Towards More Compact and Efficient De-Oiling Equipment

Abstract

Listen

Translate article

ABSTRACT The development of a compact oil-water separator is pursued through theoretical and experimental investigation of a number of separation techniques. Plate separation is an effective separation method for oil droplets with a diameter of about 50µm or larger, but ineffective for smaller droplets. Use of gasflotation or of oil coalescence in a porous medium as separation techniques leads to the removal of a part of the smaller oil droplets present in oily waters. To increase the separation efficiency each of the last two techniques will be combined withcentrifugal separation. A design of a separator based on such a combination is presented for both gas flotation and oil coalescence. INTRODUCTION De-oiling equipment at offshore oil-production platforms must preferably be light and compact, and the oil concentration of the discharged production water must be below the prescribed limit. In many cases these requirements are not met. This paper describes work on the improvementof de-oiling equipment with respect to size and efficiency. Three de-oiling techniques are studied:Plate separation. This is a suitable technique for the removal of oil droplets above a specific diameter. In practice this diameter is of the order of 50µm.Dissolved gas flotation and oil coalescence in a porous medium. With these two techniques also smaller oil droplets can be removed. PLATE SEPARATION A plate separator consists of a pack of parallel plates, through which oily water is led. To enhance the removal of the collected oil the separator is usually placed under an angle with the horizontal, and provided with corrugated platesl. The rise velocity of the oil droplets is determined by the density difference between oil and water and the friction force exerted on the rising oil droplet. It can be shown2 that this results in a stationary rise velocity:(Available In Full Paper) The use of a plate separator leads to complete removal of all oil droplets above a specific diameter, the critical oil-droplet diameter. The numerical value of this parameter depends on the dimensions of the plate separator and on the velocity of the oily water, and can be calculated as2 :(Available In Full Paper) In case the angle with the horizontal is larger than about 10 degrees, it is necessary to correct for the tilting of the separator by replacing H by H/cos ? in eq. (2). Experiments and results Experiments on plate separation were carried out in straight and in corrugated channels. A flow circuit for oily water with a maximum capacity of 5 m3/h was available for the experiments. We limit ourselves to the discussion of two typical experiments: one on the measurement of the velocity profile inside a corrugated channel and one on the experimental determination of a critical oil-droplet diameter. Fig. 1 gives a sketch of a corrugated separator channel with dimensions: length 1.50 m, width 0.50 m, height 0.020 m. At a wave-peak (A) and at a wave-trough (B) the velocity profile was measured along a vertical line, with a TNO-Delft laser-doppler anemometer.