QATAR UNIVERSITY QATAR UNIVERSITY Graduate Studies Graduate Studies College of Engineering College of Engineering CHARACHTERIZA
CHARACHTERIZATION AND TION AND TREATMENT OF SPENT CAUSTICTREATMENT OF SPENT CAUSTIC
FROM AN ETHYLENE PLANT FROM AN ETHYLENE PLANT
A Thesis in A Thesis in Environmental Engineering Environmental Engineering By By
Hasanat Mohammed Ramadan Hasanat Mohammed Ramadan
© 2013 Hasanat Mohammed Ramadan © 2013 Hasanat Mohammed Ramadan
Submitted in Partial Fulfillment Submitted in Partial Fulfillment
Of the Requirements Of the Requirements
For the Degree of For the Degree of
Master of Science in Environmental Engineering Master of Science in Environmental Engineering
December 2013 December 2013
The thesis of
The thesis of Hasanat Mohammed RamadanHasanat Mohammed Ramadan was reviewed and approved by the was reviewed and approved by the following:
following:
We, the committee members listed below accept and approve the Thesis of the We, the committee members listed below accept and approve the Thesis of the student named above. To the best of this committee’s knowledge, the Thesis student named above. To the best of this committee’s knowledge, the Thesis conforms the requirements of Qatar University, and we endorse this Thesis for conforms the requirements of Qatar University, and we endorse this Thesis for examination.
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ABSTRACT
ABSTRACT
Spent caustic solution is generated in oil refineries and petrochemical Spent caustic solution is generated in oil refineries and petrochemical plants
plants as as a a result result of of scrubbing scrubbing processes processes where where hydrogen hydrogen sulfide sulfide and and carboncarbon dioxide compounds are removed. Spent caustic is considered as one of the most dioxide compounds are removed. Spent caustic is considered as one of the most difficult streams to handle by wastewater treatment plants. Selecting the best difficult streams to handle by wastewater treatment plants. Selecting the best treatment technique is considered to be a critical task, in order to meet the treatment technique is considered to be a critical task, in order to meet the discharge limit.
discharge limit.
The present work aims to find an effective treatment process to treat spent The present work aims to find an effective treatment process to treat spent caustic solution produced from an ethylene plant. Neutralization and caustic solution produced from an ethylene plant. Neutralization and neutralization coupled with oxidation are the two processes studied. Two methods neutralization coupled with oxidation are the two processes studied. Two methods of oxidation are tested, classical oxidation by using H
of oxidation are tested, classical oxidation by using H22OO22 alone and advanced alone and advanced
oxidation by using Fenton’s reagent.
oxidation by using Fenton’s reagent. Applying neutralization alone orApplying neutralization alone or neutralization with oxidation was verified for the achievement of the required neutralization with oxidation was verified for the achievement of the required degree of treatment. For the neutralization alone, it was found that the highest degree of treatment. For the neutralization alone, it was found that the highest chemical oxygen demand removal achieved was 88 % (1699 mg/l) at pH=1 while chemical oxygen demand removal achieved was 88 % (1699 mg/l) at pH=1 while the sulfide removal was 99.8 % ( 9.9 mg/l). For classical oxidation using H the sulfide removal was 99.8 % ( 9.9 mg/l). For classical oxidation using H22OO22,,
the best removal was achieved at pH=2.5. The COD % removal was 89 % with a the best removal was achieved at pH=2.5. The COD % removal was 89 % with a COD value of 1630 mg/l. Furthermore, the sulfide removal reached a value of COD value of 1630 mg/l. Furthermore, the sulfide removal reached a value of almost 100%.
almost 100%. While for the advanced chemical oxidation using Fenton’s process,While for the advanced chemical oxidation using Fenton’s process, the best result was obtained at pH=2.5. The COD % removal was 96 % with a the best result was obtained at pH=2.5. The COD % removal was 96 % with a COD value of 542 mg/l. This was achieved with hydrogen peroxide to ferrous COD value of 542 mg/l. This was achieved with hydrogen peroxide to ferrous sulfate ratio of 1: 7.5. The sulfide removal als
TABLE OF CONTENTS
Abstract ………..………...……… III List of Figures ………….….………..………...……… VII
List of Tables ……….………...……… VIII
Abbreviation ………..………...……… IX
Acknowledgment ………..……… X
CHAPTER 1:
1. Introduction ………...……… 1
1.1. Spent Caustic ………...… 1
1.2. Sources of Spent Caustic Solution from the Ethylene Plant……. 4
1.2.1. Hot section ………...……… 8 1.2.1.1. Steam Cracker..…… ………….……… 8 1.2.1.2. Quenched Tower ..………. 9 1.2.2. Compression Section ………... 10 1.2.2.1. Gas Compressors …….………. 10 1.2.3. Cold Section.………..….………... 10
1.2.4. Treating (Caustic Tower) ……… 10
1.2.5. Fractionation Section ……….. 11
1.3. Spent Caustic Management Background……….. 14
1.3.1. Reduction ……… 14
1.3.2. Reuse within the Process ………. 15
1.3.3. Recycle Outside the Facility ………... 15
1.3.4. Treatment and Disposal ………... 15
1.4. Spent Caustic Treatment ………. 16
1.4.1. Biological Treatment of Spent Caustic ………... 17
1.4.2. Thermal Treatment ……….. 18
1.4.3. Chemical Treatment………. 20
1.5. Classification of the Chemical Oxidation Processes …………... 22
1.5.2.
1.5.2. Advanced Oxidation Processes (AOPs) ………..Advanced Oxidation Processes (AOPs) ……….. 2525 1.6.
1.6. Objectives of Present StudyObjectives of Present Study ………...………... 3535 CHAPTER 2 :
CHAPTER 2 : 2.
2. Research Methodology……….Research Methodology………. 3737 2.1.
2.1. Spent Caustic Characteristics ………...Spent Caustic Characteristics ………... 3737 2.1.1.
2.1.1. Total Suspended Solids and Total DissTotal Suspended Solids and Total Dissolved Solids…………olved Solids………… 3737 2.1.2.
2.1.2. Measurements of Chemical Oxygen Demand (COD)………..Measurements of Chemical Oxygen Demand (COD)……….. 3838 2.2.3.
2.2.3. BiologicalBiological Oxygen Demand (BOD) ……...……….Oxygen Demand (BOD) ……...………. 3939 2.2.4.
2.2.4. Total SulfideTotal Sulfide S S−2−2, , HH22S, S, HHSS−− by Titration (sulfide above 1 by Titration (sulfide above 1
mg/l
mg/l) ………...………) ………...……… 39 39
2.2.5.
2.2.5. Determination of Total SulfideDetermination of Total SulfideSS−2−2, H, H22S, HS, HSS−− (sulfide 0 to (sulfide 0 to
800
800 μμg/lg/l)………..)……….. 40 40
2.2.6.
2.2.6. Free Soda and CompleteFree Soda and Complete Alkalinity……….………Alkalinity……….……… 4040 2.2.7.
2.2.7. Total Petroleum Hydrocarbons (TPH)……….Total Petroleum Hydrocarbons (TPH)………. 4141 2.2.8.
2.2.8. Inorganic anions ………..Inorganic anions ……….. 4242 2.2.9.
2.2.9. Heavy Metals ………..………Heavy Metals ………..……… 4343 2.2.10.
2.2.10. Phenol ……….…….Phenol ……….……. 4343 2.3.
2.3. Experimental Setup and Procedure ……….Experimental Setup and Procedure ………. 4646 2.3.1.
2.3.1. Neutralization Neutralization ………..……….. 4646 2.3.2.
2.3.2. Neutralization Coupled with Oxidation ……….. Neutralization Coupled with Oxidation ……….. 4848 CHAPTER 3 :
CHAPTER 3 : 3.
3. Results and Discussion..……….…………..Results and Discussion..……….………….. 4949 3.1.
3.1. Neutralization ………...………...…………... Neutralization ………...………...…………... 4949 3.1.1.
3.1.1. Effect of pH on Sulfide Removal……….Effect of pH on Sulfide Removal………. 4949 3.1.2.
3.1.2. Effect ofEffect of Temperature on Sulfide Removal ……….………...Temperature on Sulfide Removal ……….………... 3.1.3.
3.1.3. Effect of pH on COD Removal……..……….…Effect of pH on COD Removal……..……….…
52 52 53 53 3.1.4.
3.1.4. Effect of pH on TDS Removal ……….…….…Effect of pH on TDS Removal ……….…….… 5555 3.2.
3.2. Neutralization Coupled with Oxidation: Classical Oxidation ...…… Neutralization Coupled with Oxidation: Classical Oxidation ...…… 5555 3.2.1.
3.2.1. Effect of Hydrogen Peroxide Concentrations on CODEffect of Hydrogen Peroxide Concentrations on COD Removal ………..
Removal ……….. 5656
3.2.2.
on COD Removal ………
on COD Removal ……… 5959
3.3.
3.3. Neutralization coupled with Oxidation: Advance oxidation ….. Neutralization coupled with Oxidation: Advance oxidation …...…...….. 6363 3.3.1.
3.3.1. Effect of pH in Fenton’s Reagent on the Removal of COD.…Effect of pH in Fenton’s Reagent on the Removal of COD.… 6363 3.3.2.
3.3.2. Effect Effect of of Ferrous Ferrous Sulfate Sulfate Concentrations Concentrations on on COD COD Removal... Removal... 6868 3.3.3.
3.3.3. Effect of Hydrogen Peroxide to Ferrous Sulfate Ratio onEffect of Hydrogen Peroxide to Ferrous Sulfate Ratio on COD Removal………..
COD Removal……….. 7070
3.3.4.
3.3.4. Effect of Hydrogen peroxide to COD ratio on CODEffect of Hydrogen peroxide to COD ratio on COD
Removal………...
Removal………... 7373
CHAPTER 4: CHAPTER 4: 4.
4. Conclusions and Recommendations …..……….…...…Conclusions and Recommendations …..……….…...… 7676 CHAPTER 5:
CHAPTER 5: 5.
5. Future Work ………...………....Future Work ………...……….... 7979 References ………
LISTS OF FIGURES
LISTS OF FIGURES
Figure 1
Figure 1 : Eth: Ethylene process ylene process flow diagram flow diagram (PFD) ...(PFD) ... ... 77 Figure 2
Figure 2 : : Steam Steam cracker cracker diagram ...diagram ... ... 99 Figure
Figure 3 3 : : Quenched Quenched tower tower ... ... 99 Figure
Figure 4 4 : : Gas Gas compressors compressors ... ... 1010 Figure
Figure 5 5 : : Treating Treating unit ...unit ... ... 1111 Figure
Figure 6 6 : : Fractionation Fractionation unit unit ... ... 1313 Figure 7:
Figure 7: Waste Waste management management hierarchy hierarchy ... ... 1414 Figure 8
Figure 8 : Treat: Treatment technologies ment technologies according to according to COD contents. COD contents. ... ... 1717 Figure
Figure 9 9 : : Oxidation Oxidation potential ...potential ... ... 2121 Figure 10: Summarized
Figure 10: Summarized of the of the treatment processes treatment processes of ethylene of ethylene spent caustic spent caustic ... ... 3636 Figure 11
Figure 11 : : Experimental Experimental schematic schematic diagram ...diagram ... ... 4646 Figure 12
Figure 12 : : Hydrogen sulfiHydrogen sulfide and de and pH dependent ...pH dependent ... ... 5050 Figure 13:
Figure 13: Neutralization of Neutralization of spent caustic ...spent caustic ... ... 5151 Figure 14 : Sulfi
Figure 14 : Sulfide % rede % removal at differmoval at different pH after ent pH after neutralization ....neutralization ... ... 5252 Figure 15 :
Figure 15 : Sulfide % Sulfide % removal at removal at different temperature ...different temperature ... ... 5353 Figure 16:
Figure 16: COD removal COD removal % for % for pH=1,3,5 before pH=1,3,5 before neutralization ...neutralization ... ... 5454 Figure 17: COD
Figure 17: COD removal % removal % at different at different pH after pH after neutralization ...neutralization ... ... 5454 Figure 18:
Figure 18: Effect of Effect of pH on pH on TDS removal ...TDS removal ... .... 5555 Figure 19: Blank sample with a) 0.1 H
Figure 19: Blank sample with a) 0.1 H22OO22 and b) 1 ml of H and b) 1 ml of H22OO22... 58... 58
Figure 20 : COD removal % at different H
Figure 20 : COD removal % at different H22OO22concentration ... concentration ... 6161
Figure 21 : Sulfide % removal at different H
Figure 21 : Sulfide % removal at different H22OO22concentration concentration ... 62... 62
Figure 22 :
Figure 22 : Effect of Effect of pH on pH on the COD the COD removal...removal... ... 6767 Figure 23 : Ef
Figure 23 : Effect of fect of ferrous sulfate ferrous sulfate concentration on COD concentration on COD % removal ... % removal ... 6969 Figure 24 : Effect of hydrogen peroxide to ferrous sulfate ratio on % COD
Figure 24 : Effect of hydrogen peroxide to ferrous sulfate ratio on % COD removal
removal ... .... 7272 Figure 25 : Ef
LISTS OF TABLES
Table 1: Spent caustic types and characteristics ... 4
Table 2 : Physical properties of ethylene ... 5
Table 3: WAO operational conditions ... 19
Table 4: Classical chemical oxidation ... 22
Table 4: Classical chemical oxidation ... 23
Table 4: Cont Classical chemical oxidation ... 24
Table 4: Cont Classical chemical oxidation ... 25
Table 5: Advanced chemical oxidation ... 28
Table 5: Cont Advanced chemical oxidation ... 29
Table 5: Cont Advanced chemical oxidation ... 30
Table 5: Cont Advanced chemical oxidation ... 31
Table 6 : Spent caustic characteristics used in this experiment ... 45
Table 7 : Effect of hydrogen peroxide concentrations on COD removal ... 56
Table 8 : Analyses interfere with H2O2... 58
Table 9: Effect of H2O2at different pH on COD removal ... 59
Table 12: Effect of hydrogen peroxide to ferrous sulfate ratio on COD % removal ... 71
ABBREVIATIONS
NaOH Sodium hydroxide
H2S hydrogen sulfide C2H4 Ethylene C2H6 Ethane HS- & S- Sulfide SO4 Sulfate S - Sulfide H2O2 Hydrogen peroxide O3 Ozone
OH• Hydroxyle redical
HO2• Hydroperoxyl
Fe+2 Ferrous
Fe+ Ferric
FeSO4 /H2O2 Fenton’s reagent
UV Ultra violet
TiO2 Titanium peroxide
DMDS Dimethyl disulfide
COD Chemical oxygen demand
TOC Total organic carbon
BOD Biological Oxygen Demand
TDS Total Dissolved Solids
TSS Total Suspended Solids
TPH Total Petroleum Hydrocarbons
WAO Wet Air Oxidation
CWAO catalytic wet air oxidation
ACKNOWLEDGMENTS
My sincere thanks to my supervisors, Dr. Alaa Al Hawari and Professor Ibrahim abureesh, who have supported me throughout my thesis work and provided me with advice, guidance, and continuous encouragement throughout the
work on this thesis.
Special thanks to Dr. Ahmed Al Khatat , senior lab technician of chemical engineering department, who provided us with his time and knowledge in the lab measurements. For his sincere help I’m deeply grateful.
Also I wish to express my special thanks to Qatar Pertrochemical Company (QAPCO) namely Dr. Mabrouk and Mr. Mejali Al Kuwari, who helped me throughout the period of this research and for the s upply of waste samples.
My gratitude is for my loving parents, my husband, and my family for keeping me motivated during this period of research work.
At the end, my regards to all people who supported me directly or indirectly during my graduates studies at Qatar University.
CHAPTER 1:
1. INTRODUCTION :
1.1.
Spent Caustic :
Qatar developed and built a strong industrial sector by employing the latest technological innovations in their production processes [1]. This eventually will create a positive and highly beneficial impact on the national economy. Qatar depends on two main sources of water to meet its need which are conventional water sources that come from groundwater and non-conventional water sources that come from desalination of seawater and recycling of treated wastewater [1]. Qatar is working to achieve a “zero liquid discharge” by 2016. The aim of this project is not to allow any discharge of treated wast ewater into water bodies but to reuse and recycle the produced water. The project is directed by the Ministry of Energy and Industry with the Ministry of Environment involving various industries in Qatar [2]. The zero discharge can be mainly achieved by enhancing the quality of treated waste water where it could be recycled and reused in irrigation or in operations and production processes.
In Qatar several industrial companies such as Qatar Chemical Company (Q-Chem), Qatar Petrochemical Company (QAPCO) and Qatar Petroleum (QP) generate liquid waste that is known as spent caustic solution. Spent caustic is an industrial waste solution that consists of sodium hydroxide, water, and other pollutants.
Sodium hydroxide (NaOH) solutions are used in many industries to wash out acid gases such as hydrogen sulfide (H2S) and carbon dioxide (CO2) from
different hydrocarbon streams [3]. Once these gases react with the majority of NaOH, a waste solution known as spent caustic will be produced [4]. Spent caustics are the most difficult class of liquid industrial waste to handle and to dispose due to the high concentration of pollutants in it [5]. According to the Environmental Protection Agency EPA under the Resource Conservation and Recovery Act RCRA used to classify the wastes as hazardous that create potential harmful impact to the human health and to the environment [6]. The classification of waste depends on the specific characteristics of the waste itself. The waste is considered to be hazardous if it exhibits one or more of the four characteristics [6, 7]:
1) Ignitability (D001) 2) Corrosivity (D002) 3) Reactivity (D003)
4) Toxicity (D004 - D043)
Spent caustic could be classified as D003 hazardous waste due to the reactive sulfide it contains [8]. Also, spent caustic is the highly corrosive due to the high pH value. Recent environmental regulations have a great impact on the spent caustic treatment method design since previous usual disposal methods are becoming legally prohibited [8].
Without treatment, spent caustic stream may cause environmental problems because of their alkalinity (pH>12), salinity (sodium of 5-12% wt) and
converted to elemental sulfur and/or sulfate that are preferred finishing product as it does not represent COD and maybe allowed to be discharged into the environment [14]. Moreover, spent caustics may contain toxic organo-sulfur compounds such as methanethiol and aromatic hydrocarbons like benzene [12, 13].
Spent caustics can be classified into many types depend upon the industry producing it and the source of fuel that fresh caustic wash. Table 1 summarizes
the type of spent caustic and there characteristics. Usually refineries don’t separate each type of spent caustic and they mix the three types and this is called the mixed refinery spent caustic [9].
Table 1:Spent caustic types and characteristics
Type of spent caustic
Sulfidic Cresylic Naphthenic Ref
Source Ethylene & LPG Gasoline Kerosene & Diesel
[5]
Content High Conc. of
Sulfides & Mercaptans High Conc. of Phenols & Cresols High Conc. Of Polycyclic aliphatic organic compounds [5] Effect after neutralization
Release gases Foaming and settling issue in the biological
Oil layer & foaming [5] Chemical oxygen demand (COD) (ppm) 5,000-90,000 50,000-100,000 150,000-240,000 [15] Total organic carbon (TOC) (ppm) 20-3,000 10,000-24,000 24,000-60,000 [15] Sulfides (ppm) 2,000-52,000 < 1 0-63,000 [15] Total phenol (ppm) 2-30 1,900-1,000 14,000-19,000 [15]
1.2.
Sources of Spent Caustic Solution from the Ethylene Plant:
In this study spent caustic produced from an ethylene plant will be targeted. In order to understand where spent caustic will be produced it is important to illustrate the ethylene process.
Ethylene is the chemical compound with the formula C2H4, because it contains a
hydrocarbons. Ethylene is a colorless flammable gas with a sweet odor. The physical properties of ethylene are summarized in Table 2.
Table 2 : Physical properties of ethylene [16]
Property Value
Structural formula
IUPAC name Ethene
Molecular weight 28.05 g/mol
Appearance Colorless gas
Density 1.178 kg/m at 15 °C, gas
Solubility in water 3.5 mg/100 ml (17 °C);
Thermodynamic data Phase behavior Solid, liquid, gas
Ethylene is an important building block in the petrochemical industry and its global production exceeds any other organic compound.
It can undergo many types of reactions which lead to major chemical products. Ethylene is the raw material to produce a wide range of products such as, ethylene glycol, ethylene dichloride, polyvinyl chloride, styrene, and polyethylene which is the common plastic in our daily life.
Ethane is preferred for ethylene production because the steam cracking of ethane consumes less energy than cracking heavier hydrocarbon. Ethylene process produce less emissions to the atmosphere because of that it is considered as an
environmental friendly process. The process includes a caustic tower which remove harmful gases such as NOx, mercaptans, H2S and CO2.
Figure 1 shows the process flow diagram of an ethylene plant. Ethylene is produced from ethane by a sequence of different processing steps which can be
classified as [17]: Hot section: 1. Steam Cracker. 2. Quenched Tower. Compression section : 1. Gas Compressor. Cold section: 1. Treating.
7
1.2.1. Hot Section
1.2.1.1. Steam Cracker
The Ethane Rich Gas (ERG) supplied from Qatar petroleum refinery contains high ethane content (65%) and other impurities like methane, hydrogen sulfide (H2S)
and carbon dioxide (CO2) .The ERG gas is treated to remove CO2 and H2S by
using Amine solution to achieve less than 100 ppm. The ERG gas, free of acid gases, is cooled and refrigerated to – 100 oC to separate methane. Finally pure ethane is sent to cracker unit to produce ethylene.
Figure 2 shows the furnace where the ethane gas is mixed with steam. Steam is added at controlled rates in order to increase the petrochemical yield and to minimize carbon deposits (coke) forming in the furnace and heated to about 850
o
C. The ethane is partially converted to ethylene and other hydrocarbons. In this section Dimethyl disulfide (DMDS) is added in the furnace as a coating to minimize the coke formation. Also, it is used to prevent over cracking of the gas to maintain high conversion of ethane to ethylene. This material with high temperature will generate H2S and CO2 that will be removed later using fresh
Figure 2 : Steam cracker diagram
1.2.1.2. Quenched Tower:
Figure 3 shows the quench tower unit where the effluent (cracked gases coming from the furnace) is immediately quenched by direct contact with water. The temperature drop to about 30°C is necessary to stop the cracking reaction. This quench water is then recovered and re-used.
1.2.2. Compression Section :
1.2.2.1. Gas Compressors:
Separation of hydrocarbons in distillation column requires the furnace effluent to be liquefied by increase the pressure of the gas then cool it down.
The cracked gas from the quench tower is compressed in five stages multi-stage centrifugal compressor as shown in Figure 4.
Figure 4 :Gas compressors
1.2.3. Cold Section :
1.2.3.1. Treating (Caustic Tower) :
The cracked gas stream will contain impurities (acid gases) that need to be removed. These impurities include carbon dioxide, and hydrogen sulfide that are generated as a result of addition DMDS in the steam cracker unit. Treatment of the cracked gas to remove impurities occurs between the fourth stage and the fifth stage of the compression section and it is treated in a caustic soda washing tower.
2NaOH (aq) + H2S (g) Na2S (aq) + 2H2O (1.1)
2NaOH (aq) + CO2(g) Na2CO3 (aq) + H2O (1.2)
The reason behind selecting the sodium hydroxide to remove the acid gases is its ability to remove the very small quantities of the acid gases. Then caustic tower overhead gas that is free from acid gases is treated in dryers to remove moisture and send to De-ethanizer to separate C3 and heavier components.
Figure 5 :Treating unit
1.2.3.2. Fractionation Section
The cracked gas that is free from acid gases and moisture is cooled down gradually in the fractionation section. In this section there are four distillation columns as shown in Figure 6. The first column is the de-ethanizer that separates out heavy gases such as propane (C3) from the light hydrocarbons (ethylene, ethane and methane).
De-ethanizer overhead is compressed in the 5th stage of compression and passed through acetylene reactor to remove acetylene. As the gas stream passes the catalyst the following reaction occurs:
C2H2 (g) + H2(g) C2H4(g) (1.3)
Then gas stream is sent to refrigeration chilling to condense ethane. The gas is cooled and then it liquefies.
The second distillation column is the de-propanizer column. The de-ethanizer bottom goes to the de-propanizer column to separates C3 hydrocarbon from C4.
The third column is the de-methanizer. This column separates ethylene from the lighter components which are methane and hydrogen. The methane is then used as fuel gas.
The fourth column is the C2-splitter that separates ethylene from the ethane. The ethylene stream is sent to ethylene storage and the bottoms stream which is ethane is sent back to the cracking furnace.
1.3.
Spent Caustic Management Background:
Waste management is one of the major environmental concerns in the world that involve solids, liquids and gases [18]. Large quantities of waste cannot be eliminated but, their environmental impact can be reduced by making more sustainable use of the waste [18]. The waste hierarchy shown in Figure 7 shows that disposal is the least favorable option [20, 21]. Regarding spent caustic which is a liquid waste, disposal is not an option according to Environmental Protection Agency (EPA) regulations.
Figure 7:Waste management hierarchy
The management of spent caustic should include:
1.3.1. Reduction:
Source reduction is the best practice when designing a spent caustic treating process. It is done by generating the least amount of spent caustic while
Reduce
Reuse
Recycle
Treatment & Disposal
Most favorable
maintaining the desired efficiency of the process. This can be done by using maximum caustic strength or applying multistage caustic wash [22]. But in case of reduction spent caustic still will be produced.
1.3.2. Reuse within the Process:
Reuse of spent caustic will result a decrease in the total amounts that it need to be disposed. According to Ahmad.W (2012), the reuse of spent caustics is suggested to be used for crude oil neutralization and in the biological wastewater treatment process to control the pH value [23]. The issue with the reusing spent caustic is that the concentration of sodium in the spent caustic is not steady and an appropriate amount is not easy to control [22]. Moreover phenols and napthenates are more suitable for reuse at controlled concentrations but sulfidic causes odor issues in this application [22].
1.3.3. Recycle Outside the Facility:
It has been implemented in many refineries in the US and Canada after applying additional treatment [24]. The valuable compounds from caustic such as sulfide , phenols and naphthenic acids can be removed and reused as a raw material in
many industries such as pulp and paper, tannery, mining, wood preservatives and paint industries [22]. However, this approach may need appropriate cost analysis before proceeding.
1.3.4. Treatment and Disposal:
Deep well injection is a previous practice used to dispose spent caustic but it is prohibited nowadays [4]. Recently many treatment methods can be applied and it
proofs to be effective. The treatments processes must guarantee the destruction of the contaminants in order to reach the discharge limit [22]. The removal of contaminants in liquid solution requires one or more of the available treatment methods. It is essential to decide on the most appropriate method according to the characteristics of the liquid solution, cost of the process and volume of the stream that need be treated [22].
1.4.
Spent Caustic Treatment:
The treatment processes of spent caustic must guarantee the elimination of the pollutants in order to reach the authorized limit for discharge. The elimination of pollutants in aqueous solution may need one or variou s basic treatment techniques depending on the type of compounds and concentration in solution [25]. It is necessary to choose the most adequate method according to the characteristic of the effluent.
Numerous efforts have been made to develop and to enhance the treatment of spent caustic. Treatment methods for spent caustic can be classified as biological, chemical and thermal processes [22]. According to Andreozzia,R. Caprioa, V. Insolab, and A. Marottac,R. (1999) the treatment process could be selected depending on COD concentration [26]. Figure 8 shows the relation between COD value and the appropriate treatment method [26].
Advanced oxidation processes (AOPs) is selected for COD less than 20 g/l while wet air oxidation (WAO) is implemented for COD values between 20 and 200 g/l higher than this value incineration is considered to be the best method [26].
Figure 8 :Treatment technologies according to COD contents [26].
1.4.1. Biological Treatment of Spent Caustic :
As mentioned before spent caustic stream needs excessive treatment before discharge. Biological treatment is preferred due to the low cost and the low environmental impact [27]. The treatment should be done by two steps: pretreatment followed by biological treatment. The biological treatment can be an inexpensive disposal option; however there are several drawbacks if it is applied directly to without pretreatment. These drawbacks are:
1- Noxious odors: Sulfides and mercaptans are highly odors even at the ppb level. These compounds are considered very toxic and hazardous [28].
2- It is not readily biodegradable: Often spent caustic contains a mixture of compounds that would limit the biodegradation in the biological treatment processes [29, 5].
3- Foaming: Spent caustic has compounds that have foaming characteristics when aerated or agitated during the treatment [29, 5].
0 20 200 300
AOPs
WAO
Incineration
4- High chemical oxygen demand (COD): This can cause high load to the biological process [29, 5].
5- PH swings: spent caustic is highly alkaline solution with a pH value that can reach up to 14 [28].
1.4.2. Thermal Treatment:
1) Wet Air Oxidation (WAO)
Another conventional method is the wet air oxidation (WAO), which is a high pressure treatment at elevated temperature. Here the oxidation agent is the oxygen present in the air, which is introduced into the spent caustic as steam. This reaction can accomplish either mineralization of organics into CO2 and H2O or
destroy complex molecules into simpler molecules that is easier to degrade [30, 31]. The process is very expensive, and due to severe reaction conditions, safety is a main concern. Although several tests with low pressure has been conducted without remarkable success [32]. WAO can be classified into three types based on the temperature implemented to achieve the oxidation. Table 3 shows the three types of WAO that can be applied to the different kinds of spent caustic.
Using appropriate catalysts for WAO process minimize the severity of reaction conditions and simply destroy refractory pollutants resulting in reducing capital and operational cost [33].
The operating cost of catalytic wet air oxidation (CWAO) is around half the non-catalytic WAO. However, an additional step is required to remove the metal ions from the treated effluent that would result in increasing operational costs [34]. Catalysts allow overcoming the drawbacks of the WAO; however the discovery of
low cost and stable catalysts remains the major weaknesses of CWAO for wide applications [35].
Table 3:WAO operational conditions [5]
Type of WAO Temperature (°C)
Pressure (psig)
Kind of spent caustic
Low
temperature
110-120 25 to 100 sulfides in spent caustic
Mid
temperature
200-220 300 to 600 complete treatment of sulfides and mercaptans , cresylic acids and naphthenic acids
High
temperature
240-260 700 to 1100 complete treatment of sulfides and mercaptans, cresylic acids and naphthenic acids
2) Incineration:
Incineration is a process used to convert solid, liquid or gas at elevated concentration of pollutants into more stable states at higher temperatures [36]. The economic aspect presents the disadvantage of this process because it requires high energy cost. In addition toxic emissions that results from this process are high [22].
1.4.3. Chemical Treatment:
1) Neutralization Followed by Air Stripping:
Aeration depends on two fundamental principles: equilibrium conditions and mass transfer considerations [36]. Equilibrium conditions will identify the limits of the gas transfer process. Aeration is an efficient method for H2S gas removal. The
function of aeration is not particularly to oxygenate the water, but it is to strip the dissolved gas (H2S) out of the water by changing the equilibrium conditions of the
water and thus drive the dissolved gas out [36].
Neutralization converts the spent caustic components into their original elements, such as hydrogen sulfide (H2S), mercaptan sulfur (RSH), phenol and naphthenic
acid. But it requires stripping and additional managing of volatile gases [22]. This technology is a widely understood method and the simplest and cheapest for the removal of volatile compounds. However, the effluent stream has elevated COD concentrations because a major part of the organic component is unaffected by the stripping process [32].
2) Chemical Oxidation
Chemical oxidation is a method that involves the transfer of one or more electrons from an electron donor (reductant) to an electron acceptor (oxidant), which has a higher affinity for electrons. The result of electron transfer is a chemical change of the oxidant and the reductant [37]. Oxidation technologies are established to decompose refractory molecules into simpler molecules that can be further treated by other methods. The mechanism of oxidation works by
oxidized and the oxidant that accepts the electron is reduced [38]. In natural waters, chemical oxidation processes also take place due to the presence of microorganisms that work as natural oxidants [37]. Oxidation reactions generate chemical species with an odd number of valence electrons known as radicals. These tend to be highly unstable therefore, highly reactive because one of their electrons is unpaired. The reactions that produce radicals tend to be followed by chain reactions between the radical, oxidants and other reactants until stable oxidation products are formed. The ability of an oxidant to initiate chemical reactions is measured in terms of oxidative power of an oxidant [39]. The oxidation potentials are presented in terms of the potential. The electro- potential based on half-cell reactions [38]. Figure 9 shows the potentials of the
most commonly used oxidizers [40, 41].
Figure 9 : Oxidation potential [1, 2] Hydroxyl Radical Sulfate Radical persulfate Ozone Hydrogen Peroxide Permanganate Chlorine Dioxide Chlorine Oxygen Bromine 0 0.5 1 1.5 2 2.5 3 C h e m i c a l o x i d a n t
1.5.
Classification of the Chemical Oxidation Processes
Chemical oxidation can be classified into two categories that are described below in details:
1. Classical chemical oxidation
2. Advanced Oxidation Processes (AOPs)
1.5.1. Classical Chemical Oxidation:
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
form, as Cl2; or as
aqueous solution, sodium hypochlorite NaOCl; or as a solid, calcium hypochlorite, Ca(OCl)2. o Generate halogenated DBPs o Possibly contribute to
odor and taste issues
o Require high dosage of
chlorine o Create a carcinogenic organochloride by products [42, 43]
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
sulfate (SO4-2)
By- products.
[44, 46-48]
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 reaction of the molecules other than the desired contaminants. This lead to increase the need for
oxidant requirements.
o Low oxidation potential. o Not effective for a wide
range of contaminants.
[42-44, 48]
Ozone It is a very powerful
oxidizing agent
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 [42, 44, 45, 47, 48]
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.
Available and relatively
cheap.
o The oxidation process
doesn’t produce by product
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
manner.
o large residence time
should be provided for H2O2 in the waste stream
because of the low solubility
[43-45, 48]
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
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 generated. Destruction of organic compounds occurs by hydroxyl radical
reactions coupled with direct photolysis and oxidation by
molecular ozone.
1. Supplemented disinfectant
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, [57, 59,60] 59,60] Perozone (O Perozone (O33 + H + H22OO22 Once O
Once O33 and H and H22OO22 are at are at 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 Fenton Oxidation Oxidation H H22OO22 and and ferrous iron ferrous iron (Fe(II)) (Fe(II)) Fenton’s reagent is Fenton’s reagent is powerful oxidant for powerful oxidant for
organic contaminants .It is organic contaminants .It is 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. controlled. [56, [56, 57,59] 57,59]
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
Ultrasound systems systems
Ultrasound waves are Ultrasound waves are introduced to the introduced to the
wastewater as compression wastewater as compression and expansion cycles. and expansion cycles. Micro-bubbles are Micro-bubbles are produced .These produced .These
compression cycles will compression cycles will collapse the micro-bubbles collapse the micro-bubbles create extremely high create extremely high temperature and pressure. temperature and pressure. These conditions are These conditions are capable of breaking water capable of breaking water molecular producing molecular producing hydroxyl radicals. hydroxyl radicals.
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 cost of operations [61, [61, 62] 62]
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
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 according to standard method (ASTM D5907) [63]. To measure the total solids a weighted beaker was filled with 100 ml of spent caustic sample. The sample was placed inside an oven (Heraeus) at 100℃ and left until the sample was
completely dry. The weight of dried sample was measured, subtracting the empty weight of the beaker it was found that the TS were equal to (85400 mg/l).
To measure the TDS the sample was filtered using a 0.45 m whatman filter paper. The filter was then dried at 100 ℃ until completely dry. The weight of the dried sample was measured it was found that TDS were equal to (77230 mg/l). TSS was measured by subtracting TDS from TS. It was found that TSS was equal to (8170 mg/l). All samples were done in triplicates and the mean values were reported. It was observed that most of the solids in the spent caustic sample were dissolved solids were they made 90 % of the TS.
2.1.2. Chemical Oxygen Demand (COD):
Chemical oxygen demand was measured by HACH Program [64]. This method determines COD depending on the quantity of consumed oxygen by certain impurities in water based on the reduction of dichromate solution. Three main solutions were prepared for this test. The stock COD standard solution was prepared by dissolving 0.425 g potassium hydrogen phosphate in 400 ml distilled
water which was then diluted to 500 ml to get 1000 mg/l COD. Standard solutions with known concentrations of 100, 300, 500, 700, 900, 1000 mg/l COD, were then prepared. The second solution is the digestion solution which was prepared by adding 10.2 g potassium dichromate (K 2Cr 2O7) standard grade, 167
ml H2SO4 and 33.3 g powdered mercuric sulfate (HgSO4) to 500 ml of distilled
water, dilute and complete to 1 liter. The third solution is the catalyst solution that was prepared by adding 5.5 g of powdered silver sulphate (AgSO4) to 1 kg of
concentrated sulfuric acid (H2SO4). After that, 2.5 ml of samples, standards and
blank were added to the cultured tubes. Next 1.5 ml of the digestion solution and 3.5 ml of the catalyst solution were added to the cultured tubes. The tubes were caped tightly and shacked to mix the layers. Then tubes were placed in (Heraeus) oven at 150 ℃ for 2 hours. After that, the samples were allowed to cool for about 20 minutes until reaching room temperature. Blank and standard tubes were placed in the (Varian) UV- spectrophotometer to calibrate the device before
2.1.3. Biological Oxygen Demand (BOD):
Biological oxygen demand was measured by standard method (SM 5210 B) [65]. The BOD test measures the dissolved oxygen consumed by microbial life while assimilating and oxidizing the organic matter present. A sample has brought to the desired temperature 20°C and mixed very well to homogenize the sample. 94 ml of sample has poured into sample bottle. To inhibit nitrification three drops of Ally Thiourea or ATH were added because BOD measurement shouldn’t i nclude the oxygen consumption by nitrifying bacteria. Clean magnetic stirring rod was added in the bottle. 3-4 drops of 45% potassium hydroxide solution was added to seal gasket. This will absorb the CO2. After that, the seal gasket was inserted in
the neck of the bottle and the device was switched. The Oxidirect has an optional auto start function that enables it to start. BOD bottle was placed in position into the bottle rack .The incubation period for the sample is 5 days at 20°C. After five days the BOD value was found around 431 mg/l.
2.1.4. Total Sulfides (−, H2S, H− ) (sulfide above 1 /)
Total sulfides were measured by titration method [66]. The titration method is applicable to the measurement of total and dissolved sulfides for water in concentrations above 1 mg/l [66]. Sulfide is reacted with an excess of iodine in acid solution, and the remaining iodine is then determined by titration with sodium thiosulfate.
Approximately 0.5 ml of a sample was taken into 100 ml conical flask and diluted with dematerialized water until 100 ml. Then 10 ml of 0.010N iodine was added
and mixed .Without delay 10 ml of concentrated hydrochloric acid HCl was added and the sample was shacked vigorously. Immediately sample was titrated with 0.010N 223, till faint yellow appear .Then starch was added as an indicator and titration was continued until the blue color disappeared and became like light-straw color. A blank of approximately 100 ml dematerialized water was prepared and carry it through the same procedure as the sample. Then calculate
total sulfide using equation 2.4.
−2 (mg/l) = (1000/ml of sample)× 0.1603 × (ml blank – ml sample) (2.1)
2.1.5. Determination of Total Sulfide (−, H2S, H−) (sulfide 0 to 800 /):
The Methylene blue method by HACH Program is used to measure total sulfide (0 to 800 /l) [64]. 25 ml of sample was measured and filled in the sample cell. Also, a blank was prepared by measure 25 ml of dematerialized water into second sample cell. Then 1 ml of sulfide reagent (1) was added to each cell and swirl to mix. After that, 1 ml of sulfide reagent (2) was added to each cell. Immediately swirl to mix. The blank was placed and zero key was pressed. Then the prepared sample was placed in the cell holder. The light shield was closed then result is displayed.
2.1.6. Free Soda and Complete Alkalinity:
To determine the quantity of available free soda and complete alkalinity in solution by manual titration was performed. This method is applicable for very low free soda to 0.1% and complete alkalinity as low as 0.1 % [ 64].
A flask was cleaned by distilled water and dried with clean paper. The weight of empty flask was measured using Aeadam sensitive balance. With clean pipette 10 ml of sample was taken and weighted. Then 100 ml distilled water and three drops of phenolphthalein were added. After that, titrate it against 1N sulfuric acid till the pink color of solution changes to colorless. Then three drops of methyl orange was added to the colorless solution in the flask. Titration was continued against 1N sulfuric acid till yellowish colored solution changes to orange color.
Calculate the Free soda equation (2.2):
Free soda in % wt = 4 ((6.8 )−(9 ))
(10.19) (2.2)
Complete alkalinity % wt = 4 ((6.8)+(9))
(10.19) (2.3)
Where:
P: is the weight of sample
V1: is the volume of H2SO4 used in titration after addition of phenolphthalein
V2:volume of H2SO4 used in titration after addition of methyl orange
Calculate the total alkalinity by equation 2.3
2.1.7. Total Petroleum Hydrocarbons (TPH)
Total petroleum hydrocarbon (TPH) is a measure of concentrations mineral oil, hydrocarbon oil, extractable hydrocarbons and oil and grease. TPH was measured by standard method (ASTM D7678) [67]. This method covers the range between 0.5 to 1000 mg/l. 500 ml of the sample was taken in flask and sulfuric acid drops
