Process and installation for supplying nitrogen to an apparatus

As the apparatus consumes liquid nitrogen and discharges cold gaseous nitrogen under low pressure, an additional nitrogen stream is liquified under high pressure by heat exchange with the gaseous nitrogen discharged by the apparatus 1, the liquid thus obtained is expanded to the pressure of utilization and the expanded liquid is added to the principal supply of liquid nitrogen to the apparatus. Application in the supply of nitrogen to large cryogenic crushing apparatus.

The present invention relates to a process and installation for supplying 
nitrogen to an apparatus which consumes liquid nitrogen and discharges 
gaseous nitrogen at low temperature and low pressure. 
A number of apparatus exist which employ the heat of vaporization of 
nitrogen for cooling a charge and which discharge very cold gaseous 
nitrogen whose freezing power cannot be made use of on the spot and is 
lost. 
Thus, in a cryogenic crushing operation, the liquid nitrogen is employed 
for two purposes: on one hand for cooling the charge to be crushed in the 
screw or the supply hopper, from ambient temperature to the crushing 
temperature which is generally on the order of -140.degree. to 
-150.degree. C. and on the other hand for maintaining this temperature in 
the crusher by evacuation of the dissipated energy. 
The quantity of cold required for the cooling of the charge depends on the 
product Cp .DELTA. T, in which Cp is the specific heat of the product and 
.DELTA. T the temperature difference, whereas the energy dissipated in the 
crusher depends on the aptitude of the product to be crushed, the desired 
particle size and the performances of the crusher. 
In favourable cases, these two freezing needs are in equilibrium so that if 
liquid nitrogen is introduced into the crusher, the cold gaseous nitrogen 
generated in the region of the crusher can serve to pre-cool the charge, 
which results in an optimum recovery of the negative calories. 
On the other hand, unfavourable cases exist, when the products have a high 
crushing energy, as for example certain elastomers, and when small 
particle sizes (below about 150 microns) are envisaged. 
In such cases, the consumptions of liquid nitrogen are very high in the 
crusher and the quantity of cold nitrogen generated in the crusher is very 
greatly in excess relative to that necessary for the pre-cooling of the 
charge; these negative calories cannot be recovered by the process and are 
discharged to the atmosphere. 
An object of the invention is to provide a process and installation for 
valorizing these negative calories discharged at low temperature. 
The invention therefore provides a process comprising liquefying a stream 
of additional nitrogen at high pressure by heat exchange with the gaseous 
nitrogen discharged by the apparatus, expanding the liquid thus obtained 
to the supply pressure of the apparatus, and adding the expanded liquid to 
the principal supply of liquid nitrogen to the apparatus. 
In one manner of carrying out the invention, the principal supply of liquid 
nitrogen is effected by an air-separating installation provided with a 
nitrogen liquefaction cycle, and the additional nitrogen is taken from the 
high pressure outlet of the cycle compressor. 
In another manner of carrying out the invention, the principal supply of 
liquid nitrogen is effected by a storage of liquid nitrogen and the 
additional nitrogen is taken from a conduit distributing nitrogen under 
pressure. 
The invention also provides an installation for carrying out the process 
described hereinbefore. This installation is of the type comprising a 
source of liquid nitrogen and a conduit for the principal supply of liquid 
nitrogen to the apparatus extending from said source to said apparatus and 
the installation further comprises a conduit for supplying additional 
gaseous nitrogen at high pressure, a heat exchanger for liquefying the 
additional gaseous nitrogen by heat exchange with the gaseous nitrogen 
discharged from the apparatus, means for expanding the thus liquefied 
nitrogen to the supply pressure of the apparatus, a phase separator 
receiving the expanded liquid nitrogen, and a conduit for supplying liquid 
collected in said phase separator to the apparatus.

The installation shown in FIG. 1 is adapted to supply nitrogen, and 
essentially liquid nitrogen, to an apparatus 1 (for example a cryogenic 
crushing apparatus) which, after having consumed liquid nitrogen, 
discharges gaseous nitrogen at low temperature, for example between 
-140.degree. and -196.degree. C., through a conduit 2. This installation 
essentially comprises an air separating installation 3, for example a 
double air distillation column, located in proximity to the apparatus 1, 
for example at a few tens or a few hundreds of meters from the latter, and 
producing gaseous nitrogen through a conduit 4, a nitrogen liquefaction 
cycle 5 associated with the installation 3, and an auxiliary liquefier 6 
disposed in the immediate vicinity of the apparatus 1. In this embodiment, 
the liquid nitrogen supplied to the apparatus 1 and the discharge nitrogen 
are at roughly atmospheric pressure. 
The liquefaction cycle 5 is conventional and comprises a three-stage cycle 
compressor 7 whose intake pressures are respectively about 1 bar, 6 bars 
and 20 bars (in absolute values) and whose outlet pressures are about 6 
bars, 20 bars and 50 bars, a heat exchanger 8 having three series of 
passages 9 to 11 in mutual thermal exchange relation to one another, a 
refrigerating unit 12, an expansion turbine 13 and two phase separators 14 
and 15 connected in series. 
A principal liquid nitrogen supply conduit 16, insulated under a vacuum, 
leads from the bottom of the final separator 15 to the liquid nitrogen 
inlet 17 of the apparatus 1, through an intermediate storage vessel 16A. 
There may lead from this storage vessel another conduit 16B adapted to 
supply liquid nitrogen to other users, for example for the filling of tank 
trucks. An additional gaseous nitrogen supply conduit 18 leads from the 
high pressure outlet of the compressor 7. 
The liquefier 7 comprises a heat exchanger 19 having three series of 
passages 20 to 22 and a phase separator 23. 
The nitrogen produced by the installation 3 is conducted, in the 
neighbourhood of ambient temperature, through the conduit 4 to the inlet 
of the second stage of the compressor 7. 
A part of the high pressure nitrogen issuing from the third stage of the 
compressor enters the passages 9 of the exchanger 8, emerges therefrom, is 
cooled by the refrigerating unit 12, again enters the passages 9 and a 
part thereof is cooled to the temperature of the cool end of the exchanger 
8, expanded to about 6 bars in an expansion valve 24 and divided into two 
phases by the separator 14. The liquid collected in the latter is again 
expanded to a pressure a little higher than atmospheric pressure, in an 
expansion valve 25, divided into two phases in the final separator 15 and 
it is the liquid collected in the latter which enters the conduit 16 and 
constitutes the principal liquid nitrogen supply to the apparatus 1. 
A part of the high pressure nitrogen circulating in the passages 9 is 
withdrawn from the exchanger 8, expanded to 6 bars in the turbine 13 and 
combined with the flash gas issuing from the separator 14 to provide a 
first cooling stream flowing in counter-current manner to the high 
pressure nitrogen in the passages 10, before returning to the inlet of the 
second stage of the compressor 7. The low pressure flash gas issuing from 
the separator 15 is conducted into the passages 11 in counter-current 
manner to the high pressure nitrogen and then returned to the inlet of the 
first stage of the compressor 7. 
The high pressure gaseous nitrogen taken off through the conduit 18 enters 
the passages 20 of the exchanger 19 and emerges therefrom in the liquefied 
state supercooled to roughly the temperature of the cold nitrogen 
discharged by the apparatus 1, is then expanded to a pressure slightly 
higher than atmospheric pressure in an expansion valve 26, and then 
divided into two phases in the separator 23. The liquid collected in the 
latter is conveyed through a conduit 27 to the inlet 17 of the apparatus 1 
and constitutes an additional liquid nitrogen supply for this apparatus. 
The flash gas of the separator 23 is conveyed in a counter-current manner 
in the passages 21 of the exchanger 19, and the conduit 2 is connected to 
the cold inlet of the passages 22. After reheating, the flash gas is 
discharged and the rejected nitrogen is also discharged generally by means 
of an extractor-fan 28. As a modification, these two gases may be combined 
and conveyed to the apparatus 1, for example for inert rendering purposes, 
as indicated in dot-dash lines, or the rejected nitrogen alone, which 
constitutes the principal flow, may be conveyed to the apparatus 1. 
Thus, the addition to a conventional installation having only the 
installation 3 and the cycle 5, of the conduit 18 at ambient temperature 
and the heat exchanger 19, which is of modest dimensions, permits, for a 
low investment cost, the recovery of the refrigerating power at low 
temperature by creation of an additional supply of liquid nitrogen. 
Furthermore, for reducing losses of nitrogen by flashing in the exchanger 
19, the high pressure liquid nitrogen issuing from the passages 20 may be 
expanded, as shown in FIG. 2, in two stages by inserting a high 
pressure-medium pressure expansion valve 29 and an intermediate phase 
separator 30 between the outlet of the passages 20 and the valve 26. The 
flash gas of the separator 30 at roughly 6 bars is then reheated in 
supplementary passages 31 of the exchanger 19, then returned through a 
conduit 32 to the inlet of the second stage of the compressor 7 of FIG. 1. 
The installation shown in FIG. 3 differs from that of FIG. 1 only in the 
origin of the principal stream of liquid nitrogen supplied to the conduit 
16 and of the high pressure gaseous nitrogen stream entering the passages 
20 of the exchanger. Indeed, when the air-separating installation is too 
remote from the apparatus 1, it is preferable to supply liquid nitrogen to 
the conduit 16 from a storage of liquid nitrogen 33 disposed in the 
immediate vicinity of the apparatus 1 and itself supplied by tank trucks, 
while the additional gaseous nitrogen is supplied either in the manner 
shown in FIG. 1 or by the connection of a conduit 18A to a conduit 18B 
distributing gaseous nitrogen at high pressure extending in proximity to 
the apparatus 1. 
In order to increase the yield in the recovery of negative calories, it may 
be envisaged to provide the conduit 18 or 18A with a booster which brings 
the high pressure nitrogen to a pressure higher than that of the cycle 5 
or of the conduit 18B, for example on the order of 80 bars, and/or to 
operate the apparatus 1, for example the cryogenic crusher, at a 
temperature which is lower than that strictly necessary for the process it 
carries out.