Modelling And Simulation Of Reverse Osmosis Process Of Boiler Feed Water Treatment

ABSTRACT

Model equations for prediction of process parameters of reverse osmosis for treatment of boiler feed were developed via mathematical derivation from basic equations for reverse osmosis process. A model equation relating the interfacial solute concentration (C_si) with the process pressure difference (DP) was developed. Taking the DP of a reverse osmosis as the basic independent variable, further model equations relating other process parameters such as the solute concentration polarity (G), water flux (J_w), osmotic pressure (Dp), water output rate (q), power density (Pd) and specific energy consumption (SEC) were developed. Simulation of a hypothetical reverse osmosis data using Microsoft Excel Worksheet and a Microsoft Windows 10 on a 64-bit operating system was carried out. Simulation results showed that the optimum fluid bulk concentration was C_o ¼ 0.0004 mole/cm³. The optimum rate of increase in the solute rejection factor per unit rise in ΔP was 0.45%. The optimum solute rejection factor was 97.6%. The optimum water output rate, specific energy consumption and power density were 103.2 L/h, 3.65 kWh/m³ and 6.09 W/m², respectively.

Key words: treatment of boiler feed water, modeling, process parameters, reverse osmosis, simulation, water treatment.

TABLE OF CONTENTS

COVER PAGE

TITLE PAGE

APPROVAL PAGE

DEDICATION

ACKNOWELDGEMENT

ABSTRACT

CHAPTER ONE

INTRODUCTION

1.1      BACKGROUND OF THE STUDY

• PROBLEM STATEMENT
• AIM AND OBJECTIVES OF THE STUDY
• SCOPE OF THE PROJECT
• SIGNIFICANCE OF THE STUDY
• RESEARCH OUTLINE

CHAPTER TWO

LITERATURE REVIEW

• REVIEW OF THE STUDY
• FEASIBILITY OF RO PROCESS IN WASTEWATER
• OVERVIEW OF REVERSE OSMOSIS (RO)
• REVERSE OSMOSIS PRINCIPLE
• REVERSE OSMOSIS PROCESS
• BENEFITS OF REVERSE OSMOSIS
• ADVANTAGES OF REVERSE OSMOSIS
• DISADVANTAGES OF REVERSE OSMOSIS
• BOILER FEEDWATER TREATMENT
• HISTORY OF FEEDWATER TREATMENT
• CHARACTERISTICS OF BOILER FEEDWATER
• CONDITIONING OF FEED WATER
• REVIEW OF RELATED STUDIES

CHAPTER THREE

• MATERIALS AND METHOD
• MODELING THEORY

CHAPTER FOUR

• RESULT AND DISCUSSION

4.1     Determination of the optimum bulk solute concentration for the RO operation

4.2    Impact of solute interfacial concentration on DP at variable solute concentration of RO

4.3    Impact of interfacial solute concentration on process parameters

4.4   Impact of membrane filter length on specific energy consumption

• Impact of RO process parameters on power density and specific energy consumption

CHAPTER FIVE

• CONCLUSION AND RECOMMENDATION
• REFERENCES

CHAPTER ONE

1.0                                 INTRODUCTION

1.1                      BACKGROUND OF THE STUDY

Boiler feed water is an essential part of boiler operations. The feed water is put into the steam drum from a feed pump. In the steam drum the feed water is then turned into steam from the heat. After the steam is used it is then dumped to the main condenser. From the condenser it is then pumped to the deaerated feed tank. From this tank it then goes back to the steam drum to complete its cycle. The feed water is never open to the atmosphere.

In Boiler feed water, a boiler is a device for generating steam, which consists of two principal parts: the furnace, which provides heat, usually by burning a fuel, and the boiler proper, a device in which the heat changes water into steam. The steam or hot fluid is then recirculated out of the boiler for use in various processes in heating applications.

For industrial companies using a boiler for its facility, some type of boiler feed water treatment system is usually necessary to ensure an efficient process and quality steam generation. The most appropriate boiler feed water treatment system will help the facility avoid costly plant downtime, expensive maintenance fees, and boiler failure as a result of scaling, corrosion, and fouling of the boiler and downstream equipment.

Treatment of boiler feed water via reverse osmosis process became prominent method of sourcing fresh water for industrial purpose. Reverse osmosis became a practical process with the discovery of asymmetric cellulose-acetate membranes by Loeb and Sourirajan in 1962 (Loeb & Sourirajan 2013). Before the discovery of reverse osmosis, thermal operation involving evaporation and condensation processes had been the method for treatement of boiler feed water. Today, reverse osmosis accounts for at least 64% industrial desalination of seawater while thermal process account for just about 34% (Henthorne 2019). Other low scale industrial processes for treatment of of boiler feed water include nanofiltration, membrane distillation and electrodialysis (Khawaji et al. 2018). The major drawback of the thermal method is its high energy consumption which lowers the economy of the process for commercial water desalination.

Reverse osmosis is a membrane separation method whereby miscible solutions of different concentration are separated by a membrane that is permeable to the solvent but nearly impermeable to the solute (Kim et al. 2020; Ligaraya et al. 2020; Monjezi et al. 2020). Reverse osmosis derives its name from the technical meaning of the process. It is an opposite operation of osmosis process. In osmosis process the driving force of the process is the solvent activity. The solvent diffuses in a direction of increasing solvent activity. Therefore, the solvent diffuses from the less concentrated solution region where the solvent activity is higher to the more concentrated solution region, where the solvent activity is lower (McCabe et al. 2013). The flow of solvent from the less concentrated region to the more concentrated region is driven by the osmotic pressure (Dp).

In order to achieve reverse osmosis the pressure of the concentrated solution must be raised to counter the osmotic pressure. As the pressure of the concentrated solution is raised, the osmotic diffusion is slowed down until the activity gradient across the two ends of the membrane is lowered. At the point where the activity gradient is zero, the osmotic diffusion is stopped. A further increase in the pressure of the concentrated solution will result into diffusion of solvent from the concentrated region to the dilute region (McCabe et al. 2013; Cornelissen et al. 2021). If pure solvent is on one side of the membrane, the pressure required in the concentrated region to achieve zero solvent activity gradient is called the osmotic pressure of the solution, denoted by (Dp). To achieve reverse osmosis process, the pressure applied on the concentrated region must be greater than Dp.

Membranes used in reverse osmosis are mostly flexible films of synthetic polymers prepared to have a high permeability for water molecules only. Reverse osmosis membranes are effectively non-porous and, therefore, exclude particles and even many low molar mass species such as salt ions, organics and other impurities in water (Loeb & Sourirajan 2013; Amjad 2013; McCabe et al. 2013). Only water molecule gain access across the membrane. Chemically they are typically made of either cellulose acetate or polysulfone coated with aromatic polyamides. These materials are suitable for use as reverse osmosis membrane because they have a high permeability for water and low permeabilities for dissolved salts. Reverse osmosis membranes are applied either as layers of films or as hollow fibres depending on the design of the desalination process (Amjad 2013; McCabe et al. 2013). However, for industrial applications the hollow fibres are applied.

It has been reported that 100% rejection of solute is not possible in practice for one stage reverse osmosis (McCabe et al. 2013). One stage operation can only achieve percentage solute rejection of 97–99%. To achieve approximately 100% rejection of solute, multiple stage reverse osmosis plants are usually employed in industrial treatment of boiler feed water (Cath et al. 2006; Kim et al. 2020).

1.2                             PROBLEM STATEMENT

The boiler receives the feed water, which consists of varying proportion of recovered condensed water (return water) and fresh water, which has been purified in varying degrees (make up water). The make-up water is usually natural water either in its raw state, or treated by some process before use. Feed-water composition therefore depends on the quality of the make-up water and the amount of condensate returned to the boiler. The steam, which escapes from the boiler, frequently contains liquid droplets and gases. The water remaining in liquid form at the bottom of the boiler picks up all the foreign matter from the water that was converted to steam. The impurities must be blown down by the discharge of some of the water from the boiler to the drains. The permissible percentage of blown down at a plant is strictly limited by running costs and initial outlay. The tendency is to reduce this percentage to a very small figure.

Proper treatment of boiler feed water is an important part of operating and maintaining a boiler system. As steam is produced, dissolved solids become concentrated and form deposits inside the boiler. This leads to poor heat transfer and reduces the efficiency of the boiler. Dissolved gasses such as oxygen and carbon dioxide will react with the metals in the boiler system and lead to boiler corrosion. In order to protect the boiler from these contaminants, they should be controlled or removed, trough external or internal treatment.

1.3                                       RESEARCH AIM AND OBJECTIVES

Aim of the research

The overall aim of this research is to develop comprehensive mathematical modelling for RO process based on the removal of highly toxic compounds from boiler feed water. The developed models will be used to investigate a reliable operation for the RO process via simulation studies.

Objectives of research

1. To carry out a literature survey on the modeling based on the reverse osmosis
2. To carry out a literature survey on the simulation of the RO based boiler feed water
3. To carry out a parameter estimation to estimate the best values of the separation membrane parameters and the friction
4. To carry out comprehensive simulation studies to investigate the impact of operating parameters on the performance of RO process including boiler feed water
5. To evaluate the merit of a two-stage/two-pass RO process design for removing impurities from water considering model-based techniques.

1.4                                                                              SCOPE OF THE STUDY

RO is a key treatment process in water reclamation applications for the removal of organic chemicals. Also, RO is extensively used in food processing. The research conducted in this study serves these precise purposes and explores the RO process as an alternative approach for concentration reduction of high-toxic impurities in industrial wastewater. The investigated pollutants are toxic compounds, which are considered as extremely toxic compounds with several harmful effects for humans, the environment, and the aquatic life.

The research delivers several attempts of modelling the removal of high-toxic compounds using a dual RO process.

1.5                                                 IMPORTANCE OF THE STUDY

The results show that adopting the this method of water treatment can significantly reduce the industrial machine operational cost.

Treating boiler feed water is essential for both high-and low-pressure boilers. Ensuring the correct treatment is implemented before problems such as fouling, scaling, and corrosion occur, will go a long way in avoiding costly replacements/upgrades down the line.

An efficient and well-designed boiler feed water treatment system should be able to:

1. Efficiently treat boiler feed water and remove harmful impurities prior to entering the boiler
2. Promote internal boiler chemistry control

• Maximize use of steam condensate

1. Control return-line corrosion
2. Avoid plant downtime and boiler failure
3. Prolong equipment service life

     1.6                                                RESEARCH OUTLINE

The layout of the thesis is highlighted in the following.

Chapter 1: Introduction – General background of the study. A brief description of RO process and its application in boiler feed water treatment.

Chapter 2: Literature Review: A brief description of the feasibility of RO in boiler feed water. The literature of experimental past work and membrane theory including overview of the RO modelling and simulation.

Chapter 3: Spiral Wound Reverse Osmosis Process Modelling and Validation This chapter presents all the new mathematical model developed for an individual and multistage RO process for the removal of impurities from boiler feed water.

Chapter 4:  This chapter presents an analysis study for the RO process under variable operating parameters at steady state conditions. Also, a case of two stage/two pass RO process to enhance the removal of impurities is presented.

Chapter 5: Conclusions and Recommendations for Further Research. This chapter presents the conclusions extracted from the thesis and the recommendations for future research work.