WO1999044709A1

WO1999044709A1 - Stripping of volatile substances from less volatile fluids

Info

Publication number: WO1999044709A1
Application number: PCT/GB1999/000614
Authority: WO
Inventors: Nicholas Simon Hall Taylor
Original assignee: Chemtech International Ltd
Current assignee: Chemtech International Ltd
Priority date: 1998-03-02
Filing date: 1999-03-02
Publication date: 1999-09-10
Anticipated expiration: 2000-09-02
Also published as: GB0024049D0; GB2351241B; AU3262499A; GB9804479D0; GB2351241A

Abstract

The invention provides a tower deodoriser structure (1) to be placed under a vacuum. A spray nozzle (9) is disposed in an upper region of the structure and is connectable to a supply means for a less volatile fluid containing a more volatile fluid to be removed therefrom. The spray nozzle (9) is capable of atomising the less volatile fluid prior to having the fluid undergo downward travel through the tower structure in its atomised state. Steam supply means (11) is provided in a lower region of the tower, and outlet (5) duct means for steam is provided in an upper region of the tower. A disentrainment hood (10) above the spray nozzle divides the tower interior into two parts.

Description

- 1 -

STRIPPING OF VOLATILE SUBSTANCES FROM LESS VOLATILE FLUIDS

This invention relates to a method and apparatus for the stripping of volatile substances from less volatile fluids and in particular relates to the deodorisation and steam refining of fats and oils.

Oils and fats are natural products whose impurity levels will vary not only with oil type but also with weather, soil, harvesting, feed storage and extraction conditions. Oil refining can be considered to start in the crude oil storage tank where there is gravity separation of oil insoluble material. Purification of the oils and fats then involves a number of stages some of which are optional depending on the quality of the oil supplied and all of which begin with degumming to remove phospholipids, sugars, resins and proteinaceous compounds, trace metals and others and culminate in a deodorisation step to remove volatile substances including fatty acids, mono- and diglycerides, aldehydes and ketones, hydrocarbons and pigment decomposition products.

Traditionally deodorisation has involved steam distillation under vacuum or near vacuum conditions. The effectiveness of the process depends on a combination of factors including the intimacy of mixing of steam and oil and the vacuum and temperature employed. It is a process very much concerned with efficiency - for instance, energy saving by heat exchange, reduction of steam usage, both for stripping and vacuum, baffling to prevent the loss of neutral oil entrained in distillate vapour, the loss of fatty material in vacuum condenser water and the loss of product due to saponification of triglycerides .

The basic conditions of deodorising, that is vacuum, temperature and stripping steam usage are the same for continuous deodorisers as for batch or semi- continuous units. In recent years, design developments have concentrated on oil/stripping steam contact improvement. High dispersion efficiency permits plant size reduction with consequent lowering of cost and also reduces the length of time for which the oil must be held on peak temperature. In this connection, where batch deodorisation processes are concerned, it is desirable to reduce the mass ratio of steam to oil as much as possible thereby reducing energy input to a steam: oil mass ratio of 1-2% ie . , 1:50 to 1:100. An oil mass ratio of 1:10, or at best 1:20, is generally all that is achieved when working with conventional deodorisers in which steam is bubbled in counter- current to an oil which flows down a cascade of trays. Thus, to work with much more preferred ratio of 1:50, it is necessary to optimise the time of residence of the droplets in the gas phase and maximise the surface area for improved mass transfer:

One technique which has been developed in this respect is to atomise the oil as it is supplied into a continuous steam phase under vacuum conditions. The mixture passes through a high turbulence contactor in which volatile constituents of the oils are taken up in the steam phase which is separated from the oil phase in a cyclone separator under vacuum or using thin film evaporator techniques. Thus, the oil may be injected into the throat of a venturi which atomises the oil into a flow of steam. The oil and steam then flow as a two phase flow mist. The mass ratio of steam to oil used is typically still only 1:10. Investigations have shown that such a process is controlled by the rate of diffusion within the liquid droplets. A steam ratio is needed which ensures efficiency of atomisation of the oil since such efficiency is strongly dependent upon - 3 - droplet size. However, a disadvantage is that the high steam velocity which ensues means that the subsequent residence time is very short and downstream contactors have had to be used. Such a method of working is disclosed for example in British Patent Specification No. 1205776 which provides for the mixture of volatilised oil and steam to pass along a tortuous path having a surface configuration that induces high turbulence thereby forming a homogeneous mixture of gas, oil and volatiles stripped from the oil, while increasing the velocity of movement of the mixture by expansion of the steam. Techniques hitherto devised have not been able to achieve use of steam: oil mass ratios significantly better than 1:20. In an attempt to improve steam: oil mass ratios, our European Patent specification EP-A-677313 has proposed a method of stripping volatile substances from liquid feed material in which the liquid feed material is supplied to a constricted flow of gas, causing turbulence to occur in the gas thereby to atomise the liquid feed material therein, and increase the surface area of liquid feed material at which transfer from the liquid feed material of volatile substances therein into the gas takes place, atomised droplets of the liquid feed material collecting on a wall surface surrounding the gas being returned to the flow of liquid feeding material within the gas by baffle means inclined in the direction of flow of the gas and effecting, at a position remote from that at which the turbulence occurs, liquid/gas separation to separate from the liquid feed material the gas with volatile substances from the liquid entrained therein.

Although an enhanced steam: oil mass ratio may be obtained by working in accordance with such a proposal, simplicity of plant construction and concomitant ease of operation is thought to be best achieved with - 4 - conventional tower deodorisers even if a high steam ratio has to be used. As a result of further studies, we have now devised a tower deodoriser which enables enhanced performance to be obtained in the stripping of volatile substances from less volatile fluids.

Thus, according to the present invention, there is provided a tower deodoriser comprising a tower structure, means for placing the interior of the tower structure under a vacuum, a spray nozzle disposed in an upper region of the tower structure and connectable to a supply means for a less volatile fluid containing a more volatile fluid to be removed therefrom, the spray nozzle being capable of atomising the less volatile fluid prior to having the fluid undergo downward travel through the tower structure in its atomised state, steam supply means in a lower region of the tower structure, and outlet duct means for steam in an upper region of the tower structure, in which deodoriser the spray nozzle is located under a disentrainment hood which divides the interior of the tower structure into an uppermost section leading to the outlet duct means and a main body section and provides an annular passage between it and the tower structure itself. Thus, this invention provides a means of downwardly directing a mist of atomised fluid, which is generally a natural oil or molten fat material such as cocoa butter, against counterflowing steam.

This invention also provides a method of stripping a more volatile fluid from a less volatile fluid which comprises supplying the less volatile fluid containing the more volatile fluid to the interior of a tower deodoriser via a spray nozzle located in a upper region of the tower deodoriser under a disentrainment hood which divides the interior of the tower deodoriser into an uppermost section and a main body section and defines an annular passage between it and a tower - 5 - deodoriser wall, supplying a rising body of steam to flow in countercurrent to droplets of the less volatile fluid produced at the spray nozzle and descending the interior of the tower deodoriser and withdrawing steam containing more volatile substances stripped from the feed material at an upper outlet from the tower deodoriser while removing less volatile fluid from a lower outlet at the bottom of the tower deodoriser, the tower deodoriser being subject to a vacuum during the course of stripping.

The present invention builds on the previously recognised potential effectiveness of stripping a volatile fluid from a mist of small droplets. A mist of the less volatile fluid is created for example by a standard multispray nozzle. The mist is created at the top of the tower structure, acting as a "contacting vessel", which is under vacuum and into which steam is introduced at the bottom to flow in countercurrent to the falling mist. Volatiles are taken up by the steam and removed with the steam from^" the top of the vessel, whereafter the steam passes to a scrubber and vacuum system. Such a system will work with a steam to oil ratio of less than 0.5%.

It is necessary for the present invention to ensure the droplets have a sufficient flight time before they reach the bottom of the vessel. There are two ways in which this may be achieved. Firstly, the tower may be made with a large, cross section in its upper portion. Secondly, the steam may be introduced in two parts. The first part is introduced at the bottom of the tower as already stated and a second supply of steam is from a ring surrounding the nozzle in which ring are formed a number of holes such that steam will then flow radially inwardly towards the atomising nozzle against the outflowing fine droplets, thus reducing their radial velocity. This achieves the - 6 - result that droplet contact with the wall of the vessel takes place lower down and hence a desired residence within the tower can be achieved in a tower of smaller diameter than might otherwise be contemplated. The rate of removal of the more volatile fluid is proportional to the parameter DT/R² where D is diffusivity, T is the residence time of the droplets within the tower and R is the average droplet radius . The greater this parameter, the better the transfer. It is preferred that the spray device be capable of generating droplets less than lOOμ in radius in a contacting vessel shaped so that the residence time is as large as possible.

The steam, typically constituting 0.5% by weight of the liquid flow (ie., a 1:200 steam/oil mass ratio), is introduced at the bottom of the vessel and flows in the opposite direction to the falling droplets. In spite of the low vacuum employed (typically at 5mm Hg or lower) , the steam has sufficient velocity to affect the movement of the smaller droplets and these can be carried with the flow of steam.

In order to improve efficiency while minimising the risk of carry over, the contacting vessel has a disentrainment hood. This hood is located in an upper region of the tower and causes the rising steam to ' move towards the outer wall of the vessel. Smaller droplets present in the rising steam are caused to pass through the mass of falling larger droplets creating a scrubbing effect on the smaller droplets which coalesce with the larger ones.

In addition, the disentrainment hood ensures modest steam velocity because of the larger area at the circumference . The steam can then accelerate in the space between the hood and the vessel walls while avoiding the risk of droplets entrainment at the walls that would take place if the hood were not there . - 7 -

Nevertheless, despite the vacuum employed and the low steam/oil mass ratio, the atomised droplets may enter into circulation within the vessel. This circular motion is induced by a large mass of oil entering through the nozzle at high velocity. This means that the residence time of the droplets, particularly the smaller ones, is significantly increased. A consequence of this is that, despite the entrainment hood, it may be necessary to provide an annular disentrainment device in the annular gap between the disentrainment hood and the vessel wall through which the steam is to travel on its way out of the vessel.

The annular disentrainment device can use one of a variety of different systems such as used in de-misting technology. Preferred is a wire knit mesh body made of fine metal wire. By having such body occupy a height of 15cm, a high rate of droplets removal can be achieved without generating a significant pressure drop. This means that the desired low vacuum can still be maintained in the main part of the vessel.

An alternative form of disentrainment device consists of a series of parallel plates of sinuous form mounted at an angle to the axis of the vessel. The droplets impinge on the "wavy" walls of the plates since they do not change direction as easily as gas molecules and descend therefrom into the interior of the vessel.

By operating in accordance with the present invention, it has been found possible to achieve reductions in volatiles in oil leaving the bottom of the contacting of the vessel, for example, cocoa butter in the liquid phase, of between 50-75% with a single pass . For a better understanding of the invention, and to show how the same can be carried into effect, - 8 - reference will now be made, by way of example only, to the accompanying drawings wherein Figure 1 is a vertical section through one form of tower deodoriser embodying the invention and Figure 2 is a like section through a second form of tower deodoriser.

In Figure 1 of the drawings, a tower deodoriser 1 can be seen to comprise a relatively small diameter cylindrical central portion 2 surmounted by an upper conical section 3 and surmounting a lower conical section 4. The upper conical section 3 opens into an outlet neck 5 while the lower conical section 4 communicates at its bottom with a bottom outlet 6. The body of the tower deodoriser 1, that is central portion 2, upper conical portion 3 and lower conical portion 4, is provided with an insulating jacket 7. An oil feed duct 8 to the tower deodoriser 1 passes through outlet neck 5 and terminates in a spray nozzle 9 in an upper region of central portion 2 of the tower deodoriser. In the lower part of the tower deodoriser, within the lower conical section 4, is a steam supply ring 11 formed with a plurality of outlets for release of steam into the interior of the tower deodoriser and fed by steam supplied by a steam supply duct 12. The reference numeral 13 denotes the outer limit of the body of oil droplets emitted by the spray nozzle 9 and descending within the tower deodoriser below a disentrainment hood 10.

The tower deodoriser is mounted in a support structure (not shown) by means of four support elements 14 set at 90° intervals around the periphery and below one of which is a sight glass 15 for viewing the interior of the deodoriser.

Operation of the tower deodoriser 1 takes place in the following manner. The interior of the tower deodoriser is placed under a vacuum of a few millimetres of mercury. Oil to be deodorised is - 9 - admitted to the interior of the tower deodoriser 1 through oil feed duct 8 and is sprayed into the interior of the tower deodoriser at spray nozzle 9 to conform to a spray pattern indicated by reference number 13 which is such that oil droplets do not become entrained by the inside wall of the central portion of the tower deodoriser. Steam is supplied to the steam supply ring 11 via of the steam supply duct 12 and escapes upwardly from the steam supply ring 11 to rise within the tower deodoriser and gradually come into contact with oil spray droplets. Nozzle 9 may be a standard multi spray nozzle which produces a line mist of oil droplets at the top of central portion 2 of the tower deodoriser 1. As the steam introduced at steam supply ring 11 rises in countercurrent to the falling mist of oil droplets, volatiles in the oil droplets are removed with the steam which then goes on to leave the tower deodoriser at the top and go onto a scrubber and vacuum system. The system will work efficiently with a steam to oil rate ratio of less "than 0.5%.

To achieve satisfactory contact between the oil droplets and the steam, it is essential for the droplets to have a sufficient flight time before they reach the bottom of the tower deodoriser 1. There are two ways in which this can be assisted. Firstly, the tower deodoriser 7 has the upper conical section 3 of relatively large cone angle. Secondly, the steam can be introduced in two parts . Not only would the steam be introduced through spray nozzle 9, but such steam would enter into the interior of the tower deodoriser from a ring (not shown) surrounding the spray nozzle 9 and in which are drilled a number of holes. Steam is able to flow radially from such holes inwardly towards the spray nozzle 9 against the oil droplets flowing out from the nozzle 9, thus reducing their radial velocity. These measures enable the droplets to have a point of - 10 - impact with the interior surface of the tower deodoriser 1 which is relatively low down the tower deodoriser. Hence, the construction of the invention allows a relatively long residence time to be achieved in a vessel of relatively small diameter.

The disentrainment hood 10, for its part, has the function of improving efficiency while minimising the risk of carry over of oil droplets out through outlet neck 5 as the steam from steam supply ring 11 rises gradually through the interior of the tower deodoriser. The disentrainment hood 10 first causes the steam to move towards the outer wall of the tower deodoriser 1. The effect of this is to cause entrained small oil droplets to pass through the mass of falling larger droplets, creating a scrubbing effect on the smaller droplets which coalesce with the larger ones.

Secondly, the disentrainment hood ensures a modest steam velocity because of the relatively large assessable area at the circumference of the tower deodoriser. The steam can then^" accelerate into the space between the hood and the wall of the deodoriser while avoiding the risk of droplets entrainment at the wall of the deodoriser that would take place if the hood were not there . In Figure 2 like reference numerals denote like parts in Figure 1. The tower deodoriser of Figure 2 differs from that shown in Figure 1 in that the lower conical section 4 is replaced by a dished end 40 Manufacture is then simpler to achieve and the total height of the deodoriser may be reduced. In addition, and equally usable with the tower deodoriser of Figure 1, there is provided an annular disentrainment device 16 formed, for example, of wire knit mesh occupying the annular gap between the disentrainment hood 10 and the wall of the deodoriser. The device 16 serves to separate from rising steam any oil droplets still -11- circulating therein.

Claims

- 12 -CLAIMS

1. A tower deodoriser (1) comprising a tower structure, means for placing the interior of the tower structure under a vacuum, a spray nozzle (9) disposed in an upper region of the tower structure and connectable to a ^Γûá supply means (8) for a less volatile fluid containing a more volatile fluid to be removed therefrom, the spray nozzle being capable of atomising the less volatile fluid prior to having the fluid undergo downward travel through the tower structure in its atomised state, steam supply means (11) in a lower region of the tower structure, and outlet duct means (5) for steam in an upper region of the tower structure, characterised in that the spray nozzle (9) is located under a disentrainment hood (10) which divides the interior of the tower structure into an uppermost section (3) leading to the outlet duct means (5) and a main body section (2) and provides an annular passage between it and the tower structure itself.

2. A tower deodoriser according to claim 1, wherein a further steam supply means surrounds the spray nozzle (9) .

3. A tower deodoriser according to claim 1 wherein the further steam supply means is .a steam supply ring surrounding the spray nozzle (9) and formed with a plurality of apertures for directing steam therefrom radially inwardly.

4. A tower deodoriser according to any preceding claim, wherein the tower structure comprises a cylindrical main portion (2) surmounted by an upper conical section (3) .

5. A tower deodoriser according to any preceding claim, wherein said steam supply means (11) is located in a lower conical body section (4) .

6. A tower deodoriser according to any preceding claim wherein the annular passage between the - 13 - disentrainment hood (10) and the tower is occupied by an annular disentrainment device (16) for any droplets of the less volatile fluid being carried upwardly with the steam.

7. A tower deodoriser according to claim 6, wherein the disentrainment device (16) comprises a body of wire knit mesh.

8. A method of stripping a more volatile fluid from a less volatile fluid which comprises supplying the less volatile fluid containing the more volatile fluid to the interior of a tower deodoriser via a spray nozzle located in the upper region of the tower deodoriser under a disentrainment hood which divides the interior of the tower deodoriser into an uppermost section and a main body section and defines an annular passage between it and a tower deodoriser wall, supplying a rising body of steam to flow in countercurrent to droplets of the less volatile fluid produced at the spray nozzle and descending the interior of the tower deodoriser and withdrawing steam containing more volatile substances stripped from the feed material at an upper outlet from the tower deodoriser while removing less volatile fluid from a lower outlet at the bottom of the tower deodoriser, the tower deodoriser being subject to a vacuum during the course of stripping.

9. A method as claimed in claim 8, wherein the steam is admitted to the tower deodoriser in an amount such as to constitute about 0.5% by weight of the supply of less volatile fluid into the tower deodoriser.

10. A method as claimed in Claim 8 or 9 , wherein additional steam is admitted to the interior of the tower deodoriser by means of an apertured ring surrounding the spray nozzle and directing steam radially inwardly.

11. A method as claimed in any one of claims 8 to 10 - 14 - where residual droplets of the less volatile fluid are separated from the rising body of steam in a disentrainment device in an annular gap surrounding the disentrainment hood.

12. A method as claimed in any one of claims 8 to 11, wherein the tower deodoriser comprises a tower structure including a cylindrical main portion surmounting a lower conical body and wherein the spray pattern from the spray nozzle is such that the fluid droplets reach the wall of the tower deodoriser at or adjacent transition from the central portion to the lower conical body.

13. A method as claimed in any one of claims 8 to 12, which is applied to a natural oil or molten fat material.