1. MODELLING AND SIMULATION OF A NATURAL GAS DEHYDRATION
SYSTEM IN ASPEN HYSYS USING TRIETHYLENE GLYCOL
2. INTRODUCTION
Natural gas is the most efficient fossil fuel, offering much benefit when compared to
using the coal and oil (Mokhatab et al, 2019). The natural gas is produced from
underground reservoir and contain huge quantity of light hydrocarbon such as methane
and ethane, and small amount of the heavy hydrocarbon like pentane and hexane, and
also containing non-hydrocarbon as water, nitrogen and hydrogen sulphide (Bahadori,
2014). The main purposes of natural gas treatment are to improve the purity of the gas,
separate the heavy hydrocarbon from the light hydrocarbon and to remove the
contained water vapor (Kong at al, 2018; Shoaib et al, 2018).
3. STATEMENT OF THE PROBLEM
Raw natural gas from the production wells contains many impurities such as hydrogen sulphide,
carbon dioxide, nitrogen, inert gases, metallic compounds, water etc. these impurities if not removed
from the raw natural gas may cause serious problems during production and storage of the natural gas,
hence natural gas is often treated to remove these impurities before it can be stored or used. One of
such treatment is to remove the water content of the raw natural gas as it may cause serious problems
such as corrosion of the gas pipelines, catalyst deactivation, gas hydrate formation and side reaction.
There are several ways of removing the water content of the raw natural gas, one of such is
dehydration of the raw natural gas using the absorption process with glycol as the absorbent.
Triethylene glycol will be use to absorb water from the raw natural gas and is chosen over other
desiccant because it is of low cost and can be easily regenerated, Additionally, it is an easy to use
technology, which contributes to its low failure rate and thus much lower costs (Pokrzywniak, 2007).
4. AIM AND OBJECTIVES OF THE STUDY
The aim of the study is to effectively remove the water content of raw natural gas
using triethylene glycol as the absorbent.
In order to achieve this aim, the following objectives shall be pursued:
i. Analyse the effect of triethylene glycol on Natural Gas Dehydration.
ii. Investigate the effect of operating parameters on the efficiency of the process.
iii. Improve on triethylene glycol regeneration and recovery process for recycling.
iv. Simulate the absorption process using Aspen Hysys using available industry data.
v. Optimize the process and improve its efficiency
5. SIGNIFICANCE OF THE STUDY
Natural gas is employed at homes as a domestic cooking gas and is the preferred choice over other
fuels such as kerosene because of its low carbon emission as it is primarily made up of methane. Some
generators also make use of natural gas as their fuel, these types of generators produce less noise and
emit less amount of carbon monoxide to the environment when compared to generators using
gasoline.
This study seeks to carry out the dehydration of raw natural gas using the glycol process as it is
considered the most successful and common process in the chemical industry, this is because glycol
liquid has high affinity toward water vapor (Stewart et al, 2011). If the wet natural gas is effectively
dehydrated, it will serve several purposes such as generation of electricity using gas turbine, heating as
it is more effective than electric heating pumps, it is also use in the industry as raw materials for the
production of other chemicals. Moreover, natural gas is used for cooling purposes, manufacturing,
cogeneration and trigeneration (simultaneous use of electrical energy and heating).
6. REVIEW OF PAST WORKS ON GAS DEHYDRATION
Siti, (2012) carried out a research on the simulation of natural gas dehydration using glycols solution, the effect of
different types of glycol in terms of their ability in dehydrating the wet natural gas to give the minimum water contents in
the dry gas were studied. It was observed from this studies that triethylene glycol gives a better absorption rate of water
from the wet gas when compared to ethylene and diethylene glycol. The effect of operating condition such as the number
of equilibrium stages in the contactor were also studied and it was found that the increase in the number of stages of the
contactor allows for more water to be absorbed from the wet gas thereby reducing the residual water content in dry gas.
Abdulrahman et al. (2013), carried out a similar research on the development of a dehydration of natural gas process
using triethylene glycol by exploring the system response to different TEG flowrates using Aspen Hysys process
simulator, and observed that the flow rate of TEG into the absorption column significantly affect the rate of water
absorption by the TEG.
Ghati (2013), carried out a dehydration of natural gas process with triethylene glycol as absorbing solvent. this
experiment was carried out by changing the number of equilibrium stages in the absorption column using Aspen Hysys.
It was observed that the increase in the number of equilibrium stages increases the rate of absorption.
7. DEFICIENCY OF PAST WORKS ON GAS DEHYDRATION
Previous works on gas dehydration focus mainly on analysing the effect of different glycols on
natural gas dehydration, the effectiveness of dehydrating natural gas with glycol as compared
to others desiccants, the effect of number of theoretical stages in the absorption process, as well
as the effect of operating parameters such as inlet gas flow rate, inlet glycol flow rates,
operating temperature and pressure of the dehydration without taking into consideration the
regeneration of the glycols used to carry out this process as it is more economical to reuse the
glycol used for the dehydration process than regular injection of new lean glycol.
8. THE NEW INNOVATION INTRODUCED
The regeneration of the glycol used in the dehydration of the wet natural gas is very important in the
production process of natural gas in the industry as it reduces production cost of the dry natural gas since it
is economical to reuse the glycol than always injecting new one. This research will carry out the simulation
of the dehydration process of wet natural gas using triethylene glycol as the absorbent (absorption process)
as well as the regeneration of the triethylene glycol by removing the absorbed water from it and then
sending it back to the contactor for further dehydration process (stripping). It will be carried out in three
stages: (1) by carrying out the dehydration process of the wet natural gas at constant flow rate and
regenerating the glycol used, sending it back for further dehydration process, (2) by changing the flow rates
while still regenerating the glycol and sending it back for used in the process and lastly (3) by using different
flow rates without regeneration of the glycol as new glycol will always be used for the process. From here it
will be analysis which of these three glycol dehydrating approach is most effective and economical in the
dehydration process.
9. OVERVIEW OF GAS DEHYDRATION UNIT
A typical dehydration process in natural gas processing plant can be divided into two major parts, gas dehydration and
absorbent regeneration. In dehydration using the absorption process, water is removed from the gas using glycol and in the
regeneration; water is removed from the absorbent (glycol) before it gets back to the absorption column
A Typical dehydration unit in gas processing plant is shown in the figure below:
10. DESCRIPTION OF A NATURAL GAS DEHYDRATION PLANT
In a natural gas dehydration plant that uses glycol as the absorbent to remove water from wet natural gas, lean
glycol is usually fed to the top of an absorber (also known as a "glycol contactor") where it is contacted with the
wet natural gas stream entering from the bottom of the absorber. The glycol removes water from the natural gas by
physical absorption and is carried out to the bottom of the column. Upon exiting the absorber, the glycol stream is
often referred to as "rich glycol". The dry natural gas leaves the top of the absorption column and is fed either to a
pipeline system or to a gas plant.
After leaving the absorber, the rich glycol is fed to a flash vessel where hydrocarbon vapors are removed. After
leaving the flash vessel, the rich glycol is heated in a heat exchanger and fed to the stripper (also known as a
regenerator). The glycol stripper consists of a column, an overhead condenser, and a reboiler. The glycol is
thermally regenerated to remove excess water and regain the high glycol purity. The hot lean glycol is cooled by
cross-exchange with rich glycol entering the stripper. It is then fed to a pump where its pressure is elevated to that
of the glycol absorber before being fed back into the absorber. (source: https://en.wikipedia.org/wiki/
Glycol_dehydration).
11. LIST OF MATERIALS AND COMPONENTS
i. Contactor or Absorbent Column: This is where the gas flows-in from bottom to top and contacts with the countercurrent of the lean
glycol liquid from the tower top; then the water vapor in the gas is absorbed by the glycol liquid.
ii. Flash Drum: This is where gas molecules in the rich glycol are let out before passing through the heat exchanger to increase the
temperature.
iii. Stripper of Still Column: This is where the absorbed water in the glycol is given off so that the glycol can then be reused. It is also
referred to as the regenerator or the regeneration column.
iv. Reboiler: The rich glycol, after being concentrated in the regeneration device, overflows to the reboiler from the still column.
v. Surge Tank: This is used to absorb sudden rise of pressure, as well as to quickly provide extra water during a brief drop in pressure.
vi. Glycol pump: It pumps back the regenerated glycol to the absorption column to be used for further dehydration processes.
vii. Glycol particulate filter: This is where all particles in the rich glycol are filtered off before been sent to the regeneration column.
viii. Heat exchanger: The rich glycol solution, which comes out from the flash separator bottom, enters into the regeneration device after it
passes the filter and poor or rich glycol heat exchanger.
ix. Inlet scrubber: Water-bearing gas (greenhouse gas) first enters into the inlet scrubber, in order to remove the liquid and solid impurities
carried in the gas.
12. METHODS
This research will be categorized into two main phases, these are the simulation of natural gas dehydration unit and the regeneration of the
triethylene glycol unit. The simulation of the process will be done using various industry data such as the composition of the raw natural gas,
the operating parameters like operating temperature and pressure of the process and the flow rate of the glycol and raw natural gas. The
regeneration of the glycol used will be done after the simulation of the dehydration process had been done and the dry natural gas is given off at
the top of the absorber as the product.
The schematic below shows how the research will be carried out from start to end.
Introduction
Initiating problem
Gas dehydration Thermodynamics
Process simulation
Discussion
Conclusion
