Refinery wastewater treatment: Case study Khartoum Refinery

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ن الرحيمه الرحمل بسم الUniversity OF NILAIN Faculty of Engineering Department of Chemical Engineering Refinery wastewater treatment: Case study Khartoum Refinery A Thesis submitted for partial fulfillment of the reguirement for the degree of master of science in Chemical Engineering By: ABD ELWAHID IBRAHIM ELGADI MOH B.Sc. Industrial chemistry April 2005 Supervisor: Dr. HASSAN MOHE ELDEEIN

Transcript of Refinery wastewater treatment: Case study Khartoum Refinery

Page 1: Refinery wastewater treatment: Case study Khartoum Refinery

بسم الله الرحمن الرحيم

University OF NILAIN

Faculty of Engineering

Department of Chemical Engineering

Refinery wastewater treatment:

Case study Khartoum Refinery

A Thesis submitted for partial fulfillment of the

reguirement for the degree of master of science in

Chemical Engineering

By:

ABD ELWAHID IBRAHIM ELGADI MOH

B.Sc. Industrial chemistry April 2005

Supervisor:

Dr. HASSAN MOHE ELDEEIN

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Dedication

To my mother

to my father

to my brothers and sisters

and to my friend

who encouraged me to accomplish this research

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Acknowledgments

I acknowledge with gratitude the immense support given to me, all through this

research project , by Khartoum Refinery and its staff; I am particularly

indebted to engineer Mohammed Abdu who had most courteously coached me

within the refinery explaining its technical equipments, its functions and its

overall performance; he was so kind as to allow me browse through the waste

water treatment unit and get acquaintedwith its technical mechanics and its

particular role within the refinery.But foremost I highly indebted to my

supervisor

Dr.Hassan Mohe Eldeen Mhom

without his personal counselling and keen guidance, this research project

would not have been accomplished .

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List of Contents

1 Dedication I

2 Acknowledgment II

3 List of Contents III

4 List of Tables VI

5 List of Figures VII

6 Abstract VIII

IX مستخلص 7

8 List of Abbreviations X

Chapter One

Introduction

1.1 Background I

1.2 Water pollution hazards 2

1.3 Objectives 3

1.3.1 General Objectives

1.3.1 Specific objective

Chapter Two

Literature Review

2.1 Sour Water 4

2.2 2.2 Hydrogen sulfide

4

2.2.1 Hazard and risk of hydrogen sulphide 5

2.3 Ammonia 6

2.3.1 Uses of Ammonia 6

2.3.2 Hazard and risk of ammonia 7

2.4 Pollution 7

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2.5 Steam 8

2.6 Site visit 8

2.7 Sources of sour water 9

2.7.1 Sources of sour water from Crude Distillate Unit (CDU) 9

2.7.2 Sources of sour water from (RFCC) 9

2.7.3 Sources of sour water from (DHT) 9

2.8 Sour water unit sections 10

2.8.1 Degassing Section 10

2.8.2 De-oiling Section 10

2.8.3 Stripper section 10

2.8.4 Incineration Section 11

2.9 Stripping of Sulfide and Ammonia 12

2.10 Process Description 13

2.10.1 Degassing 13

2.10.2 Acid Gas Stripping 14

2.10.3 Ammonia Stripping 14

2.11 Spent Caustic Treatment 16

2.12 Activated Carbon 18

Chapter Three

Material and Methods

3.1 Khartoum Refinery 21

3.1.1 General 21

3.2.2 Sour water stripping unit 21

3.1.2.1 Outline of the Unit 21

3.1.2.1 Process Design 21

3.2 Methodology 23

3.2.1 The experiment lab work includes 23

3.2.2 Oil content 23

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3.3 Hydroxyl Benzene (phenol) content 24

3.4 ammonia nitrogen 25

3.5 Sulfide (Iodimetry Method) 26

Chapter Four

Results and Discussion

4.1 Results 28

4.2 Discussion of Results 28

4.2.1 Sour water stripping unit 28

Chapter Five

Conclusions and Recommendations

5.1 Conclusions 29

5.2 Recommendations 29

References

30

Appendix 32

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List of Table

Tables Title Pages

2.2.1 Hazard and risk of hydrogen sulphide 5

2.3.2 Hazard and risk of ammonia 7

2.13 refinery prevention and control techniques 19

3.1.2.2.1 Material balance of the material 22

4.1.3 analysis results of sour water sample 37

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List Of Figures

Figures Title Pages

2.10 Sour Water Process 15

2.4 Ammonia Purification & Recovery 16

2.14 Sour water flow diagram 20

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Abstract

Khartoum refinery was considered as a study case, liquid waste produced

was studied, the unit that used as well as to treat the waste, and the, evaluation of

the performance of these units.

Laboratory tests were conducted to identify the characteristics of wastes

in each stage.

The effluent drain to the environment was studied and compared with the

permissible limits .The analysis carried include: oil content, phenol content, power

of Hydrogen, Ammonia, and Sulphide.

The results obtained revealed the following:

The oil content can reach more than 20000mg/l and this is more than, the

design capacity of the treatment unit but it can be reduced to less than

10000mg/l after treatment.

The waste water treatment unit works with capacity more than the

designed, so the efficiency of this unit is not enough.

There is a lot of polluting effluent with concentration more than the

allowed (i.e. Amonia,pH)

The above indicates some design problem and the study recommended

to add additional units to solve the above mentioned problems and

obstacles.

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المستخلص

يهدف البحث لدراسة كيفية معالجه المخلفات السائله الناتجه من عملية تكرير البترول و

خواص هذه المخلفات السائله و تحديد مصادرها و الطرق المستخدمه لمعالجتها و أثر هذه المخلفات

على البيئه .

و قد أخذت مصفاة الخرطوم كمنطقة للدراسه و تمت دراسة خواص المخلفات السائله الناتجه

راء التحاليل المعمليه منها و كذلك الوحدات المستخدمه فيها , و تقييم الأداء لهذه الوحدات و تم إج

لمعرفة الخواص المخلفات في كل مرحله من مراحل الوحده و المخلفات الخارجه منها و التي يتم

تصريفها إلى البيئه المجاوره و مقارنة المخرجات مع الحدود المسموح بها , و قد غطت التحاليل عدة

روجيني , الأمونيا و الكبريتيد .مؤشرات كيميائيه تشمل محتوى الزيت , الفينول , الرقم الهيد

-و قد أوضحت نتائج الدراسه الآتي :

ملجم / لتر و ذلك أكبر من الطاقه التصميميه للوحده 02222محتوى الزيت يصل إلى أكثر من

ملجم / لتر و ذلك مما جعله مطابقاً للمواصفات القياسيه 02222و لكنه ينخفض إلى أقل من

المسموح بها .

وحدة معالجة المخلفات الملوثه تعمل بطاقه أكبر من الطاقه التصميميه لها , لذا نجد نجد أن

أن كفائة الوحده لا تعمل بالصوره المطلوبه .

محتوى بتركيز أكثر من المسموح به مثل هنالك العديد من الملوثات تخرج من وحدة المعالجه

9.680اصفه القياسيه بواقع مجم / لتر و هي أكثر من المو 09.680الأمونيا يساوي

ملجم/لتر وهو أكثر من المواصفه القياسيه 869ملجم/لتر( و كذلك )الرقم الهيدروجيني يساوي

ملجم/لتر( . 069بواقع

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List of Abbreviations

WWT Waste Water Treatment

VOC Volatile Organic compounds

PAC powder Activated Carbon

API American Petroleum Institute

CPI Corrugated Plate Interceptors

DAF Dissolved Air Floatation

RAS Return Activated Sludge

W.A.S Waste Activated Sludge

S.A.S Surplus Activated Sludge

TOC Total Organic Carbon

LP Low Pressure

COD Chemical Oxygen Demand

BOD Biological Oxygen Demand

IAF Induced Air Floatation

RBC Rotating Biological Contactors

AC Activated Carbon

GAC Granular Activated Carbon

P-C Physical-Chemical O&G Oil& Grease

OSHA Occupational Safety & Health Administration

EPCRA Emergency Planning and Community Right-to-Know

SDWA Safe Drinking Water Act

CWA Clean Water Act

DDT Dichlorodiphenyltrichloroethane

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Chapter One

Introduction

1.1 Background

General Sour Water is the wastewater that is produced from atmospheric and vacuum crude

columns at refineries. Hydrogen sulfide and ammonia are typical components in sour water that

need to be removed before the water can be reused elsewhere in the plant. Removal of these

components is done by sending the sour water from the process to a stripping tower where heat,

in the form of steam, is applied. The ammonia and hydrogen sulfide contained in the water is

released by the heat and exits the top of the tower. The ideal Ph value for stripping H2Sis below,

since above 5, sulfide is primarily found in the form of ions (H2S or S2). Alternatively, efficient

ammonia stripping requires a pH above 10 to prevent the formation of ammonium (NH4+) ion

that cannot be stripped. Although the most favorable strategy for sour water stripping is a three

step process where two separate stripper tower are used , One for removing hydrogen sulfide

and other and other for removing ammonia .Economics usually dictates a compromise. Having

only one stripper tower and using a pH around 8 allows adequate removal of both gases. There

are three distinct processing steps in the sour water stripping process: degasification, hydrogen

(acid-gas) stripping and ammonia stripping. During the degasification stage the sour water feed

from the plant is cooled and fed to a degasser where dissolved hydrogen, methane and other

light hydrocarbons are removed. These removed gases are known as sour gas and pumped into a

storage tank that serves to dampen the flow rate and facilitates removal of entrained oil and

solids. The next step in the process is known as hydrogen sulfide stripper, the degassed sour

water is fed to the acid gas or hydrogen sulfide stripper, which is steam-reboiled distillation

column. The hydrogen sulfide, which is stripped overhead, is of high purity, an excellent feed

for sulfur recovery units or sulfuric acid plants. Next, the hydrogen sulfide stripper stream,

containing all the ammonia in the feed water and some hydrogen sulfide, is fed directly to the

ammonia stripper, which is a refluxed distillation column. In this column, essentially all

ammonia and hydrogen sulfide are removed from the water. After exchanging heat with the

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hydrogen sulfide stripper feed, the stripped water is cooled and sent off for either reuse or

treatment.

1.2 Water pollution hazards

Refineries are also potential major contributors to ground water and surface water

contamination. Some refineries use deep-injection wells to dispose of wastewater generated

inside the plants, and some of these wastes end up in aquifers and groundwater. These wastes

are then regulated under the Safe Drinking Water Act (SDWA).

Wastewater in refineries may be highly contaminated given the number of sources it can come

into contact with during the refinery process (such as equipment leaks and spills and the

desalting of crude oil). This contaminated water may be process wastewaters from desalting,

water from cooling towers, storm water, distillation, or cracking. It may contain oil residuals

and many other hazardous wastes. This water is recycled through many stages during the

refining process and goes through several treatment processes, including a wastewater treatment

plant, before being released into surface waters.

a- The wastes discharged into surface waters are subject to state discharge regulations and

are regulated under the Clean Water Act (CWA). These discharge guidelines limit the

amounts of sulfides, ammonia, suspended solids and other compounds that may be

present in the wastewater. Although these guidelines The main task of the unit is to

remove the hydrogen sulphide, ammonia and other gaseous contaminants dissolved in

water before discharged to sewage.

b- Characterize sour water generated at the Khartoum refinery.

c- Evaluate of the current treatment process performance.

are in place, sometimes significant contamination from past discharges may remain in surface

water bodies.

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1.3 Objectives

1.3.1 General Objectives

This research aims to study the treatment and the utilization of the sour water in Khartoum

Refinery Company, This general aim will be achieved by the following specific objectives.

1.3.2 Specific objective

• Characterize sour water generated at the Khartoum refinery.

• Evaluate of the current treatment process performance.

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Chapter Two

Literature Review

2.1 Sour Water

Sour water is any water from a refinery that contains hydrogen sulfide and ammonia. In

addition, sour water may contain ammonia, phenol and cyanide. Historically, sour water from

refinery process units is treated to remove hydrogen sulfide prior to hospital. Today, the

removal of ammonia from sour water is more important due to recent regulation reducing

nitrogen into estuaries. Selenium is also important as regulators seek to reduce selenium

because of the mutagenic effects found in wildlife with high selenium concentrations. While

ammonia, selenium, phenol, salts and other constituents are removed while during hydrogen

sulfide treatment, removal efficiencies of these chemicals are lower due to the physics and

chemistry of hydrogen sulfide treatments systems.

Water that contains sulfide is called (sour water). And this sour water is a secondary product

from the crude oil refinery and the operation unit of separation process. It consists mainly of

ammonia and hydrogen sulphide (NH3, S2H) and small percentage of gases.

Sour water is a pollutant, because it consists of the above mentioned component. These

components are of bad odour and may cause respiratory diseases if inhaled in large quantities.

As well as its disposal in the soil may cause contamination, this may penetrate to the surface

water, or being carried by the rain.

The sour water as the feed firstly goes through the flashing section to remove light hydrocarbon

gases.

2.2 Hydrogen sulfide

Hydrogen sulfide is the chemical compound with the formulaH2S. It is a colorless gas with the

characteristic foul odor of rotten eggs, it is heavier than air, very poisonous, corrosive,

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flammable and explosive. Hydrogen sulfide often results from the bacterial breakdown of

organic matter in the absence of oxygen, such as in swamps and sewers. This process is

commonly known as anaerobic digestion. H2S also occurs in volcanic gases, natural gas, and

some well waters. The human body produces small amounts of H2S and uses it as a signaling

molecule.

2.2.1 Hazard and risk of hydrogen sulphide

Table (2.1) Characteristics of H2S

Concentration (ppm) Heath Response

.Irritation of the eyes, nose and throat 10ــــــــــــ 0

50ـــــــــــ 10

Headache

Dizziness

Nausea

Coughing and breathing difficulty.

200ــــــــــــ 50

Severe respiratory tract irritation

Eye irritation / acute conjunctivitis shock

Convulsions

Coma

Death in severe case.

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2.3 Ammonia

Ammonia is a compound of nitrogen and hydrogen with formula NH3. It is a colorless gas with

characteristic pungent odour .It dissolve in water by different ratios depend on temperature and

pressure in standard condition (P =1 atom & T = 250C) it solve in water by (18 -20) %.

Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as

a precursor to food and fertilizers. Ammonia, either directly or indirectly, is also a building-

block for the synthesis of many pharmaceuticals and is used in many commercial cleaning

products. Although in wide use, ammonia is both caustic and hazardous. In 2006, worldwide

production was estimated at 146.5 million tones. It is used in commercial cleaning products.

Ammonia, as used commercially, is often called anhydrous ammonia. This term emphasizes the

absence of water in the material. Because NH3 boils at -33.34 °C, (-28.012 °F) the liquid must

be stored under high pressure or at low temperature. Its heat of vaporization is, however,

sufficiently high so that NH3 can be readily handled in ordinary beakers, in a fume hood (i.e., if

it is already a liquid it will not boil readily). "Household ammonia" or "ammonium hydroxide"

is a solution of NH3 in water. The strength of such solutions is measured in units of Baume

(density), with 26 degrees Baume (about 30 weight percent ammonia at 15.5 °C) being the

typical high concentration commercial product. Household ammonia ranges in concentration

from 5 to 10 weight percent ammonia.

2.3.1 Uses of Ammonia

1. In the manufacture of rayon and urea

2. In the manufacture of fertilizers such as urea ammonium phosphate, ammonium nitrate,

ammonium sulphate etc.

3. In ice plants, as a refrigerant

4. In furniture industry, as a cleansing agent for furniture and glass surfaces.

5. In the manufacture of nitric acid.

6. In the manufacture of sodium carbonate.

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2.3.2 Hazard and risk of ammonia:-

Ammonia has a sharp, distinct, penetrating odour detectable at very low concentrations. At

moderate levels of concentration, ammonia can irritate the eyes and respiratory tract; at high

concentrations, it can cause ulceration to the eyes and severe irritation to the respiratory tract.

Table (2.2) of Ammonia

Concentration (ppm) Heath Response

.Nose and throat irritation after ten minutes of exposure 50ــــــــــــ 24

.Irritation of nose and throat after five minutes exposure 134ـــــــــــ 72

700 Immediate and severe irritation of respiratory system.

5000 Respiratory spasm, rapid suffocation.

Above 10.000 Pulmonary edema, potentially fatal accumulation of fluid in

lungs and death.

2.4 Pollution

Water pollution occurs when a body of water is adversely affected due to the addition of large

amounts of materials to the water. When it is unfit for its intended use, water is considered

polluted. Two types of water pollutants exist, Point sources and non point source. Point source

of pollution occurs when harmful substances are emitted directly into a body of water. A

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nonpoint source delivers pollutants indirectly through environmental changes. An example of

this type of water pollution is when fertilizer from a field is carried into a stream by rain, in the

form of run-off which in turn affects aquatic life. The technology exists for point sources of

pollution to be monitored and regulated, although political factors may complicate matters.

Nonpoint sources are much more difficult to control. Pollution arising from nonpoint sources

accounts for a majority of the contaminants in streams and lakes.

2.5 Steam

Providing steam for process and space heating applications is one of the main energy costs sites

in factories. Steam costs are also major contributor to energy bills in building complexes in

other sectors. Steam is used for heating and process work as it is an ideal carrier of heat. Its

three main advantages as a heat transfer medium are as follows:

It transfers heat at a constant temperature - extremely useful when dealing with heat sensitive

materials, the temperature of steam is dependent upon the steam pressure – resulting in a simple

method of temperature control, it is compact in terms of heat content per unit volume – this

means that heat can be conveyed in simple piping systems. The boiler is tended to be ignored as

long as it produces hot water and steam reliably and safely. Additionally, steam is often used

carelessly resulting in systems becoming poorly maintained and thus inefficient. Even in the

best regulated establishments there is bound to be some unavoidable wastage of heat should be

properly utilized. a 10% energy saving could be achieved by improvements in the design and

operation of boilers and their distribution system .

2.6 Site visit

This study has been carried in Khartoum Refinery Company, the refinery is located about

seventy-five kilometers north of Khartoum; at the plant and either reused or discharged directly

into receiving waters.

Agricultural, including commercial livestock and poultry farming, is the source of many organic

or inorganic pollutants in surface waters and ground water, these contaminants include both

sediment from erosion cropland and compound of phosphorus and nitrogen that partly originate

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in animal wastes and commercial fertilizers. Animal wastes are high in oxygen demanding

material, nitrogen and phosphorus, and they often harbor pathogenic organisms.

Waste from commercial feeders are contained and disposed of on land; their main threat to

natural waters, therefore, is from runoff and leaching. Control may involve setting basins for

liquid, limited biological treatment in aerobic anaerobic lagoons, and Varity of other methods'"

2.7 Sources of sour water

Atmospheric Crude, catalytic cracking & reforming, distillation column produce sour water as

condensates from steam used in injection and stripping removed by overhead condensing

system. Another major source of sour water is hydro-treated wash water, there are:-

(i) Water from accumulator of main fractionators overhead.

(ii) Sour water of residual fluidized catalytic cracking (RFCC).

(iii) Water used as injection in diesel hydrotreated unit (DHT) to dissolve corrosive salt.

2.7.1 Sources of sour water from Crude Distillate Unit (CDU)

(i) Water used to wash the crude oil.

(ii) Chemical injected.

2.7.2 Sources of sour water from (RFCC)

(i) In the top of fractionators where stripping steam is used in the bottom.

(ii) In reactor where atomized steam is used.

2.7.3 Sources of sour water from (DHT)

This is a hydrotreating unit where the product is purified by using hydrogen. Among the

impurities removed are sulpher, nitrogen, oxygen and chloride. From such process, sour water

can be combined .

2.8 Sour water unit sections

The unit consists of four Sections as follow:

1. Degassing Section:

2. De-oiling Section.

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3. Stripping Section.

4. Incineration Section.

Beside these main sections the unit has the following station

1. Condensate Recovery unit.

2. Waste gas sweetening.

3. Caustic Soda Injection.

2 .8.1 Degassing Section

The sour water as the feed firstly goes through the flashing section to remove light hydrocarbon

gases.

2.8.2 De-oiling Section

a. (Tank-in-tank) regulating tank type is a separator tank which integrates hydraulic

cyclone.

b. De-oiler cyclones are driven by inlet water pressure and utilise a pressure drop across

the Cyclone to provide the energy or driving force to cause oil-water separation.

c. This equipment promotes three-phase separation for oily waste water

2.8.3 Stripper section

The solubility of NH3 and H2S in water will rise with the decrease of temperature, Ionization

reaction may take place for partial H2S and NH3 dissolved in water, i.e.,

NH3 → NH4++ OH-..................................................... (2-1)

H2S → HS-+H+ ....................................................... (2-2)

Under ambient temperature, H2S and NH3 dissolve in water, and is ionized to ion existing in

water, after temperature is increased, the above formula shifts toward right side.

Sour water stripping is based on this theory to heat sour water up to the specified temperature,

this destroys the equilibrium of H2S and NH3 in water, promotes them change from liquid phase

to gas phase, thus realize the target of purifying waste water.

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The stripper is equipped with high performance float valve trays which are characterized

with low pressure drop, high efficiency and large operation flexibility; it is suitable for

long term operation.

Stripping energy is supplied through bottom re-boiler.

Caustic Soda injected in the column to control the pH for more efficient separation.

2.8.4 Incineration Section

Sour gas incinerator uses hot incineration to get sour gas burnt at 1350~1400℃ tail gas

discharged to the atmosphere by stack:

H2S +3/2O2→ SO2 + H2O……………………………………..… (2-3)

2NH3+3/2O2→3H2O +N2 …………………………………….…. (2-4)

Also sour gas from RFCC and DCU will be incinerated in this unit.

The system contains toxic and hazardous medium, such as ammonia, SO2 and H2S,

thus inspection leak and personal protection should be done well.

Strictly control the temperature of sour gas system, to avoid the pipelines to be

blocked with ammonia salt crystallization, to make sure long-term running of the

unit.

Strictly control the temperature of sour gas incinerator chamber, to make sure the

ammonia in sour gas completely burned and decomposed, meanwhile convert H2S

into SO2.

The sour gases completely burnt and converted to N2 and SO2.

Feed water tanks are sealed with water from the top, to avoid light hydrocarbon gas

leakage, thus to protect the environment from polluting.

High efficient absorbent is used to deal with malodors gases from the feed tanks.

Closed drain system is applicable to reduce the pollution.

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Recovery of condensate is applicable, then much part of the energy consumption for

boiler feed water treatment is saved.

Air cooler is used wherever is possible within the unit to reduce conserve circulating

cooling water.

Mature and suitable technology shall be selected following the principle of advanced,

safe and reliable, easy to operate and environment friendship to meet the goal of saving

investment, land occupation and lowering energy consumption.

Source of feedstock: the feedstock is sour water from existing RFCC, DCU, GDHT and

DHT.

Unit Sections the unit consists of four sections: Degassing section, Deoiling section and

Stripping Section, sour gas burnt section.

The project investment is reduced by relying on the existing utility system as much as

possible.

2.9 Stripping of Sulfide and Ammonia

Interestingly, the physical conditions for the efficient removal of both hydrogen sulfide

and ammonia from the sour water are not possible in a traditional hydrogen sulfide treatment

system. The ideal pH for stripping hydrogen sulfide is below 5.5 and at a pH above 5.5 sulfides

is primarily found in the form of ions and cannot be as effectively removed. Conversely,

efficient ammonia stripping requires a pH above 10 to prevent the formation of ammonium ion

that cannot be stripped.

Most refineries target a pH of about 8 in the steam stripper which allows removal of

both ammonia and hydrogen sulfide but at lower efficiencies. Selenium is not removed during

the stripping process and it finds its way into the stripped water. After stripping the stripped

sour water usually contains 50-100 ppm of ammonia plus the selenium. Some of the ammonia

found in the stripped water is resistant to removal. It has been hypothesized that acidic materials

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such as thiosulfuric acid, thiocyanic acid and weak organic acids may cause ammonia fixation.

This issue may go away if the pH of the stripped water increases to above10.

There have been proponents of using dual strippers to increase the removal of ammonia

from sour water. The first stripper removes the hydrogen sulfides under acidic conditions and

the second stripper would remove the ammonia under caustic conditions. While hydrogen

sulfide and the removal of ammonia from sour water would increase, and ammonia fixation

avoided, the selenium would remain in the stripped sour water.

The process of these units is two columns low-pressure stripping technology. This

process use two –columns to treat sour water in low-pressure, degassed and deoiled sour water

is fed into the top section of stripper. Hydrogen sulfide and ammonia contained in water are

stripped by steam introduced at the stripper bottom. Top of the stripper is controlled by

overhead reflux rate. Acid gas from overhead knockout drum is sent to Ammonia Unit to

recovery Ammonia solution (acid gas will be burn directly in acid gas incinerator before the

Ammonia solution is put on stream). Part of purified water is sent back to upstream units for

reuse.

2.10 Process Description

There are four processing steps in sour water treatment:

1. Degassing.

2. Acid gas stripping.

3. Ammonia stripping.

4. Ammonia purification and recovery.

2.10.1 Degassing.

The water feed to the plant -from single or multiple sources is combined with recycle

stream from the ammonia stripper and is cooled and feed to a degasser where dissolved

hydrogen, methane, and other light hydrocarbons are removed.

The release of acid gas and possible air pollution are minimized. The degassed sour

water is pumped to a storage tank that serves to dampen flow rate and composition changes.

The tank also facilities removed of entrained and solids.

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2.10.2 Acid Gas Stripping

From the feed tank, the degassed sour water is pumped to the plant, where it is heated by

feed/bottoms exchange and fed to the acid gas or hydrogen sulfide striper. This stripper is a steam

re-boiled distillation column. The Hydrogen Sulfide -which is stripped overhead- is of high purity

an excellent feed for sulfur recovery units or sulfuric acid plants. It contains negligible ammonia,

and because the plants feed has been degassed- only traces of hydrocarbons. It does contain how

ever- any carbon dioxide that is present in the feed.

2.10.3 Ammonia Stripping

The Hydrogen Sulfide stripper bottoms stream -containing all the ammonia in the feed

and some hydrogen sulfide- is fed directly to the ammonia stripper, which is refluxed

distillation column. In this column, essentially all ammonia and H2S removed from the water,

which leaves as the column bottoms stream.

After exchanging heat with the hydrogen sulfide stripper feed, this stripped water is

cooled and sent off-plant for re-use or treating. The ammonia and H2S stripped from the water

in the ammonia stripper are passed through an overhead condenser and partially condensed.

Figure (2.3): Sour Water Process

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The vapour product from the overhead condenser in the ammonia stripping section is an

ammonia-rich gas which may be handled in a variety of ways:

• Ammonia Incineration:

For some plants, actual ammonia recovery may be neither desired nor economical. In

such cases, the ammonia product may be incinerated, either directly off the reflux drum or after

being scrubbed with water to reduce the hydrogen sulfide content.

• Anhydrous Ammonia

For production of anhydrous ammonia, the gas is first passed through a two-stage

Scrubbing system to remove hydrogen sulfide. It is then liquefied to produce the anhydrous

ammonia.

• Aqueous Ammonia

For production of aqueous ammonia, a one-or-two stage scrubbing system may be used to

remove hydrogen sulfide. The ammonia gas is then dissolved in water to yield the desired

product grade.

Selection of the appropriate ammonia recovery option will be totally dependent on the site

economics.

2.10.4 Ammonia purification and recovery

Figure (2.4): Ammonia Purification and Recovery

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2.11 Spent Caustic Treatment

Caustic solutions are widely used in refining. Typical uses are to neutralize and extract:

a. Acidic materials that may occur naturally in crude oil.

b. Acidic reaction products that may be produced by various chemical treating processes.

c. Acidic materials formed during thermal and catalytic cracking such as hydrogen sulfide,

phenolic, and organic acids.

Spent caustic solutions may therefore contain sulfides, mercaptidessulfates, sulfonates,

phenolates, naphthenates, and other similar organic and inorganic compounds.

At least four companies process these spent caustics to market the phenolic and the sodium

hyposulfide, however, the market is limited and most of the spent caustics are very dilute so the

cost of shipping the water makes this operation uneconomical. Some refiners neutralize, the

caustic with spent sulfuric from other refining processes, and charge it to the sour water

stripper,this removes the hydrogen sulfide. The bottoms from the sour water stripper go to the

desalter where the phenolics are extracted by the crude oil.

Spent caustics usually originate as batch dumps, and the batches may be combined and

equalized before being treated and/or discharged to the general refinery waste waters. Spent

caustic solutions can also be treated by neutralization with flue gas. In the treatment of spent

caustic solutions by flue gas, hydroxides are converted to carbonates.

Sulphides, mercaptides, phenolates, and other basic salts are converted by the flue gas stripping.

Phenols can be removed and used as a fuel or can be sold.

Hydrogen sulfide and mercaptans are usually stripped and burned in a heater. Some sulfur is

recovered from stripper gases, the treated solution will contain mixtures of carbonates, sulfates,

sulfites, thiosulfates and some phenolic compounds. reaction time of 16-24 hours is required for

the neutralization of caustic solution with flue gas.

The oxidation phase of spent caustic treatment is aimed at the sulfide content of these wastes

and achieves 85-99 percent sulfide removal. In this process, sulfides are oxidized primarily to

thiosulfates although in some variations there is partial oxidation of the sulfur compounds to

sulfate.

Oxidation processes are not applied to phenolic caustics, as phenols inhibit oxidation. It should

be noted that those processes which oxidize the sulfide only to thiosulfate, satisfy half of the

oxygen demand of the sulfur, as thiosulfate can be oxidized biologically to sulfate.

Neutralization of spent caustics is applied to both phenolic and sulfidic caustic streams: the

sulfidic caustics are also steam stripped, after neutralization, to remove the sulfides. When

phenolic spent caustics are neutralized, crude acid oils or "crude carbolates" are sprung and thus

removed from the waste water. The major part of the phenols will appear in the oil fraction, but

a significant part may remain in the waste water as phenolates.

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Fluid bed incineration is also now being used, this process was developed under an EPA

demonstration grant and at least two large units are under construction. Once the incinerator is

started up, the sludge should provide the necessary heating value to keep the system operating.

Oxidizing fuels may be required when the sludge is burnt, as ash remains in the bed of the

incinerator. A constant bed level is maintained, so the sand bed originally in the incinerator is

gradually replaced by the inert sludge ash .The gasses pass through a scrubber, so the fines and

particulate matter can be recovered. The ash and fines can be landfilled. This landfill is cleaner

than a sludge landfill, because there are no organic materials present to contaminate ground

water or run-off. In the past ocean dumping, deep well injection, evaporative lagoons, and

simple dilution have all been used. These methods will no longer be acceptable.

2.12 Activated Carbon

The activated carbon (AC) process utilizes granular activated carbon to adsorb

pollutants from waste water.

The adsorption is a function "of the molecular size and polarity of the adsorbed substance.

Activated carbon preferentially adsorbs large organic molecules that are non-polar.

An AC unit follows a solids removal process, usually a sand filter which prevents plugging of

the carbon pores. From the filter the water flows to a bank of carbon columns arranged in series

or parallel. As the water flows through the columns the pollutants are adsorbed by the carbon,

gradually filling the pores ,at intervals, portions of the carbon are removed to a "furnace where

the adsorbed substances are burnt off , the regenerated carbon is reused in the columns, with

some makeup added, because of handling and efficiency losses.

Activated carbon processes currently have only limited usage in fining industry; however, there

are new installations in the planning construction stages. the increasing use of activated carbon

has occurred because activated carbon can remove organic materials on an economically

competitive basis with biological treatment. Activated carbon regeneration furnaces have high

energy .

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2.13 Refinery Prevention and Control Techniques :

Table (2.4): Common refinery prevention and control techniques W

aste

wat

er t

reat

men

t

Oil & graese

-oil skimming(API separators)

gravity separation(settling tanks and coalescing

plate separators)

-dissolved/induced air floatation(DAF/IAF)

-Biological treatment

sulphide -chemical treatment

-clarification

Organic compounds

-Biological treatment

-activated carbon adsorption

-clarification

Metals and solids

-source reduction

-gravity separation settling tanks and coalescing

plate separators physical treatment (flocculation)

-DAF/IAF

-granular media filtration

Caustics and acids -clarification

-neutralization

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Flow chart of the Unit

Figure (1-1) Sour water flow diagram sheet

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Chapter Three

Material and Methods

3.1 Khartoum Refinery

3.1.1 General

Khartoum Refinery Company (KRC) limited is a modern refinery, it is a joint venture

between china and Sudan, investment by China National Petroleum Corporation (CNPC) and

Ministry of Energy and Mining of Sudan (MEM) on 50%basis, construction was completed in

January 2000, on May 16th 2000 all units realized once through commissioning, and consider

may 16th is production day, with capacity 50,000bbl/day.

The refinery is located approximately 70 kilometer north of Khartoum, 12km East from

the river Nile and 5km East from Khartoum atbra road with an area of 121 fadans, which lies on

a semi rocky desert land (Gari area),that surrounded by small village with small population

scattered along the area, the inhabitants mostly are farmers and sheep keepers ,some of them

have been hired as casual labour in the KRC

The main production units in the KRC are:-

- Crude Distillation Unit (CDU).

- Reforming Unit (RFF)

- Diesel Hydrotreating Unit (DHT)

- The Utilities such as: water system unit, power plant unit and storage tank& tank farm (OMS)

Oil Movement System

- Delay Coking Unit (DCU)

- Residual Fluid Catalytic Cracking (RFCC)

3.1.2 Sour water stripping unit

3.1.2.1 Outline of the Unit

The sulfur-containing sour water stripper of Khartoum Refinery is capable of treating 65

tons of sour water each year. The stripper consists of three parts: the deaeration, deposition and

deoiling of sulfur-containing sour water; single column stripping (ammonia extraction of side

line) of sulfur-containing sour water; and ammonia make-up.

3.1.2.2 Process Design

After the air of sulfur-containing sour water has been extracted in the raw water

deaeration tank of the unit, the sour water is to be removed of its oil in the raw water deoiling

tank and deposited for separation in the raw water tank and then it goes through two passages.

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One passage, for cold feeding, is directed to the top of the sulfur containing sour water

stripper after cooling. The other passage, for hot feeding, is directed to the middle part of the

sulfur-containing sour water stripper after many a time heat transfer.

After sulfur-containing sour water has been stripped by the steam at the bottom of the

stripper, highly-concentrated sulfureted hydrogen gas is to be separated from the top of the

stripping column and it will be sent to the torch for burning after liquid separation. A highly-

concentrated ammonia gas section is formed in the middle part of the column. Ammonia-rich

gas is extracted from the side line. After three-step condensation, it passes through the

desulfurizing tank. Ammonia gas of higher concentration is obtained after desulfurization. It

will be mixed with softening water to form ammonia water that is to be output from the unit.

3.1.2.2.1 Material balance of the material:

Table (3.1): Material balance of the unit

No. Material t/y

Input

1 Raw material sour water 650000

2 Softening water 9440

3 Stripping steam 88000

Total 747440

Output

1 Purified water 9440

2 Sulfureted hydrogen gas 800

3 Ammonia water 3520

4 Loss 1200

Total 747440

3.2 Methodology:-

The methodology followed in this research amounts to

• Field visits and observation.

• Discussion with refinery personal concerned with sour water treatment unit.

• Experiment lab work sour water analysis.

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3.2.1 The experiment lab work includes:

1- Oil Content.

2- Phenol Content.

3- Ammonia Nitrogen.

4- Sulfide.

3.2.2 Oil content :

Oil content can be tested by testing aromatic content. To extract oil of industrial water

by petroleum ether (60-90°C) as solvent, oil content is determined by measurement of the

absorbance at 225nm.

Oil standard sample

Under acid condition, use 30-60°C petroleum ether to leach out the oil substance in the

water and then dehydrate through anhydrous sodium sulfate. After filtering put it on the 80°C

water bath to evaporate the most of petroleum ether, then put it into a 70°C oven, evaporate all

petroleum ether, then the standard oil sample is got.

Instrument

- Ultraviolet spectrophotometer.

- Separating funnel = 500ml.

- Funnel diameter is 60mm.

- Measuring flash.

Analysis step:

Test of water sample

1- To get 150-200ml water sample, and put into separating funnel 5ml 1-3 sulfuric acid

solution.

2- Add 20ml of ether; clean the sampling bottles, measuring cylinders then pour into the

separating funnel violently shake 2-3 minutes, and continuously start up piston to discharge gas,

keep still for 2-3 minutes, separate layers, put the bottom layer water sample into the original

sample bottle.

3- Put anhydrous Sodium sulfate into a funnel with filtering paper1/3 height, such out the

filtered object from the petroleum ether and filter it into 50mlmeasuring flask.

4- Transfer the water sample into the separating funnel again repeat (2) & (3),then put 10ml of

petroleum ether to clean the funnel and filtering bottle,collect the extracting liquid of petroleum

ether into one measuring bottle,use petroleum ether to dilute it to the graduation shake it to be

uniform.

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5- Test the absorbency of the sample in (4).

A= Reading 200*02.003.7* \ V.

Calculate the oil content:

A= oil content correspondingly to relative absorbency.

V= volume of water sample ml.

Oil content = 0.532*200*02.003.7 \ 15.1 = 140.36 mg/l.

3.3 Hydroxyl Benzene (phenol) content

Principle:

Distil off sample and separate from disturbance matter and fixed agent by distillation.

Distilled sample reacts with tetra amino antipyrine in the medium with pH 10±0.2 and existence

of potassium ferri-Cyanide to produce orange antipyrine dye. After developing color, test

absorbency at wavelength of 510nm with 30mm thick cell, expressed in phenol content mg/l.

Preparation of water without hydroxyl benzene (phenol)

Add 0.2g of active Carbon powder which has been activated at 200°C for 30 minutes after

sufficiently shaking up, set for filtering with filter paper of double layers and at medium speed.

Add Sodium hydroxide and drop Potassium permanganate solution to make solution purple,

transfer into glass vaporizer for distillation to collect distillate service.

Instruments

- 500ml whole glass vaporizer.

- Spectrophotometer.

Test steps

- Transfer 250ml sample into 500ml glass vaporizer, add several glass balls to prevent bumping

, add several drops of (0.5g/l) methyl orange indicator.

- Adjust with 1+9 Phosphorous acid solution to pH= 4 (solution indicates orange), add10%

Copper sulfate solution of 5ml.

- Connect a condenser and heat for distillation till 225ml distillate is produced, then stop heating

and cool down, add 25ml distilled water without phenol into distillation flask, then continue to

distill till 250ml distillate is produced.

- Take 50ml distillate into a 50ml color comparison tube, add 0.5ml Buffer solution with pH=

10.7, shake up at this time pH is about 10±0.2, add 2%4-amonio antipyrine solution of 1ml

shake up, add 8% Potassium ferri-cyanide solution of 1ml, shake up sufficiently, settle for 10

minutes.

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- At wavelength of 510nm with 20nm cell taking blank reagent as reference, test the absorbency

of the solution.

- At the same time, take 250ml distilled water without phenol with the sametest steps for blank

test.

Result expression

Phenol content (mg/l) = absorbance (Reading (*15.196* 0.11 / V )

V= sample volume in ml.

3.4 ammonia nitrogen

Principle:

Control the sample pH at the range of 6-7, adds Magnesia to make the sample alkalescent ,

Ammonia which is distilled off is absorbed by boric of receiver. Take methyl red-methylene

blue as indicator, titrate ammonium of distillate with standard acid solution.

Instrument

- Vaporize composed of distillation flask of 500-800ml blow out prevention nozzle and vertical

condensation tube, the end of condensation tube can be connected with a burette with proper

length, dip the outlet tip below 2cm of absorption liquid level.

Test steps

- Take phosphoric acid ( H3PO4 ) 2% indicator solution 50ml into receiver of vaporizer to

ensure the outlet of condensation pipes is below boric acid solution level; put 50mlof sample

into the distillation bottle. Add 3 drops of Bromothymol (Cr7H28O5Br25) (0.5g/l) blue indicator,

if any necessary adjust pH to 6.0 (shows yellow)-(7.9 shows blue) with 1 mol/l Sodium

hydroxide or 1% hydrochloric acid, then add water to make the volume of liquid in the

distillation flask at about 350ml. add light Magnesia (MgO) and a few of explosion proof glass

ball into distillation flask, connect immediately distillation flask with condensation tube.

- Heating vaporizer to make the collecting speed of distillate at about 10ml/mit,stop distillation

when effluent is about 200ml.

- Titrate distillate with 0.10mol/l standard hydrochloric acid solution to reach purple end point,

write down the consumption (V1).

Blank test

Make blank test according to test steps of sample, but use water 250ml instead ofsample.

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Result

CN = V1− V2 / V0 * 01.14*1000.......... .......... (3.3 )

Where: V0= sample volume in ml.

V1= volume of standard hydrochloric acid solution consumed for titrating sample in ml.

V2= volume of standard hydrochloric acid solution for titrating blank sample in ml.

C = concentration of standard hydrochloric acid titration solution .

CN = 50− 35 / 150 * 01.14*1000 = 7638.81 mg/l.

3.5 Sulfide (Iodimetry Method)

Principle

Sulfide reacts with Zinc acetate to produce settling of white Zinc. Sulfide dissolve this settling

in acid and let it react with standard Iodine, then use sodium

Hyposulfite to titrate the excessive amount of iodine.

𝑍𝑛+2 + 𝑆−2 → 𝑍𝑛𝑆 ↓

𝑍𝑛+2 + 𝐻𝑆−1 → ZnS↓ + H

𝐻2S + I → 2HI + S

𝑍𝑛+2 + 𝐻2S → ZnS↓ + 2𝐻+1

ZnS + 𝐻2𝑆𝑂4→ Zn𝑆𝑂4+ 𝐻2S

𝐼2+ 2𝑁𝑎22𝑆2𝑂3→ 2NaI + 𝑁𝑎2𝑆4𝑂6

Instruments

- Acid burett

- Iodine flask.

- Vacuum pump, Buckner filters.

- Middle-speed quantitative filter paper.

Test steps

- Pour 10ml of 10% Zinc acetate solution and 5ml of 1mol/l Sodium hydroxide solution into a

sample-taking bottle of 250ml, take 250ml of water sample into this bottle to determine sulfide

of water sample.

- Take 100ml of water sample with immobile liquid, use middle-speed filter paper and

vacuum. Pump to filter white settling and wash sediment with distilled water. Put settling and

filter paper into an Iodine flask of 250ml, add 50ml of distilled water. Agitate and break up

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filter paper sufficiently, then add 5.0ml of 1-5 sulfuric acid solution as well as 10ml of

0.05mol/l Iodine water. Cover with a bottle block immediately and seal with distilled water,

settle in a dark place for 5 minutes, and titrate with 0.05mol/l sodium hyposulfite to light

yellow, add 1ml of 1% ferula indicator, continue titrating till blue disappears completely.

- Record consumption V2 (ml) of Sodium hyposulfite, make a blank test in the same way.

Result

𝑆−2(Ml/l) = ( V1 –V2) *C*16 *1000 /V..........(4.3)

Where: V1≡ volume of Sodium hyposulfite standard solution for blank test (ml).

V2≡ volume of Sodium hyposulfite for titrating water sample (ml).

V ≡ volume of water sample (ml).

𝑆−2(Ml/l) = (100 –5.05) *0.005*16 *1000 /50 = 4.24 mg/l.

Note

In order to present the volatilization of hydrogen sulfide, immobile liquid10ml of 10% Zinc

acetate solution, 5ml of 1mol/l Sodium hydroxide solution are used to fix sulfide in water of

250ml) must be added into the sample-taking bottle before sample taking

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Chapter Four

Results and Discussion

4.1Results

The results obtained from the analysis which have been already mentioned

in the last chapter are shown as below:

4.1.3 Sour water stripping unit

Table (4.3): analysis results of sour water sample taken from different places in

Sour water stripping unit (7/1/2018)

item Before by (mg/l) After by (mg/l) range

Oil content 242.64 140.36

Ammonia nitrogen 6738.81 286.92

Sulfide content 3304.00 4.24

pH value 9.76 9.81

4.2 Discussion of Results

4.2.1 Sour water stripping unit

• Sour water enters the unit with high concentration of ammonia (6738.8mg/l)and leave the unit

as purified water with concentration about (286.92 mg/l) ,but it must be less than 200, since this

will effect the biochemical treatment unit.

• And enter the unit with concentration of sulfide (3304mg/l) and leave the unit as purified

water with concentration about (4.24mg/l) which is with in the range of permission.

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Chapter Five

Conclusions and Recommendations

5.1 Conclusions

The conclusions of the study are:

-The purified water can be used within the refinery in desalting, cooling and washing process.

-Excess amount of oxygen is used to insure that ammonia is burned completely in the

incinerator

-Nitrogen is used as insulator in the gaseous drum.

-Steam tracing is used around the gaseous line.

-Hydrogen sulfide is conveyed to the Sulphur recovery unit.

5.2 Recommendations

• Sell ammonia or as another alternative one can burn it instead of dissolving in water and

disposal in dumping area .because excessive ammonia has a great effect on ground water, soil

and aqueous life.

• Treat contaminated oil it must be treated instead of disposing it in dumping area.

• Use zeolite instead of activated carbon, the results showed that the zeolite performed some

better in removal of ammonia compounds than activated carbons.

• expand the capacity of the unit.

•As the height of the sour water stripper is high, it should be joined with alarm system to avoid

the risk of ammonia and hydrogen sulfide leak.

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References

1. BabikerAli, B, M (2003), Environmental pollution in Khartoum Refinery with emphasis on

waste water treatment, M.Sc. U of KH.

2. Dr.salah F. sharif, wastewater treatment of effluents from refinery & petrochemical

industry,MS.c.phD . Environmental pollution control , Publishedby Arab Petroleum

Training Institute(APTI).

3. ( Environmental Impact of the Petroleum Industry) Published by the Hazardous Substance

Research Center /South & Southwest Outreach Program June 2003.

4. G. M. Wilson and W. W. Y. Eng, "Research Report RR-118 GPSWAT GPS Sour Water.

5. http://en.wikipedia.org/wiki/Wastewater_quality_indicators(8-12-2007).

6. http://www.umich.edu/~gs265/society/waterpollution.htm (1-2-2008).

7. http://en.wikipedia.org/wiki/water pollution (8-12-2007).

8. http://www.chevron.com (1-2-2008).

9. http://en.wikipedia.org/wiki/oil -Refinery (8-12-2007).

10. http://en.wikipedia.org/wiki/API_oil-water_separator (8-122007).

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Equilibria for Sour Water Systems at High Temperature," Gas Processors Association,

Tulsa, Oklahoma, 1983.

12. Lawrence K- wang (et al), 2006, wastewater treatment in the process industries, published

by CRC press Taylor& Francis Group BOCO Raton London –Network.

13. P. C. Gillespie, W. V. Wilding and G. M. Wilson, "Research Report RR-90 Vapor-

LiquidEquilibrium Measurements on the Ammonia-Water System from 313K to 589 K,"

Gas Processors Association, Tulsa, Oklahoma, 1985 .