GB2146943A

GB2146943A - Producing composite pellets

Info

Publication number: GB2146943A
Application number: GB08423593A
Authority: GB
Inventors: Shuji Miyahara; Harumi Kimuro; Saburo Yamashita
Original assignee: IHI Corp
Current assignee: IHI Corp
Priority date: 1983-09-19
Filing date: 1984-09-18
Publication date: 1985-05-01
Also published as: JPS6065710A; DE3434162C2; FR2552213B1; DE3434162A1; FR2552213A1; GB8423593D0; GB2146943B

Abstract

Liquefied carbon dioxide is caused to expand through a first nozzle 1a whereby at least a portion of it solidifies into powdery dry ice. Water is sprayed through a second nozzle 1b and coats the dry ice particles with a layer of ice. A liquid other than water may be used. The resulting powder 6 is then pelletized in a pelletizer such that pellets comprising a core in the form of a dry ice particle surrounded by a frozen liquid coating are produced. <IMAGE>

Description

SPECIFICATION Method and apparatus for producing composite pellets The present invention relates to a method and apparatus for producing composite pellets including dry ice which may be used for, for example, the abrasive blasting of sheet steel or the like or the preservation of perishables. Various methods have been proposed for producing pellets of dry ice for these purposes.

In one method, carbon dioxide gas is compressed at low temperature into liquefied CO2 (LCO2) which is then subjected to adiabatic expansion so that powdery or snow-like solid CO2, referred to as dry ice snow, is produced due to the heat of vapourisation and heat of sublimation of the CO2, Thereafter, a small quantity of LCO2 is added to the dry ice snow which is then compressed to produce large lumps or blocks of dry ice. These blocks are then crushed by a crusher or the like into dry ice pellets. In another method, a press is used to mould dry ice snow into blocks. In a further method, dry ice snow is fed into a rotary type extruder to compress and mould it into pellets.

However, in the method in which blocks of dry ice are crushed, a high proportion of powder tends to be produced so that the yield may be as low as 20%, resulting in an increase in cost. Moreover, blocks of dry ice cannot be stored for a long time before they are crushed. When press-moulding dry ice snow or using a rotary type extruder, the surface of the dry ice pellets is very rough, rather like frost, and is very soft so that the dry ice pellets cannot be used effectively and efficiently for abrasive blasting.

Thus, dry ice pellets produced by the various known methods have various disadvantages and a common problem is that dry ice pellets are generally at a temperature of about -80 C at which they can be used for preserving perishables, but when the temperature exceeds -80 C, they rapidly melt and sublimate. Thus, the rate of comsumption of dry ice pellets increases considerably at temperatures highear than -80 C.

Thus, it is an object of the present invention to provide a method and apparatus for producing pellets including dry ice which may be used for abrasive blasting orforthe preservation of perishables.

According to the present invention, a method of producing composite pellets comprising dry ice and a frozen liquid includes forming a powder comprising dry ice and the frozen liquid and pelletizing the said powder. The method preferably includes causing liquefied carbon dioxide to expand thereby cooling and solidifying at least a proportion of it into a powder and spraying the said liquid into the stream of carbon dioxide thereby freezing the said liquid. It is preferred that the solidified carbon dioxide powder particles are sprayed with the said liquid, e.g. water, and thereby substantially coated with the said frozen liquid, e.g. ice.

Thus, in the method of the present invention, the surface of the composite pellets includes a frozen liquid and is thus harder than pellets of dry ice oniy.

The melting and sublimation of the dry ice is thus reduced to a minimum at temperatures greater than -80 C. The composite pellets produced by a method in accordance with the present invention are also harder than conventional dry ice pellets and are thus more suitable for abrasive blasting.

The present invention also embraces an apparatus for carrying out such a method and thus, in accordance with a further aspect of the present invention, apparatus for producing composite pellets comprising dry ice and a frozen liquid comprises means for spraying liquefied carbon dioxide, means for spraying the said liquid into the stream of carbon dioxide, whereby a powder comprising dry ice and the said frozen liquid is formed, and a pelletizer for pelletizing the said powder.

Further features, details and advantages of the present invention will be apparent from the following description of a preferred embodiment which is given by way of example with reference to the accompanying drawings, in which Figure 1 is a schematic view illustrating the underlying principle of the present invention; and Figure 2 is a schematic view of an apparatus for producing composite pellets of dry ice and frozen liquid in accordance with the present invention.

Referring firstly to Figure 1, liquefied CO2 (LCO2) which is temporarily stored in a storage tank 2 is fed by a pump 3 into a snow-mixture making device 1 and is sprayed downwardly in an atomised stream from the top of the snow-mixture making device 1.

Water (H20) which is stored in a water storage tank 4 is introduced by a high pressure pump 5 into the snow-mixture making device 1 and is atomised at a predetermined rate at a position below the position at which LCO2 is atomised such that atomised water particles are surrounded by atomised carbon dioxide particles. As a result, a part of the LCO2 is subjected to an adiabatic expansion so that the remainder solidifies into a powder or dry ice snow due to the heat of evaporation and heat of sublimation of the CO2. Concurrently, the water absorbs the negative heat and is frozen over the surfaces of the dry ice snow. Thus a snow mixture 6 consisting of dry ice snow and ice is produced.

Referring now to Figure 2, disposed below the snow-mixture making device 1 is a pelletizer 7 for making pellets which may be elongate and which are generally of chalky appearance. The device 1 communicates with the water storage tank 4 via the high pressure pump 5 and a hose 8 and communicates with the liquefied carbon dioxide storage tank 2 via the pump 3 and a hose 9. A tank truck 10 is used to transport LCO2 into the storage tank 2. Thus, in the method of the present invention, LCO2 is stored in lieu of dry ice. Only when dry ice snow is to be produced is LCO2 withdrawn from the storage tank 2.

As best seen in Figure 1, a liquefied carbon dioxide spraying nozzle 1 a and a water spraying nozzle 1 b are coaxially disposed within the snow-mixture making device 1 and are directed downwards. Water is sprayed into the atomised or sprayed LCO2. The portion of the hose 8 which is disposed within the device 1 is wrapped with a heat insulating material 11 and heated by a heater (not shown) so that water in the hose 8 does not freeze.

Various types of pelletizers 7 may be used in the method of the present invention. For instance, the mixture can be moulded into a plurality of pellets by using a press mould. Alternatively, the mixture is fed into the space defined between a rotary inner barrel and an outer barrel with many pellet-forming holes therein so that the mixture is forced through these holes and thus pelletized.

In order to produce composite particles or pellets of dry ice and ice, LCO2 stored in the storage tank 2 is pumped by the pump 3 into the snow-mixture making device 1 and sprayed through the nozzle la.

Concurrently, water stored in the water storage tank 4 is pressurised by the high pressure pump 5 and is passed to the snow-mixture making device 1 in a predetermined ratio to the feed rate of the LCO2 and is sprayed through the nozzle 1 b into the mist of LCO2. As a result, a powdery snow mixture consisting of CO2 and H20 is produced. The mixture thus obtained is fed into the compression pelletizer 7 whereby pellets of dry ice and ice are produced.

The pellets thus produced can be used for abrasive blasting of sheet steel or the like or can be used as a cooling agentforthe preservation of various perishables or transportation under low temperature of ice cream, milk and the like.

The composite pellets obtained in accordance with the present invention are in the form of a dry ice particle covered with ice, the temperature of the dry ice particle being about -80 C. The surface of these pellets is glossy and smooth unlike dry ice particles.

Even when the temperature is higher than about -80 C, the dry ice particles cannot vaporise since they are covered with ice, unless the ice melts, so that the consumption of the dry ice is reduced to a minimum.

The ratio of the LCO2 to water may be varied as needs demand and at any ratio the dry ice particles are covered with hard ice.

The snow-mixture has been described as being produced by mixing liquefied carbon dioxide with water and pelletizing the frozen mixture thereof, but it is to be understood that a liquid other than water may be used. It is advantageous if a process liquid is used, since no impurity will be contained in the liquid.

As described above, the present invention employs a method of mixing liquefied carbon dioxide with a liquid to produce a snow-mixture when is then pelletized into composite pellets consisting of dry ice and a frozen liquid. The surfaces of these composite pellets are coated with frozen liquid and are thus glossy and hard. As a result, as compared with an abrasive blasting method in which only dry ice is used, the efficiency of abrasive blasting using composite pellets made in accordance with the present invention is considerably improved. Furthermore, as long as ice remains coating the surfaces of the dry ice particles, the premature vaporisation of the dry ice can be prevented so that the composite pellets of dry ice and frozen liquid in accordance with the present invention may be kept for a long time.

Moreover, liquefied CO2 and a liquid are used for production of the snow-mixture so that the ratio of the dry ice to the frozen liquid in the mixture can be more easily varied than if one were mixing different solids to form the snow-mixture. Furthermore, CO2 for the production of the composite pellets can be stored for as long as desired since it is stored in liquid form rather than as dry ice.

Claims

1. A method of producing composite pellets comprising dry ice and a frozen liquid which includes forming a powder comprising dry ice and the frozen liquid and pelletizing the said powder.

2. A method as claimed in Claim 1 which includes causing liquefied carbon dioxide to expand thereby cooling and solidifying at least a proportion of it into a powder and spraying the said liquid into the stream of carbon dioxide thereby freezing the said liquid.

3. A method as claimed in Claim 2 which the carbon dioxide powder particles are sprayed with the said liquid and thereby substantially coated with the said frozen liquid.

4. A method as claimed in any one of the preceding Claims in which the said liquid is water.

5. Apparatus for producing composite pellets comprising dry ice and a frozen liquid comprising means for spraying liquefied carbon dioxide, means for spraying the said liquid into the stream of carbon dioxide, whereby a powder comprising dry ice and the said frozen liquid is formed, and a pelletizer for pelletizing the said powder.

6. A method of producing composite pellets substantially as specifically herein described with reference to the accompanying drawings.

7. Apparatus for producing composite pellets substantially as specifically herein described with reference to the accompanying drawings.

8. A method for producing composite pellets consisting of dry ice and frozen liquid which comprises, during a process in which liquefied carbon dioxide is subjected to adiabatic expansion so that snow-like solids are produced by heat of vaporisation and heat of sublimation, spraying a liquid into said liquid carbon dioxide to produce snow-mixture consisting of dry ice and frozen liquid; and compressing said snow-mixture into pellets.

9. An apparatus for producing composite pellets consisting of dry ice and frozen liquid comprising means for spraying liquefied carbon dioxide, means for spraying a liquid into the sprayed liquefied carbon dioxide, a snow-mixture making device for producing snow-mixture consisting of snow-like dry ice and frozen liquid, and a compression pelletizer communicated with said snow-mixture making device for producing pellet-like mixture consisting of dry ice and frozen liquid from said snow-mixture.