Review on Measurement Techniques for Drop Size Distribution in a

Review pubs.acs.org/IECR

Review on Measurement Techniques for Drop Size Distribution in a Stirred Vessel Mohd Izzudin Izzat Zainal Abidin, Abdul Aziz Abdul Raman,* and Mohamad Iskandr Mohamad Nor Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia S Supporting Information *

ABSTRACT: A literature review on measurements of drop size distribution in liquid−liquid dispersion produced in a stirred vessel is presented in this work. The methods of measurement can be classified into in situ and external measurement. Two main groups of measurement techniques, namely, a laser system and image analysis, are reviewed. Several issues regarding the applications of the techniques and possible ways to overcome the problems are discussed. The suitability of different techniques depends on the operating conditions and properties of the drops. Laser systems provide fast in situ measurements which are useful for online monitoring and detecting process changes but unable to deliver reliable drop size and distribution values. In situ image analysis techniques could give accurate measurement of drop size, but a long time is required to analyze drops from a large number of images. However with development of automated image analysis, analysis time can be reduced. Therefore real-time monitoring and process control by image analysis techniques can be possible.

1. INTRODUCTION

Existing data on drop size and distributions are limited for unstable dispersions and highly dispersed phase fractions caused by the difficulty of performing measurements under such conditions.12 Available measurement techniques such as image analysis techniques are not suitable at high concentration because there is a tendency of drop overlapping in the images13 and fast acting equipment are required to capture rapid changes in unstable dispersions. In this work, the suitability and limitations of laser systems and image analysis techniques on drop size measurements are focused. Understanding the mechanism and limitations of the measurement techniques is important in order to choose suitable techniques depending on the operating conditions and properties of the drops. 1.1. Drop Breakup and Coalescence. Drop breakup or deformation is caused by mechanical forces induced by the surrounding fluid while resisted by surface and internal viscous forces. Drop deformation occurs when the mechanical forces are bigger than the combined resisted force.1 The main disruptive forces are turbulent pressure fluctuations, viscous stress due to velocity gradients in the surrounding continuous phase,14 and interfacial stability,15 while the main cohesive stress is interfacial tension and internal viscous stress.8 The drop coalescence process involves drainage and eventual rupture of the intervening liquid film which depends on the physical properties of the fluids.16 Factors controlling the drop breakup and coalescence processes such as viscosity, interfacial tension, dispersed phase fraction, and impeller speed will govern the rate of drop breakup and coalescence. The complex hydrodynamic behavior between the interacting phases can be described by population balance based modeling. It can be

Liquid−liquid dispersion is produced by mixing two immiscible liquids together with the aid of an agitator. An immiscible liquid−liquid system refers to two or more mutually insoluble liquids which present as separate phases.1 Those phases are classified as dispersed phase and continuous phase, where the dispersed phase which is usually smaller in volume will be dispersed in the continuous phase.1 The dispersion process is very complex because it involves the simultaneous process of drop breakup and drop coalescence, where both phenomena have to be considered.2 Stirred vessel, rotor−stator mixer, inline mixer, static mixer, valve or jet homogenizers, and extraction columns are used in industrial processes to contact the liquid−liquid system.3 A stirred vessel is commonly used to produce liquid−liquid dispersion in the chemical industry. Numerous investigations are conducted by researchers on the mixing mechanism and analysis of the flow pattern and its relation with liquid−liquid dispersion in the stirred vessel.2,4−6 Mass and heat transfer between the two immiscible fluids are involved in industrial processes. The interfacial area between the fluids will govern the amount of heat and mass transferred between its boundaries.7 Thus, knowing the interfacial area in a dispersion is an important factor to determine the interphase reaction rate and the mass-transfer rate between the liquid phases.8,9 Research has been done in the system to find optimum conditions in order to produce as large an interfacial area as possible to enhance the mass- and heat-transfer rates.10 Since the drop size distributions are related to the interfacial area and transfer rate, techniques of drop size measurement have been developed on the basis of different physical principles. Having an accurate technique is important to produce reliable models of drop size and distributions, which can help in industrial scale-up. Accurate and reliable models which can help in the scale-up of reactors can save cost.11 © 2013 American Chemical Society

Received: Revised: Accepted: Published: 16085

May 15, October October October

2013 18, 2013 22, 2013 22, 2013

dx.doi.org/10.1021/ie401548z | Ind. Eng. Chem. Res. 2013, 52, 16085−16094

Industrial & Engineering Chemistry Research

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Figure 1. Classification of measurement techniques.

are laser systems and image analysis techniques. In situ measurement is important in order for online analysis to be conducted. It allows the transient drop size to be determined, and therefore the rate of drop deformation can be obtained.25 Fast acting in situ instruments that could conduct measurements in very short time allow real-time monitoring and control of the dispersion process. 2.1.1. Laser Systems. Drop size measurement using laser systems is an indirect method, where the equipment measures the variation of some physical parameter of the dispersion instead of measuring the diameter of the drops. The physical parameter is then converted to the drop size and distributions.25 Laser systems can further be classified into different groups that depend on their principle of measurements, which are Fraunhoffer diffraction and light backscattering. 2.1.1.1. Light BackScattering. Light backscattering techniques determine the chord length distribution (CLD) which is then transformed into particle size distribution (PSD). FBRM and ORM are the commonly used instruments in which this measurement principle is applied. 2.1.1.1(a). Focus Beam Reflectance Measurement. The FBRM consists of a probe which can be installed easily, an electronic measurement unit, and a computer for data analysis.27 Near-infrared light is transmitted through fiber optics to the probe tip where an optical lens which rotates at high velocity focuses the laser beam near the sapphire window. The focused beam scans a circular path at the interface between the probe window and the particle system where it will be reflected when it scans across the particle’s surface. The reflectance time is then measured by the probe. Thus, the chord length of the scanned particles is determined as the product of reflectance time and the velocity of the laser scan (2−16 m/ s).26 In other words, chord length is a straight line connecting two points on the edge of a particle. The measurements of CLD is adequate for monitoring process dynamic changes related to particle count, distribution, shape, concentration, and

done in two ways, which are stagewise and differential models.17 In several processes, only drop breakup occurrences are preferred. Therefore several researchers18−20 only conducted studies involving drop breakup. For example, in suspension polymerization, only drop breakup occurrences are preferred to obtain a narrow size distribution of the polymer particles.21 Thus, a suspending agent is used to minimize drop coalescence. The final particle size is determined by the drop size and drop size distribution during early stages of the suspension polymerization with the aid of suspending agents.22 Liquid−liquid dispersion can be characterized by parameters which can represent the entire dispersion such as the median drop size; the Sauter mean diameter, d32; and the diameter of the largest drop, dmax.23 d32 is commonly used in characterization of liquid−liquid dispersion because it relates the area of the dispersed phase to its volume. Therefore, it can be related to mass transfer and the chemical reaction rate.24

2. METHODS OF MEASUREMENT Different techniques are available to determine the drop size and distributions. The measurement techniques for drop size in a liquid−liquid dispersion can be divided into in situ and external measurements. In in situ measurement, the drop size is measured directly inside the vessel where the dispersion is produced, while external measurement requires sampling of the dispersion and is analyzed outside of the vessel. Both measurement techniques can be further classified into different groups based on the principal of measurements such as laser systems and image analysis,11 which are the most common techniques used in liquid−liquid dispersion. Figure 1 shows the classifications of external and in situ measurement techniques. 2.1. In Situ Measurement Techniques. In situ measurement allows the measurement to be conducted at the temperature and pressure of the operating conditions. It can further be divided into direct and indirect measurements, which 16086



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Figure 2. Illustrations of FBRM probe (right) and ORM probe (left).

carefully positioned to be parallel to the flow stream going out from the impeller, the effect can be minimized. Multiple mathematical unfolding techniques are required to transform the CLD measured by the light scattering techniques into PSD. Certain inaccuracies might come from the use of CLD to represent the size distributions of the dispersion.11 This is because the CLD obtained by FBRM is complex, which not only depends on the size distribution but also on the particles’ optical properties and shapes.27 Therefore light scattering techniques are very sensitive to the surface structure of the particles and not suitable for measurements of particles with a smooth surface exterior. This is because the reflection of the laser beam from the smooth surface is not diffuse over the whole surface area but will be punctuated which may undersized particles. However, the result can be improved by producing synthetic roughness on the drop surface by introducing particles such as titanium dioxide (TiO2). Adding synthetic roughness was proved to reduce the deviation between measurements conducted by FBRM and a reliable image analysis technique.11 One of the limitations of a light scattering technique is the inability to determine the actual shape of the drops where it assumes that the drops are spherical.12 2.1.1.2. Laser Diffraction. Laser diffraction technique is one of the recent techniques used by researchers18,19,30−32 for drop size measurements in a stirred vessel. It can be used in situ or externally to measure the drop size and distribution. Laser diffraction technique is based on the measurement and interpretation of angular distribution of light diffracted by the droplets using Fraunhoffer diffraction theory.25 Particle sizes are obtained by measuring the intensity of light scattered as a laser beam passes through the particles. A Malvern Mastersizer is a laser diffraction instrument which is commonly used by researchers. The analyzer is capable of measuring droplets in the range of 0.02−2000 μm with an accuracy of ±1% on volume median diameter. Calibration is not needed, but it requires the information on refractive index of the dispersed phase used in creating the dispersion.19 It has a short measuring time, thus permitting online analysis for measurement of transient drop size distributions with minimal possible instrumental, sampling, and dispersion errors. Fast measurements also allow this technique to detect changes in drop size when there is a change in the dispersion process, such as varying impeller speed.9 Online monitoring of drop size distributions in an agitated vessel using a Malvern Mastersizer

rheological behavior.27 In this technique, no assumption on particle shape is made for the measurement. Therefore, it could avoid complex mathematical assumptions which could add significant errors. The FBRM conducts fast measurements where thousands of chord lengths are obtained every second. Thus, a robust CLD can be determined to illustrate changes in particle dimensions, population, and shape in time,28 making it suitable for online measurement. In a study of hydrate formation from water-in-oil (W/O) emulsion, FBRM can successfully identify system changes, through detection, for example, of hydrate and ice nucleation, and could distinguish the extent of ice and hydrate agglomeration.26 2.1.1.1(b). Optical Reflectance Measurement. Another technique that applied the light backscattering principle is ORM, where a laser beam is used with a focal point of 0.6 μm in diameter at a distance of less than 1.0 mm from the instrument front. The short distance between focal point and the probe helps to reduce the distance traveled by the laser beam through the liquid mixture. Thus, it can measure the chord length in high-concentration dispersions (up to 50%).28 The beam rotates at a known velocity, and every time it intercepts a drop, light is scattered back in the beam path and transmitted to a detector where it will be transformed into an electrical signal. The chord length is then determined from the electric signal duration and the speed of the rotating beam. Therefore the chord length distributions can be built up over a period of time. Validations of ORM measurements were conducted with a known particle size ranging from 20.6 to 230 μm, and the results shows a high deviation for a mean particle size of 20.6 μm (6.65%) compared to that for 58.5 μm (0.17%).12 This is because for a particle size less than 20 μm, the light backscattering instruments may receive backscattered light instead of reflected light from particles which tends to oversize the particles.29 The ORM was used successfully in analyzing system changes such as a change of drop size distributions (DSD) at different power inputs. It shows almost similar proportionality to the mean Sauter diameter over the power input when compared with image analysis techniques (endoscope).11 The limitation of the ORM is the large size of the probe (30 mm) compared to FBRM and an endoscope (Figure 2), which may interfere with the drop size inside the vessel. However, if the probe is 16087



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Figure 3. Illustrations of endoscope probe (left) and PVM probe (right).

has been conducted by Chatzi et al.25 and Sis et al.33 Sis et al.33 applied the instrument for in situ measurement of dodecane in water (see Table 1) emulsion in turbulent flow. It is achieved by placing the flow-through cell in the path of the light beam of the light scattering device. The limitation of this method is that low concentrations of dispersion (0.6

endoscope stereomicroscope FBRM, ORM, endoscope ORM, endoscope external Malvern FBRM, Malvern, external microscope laser granulometer Malvern Malvern stereomicroscope PVM Endoscope External Microscope External Malvern External Microscope External Microscope External Malvern

37.1−51.5 38.7 36 44.7 10−50 N/A 10−13.4 14.3−25.7 72.2 45.8−46.6 35−37 5 3.2 19.9 3.2 N/A N/A

0.427−1.151 0.93 0.55 5.5 338.8 N/A 0.947−0.208 0.59−0.94 8.5 10−1000 53.8−188.9 N/A 39 1.56 39 0.677−319 10−1000

φ = dispersed phase fraction; γ = interfacial tension; μd = viscosity.

Table 2. Summary of Parameters and Characteristics of Different Measurement Techniques techniques

measurement range (μm)

accuracy

dispersed phase fraction

drops optical properties

in situ

invasive/ noninvasive

determination of particle shape

FBRM ORM laser diffraction endoscope PVM stereomicroscope

1−1000 1−1000 0.02−2000 5−5000 20−2000 20−2000

low low high high high high

high high low high high high

external surface external surface N/A refractive Index refractive Index refractive Index

yes yes yes yes yes yes

invasive invasive noninvasive invasive invasive noninvasive

no no no yes yes yes

5−3000

high

medium

refractive Index

no

noninvasive

yes

external microscope

stabilizer concentration in lowering the surface tension decays when the viscosity of the drops increases.16 In a study conducted by Tobin et al.47 a few drops of SDS solution were added to their sample in order to immobilize the drops. The SDS solution is able to efficiently stabilize the extracted sample from further coalescence. The effectiveness is also confirmed from visual observations by O’Rourke and MacLoughlin.41 According to El-Hamouz et al.,18 their sample which was stabilized by sodium laureth sulfate solution can be kept until 24 h without any changes in the drop size. 2.2.2. External Image Analysis: Microscope. Extracting the dispersion sample and observing it under a microscope is one of the earliest and simplest methods to determine the size of the drops and to determine the drop size distributions. a major advantage of using a microscope to determine the drop size is that it is a direct method with straightforward calibration. The limitations of this method are well-understood, being caused mainly by the nature of light wave and by optical aberrations.57 External image analysis using a microscope is able to measure drops in the range of 17−1000 μm but varies according to the lens used in the microscope.41 Usually, a microscope is used together with image analysis software which allows enhancement of the image quality, thus improving the precision of the drop size measurement. During analysis, the microscope can be adjusted to obtain clear and sharp images of the drop. Table 1 shows the details of liquid−liquid dispersion systems for various in situ and external measurement techniques. From Table 1, one can see that laser diffraction and image analysis

probe diameter (mm) 25 30 N/A 7 19 0.4 (light probe) N/A

techniques are successfully applied at high-viscosity dispersed phase.19,38,41,61,62 This is because drops with higher viscosity have higher refractive index value, since the viscosity of the dispersed phase is linearly related to the refractive index.63 Therefore it is suitable to use laser diffraction or image analysis techniques to conduct measurement for drops with high viscosity. 2.3. Comparisons of In Situ and External Measurement. Although laser systems could conduct fast online measurements, several researchers still pair a laser technique with image analysis techniques to ensure the validity of their result. Comparison between external measurement using a microscope and in situ measurement using a particle vision measurement probe. which is essentially a video microscope, was conducted by O’Rourke and MacLoughlin.41 Precautionary steps were taken during sampling to minimize or eliminate the error during sampling such as using surfactants (SDS) to immobilize the dispersion sample and adding iodopentane to create a droplet phase with a density near the density of continuous phase. Hence, an almost neutral buoyant system is produced. They reported that both techniques were in agreement with each other wherein the difference in results yielded by both techniques does not exceed 10%. The external sampling method can provide more droplet images which are more than 500 droplets compared to the PVM method which only captured less than 400 measurable droplets. In the PVM technique, there is a tendency to underestimate a larger droplet size and overestimate a smaller 16091



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drop size.41 Thus, as the mean droplet size decreases, the probability of PVM measurement capturing and measuring the entire range of droplet size increases and consequently closer agreement between the microscope and PVM method is expected. From the work in ref 41 it was concluded that a satisfactory level of agreement was achieved by both techniques and the PVM technique could be improved by increasing the amount of measurable droplets. Therefore, although prone to errors, external image analysis can be used to produce reliable drop size and drop size distribution if proper care is taken during sampling and measurement. Comparisons between in situ laser techniques and external and image analysis technique were conducted27 using light scattering (FBRM), light diffraction (Malvern Mastersizer), and image analysis techniques (microscope). They reported that laser diffraction technique gives results very similar to those with image analysis technique for spherical particles. The mean diameter of glass beads (20−36 μm by sieving) measured by laser diffraction and image analysis are 35.2 and 35.9 μm, respectively. However, the result provided by FBRM techniques shows very large deviation compared to other techniques. It shows that FBRM technique is not suitable to be used to determine the size of the drops. The parameters and characteristics of different in situ and external measurement techniques are shown in Table 2. From the comparisons, although external measurement techniques are prone to errors, it shows good agreement with in situ measurement techniques. Thus, with proper care to minimize errors during measurement such as adding surfactant and proper sampling techniques, external measurement techniques could also give reliable results for drop size and distribution. From Table 1, one can see that surfactants are used in most of the external measurements. However, external measurement techniques could not conduct real-time monitoring and online measurement of drop size.

(4) Although external measurement techniques are prone to errors, with proper precaution steps, such as adding surfactants, it could produce reliable result similar to in situ measurement techniques. (5) The measurement techniques depend on the optical properties of the drops to be measured, which are the refractive index and the smoothness of the exterior surface.

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ASSOCIATED CONTENT

S Supporting Information *

Table listing details of dispersion systems for coalescence prevention. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (603) 03-79675300. Fax: (603) 7967 5319. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the University of Malaya High Impact Research Grant (HIR-MOHE-D000038-16001) from the Ministry of Education Malaysia which financially supported this work.

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NOMENCLATURE CLD chord length distribution d32 Sauter mean diameter (m) dmax Maximum stable diameter (m) DAT data acquisition time FBRM focused beam reflectance measurement fps frames per second N/A not available ORM optical reflectance measurement PSD particle size distribution PVA poly(vinyl alcohol) PVM particle video microscope SDS sodium dodecyl sulfate SLES sodium laureth sulfate

3. CONCLUSION In situ or external measurement techniques have their own advantages and limitations which mostly depend on the operating conditions and optical properties of the working liquid. On the basis the review, several conclusions can be made: (1) In situ measurements which applied laser systems can provide fast measurements which are suitable for online monitoring and for unstable dispersions with a high rate of change. However, FBRM and ORM techniques are not suitable to determine the exact size of the drops. (2) In situ image analysis techniques could provide an accurate value of the drop sizes. With the development of camera and lenses, this technique could obtain a high number of images in a short time, but it requires a lot of work and time to extract data of drops from the amount of images. Since the measurement analysis time is much higher than its data analysis time, it is not suitable for online or real-time analysis. However, with the development of an automated image analysis program, the analysis time can be shortened, allowing real-time monitoring and process control to be possible. (3) Image analysis techniques should be used as secondary tools to validate the results obtained by light scattering techniques. It also can be used to calibrate the light scattering techniques. For example PVM can be used to calibrate FBRM techniques.

Greek Symbols

μ dynamic viscosity (mPa·s) ρ density (kg·m−3) φ dispersed phase fraction

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REFERENCES

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