Abstract:
The invention concerns a method for driving at least a compression machine ( 7, 8 ) of an air distillation unit ( 3 ) which supplies oxygen and/or nitrogen and/or argon to an industrial plant ( 1 ) producing water vapour. In normal running conditions, the compression machine is driven at least partly by a steam turbine ( 13 ) fed with said water vapour, which is input at an input port ( 15 ) of the turbine. The turbine has two input ports ( 15, 16 ) which correspond to different intake pressures. During at least one operating phase of said plant ( 1 ), the turbine is partly supplied with water vapour from an auxiliary water vapour source ( 21 ) and input at the turbine other input port ( 16 ). The invention is useful for supplying air gas to a synthetic hydrocarbon production plant.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method of driving at least one compressor of an air distillation unit which delivers oxygen and/or nitrogen and/or argon to an industrial unit producing steam, this method being of the type in which, in the steady state, the compressor is at least partly driven by steam expansion means with production of external work, said means being supplied with steam coming from said industrial unit, this steam being introduced into an inlet of said expansion means. 
   2. Related Art 
   Certain industrial processes that consume oxygen and/or nitrogen and/or argon, such as synthetic hydrocarbon processes (referred to as gas-to-liquid or GTL processes), are exothermic and generate steam. When the pressure and/or the temperature of this steam make it unusable on the site, the steam is generally utilized as a drive supply, via a steam turbine, of at least one compressor of the air distillation unit that produces oxygen. The steam turbine may be a backpressure turbine, exhausting at a pressure above atmospheric pressure, or a condensing turbine, exhausting at a pressure below atmospheric pressure and associated with a water condenser, cooled by water or by the ambient air, and with a pump for recycling the water back into the steam production boiler. 
   However, the steam is fully available only in the steady state, which poses the problem of starting up the entire plant. 
   EP-A-0 930 268 discloses an air separation apparatus whose main compressor is coupled to an electric motor and a steam turbine that receives steam at two different pressures. During startup, the main compressor and the electric motor operate using electricity generated by a gas turbine. 
   SUMMARY OF THE INVENTION 
   The object of the invention is to solve this problem in a particularly flexible manner, while eliminating the electric motor. 
   For this purpose, the subject of the invention is a method of driving at least one compressor of an air distillation unit that produces oxygen and/or nitrogen and/or argon, this method being of the type in which, in the steady state, the compressor is driven only by steam expansion means with production of external work, this steam being introduced into an inlet of said expansion means, said steam expansion means having two inlets that correspond to different, respectively high and medium, intake pressures and, during at least the startup of said industrial unit, said expansion means are at least partly supplied with auxiliary steam coming from an auxiliary steam supply and introduced into an inlet of these expansion means. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers. 
     Examples of how the invention is implemented will now be described in conjunction with the drawings in which: 
       FIG. 1  shows schematically a combined plant according to the invention; 
       FIGS. 2A to 2C  illustrate three successive startup phases of this plant; and 
       FIGS. 3A and 3B  similarly illustrate the startup of an alternative plant. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   The method according to the invention may have one or more of the following features:
         the auxiliary steam comes from an auxiliary steam supply and is introduced into the other inlet and/or the same inlet of these expansion means;   said operating phase includes the startup phase of the said industrial unit;   as the industrial unit is progressively brought up to the normal operating conditions, the industrial unit being supplied with oxygen and/or nitrogen and/or argon by the air distillation unit, the steam produced by the latter is used to deliver some of the energy for driving the compressor via said expansion means;   in the steady state that turbine means are predominantly supplied with steam coming from said industrial unit;   the inlet corresponding to the high intake pressure of said expansion means is supplied practically permanently;   the auxiliary steam is at the medium intake pressure and the following are supplied in succession:
           the two inlets with the auxiliary steam;   the high-pressure inlet with the steam coming from said industrial unit and the medium-pressure inlet with the auxiliary steam; and   in the steady state, at least the high-pressure inlet with the steam coming from the industrial unit;   
           the auxiliary steam is at the high pressure and the following are supplied in succession:
           the high-pressure inlet with the auxiliary steam;   the high-pressure inlet with the auxiliary steam and the medium-pressure inlet with the steam coming from said industrial unit; and   in the steady state, the two inlets with the steam coming from said industrial unit;   
           the auxiliary steam is at the high pressure and the following are supplied in succession:
           the high-pressure inlet with the auxiliary steam; and   the high-pressure inlet with the auxiliary steam and the medium-pressure inlet with the steam coming from said industrial unit;   
           at least two compressors coupled to a single shaft, namely a main air compressor and another gas compressor, especially an air booster, of the air distillation unit are driven in a similar manner; and   said expansion means comprise a steam turbine having a body provided with two inlets.       

   The subject of the invention is also a combined air distillation/steam production plant of the type comprising, on the one hand, at least one air distillation unit, which comprises at least one compressor driven only by steam expansion means with production of external work, and a cold box containing an air distillation apparatus and a heat exchange line that is designed to cool the compressed air down to a temperature allowing it to be distilled, and, on the other hand, an industrial unit which is optionally supplied with oxygen and/or with nitrogen and/or with argon produced by the air distillation unit and which produces steam, at least some of this steam feeding the steam turbine, said expansion means having two inlets that correspond to two different, respectively high and medium, intake pressures, the plant including an auxiliary steam supply, and this supply delivering steam at a pressure that corresponds to one of the two inlets of said expansion means and being designed to be connected to this inlet, whereas the industrial unit produces steam at a pressure that corresponds to the other inlet of said expansion means and is designed to be introduced into this other inlet. 
   The combined plant shown in  FIG. 1  consists, on the one hand, of a GTL unit  1  that produces, among other things, high-pressure steam in a line  2 , and on the other hand, an air distillation unit  3  that supplies the unit  1  with high-pressure gaseous oxygen HPGOX via a line  4  and also with high-pressure gaseous nitrogen HPGN via a line  5  equipped with a nitrogen compressor  6 . In practice, several units  3  in parallel may be provided. 
   The unit  3  essentially comprises a first compressor, consisting of a main air compressor  7  (or, in an alternative embodiment, several compressors in parallel), a second compressor consisting of an air booster  8  (or, in an alternative embodiment, several boosters in parallel) and a cold box  9 . The latter essentially comprises an air distillation apparatus  10 , for example a double column comprising a medium-pressure distillation column and a low-pressure distillation column that are coupled via a condenser-reboiler, and a heat exchange line  11 . 
   The compressor  7  and the booster  8  are mounted on a single shaft  12  coupled to a steam turbine  13  via a disconnectable coupling device  14 . The turbine  13  has two inlets, namely a high-pressure inlet  15 , located at the intake of the turbine, and a medium-pressure inlet  16 , located between the high-pressure feed and the exhaust of the turbine. Indicated at  17  is the exhaust port of the turbine, at atmospheric pressure or at a pressure above or below atmospheric pressure. 
   The apparatus  10  produces low-pressure liquid oxygen LOX and this liquid oxygen is brought to the high production pressure by a pump  18  before being vaporized and warmed in the exchange line  11 . The apparatus  10  also produces low-pressure and/or medium-pressure gaseous nitrogen GN, which is warmed in the exchange line and then compressed at  6  to the high production pressure. 
   In operation, atmospheric air, compressed to the medium pressure at  7 , is precooled in an air or water precooler  19 , purified of water and of CO 2  in an adsorption-type purifier  20  and separated into two streams, namely a first, medium-pressure, air stream, which is cooled at  11  down to close to its dew point before being distilled at  10 , and a second air stream that is boosted at  8  to a high pressure allowing the high-pressure liquid oxygen in the exchange line to vaporize. 
   The unit  3  also includes an auxiliary boiler  21  that produces medium-pressure auxiliary steam in a line  22 . This line  22 , provided with a valve  23 , is connected to the medium-pressure inlet  16  of the turbine  13 , whereas the line  2  is connected to the high-pressure inlet  15 . 
   To give an example, the high-pressure steam is at approximately 60 bar and the medium-pressure steam is at approximately 15 bar. 
   In addition, a branch line  24  fitted with a valve  25  connects the inlets  15  and  16 . 
   During plant startup, the unit  1  produces no high-pressure steam and then it does produce this in increasing amounts until the nominal steady state is reached. The startup operation comprises three successive phases:
         1st Phase ( FIGS. 1 and 2A ): the turbine  13  is supplied only with medium-pressure steam.       

   However, to balance the upstream expansion part of the turbine, this steam is introduced both into the inlets  15  and  16  via the line  24 , the valve  25  of which is open;
         2nd Phase ( FIGS. 1 and 2B ): the valve  25  is closed. The high-pressure steam, with an increasing flow rate, is introduced via the line  2  into the inlet  15  and the necessary additional energy is delivered by a flow of medium-pressure steam, of decreasing flow rate, introduced into the inlet  16 ; and   3rd Phase ( FIGS. 1 and 2C ): the valve  25  remains closed; when the steady state is reached, the flow of high-pressure steam at the nominal flow rate is introduced into the inlet  15  and drives the compressors  7  and  8 .       

   Optionally, additional medium-pressure steam may be delivered, continuously or periodically, to the inlet  16 , as indicated by the dot-dash line. 
     FIGS. 3A and 3B  relate to the case in which the auxiliary steam is at a pressure above that of the steam produced by the unit  1 . 
   In this case, the first startup phase ( FIGS. 1 and 3A ) consists in introducing the auxiliary steam into the inlet  15 . In the second phase, illustrated by the dot-dash line in  FIG. 3A , the medium-pressure steam is introduced, with an increasing flow rate, into the inlet  16  via the line  2 , while the flow rate of the make-up steam is correspondingly reduced. 
   In the third phase ( FIGS. 1 and 3B ), corresponding to the steady state, the valve  23  is closed. The medium-pressure steam is introduced (nominal flow rate) into the inlet  16 . As previously, it may then be advantageous to provide the branch line  24  with its valve  25  so as also to introduce the medium-pressure steam into the inlet  15  and thus balance the expansion part upstream of the turbine. 
   It may thus be seen that, in both cases, all the steam available in the line  2  is used in the turbine  13  without prior expansion, and therefore without loss of energy. In addition, at any instant, the additional energy for driving the compressors is delivered by the auxiliary steam, the characteristics of which may be chosen relatively freely. 
   In an alternative embodiment, the compressor  6  may be coupled to the shaft  12 , as a replacement of the booster  8  or as a supplement thereof. 
   Also as an alternative embodiment, if an auxiliary oxygen supply is available on the site, for example a liquid oxygen storage tank, it is possible to start up the unit  1  with this oxygen. During startup of the unit  3 , the turbine  13  is then supplied to a minor extent with the steam coming from the unit  1  and to a major extent with the auxiliary steam coming from the supply  21 . The ratio is for example 30%/70%. The proportion of auxiliary steam is then progressively lowered until the steady state is reached, in which it becomes the minor proportion, especially less than 30%, and more preferably still less than 10% or even zero. 
   The turbine may be composed either of a single body with two inputs, which correspond to different intake pressures, or two bodies each having one inlet. In the latter case, one body of the turbine is supplied with steam coming from the industrial unit  1  and the other body with auxiliary steam. The two bodies of the turbine are then either mechanically linked together or mechanically linked to the body of at least one compressor of the air distillation unit. 
   It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above. 
   A process and apparatus is provided for an integrated air distillation unit and industrial unit wherein at least one compressor of the air distillation unit is driven only by steam expansion means during both start-up and steady state operation.