EP0166655B1 - Process and apparatus for cooling a powder by means of a refrigerant fluid - Google Patents

Abstract

A device for injecting a liquefied refrigerating fluid directly into a mass of detergent powder within a hopper the device being connected to a tank storing this fluid under pressure.

Description

The present invention relates to a method and an installation for cooling, by means of a refrigerant, a powder and in particular a detergent powder.

In the powder manufacturing industries, it is necessary to cool them for various reasons relating both to the particular constitution of the powders and to the requirements posed by the packaging operations. This is for example the case of so-called “economical” washing powders, which work at low temperature (30 to 60 ° C) instead of boiling and to which are added washing agents, surfactants or other specific additives which cannot withstand high conditioning temperatures. Generally, washing powder is produced continuously, at tonnages ranging from 10 to 50 tonnes / h, on a single installation. The raw materials (liquids and solids), after having been mixed and then dried (for example with hot air), are transported, by means of conveyor belts, in the form of a powder having an average particle size of 500 microns. The powder then arrives at the bottom of a vertical or slightly inclined cylinder, while still being at a temperature of 70 to 90 ° C. The powder is then sucked upwards, into this cylinder, with colder ambient air and it arrives at a point located 10 to 40 meters higher, point where it separates from the air by gravity. The powder then falls into one or more receiving hoppers from where it is taken up to receive the specific additives, before being sent to the packaging workshop.

The desired temperature in the detergent hopper is about 25 to 40 ° C. However, as soon as the temperature of the air sucked in at the bottom of the lifting cylinder by air flow is relatively high (in summer in particular), the cooling of the powder produced by heat exchange with this sucked air-turns out to be insufficient. It was therefore necessary to implement additional cooling methods to allow the desired temperature to be reached in the receiving hopper.

According to a known method in which the cooling is carried out by a mechanical group, a refrigerating group is used which cools either the powder alone (by means of a solid / liquid exchanger for example) or the air alone, or again at the same time air and powder. This process has the disadvantage of having a low yield, of being very expensive, unreliable and of not being flexible.

Another method which has been tried is to spray liquid nitrogen at the bottom of the lifting cylinder by air flow. To this end, liquid nitrogen is relaxed and vaporized under very low pressure, at the base of the lifting cylinder and at the inlet of the aspirated air. The liquid nitrogen spraying device can consist of a simple torus pierced with holes directed upwards or by a ramp of liquid nitrogen inlet tubes oriented upwards. The exchange of frigories with the air and the powder sucked up is relatively homogeneous because of the Reynolds number and the height of the lift cylinder with air flow.

The latter method has the disadvantage, however, that both the air and the powder are cooled. Furthermore, any nitrogen leakage, as a result of any shutdown or deficiency of the fan, or any blockage of the liquid nitrogen injection device by fallout of powder agglomerate constitutes a risk for users. . It is therefore imperative to provide security systems.

US patent 4222527 describes a cryo-grinding process in which a particulate product to be ground is first cooled in a cylindrical hopper using liquid nitrogen. This is sent by a cane pierced with openings facing upwards and placed at the base of the hopper. The product thus precooled is then ground under injection of liquid nitrogen, at low temperature.

Such a system has drawbacks which make it inapplicable to its use in the cooling of detergent powders. Placed in a hopper containing a significant thickness of powder, the cane pierced with openings becomes clogged quickly. Furthermore, its shape does not allow homogeneous cooling of the powder.

The present invention aims to remedy these drawbacks by providing a method and an installation of remarkably simple design, of very easy implementation, making it possible to obtain selective cooling of the powder alone with a variable refrigerating contribution, and this, in the best security conditions.

To this end, this method of cooling a powder, in which the powder is poured into a hopper through its upper opening, then cooled inside said hopper before being discharged through a lower opening thereof, this powder forming a mass of predetermined height inside said hopper, the powder being cooled using a refrigerant under pressure in the liquefied state, is characterized in that the fluid is injected in the form of elementary jets in the powder mass using a tubular frame provided with orifices, in particular slots, oriented towards the bottom of the hopper through which said jets exit, pointing towards the axis of the hopper, and in this that said tubular frame is placed in the hopper at a height, from the lower opening of the hopper, which is between half and a third of the height of the mass of powder.

The invention also relates to an installation for cooling, by means of a refrigerant, a powder, in particular a detergent powder comprising a hopper containing said powder, in which the powder falls in the hopper before its distribution towards a post located downstream, comprising, in the powder reception hopper, a device for injecting a liquefied refrigerant, connected to a reservoir of this pressurized fluid, said injection device consisting of a tubular frame of polygonal or circular shape in plan, extending horizontally across the hopper, pierced with orifices, in particular with slots, regularly distributed in the lower part of the wall of said tubular frame, each of these orifices allowing an elementary jet of refrigerant to escape directed downward and towards the vertical axis of the hopper, that is to say towards the zone where the pressure of the powder is the highest, the section and the number of these orifices depending on the desired refrigerant flow, the tubular frame ensuring the injection being placed at a distance from the bottom of the receiving hopper, which is between half and a third of the height of the has mass of powder in the hopper.

The installation also includes a regulation assembly comprising a temperature sensor for reading the temperature of the powder at the outlet of the hopper, and means for controlling the flow of the refrigerant towards the injection device, depending on the powder temperature recorded by the sensor.

The method and the installation according to the invention make it possible to cool the powder in the hopper without modifying its physicochemical characteristics and its particle size, whatever the flow rate of the powder at the outlet of the hopper.

The refrigerant used in the process and the installation according to the invention is chosen so as to be inert with respect to the powder and it can preferably be constituted by liquid nitrogen.

Compared to known cooling methods, the method according to the invention offers the advantage of making it possible to achieve a very significant saving in liquid nitrogen. Furthermore, this process has a great flexibility of implementation because the flow rate of liquid nitrogen can be easily adapted to the conditions of production and in particular to the flow rate of the cooled powder at the outlet of the hopper. The method according to the invention also makes it possible to make an additional saving on the heating energy of the air evacuated at the top of the lifting cylinder with air flow, with a view to using this air for drying, given that the air used for entraining the powder in the lifting cylinder is not cooled. The process does not require the obligation to install an oxygen detection because the cooling is not carried out in the workshops. Finally, the injection of the cryogenic fluid within the powder stored in the receiving hopper causes an inerting effect with respect to this powder, while in the conventional process the nitrogen gas is entrained outside with the air.

• La figure 1 est une vue en coupe verticale schématique d'une installation de refroidissement d'une poudre contenue dans une trémie de réception.
• La figure 2 est une vue en plan d'un dispositif d'injection d'azote liquide dans la poudre.
• La figure 3 est une vue en coupe verticale partielle, faite suivant la ligne III-III de la figure 2, du dispositif d'injection d'azote liquide.

An embodiment of the present invention will be described below, by way of non-limiting example, with reference to the appended drawing in which:

• Figure 1 is a schematic vertical sectional view of an installation for cooling a powder contained in a receiving hopper.
• Figure 2 is a plan view of a device for injecting liquid nitrogen into the powder.
• Figure 3 is a partial vertical sectional view, taken along line III-III of Figure 2, of the liquid nitrogen injection device.

The cooling installation shown in FIG. 1 is intended to cool a mass of powder 1 contained in a reception hopper 2. This reception hopper 2, of frustoconical or pyramidal shape, is located below the upper end of a vertical or slightly inclined lifting cylinder 3, into which the powder is aspirated with ambient air. At the upper end of the air flow lifting cylinder 3, the powder separates from the air flow and falls into the hopper 2, as indicated by the arrow in solid line f, while the air continues to flow. outward movement, as indicated by the arrow f1 in dashes. Thus, in a first phase of the process (phase not necessary, the process according to the invention being able to start in the next phase only), the powder is first of all precooled by the suction air (system called “air-lift” _" ). At the base of the receiving hopper 2, there is a dosing extractor device 4, for example of the rotary valve type, which ensures the distribution, on a conveyor belt, of an appropriate flow of powder 1 cooled to a determined temperature.

According to the invention, the cooling of the powder 1 is carried out within the powder itself, while it is contained in the receiving hopper 2, and this by means of a device for injecting a cryogenic fluid. liquefied, for example liquid nitrogen. This injection device 5 is advantageously constituted by a tubular frame of polygonal or circular shape in plan, extending horizontally across the hopper 2 and which is connected, externally, to a source of liquid nitrogen. The shape of the tubular frame is adapted to the section of the hopper so as to be at a distance from the edges of the hopper sufficient to prevent liquid nitrogen from coming into contact with its walls before vaporization. In a sectional view (fig. 2), the surface B between the edges of the hopper and the tubular frame is substantially equal to the surface A located inside the frame. This tubular frame 5 is pierced with orifices, in particular with slots 6, regularly distributed over its wall. The section and the number of these slots 6 depend on the desired flow rate of liquid nitrogen. These slots generally have a section equal to about half that of the frame, with symmetry in the cutting plane (fig. 3) so as to inject the same amount of nitrogen towards surfaces A and B. (height D / 2 of a slot for a height D of the frame).

Liquid nitrogen is injected under pressure in order to be able to penetrate the powder. The slots 6 are preferably formed in the lower part the inner wall of the tubular frame 5 so that each of these slots 6 lets out an elementary jet of liquid nitrogen directed downwards and towards the vertical axis of the hopper, that is to say towards the zone where the powder pressure is highest.

Preferably, the tubular frame 5 ensuring the injection of liquid nitrogen must be at a height, starting from the bottom of the receiving hopper 2, which is between half and a third of the height of the mass of powder. 1.

The device 5 for injecting liquid nitrogen into the powder mass 1 makes it possible to obtain the desired liquid nitrogen flow rates, while avoiding the "filling or rising of cold liquid or gaseous nitrogen in cracks which are created. in the powder mass 1. The injection device 5 must also be designed to allow good homogeneity of the distribution of the frigories provided, while not clogging, that is to say it must not be blocked by icing due to the 10 to 15% humidity of the powder 1 or by the powder itself.

The injection device 5 is connected, by means of an isolated cryogenic line 7, to a tank (or evaporator) of liquid nitrogen 8 under pressure, due to the height at which the injection is to be carried out liquid nitrogen in the powder 1.

The cooling installation according to the invention also comprises a set for regulating the flow rate of liquid nitrogen injected into the powder 1. This set includes a regulator proper 9 which is connected to a temperature probe 10 engaged in the part lower of the hopper 2, to continuously measure the temperature of the powder 1 at the outlet of this hopper. This regulator in turn acts on one or more valves connected to the cryogenic line 7. In the non-limiting embodiment shown in the drawing, the regulator 9 controls two valves 11 and 12 connected in parallel.

The fact of using two valves 11, 12 makes it possible to carry out regulations of the "all, little or nothing" type. The “all or little” regulation (opening of one of the two valves 11 and 12 or of the two valves at the same time) guarantees a lower diphasic rate, due to a permanent flow of the gas phase in the cryogenic line 7 In addition, the jolts and overpressures in the powder 1 are limited, resulting in a very strong reduction in the surges of “fines and the overpressures exerted on the extractor dosing device 4. This surge of fines is also avoided due to the frustoconical pyramidal shape of the hopper, which allows a decrease in the speed of the gas during its ascent, taking into account the increase in surface of the hopper upwards.

It can be seen from the above description that the cooling is carried out, by direct injection of liquid nitrogen, in a place where only the powder 1 is present, the temperature of this powder having already been lowered beforehand as a result of its passage through the air-flow lifting cylinder 3. This direct injection of liquid nitrogen, via the injection device 5, within the powder mass 1 allows regulation of the cooling power over a very wide range.

The comparative results obtained by injection of a cryogenic fluid at the base of the lifting cylinder by air flow, according to the conventional process and by injection of this fluid in the upper part, will be given by way of example by way of example. in the receiving hopper 2 located 30 meters above the ground. For a fixed powder flow rate of 30,000 kg / h, a powder temperature in the lower part of 80 ° C, air in the lower part of 28 ° C and a desired powder temperature of 35 ° C, a specific heat powder of 0.33 kcal / kg and a specific heat of the air of 0.24 kcal / kg is consumed, according to the conventional process consisting in injecting liquid nitrogen at the base of the lift cylinder with air flow , an hourly flow rate of 4,111 I / h whereas with the process according to the invention, in which the injection of liquid nitrogen takes place at the top, in the hopper 2, the hourly flow rate is only 1,739 I / h. In other words, the process according to the invention makes it possible to reduce by around 58% the hourly consumption of liquid nitrogen necessary to obtain, at the outlet of the receiving hopper 2, a powder having the desired temperature of 35 ° C.

Although the application of the process and the installation according to the invention to the cooling of a detergent powder has been described in the foregoing, before the packaging thereof, it goes without saying that they can also be used in other industrial fields, for example for the packaging of dye powders, in other words each time the temperature of the powder must be lowered to a desired value.

Claims (9)

1. Process for cooling a powder (1), in which the powder is poured into a hopper (2) via its upper opening, then cooled inside the said hopper before being discharged via a lower opening thereof, this powder forming a mass of predetermined height within the said hopper, the powder being cooled by means of a refrigerant fluid under pressure in the liquefied state, characterised in that the fluid is injected in the form of elementary jets into the mass of powder by means of tubular framework provided with orifices, particularly slots (6) facing towards the bottom of the hopper, through which emerge the said jets directed towards the axis of the hopper, and in that the said tubular framework is positioned in the hopper as a height from the lower opening of the hopper, which is comprised between a half and a third of the height of the mass of powder.

2. Process according to claim 1, characterised in that the edges of the tubular framework are placed at a sufficient distance from the walls of the hopper to prevent liquid nitrogen from coming into contact with these prior to vapourisation.

3. Process according to any one of the preceding claims, characterised in that a fluid which is inert regarding the powder, and preferably liquid nitrogen, is used as the liquefied refrigerant fluid.

4. Plant for the cooling, by means of a refrigerant fluid, of a powder (1), in particular of a detergent powder, comprising a hopper (2) containing the said powder, in which the powder falls into the hopper prior to its distribution in the direction towards a station situated downstream comprising within the hopper (2) for reception of the powder (1), a device (5) for injection of a liquefied refrigerant fluid, connected to a reservoir (8) of this fluid under pressure, characterised in that the said injection device (5) is constituted by a tubular framework of polygonal or circular shape in plan view, extending horizontally across the hopper (2) pierced by orifices, particularly slots (6), distributed evenly in the lower part of the wall of the said tubular framework (5), each of these orifices allowing a discharge of an elementary jet of refrigerant fluid directed downwards and towards the vertical axis of the hopper, that is to say towards the section in which the pressure of the powder is a maximum, the cross-section and the number of these orifices depending on the required flow rate of refrigerant fluid, the tubular framework (5) providing the injecting being positioned at a distance from the base of the reception hopper (2) which is comprised between half and a third of the height of the mass of powder (1) in the hopper.

5. Plant according to claim 4, characterised in that it equally comprises a governing assembly comprising a temperature sensor (10) for detecting the temperature of the powder (1) at the outlet of the hopper (2) and means (9, 11, 12) of controlling the flow rate of the refrigerant fluid in the direction towards the injection device, as a function of the temperature of the powder (1) detected by the sensor (10).

6. Plant according to claim 5, characterised in that the means of controlling the flow rate of refrigerant fluid comprise a regulator (9) acting on two valves (11, 12) connected in parallel.

7. Plant according to one of the claims 4 to 6, characterised in that the hopper (2) is preceded by suction means for the powder, which draw the powder by suction from the bottom towards the top of the hopper by means of an air flow which effects a preliminary cooling of this powder.

8. Plant according to one of the preceding claims, characterised in that the geometry of the tubular framework (5) with respect to the corresponding edges of the hopper (2) is such that the area (B) between the edges of the hopper (2) and the tubular framework (5) is substantially equal to the area (A) located within this framework (5).

9. Plant according to claim 8, characterised in that the orifices of the tubular framework (5) have a cross-section allowing the same quantity of nitrogen to be injected towards the areas (A) and (B).

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