Evaluation of an in-line rotor stator mixer

Abstract

Background & purpose
Emulsions are droplets of one immiscible fluid that is encircled by another immiscible fluid, the formation of an emulsion requires energy applied to a certain volume, two immiscible fluids and an emulsifying agent. The excess of energy comes from high shear forces from the mixing blades which rotate at high speed. It is desired to know what fluid dynamics, in terms turbulent eddies are present in the mixing system in order to optimize production of different emulsified liquids. These eddies are quite cumbersome to measure but adequate estimations can be done by studying the emulsion drop size distribution when mixing parameters and fluid composition are altered. Emulsification under turbulent flow conditions are often divided into two different hydrodynamic flow patterns or regimes due to the relationship between turbulent eddies and emulsion drop size. The first regime is called turbulent inertial were fluctuation in the dynamic pressure is the driving force for fragmentation of emulsion while in the second regime is characterized by the presence of a viscous stress inside the smallest eddies that is strong enough to cause droplet brake-up. This regime is knows as turbulent viscous.
The purpose of this thesis was to study different flow behaviour in an in-line rotor stator mixer and determine the presence of different regimes and certain emulsification mechanisms when different mixing settings and fluid composition are applied. The investigated process parameters were tip speed, flow rate and stator hole size while the altered fluid composition parameters was oil content and continuous phase.

Method
A 200 L coarse emulsion consisting of either 0.5/60% (v/v) oil in water or 50/60/70% oil in 65°Brix sugar solution passed an in-line mixer a specific number of times. The drop sizes distribution was measured by laser diffraction after each pass. The tip speed, flow rate and stator hole size was altered and the droplet sizes of the emulsions were compared. The decrease in droplet size after a specific number of passes was analysed and the empirical data was used to find correlation that could yield information about the presence and speed of different emulsification mechanisms.

Result & Discussion
Smaller droplets were obtained when the continuous phase was substituted from water to sugar solution. The tip speed had a significant impact on the drop size distribution while the flow rate had none. The droplet size increased significantly when stator hole size was decreased from 4mm to 2mm. The increase of volume fraction of oil lead to a decrease in droplet sizes when the continuous phase was water, the opposite occurred when sugar solution was used as continuous phase.
A change in regime (from turbulent inertial to turbulent viscous) could be seen when the fluid composition was altered from 0.5% in water to 60% in sugar solution. The increased in drop size distribution when oil content was increased in sugar solution could be a consequence of either a greater rate of coalescence due to smaller distance between droplets or hindered allocation of Polysorbate 80 due to higher viscosity. Opposite behaviour is observed when oil content is increase in water from 0.5% to 60%, most likely to change in regime from turbulent inertial to turbulent viscous.

Popular Abstract

When two immiscible fluids (e.g. oil and water) are intensely mixed, they sometimes appear to mix. What actually happens is that small droplets are formed by one of the fluids, these droplets are called emulsion and they appear in various branches within the field of chemical engineering. Their size can be used to determine how fast a drug is administrated in the body, generate proper texture or “mouth feeling” for different types of food or optimize hygiene products ability to dissolves oil soluble fluids when were washing our hands.
Emulsified fluids were the droplet sizes are really small (less than 20 micrometer) required a powerful mixing apparatus. One type of mixers that satisfies this requirement in called rotor stator mixers that work like a conventional mixer with the exception that the mixed fluid is pushed throughout small holes that encircled the dynamic mixing device a.k.a. stator, hence the mixers are called rotor stator mixers.

The purpose of this thesis was to investigate the emulsification capabilities, i.e. how small droplets are obtained by a rotor stator mixer made for commercial production of e.g. mayonnaise that is produced by Tetra Pak processing systems. The mixer is also able to generate pumping pressure which means that it was also used as a pump. The pilot scale set up was constructed in a manner called in-line
mixing mode. This means that the fluid can only pass the mixer one time before it is pumped to the next unit operation.
The investigation was conducted by altering the mixing conditions in terms of how fast the dynamic part (rotor) was rotating, the flow rate of the mixed fluid and the stator hole sizes that the stator consisted of. The recipe of the mixed/emulsified fluid was also modified, the concentration of oil was changed and water was substituted to a sugar solution in order to increase the viscosity of the produced fluid.
The main result shows that smaller droplets are obtained when the rotor is rotating faster, this patter is to be expected since the chance that the oil droplets are stretched and torn apart to two smaller droplets increased when the fluid is subjected to more vigorous stirring.

Alteration in flow rate gave no significant difference in droplet size, this could be explained by the high residence time that the fluid has in the mixer or that the interval of the chosen flow rates (5-10 m3/h) is to small compared to the time that the droplets needs to be subjected to disruptive forces (Walstra & Smulders, 1998). When the stator hole size was altered form 4 mm to 2 mm in diameter, the droplet sizes increased. A potential explanation for this behaviour could be that jet that is created when the fluid is pushed through 4 mm stator holes is bigger that the jet created when 2 mm stator holes are applied. Bigger jet usually leads to more disruptive forces and smaller droplets.

The recipe parameters generated some interesting results, when the oil content in water was increased 120 times the droplet sizes of the system decreased. This indicates that the number of droplets contributes to the viscosity of the mixed fluid. However, when the oil content was increased in sugar solution larger droplets were obtained. This shows more coalescence, i.e. the process were to small droplets collide and form a big droplet occurs more frequently when the oil content in increased. Plausible explanation for this phenomenon could be that the chances for coalescence to occur are greater with increased concentration of oil since the average distance between the droplets decreases. When to fluid that the oil is dispersed in is changed from water to a sugar solution that contained 65g sugar in 100 g water (a.k.a. 65° Brix), the droplets decreases significantly. The brake-up of droplets become more efficient with higher viscosity, the cost of this is higher power consumptions to ensure the same rotational speed of the rotor in a more viscous fluid.

To summarize, this diploma work has shown how different process setting and recipe parameters effect the droplet size of a technically produced emulsified fluid. The results show how the different chemical processes that occur during mixing changes when emulsions are produced in different way or when they contains different chemical compounds. The results can be used to generate different types of simulation that is fitted to show an increase of droplet sizes when dealing with an increased when the oil content in increased in high viscous fluid and the stir the simulation to the opposite behaviour when similar alteration are done in low viscous fluids. It has also provided information about the required amount of passes which aid with the optimization of production time of different types of emulsified fluids.
This work can be complemented by running additional trail to validate the previous suggested mechanism (brake-up vs. coalescence) by expanding the interval to experimental set points or increase the certainty of the methods.

• author: Chaudhry, Zishan
• supervisor: Björn Bergenståhl
• organization: Food Technology and Nutrition (M.Sc.)
• course: KLT920 20151
• year: 2015
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Mixing technology, emulsification, food engineering, livsmedelsteknik
• language: English
• id: 8411926
• date added to LUP: 2019-04-11 14:11:40
• date last changed: 2019-04-11 14:11:40