10. DESARROLLO DEL TRABAJO HERRAMIENTA ANÁLISIS JERÁRQUICO
10.2. Criterios y subcriterios en el proceso de toma de decisiones para incursionar a nuevos
10.2.3. CRITERIOS PAÍS
Classical chemical oxidation is a direct chemical oxidation process that is achieved by the addition of an oxidation agent to the contaminated aqueous solution to oxidize it. The most common chemical oxidants are chlorine (Cl2), chlorine dioxide (ClO2), oxygen (O2), persulfate, permanganate (KMnO4), ozone (O3), and hydrogen peroxide (H2O2). Moreover, advantages and disadvantages of each oxidant are summarized in the Table 4.
Table 4: Classical chemical oxidation
Chemical Oxidant
Advantages Disadvantages Ref.
Chlorine Strong oxidant
Strong disinfectant
generate persistent deposit
cheap oxidant
very simple to injected into the system
used wildly in the past
Available in gaseous
o Require high dosage of chlorine
Table 5: Classical chemical oxidation (cont.)
Chlorine dioxide
Strong oxidant
Strong disinfectant
Gaseous that is very soluble in water.
pH values in the range of 3.5 to 5.5 are preferred
Used efficiently to taste and odor for specific types
Does not generate halogenated DBPs
Does not react with ammonia
o Produces chlorite as an inorganic DBP
o It might create unwanted odors
o hard to maintain a persistent disinfection
o Difficult to handle and transport because it is unstable at high
concentrations and can explode if it exposed to heat, light.
[42]
Oxygen Easy to feed in the system
Does not generate
halogenated by-products.
Low operation costs
o Quite weak oxidant for the majority of water
treatment system.
o Oxygen require certain operation conditions and it is not working under normal temperature and pressure
o It is requires large investments in installations.
[42, 43]
Persulfate It is much more stable and it does not react quickly by nature.
High oxidation potential
applicable to wide range of organics
It can be catalyzed by heat, ultraviolet light, high pH, hydrogen peroxide,
and transition metals
highly reactive at pH <3, but it is also highly
reactive at pH > 10
Fewer mass transfer and mass transport limitations
o New technology and few studies are tested in the field.
o Might degrade soft metal
o Undesirable long lasting
Table 6: Classical chemical oxidation (cont.)
Chemical Oxidant
Advantages Disadvantages Ref.
Permanga nate
Easy to feed in the system
Does not generate halogenated DBPs
Efficient for certain types of odor and taste
Applicable over wide pH range.
widely available
Cheap oxidant
Easily transport and handling.
Less health and safety problem. (no gas/heat production)
o Produces manganese dioxide by product that should be removed
o Can result a pink water color if dosage not controlled
o Limited disinfection capabilities
o Reduction in the oxidant efficiency due to the
o Not effective for a wide range of contaminants.
Recommended pH =7.5 or higher
Applicable to wide range of organics.
Very strong disinfectant
Efficient for taste and odor
Does not generate
halogenated DBPs except in bromide-rich waters
May used to support in the coagulation and
flocculation
o Does not create a persistent disinfectant
residual
o Quietly costly
o generate bromate in bromide-rich waters
o Ozone can react with a variety of contaminants, but it also can react with
many other molecules.
o Process efficiency is dependent on gas liquid mass transfer, which is quite difficult to maintain due to the low solubility of ozone in the aqueous solutions. This Cause non uniform distribution through the complete substance.
o Lack of studies on large scale operation
Table 7: Classical chemical oxidation (cont.)
Chemical Oxidant
Advantages Disadvantages Ref.
Hydrogen Peroxide
One of the cheapest oxidizers
It has high oxidizing potential
It is water-soluble.
It does not produce toxins or color byproducts
Applicable over a wide range of organic
contaminants
Can be combined with ozone or UV to increase the efficiency
It can store, operate and transport safely.
o Not effective for complex materials
o Mass transfer limitations between the hydrogen peroxide with the organics
o Over-oxidation reaction could happen. So it should be used in controlled
From Table 4 it can be seen that chemical oxidants offer a diversity of benefits in the treatment process. Some are simple and others are more difficult. The classical chemical oxidation is a multipurpose process that is implemented in many applications either to treat the wastewater or to improve the quality of water. Each oxidant has specific advantages that need to be evaluat ed before employing [41].
1.5.2. Advanced Oxidation Processes (AOPs)
Advanced oxidation processes are considered as promising methods for the treatment of spent caustic. The mechanism of AOPs is the process of forming sufficient quantity of highly reactive Hydroxyl radicals (HO•) at near ambient temperature and pressure. Once it is generated, it can attack the complex chemical contaminants in water and oxidize most of them [49].When AOPs are applied in
controlled conditions, they can reduce the concentration of contaminants from hundreds of ppm to less than 5 ppb and therefore bringing the COD and TOC to discharge limits [50].
In some cases these AOPs must be complemented with other treatment techniques in order to achieve the final treatment level. This leads to a more complex process and to an increase in the treatment cost [51].
A large number of methods are classified under the AOPs. The generation of the Hydroxyl radicals are achieved by the use of one or more strong oxidants (H2O2, O2, and O3) and/or catalysts (titanium dioxide, transition metal ions ) and/or energy sources (ultraviolet radiation) [49].
Advanced oxidation processes have several advantages which are [52, 53]:
Fast reaction rates
Simple and easy to implement.
Possible to reduce toxicity and possibly to complete mineralization of organic pollutants to CO2 and H2O without generating sludge.
Treatment of various organic compounds at the same time.
While these processes have disadvantages which are [52, 53]:
Some processes might be high in capital cost
Need high controlled conditions
Some applications require quenching of excess peroxide is required.
The selection of a certain advanced oxidation process depends on the application.
The selection could be based on the type of compounds to be removed, treatment objectives, concentrations, site considerations, and cost.
However, it has been observed that none of the methods can be used individually in treatment applications due to substantially lower energy efficiencies and higher costs of operation and usually a combination of different AOPs has been found to be more efficient for the treatment [54, 55]. The main processes found in literature
for producing these radicals are summarized in Table 5. It can be note that UV system has major drawbacks such as mass transfer limitation, turbidity that can inhibit UV light diffusion, and some compounds (nitrate) can absorb UV light. All of these will result lowering process efficiency.
Also, ozone with hydrogen peroxide system is like UV with hydrogen peroxide system. However, this system is less affected by feed characteristics.
One method can be used to improve contaminants removal is the implementation of ozone, hydrogen peroxide, and ultraviolet radiation system (O3/H2O2/UV).
However, in this case the cost of treatment system will be huge because of the usage of the two oxidants. This system is recommended when wastewater pollutants weakly absorb UV radiation light.
28
Table 8:Advanced chemical oxidation
Chemical oxidant
Brief description Advantages Disadvantages Ref.
UV/O3 When low pressure UV light is applied to ozonated water hydroxyl radicals are
2. More effective than O3 or UV alone.
3. More efficient generating OH radical than H2O2& UV for equal oxidant concentrations.
1. Energy and cost intensive process.
2. Potential for bromate formation but it can be controlled through adjustment of pH.
3. Turbidity can interfere with UV light.
4. Ozone diffusion can result in mass transfer limitations.
5. May require ozone off gas treatment.
[57-60]
UV /H2O2 H2O2 is injected and mixed followed by a reactor that is equipped with UV light.
During this process, UV is used to cleave
the O-O bond in hydrogen peroxide and generate the
hydroxyl radical
1. UV decompose H2O2 to produce (2OH) free radicals, 2. No sludge generation,
3. UV/H2O2 process is efficient in mineralizing organic pollutants 4. No potential for promate
formation
5. No off gas treatment requires.
6. Not limited by mass transfer relative to O3 processes.
1. it cannot utilize solar light as the source of UV light
2. H2O2 has poor UV absorption characteristics because of that special reactor designed for UV is required.
3. Turbidity can interfere with UV light penetrating
4. Less stoichometric efficient in generating OH radical than O3/H2O2 process.
5. Interference compounds like nitrate can absorb UV light
[57, 60]
29 29
Table 9:
Table 9:Advanced chemical oxidationAdvanced chemical oxidation (Cont’d)(Cont’d) Chemical
Chemical oxidant oxidant
Brief
Brief description description Advantages Advantages Disadvantages Disadvantages Ref.Ref.
UV/ TiO
UV/ TiO22 a titanium peroxide is aa titanium peroxide is a semiconductor absorbs UV semiconductor absorbs UV light and
light and causing causing toto generate hydroxyl radicals.
generate hydroxyl radicals.
1.
1. Chemical stability of TiOChemical stability of TiO22 in in aqueous media and high potential to aqueous media and high potential to produce radica
produce radicals.ls.
2.
2. Easy availability and low price.Easy availability and low price.
3.
3. Possible use of solar irradiation.Possible use of solar irradiation.
4.
4. TiOTiO22 is a is a cheap, readily availablecheap, readily available material
material 5.
5. TiOTiO22 is capable for oxidation of a is capable for oxidation of a wide range of organic compounds wide range of organic compounds 6.
6. No potential for No potential for bromate formabromate formationtion 7.
7. Can be performed at high UVCan be performed at high UV wavelength than other UV oxidation wavelength than other UV oxidation processes.
processes.
8.
8. No off gas tre No off gas treatment requireatment required.d.
1.
1. impossible to achieve uniformimpossible to achieve uniform irradiation of the entire catalyst irradiation of the entire catalyst surface
surface 2.
2. Pretreatment is essential to avoidPretreatment is essential to avoid fouling of the TiO
fouling of the TiO22 catalyst. catalyst.
3.
3. If TiOIf TiO22 is added as slurry then a is added as slurry then a separation step is required.
separation step is required.
4.
4. Need more study Need more study to determine theto determine the optimum TiO
optimum TiO22 dose dose 5.
5. Reaction efficiency is highlyReaction efficiency is highly depending on pH because of that depending on pH because of that close monitoring and control is close monitoring and control is required.
required.
6.
6. Require onsite storage orRequire onsite storage or regeneration method.
regeneration method.
7.
7. No full scale exists. No full scale exists.
[57, once applied, they react to once applied, they react to form hydroxyl radicals.
form hydroxyl radicals.
1.
1. Peroxone process is much rapidPeroxone process is much rapid than using O
than using O33 or H or H22OO22 alone. alone.
2.
2. It is extremely efficient to treatIt is extremely efficient to treat complex compounds
complex compounds 3.
3. HH22OO22 is stable in acidic medium is stable in acidic medium
1.
1. Greatly dangerous and shouldGreatly dangerous and should carefully handle when it is used carefully handle when it is used and stored.
and stored.
2.
2. Not very efficient when it is used Not very efficient when it is used to oxidize iron and manganese.
to oxidize iron and manganese.
3.
3. Potential to produce byproductsPotential to produce byproducts such as aldehydes, ketones, such as aldehydes, ketones, peroxides.
peroxides.
4.
4. May require treatment of excessMay require treatment of excess H
H22OO22..
5.
5. May require gas treatment.May require gas treatment.
[58, [58, 59]
59]
30 30
Table 10:
Table 10:Advanced chemical oxidation (Advanced chemical oxidation (Cont’dCont’d)) Chemical
Chemical oxidant oxidant
Brief
Brief description description Advantages Advantages Disadvantages Disadvantages Ref.Ref.
Fenton a mixture of ferrous iron a mixture of ferrous iron (catalyst) and hydrogen (catalyst) and hydrogen peroxide (oxidizing agent).
peroxide (oxidizing agent).
1.
1. Fenton may lead to completeFenton may lead to complete destruction of the contaminants destruction of the contaminants under ideal conditions to nontoxic under ideal conditions to nontoxic compounds.
compounds.
2.
2. the iron used cathe iron used can be n be removed fromremoved from the solution
the solution 3.
3. The generation of OHThe generation of OH∙∙ cause to acause to a rapid reaction to many
rapid reaction to many contaminants.
contaminants.
4.
4. High oxidation potential that canHigh oxidation potential that can target complex organic compounds.
target complex organic compounds.
5.
5. Treatment of both organic andTreatment of both organic and inorganic substances under inorganic substances under laboratory conditions as well as laboratory conditions as well as realreal effluents
effluents 6.
6. Can oxidize wide range ofCan oxidize wide range of contaminants.
contaminants.
7.
7. Iron and hydrogen peroxide areIron and hydrogen peroxide are cheap and safe.
cheap and safe.
8.
8. Hydrogen peroxide easy to storageHydrogen peroxide easy to storage and to handle
and to handle
1.
1. Need to reduce the pH, followed Need to reduce the pH, followed by neutralization.
by neutralization.
2.
2. Hazards associated with usingHazards associated with using H
H22OO22
3.
3. Hydroxyl radical with highHydroxyl radical with high concentration
concentration might react might react with thewith the other species
other species 4.
4. The reaction is exothermic andThe reaction is exothermic and might cause an increase in the might cause an increase in the temperature but it can be temperature but it can be controlled.
31 31
Table 11:
Table 11:Advanced chemical oxidation (Advanced chemical oxidation (Cont’dCont’d)) Chemical
Chemical oxidant oxidant
Brief
Brief description description Advantages Advantages Disadvantages Disadvantages RefRef
Ultrasound
Usually ozone or hydrogen peroxide is Usually ozone or hydrogen peroxide is used along ultrasound to promote used along ultrasound to promote hydroxyl radical’s generation wh hydroxyl radical’s generation whichich enhances pollutants’ removal.
enhances pollutants’ removal.
Higher ultrasound frequency will Higher ultrasound frequency will provide shorter time for the provide shorter time for the
microbubble to collapse resulting in microbubble to collapse resulting in lower possibility of hydroxyl radicals lower possibility of hydroxyl radicals to recombine which result in higher to recombine which result in higher generation rate of hydroxyl radicals.
generation rate of hydroxyl radicals.
The main disadvantages are no The main disadvantages are no commercial plant using this system commercial plant using this system has been built yet and the amount of has been built yet and the amount of oxidant either ozone or hydrogen oxidant either ozone or hydrogen peroxide required to increa
peroxide required to increase hydroxylse hydroxyl radical is large which increases the radical is large which increases the cost of operations
The treatment of spent caustic by application of Fenton’s method is highly recommended. As seen in table 4 Fenton’s reaction has several advantages that make this method to be widely implemented in the treatment processes. It has high efficiency and its ability to treat various contaminants and can lead to complete destruction of contaminants [36].
There are many researches that were done to treat refinery spent caustic. It is very difficult to treat with conventional wastewater processes because of that it has been incinerated. On the other hand, ethylene spent caustic is highly diluted than refinery that make it possible to treat and dispose it in a save manner.
Ethylene spent caustic solutions are disposed of through wet air oxidation.
However, the major problem is the exothermal reaction that needs to control the heat buildup in the process. Also, it is a very expensive process for the treatment.
There are several researches that study the treatment of ethylene spent caustic by Fenton’s method. Sheu and Weng, (2001) studied a new method of treatment of spent caustic from a naphtha cracking plant by neutralization followed by oxidation with Fenton’s reagent. Spent caustic contains high H2S concentration and some mercaptans, phenols and oil. Over 90% of dissolved H2S were converted to gas by neutralization at pH=5 and T = 70 oC. The remaining residual sulfides were oxidized to less than 0.1 mg/l by Fenton’s reagent. The total COD removal of spent caustic is over 99.5% and the final COD value of the effluent can be lower than 100 mg/l. As a result, the spent caustic treatment becomes economical and effective [11]. Moreover, Nunez, et all (2009) studied electro-Fenton process. The efficiency of the Electro-Fenton process was
spent caustic samples. Approximately 97% COD removal was achieved for sulphide treatment, as the sulphide was highly affected by both the pH reduction and the oxidation by Fenton’s reagent. In the real spent caustic sample, 93%
COD reduction was obtained. The process designed includes a pH reduction unit followed by an Electro-Fenton’s reactor. Its advantages regarding safety and costs make it a process that has to be considered in petroleum refineries [32].
Nowadays, real treatment plant by Fenton’s method exists. The treatment by Fenton’s is done by a company called “FMC Foret”. They modified Fenton’s reaction to treat spent caustic and they named the process as Oxidation with Hydrogen Peroxide (OHP) [62]. However, there are differences between this method and the Fenton’s reaction. The first difference is the catalyst used in FMC Foret, the catalyst used is ferrous salt without specifying the type of salt. The second difference is the operational conditions; Fenton’s reaction operates at ambient temperature and pressure while FMC Foret operates at mild conditions [62]. Spent caustic is first pumped to an acidification tank to adjust the pH value to 3-5 so Fenton’s reaction can take place [11]. After that, the feed is pumped to raise the pressure to 2-2.5 bar. The pressurized spent caustic is then fed into a heat exchanger to raise the temperature to 110-120 °C [62]. Then the reactor effluent is send to a heat exchanger to cool the product. The effluent is then sent to neutralization tank where the pH is adjusted to a value around 7. As a result of neutralization, the ferric ion generated in the reaction will precipitate [61]. Finally the treated effluent is decanted then sent to biological treatment for post treatment. The main advantage of this process is the ability of treating influents with different organic content and some inorganic contaminants such as sulfides
and mercaptans. Also, COD removal can reach up to 95 % as well as the process is easy to install with low capital cost [62].
1.6. Objectives:
The main objectives of this thesis is to characterize and treat spent caustic produced from ethylene plants. The treatment process targeted a COD value less than 1000 mg/l and a sulfide concentration of 2 mg/l. These values were chosen since these are the limits that should enter the biological process which proceeds the chemical process.
Two treatment processes will be studied:
1. Neutralization: neutralization will release all carbonates as carbon dioxide and all sulfides as hydrogen sulfide this in return will result in the COD reduction.
2. Neutralization coupled with oxidation: oxidation will further reduce the contaminant’s concentration. Neutralization is applied before the oxidation because of high concentration of acid gases (H2S and CO2) that would
react with ferric ion causing a loss of iron catalyst
For the neutralization coupled with oxidation two methods will be teste d:
1. Classical oxidation by using hydrogen peroxide alone.
2. Advanced oxidation by Fenton’s reagent.
The effect of different parameters on the treatment process will be investigated namely, pH value, temperature, oxidants and catalyst concentration. The effect of these parameters on COD and sulfide removal will be measured. Figure 10 summarizes the treatment processes of ethylene plant spent caustic that were included in this study.
36
Figure 10:Summarized of the treatment processes of ethylene spent caustic
CHAPTER 2:
2. RESEARCH METHODOLOGY
2.1. Spent Caustic Characteristics
A spent caustic sample was obtained from Qatar Petrochemical Company (QAPCO). The pH value and conductivity were initially measured using a pH/
conductivity meter (WTW/Germany). The pH and conductivity values were 13.5-14 and 136.2 ms /cm, respectively.
2.1.1. Total Suspended Solids and Total Dissolved Solids:
Total dissolved solids (TDS), and total suspended solids (TSS) were determined
Total dissolved solids (TDS), and total suspended solids (TSS) were determined