Studies on the oxidation of monoethanolamine using UV and H20 2 with post-biological treatment

In natural gas processing, alkanolamine solvents such as monoethanolamine (MEA)
are wide! y used for removal of sour gases from natural gas. In natural gas processing
plants, large volumes of alkanolamine solutions are routinely generated during
periodic maintenance, cleaning and vessel safety inspections. Due to intermittent
generation, high organic content and biological recalcitrance, these chemicals are
generally not treatable in the conventional wastewater treatment systems available in
these facilities and high costs can be incurred to segregate and dispose alkanolaminecontaminated
wastewater. Advanced oxidation processes (AOPs) have been studied
extensively as a promising pollution abatement strategy to rapidly oxidize many
organic pollutants. The combination of UV radiation and hydrogen peroxide, called
the UV/H20 2 process, is a widely studied AOP, and the degradation of syntheticallyprepared
MEA solution using the UV/H20 2 process is investigated in this work.
The effects of various parameters on the organics degradation of MEA under
UV IH202 treatment was studied using the Taguchi approach to design of experiments
based on the L-16 (45
) modified orthogonal array. Experiments were conducted in a
jacketed glass reactor using low-pressure UV lamps. Chemical oxygen demand
(COD) was used as a measure of the degree of degradation of organics in the MEA
solution. The parameters studied were UV dose, temperature, initial pH and initial
H20 2 dose. The optimum conditions, predicted response (COD removal) and
confidence interval were determined and a confirmation experiment was conducted.
The results indicate that the main and controlling factor for the removal of COD in the
experimental conditions used in this study is the UV dose. Other parameters did not
have any statistically significant effect on the COD removal at the ranges used in this
study. The response at optimum conditions was verified by confirmation experiment.

A more detailed study of UV/H20 2 degradation on MEA was conducted to
determine the effects of various parameters, i.e. initial pH, temperature, UV dose
(photon flux), and initial H20 2 dose at a broader range than the Taguchi study. In
addition, the solution pH, H20 2 concentration, MEA concentration and some
breakdown products were also investigated. The UV incident photon flux (i.e. UV
dose) was quantified using hydrogen peroxide actinometry. It was found that COD
removal and HzOz decay are increased by raising the initial pH of the solution and
more than 90% COD removal is achievable at high initial pH (8-9) after 60 minutes.
Variation of solution temperature in the range studied did not have any appreciable
effect on COD removal nor HzOz decay. The COD removal and HzOz decay
increased with higher UV photon flux. Although the Taguchi study found no effect of
initial H20 2 dose on COD removal at low H20 2 dosage, it was found that increase of
initial H20 2 dose above 0.16 M concentration retarded the COD removal rate due to
the scavenging of hydroxyl radicals by excess H20z. The pseudo first-order kinetic
constants for MEA degradation were estimated and ranged between 0.0090 to 0.0922
min·1 depending on the reaction conditions. The effects of the parameters on the
kinetic constants were also evaluated. Several intermediate breakdown products were
identified, including formate and nitrate. The formation of these acidic species
resulted in the pH depression that was observed during the course of reaction.
Significant concentration of ammonia was also formed during the course of the
reaction.
A quasi-mechanistic kinetic rate model for the reduction of gross orgamc
content (based on COD) during MEA oxidation using UV/H20 2 process was also
developed. The kinetic model incorporates a set of literature rate constant values for
the principal reactions involved in the photolysis of hydrogen peroxide by UV
radiation to which is added the n-th order reaction of the substrate (COD) with ·OH
radical. The kinetic model was validated using experimental COD and H20 2
degradation data and exhibited good agreement with measured values. The model
results confirm that the increased COD removal at higher pH was a result of the
formation of pH -dependent species and not due to the effect of H20 2 dissociation into
hydroperoxide ion at high pH.

The biodegradability of MEA solution (in terms of COD) that has been
partially degraded via UV/H20 2 was studied using batch growth reactor operated
under aerobic conditions. The kinetic rate constants based on the Monod model
formed the basis of comparison and were calculated by fitting the growth and
utilisation data to a sigmoid equation. The acclimatization times were also studied.
The results indicated that, for MEA solution which was partially treated with
UV /H20 2 at 30% COD removal, the biomass growth rate, substrate utilisation rate and
biomass yield was reduced compared to untreated MEA. The acclimatization time for
aerobic biodegradation was unaffected. The only parameter that showed
improvement was the half-saturation coefficient. This effect may be attributed to
formation of some unidentified inhibitory compound at the level of pretreatment that
was applied. Biodegradation of both MEA and partially treated MEA was found to
generate high levels of ammonia as by-product.