WO2001070382A1

WO2001070382A1 - Apparatus for mixing a solid and a liquid

Info

Publication number: WO2001070382A1
Application number: PCT/AU2000/000225
Authority: WO
Inventors: Rakesh Kumar Aggarwal
Original assignee: Rakesh Kumar Aggarwal
Priority date: 1998-10-26
Filing date: 2000-03-22
Publication date: 2001-09-27
Also published as: AU5598699A

Abstract

Apparatus for in-line and continuously mixing a particulate solid and a liquid, while removing air at the same time, includes a mixing vessel (24), into which a liquid is introduced tangentially through a tangential entry (30). The rotation of the liquid induces a vortex (31) whose depth is limited so that it does not reach the outlet (29) of the mixing vessel (24). The particulate solid is deposited by an auger feeder (28) onto the vortex (31), and spins outwards due to centrifugal force, while interstitial air moves towards the centre and discharges into the vortex (31). The mixed liquid is withdrawn from the outlet (29) of the mixing vessel (24) by means of a pump (25). The pump (25) provides further mixing, and pressure for transporting the liquid. The level in the mixing vessel (24) is controlled by a level sensor (23), and a control valve (22) in the incoming line (21) of the apparatus.

Description

APPARATUS FOR MIXING A SOLID AND A LIQUID

The present invention relates to apparatus and a method for mixing a solid and a liquid, and more particularly relates to a solids/liquid mixing device which removes air from the product at the same time as mixing takes place. The apparatus may also be used for mixing a plurality of liquids.

Many processes in the food, pharmaceuticals and chemical industry call for the mixing of solids into liquids, to prepare a solution or suspension of a precise composition.

Quite often this is done discontinuously, in many steps. Firstly, a tank is filled with a known quantity of liquid. Next, the required amount of solid material is dumped into the tank. Thirdly, the contents of the tank are vigorously mixed to achieve a uniform distribution and an homogeneous solution. Sometimes, a pump is used to circulate the product and achieve mixing. The product is then ready for use.

The above process has many disadvantages, some of which are listed below:-

(a) It is a batch process which normally has to integrate with a continuous, process thus necessitating two independent mixing units.

(b) Vigorous mixing can lead to aeration of the product, which may not be desirable.

(c) Sometimes an additional solvent may be required to prepare a solution, for example the addition of lactose in milk is traditionally achieved by making a lactose solution in water, which water ultimately needs to be evaporated, costing energy.

There is some proprietary equipment available on the market for industrial application, which relies upon closing the solids in the eye of a vertically mounted impeller, and accurately controlling the feed in and feed out of liquid. Such equipment is fairly complex and relies upon extensive instrumentation and controls. This leads to aeration of the product.

Other available equipment relies upon vigorous agitation to achieve mixing. This introduces air into the product and results in a very high energy consumption. This may damage the shear sensitive products.

The mixer in AU-B-30882/95 uses the same principle, but does not provide for air removal and in-line use. It relates to batch process equipment.

It is an object of the present invention to provide a means of mixing solids into liquids as a continuous process, with a low energy cost and while removing air at the same time.

The invention provides apparatus for mixing a solid and a liquid, including a mixing vessel having an inlet for said liquid, means to introduce said solid into said vessel, and an outlet from said vessel for a mixture of said solid and said liquid, characterised in that said liquid is introduced into said vessel through a tangential entry, to rotate the liquid in said vessel, the rotation of said liquid causing the formation of vortex in said vessel, and in that said solid is deposited into said vortex.

The invention also provides a method of mixing a solid and a liquid, including the steps of:-

introducing said liquid tangentially into a mixing vessel, such that rotation of the liquid in said vessel takes place, such that a vortex is created in said liquid in said vessel; and

introducing said solid into said vortex.

The invention further provides an apparatus for mixing liquids and particulate solids comprising a vessel with inlet and outlet for said liquid, said liquid said inlet for introducing said liquid into the said vessel, a means to create a rotating said liquid in a manner that the air core in the middle of the said rotating said liquid does not reach the said outlet of the said vessel, said particulate solids inlet means for introducing a stream of said particulate solids into the said rotating liquid, and removing the mixed said liquid and said particulate solids from the said outlet of the vessel.

Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and the advantages of the invention may be realised and obtained by means of the instrumentation and combinations particularly pointed out in the description and their equivalents.

To achieve the foregoing objects, and in accordance with the purposes of the invention as embodied and broadly described herein, there is provided an apparatus and process to achieve an in-line mixing of solids and liquids. The apparatus consists of a solids (or liquid) dosing equipment and an appropriate method of generating a centrifugal force in liquid. The centrifugal force thus generated is limited in magnitude so as to avoid the air core in the eye or vortex from reaching the liquid outlet port. The solids (or liquids) are dropped on the surface of the liquid. The solids that have a density higher than the liquid are subjected to a centrifugal force according to Stokes' law, and move towards the wall of the vessel. The gases or air which are generally lighter than the liquid tend to move towards the centre again due to the centrifugal forces and the Stokes' law or principle. The rotating liquid creates an interface with the solids surface and that facilitates suspension or solution of the solid. This mixer has the following advantages:-

(a) It is an in-line continuous process thus does not need holding tanks and vessels;

(b) The mixing action is very gentle and the centrifugal force used for mixing also removes the air, it does not vigorously mix, and does not aerate the product; and

(c) It does not need an additional solvent like batch processes do.

Other objects and advantages are:-

(a) The energy consumption is extremely low in this process;

(b) The equipment is washed in-line and does not need separate washing;

(c) The equipment has a small footprint and needs very little space compared to batch mixing which needs tanks and pumps; and

(d) The control system to integrate this concept in an automatic plant are very simple.

The invention may be seen to be apparatus and a method for in-line and continuously mixing a particulate solid and a liquid, while removing air at the same time. It may include a mixing vessel, into which a liquid is introduced tangentially through a tangential entry. The rotation of the liquid induces a vortex whose depth is limited so that it does not reach the outlet of the mixing vessel. The particulate solid is deposited by an auger feeder onto the vortex, and spins outwards due to centrifugal force, while interstitial air moves towards the centre and discharges into the vortex. The mixed liquid is withdrawn from the outlet of the mixing vessel by means of a pump. The pump provides further mixing, and pressure for transporting the liquid. The level in the mixing vessel is controlled by a level sensor, and a control valve in the incoming line of the apparatus.

An embodiment of the invention, which may be preferred, will de described in detail hereinafter, with reference to the accompanying drawing, in which:- Fig.1. is a schematic diagram of an embodiment of a solids and liquids mixing apparatus according to the present invention.

Fig. 1 , which is incorporated in and constitutes a part of the specification, illustrates a preferred embodiment of the invention and together with the general description given above and the detailed description of the preferred embodiment and the description of an actual trial unit given below, serve to explain the principles of the invention.

According to the embodiment of Fig. 1 , the components of the apparatus of the preferred embodiment, and the reference numerals denoting them, are:

28 Auger feeder

27 Bag filter

24 Mixing vessel

22 Control valve (for the liquid line to maintain level in the mixing vessel 24) 23 Level transmitter or sensor

25 Mixing pump

26 Product outlet line 21 Liquid incoming line

30 Tangential entry to mixing vessel 24 29 Outlet port of mixing vessel 24

31 A typical shape of the vortex formed due to liquid entering the mixing vessel 24 through tangential entry 30

The apparatus of Fig. 1 includes a liquid incoming line 21 in which is located a control valve 22. Line 21 has a tangential entry 30 to mixing vessel 24. Reference numeral 31 denotes the typical shape of the vortex formed in mixing vessel 24 as a result of the liquid entering the vessel through tangential entry 30. Auger feeder 28 is connected to mixing vessel 24. A bag filter 27 is connected to the top of the mixing vessel 24. A level sensor or transmitter 23 is mounted on a mixing vessel 24, and is connected to control valve 22. Mixing vessel 24 has an outlet port 29 which is connected to mixing pump 25 which in turn is connected to a product outlet line 26. The auger feeder 28 is typically a commercially available unit. The mixing pump 25 is a commercially available pump of required flow rate and head. The mixing vessel 24 is typically a cylindrical vessel with a conical base, having, preferably, a diameter of 150mm diameter and a length of 1000mm.

From the description above, a number of advantages of the apparatus become evident:

(a) It is an in-line continuous process, and thus does not need holding tanks and vessels;

(b) The mixing action is very gentle and the centrifugal force used for mixing also removes the air, and thus it does not vigorously mix, and does not aerate the product which may not be desirable;

(c) It does not need an additional solvent like batch processes do.

(d) The energy consumption is extremely low for this process.

(e) The equipment can be washed in-line and does not need separate washing.

(f) The control system to integrate this concept in the automatic plant are very simple.

The operation of the apparatus of this embodiment of the present invention as in-line equipment is identical to any in-line equipment for liquid processing. It continuously receives a stream of liquid, processes the incoming liquid in real time and discharges the liquid continuously after processing.

There is a liquid incoming line 21 which supplies liquid through the control valve 22. Control valve 22 controls the level in mixing vessel 24. Liquid enters mixing vessel 24 via a tangential entry 30. The tangential entry velocity thus developed is used to create a rotating motion and create a centrifugal force. The centrifugal force is controlled such that a vortex 31 is formed. Reference numeral 31 shows a typical shape of the vortex. The vortex 31 is limited in depth such that it does not reach outlet port 29 of mixing vessel. The control valve 22 maintains the level in mixing vessel 24 by using the signal from the level transmitter or sensor 23.

The solids are dosed at a controlled rate by auger feeder 28. The dust thus generated is trapped by the bag filter 27, while the excess air is allowed to escape. Solids make contact with the rotating liquid in mixing vessel 24. The centrifugal force spins the particulate solids towards the inner wall of mixing vessel 24. All particulate solids have interstitial air. This air, which is lighter, moves towards the middle of mixing vessel 24 and discharges in the vortex cone 31 as shown in Fig.1.

The solids mixed with the liquid move to an outlet port 29 of mixing vessel 24 and are drawn to the pump 25. The pump 25 has a mixer, size reduction equipment and a pressure generator. The mix coming out at product outlet line 26 is an homogenous mix of the solids in the liquid.

The size of the mixing vessel 24 and the flow rate of liquid through the apparatus is determined by the characteristic of the liquid and solid(s).

While it is believed that the physics involved in the operation of the apparatus of this invention may be explained by:-

(a) Stake's law of centrifugal forces;

(b) The principle of vortex formation in rotating liquids;

(c) The principle of cyclone operation; this description is not bound by such an explanation.

The configuration of the apparatus according to the present invention is not limited to the preferred embodiments disclosed.

A trial unit of apparatus for mixing a solid and a liquid was built in accordance with the schematic diagram of Fig. 1.

The trial unit was used to add lactose powder to milk for protein standardisation. The milk flow rate was 6,000 litres per hour and the lactose powder to be added was 300kg per hour.

Pump 30 was rated at 6,000 litres per hour, feeding to the evaporator. Line 21 (size 40 mm) fed to the mixing vessel 24. Accordingly, 300kg of lactose was added to 6,000 litres of milk per hour. Pump 24 is a centrifugal pump with a 1.0 kW motor running at 1350 rpm.

The alternate technology required two 30,000 litre tanks with necessary mixing, pasteurising equipment and with provision for cleaning using hot detergents. The product was still prone to bacterial contamination. Since the solution was made using water, the water had to be evaporated which increased energy usage as well.

In another experiment, the trial unit was used for mixing starch into water. This was equally successful.

In another trial it was used for mixing skim milk powder into the water. This is a very difficult application and always tends to generate foam. In the trial unit, no foam was formed at all.

In another experiment, the trial unit was used for mixing lactose into skim milk concentrate (viscosity equal to 50cps), and the unit performed in accordance with expectation. Thus, the addressee will see that the apparatus and method of this invention provides:-

(a) An in-line continuous process thus does not need holding tanks and vessels;

(b) A gentle mixing action and removal of air, without aeration of the product, which would have played havoc with the vacuum control of a multi effect evaporator;

(c) No requirement for an additional solvent like batch processes require;

(d) The equipment may be washed in-line and does not need separate washing;

(e) The equipment has a small footprint and needs very little space compared to batch mixing which needs tanks and pumps;

(f) The control systems to integrate this concept into an automatic plant are very simple; and

(g) It may be used with a large variety of products.

While the above description contains many specificities, these should not be construed as limitations on the scope of the invention, but rather as an exemplification of one preferred embodiment thereof. Many other variations are possible. For example, if the flow rate required is lower than what is required to form a proper vortex, a part of the mixed liquid can be recirculated through the liquid inlet line.

In another example if the flow rate in the line is too large, only a part of the liquid stream can be passed through the apparatus.

Alternative means of level control can be used too. As an example, if it is connected to an existing balance tank, the height of the apparatus may be such designed that the level control is not required at all.

Alternative means to tangential entry can be used for generating a rotating liquid. This could be a mechanical means like a rotating mechanical device which induces a rotation in the liquid.

The apparatus may be used for mixing liquids as well. In place of the deposition of solids particulate matter, a different liquid may be deposited into mixing vessel 24.

Additional advantages and modifications will readily occur to those skilled in the art. The invention in its broader aspects is, therefore, not limited to the specific details, representative apparatus and illustrative examples shown and described. Accordingly, departures from the spirit or scope of the invention can be made.

Claims

1. Apparatus for mixing a solid and a liquid, including a mixing vessel having an inlet for said liquid, means to introduce said solid into said vessel, and an outlet from said vessel for a mixture of said solid and said liquid, characterised in that said liquid is introduced into said vessel through a tangential entry, to rotate the liquid in said vessel, the rotation of said liquid causing the formation of vortex in said vessel, and in that said solid is deposited into said vortex.

2. Apparatus according to claim 1 , characterised in that vortex is controlled such that the air core at the centre of said vortex does not reach said outlet.

3. Apparatus according to claim 1 or claim 2, characterised in that said solid is a particulate solid.

4. Apparatus according to claim 3, characterised in that said particulate solid is deposited into said vessel in a stream.

5. Apparatus according to claim 3 or claim 4, characterised in that said particulate solid is deposited into said vessel through an auger feeder.

6. Apparatus according to any preceding claim, characterised that sensing means is provided to sense the level of liquid or mixture in said vessel, and in that a control valve is located in an incoming line communicating with said inlet, said control valve acting to control the flow of said liquid in accordance with the information from said sensing means and in relation to a predetermined level in said vessel.

7. Apparatus according to any preceding claim, characterised that a pump is provided to pump mixture from said outlet.

8. A method of mixing a solid and a liquid, including the steps of:-

introducing said solid into said vortex.

9. A method according to claim 8, characterised in that said solid is a particulate solid.

10. A method according to claim 9, characterised in that said particulate solid is deposited into said vortex by an auger feeder.

11. A method according to any preceding claim, further including the step of removing the mixture of said solid and said liquid from said vessel.

12. A method according to any preceding claim, characterised in that said vortex is controlled such that the air core therein does not reach the outlet of said vessel, through which a mixture of said solid and said liquid is removed from said vessel.

13. A method according to any preceding claim, further including the step of controlling the level of said liquid and/or said mixture of said liquid and said solid in said vessel.

14. An apparatus for mixing liquids and particulate solids comprising a vessel with inlet and outlet for said liquid, said liquid said inlet for introducing said liquid into the said vessel, a means to create a rotating said liquid in a manner that the air core in the middle of the said rotating said liquid does not reach the said outlet of the said vessel, said particulate solids inlet means for introducing a stream of said particulate solids into the said rotating liquid, and removing the mixed said liquid and said particulate solids from the said outlet of the vessel.