Process and plant for producing a variable gas output

Details Process and plant for producing a variable gas output Process and plant for producing a variable gas output

1999-10-08

2001-08-14

Spitzer, Robert H. (Department: 1724)

Gas separation: processes

With control responsive to sensed condition

Concentration sensed

C095S022000, C095S023000, C095S045000, C095S130000, C096S004000, C096S111000, C096S113000

Reexamination Certificate

active

06273936

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a process for separating a compressed feed gas, especially compressed air, for the purpose of delivering to a user a variable output of a gas product, especially nitrogen, by means of a variable-capacity separation apparatus fed by a compression apparatus.
The invention applies especially to the production of nitrogen from atmospheric air by distillation or by permeation.
BACKGROUND OF THE INVENTION
The use of variable-output air separation apparatuses is tending to increase, because of the advantages of standardization that they afford: the design costs are amortized over more apparatuses, the grouping of the procurements makes it possible to reduce their cost, the mass-production effect decreases the unit manufacturing cost of the apparatuses, it is possible to stock the apparatuses in order to reduce the delivery time to customers since it is known that they can meet a wide range of demands, the maintenance operations are simplified and the stock of spare parts reduced, etc.
However, variable-output separation apparatuses pose problems when adapting the apparatus itself and its feed compressor to variable operating conditions, so as to maintain for as long as possible the separation performance and the specific energy consumption.
Thus, it is known to control small air distillation apparatuses on an on/off basis: they stop when the nitrogen pressure in the user network increases up to a predetermined high threshold and restart when this pressure comes back down to a predetermined low threshold.
The drawback of this method is that the distillation apparatus takes a great deal of time to restart and, throughout the duration of restarting its compressor is consuming its nominal energy with no nitrogen being produced. If it is desired to reduce the number of stop/restart cycles, a buffer tank of large volume must be installed.
It is also known to vent the excess nitrogen output. The specific energy is then, of course, increased accordingly.
SUMMARY OF THE INVENTION
The object of the invention is to allow satisfactory operation of the gas separation apparatuses over a relatively wide range of variation of the output produced with respect to the nominal output.
For this purpose, the invention proposes a process in which:
only a portion of the flow of feed gas is compressed by means of at least a first, variable-rotation-speed compressor (
2
), a predetermined flow of this gas being compressed by at least a second, fixed-rotation-speed compressor (
2
A) connected in parallel with the first compressor (
2
);
at least one parameter representative of the gas output demanded by the user is measured; and
the speed of rotation of the first compressor (
2
) is controlled according to this measurement.
The process according to the invention may furthermore include one or more of the following characteristics, taken separately or in any of their technically possible combinations:
the portion of the flow of feed gas delivered by the first compressor covers the entire range of capacity variation of the separation apparatus;
the parameter is the pressure and/or the flow rate of the feed gas or of the gas product, and/or the purity of the gas product;
the output of a residual gas from the separation apparatus or the output of the gas product is controlled according to the speed of rotation of the compressor;
The subject of the invention is also a plant intended for the implementation of the process defined above.
This plant, of the type comprising a variable-capacity separation apparatus and an apparatus for compressing the feed gas, the delivery side of which is connected to the inlet of this apparatus, is characterized in that:
the compression apparatus (
2
;
2
A) comprises, in parallel, at least a first, variable-rotation-speed compressor (
2
), provided with a variable-speed drive (
9
), and at least a second, fixed-rotation-speed compressor (
2
A);
and in that it comprises means (
8
) for measuring at least one parameter representative of the gas output demanded by the user, which means are connected to means (
10
) for controlling the variable-speed drive (
9
).
The subject of the invention is also a range of gas separation plants, characterized in that it comprises:
a common separation apparatus, at least one common variable-speed compressor and at least one fixed-speed compressor; and
means for driving the compressor at different speeds depending on the plant in the range.
These means may comprise a common motor and a range of drives having different ratios depending on the plant in the range, or a common variable-speed motor controlled by an electronic variable-speed drive.


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