Recent experimental works have demonstrated the benefits of adding nanofibers to microfiber nonwoven filter media. In this work, single fiber efficiencies and drag are applied to model filter performance for steady-state coalescence of oil drops from air streams. The model results show the same trends as observed in the experiments, namely that the addition of small amounts of nanofibers significantly increase the Quality Factor. New results from the model and experiments show that there is an optimum amount of nanoflber.

INTRODUCTION

Recent work shows improved performance of nonwoven filter media by the addition of small amounts of nanofibers. (1) The purpose of this work is to determine whether there is an optimum amount of nanofibers to add to the filter media.

Our approach to this project is to model the filter using single-fiber capture mechanisms and single-fiber drag forces. The coalescence filter is assumed to operate at steady state with a uniform saturation of 10% (a typical value from our experimental data). The filter performance is determined using the Quality Factor. The model results are compared with experimental data.

Our model results show that there is an optimum amount Of nanofiber. The highest Quality Factors occur when the ratio of nanofiber surface area to micro fiber surface area is in the range of 1.0 to 2.0. Our experimental results agree with the optimum occurring in the same ratio. Qualitatively, the model and experimental results are similar, but the model over-predicts the value of the Quality Factor due to simplifying assumptions used to developing it.

Coalescing filters are used throughout industry to separate small liquid droplets from gas streams or from another liquid phase. Several factors influence the efficiency and economics of the separation. In general, droplets in the 0.1- to 0.8-micron ([micro]) range are the most difficult to remove. Polymer nanofibers, made in our laboratory, provide a flexible and adjustable system for optimizing the filter structure to capture particles in the size range that has the highest probability for passing through the filter.

Unlike other filter media whose primary purpose is to stop particles from moving with the fluid stream, coalescing filter media have the additional requirements of making the drops coalesce into larger drops and of providing a means for them to drain out of the medium. In operations such as gas compression, coalescing filters may be used upstream of the compressor to protect the equipment. They may also be used downstream to collect compressor oil. The compressor oil is typically an expensive synthetic oil used in the compressor as a coolant, sealant and lubricant. Coalescence filters are used to recover and recycle the oil back to the compressor. Recovering even smaller droplets also reduces airborne emissions in many processes and helps in regulatory compliance.

There are a number of mechanisms that control the coalescence filtration process. (2) The process is sketched in Fig. 1. Single-fiber capture mechanisms (3) control the rate at which drops are captured within the filter media. The filter media act to slow the movement of drops, helping them to collide. Microscopic observation of the coalescence process shows that most of the drops visible to the microscope (20- to 200-[micro] range) are captured on the fibers. (4) When the captured drops form beads on the fiber that are large enough to see with an optical microscope, bead growth is rapid. (5) Drag of the gas phase, together with gravity forces, causes the enlarged drops to migrate out of the filter media.


Several parameters, including pressure drop and capture efficiency, characterize the performance of filter media. It is convenient to have one parameter that accounts for multiple effects. Brown (3) recommends using the Quality Factor, QF, defined by:

QF = -ln([C.sub.out]/[C.sub.in])/[DELTA]P

where ([C.sub.out]/[C.sub.in]) n is the penetration defined as the ratio of the partide concentration passing through the filter to the particle concentration entering the filter, and [DELTA]p is the pressure drop. The nature of capture efficiency is such that if you double the thickness of a filter medium, the penetration decreases by the square of the thickness, hence the logarithm of the penetration is proportional to the thickness. Conversely, the pressure drop is directly proportional to the filter thickness. Hence, ideally, the Quality Factor is independent of the medium thickness and provides a means of direct comparison between various media.

The numerical model applies volume-averaged continuum equations to account for conservation of mass for the gas and liquid phases. Capture rates are calculated for the dominant mechanisms of Brownian diffusion and direct interception using literature correlations. (3)

The gas-phase momentum balance is applied to determine the pressure drop. Drag correlations for flow around fibers are determined from literature correlations) The capture and drag correlations account for continuum, slip or molecular flow regimes, depending on the Knudsen number for the materials.