Abstract
The objective of this work is to study the effect of non-ionic surfactants on hydrophobic VOCs absorption
process in bubble column. The absorbate and absorbent used in this work are benzene and aqueous solution with
surfactants, respectively. Moreover, the ranges of surfactant concentrations and gas flow rates applied are 0.1, 1, 5
CMC and 0.5, 1.3, 2.2, 3.0 ml/s, respectively. The analytical parameters were concerning to the treatment efficiency,
bubble hydrodynamic parameters, and also the mass transfer parameters. This result has shown that the VOCs
absorbed amounts obtained with liquid phase containing with non-ionic surfactants are greater than those obtained
with tap water. The benzene solubility in liquid phase can be augmented by injecting the surfactants. Moreover, the
absorbed amount obtained relates to the overall mass transfer coefficient (KLa) that the product between the interfacial
area (a) and the liquid-side mass transfer coefficient (KL). It can be stated that these two parameters (a and KL)
compensate to each other. Therefore, the appropriate amount of surfactants and bubble hydrodynamic condition are
necessary in order to increase the benzene solubility, interfacial area, KL coefficient, and thus obtain the fine
absorption of hydrophobic VOCs in bubble column.
Keywords : Volatile organic compounds, Surfactant, Absorption, Hydrodynamic, Mass transfer
Introduction
Volatile organic compounds (VOCs) are
widely used in the industrialized countries as solvents.
VOCs are large family of compounds. Some (e.g.,
benzene) are toxic and carcinogenic, and are regulated
individually as hazardous pollutants [1]. Therefore, the
removal of VOCs from waste water or exhaust air is of
great interest. Normally, VOCs emissions may be
reduced by different methods: adsorption, thermal and
catalytic oxidation, absorption in a liquid, membrane
separation, bio-treatment, etc. These techniques have
their pluses and minuses [2]. Up to now, there is a
continuing research for the techniques which do not
suffer from any limitations. Among the different
techniques, the absorption process, which allows the
transfer of a pollutant from the gas phase to a liquid
phase (absorbent), with or without any chemical
reaction, is one of the well-known methods. From [3],
the VOCs removal efficiencies obtained with the
absorption process can be greater than 98%.
Concerning to the hydrophilic and hydrophobic VOCs
emission normally generated in real operating
conditions, it can be noted that, in the case of
hydrophobic VOCs, the normal absorbent used (water)
can not be used due to its corresponding solubility in
water. Therefore various types of substances have been
studied [4].
In order to enhance the absorption efficiency,
the VOCs gas is released in form of small bubble to
yield a large surface area and also an efficient mass
transfer between gas and liquid phases in bubble
column. Thus, the mass transfer from the gas phase
(bubble) to the liquid phase is a key parameter of the
process. However, a few studies have focused to this
absorption process which is one type of absorption
technology possibly applied in the real operating
conditions [1, 4]. Moreover, there is a very limited
number of qualitative data related to the effect of
bubble hydrodynamic (bubble diameter, bubble rising
velocity and its formation frequency) and mass transfer
parameter (interfacial area and mass transfer
coefficient) on the VOCs absorption efficiency. In
practice, the overall mass transfer coefficients (kLa) are
often global and thus insufficient to understand the
gas-liquid mass transfer mechanism, which is directly
to the associate efficiency [5].
To fill this gap, this research will be mainly
focused on the study of hydrophobic VOCs absorption
process in bubble column in terms of bubble
hydrodynamic and mass transfer parameters. Moreover,
the effect of gas flow rate and also of the non-ionic
surfactant contamination as absorbents will be
investigated. In this research, the non-ionic surfactant was
chosen as absorbent due to its higher dissolution rate and
greater solubility of hydrophobic VOCs with less
foaming [6]. Moreover, the point of investigation was a
screening commercially available component, with the
requirement of being safe and cheap [4]. The local
experimental methods for measuring the bubble
hydrodynamic parameters (bubble size, bubble formation
frequency and bubble rising velocity) and overall mass
transfer coefficient will be used in order to enables the
VOCs mass transfer efficiency.
The experimental set-up was schematically
represented in Figure 1. The experiments are carried out in
a small bubble column 4.4 cm. in diameter and 30 cm. in
height. In this study, the bubble column reactor was a
closed system. Tap water and the aqueous solution with
non-ionic surfactant were used as absorbents. The bottom
surface of bubble column was drilled for installing the rigid
orifice diffuser in order to inject the benzene gas bubbles as
VOCs gas. The average gas flow rate was measured by
using soap film meter. In this work, the bubbles were
generated by a single puncture rigid orifice made from
PVC plastic and located at the center of bubble column.
Moreover, the top and bottom positions of reactor have the
septum (sampling point) for collecting the inlet and outlet
VOC gases. Then the associated concentrations will be
measured by using gas chromatography with a Flame
Ionization Detector (FID). In order to analyze the bubble
hydrodynamic, the high speed camera (100 image/sec) and
image treatment program were used to determine the
bubble diameters (DB), bubble formation frequencies (fB)
and bubble rising velocities (UB) [6-8].
Liquid phase characterization
The liquid phases under test are: tap water and
aqueous solutions with nonionic surfactant at three
different concentrations. Given that these liquids are
dilute aqueous solutions, their density and viscosity are
assumed to be equal to those of tap water (997 kgm−3 and
10−3 Pa s, respectively). The chemical properties (the
concentration of surfactant, static surface tension, the
Critical Micelle Concentration (CMC) are reported in
Table 1. The associate surface tension values were
measured experimentally by using Tensiometer (K10T,
Kruss, Germany).