Enhancement of Oil-Water Separation by Dissolved Air Flotation

Author: Aliff Mohamad Radzi

Department of Chemical & Process Engineering, Faculty of Engineering and Physical Sciences, University of Surrey, Guildford, GU2 7XH, UK

A dissolved air flotation (DAF) process is used to â??polishâ?? liquid effluent streams by removing small oil droplets or fine particles from liquids, often after a primary separator. The main objective of the experiments reported in the paper is to remove the oil droplets in the range of 15-100 μm from an oil-in-water mixture by manipulating several individual parameters.
The DAF rig consists of a water tank, an oil tank, a DAF flotation tank, a static mixer and an effluent tank. The static mixer is used to break up the oil droplets to produce the desired oil/water feed. The feed flows into the flotation tank that is consisting of two main areas, which are the contact and the separation zones. The contact zone is where the oil droplets collide and attach with the micro bubbles generated from the rapidly depressurized water, which flow from the three nozzles located at the bottom of the contact zone. The agglomerate so formed has low density compared to the continuous phase. Hence, float well. The agglomerate flows directly to the separation zone, where the separation of the oil droplets from the continuous phase mainly happens. The two layers of oil and continuous phase are easily observed here. The flotation tank was built from Perspex as a scale model of an industrial DAF tank and has the dimensions 0.75m x 0.3m x 0.5m, length, width and height respectively.
It was decided that vegetable oil should be tested as it relevant to the food industries. However, in order to resemble the separation of oil droplets in an oil production platform, experiments were also carried out using a mineral oil, of which lamp oil is the safest example. It should be noted that the two oils have different chemical compositions. Saline water in this experiment was made by adding 3.5% by concentration of NaCl, to mimic the average salinity of produced waters.
Samples were taken at the inlet and outlet of the rig after steady state had been achieved. An oil-in-water measuring technique (FastHEX) and a Coulter Counter were used and they have produced similar results. The analysis has shown that each of the parameters tested had a significant effect on the efficiency of oil removal.
The lab scale DAF succeeded to remove the oil droplets up to 80 % efficiency. Lower inlet oil concentration produced higher removal efficiency. Three different saturation pressures were tested and the pressure of 4 barg obtained the best removal efficiency compared to 3 barg and at No DAF. 5 barg was not tested because of the safety limitation at the rig. Temperature was adjusted to see the effects of viscosity on the separation process. No very significant results were obtained but higher temperature gave slightly better removal efficiency.
This paper focuses on the type of oils that need to be removed from the oil-in-water mixture. Experiments with lamp oil obtained less than 40% removal efficiency at an oil droplet size range of 15-40 µm, with the average lying at 25%. The droplet had Sauter mean diameter of 28 µm. Higher removal efficiencies of oil were obtained in the size range 45-55
µm. Here, the average efficiency was approximately at 50%. The coalescence of oil droplets appeared start to occur above 55 µm. Coalescence caused bigger oil droplets to agglomerate and make them difficult to attach with the bubbles. A little fall of the removal efficiency recorded when the flow rate to the DAF tank was increased. Higher flow rate into DAF tank means the oil droplets and bubbles have lower residence to collide and attach. Tests were conducted at 10, 15, 20 and 25 l/min.
A wider oil droplet size distribution was produced up to 80 µm for vegetable oil. Most of the oil droplets were produced at 15-10 µm with 30 µm Sauter mean diameter. Better separation efficiencies were recorded from the vegetable oil regardless of any parameters manipulated. This seems contradict the droplet viscosity being the main factor of the separation. However, the spreading coefficient appears more significant because vegetable oil
has a large and positive spreading coefficient, (14.6 x 10-3 N/m), whereas lamp oil has a negative

spreading coefficient (-7.11 x 10-3 N/m). A positive spreading coefficient allows the droplets to attach easily to micro bubbles. The best removal efficiency of the vegetable oil is at 78.8% with the aid of 4 barg of DAF and low inlet oil concentration. Vegetable oil exhibited almost

40% differences compared to lamp oil. The removal efficiency for vegetable oil increases as the droplet size increases. The increasing trend started after 45 µm. This reached 100% at certain droplet size. No coalescence was detected for the vegetable oil, hence the average efficiency value maintained at higher value. The effect of the mixtureâ??s flow rate was investigated by increasing it from 10 l/min to 25 l/min on 5 l/min bands. As the flow rate
increases, the removal efficiency decreases. The effect becomes strong between 20 l/min and
25 l/min where the difference in efficiency was approximately 20%.
To conclude, DAF is a promising secondary separation unit, which can be used to remove oil droplets with a positive spreading coefficient in a range of 15-100 µm from the oil-in- water mixture. This process also can be improved by carefully optimizing several operating parameters.

Keywords: Dissolved air floatation (DAF), droplet size distribution, oil droplets separation, wastewater treatment, separation processes, spreading coefficient