GB2120392A

GB2120392A - The measurement of the sizes of particulate matter dispersed in an aerosol

Info

Publication number: GB2120392A
Application number: GB08310990A
Authority: GB
Inventors: Dr Ray William Kenneth Allen; Dr Keith Sydney Robinson
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1982-04-22
Filing date: 1983-04-22
Publication date: 1983-11-30
Also published as: GB2120392B; GB8310990D0

Abstract

A method for measuring the sizes of particulate matter dispersed in an aerosol comprises the operations of passing an aerosol the sizes of the particulate matter dispersed in which are to be measured sequentially through a plurality of beds 1, 2, 3 of foraminous material, measuring the amount of particulate material collected by each bed of foraminous material in a given interval of time, deriving therefrom the penetrations of the particulate material through each of the beds of foraminous material prior to the final one, and deriving therefrom an estimation of the sizes of the particulate matter in the aerosol. Removable rings 4, 5 of cotton lint or glass wool extract the liquid collected by filter beds 1 and 2 and are weighed at the end of a predetermined time together with the final filter bed 3 which collects the particulate matter remaining in the aerosol after passage through beds 1 and 2. <IMAGE>

Description

SPECIFICATION The measurement of the sizes of particulate matter dispersed in an aerosol The present invention relates to the measurement of the sizes of particulate matter dispersed in an aerosol, and in particular to the measurement of the size distribution of the droplets in a mist.

For some purposes, it is desirable to know the sizes of the droplets contained in a mist, but in practical situations, for example field measurements in chemical or other industrial plants, the measurement of mist droplet size is a very difficult task. Possible techniques which can be used for this purpose are: (a) Optical techniques using light scattered by or from the droplets. These are expensive and they are not suited to field use because of the difficulties associated with maintaining critical optical alignments and the need to keep optical surfaces clean.

(b) Methods utilising slides coated with powders such as magnesium oxide which are inserted into the mist, the sizes of craters caused by droplets impacting into the powder then being measured and correlated with drop sizes using standard equations. These methods are timeconsuming and subject to errors with small drops and concentrated mists.

(c) Cascade impactors. These consist of multiple stage devices which cause a fractionation of droplets on the basis of their inertia. Such units are used in the field but they are expensive and not suitable for use with concentrated mists. Also, because of the need to collect small quantities of liquid on the plates of the cascade impactor, they generally need some form of physico-chemical tracer to compensate for the effects of evaporation.

The present invention provides a method and apparatus which depends only on simple weighing operations to give the particle size distribution in an aerosol.

According to the present invention there is provided a method for measuring the sizes of particulate matter dispersed in an aerosol comprising the operations of passing an aerosol the sizes of the particulate matter dispersed in which are to be measured sequentially through a plurality of beds of foraminous material, measuring the amount of particulate material collected by each bed of foraminous material in a given interval of time, deriving therefrom the penetrations of the particulate material through each of the beds of foraminous material prior to the final one, and deriving therefrom an estimation of the sizes of the particulate matter in the aerosol.

Also according to the invention there is provided an apparatus for carrying out the method, including a duct for directing an aerosol the sizes of particulate matter dispersed in which are to be determined sequentially through a plurality of foraminous bodies removably mounted in the duct, the initial ones of which are such as only partially to extract particulate matter from the aerosol and the final one of which is such as to remove from the aerosol the remaining particulate matter.

Each of the foraminous bodies may, for example, be constituted by a close-packed array of spherical bodies or by a pad of fibrous material.

For the purpose of this specification, the term particulate matter is intended to include liquid droplets as well as solid particles. Thus the aerosol may be a mist.

In a preferred embodiment of the invention for use in determining the sizes of the droplets in a mist, each of the initial beds of foraminous material is surrounded by a body of absorptive medium which removes by capillary action the liquid from each of the said beds of foraminous material as it accumulates in that bed, so as to maintain the liquid trapping characteristics of the said beds constant throughout a given measurement period. The bodies of absorptive material preferably are detachable from their respective beds of foraminous material so that they can be weighed to measure the amount of liquid material extracted from the mist by the beds of foraminous material.

The invention will now be explained and described, by way of example, with reference to the accompanying drawing of an apparatus for use in carrying out the invention.

Referring to the drawing, a filter unit for determining the sizes of the droplets in a mist comprises a first filter bed 1, a second filter bed 2 and a third filter bed 3. The filter beds 1 and 2 are surrounded by annular rings 4 and 5, respectively, which combine to form part of a duct 6 which directs a stream of a mist 7, the droplet sizes of which are to be determined, through the filter beds 1 , 2 and 3. The filter beds 1 and 2 are both made of a single layer 8 of glass beads 9 supported between two perforated screens 10 and 1 liThe beads 9 have a diameter of 7.5 mm and the overall dimensions of the filter beds 1 and 2 are 25 mm diameter by 7.5 m long. The filter bed 3 consists of a layer 12 of fine glass beads 1 3 followed by a layer of glass fibre 14.The rings 4 and 5 surrounding the filter beds 1 and 2 are made of a porous material which can extract and hold a large volume of liquid from the filter beds 1 and 2 by capillary attraction. Any suitabie material such as cotton lint or glass wool can be used for the rings 4 and 5.

In use, the porous rings 4 and 5 and the filter bed 3 are weighed together, the unit is assembled and the mist 7 is passed through the filter beds 1, 2 and 3 for a known period of time. The unit is then dismantled, the porous rings 4 and 5 and the final filter bed 3 are weighed together again, and the weights of liquid collected by the filters 1, 2 and 3, W1, W2 and W3 respectively, are determined.

The penetrations of each of the beds 1 and 2 are given by the relations

These two quantities enable the size distribution of the droplets to be determined as follows: The equation for the pentration P of particulate material through a filter bed has the general form

where CO and Cj are the incident and emerging particle concentrations, d is the particle diameter, L is the length of the filter bed and k is a function of the density of the particulate material, the velocity of the carrier gas, the physical properties of the filter bed, and other factors. It can be determined by calibration for a given system.

For simple cases such as spherical drops f (d) = d2. Hence for a given drop size d1, P1=W1exp (dr2 k L) where Wi is the weight per cent of drops of diameter din the mist.

Assume W, is a function of two parameters, a and b, so that: W, = f (a, b)

and for two demisters L1 and L2 in series

These two equations have two unknowns which can be solved using iterative techniques to give the parameters a and b and hence a two parameter fit to the droplet size distribution from experimentally determined laboratory calibrations of k.

In a second embodiment of the invention, which is not illustrated because its general form is similar to that of the first embodiment, the filter beds 1, 2 and 3 are made up of pads of fibrous material instead of arrays of close-packed beads.

Otherwise the construction is the same, as is the mode of use.

The equation which gives the penetration P for a mesh layer, however, does have a somewhat different form. The penetration P for a short filter bed and a single size of particles is given by: p -- ke)N where e is the collection efficiency of a single bed of mesh, k is a mesh factor.

N is the number of layers of fibre in a bed.

For a mist of particles of varying diameters, anc assuming a two-parameter distribution, we have Wi = f (X7 N) where Xand Nare the parameters

where d, is a size chosen such that of the expected particles, 0.5% will be undersize, and d2 is chosen such that 0.95% of the particles will be undersize and PT is the penetration over the range of particle sizes.

For 2 demisters in series

On the other hand, the measured values of the penetrations are given as before, and the droplet sizes can be determined using a similar iterative method, as outlined below: 1. Assume a Rosin-Kammlerdistribution:-

2. Guess values for X and m.

3. Recreate the size distribution and divide it into 30 ranges.

4. Calculate penetration for the mean size of each size range for a demister with n, beds.

5. Sum the pentrations for all the values of dmean 6. Repeat for a demister with n, + n2 beds.

7. Compare the calculated penetrations with the experimental ones.

8. Use the Neutron-Rapherson method four a better estimate of the parameters Xand N.

9. Repeat from step (3) until the solution converges.

Claims

1. A method for measuring the sizes of particulate matter dispersed in an aerosol comprising the operations of passing an aerosol the sizes of the particulate matter dispersed in which are to be measured sequentially through a plurality of beds of foraminous material, measuring the amount of particulate material collected by each bed of foraminous material in a given interval of time, deriving therefrom the penetrations of the particulate material through each of the beds of foraminous material prior to the final one, and deriving therefrom an estimation of the sizes of the particulate matter in the aerosol.

2. An apparatus for measuring the size of particulate matter dispersed in an aerosol, comprising a duct for directing an aerosol the sizes of the particulate matter dispersed in which are to be measured sequentially through a plurality of foraminous bodies removably mounted in the duct, the initial ones of which are such as only partially to extract particulate matter from the aerosol, and the final one of which is such as to remove from the aerosol the remaining particulate matter.

3. An apparatus according to Claim 2, wherein each of the initial foraminous bodies is surrounded by a body of absorptive medium which is adapted to remove by capillary attraction the liquid from each of the said beds of foraminous material as it accumulates in that bed.

4. An apparatus according to Claim 2 or Claim 3, wherein the beds of foraminous material are constituted by a close-packed array of spherical beads.

5. An apparatus according to Claim 2 or Claim 3, wherein the beds of foraminous material are constituted by pads of fibrous material.

6 A method of measuring the size of particulate matter dispersed in an aerosol substantially as hereinbefore described.

7. An apparatus for measuring the size of particulate matter dispersed in an aerosol substantially as hereinbefore described.