# Novel Photocatalytic Reactor Development for  Removal of Hydrocarbons from Water

Authors: Morgan Adams; Ian Campbell; Peter K. J. Robertson
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

Hydrocarbons contamination of the marine environment generated by the offshore oil and gas industry is generated from a number of sources including oil contaminated drill cuttings and produced waters. The removal of hydrocarbons from both these sources is one of the most significant challenges facing this sector as it moves towards zero emissions. The application of a number of techniques which have been used to successfully destroy hydrocarbons in produced water and waste water effluents has previously been reported. This paper reports the application of semiconductor photocatalysis as a final polishing step for the removal of hydrocarbons from two waste effluent sources. Two reactor concepts were considered: a simple flat plate immobilised film unit, and a new rotating drum photocatalytic reactor. Both units proved to be effective in removing residual hydrocarbons from the effluent with the drum reactor reducing the hydrocarbon content by 90% under 10 minutes.


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## Body

## 1. Introduction

Hydrocarbons can contaminate the aqueous environment
through several routes, for example, as by-products of the oil and gas industry
such as drill cuttings, or as surface run off from petrol stations and
garages. Pollution resulting from
hydrocarbons [1, 2] contaminating the marine environment must be addressed due
to the potential toxic effects associated with these compounds can cause considerable harm to a range of targets
within the environment [3–5]. Produced water presents a significant
environmental problem to the oil industry internationally. In one year over 8500 tonnes of oil was discharged from oil and gas installations to the North Sea from produced water discharges [6]. This is a
particular problem now as the offshore industry is moving towards zero
discharges from platforms.Drill cuttings are one of the by-products
of oil exploration and recovery, with around8000
m
3 of diesel and
low toxicity oil contaminated drill cuttings deposited around the base of
platforms in the North Sea [7]. Drill cuttings consist of small pieces of
rock which are generated when drilling a well, which vary in size from gravel
to fine silt. These cuttings are carried
from down hole to the oil platform by drilling fluid which not only lubricates
and cools the drill bit, but also prevents blowouts. On the platform, the cuttings are separated
from the fluid with the fluid being reinjected.
Most of the cuttings will also, at some point, come into contact with
hydrocarbons which are difficult to remove in an environmentally friendly
manner. In the past, almost all of the drill cuttings from the North Sea were dumped overboard onto the seabed.Since the detrimental environmental
impacts [8] of these cuttings were established, the government legislation has
reduced the amount of drill cuttings permitted to be discharged into the sea
and is moving towards banning the practice altogether [9, 10]. Consequently, the oil and gas industries have
investigated alternative methods for disposing of drill cuttings. Processes
that have been investigated include reinjecting the cuttings back into the well
(well injection) [7] or shipping the cuttings to shore for treatment. The on-shore treatment and disposal options
which include techniques such as thermal desorption, thermal distillation, solvent
extraction, solidification, incineration/combustion, and landfill [11].With thermal
desorption [12], the drill cuttings are treated by heating the materials, which
results in the vaporisation of water and hydrocarbons. This vapour is separated
and subsequently recondensed giving an oil/water liquid and clean drill
cuttings. The drill cuttings are then bagged and used for landfill whilst oil
and water are separated. The recovered oil is burned in oil-fired power
stations whilst the water is filtered to remove any residual hydrocarbons
before being discharged into the sea.  This water may still have residual hydrocarbon
content.Hydrocarbon contamination of water can also occur at
the distribution stage as well as extraction. A particular example of such
contamination results from “surface runoff” from Garage Forecourts. This effluent frequently contains petrol, diesel,
oils, brake fluids, and also dust from brake parts and exhaust particles. The
effluent is collected from drainage tanks for specialised treatment and
disposal. Typically, the waste water effluent is passed
through filtration systems but cannot fully remove volatile organic compounds
(VOCs), and hence secondary treatment is often required.Semiconductor
photocatalysis is a rapidly developing process which may have a significant
impact on the reduction and removal of these harmful and toxic compounds from
produced water and waste water effluents and has even been used for potable
water treatment [13–17]. This technology should be therefore a highly
feasible process for the treatment of both produced waters and waste water
runoff from garage forecourts. Although the use of the technology for
removal of a vast range of compounds from water has been previously reported,
one of the main challenges to date has been the up-scaling of the process to a
size where it can practically treat large volumes of water. Many processes
reported are usually treating litres per hour or even tens of litres per hour.
Most practical effluent treatment processes require at least 5–10 m
3
hr
−
1, and some offshore oil and gas platforms generate up to 10
5
m
3
hr
−
1. A review by Alfano et al. [18] provides an excellent overview of many of the
practical processes that have been developed for photocatalytic water treatment,
particularly those utilising solar energy. In this paper, we report the
development of both flat bed and drum reactor designs for the treatment of two
real contaminated water samples, that is, a pretreated produced water and a
waste water sample from a garage forecourt. The basic concept of these processes
has been assessed and described herein.In
this paper, we describe a method of substituting the water filtration system, as an addition to the current system, to
achieve very low levels of hydrocarbons in water.


## 2. Materials and Methods

### 2.1. Multiplate Thin Film Reactor Design

There
are several important parameters in the reactor design; one of the most
significant of which is the active photocatalyst coating and the underlying
substrate material. In addition, the coating preparation and the surface area
of the catalyst available to the pollutant molecules are also crucial considerations.
Two substrate materials were studied in this investigation: polymethylmethacrylate
(PMMA) and titanium metal.It is
well known that certain forms of PMMA are transparent to ultraviolet radiation
making the material ideally suited for an optical type chemical reactor where
ultraviolet light is used to activate the photocatalyst. The other material
under investigation is titanium. Although expensive, there is an important
property in that when the titanium is oxidised, titanium dioxide is produced
which is the photocatalyst being used in the reactor. There may also be
advantages in that; there is likely to be good adhesion between the titanium
and titanium dioxide.The initial
system developed for treating contaminated water was based on a thin film
photocatalytic reactor.  Figure1 shows
the prototype design of the photocatalytic reactor where the UV source was mounted on a support frame
above the coated plate. The plate was placed on a water tight channel which had
an effluent delivery tank at the head and an effluent collection reservoir at
the base. The plate was mounted at an angle inducing effluent flow when
introduced at the top of the unit.Figure 1
Prototype flat plate reactor treatment system under a UV source.The thin film plates of either PMMA or titanium were
coated withTi
O
2 in a 50 mL methanol suspension, containing between
200 and 250 mg of photocatalyst. This
was achieved by stirring the solution for 10 minutes to obtain an evenly
distributed mixture; this was then applied to the PMMA or titanium plates. The plate to be coated was placed in a
shallow vessel with the Ti
O
2/methanol solution applied centrally,
and the vessel gently tipped from side to side to produce an even coating.In this type of unit, the plates and plate
reservoirs ultimately could be jointed creating a “concertina” multiple plate
reactor module for large-scale water treatment (see Figure2).Multiple thin film plate
reactor stack (a) lab-based unit, (b) concept design for scaled-up unit.
(a)
(b)The contaminated water sample used to assess the
efficiency of this reactor was a sample of effluent taken from a thermal
desorption plant used to treat drill cuttings. The water sample typically
contained 100–200 ppm hydrocarbons.
This is significantly higher than the permissible discharge consent level for
the UK
controlled waters which is currently 30 mg/L [19].We have
previously reported the use of fluorescence spectroscopy for in situ monitoring
of hydrocarbons in the marine environment. As part of this study, the
applicability of absorption spectroscopy as an alternative technique to
fluorescence spectroscopy was assessed. Figure3 shows the results obtained for
the analysis of a produced water sample using absorption and fluorescence
spectroscopy, when the sample was treated using a flat plate reactor. As can be
seen from the figure for this study, a good correlation between the two
techniques was obtained and hence absorption spectroscopy was used for
monitoring the hydrocarbon reduction for this investigation. The analysis was
performed using a Novaspec
II absorption spectrometer monitoring the
decrease of the broadband peaking at 335 nm.Figure 3
Demonstration of the decrease in both absorbance and fluorescence activity of produced water with reaction time.


### 2.2. Drum Reactor Design

The drum reactor was designed to be a single pass
continuous flow system for produced water/effluents. If after one pass the water was still above
the discharge level for hydrocarbons, the water was allowed to run into a lower
reservoir. Typically, the residence time in each drum was just over 3 minutes,
with a total treatment time after passing through three drum modules being
around 10 minutes. If at this stage the sample was still contaminated, it was
then recirculated.The addition of hydrogen peroxide to the photocatalytic
system has been previously reported to enhance photocatalytic degradation rates
through the generation of additional OH radicals via the conductance band
reaction with the peroxide molecule [20, 21]. This was also found to be the
case for the degradation of hydrocarbons in both our systems so was introduced
into the final reactor setup. The hydrogen peroxide concentration was 0.5% v/v
total concentration in the effluent. This recirculation process was continued
until the hydrocarbons had been removed.TheTi
O
2 utilised in the reactor was a
Hombikat C material supplied by Sachtleben Chemie, Duisburg, Germany. The reactor drums were irradiated using 36 W Philips PL-L sunlamp
UV tubes supplied by RS Components Ltd, Northants, UK. Figures 4(a) and 4(b) show the
patented photocatalytic drum reactor configuration [22]. Sampling was achieved via the open air vents
at the fluid inlet side of the reactor drum.(a) Photocatalytic reactor drum setup
with patented paddle design [20] and (b) drum
reactor configuration in UV box.
(a)
(b)The
waste water sample was taken from an interceptor waste water collecting
effluent from a Garage Forecourt. This sample contained a mixture of
hydrocarbons at a total COD level of between 3500 and 4000 ppm.The
destruction of the hydrocarbons was monitored by both measuring the chemical
oxygen demand of the sample and also by gas chromatography/mass spectrometry (GCMS)
using a Hewlett Packard model 5890 series II GC connected to a Hewlett Packard
model 5971A mass selective detector.


## 2.1. Multiplate Thin Film Reactor Design

There
are several important parameters in the reactor design; one of the most
significant of which is the active photocatalyst coating and the underlying
substrate material. In addition, the coating preparation and the surface area
of the catalyst available to the pollutant molecules are also crucial considerations.
Two substrate materials were studied in this investigation: polymethylmethacrylate
(PMMA) and titanium metal.It is
well known that certain forms of PMMA are transparent to ultraviolet radiation
making the material ideally suited for an optical type chemical reactor where
ultraviolet light is used to activate the photocatalyst. The other material
under investigation is titanium. Although expensive, there is an important
property in that when the titanium is oxidised, titanium dioxide is produced
which is the photocatalyst being used in the reactor. There may also be
advantages in that; there is likely to be good adhesion between the titanium
and titanium dioxide.The initial
system developed for treating contaminated water was based on a thin film
photocatalytic reactor.  Figure1 shows
the prototype design of the photocatalytic reactor where the UV source was mounted on a support frame
above the coated plate. The plate was placed on a water tight channel which had
an effluent delivery tank at the head and an effluent collection reservoir at
the base. The plate was mounted at an angle inducing effluent flow when
introduced at the top of the unit.Figure 1
Prototype flat plate reactor treatment system under a UV source.The thin film plates of either PMMA or titanium were
coated withTi
O
2 in a 50 mL methanol suspension, containing between
200 and 250 mg of photocatalyst. This
was achieved by stirring the solution for 10 minutes to obtain an evenly
distributed mixture; this was then applied to the PMMA or titanium plates. The plate to be coated was placed in a
shallow vessel with the Ti
O
2/methanol solution applied centrally,
and the vessel gently tipped from side to side to produce an even coating.In this type of unit, the plates and plate
reservoirs ultimately could be jointed creating a “concertina” multiple plate
reactor module for large-scale water treatment (see Figure2).Multiple thin film plate
reactor stack (a) lab-based unit, (b) concept design for scaled-up unit.
(a)
(b)The contaminated water sample used to assess the
efficiency of this reactor was a sample of effluent taken from a thermal
desorption plant used to treat drill cuttings. The water sample typically
contained 100–200 ppm hydrocarbons.
This is significantly higher than the permissible discharge consent level for
the UK
controlled waters which is currently 30 mg/L [19].We have
previously reported the use of fluorescence spectroscopy for in situ monitoring
of hydrocarbons in the marine environment. As part of this study, the
applicability of absorption spectroscopy as an alternative technique to
fluorescence spectroscopy was assessed. Figure3 shows the results obtained for
the analysis of a produced water sample using absorption and fluorescence
spectroscopy, when the sample was treated using a flat plate reactor. As can be
seen from the figure for this study, a good correlation between the two
techniques was obtained and hence absorption spectroscopy was used for
monitoring the hydrocarbon reduction for this investigation. The analysis was
performed using a Novaspec
II absorption spectrometer monitoring the
decrease of the broadband peaking at 335 nm.Figure 3
Demonstration of the decrease in both absorbance and fluorescence activity of produced water with reaction time.


## 2.2. Drum Reactor Design

The drum reactor was designed to be a single pass
continuous flow system for produced water/effluents. If after one pass the water was still above
the discharge level for hydrocarbons, the water was allowed to run into a lower
reservoir. Typically, the residence time in each drum was just over 3 minutes,
with a total treatment time after passing through three drum modules being
around 10 minutes. If at this stage the sample was still contaminated, it was
then recirculated.The addition of hydrogen peroxide to the photocatalytic
system has been previously reported to enhance photocatalytic degradation rates
through the generation of additional OH radicals via the conductance band
reaction with the peroxide molecule [20, 21]. This was also found to be the
case for the degradation of hydrocarbons in both our systems so was introduced
into the final reactor setup. The hydrogen peroxide concentration was 0.5% v/v
total concentration in the effluent. This recirculation process was continued
until the hydrocarbons had been removed.TheTi
O
2 utilised in the reactor was a
Hombikat C material supplied by Sachtleben Chemie, Duisburg, Germany. The reactor drums were irradiated using 36 W Philips PL-L sunlamp
UV tubes supplied by RS Components Ltd, Northants, UK. Figures 4(a) and 4(b) show the
patented photocatalytic drum reactor configuration [22]. Sampling was achieved via the open air vents
at the fluid inlet side of the reactor drum.(a) Photocatalytic reactor drum setup
with patented paddle design [20] and (b) drum
reactor configuration in UV box.
(a)
(b)The
waste water sample was taken from an interceptor waste water collecting
effluent from a Garage Forecourt. This sample contained a mixture of
hydrocarbons at a total COD level of between 3500 and 4000 ppm.The
destruction of the hydrocarbons was monitored by both measuring the chemical
oxygen demand of the sample and also by gas chromatography/mass spectrometry (GCMS)
using a Hewlett Packard model 5890 series II GC connected to a Hewlett Packard
model 5971A mass selective detector.


## 3. Results and Discussion

### 3.1. Flat Plate Reactor

Initial experiments focused on
the optimisation of theTi
O
2 coating on the plate substrate were
performed by preparing different coatings using the slurry method described
above. It can be seen from Figure 5 that the most effective coatings were achieved
using suspensions of between 200 and 250 mg of Ti
O
2 (Degussa P25) in 50 mL
of pure methanol followed by air drying at room temperature. Experiments were
also carried out using elevated temperatures for evaporating off the methanol. As
can be seen from Figure 5, the effect of catalyst loading on the system was
only marginal, which would be expected in this type of unit where mass transfer
kinetics would be expected to predominate [23–25].Figure 5
Comparison ofTi
O
2 loading and heat treatment.After establishing loading parameters, the reaction
rate was studied as a function of plate angle and substrate material. It can be seen from the plot that the PMMA
substrate plate consistently outperformed the titanium substrate when both are
coated with the optimalTi
O
2 loading at a sub 15
° angle. The
primary reason that the shallower plate angle had greater destructive
efficiency is most likely due to a greater contact time of the effluent on the
catalyst plate due to the slower flow rates and hence longer residence time (see
Figure 6).Figure 6
Comparison of substrate material with
reactor plate angle at the optimumTi
O
2 loading.To
determine increase in efficiency of the reaction by the addition of an alternative
electron acceptor to oxygen (air), hydrogen peroxide solution was added at an
optimum initial concentration of 0.5% to the produced water sample. Figure7 shows the results of bubbling air only
through the produced water, using a combination of air and hydrogen peroxide
and hydrogen peroxide only. It can be seen that a significant enhancement was
obtained with the addition of the peroxide alone. In addition bubbling air
through this system did not lead to any additional enhancement of the
destruction of the hydrocarbons in the produced water. This supports similar
observations previously reported by our own group and others [26–31] and indicates
that the rate of aeration of the solution by ambient air is faster than the
rate of oxygen consumption associated with the photocatalytic destruction of
the hydrocarbons.Figure 7
Effect of bubbling air and adding hydrogen peroxide on the destruction
rate of produced water with the plate reactor.


### 3.2. Drum Reactor

Initial experiments using the drum reactor were
configured for continuous flow effluent treatment. The photocatalyst was initially washed with
distilled water to remove excess particulatesTi
O
2 from the surface of the pellets which could affect the photocatalytic reaction
by providing a higher surface area of catalyst within the reactor drum. This would also cause secondary problems for
the setup of the reactor as the excess particulates could block the effluent
transfer pathways.Figure8 shows the GC/MS results which clearly show
a 90% overall destruction over 10 minutes of VOCs present in the waste water
effluent treated through a total of 600 g of the Ti
O
2 catalyst. This was achieved by passing the waste water
effluent through three consecutive reactor drums each containing 200 g loads of
photocatalyst. It can be seen that with 10 minutes and after passing through
the third drum, the level of hydrocarbons in the water sample had virtually
disappeared.GC/MS chromatogram of waste water effluent, (a) pure sample with no
treatment, (b) 1st pass through drum reactor, (c) 2nd pass through drum reactor, and (d) 3rd pass through drum reactor (accumulative
total) catalyst treatments over a 10-minute irradiation.
(a)
(b)
(c)
(d)The chemical oxygen demand (COD) of the water
samples was also measured as an indicator of the total hydrocarbon level in the
untreated and treated samples. Table1 shows the mean COD values obtained from
5 experimental runs with each point being an average of 3 samples. It can be seen that the COD value decreases
very quickly during the time it takes to process through the 3 consecutive
reaction drums (10-minute reaction time).
As these experiments were performed on different days and the waste
water effluent was decanted from a large storage drum, it is possible that the
effluent content was not 100% consistent.Table 1
Mean COD values for pretreatment, 1st pass and  2nd pass through the drum reactor.
Sample pass
Mean COD mg/L
0 Pass pretreatment
3618
1st Pass through drum reactor
2166
2nd Pass through drum reactor
868


## 3.1. Flat Plate Reactor

Initial experiments focused on
the optimisation of theTi
O
2 coating on the plate substrate were
performed by preparing different coatings using the slurry method described
above. It can be seen from Figure 5 that the most effective coatings were achieved
using suspensions of between 200 and 250 mg of Ti
O
2 (Degussa P25) in 50 mL
of pure methanol followed by air drying at room temperature. Experiments were
also carried out using elevated temperatures for evaporating off the methanol. As
can be seen from Figure 5, the effect of catalyst loading on the system was
only marginal, which would be expected in this type of unit where mass transfer
kinetics would be expected to predominate [23–25].Figure 5
Comparison ofTi
O
2 loading and heat treatment.After establishing loading parameters, the reaction
rate was studied as a function of plate angle and substrate material. It can be seen from the plot that the PMMA
substrate plate consistently outperformed the titanium substrate when both are
coated with the optimalTi
O
2 loading at a sub 15
° angle. The
primary reason that the shallower plate angle had greater destructive
efficiency is most likely due to a greater contact time of the effluent on the
catalyst plate due to the slower flow rates and hence longer residence time (see
Figure 6).Figure 6
Comparison of substrate material with
reactor plate angle at the optimumTi
O
2 loading.To
determine increase in efficiency of the reaction by the addition of an alternative
electron acceptor to oxygen (air), hydrogen peroxide solution was added at an
optimum initial concentration of 0.5% to the produced water sample. Figure7 shows the results of bubbling air only
through the produced water, using a combination of air and hydrogen peroxide
and hydrogen peroxide only. It can be seen that a significant enhancement was
obtained with the addition of the peroxide alone. In addition bubbling air
through this system did not lead to any additional enhancement of the
destruction of the hydrocarbons in the produced water. This supports similar
observations previously reported by our own group and others [26–31] and indicates
that the rate of aeration of the solution by ambient air is faster than the
rate of oxygen consumption associated with the photocatalytic destruction of
the hydrocarbons.Figure 7
Effect of bubbling air and adding hydrogen peroxide on the destruction
rate of produced water with the plate reactor.


## 3.2. Drum Reactor

Initial experiments using the drum reactor were
configured for continuous flow effluent treatment. The photocatalyst was initially washed with
distilled water to remove excess particulatesTi
O
2 from the surface of the pellets which could affect the photocatalytic reaction
by providing a higher surface area of catalyst within the reactor drum. This would also cause secondary problems for
the setup of the reactor as the excess particulates could block the effluent
transfer pathways.Figure8 shows the GC/MS results which clearly show
a 90% overall destruction over 10 minutes of VOCs present in the waste water
effluent treated through a total of 600 g of the Ti
O
2 catalyst. This was achieved by passing the waste water
effluent through three consecutive reactor drums each containing 200 g loads of
photocatalyst. It can be seen that with 10 minutes and after passing through
the third drum, the level of hydrocarbons in the water sample had virtually
disappeared.GC/MS chromatogram of waste water effluent, (a) pure sample with no
treatment, (b) 1st pass through drum reactor, (c) 2nd pass through drum reactor, and (d) 3rd pass through drum reactor (accumulative
total) catalyst treatments over a 10-minute irradiation.
(a)
(b)
(c)
(d)The chemical oxygen demand (COD) of the water
samples was also measured as an indicator of the total hydrocarbon level in the
untreated and treated samples. Table1 shows the mean COD values obtained from
5 experimental runs with each point being an average of 3 samples. It can be seen that the COD value decreases
very quickly during the time it takes to process through the 3 consecutive
reaction drums (10-minute reaction time).
As these experiments were performed on different days and the waste
water effluent was decanted from a large storage drum, it is possible that the
effluent content was not 100% consistent.Table 1
Mean COD values for pretreatment, 1st pass and  2nd pass through the drum reactor.
Sample pass
Mean COD mg/L
0 Pass pretreatment
3618
1st Pass through drum reactor
2166
2nd Pass through drum reactor
868


## 4. Conclusion

The results of this study have demonstrated that
both reactor designs proved effective for the removal of hydrocarbon
contamination from waster water effluents. With the thin film plate reactor,
the assessment of PMMA and titanium metal plates as substrate materials was investigated
under different conditions. By varying
the mounting angle of the plates, the PMMA plate mounted at 15 degrees produced
60% destruction after 15 minutes. This
demonstrated that the lower plate angle increases the retention time of the
pollutant and therefore the chance of a successful catalyst–pollutant interface. An investigation into the effect of adding
air andH
2
O
2 to the system showed that the addition of
air alone to the reactor produced 40% degradation, compared to the 80%
degradation of H
2
O
2, over 135 minutes.With the development of the pelletisedTi
O
2,
it was possible to develop an alternative reactor configuration with a smaller
foot print. Conventional powder catalyst systems have traditionally posed
removal problems with filtration, and settling is required to remove powder
from the effluent. This limits the type
of reactor design to batch, as it is impractical to provide online filtration
for a continuous flow reactor system.The drum reactor reported in this study was
configured for continuous flow through 3 reactor tubes (see Figure4(b)) each
containing the same quantity of catalyst; GC/MS results showed the effective 90% removal of VOCs over 5 minutes. The
mechanical mixing action of the paddle array within the reactor tubes greatly
increases the pollutant-catalyst interface, enhances mass transport, and also
removes the need for additional air to be added to the system. The patented
paddle array also maintains an even spread of the catalyst pellet within the
drum which would normally suffer from “corkscrew” effect of the turning drum. Using
a standard indicator for organic compounds in water, chemical oxygen demand,
the drum reactor showed an 85% reduction of organic content.It should finally be noted that for both reactors
assessed in this paper, the processes had been developed as “polishing” units and
a complementary technology to existing techniques. The technique would not be
viable for more heavily contaminated water samples as the kinetics of the
process would require very significant reaction times and the photocatalytic
process cannot compete with existing technologies, where semiconductor
photocatalysis has demonstrated a particular effectiveness in such a final
polishing step for removal of more resilient compounds that traditional waste
water technologies are not capable of removing.



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*Source: 674537-2008-10-15.xml*