GB2050806A

GB2050806A - Continuous-flow treatment of particulate material

Info

Publication number: GB2050806A
Application number: GB7917104A
Authority: GB
Original assignee: Johnston J S
Current assignee: Johnston J S
Priority date: 1979-05-16
Filing date: 1979-05-16
Publication date: 1981-01-14

Abstract

In a method and apparatus for continuous-flow treatment of flowable unpumpable particulate material with a treating fluid at other than ambient temperature and/or at other than atmospheric pressure, a downward flow of a column of the material under the effects of increasing weight and the action of a downward conveyor creates an increase of pressure within the material flow for injection of the treating fluid, and subsequently an upward flow of the material creates, under the effects of decreasing weight and an upward conveyor creates a reduction of the pressure, e.g. at or about atmospheric, whereby need for mechanical airlocks at the entry and exit of a pressurised treating chamber is eliminated. <IMAGE>

Description

SPECIFICATION Continuous4low treatment of particulate material This invention relates to a method and apparatus for the treatment with a treating fluid of flowable unpumpable particulate materials.

There is a need, for example in the food, pharmaceutical, and chemical industries, for treatment of such material with a fluid at a temperature other than ambient, and a pressure other than atmospheric. Hitherto, this treatment has usually been performed batch-wise.

Particularly in the food industry there may be preliminary operations to be carried out prior to treatment, e.g. sorting, preparing, washing, and transporting of the material to the place of treatment. There may also be other operations subsequent to the treatment stage, for example packaging.

These preliminary operations and subsequent operations may well lend themselves to being continuous operations, and in such circumstances the batch-wise treatment of the material causes disruption of flow. Consequently methods of treatment have been developed in the prior art which incorporate handling a large number of small batches, as at least an approach towards continuous processing.

Feeding of the material to, and discharging of it from, the batch processing stage is obtained by, for example, rotary valve systems having a number of sealable pockets containing individual batches, or by batch chamber systems having inlet and outlet shut-off valves and intermittent pressurising. Such systems entail both high initial cost and high maintenance cost, and the pockets or valves used can cause damage to the treated material, e.g.

bruising or splitting of cereal grains and the like.

Further, such batch systems subject the treated material to a very rapid transition of pressure and temperature which, for certain materials, may result in a form of case-hardening tending to inhibit transfer of treating fluid and/or change of temperature and/or change of pressure to the inner part of each particle of material. Thus, the advantage of temperature and pressure treatment may thereby be nullified. In some processes it is desirable to subject a material to lower than atmospheric pressure to "open" the surface of the particles and remove gas, liquid or vapour phase preparatory to conditioning by heat and pressure, and application of other treating materials, such as flavours and coatings in the case of food materials. Again, after such treatment it is often of advantage to flash dry and cool the treated material under lower than atmospheric pressure.For all of these process steps the use of a single batch operation, or semi-continuous multiple batch operations, is relatively clumsy, timeconsuming and labour intensive.

It is the object of the present invention to provide an improved method, for treating flowable unpumpable particulate material with a treating fluid at other than ambient temperature and/or at other than atmospheric pressure, wherein there is continuous flow of the material. It is also the object of the invention to provide apparatus for carrying out said method.

According to the present invention a method for the treatment of a flowable particulate material with a treating fluid at other than atmospheric pressure comprises the steps of continuously feeding the material at an initial pressure to an inlet of a conveying means and passing the material along a first portiqn of the conveying means having a vertical component of direction such that the pressure in the material due both to gravity and to the conveying action varies progressively from the initial pressure, feeding treating fluid at other than atmospheric pressure into the material at a position along the first portion of the conveyor at which the pressure in the material has been so varied, thereafter passing the treated material continuously along a second portion of the conveying means having a vertical component of direction such that the pressure in the material due both to gravity and the conveying action, progressively varies towards the initial pressure, and continuously discharging treated material from the downstream end of said second portion of the conveying means.

Further, according to the invention, apparatus for carrying out the above method comprises a conveying means having an inlet and a first portion having a vertical component in one direction and a second portion having a vertical component in the opposite direction, and means for feeding treating fluid at other than atmospheric pressure to said first portion remote from the inlet.

In order that the nature of the invention may be readily ascertained, some embodiments of apparatus, and their method of use, in accordance with the invention, are hereinafter particularly described by way of example, with reference to the figures of the accompanying drawings, wherein: Figure 1 is a schematic cross-section of a first embodiment of apparatus; Figure 2 is a side elevation, with part shown in section, of a second embodiment of apparatus; Figure 3 is a side elevation of a third embodiment of apparatus; Figure 4 is a side elevation of a fourth embodiment of apparatus; Referring now to Figure 1,there is shown an apparatus which may be employed for example in the pressure cooking or air-dried cereal grain.

The apparatus comprises a first screw conveyor having a casing 1 with a conveyor screw 2 therein driven by a motor 3. The casing 1 is vertically disposed and at its upper end it has an inlet 4. A second screw conveyor has a casing containing a screw 6 driven by a motor 7. The casing 5 is vertically disposed and at its upper end it has an outlet 8. At its lower end it has a widened casing portion 9 containing a pick-up screw 10 and communicating through an intermediate casing 11 with the bottom end of the casing 1.

The cereal grain to be treated is fed in at the inlet 4 at a rate sufficient to keep the internal volume of the conveyor filled at all times. The column of material present in the conveyor is thus subjected to two forces acting downwardly, i.e. (i) the weight of the column of material itself, lessened by the fact that the material is at least to some extent supported by the flights of the conveyor, and (ii) the downward conveying thrust acting in the direction of the arrow "A". There is thus a pressure gradient in the column of material, from approximately atmospheric pressure at the upper end adjacent the inlet 4, to a pressure considerably above atmospheric at the bottom end, in intermediate casing 11.At a first point 12 of the casing 1 there is injection of steam at a first pressure, and at a second lower point 13 of the casing 1 there is injection of steam at a second and greater pressure. The material, in passing from inlet 4 to casing 11, is thus subject to an increase of pressure, an increase of temperature, and the addition of water vapour, serving for a first stage of the cooking operation.

The material from the casing 11 is picked up by the pick-up screw 9 and passes to the second conveyor screw 6 which conveys the material upwardly until it can fall out of the outlet 8. As the material passes up the column the pressure due to the weight of the material above gradually drops, until the material at the outlet 8 is at substantially atmospheric pressure.

During the transfer of the material from the pick-up screw 10 to the second conveyor, further steam may be injected at 14, and still further steam, at a somewhat lower pressure, may be injected at 15 for further cooking of the material. It will be appreciated that in this embodiment the pressure on the material rises to a peak and then drops off substantially immediately after reaching peak pressure.

Referring now to Figure 2, there is shown a second embodiment wherein the material enters an inlet 16 and is fed down a conveyor casing 17 by a screw similar to that shown at 2 in Figure 1, driven by a motor 18. At its lower end the casing 17 opens into an inlet end of an intermediate casing 19 containing a conveyor screw 20 driven by a motor 21. The outlet end of the casing 19 is connected by a conduit 22 to a pick-up chamber 23 of a conveyor casing 24 having a pick-up screw and conveyor screw similar to those shown at 10 and 6 respectively in Figure 1, and driven by a motor 25. At the upper end of the casing 24 there is an outlet 26 to atmosphere. In this construction, the material fed in at inlet 16 progresses at increasing internal pressure down to the bottom of the casing 17.At intermediate points there is injection of steam at 27 at a first pressure, and at 28 at a second greater pressure, for pre-heating of the material. The material then enters the intermediate casing and remains at constant pressure for the period of time necessary to pass from one end of the conveyor to the other. During this dwell time, for prolonged cooking, steam is injected at points 29, 30 and 31, all at the same pressure. The material reaching the outlet end of the intermediate casing 19 is then picked up and conveyed upwardly in the casing 24, and is dried during its upward travel by hot air injected at points 32 and 33.

Referring now to Figure 3, there is shown an apparatus comprising an upward conveyor 34 having an inlet 35 and containing a conveyor screw (not shown) driven by a motor 36. An upper end outlet 37 leads to the top (inlet) end of a downward conveyor 38 containing a screw (not shown) driven by a motor 39. The lower end of the downward conveyor has an outlet 40. At the upper end of the upward conveyor there is provided a union 41 connected to means (not shown) for progressively sucking off air. The material is fed to the inlet 35 in such a manner as to keep the inlet conduit filled and thus effectively sealing the conveyor 34 against ingress of air. Due to the suction applied at 41, there is a decreasing pressure gradient from bottom to top of the conveyor 34, and also the weight of the column of material decreases with height in the column.The material to be treated is thus progressively subjected to lower pressure and its air content, and vapour content (if any) will be drawn off as it reaches the upper end of the conveyor 34. After transfer of the material to the downward conveyor 38, the material passes downwardly under gradually increasing pressure due to the weight of the material in the column and the downward drive of the conveyor. At points 42 and 43 steam is injected for conditioning of the material. This embodiment may be used, for example, for killing the "beany" flavour of air-dried soya beans.

Referring now to Figure 4, there is shown an embodiment which comprises four structures, of the kind described with reference to Figure 3, arranged in cascade. It is assumed that each conveyor is of the nature already described, and it is sufficientto show only the directions of flow as indicated by the arrows. By way of example, to fully cook, flavour, coat and cool a whole particulate grain, continuously and without abrupt changes of pressure, the process would be as follows: Suction is applied at 44, at the top of the first conveyor, to eliminate any air in the interstices of the material being fed upwardly. Steam is injected at 45 into the material passing down the second conveyor to commence the cooking operation.The cooking material then passes upwardly in the third column, and during this time there is a slow swelling of the material due to the reduction of internal pressure on the material of the column as it rises up the column.

The material then passes downwardly in the fourth conveyor and is again subjected to gradually increasing pressure, due to gravity and the direction df conveying, to finish the cooking stage. The material is then carried upwardly again in the fifth conveyor, and suction is applied at 46 to remove rapidly any excess moisture which may be present in the material.

Flavouring and coating substances are then injected into the sixth conveyor and become impregnated into the cooked dried material due to the increasing pressure on the material as it passes downwardly. The seventh and eighth conveyors, disposed in series, are both used for cooling the flavoured coated product delivered from the lower end of the sixth conveyor. The seventh and eighth conveyors may be, for this purpose, provided with jacketing for the conveyor casing to receive a cold mixture of ethylene glycol and water as a heat exchange medium.

The apparatus lends itself particularly to heat exchange between a heating or cooling medium and the material to be treated. Where suction is applied during the upward movement of the material in a conveyor, the suction causes evacuation of air or other gas from the material, thus removing an effective insulating material from the interstices between the particles and improving access to the material for heat exchange. If relatively small diameter helical conveyor screws are used, and the conveyor casing and/or the screw is heated or cooled by a heat exchange medium, there is direct application of the heating or cooling to a relatively shallow layer of the treated material, giving faster convection and conduction of heat, and thus more uniform results and reduced dwell or treatment times.

The invention can provide truly continuous careful heat treatment, under such pressure as may be required, of both as-harvested and air-dry foods including grains, legumes, nuts and diced and particulate vegetables. The apparatus allows for gradually increasing pressure contingent with high or low temperature treatment, in liquid or vapour phase, or pressure may be reduced for drying or for desolventising, and subsequent moisturising or inoculating with increased pressure. The invention is applicable to hot or cold treatment under increased or atmospheric or reduced pressure, by indirect thermal transfer for heat desolventising, dehydrating, drying and cooling of flowable particulate solids as appertaining to the food, pharmaceutical and chemical industries.The apparatus is of comparatively low initial and maintenance cost and provides smooth non-destructive transporting of the material during progressive variations in applied temperatures and pressures, in the processing of particulate solid materials. The transition of conditions from low to high temperature and pressure after feeding is achieved truly continuously and gradually without loss of heat by, e.g. vertically disposed helical screws rotating in tubular casings, whereas in the prior art the semi continuous rotary valves or intermediate valve chambers used impose a sudden increase in temperature and pressure, and an expensive waste of heating medium.Transition from high to low temperature and pressure is again truly continuous and gradual, thus avoiding the excessive flash-off and fragmentation of discharging product which occurs when using rotary valves or intermedlate valved chambers. The initial cost of small diameter conveyor screws is low, maintenance is minimal and useful life is long, compared to the equivalent costs of separate large vessels and valves.

The conveyors may have plain or jacketed single or multiple pitch helical flighted rotatable screws, mounted on solid or hollow shafts, driven by variable speed motors integrally mounted above or below, supported by bearing journals at top and bottom ends, and contained within plain or jacketed stationary casings. Alternatively, the flights may be replaced by plain or jacketed paddles.

Well known economic advantages of vertically disposed tubular casing screw eievators include the reduction in screw shaft diameter and increase in practical length, because of the absence of bending stresses on the screw and the casing. When jacketed screws and casings are used, there will be very considerable increases in heat transfer coefficients, due to more effective dropwise condensation of vapour and the absence of condensate hold up in cavities. In addition to savings in floor space, screw elevators have the processing advantages of screw conveyors, including simple design and light construction for handling, and exceilent facility for the important operations of cleaning in place, and manual cleaning. They are also self feeding, and may be vibrated to improve access for heating or cooling as well as assisting transport.

Claims

1. A method for the treatment of a flowable particulate material with a treating fluid at other than atmospheric pressure comprising the steps of: (i) continuously feeding the material, at an initial pressure, to an inlet of a conveying means and passing the material along a first portion of the conveying means having a vertical component of direction such that the pressure in the material, due to both gravity and the conveying action, varies progressively from the initial pressure, (ii) feeding treating fluid at other than atmospheric pressure, into the material at a position along said first portion of the conveying means at which the pressure in the material has been so varied, (iii) thereafter passing the material continuously along a second portion of the conveying means having a vertical component of direction such that the pressure in the material, due both to gravity and the conveying action, progressively varies towards the initial pressure, and (iv) continuously discharging treated material from the downstream end of said second portion of the conveying means.

2. The method claimed in claim 1 wherein treating fluid is fed into the material at a position along the second portion of the conveying path.

3. The method claimed in either of claims 1 and 2 wherein between steps (ii) and (iii) the material is passed continuously along a third portion, intermediate the first and second portions of the conveying means, having no vertical component of direction.

4. The method claimed in claim 3 wherein treating fluid is fed into the material at a position along said third portion of the conveying means.

5. The method claimed in any one of claims 1 to 4, including the further step of applying suction to the material at a point of the conveying means between said first and second portions.

6. Apparatus, for the treatment of a flowable particulate material with a treating fluid at other than atmospheric pressure, comprising: a) a conveying means having an inlet, a first portion having a vertical component in one direction, and a second portion having a vertical component in the opposite direction, and b) means for feeding treating fluid at other than atmospheric pressure to said first portion remote from said inlet.

7. Apparatus, as claimed in claim 6, comprising means for feeding treating fluid at other than atmospheric pressure to said second conveying means portion remote from the first conveying means portion.

8. Apparatus, as claimed in either of claims 6 and 7, wherein the conveying means includes a third portion, having no vertical component of direction, between said first and second portions.

9. Apparatus, as claimed in claim 8, comprising means for feeding treating fluid, at other than atmospheric pressure, to said third conveying means portion.

10. Apparatus, as claimed in any one of claims 6 to 9, comprising means for applying lower than atmospheric pressure to the conveying means at a point intermediate the first and second portions.

11. The method for the treatment of a flowable particulate material substantially as described herein with reference to any one of Figures 1 to 4 of the accompanying drawings.

12. Flowable particulate material treated in accordance with the method claimed in any one of claims 1 to 5 and 11.

13. Apparatus for the treatment of a flowable particulate material substantially as described herein with reference to any one of Figures 1 to 4 of the accompanying drawings.