particular compressed air network with compressor rail)
The preferred field of application of the invention is a cryogenic air separation process for supplying an integrated power station process (IGCC—Integrated Gasifier Combined Cycle Process). Energy production is served by a gas turbine system which has a gas turbine (gas turbine expander), a gas turbine compressor driven by the gas turbine and a combustion chamber. One or more products of the air separation are used in the energy production system. For example, oxygen produced in the air separator can be used to produce a fuel gas with which the combustion chamber is charged; in this case the oxygen serves in particular as oxidizing agent in coal or heavy oil gasification. Alternatively, or additionally, nitrogen introduced from the air separator into the gas turbine stream can be used by feeding it into the combustion chamber or the gas turbine or mixing it with the gas turbine exhaust gas between combustion chamber and gas turbine of the combustion chamber. In some cases, nitrogen can also be used to transport coal into a synthesis gas plant.
The gas turbine compressor supplies firstly the air required for the combustion process and secondly a portion of the feed air for the air separator (the “first feed gas stream”); it thus represents the “first feed gas compressor” in the context of the invention. In the process of the invention another portion of the feed air for the air separation (the “second feed gas stream”) is compressed by a separate air compressor (the “second feed gas compressor”), which is driven independently of the gas turbine system, for example by means of an electric motor or a steam turbine. The ratio between the first and second feed air stream can in principle have any value. In practice, it is generally 30:70 to 70:30, preferably 40:60 to 60:40.
If the gas turbine fails owing to an operating fault, after a very short time the appropriate feed air rate for the air separator is lacking. Together with the fall in air rate and air pressure, the load of the rectification column(s) of the air separator also falls, the liquid (holdup) drops from the plates or the packings to the bottom and all product purity grades are lost. Hitherto, no process has been known for maintaining the separation process in the air separator after such a fault. Further operation of consumers of the air separator, for example the gasification plant, is only possible temporarily using an external emergency supply involving expensive pressurized tanks and evaporating liquid products.
It is an object of one aspect of the invention, therefore, to provide a process and apparatus for gas separation of preferably the type mentioned at the outset which can operate further in the event of loss of one of the two feed gas compressors. Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
According to one aspect of the invention, in the event of loss or partial loss of the compressor system, a first auxiliary stream which has approximately the composition of a first product stream or approximately the composition of the feed gas is compressed to approximately the first pressure and is recirculated to the separation plant.
In the context of the present invention, the feed gas rate which is lacking owing to a fault in the compressor system is at least partially replaced by an auxiliary stream from a different source. In this manner, the decrease in the rate of feed gas is at least partially compensated and the separation process (for example a rectification) can be maintained. It is then possible to bring the separation plant in a controlled manner into a partial load mode and thus to decrease the auxiliary stream rate, if appropriate to zero.
The designation “approximately” with respect to the composition of the first auxiliary gas denotes a deviation of the proportion of each component of a maximum of 5 mol %, preferably a maximum of 1 mol %. In the specification of pressure, “approximately” permits deviations of the order of magnitude of the pipeline losses and of the flow losses.
The other source from which the auxiliary stream comes can be, for example, the separation plant itself. In this case, in the event of a fault, at least one first product stream of the separation plant is recirculated to the separation plant. Especially when the product or products are produced in any case under pressure, the product stream to be recycled can be taken off downstream of the existing product compressor or from an intermediate stage of the product compressor, if appropriate expanded to the feed gas pressure (the “first pressure”) and fed back to the separation plant, for example to the rectification column or to a rectification column of an air separator.
Thus, although the product rate is also decreased (in the event of total loss of the compressor system the separation plant can under some circumstances initially no longer deliver any product at all), this disadvantage is, however, more than compensated for by the virtually uninterrupted further operation of the separation plant. If there is such a need, the consumers can be supplied by an emergency supply system (backup system) in the period of decreased or absent product delivery. Such a system has, for example, a storage unit in the form of at least one liquid tank and/or a gas pressurized store. The storage unit can be filled from outside the separation plant (for example using tanker vehicles or pipelines) and/or by product produced in usual operating mode of the separation plant. If the storage unit is in any case at a pressure which is at least approximately equal to the “first pressure” (the outlet pressure of the compressor system), there is no requirement for a separate pressure boosting system for recycling the corresponding auxiliary stream to the separation plant.
If such an emergency supply system is present, an emergency supply stream produced there can also be recirculated to the separation plant, instead of to the consumer, alternatively or additionally to one or more product streams from the separation plant.
Preferably, in the event of loss or partial loss of the compressor system, two product streams of different composition are recirculated to the separation plant, for example a nitrogen stream and an oxygen stream in the case of air separation. As a result, it is possible within the scope of modern process control technology to adapt the composition in the recycle stream substantially to the feed gas. In the optimum case, not only the total rate of the recycled product streams, but also their relative rates, are set so that they correspond as far as possible exactly to the missing feed gas rate. Thus the loss of the first feed gas compressor has no direct effect on the separation process—the separation plant experiences virtually nothing from this serious operating fault in the compression of the feed gas. Obviously, the two recycled streams (“auxiliary streams”) can also be formed in part or completely by emergency supply streams.
When a first and a second feed gas compressor are connected in paral
Kunz Christian
Zapp Gerhard
Capossela Ronald
Linde Aktiengesellschaft
Millen White Zelano & Branigan P.C.
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