Abstract:
The distillation unit includes at least one cryogenic distilling column and at least one heat exchanger, arranged in a chamber with a double casing in which the intermediate partition contains a solid insulator. The unit can be on board offshore floating terminals and drilling platforms.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a cryogenic distillation unit, in particular for distilling air, of the type comprising, on the one hand, distillation equipment including at least one cryogenic distillation column and at least one heat exchanger and, on the other hand, means for thermally insulating this equipment. It applies, for example to oxygen-production units on board offshore oil platforms or barges. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 4,038,060 discloses a cryogenic distillation apparatus for a radioactive gas which prevents dangerous leakage by surrounding the apparatus with a double wall filled with perlite and utilizing an adsorbent within the inner wall for adsorbing radioactive gases in case of an accident. 
     In conventional technology, cryogenic distillation columns and all the associated equipment (heat exchangers, cryogenic pumps, cryogenic valves, connecting pipes, etc.) operating at low temperature are arranged in a “cold box”, generally of parallelepipedal shape, filled with an insulator in loose bulk form such as expanded perlite. This insulator thermally protects each component both from the external temperature and from that of other components which are at a different temperature. This type of material derives its insulating properties from both low thermal conductivity (&lt;0.05 W/m.° C.) and a high head loss, which is favourable in terms of the convection phenomenon. 
     However, this technology has a drawback from the point of view of safety. By way of explanation, if there is a significant leak of gas or cryogenic liquid, for example due to an accidental pipeline break, the large head loss of the perlite hinders discharge of this fluid to the atmosphere. This results in abrupt cooling of the wall of the cold box and the structure supporting it, which may lead to breaks due to embrittlement and spillage of the cryogenic fluid to the surroundings, which is dangerous to the personnel. 
     Offshore oil platforms produce residual gases. For economic and environmental reasons, it is becoming increasingly necessary to recover these gases. One method consists in converting them into heavier hydrocarbons, which are in liquid form and are therefore easier to transport, using the Fischer-Tropsch process, which consumes large amounts of oxygen. 
     It would therefore be beneficial to be able to install an air distillation column on board a platform or a barge, but the safety problem explained above has to be solved because the distillation unit is then fixed on a metal structure and, furthermore, the personnel cannot evacuate quickly from the surroundings of this unit. 
     SUMMARY OF THE INVENTION 
     The object of the invention is therefore to provide a cryogenic distillation unit comprising effective protection against leaks of cryogenic liquid. 
     To this end, the invention relates to a cryogenic distillation unit according to claim  1 . 
     The distillation unit according to the invention may comprise one or more of the following features: 
     the solid insulator located around and above the distillation equipment is an insulator in loose bulk form, in particular expanded perlite; 
     a solid insulator supporting the inner wall of the enclosure is interposed between the floors of the two walls; 
     the distillation column and the heat exchanger are supported by legs which pass with a leaktight seal through the floor of the inner wall and bear on that of the outer wall; 
     at least one shaft is provided for access to auxiliaries of the distillation equipment, such as pumps or valves, this shaft passing through the double wall and being closed off by a removable plug; 
     the heat exchanger, and optionally other internals of the enclosure whose operating temperature is different from that of the distillation column are provided with additional insulation, in particular by lagging with an insulator in sheet or strip form; 
     the inner wall is equipped with a valve for limiting its internal pressure; 
     the enclosure is provided with means for introducing a dry gas, in particular an inert gas, inside the inner wall and/or inside the wall cavity; 
     the wall cavity is equipped with a valve for limiting its internal pressure; 
     the enclosure is fixed to an offshore structure such as a oil platform or a barge; and 
     the enclosure is fixed to a support which is itself fixed to a base of the said structure by means of piles. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An illustrative embodiment of the invention will now be described with reference to the appended drawing, in which: 
     FIG. 1 schematically represents a vertical section of a distillation unit according to the invention; and 
     FIG. 2 is a view taken in section on the line II—II in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The air distillation unit  1  represented in FIGS. 1 and 2 essentially comprises a double enclosure  2 , in the interior  3  of which a double air-distillation column  4 , heat exchangers  5 , connecting pipes  6  and cryogenic accessories such as cryogenic pumps  7 , cryogenic valves  8 , etc. are arranged. 
     The column  4  consists of two distillation columns, respectively a medium pressure column and a low pressure column, as is well known in the art. These two columns  4 A and  4 B are arranged side by side, as can be seen in FIG. 2, but have been shifted to the central plane in FIG. 1 in order to make the drawing clearer. The distillation unit furthermore includes, of course, all the usual accessories and equipment. 
     The enclosure  2  is double walled. The inner wall  9  comprises a flat floor  10 , a cylindrical side  11  and a domed roof  12 , and is made of stainless steel. The outer wall  13  comprises a flat floor  14 , a cylindrical side  15  and a domed roof  16 , and is made of carbon steel. The floor  14  may, however, also be made of stainless steel. 
     The first solid insulator  17 , consisting of blocks of an insulating material such as foam glass, is arranged over the entire surface of the floor  10 , between it and the floor  14 . The second solid insulator  18 , consisting of loose bulk expanded perlite, fills the remainder of the wall cavity of the enclosure, around and above the components of the unit  1  which are arranged in the space  3 , which is free of any solid insulator. 
     The components arranged in the space  3  are carried by legs  19  which pass with a leaktight seal through the floor  10  and bear on the floor  14 . Thus, the insulator  17  supports the inner wall  9 , but it is the floor  14 , and therefore the support  20 , for example consisting of a steel framework, on which this floor is laid, which support the weight of the components of the unit which is located inside the enclosure  2 . In turn, this support  20  is fixed to the supporting structure  21 , for example a metal deck of an oil platform or barge, by means of piles  22  which form an air gap  23  between them. 
     An access shaft  24 , closed off by a removable plug  25 , passes through the double roof  12 ,  16  of the enclosure  2  and leads to those components which require maintenance, for example the pumps  7  and the valves  8 . 
     Each component which operates at a temperature different from that of the column  4  is provided with additional insulation  26  in the form of lagging with glass wool or the like. These are, in particular, the heat exchangers  5 , the pipes  6  connected to the hot end of the latter, and the shaft  24 . The convection phenomena are thus minimized. 
     A pipe  27  for feeding dry nitrogen gas at ambient temperature, which is provided with control valves  28 , makes it possible to maintain a slight overpressure in the space  3  and the wall cavity of the enclosure. This makes it possible, on the one hand, to inert these volumes, and, on the other hand, to prevent the appearance of condensation phenomena in them, which would run the risk of depressurizing them. 
     In order to avoid any risk of excessive overpressure, the roofs  12  and  16  are each equipped with a pressure limit valve  29 . It will be noted that the cylindrical and domed shape of the enclosure is favourable to keeping it under overpressure. 
     In the event of a significant leak of cryogenic liquid, this fluid is held back effectively by the internal wall  9  made of stainless steel, which is a resilient metal, and the gas resulting from possible vaporization of liquid is discharged through the valve  29  in the roof  12 . 
     Furthermore, virtually no cooling effect is encountered outside the outer wall, and correspondingly outside the support  20  or the underlying metal structure  21 . The risks of elements breaking through embrittlement are thus avoided. 
     It will be noted that, by virtue of the presence of the shaft  24 , it is possible to access the parts such as the parts  7  and  8  without having to remove the perlite  18  for each intervention.