WO2001068230A1

WO2001068230A1 - Pellet-freezing device

Info

Publication number: WO2001068230A1
Application number: PCT/EP2001/003485
Authority: WO
Inventors: Pascal Schvester
Original assignee: L'air Liquide, Societe Anonyme Pour L'etude Et L'exploitation Des Procedes Georges Claude
Priority date: 2000-03-15
Filing date: 2001-03-13
Publication date: 2001-09-20
Also published as: DE10012551B4; DE10012551A1; AU2001254745A1; AR027661A1

Abstract

The invention relates to a device for the pellet-freezing of liquid or flowable substances in a flow of coolant, in particular comprising liquefied gas, proposing a separating device (3) which has a coolant/pellet discharge conduit (4) and a pellet-collection conduit (5) which is arranged downstream in the direction of flow, the discharge conduit (4) being adapted to the flow of coolant in such a way that the pellets are only slightly immersed, and the collection conduit (5) being arranged at a distance a from the discharge conduit (4) which is such that the pellets (12) can be collected separately from the coolant.

Description

The invention relates to a device for the pellet-freezing or granulation of liquid or flowable substances in a liquid coolant, in particular a cryogenic liquid such as a cold-liquefied gas.

Devices of this nature are used, for example, in the food sector to process liquid or pasty ingredients to form frozen granules (pellets) . For this purpose, the liquid substance is introduced in drops into a flow of coolant inside a thermally insulated vessel, remains in the flow of coolant over a certain flow path and therefore freezing duration, and then the frozen grains of substance are removed from the flow of coolant by means of a separating device.

EP 0,919,279 Al describes a pellet-freezing device in which, to remove the grains of substance from the coolant, a revolving, liquid-permeable conveyor belt is provided, onto which the flow of coolant containing the frozen grains is guided, and which conveys the pellets beyond an opening in the pelletizing vessel to a collection container. The permeable belt material used is an endless belt which is in the form of a mesh or is perforated. This device has the drawback that the conveyor belt not only conveys grains, but also transmits heat from the outside, which is at ambient temperature, into the pelletizing vessel in which extremely low temperatures prevail. This is the case in particular if the belt is made from a metal such as stainless steel, since metals have a high heat absorption capacity and a good thermal conductivity. The resultant heating of the vessel interior leads to an increased coolant consumption and therefore higher production costs. Moreover, there is a risk of the conveyor belt icing up as a result of moisture which is absorbed from the outside. Such a layer of ice on the belt can lead to pellets sliding or being thrown off the conveyor belt and being lost in the vessel sump. Measurements aimed at this problem have shown that up to 5% of the pellets produced are lost in this way.

EP 0,641,522 A2 describes a pelletizing device in which, to separate the pellets from the coolant, a rotating worm belt with a screen base is provided inside the insulated vessel, into which worm belt the pellet/coolant flow is introduced and, at the outlet end of which, the pellets emerge via an opening in the vessel. This device has the advantage over those described above that it does not act as a heat pump and a smaller proportion of the production volume is lost.

Nevertheless, even with this device, a certain proportion of coolant is conveyed along the worm belt and is lost outside the vessel. Consequently, in this device too, the coolant consumption is increased unnecessarily, which has an extremely bad effect given the high costs of, for example, cold-liquefied nitrogen

(LN₂) .

The invention is based on the object of providing a device for pellet-freezing by means of which it is possible to achieve improved separation of the frozen grains of substance from the coolant combined with a reduced coolant consumption, reduced operating problems and a higher production yield. This object is achieved by means of a device in accordance with the features described in Claim 1. Advantageous configurations and refinements form the subject matter of the subclaims .

The device described in Claim 1 has a separating device, comprising a coolant/pellet discharge conduit and a pellet-collection conduit which is at a distance downstream of the latter, as seen in the direction of flow. The cross-sectional width of the discharge conduit is adapted to the flow of coolant in such a way that the pellets are only slightly immersed, i.e. to the extent of less than half their diameter

(grain size), in the coolant in the discharge conduit.

Therefore, the depth of the flow of coolant is reduced in a controlled manner by means of the cross-sectional width. As a result, a retaining force which is otherwise present between the coolant and the pellets is reduced, while the forces which exist as a result of surface tension between the flow of coolant and the conduit remain active. The punctiform contact of the pellets with the conduit base enables them to become detached from the discharge conduit more easily. Consequently, when the flow of pellets and coolant emerges from the discharge conduit, the pellets follow a different orbit from the coolant. As a result, the pellets are easy to collect separately from the coolant and thus can be cleanly separated from the coolant using extremely simple means. The collection conduit is arranged below and at a vertical distance a from the discharge conduit, so that all the pellets can be collected in the pellet-collection conduit. Due to the shorter orbit of the coolant, it falls, in the manner of a waterfall, through the gap between the conduits, into the sump of the pelletizing vessel, from where it can be recovered in full. No coolant escapes from the pelletizing device, since clean separation takes place entirely inside the vessel.

According to an advantageous refinement of the invention, the distance a between the outlet of the discharge conduit and the collection conduit is adjustable. As a result, the device can be adapted to different throughput quantities (coolant/pellet volumetric flow rates), to different flow velocities and to various other conduit settings . The clean and complete separation of the pellets is thus possible over a greater operating range of the device.

According to a further advantageous configuration of the invention, the discharge conduit is inclined at an adjustable angle of inclination, so that the discharge velocity of the mixture can be increased as desired, leading to a longer orbit on leaving the conduit. This also increases the difference between the coolant orbit and the orbit of the pellets. A higher degree of separation and therefore a higher yield of produced pellets can be achieved in this way.

According to a further advantageous configuration of the invention, the pellet-collection conduit is inclined at an adjustable angle of inclination. This facilitates the collection operation, since the pellets are conveyed down the collection conduit more easily and more readily as a result of the force of gravity. According to a further advantageous configuration of the invention, a downwardly rounded flow-guiding lip is provided at the outlet of the discharge conduit. The significantly facilitates the separating operation, since the flow lip, with the aid of the surface tension between the conduit base and the lip, provides the flow of coolant with an additional downward guidance, while the pellets continue to fall off the conduit along the previous orbit. The separation rate and the yield of the device are thus improved.

According to a further advantageous configuration of the invention, the discharge conduit has an end section, the width of which increases towards the outlet. The increasing width reduces the flow velocity towards the end of the discharge conduit, so that the coolant falls downwards in a thin film, in the manner of a waterfall. At the same time, however, the flow velocity of the pellets remains approximately the same, due to the higher moving mass, so that clean separation is possible even in the case of heterogeneous pellets of considerably varying sizes. Moreover, in this way the distance between the outlet of the discharge conduit and the collection conduit can be reduced, so that the structural volume of the device as a whole is reduced. According to one configuration which is advantageous in this respect, the end section is widened by an angle which is such that the flow velocity at the end of the discharge conduit is less than 2/3 of that in the straight section of the conduit. It has been found that with a widened section of this nature, optimum separating results can be achieved.

According to a further advantageous configuration of the invention, the base of the discharge conduit is designed with flow grooves which run in the direction of flow, preventing smaller- diameter pellets from being entrained downwards by the flow of coolant, as a result of being partially or entirely immersed in the coolant. Preferably, the width of the flow grooves is less than 1 mm, i.e. smaller than the smallest pellets which are to be produced. This measure enables an even higher yield to be achieved using the device. Three exemplary embodiments of the invention, which are described in more detail below and with reference to the figures, are illustrated in the drawing, in which:

Fig. 1 shows a side view of a simplified illustration of a first exemplary embodiment of a pelletizing device with separating device according to the invention; Fig. 2 shows a simplified side view of the separating device shown in Fig. 1 as an excerpt; Fig. 3 shows an excerpt providing a simplified side view of a second exemplary embodiment of a separating device with a flow-guiding lip; Fig. 4 shows an excerpt illustrating a simplified plan view of a third exemplary embodiment of a separating device, with a widened end section; and Fig. 5 shows a coolant/pellet discharge conduit with flow grooves, in cross section. Fig. 1 shows a simplified side view of a first exemplary embodiment of a pellet-freezing device according to the invention. A dropping device 2 for introducing drops of a liquid substance which is to be pelletized is provided on a thermally insulated pelletizing vessel 1. Inside the pelletizing vessel 1, the drops of the substance which are to be frozen enter a pelletizing trough 14 through which coolant 13 flows and inside which the drops remain for a period which is determined by the length of the trough 14, until they have fully frozen to form pellets 12 (frozen grains or granules of substance) . Then, the pellets 12, together with the coolant 11, pass into the separating device 3, which is formed from a coolant/pellet discharge conduit 4 and a pellet-collection conduit 5 which is arranged downstream in the direction of flow. The discharge conduit 4 has an adapted cross-sectional width, so that the pellets 12, as they flow through the conduit 5 together with the coolant 11, are not fully immersed in the latter. Only a small part of the pellets 12 is surrounded by the coolant 11, owing to the wide cross section, which is adapted to the flow volume (volume of the coolant/pellet flow), of the conduit 4. In this exemplary embodiment, the discharge conduit 4 is inclined downwards in the direction of flow, by the angle α. At the outlet 6 of the conduit 4, the pellets 12 and the coolant 11 leave the conduit at the same time, but fall over different orbits A, B. The orbit A of the coolant 11 is shorter than the orbit B of the pellets 12, because the pellets 12 come off the conduit 4 more easily than the coolant 11, since they are only in punctiform contact with the conduit 4. If the pellets 12 were fully immersed in the coolant 11, they would have the same orbit, since, irrespective of their different masses, different bodies which are travelling at the same flow velocity and are ejected at the same angle follow an identical orbit. In this case, it would be impossible to separate the pellets 12 from the coolant 11. The adapted width of the discharge conduit 4 is therefore of central importance for the separating device to function. The falling pellets 12 are collected in the pellet-collection conduit 5 and, from this, are conveyed onwards into the collection vessel 15. In this exemplary embodiment, the collection conduit 5 is inclined downwards at an angle of inclination β, with the result that the pellets roll down the conduit 5 more easily and more readily. However, the conduit 5 may also be arranged horizontally. The distance a between the collection conduit 5 and the outlet 6 of the discharge conduit 4 is such that the collection conduit lies precisely inside the orbit B of the pellets 12. The liquid coolant 11 - generally a liquefied nitrogen - with its shorter orbit A falls into the bottom area of the pelletizing vessel 1, from which it can be pumped out, via the pump 13, to the pelletizing trough 14. No coolant 11 is lost.

Fig. 2 shows an excerpt illustrating the separating device 3 from Fig. 1. In the coolant/pellet discharge conduit 4, the pellets 12 are only immersed slightly in the flowing coolant 11. In this case, they are only about one third surrounded by the coolant 11 and are therefore easier to detach from the base of the conduit 4. At a given, equal flow velocity V of the pellets 12 and the coolant 11, different orbits A and B therefore result. The discharge conduit 4 is inclined at the angle and the collection conduit 5 is inclined at the angle β. The first inclination increases the flow velocity and lengthens the orbits, while the second inclination makes it easier to convey the pellets onwards.

Fig. 3 shows a simplified side view, as an excerpt, of a separating device 3 of a second exemplary embodiment of the invention. Compared to Fig. 2, this device differs only by the flow-guiding lip 7 at the outlet 6 of the discharge conduit 4. This improves the results of separation, because the lip 7 further shortens the orbit A of the coolant 11 compared to the orbit B of the pellets 12. Fig. 4 shows a simplified plan view, as an excerpt, of a separating device 3 of a third exemplary embodiment of the invention. Unlike in the previous examples, the discharge conduit 4 is in this case designed with a widened end section 8. This ensures that the coolant 11 falls downwards at the outlet 6 in a thin, waterfall-like film, at a reduced flow velocity and therefore with a shortened orbit. The pellets 12 do not follow this widened section and fall in the same orbit B as in the examples described above. In this way, the distance a can be kept as short as possible, and even pellets 12 of a relatively small diameter can in this way be separated cleanly from the coolant 11.

Fig. 5 shows a conduit in detail, in a simplified cross-sectional representation, with a grooved base as a further improvement. The flow grooves 9, alternatively drainage grooves, running in the direction of flow in the base of the pellet/coolant discharge conduit 4 make it possible to separate even small-diameter pellets 12, as can be seen from the drawing, from the flow of coolant. Consequently, these small pellets are prevented from being completely surrounded by the coolant 11 and being entrained by the coolant 11 into the sump of the vessel 1.

List of reference symbols

1 Thermally insulated pelletizing vessel

2 Dropping device

3 Separating device

4 Coolant/pellet discharge conduit

5 Pellet-collection conduit

6 Outlet

7 Flow-guiding lip

8 Widened end section

9 Flow grooves

11 Coolant

12 Pellets (frozen grains of substance)

13 Coolant pump

14 Pelletizing trough

15 Collection vessel

Angle of inclination, discharge conduit β Angle of inclination, collection conduit δ Widening angle, discharge conduit a Distance from outlet of discharge conduit to collection conduit

A Orbit of coolant

B Orbit of pellets

V Flow velocity of coolant/pellet flow

Claims

1. Device for the pellet-freezing of liquid or flowable substances in a liquid coolant, in particular in a cryogenic liquid, having a thermally insulated pelletizing vessel (1) , having means for generating a flow of coolant, having a dropping device (2) for adding the substance in drops of a predetermined size to the flow of coolant in a pelletizing trough (14), and having a separating device (3) for separating the drops of substance which have been frozen into pellets from the flow of coolant, characterized in that the separating device (3) has a coolant/pellet discharge conduit (4) with a cross-sectional width which is adapted to the flow of coolant in such a way that the pellets (12) are only slightly immersed in the flow of coolant, and a pellet-collection conduit (5) arranged downstream in the direction of flow, and the collection conduit (5) is arranged below and at a vertical distance a from the outlet (6) of the discharge conduit (4), in such a way that the pellets (12) can be collected separately from the coolant using the collection conduit (5) .

2. Device according to Claim 1, characterized in that the distance a between the outlet (6) of the discharge conduit (4) and the collection conduit (5) is adjustable .

3. Device according to Claim 1 or 2, characterized in that the discharge conduit (4) is inclined at an angle of inclination α, and the angle of inclination is adjustable.

4. Device according to one of the preceding claims, characterized in that the collection conduit (5) is inclined at an angle of inclination β and the angle of inclination β is adjustable.

5. Device according to one of the preceding claims, characterized in that a downwardly rounded flow-guiding lip (7) is provided at the outlet (6) of the discharge conduit (4).

6. Device according to one of the preceding claims, characterized in that the discharge conduit (4) has an end section (8) with a cross section of increasing width.

7. Device according to Claim 6, characterized in that the end section (8) is widened by a widening angle δ, which is selected in such a way that the flow velocity at the end of the discharge conduit (4) is less than 2/3 of the flow velocity in the straight section of the discharge conduit (4).

8. Device according to one of the preceding claims, characterized in that the base of the discharge conduit (4) is designed with flow grooves (9) which run in the direction of flow.

9. Device according to Claim 8, characterized in that the width of the flow grooves is less than 1 mm.