GB2276559A - Processing of liquid/solid mixtures using pulsations - Google Patents

Abstract

A method of processing a mixture of at least one liquid and at least one solid in particulate form, in which the mixture is contained within a vertical vessel 1 divided into compartments 12 - 17 by a series of apertured baffles 10 located at axial intervals, in which adjustable pulsations are applied to the mixture, and in which the frequency and amplitude of the pulsations are chosen so that so-called "chaotic" flow takes place within the compartments, promoting a steady-state axial distribution of the solid/liquid concentration, namely a common value within any one compartment and a predetermined ratio between any two adjacent compartments. The method may be of batch type, or liquid may continually flow through the vessel. The baffles may typically be coaxial with the vessel 17 and ring-shaped, so that communication between adjacent compartments is by way of the central aperture in the baffle separating them. Pulsations may be applied by means of an oscillator 9 acting on a plate 6 resiliently-suspended by springs 7, the plate being connected to the bottom of the vessel by bellows 5. The frequency of pulsations may be 2 - 25 Hz. The processing may be mixing, chemical reaction, dissolving crystallisation or separation. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO THE PROCESSING OF MIXTURES This invention relates to the processing of mixtures of liquids and particulate solids. By the term processing we include for instance firstly mixing, especially where there may be a requirement to handle a broad range of particle size or density and a high volume fraction of solids loading. Secondly processing in which a chemical reaction takes place between the constituents of the mixture, leading to a liquid, solid or gaseous product, especially where efficient heat/mass transfer is particularly required. Thirdly, dissolving or crystallising.

Fourthly, the use of the process to achieve separation from mixtures of particles of different sizes, shapes and densities.

Four preferred features of the invention are, firstly, that the mixture is processed in an elongated vessel arranged either vertically or steeply sloping. Secondly, that the vessel is divided into a succession of compartments separated by baffle means located at axial intervals within the vessel. Thirdly, that the mixture is subjected to a pulsating motion which, in combination with the gravity forces inherent in a vertical vessel, causes the mixture to reach a steady state in which a substantially uniform characteristic is achieved within each compartment but there is a predetermined relationship between the values of that characteristic as you proceed from each compartment to the next. Fourthly, that parameters are chosen so that chaotic liquid flows are promoted within the vessel.By chaotic flow we mean a flow where adjacent fluid elements would separate exponentially, rather than linearly as in the case of a steady, simple shearing flow. In order to generated a chaotic flow in a baffled tube the oscillatory Reynolds No. ReO should probably exceed of the order 150 where:

Where D is the tube diameter, X is the angular frequency of oscillation, xO is the centre-to-peak amplitude of oscillation, and v the kinematic viscosity.

The invention is to be distinguished from processing methods involving both baffles and oscillations as described, for instance, in Patent GB-B-2187970, where the axis of the elongated processing vessel is essentially horizontal. The invention is also to be distinguished from the many known so-called "fluidised bed" methods of achieving continuous-ion-exchange and other processes which require a continuous net recirculating or other flow of fluid through a process vessel, and also at least one bed structure - typically a succession of them - on which to support fractions of the particulate solid at those times during the process cycle when they are not in a fluidized state.The invention is again to be distinguished from the kind of apparatus and processing method described in Patent Specification SU-A-899107, in which a mixing vessel subject to pulsation is fitted with a series of axially-spaced baffles, but in which those baffles are shaped so as to impose a deliberate component of rotary velocity (about the axis of the vessel) upon fluid which passes axially through them, so imposing a partiallyordered flow instead of a chaotic one.

The invention is defined by the claims, the contents of which are to be read as included within the disclosure of this specification, and the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a diagrammatic and simplified view of a processing vessel in vertical section, and Figures 2 and 3 are graphs.

Figure 1 shows an elongated vessel 1 fixed by a clamp 11 and arranged with its axis 2 vertical, with its top 3 open and its base closed by a plug 4 which is permeable to the liquid but not to the solid constituents of the mixture to be processed. A liquid-tight bellows 5 connects the base of vessel 1 to a plate 6 which is resiliently suspended by springs 7 and carries a transducer 8 to produce an output indicative of the frequency and amplitude of the vertical oscillations imparted to the plate 6, and thus to the mixture within vessel 1, by an oscillator indicated at 9. Ring-shaped baffles 10, mounted at vertical intervals from the inner wall of vessel 1, divide the vessel into a vertical succession of compartments 12-17.The radial extent of the baffles 10 is such that although it is sufficient for them to exert a substantial mixing action of the contents of the vessel when they are pulsated by the vibrator 9, and for the baffles to define distinct compartments (12-17), in which stable liquid/solid regimes can exist when a steady state is reached.

Nevertheless the central clearance defined by each baffle must be large enough for both the liquid and the solid particles to move freely between compartments whenever a steady state does not exist. Assuming vessel 1 is transparent, measurement of solids concentration within any of the compartments 12-17 may be achieved by an optical densitometer (18) moveable up and down the outer wall of the vessel. While the apparatus may typically be used in 'batch' operation to achieve a useful steady-state condition of a single charge of mixture admitted at the start by pouring into the open top 3, by use of interchangeable inlet/outlet means (shown in outline in broken lines at 19 and 20) the liquid content (and, if desired, the solid content too) of the charge may if desired be changed continuously as operation proceeds.

Experiments with such apparatus have been conducted in which vessel 1 was 0.5m tall and of internal diameter 23mm, and the bellows 5 were of flexible stainless steel. Baffles 10 had a radial width of 8mm, so leaving a central orifice of diameter 7mm, and were spaced 35mm apart axially. The batch charge was a 1% w/w concentration of ion-exchange beads in water, the mean initial diameter of the beads being 0.5mm, and their density 1100 kg/m3. Oscillator 9 was of electromagnetic type, and was not positively attached to the underside of plate 6, so that care had to be taken not to exceed "g"-force on the underside of the plate, and so risk loss of contact between items 6 and 9.

Oscillator 9 was operated over a range of frequencies and amplitudes. It was found that peak-to-peak amplitudes of less than 3mm were insufficient to levitate the solid beads, and that 6mm was the maximum amplitude achievable without exceeding "g"-force on the bellows, but that if there had been a positive connection between oscillator 9 and the underside of plate 6 a maximum complete stroke of the oscillator of up to 18mm might have been possible. Frequency was varied in the range 3 to 10 Hz.

It was found that provided the beads were heavier than the liquid and were levitated by choosing an amplitude of oscillation of not less than 3mm, and a frequency giving rise to a maximum oscillating velocity of not less than about lOmm/s, a steady state was achieved in which the bead/water concentration was constant in each individual compartment (12-17), with good mixing within each such compartment, the concentration being highest in compartment 12 and falling off roughly exponentially for each successive compartment. It was found that adjustment of oscillation conditions - including asymmetrical oscillations could be used to control the segregation for example of larger particles in the lower compartment and smaller particles of similar density in higher compartments. Increasing the maximum velocity of the oscillations tended to diminish the steepness of the exponential drop in solids concentration, already referred to, leading eventually to a condition in which there was a substantial constant solids concentration in all the compartments 12-17. If the particulate solids should instead have been buoyant relative to the liquid the reverse of the above result could have been expected, namely an exponential rise in solids concentration through compartments 12-17 at lower oscillation velocities, leading to a greater uniformity of concentration as the velocity rose.

The effect of the oscillations can also be influenced by structural features. For instance by making the baffles asymmetrical by giving them different configurations on their two axially-facing surfaces.

The capacity of the spaced baffles 10 to promote graduated and/or uniform mixing of solids and liquid within the compartmented region of the tube was also evident from another observation of the experiments using heavier-than-liquid particles. However much power was supplied to the oscillator 9, up to and above the level achieving uniform solids/liquid concentration within all compartments, above the level of the topmost baffle 10a mixing always deteriorated and concentration fell off. It proved impossible to cause any particles at all to remain suspended in the liquid above the level shown in broken lines at 21, which lies at a height (h) (identified in the experiments as the 'disengagement height') above the top baffle lOa.In the experiments h had a value of about 70mm, rising to only lOOmm even when the key experimental parameters were widened to include the following alternatives and ranges: Baffle Orifice Diameter 12mm and 7mm Baffle Separation Distance 35mm and 70mm Particle Type Resin (terminal velocity in water 45mm/s) and sand (78mm/s) Amplitude Range 2-8mm Frequency Range 2-25Hz Particle Concentration Up to 50X v/v Figure 2 is a graph illustrating, approximately, the typical range of operating conditions likely to result in chaotic flow (as already defined) in a fluid-filled baffled vessel in which the fluid is subject to oscillations. The y-axis indicates the peak-to-peak amplitude of the oscillations, from zero at the origin to a maximum of say a few centimetres at A.The x-axis represents the frequency of the vibrations, from zere at the origin to a maximum of say a few kHz at point B. At high amplitudes and low frequencies, falling for instance within the approximate area enclosed by envelope 25, where the amplitude of the oscillations will be of the order of centimetres and the frequency in the range say 0.1 - 1.0 Hz, mixing within the vessel will have the character commonly known in the art as "quasi-steady". At the other extreme, within the region enclosed by the approximate envelope 26, where the amplitude may be as low as a few microns but the frequency will typically be within the range say 30 Hz - 1 kHz, the mixing will have the character generally known as acoustic.The invention applies especially to the area in between, represented roughly by envelope 27, and covering a typical frequency range of say 2 - 20 Hz and an amplitude of a few millimetres.

Assume that one of the series of compartments (referenced 12-17 in Figure 1) is referred to generally as compartment n, and the adjacent compartment as n+l, then the ratio y of the concentration C between the adjacent cells may be defined as Cn+ Y = therefore, Cn y(n-l ) C1 and

Research with apparatus according to the present invention has indicated the surprising conclusion that for constant amplitude and frequency of oscillation, y is substantially constant for all pairs of adjacent compartments throughout the series. From this it follows that particle concentration distribution can be defined by y alone and is dependent only upon the velocity of oscillation, i.e. the same particle distribution would result from a N1.Hz frequency and N2.mm amplitude as for a N2.Pz frequency and Nl.mm amplitude, provided of course that N1 and N2 both lie within the approximate area of envelope 27 of Figure 2, so that chaotic flow results. In Figure 3, the readings for which were taken from apparatus as described with reference to Figure 1, y has been plotted, for a range of amplitudes and frequencies of oscillation, against the ratio of the oscillatory velocity to the terminal velocity of the particles. The nearly-linear character of the function 28, derived from the individual readings in Figure 7, is evidence that when the method of the present invention is performed, y varies steadily and directly with the ratio V oscillatory/V terminal.

It should be noted that although the invention has been described with reference only to ring-shaped baffles, the invention can also be effected using many other shapes of baffle, e.g. in particular multi-orificed baffles. Whatever sort of orifice or orifices each baffle presents, it is generally advantageous for the edges of the orifices to be sharp, and for the baffles to be arranged so that the orifice edges point in a direction that can be resolved into a substantial component in a plane lying transverse to the length of the vessel. Sufficient sharpness may be achieved in various ways, e.g. by forming the edge to an evident point or by forming the baffle from thin material. As one indication of sharpness the formula xo/r > l, and preferably > 10, may be applied where r is the radius of curvature at the edge and xO is the maximum amplitude of the applied oscillations.

Claims (19)

1. A method of processing a mixture of at least one liquid and at least one solid in particulate form, in which the mixture is contained within an elongated and substantially vertical vessel within which baffle means are present at axial intervals so dividing the vessel into communicating but axially-successive compartments, in which adjustable oscillations are applied to the mixture, and in which the combination of those oscillations and gravity results in so-called "chaotic" mixing of liquid and solid within the compartments and in a steady state axial distribution of a given property of the liquid/solid mixture, there being a substantially common value of that property within any one compartment and a predetermined relationship between the values in two adjacent compartments.

2. A method according to Claim 1 in which the predeterminable relationship takes the form of a gradation of the value of the given property in successive compartments.

3. A method according to Claim 1 in which the predeterminable relationship is equality between the values of the given property in successive compartments.

4. A method according to Claim 1 in which the given property is the concentration of the solid in the liquid.

5. A method according to Claim 1 in which the solid particles of the mixture are of the same density, and the given property is average particle size.

6. A method according to Claim 1 of batch type, in which the charge of mixture within the vessel remains unchanged during the process.

7. A method according to Claim 1 in which liquid continually flows through the vessel during the process.

8. A method according to Claim 1 in which the oscillations are applied to the liquid in the vessel by means of at least one communicating and driven piston, bellows, diaphragm or the like.

9. A method according to Claim 8 in which the oscillations are so applied by means communicating with one end of the vessel.

10. A method according to Claim 8 in which the oscillations are applied by means communicating with both ends of the vessel.

11. A method of processing according to Claim 1 in which the oscillations are of asymmetric character, whereby to modify the action they impose upon the mixture.

12. A method according to Claim 1 in which the predetermined relationship between the valves of the given property in a pair of adjacent compartments is the same for substantially all such pairs of compartments within the vessel.

13. A method according to Claim 1 in which the oscillatory Reynolds number applicable to the oscillations exceeds about 150.

14. A method according to Claim 1 in which the frequency of the pulsations lies substantially within the range 2 - 20 Hz.

15. A processing vessel for carrying out a method of processing according to any of the preceding claims.

16. A processing vessel according to Claim 15 in which the baffle means are formed integrally with the vessel.

17. A processing vessel according to Claim 15 in which the baffle means are formed separately from the vessel but are fixed to it.

18. A processing vessel according to Claim 15 in which the baffle means are ring-shaped baffles coaxial with the vessel and mounted on its inner wall.

19. A processing vessel according to Claim 18 in which the ring-shaped baffles are asymmetrical, with the surface of the ring that faces in one axial direction having a different configuration from the corresponding surface facing in the opposite direction.