Process and device for regulating a flow of liquid CO.sub.2, and application thereof in a cooling tunnel

According to the process, there is maintained in a conduit (3), on the downstream side of a valve (5), up to the vicinity of the point A of injection of the CO.sub.2, an intermediate pressure (PI) higher than the pressure (PT) of the triple point of the CO.sub.2. Application in the treatment of waste waters or the deep freezing of food.

The present invention relates to a process and device for regulating a flow 
of liquid CO.sub.2 in a thermally-insulated conduit provided with a valve 
controlled in a continuous manner. 
CO.sub.2 (carbon dioxide) is employed for many processes in the industrial 
field (for example: carbonatation in chemistry, neutralization of basic 
agents and adjustment of pH in the treatment of water), and in the 
agricultural food field (for example: rapid cooling and deep freezing, 
control of temperature). 
In these applications, bearing in mind the needs, the CO.sub.2 is most 
often delivered in the liquefied form and stored in this form in a 
reservoir. 
The characteristics of the processes require the possibility of adapting 
the flow of CO.sub.2 to the charge to be treated; thus it is necessary to 
regulate the flow of CO.sub.2 as a function of parameters characteristic 
of the process: measurement of the pH in the treatment of water, 
measurement of the temperature in cryogenic treatments. 
The regulating method which is theoretically the most precise and the 
cheapest as concerns the consumption of CO.sub.2 is that consisting in 
continuously regulating the flow of liquid CO.sub.2 by means of a 
controlled valve having a variable opening controlled by a regulator 
having a derived, integral and proportional action. The principle of such 
a valve is to present a restriction to the flow of the liquid. The section 
of this restriction is adjusted by means of a closure member which moves 
in a continuous manner between two end positions under the effect of 
electric or pneumatic power. 
The CO.sub.2 arrives on the upstream side of this valve at a pressure close 
to that of the reservoir, namely 11 to 60 bars as the case may be. The 
restriction of the section results, according to the laws of flow of 
fluids, in a loss of pressure which increases with decrease in the section 
of the passage defined by the closure member. When temporarily the 
operation of the process is such that the need of CO.sub.2 is minimum, the 
valve takes up a position close to its complete closure. The restriction 
of the section is then maximum and the pressure drop across the valve is 
sufficiently large to ensure that the pressure of the CO.sub.2 on the 
downstream side of the valve assumes values lower than 5.2 bars. 
This value of 5.2 bars corresponds to the pressure of the triple point of 
CO.sub.2 which is a value within which the liquid CO is instantaneously 
converted into a mixture of gaseous CO.sub.2 and solid CO.sub.2 (carbon 
dioxide snow). 
Now, the constructional characteristics of regulating valves employed for 
these processes are such that the small diameter and the tortuous shape of 
the piping immediately on the downstream side of the closure member result 
in an immediate clogging as soon as the carbon dioxide snow appears. 
Consequently, in practice, these regulating valves are only rarely of 
utility for the regulation of a flow of liquid CO.sub.2, and the solutions 
usually adopted employ other techniques: the rather imprecise open-closed 
regulation or, when the application does not require liquid CO.sub.2, the 
use of a vaporizer on the upstream side of the regulating valve, which is 
an expensive technique as concerns investment and power expenditure. 
An object of the invention is to permit the use of a valve which is 
controlled in a continuous manner in all cases. 
The invention therefore provides a process comprising maintaining in the 
conduit, on the downstream side of the valve up to the vicinity of the 
point of injection of the CO.sub.2, an intermediate pressure higher than 
the pressure of the triple point of the CO.sub.2. 
Preferably, before connecting the conduit to the reservoir of liquid 
CO.sub.2, there is injected into this conduit, on the upstream and 
downstream sides of the valve, gaseous CO.sub.2 at a pressure between said 
pressure of the triple point and said intermediate pressure. 
The invention also provides a device for carrying out said process. In this 
device, the conduit comprises, on the downstream side of the valve, a 
conduit section leading to a discharger. 
According to advantageous features: 
the discharger (4) comprises an outlet orifice on the axis of its closure 
member; 
a pipe having substantially the same diameter as the outlet orifice of the 
discharger extends from this orifice to the point of injection of the 
CO.sub.2 
The invention also provides a cooling tunnel comprising a plurality of 
points of injection of CO.sub.2 and a device such as that defined 
hereinbefore in which said conduit section includes a plurality of 
branches each leading to a discharger, each discharger being disposed at 
one of said points of injection. The dischargers may be adjusted at 
different opening pressures.

The regulating device shown in FIG. 1 is adapted to deliver a variable flow 
of CO.sub.2 at an injection point A from a storage reservoir 2 in which 
there is maintained a pressure PS which is distinctly higher than the 
pressure PT of the triple point of the CO.sub.2 (5.2 bars), and generally 
between 11 and 60 bars. The point A is at a given pressure PO, for example 
substantially equal to atmospheric pressure, but in any case lower than 
PT. 
The regulating device 1 comprises a thermally-insulated liquid conduit 3 
leading from the lower part of the reservoir 2 to a discharger 4. A 
controlled valve 5 is inserted in this conduit and defines in the latter 
an upstream section 6, from the reservoir to the valve, and a downstream 
section 7, from the valve to the discharger. This valve comprises a 
closure member whose position may vary in a continuous manner between a 
position of maximum opening and a position of total closure of the valve, 
by the action of a motor 8. The latter is controlled by a control 9 which 
receives from a measuring instrument 10 (for example a pH meter or a 
thermometer) a signal which represents the control magnitude. 
The discharger 4 (FIG. 2) comprises a housing 11 divided into two chambers 
by a membrane 12. A coil spring 13, whose force is adjustable by means of 
a screw 14, is disposed in one of these chambers, while the other chamber 
(the lower chamber as viewed in FIG. 2) receives the fluid contained in 
the conduit section 7. In this other chamber, a closure member rod 15 is 
connected to the membrane and terminates in a closure member 16 
cooperative with a seat 17 located at the entrance of the outlet orifice 
18 of the discharger. The elements 13 to 18 are all coaxial. 
Consequently, the closure member 16 is lifted from its seat if, and only 
if, the pressure prevailing in the lower chamber of the discharger exceeds 
the pressure corresponding to the force of the spring 13. It is therefore 
possible to adjust the screw 14 in such manner that this opening occurs 
when the pressure in the section 7 is at least equal to an intermediate 
pressure PI higher than the pressure PT. 
A gaseous CO.sub.2 conduit 19 leads from the upper part of the reservoir 2 
and includes, from the upstream end to the downstream end, a stop valve 20 
and a pressure reducing valve 21. The latter delivers on the downstream 
side a pressure P2 higher than PT but lower than PI. On the downstream 
side of the pressure reducer 21, the conduit 19 is divided into two 
branches 22 and 23 which respectively lead to the sections 6 and 7 
respectively of the conduit 3. Each branch is provided with a check-valve 
24 which allows the flow of fluid only in the direction from the pressure 
reducer 21 to the conduit 3. 
In operation, the device is first of all set at pressure P2 by opening the 
valve 20. In this way, it is guaranteed that the pressure will not drop 
below the triple point of the CO.sub.2 at any point of the device. 
Then, by means of an initiating control (not shown), the liquid CO.sub.2 is 
admitted into the conduit 3. The discharger 4 is opened when the pressure 
in the section 7 is higher than the value PI and a jet of carbon dioxide 
snow then issues from the orifice 18. The discharge of this snow occurs 
with no hindrance owing to the location of the orifice 18 on the axis of 
the membrane-closure member system. 
In a variant (FIG. 3), in the case where the carbon dioxide snow needs to 
be conducted in a short portion of the piping before reaching the 
injection point A, there is connected to the orifice 18 a pipe 25 which 
has no internal asperity or pronounced bend. The inside diameter of the 
pipe 25 is throughout its length substantially equal to the inside 
diameter of the orifice 18 and offers no restriction of section to the 
flow of the gas-solid mixture. 
An application of the variant shown in FIG. 3 is diagrammatically 
illustrated in FIG. 4. It concerns the regulation, by means of a 
measurement of the pH, of a flow of liquid CO.sub.2 injected into in a 
waste water duct 26 so as to neutralize a basic effluent therein. 
The pipe 25 opens into a venturi 27 adapted to inject and spray the carbon 
dioxide snow into the stream of water. 
FIG. 5 shows a variant of the device of FIG. 1 in which the conduit section 
7 is divided into three branches 7A to 7C each leading to a respective 
discharger 4A to 4C. This permits delivering the CO.sub.2 at a plurality 
of injection points and, by adopting different pressure settings for each 
discharger, injecting individually adjustable flows of CO.sub.2 for each 
injection point. 
This possibility is of particular interest for, for example, producing more 
or less cold regions in a longitudinal tunnel for deep freezing food 
products, as illustrated in FIG. 5. 
In this application, the valve 5 is regulated by means of a single 
measurement of temperature taken close to the outlet of the tunnel. The 
CO.sub.2 is injected in parallel through the dischargers 4A to 4C so as to 
distribute the freezing product throughout the length of the tunnel 28. In 
a permanent operation mode, the discharger 4A located adjacent to the 
entry 29 of the products to be treated, fed by a conveyor belt 30, 
produces a flow of CO.sub.2 larger than the others owing to its adjustment 
at a lower pressure PI-A. Further, when the installation operates at close 
to its minimum output (standby position between two treating stages), this 
first discharger could be the sole discharger to operate. 
The arrangement shown in FIG. 5 permits obtaining in a simple and cheap 
manner a reliable and precise regulation of the deep freezing procedure. 
The invention finds application in many other processes consuming CO.sub.2. 
It is particularly well adapted in applications requiring a considerable 
flow of CO.sub.2 (at least 100 kg/h), delivered in a quasi-continuous 
manner and at a rate which is variable in a ratio of about 1 to 5.