FR2858416A1 - Cryogenic liquid analyzer comprises assembly of parallel plates with sawtooth surfaces to resist passage of liquid - Google Patents

Abstract

An assembly of plates is arranged parallel to each other and, between two adjacent plates, sawtooth waves. There are means for sending a liquid through the waves in the direction in which the waves offer the greatest resistance to passage of the liquid, and means of removing the vaporized liquid. Vanes are mounted on the outside surface of the external plates and the plates can be heated. The analyzer also includes means of removing unvaporized liquid. An independent claim is for analysis using the above analyzer, where a contaminated cryogenic liquid is brought to the analyzer and analyzed.

Description

I

  The present invention relates to methods for analyzing cryogenic liquids. Sampling for analysis is necessary for control, monitoring and management of production units.

  In the case of cryogenics, the monitoring of the content of low volatility impurities (typically N20, CO2, CnHm) is an essential point affecting the operation of equipment and safety.

  Indeed, at cryogenic temperatures, some of these impurities are likely to be deposited in the waves of the vaporizer-condenser columns.

  The control of the content is therefore necessary both from the point of view of the quality of the products and the safety of the installations.

  When one is interested in the analysis of low volatility impurities, the difficulty lies in obtaining a vaporized sample as representative as possible of the fluid to be analyzed.

  Indeed, commonly used analysis methods (gas chromatography, IR spectrometry, etc ...) require a warming of the sample at a temperature close to room temperature. For that, it is necessary to vaporize the cryogenic liquid then to reheat it.

  The analysis of a cryogenic liquid therefore consists in conveying to the analyzer a gaseous sample of composition identical to that of the liquid to be analyzed. In order for this sample to be representative of the liquid, the portion taken for the analysis must be rapidly and completely vaporized.

  To do this, it is first necessary to limit the vaporization of the liquid in the transport line to the vaporization zone, then to generate a vaporization front that is schematically as straight as possible so that the pollutants present in the liquid pass entirely into the fraction of the vaporization zone. analyzed gas.

  During a bad vaporization, the vaporization front is at an angle.

  In the vaporization zone, the amount of stagnant liquid should be as small as possible. Indeed, during vaporization of the liquid, the less volatile molecules will tend to concentrate in the liquid.

  The vapor phase will be progressively enriched according to the formula: Y vapor = K * Y liquid with Y vapor concentration in the vapor phase Y liquid concentration in the liquid phase K: liquid-vapor equilibrium coefficient of the molecule considered If the solubility limit is reached, there is enrichment of the liquid until accumulation of crystals in suspension.

  Otherwise, the enrichment in the liquid is stopped when the concentration in the gas phase is equal to that in the liquid phase feeding the vaporization zone.

  In the vaporization zone, the crystals can be randomly entrained (for example, disturbance of the gas withdrawal) in the vapor phase, causing a sudden sublimation thereof and causing instability of the measurement.

  Samples are taken at the level of the vaporizer-condenser of the air separation apparatus, on cryogenic liquids (and more particularly liquid oxygen with regard to safety).

  Two systems mainly used are the capillary outlet and the liquid elevator (the latter is the only device allowing analysis on static liquids).

  * The liquid lift This sampling system is based on the thermosiphon effect: part of the liquid taken off vaporizes thanks to the existing AT between the bath and the hotter capacity because closer to the cold box, bringing with it a quantity of 25 liquid. This liquid, taken continuously and circulating in the elevator, is vaporized in an exchanger.

  The pressure difference between the two taps of the intake is obviously crucial for the circulation of the liquid (hence the presence of a return tapping higher in the bath, to take advantage of the hydrostatic pressure of the liquid).

  * The capillary setting This method is applied to homogeneous liquids, so in general in circulation.

  The liquid is removed by means of a capillary (tube with a diameter of about 0.5 mm) and then fed into a 2x4 mm tube (place of vaporization) to the analyzer.

  The installation and use of this system obviously requires great precision.

  The important points determining the quality of the catch are the representativeness of the sample, the control of the zone of vaporization, even the aging of the equipment.

  The problem of representativeness arises during sampling of static liquids: the systems developed make it possible to take a sample of a liquid fraction in circulation at most equal to 1% of the total liquid.

  Aging mainly affects the liquid elevator: the humidity gradually penetrates into its isolation box causing formation and accumulation of ice. The heat inputs (greater conduction through the ice than through the glass wool) then become such that the circulation of the liquid can be affected.

  This being the case, the capillary catch can also be sensitive to it (blockages at the level of the tube).

  These various problems have prompted the development of new sampling systems.

  As illustrated in Figure 1a) and Figure 1b) the flash is an exchanger consisting of an outer fin tube, followed by a mixing and homogenizing sphere. The mixture is conveyed by an inner tube which touches the walls of the finned tube and ends at the center of the mixing ball.

  This exchanger is a tube of diameter 0 = 25mm, provided with internal and external fins. Different lengths of tubes are used (20 cm, 60 cm, and 1 m).

  The exchanger is placed vertically with the inlet of the liquid at the top to allow it to run along the fins and avoid a cold setting of the exchanger (caused if the entry was made at the foot of the exchanger).

  The design of this exchanger is not optimal for vaporization: the internal fins are present throughout the tube, while the external ones start only about ten centimeters from both ends (at which a fitting had to be soldered to connect the exchanger to the circuit). A cold point is most likely created at the entrance to the exchanger, with accumulation of pollutant.

  An opposite configuration would certainly have been preferable: over a few centimeters, the external fins would have brought the fluid energy from convection, and the drop of liquid would fall in a second time on the network of internal fins to vaporize .

  Figures 2 to 4 show an internal and external fin exchanger used to vaporize a cryogenic liquid.

  The object of the invention is to define an analysis device having a better performance / cost ratio than those of FIGS. 1 to 4.

  According to an object of the invention, there is provided a cryogenic liquid analyzer comprising an assembly of plates arranged parallel to each other and between two adjacent plates; sawtooth waves, means for sending a liquid to the waves in the sense that the directions provide the most resistance to the passage of the liquid and means for withdrawing the vaporized liquid.

  According to other optional features, the analyzer comprises - fins placed on the outer surface of the outer plates.

  - Means for heating the plates.

  means for removing the non-vaporized liquid.

  According to another object of the invention, there is provided a method for analyzing a cryogenic liquid comprising the steps of i) bringing a contaminated cryogenic liquid to an analyzer according to one of

of the preceding claims

  ii) analyze the liquid in the analyzer.

  The wave is a sawtooth wave (serrated), and the passage of the fluid is deliberately in the direction of the wave that provides the most resistance to improve the exchange (passage hardway).

  The invention will be described in more detail with respect to the figures. Figure 5 illustrates an exchanger according to the invention.

  Figures 6,8 and 9 illustrate the exchanger of Figure 5 provided with outer fins.

  Figure 7 illustrates the exchanger of Figure 5 without fins.

  It is very important to maintain a sufficient amount of liquid during transport to the spray system to properly wet the line and avoid any deposits inside it. The configuration of the ASUs suggested a line length of 1 to 2 meter (s) before reaching the exchanger.

  The heat insulation chosen was a combination of two layers of insulating foam tape coupled with an insulating sheath 3.5 cm thick.

  This thickness of insulation reduces the flux exchanged by more than 40%, lowering it to about 15W.m -1.

  The exchanger model of Figure 6,8 and 9 is the one with external fins. The percentage difference between the theoretically generated content and that measured by the analyzers is 12.6% at 0.8L.h-1 and 12.4% at 1L.h-1.

  This difference is satisfactory for the use of the device.

  For this exchanger and the other models hardway wave later tested later, the power supply was made laterally and by a single input 15 to break the current on the wall and distribute as widely as possible the liquid to vaporize. Six inputs are possible at the top of the exchanger.

  The gas outlet is at the base of the exchanger and the purge of non-vaporized liquid.

  It is also possible to use the Hardway single wave heat exchanger without external fins, as shown in FIG. 7.

  The exchanger without external fins of Figure 7 may be heated with heating resistors on the walls or by circulating gas against the current. The exchanger may be provided with heating resistors applied to the walls of the exchanger with Teflon plates.

  The introduction of the heating has brought an improvement in the operation of the device, under the usual operating conditions (Tamb = 1820 C).

  The fins may consist of metal plates about ten centimeters long, placed perpendicular to the surface of the exchanger and spaced several centimeters (Figure 8).

Claims (5)

  A cryogenic liquid analyzer comprising an assembly of plates disposed parallel to one another and between two adjacent plates; sawtooth waves, means for sending a liquid to the waves in the sense that the waves provide the most resistance to the passage of the liquid and means for withdrawing the vaporized liquid.   An analyzer according to claim 1 comprising fins disposed on the outer surface of the outer plates.   3. Analyzer according to claim 1 or 2 comprising means for heating the plates.   4. Analyzer according to one of the preceding claims comprising means for removing the non-vaporized liquid.   A method of analyzing a cryogenic liquid comprising the steps of i) supplying a contaminated cryogenic liquid to an analyzer according to   one of the preceding claims   ii) analyze the liquid in the analyzer.