Chapter value · Environmental limits · Safety and HAZARD

Chapter I


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The research worked
done on “Modeling and optimization of LPG (Liquid Petroleum Gas) sweetening unit”.
The area of research selected due to its commercial, industrial safety, hazard,
equipment integration and product handling issues. A brief explanation of these
are provided below

Commercial and industrial value

Environmental limits

Safety and HAZARD

Pipeline and equipment corrosion and

Handling off-spec LPG

1.1 Commercial and market

LPG (Liquid Petroleum
Gas) is one of the important petroleum products by its usage in residential,
industrial, commercial and as auto gas. LPG is most preferred fuel due to its non-toxic,
clean, easy material handling and cost efficient. Credence Research Inc.(PARTNERS) reported
that the global LPG market was valued at US$ 257.8 bn in 2016 and expected to
reach US$ 339.2 bn by 2024, expanding at CAGR of 3.5% from 2016 to 2024. The
main drives for the increase in demand due to the consumption by residential
sector and increase in adoption of LPG as an Auto Fuel. As per IHS (MARKIT) Global LPG demand will continue to grow
and mainly driven by Asia and Middle East.

Market analysis of the
global LPG demand study conducted by (MEDIA) reveals that    residential usage is 47%, Commercial and
chemical usage account for 27%, industrial  usage is 8% and rest by others. Recently there
is remarkable shift in LPG usage as auto gas. The major advantages of LPG as an
auto gas over other conventional fuel primarily due to its performance, engine
life, fuel economy, cost saving and environmental benefits. With the use of LPG
as fuel, 75% less Carbon Monoxide, 85% less hydrocarbon, 40% less oxides of
nitrogen and 87% less ozone forming potential compared to gasoline (AEGPL).

1.2 Environmental Limit

Burning propane, butane
and other LPG products containing H2S will oxidize to SO2,
which is harmful to the people under expose. Emissions
from LPG vehicle used indoors too pose a health hazard to workers. The emission
is indirectly regulated by all occupational health and safety air quality
standards. There are many regulatory authorities in different jurisdictions.
Occupational Safety and Health Administration (OSHA) within the Department of Labor sets air quality standards
at the federal level in the U.S. The corresponding regulatory authorities in
Canada are located within provincial Ministries of Labour. Most air quality
standards in North America are based on the guidelines established by the
American Conference of Governmental Industrial Hygienists (ACGIH). The ACGIH Threshold Limit Values (TLV) for pollutants
present in the LPG exhaust is listed in Table 1.
The TLVs are time-weighted averages for an 8-hour workday.

Table 1 Threshold Limit Values for
LPG Exhaust Pollutants    

S. No


TLV(v ppm)


Carbon Monoxide



Nitric Oxide



Nitrogen Dioxide






Sulfur Dioxide



1.3 Safety and HAZARD

As per IS 4576, limit
for LPG H2S content is not more than 5 ppm (IS).
The detailed specification of LPG is provided in Annexure
1.  H2S is a flammable and toxic gas, the
flammability limits is 4.3% (43,000 ppm) to 4.6% (46,000 ppm), which is far,
exceed the concentrations of concern for personnel protection. H2S
is heavier than air; it will tend to accumulate near the ground when leaked
into the atmosphere. As per NIOSH (National Institute of Occupational safety
and health association) IDLH (Immediately Dangerous to life or Health) value of
100 ppm. At concentrations above the
IDLH level, a person sense of smell quickly deadened. A bodily response in breathing
various concentrations of H2S is provided in Table 2. American Conference of Governmental Industrial Hygienists (ACGIH), a
broadly recognized authority on the health effects of toxic gases, changed its
recommended threshold values (TLVs) for airborne hydrogen sulfide (H2S)
exposure. It recommends TWA of 1 ppm
and STEL of 5 ppm. Many companies
adopted these limits in their industrial and safety hygiene procedures due to health and legal perspective.

As mentioned in chapter 1.1, globally 8 to 11% of LPG is being used for
industrial application which includes food processing industry, glass blowing,
fast food centers etc where the open flame is directly exposed either on the
food products and personal exposure. Any marginal quality deviation in the LPS
H2S specification can lead to sever
safety and health hazard.

1.4 Pipeline and
equipment corrosion

A study conducted by NACE reported (NACE
2002)  that total
annual estimated direct cost of corrosion in the U.S. is  staggering $276 billion, which is approximately
3.1% of the nation’s Gross Domestic Product (GDP). Nation’s 163 refineries
supplied more than 18 million barrels per day of refined petroleum products in
1996, with a total corrosion-related direct cost of $3.7 billion. Maintenance
expenses make up $1.8 billion of this total, vessel expenses are $1.4 billion,
and fouling costs were approximately $0.5 billion annually. Hence, it is
important to understand the corrosion takes considerable percentage of margin and
it is necessary to take mitigate measures to control the corrosion in equipment
and pipelines.

H2S corrosion

corrosion in equipment and pipelines are a major challenge in upstream,
downstream and petrochemical industry. Generally, carbon steels metals are most
susceptible to corrosion even at traces of H2S present in the
stream. H2S can cause localized corrosion that promotes the sulfide
stress corrosion cracking (SSCC), hydrogen induced cracking (HIC) and hydrogen
embrittlement (HE). Predominantly, H2S causes the sulfide stress
corrosion cracking in high strength steel, even at low temperature and low
partial pressure. Very small amount of H2S (0.005 ppm) can act as a
catalyst and enhanced the corrosion on pipe surface. The H2S
corrosion is more dangerous than the CO2 corrosion because it fails
without notice in short period. The H2S corrosion mechanism provided


Presence of hydrogen
sulphide in LPG even at concentration of 5 ppm, causes changes in the copper
strip test gives green-pink-purple corrosion products, mainly consisting of CuS
and Cu2S. The H2S present in LPG oxidize elemental
sulphur (W. Sun 2009).

For carbonyl sulphide
(COS), which does not cause corrosion of test copper strips even at
concentrations of up to 100 ppm, but in the presence of water, it hydrolyses to
H2S and CO2 , which may accelerate the corrosion
processes (Smith and Joosten).

Figure (1) Corrosion of oil and gas equipment under the influence of H2S.

CO2 corrosion

In LPG pipelines and
equipment internal general corrosion begins with CO2 corrosion, the
reaction of iron from the pipe with aqueous bicarbonate to produce scale (iron
carbonate), water and carbon dioxide(Rennie 2006).
Carbon dioxide corrosion is also known as sweet corrosion and it is one of the
most important problems in the oil and gas industries. The severity of
corrosion depends on many factors such as the concentration of CO2,
temperature, pressure and velocity in the solution(M. Nordsveen 2003). In CO2 corrosion of carbon
steel, when the concentrations of Fe2+ and CO32-
ions exceed the solubility limit, they can precipitate to form solid ion
carbonate according to the reaction below

The metal anodic
dissolution reaction takes as follows

The reaction takes
place through intermediate reactions involving hydroxyl ions (OH-)
and its individual rate decreases with decreasing pH and a time will reach
where cathodic reaction becomes predominant and it acts as a rate controlling
step on the metal surface.

In the CO2
medium, the rate of cathodic reaction is mainly affected by the partial
pressure of CO2.

Thus dissolved CO2 have the tendency to form weak carbonic
acid (H2CO3) in the solution which increases the cathodic
reaction kinetics by dissociation to bicarbonate and hydrogen ions.

Figure (2) Carbon dioxide corrosion of carbon steel

1.5 Handing Off-Spec

Off-spec production for any type of product such as gasoline, jet,
diesel, chemical and LPG and this is termed as slop. Slop is created when
stream fails refinery product specification. Off-spec (slop) production occurs
during start-up, shutdown, transportation and flushing(Poe
and Mokhatab 2016). Disadvantage of slop is mainly due to its high cost. Refineries either
lose money because the slop production is unrecoverable or it costs significant
amount of money to recover the value. Unlike other products, handling off-spec
LPG having limited options for correction by blending, reprocessing and
disposal. On some occasions refineries forced to adopt non-recovery options
such as flaring or use as fuel gas which leads to significant value degradation(Jones
and Pujadó 2006).

When refineries take a liquid product and use it as fuel gas equivalent,
there is significant value degradation occurs. On barrel to barrel basis, the heat
content of the LPG gas is significantly lower than the value of the liquid
fuel. The products downgrade ranging from $15/bbl upto $60/bbl or more.

Therefore it is apparent that on product value, commercial importance,
usage, safety & environment and operational challenges, sour LPG sweetening
is an important unit, needs attention in terms of quality consistency and

From the literature survey it is observed that several publications
available on Natural Gas, Fuel gas, flue gas sweetening but very few
literatures available on LPG sweetening and optimization.

Considering the above industrial, commercial value
and significant safety & environmental factors associated with LPG, the
research work focused on

A thorough literature review on the LPG sweetening
process, amine absorption, solvent selection, process and performance
parameters, kinetic modeling and optimization.

Identified an industrial LPG absorber and collected
actual operating conditions, design details and feed and product quality information.

Amine absorption flow sheet model development using
amine and acid gas package.

Identified the performance parameters of the sweetening

Validating the model parameters and output data with
actual industrial data.

Optimization of absorber parameters for maximum
product quality.

Usage of the model in actual industrial application.

Research contribution

models are the most versatile way to model the chemistry of the processes,
which is necessary for an optimal chemical reactor optimization and design.  It is used
in data interpretation, process optimization and control. It is also used to
predict the behavior of reacting systems where conditions significantly
different from those that have already measured (ProMax,Petro-SIM,CHEMCAD,ASPEN Plus & ASPEN HYSYS. (CuiQing and ShaoMei 2004; Aylott and Van
der Merwe 2008; Hanyak 2012; Schefflan 2016)

Process performance and operation

A process simulator is software used for the
modeling of the behavior of a chemical process in steady state or dynamic
conditions by means of pressures, temperatures and flows. The process
simulators are used to predict the behavior of process, identifying the process
behavior at different operating conditions, optimize the process for maximum
efficiency, equipment sizing, CAPEX and OPEX.

Process simulation as discipline uses
mathematical models as basis for analysis, prediction, testing and detection of
a process performance. A model based engineering (MBE) approach applies
advanced process models in combination with observed laboratory and plant data.

Advanced process control applications

Steady state and dynamic simulation models are used
in advanced process control projects. The first step is to develop a steady
state model of the process where APC is planned. Then, the model is calibrated to reflect the real
plant conditions and after adding all dynamic data (volumes, valve sizes, k
factors, controllers, etc) and setting up the right pressure-flow relations, it
can be simulated in dynamic mode using the steady-state data as initialization (Deshpande and Ash 1988). The Steady-State model can be used to identify new
instrumentation needs, to check the feasibility of the inferential and to
estimate the potential benefits. It can also be used to detect ill-conditioning
of the selected APC variables(Foss, Lohmann et al. 1998).
Dynamic model that will be able to reproduce all non linearity and dead times
of the process when changing the process conditions or introducing

Process design in FEED (Front End Engineering Design)

Process simulation models are used for to compare
the various process scheme and select the optimum scheme with respect to
product quality, energy consumption and equipment size and cost. It provide
inputs to engineering disciplines such as Mechanical, Piping, Instrument,
Electrical, Pipeline, safety, etc. Simulation models used to generate heat and
material balance for different cases and it gives input different equipment
such as pumps, compressors, chillers, cooling system, utility system, safety,
pressure relief, fire network sizing calculations. Simulation models provided
input for designing piping, instrumentation, ESD and process control system
related to the process.

Outline of the thesis

The thesis divided into seven chapters are provided
in this


1.0 Introduction

1.1 Commercial and market value

1.2 Environmental Limit

1.3 Safety and HAZARD

1.4 Pipeline and equipment corrosion

1.4.1 H2S

1.4.2 CO2

1.5 Handing Off-Spec LPG

1.6 Research contribution

1.6.1 Process performance and operation

1.6.2 Advanced process control applications

1.6.3 Process design in FEED (Front End Engineering Design)


2.0 Literature survey

2.1 Acid gas treating process


Physical Absorption

2.2.1a. Selexol process

2.2.1.b Rectisol process

Chemical Absorption

2.3 Kinetic modeling

2.4 Amine treatment process

2.5 Amine selection

2.6 Performance parameters

2.7 Process simulation

2.8 Optimization of LPG sweetening and similar
treating process


LPG amine absorption process

Comparison of different models

Process simulation models,

Thermodynamic methods and selection criteria


Information related to industrial LPG amine absorption

Simulation model development,

Industrial absorber operating and design parameters

Quality data on feed and products composition, amine
concentration, lean and rich amine loading,

Thermodynamic method selection, flow sheet modeling
and convergence.


Model validation,

Input and output parameters of the model



Optimization of the flow sheet model

Single objective optimization and case study
optimization method

Optimization of input and output parameters

Model findings, results, model finding and


Results and discussion


Applications of research are provided