Enhancing filtration by electroacoustic methods

Fouling of filter media and physico-chemical properties of suspensions decrease the efficiency of filtration devices in a wide range of process industries. Environmental protection causes increasing demand to clean effluent waters to higher standards and to recycle process waters more completely. Conventional deliquoring processes are mainly based on a single driving force, usually gravity, underpressure or pressure. Today, multiforce deliquoring processes based on a combination of ultrasonic and/or other nonmechanical forces, like an electric field, are being developed. These new technological applications, namely electro-acoustic deliquoring techniques, will most probably enable higher deliquoring rates and final solid contents than conventional methods have been able to yield. Results from an experimental study of electric and/or ultrasonic field assisted filtrations are presented in this thesis. Both electric and ultrasonic fields can reduce fouling of the filtration medium and have a significant influence on filtration capacity. The extent of filtration improvement is affected mostly by particle size, surface charge, acoustic frequency, intensity and field strengths. Theoretical examinations of the use of electric and/or ultrasonic fields to enhance filtration efficiency are laid out. Some aspects regarding orthokinetic interaction in acoustic agglomeration have been considered, and energy consumptions of the filtrations of different suspensions used in experiments were also determined. Using electric field as a pre-treatment, biolfiber suspension filtration can be enhanced 4-fold and energy consumption of electric field enhancing the filtration (kWh kg1 separated water; product final dry solid content 23 % by mass) was only about 17 % of the total energy consumption of conventional vacuum filtration. Pre-treatment units can be connected to the filtration unit, for instance before the filter drum. Possible pre-treatment apparatuses could be electroflotation equipment or a pre-treatment tube technique introduced in this Ph.D. Thesis…