Introduction
Volatile organic compounds (VOCs) are some types of organic
chemicals with high vapor pressure at room
temperature.1 VOC substances include alkanes,
aromatics, ketones, paraffins, alcohols, esters, ethers and so
on.2These hazardous chemicals are emitted from exhaust
of industrial plants and involve in atmospheric photochemical reactions.
Since these air pollutants cause long-term health problems and
environmental issues, control of VOCs emission seems to be a major
concern in quality of air. To control VOC emission into atmosphere, many
technologies have been proposed such as biodegradation, condensation,
catalytic oxidation, adsorption, absorption, etc.3When VOC high removal efficiency and good operation
adaptation is concerned, the adsorption process is considered as an
effective tool to treat VOCs. 4 In adsorption process,
VOCs are held on surface of adsorbent and its pores using the vander
Waals force. Activated carbon is widely used as a suitable adsorbent for
VOCs recovery due to its large surface area, high adsorption capacity,
non-selective nature and low cost comparing to zeolites.5-6However, activated carbon could not act as a
catalyst properly and more appropriate adsorbent may be explored. When
VOCs are passed over activated carbon, they are adsorbed on the carbon
surface and treated air is exhausted to at atmosphere. When all surfaces
of activated carbon are occupied, the adsorbent undergoes regeneration
to release VOCs by heating it with steam in the temperature swing
adsorption (TSA) system. As long as activated carbon gets warmer, it
holds less VOCs and then the regeneration stream, a mixture of VOC and
steam, exits from the bed and condensed. After cooling stage, the carbon
is now ready to be re-used for adsorption.
Shah et al.,7 investigated adsorption of acetone and
methyl ethyl ketone (MEK) on activated carbon and its regeneration via
hot air in a TSA system. They observed 95 % adsorption capacity for
acetone at 80 °C and continuous degradation of the adsorption capacity
for MEK. Wang et al.,8 investigated adsorption process
for automotive painting components on beaded activated carbon and used
hot nitrogen for desorption. They observed competitive adsorption for
mixture and displacing of low boiling point compounds with high boiling
point compounds. They found that high boiling point compounds may show
“heel accumulation” on activated carbon due to their low desorption
rate. Tefera et al.,9 studied two-dimensionally model
for adsorption of acetone, benzene, toluene and 1,2,4-trimethylbenzene
in a fixed-bed cylindrical adsorber over beaded activated carbon. They
investigated the effect of operating parameters (namely; temperature,
superficial velocity, VOC load, particle size, etc) on the process
efficiency and properly simulated transport phenomena in adsorber
column. Kim et al., 10 used X- or Y-type Faujasite and
Mordenite zeolites to adsorb VOCs and microwave heating for desorption.
The highest adsorption capacity was attributed to Faujasite zeolite due
to its large surface area and mesoporous volume. They finally concluded
that pore structure of zeolites controls adsorption properties of
zeolites. Wang et al., 11 synthesized a series of
polymeric adsorbents to investigate dichloromethane and 2-butanon
adsorption/desorption performance. They found that high surface area
enhances adsorption of medium to high concentration of VOCs and
meso-pores increase desorption efficiency. Al-Ghouti et
al.,12 characterized diethyl ether adsorption using an
innovative refrigerator. They described adsorption data by the Langmuir
model and chemisorption was attributed as the dominant mechanism for
diethyl ether adsorption over activated carbon. An et al.,13 used the Grand Canonical Monte Carlo to simulate
adsorption of VOCs on activated carbon. They found that although acetone
and methanol can reach to their best adsorption capacity on activated
carbon, benzene and toluene functional groups on reduces their
adsorption performance. Laskar et al., 14described
competitive adsorption of water and multi-component of VOC over
activated carbon. They properly fitted experimental data with that of
model and proposed the modified Dubinin-Radushkevich and Qi-Hay-Rood
models for VOCs and water vapor adsorption, respectively. Gabrus and
Downarowicz6 developed two combined TSA systems for
ethanol recovery from wet air over molecular sieve 3Aand activated
carbon Sorbonorit-4.They utilized the Thomas model to predict
breakthrough curve of ethanol and water in liquid and vapor phase.
Up to now, removal of VOCs is studied extensively and most of
researchers only focused on adsorption state than cyclic one. In this
study, industrial cyclic TSA unit is mathematically modeled to
simultaneous removal of diethyl ether and ethanol from air. The
breakthrough curves are plotted and compared to industrial data while
good agreement is seen between them. With all progresses in modeling,
simulation and optimization of TSA systems, there is a still shortcoming
in multi-objective optimization. Based on open literature, there is
scarce detail study attempting to model a TSA unit satisfying
multi-objective optimization. Since, the regeneration step is the most
important stage in the TSA unit, four variables relating to the
regeneration step are considered as effective parameters. In order to
reach the lowest operating costs and highest ethanol and diethyl ether
recovery in the TSA unit, multi-objective optimization is conducted as a
new study using the response surface methodology (RSM). Eventually, a
set of regeneration variables and their optimal values are obtained and
a model is proposed for individual objectives.