Chapter I1.0IntroductionThe research workeddone 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 theseare provided below· Commercial and industrial value· Environmental limits· Safety and HAZARD· Pipeline and equipment corrosion and· Handling off-spec LPG1.1 Commercial and marketvalueLPG (Liquid PetroleumGas) 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) reportedthat the global LPG market was valued at US$ 257.8 bn in 2016 and expected toreach US$ 339.
2 bn by 2024, expanding at CAGR of 3.5% from 2016 to 2024. Themain drives for the increase in demand due to the consumption by residentialsector and increase in adoption of LPG as an Auto Fuel. As per IHS (MARKIT) Global LPG demand will continue to growand mainly driven by Asia and Middle East. Market analysis of theglobal LPG demand study conducted by (MEDIA) reveals that residential usage is 47%, Commercial andchemical usage account for 27%, industrial usage is 8% and rest by others. Recently thereis remarkable shift in LPG usage as auto gas.
The major advantages of LPG as anauto gas over other conventional fuel primarily due to its performance, enginelife, fuel economy, cost saving and environmental benefits. With the use of LPGas fuel, 75% less Carbon Monoxide, 85% less hydrocarbon, 40% less oxides ofnitrogen and 87% less ozone forming potential compared to gasoline (AEGPL).1.2 Environmental LimitBurning propane, butaneand other LPG products containing H2S will oxidize to SO2,which is harmful to the people under expose. Emissionsfrom LPG vehicle used indoors too pose a health hazard to workers. The emissionis indirectly regulated by all occupational health and safety air qualitystandards.
There are many regulatory authorities in different jurisdictions.Occupational Safety and Health Administration (OSHA) within the Department of Labor sets air quality standardsat the federal level in the U.S. The corresponding regulatory authorities inCanada are located within provincial Ministries of Labour.
Most air qualitystandards in North America are based on the guidelines established by theAmerican Conference of Governmental Industrial Hygienists (ACGIH). The ACGIH Threshold Limit Values (TLV) for pollutantspresent 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 forLPG Exhaust Pollutants S. No Substance TLV(v ppm) 1 Carbon Monoxide 25 2 Nitric Oxide 25 3 Nitrogen Dioxide 3 4 Butane 800 5 Sulfur Dioxide 2 1.3 Safety and HAZARDAs per IS 4576, limitfor LPG H2S content is not more than 5 ppm (IS).The detailed specification of LPG is provided in Annexure1.
H2S is a flammable and toxic gas, theflammability limits is 4.3% (43,000 ppm) to 4.6% (46,000 ppm), which is far,exceed the concentrations of concern for personnel protection. H2Sis heavier than air; it will tend to accumulate near the ground when leakedinto the atmosphere.
As per NIOSH (National Institute of Occupational safetyand health association) IDLH (Immediately Dangerous to life or Health) value of100 ppm. At concentrations above theIDLH level, a person sense of smell quickly deadened. A bodily response in breathingvarious concentrations of H2S is provided in Table 2. American Conference of Governmental Industrial Hygienists (ACGIH), abroadly recognized authority on the health effects of toxic gases, changed itsrecommended threshold values (TLVs) for airborne hydrogen sulfide (H2S)exposure. It recommends TWA of 1 ppmand STEL of 5 ppm. Many companiesadopted 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 forindustrial application which includes food processing industry, glass blowing,fast food centers etc where the open flame is directly exposed either on thefood products and personal exposure. Any marginal quality deviation in the LPSH2S specification can lead to seversafety and health hazard.
1.4 Pipeline andequipment corrosionA study conducted by NACE reported (NACE2002) that totalannual estimated direct cost of corrosion in the U.S.
is staggering $276 billion, which is approximately3.1% of the nation’s Gross Domestic Product (GDP). Nation’s 163 refineriessupplied more than 18 million barrels per day of refined petroleum products in1996, with a total corrosion-related direct cost of $3.
7 billion. Maintenanceexpenses 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 isimportant to understand the corrosion takes considerable percentage of margin andit is necessary to take mitigate measures to control the corrosion in equipmentand pipelines. 1.
4.1H2S corrosionH2Scorrosion in equipment and pipelines are a major challenge in upstream,downstream and petrochemical industry. Generally, carbon steels metals are mostsusceptible to corrosion even at traces of H2S present in thestream. H2S can cause localized corrosion that promotes the sulfidestress corrosion cracking (SSCC), hydrogen induced cracking (HIC) and hydrogenembrittlement (HE). Predominantly, H2S causes the sulfide stresscorrosion cracking in high strength steel, even at low temperature and lowpartial pressure. Very small amount of H2S (0.
005 ppm) can act as acatalyst and enhanced the corrosion on pipe surface. The H2Scorrosion is more dangerous than the CO2 corrosion because it failswithout notice in short period. The H2S corrosion mechanism providedbelow. Presence of hydrogensulphide in LPG even at concentration of 5 ppm, causes changes in the copperstrip test gives green-pink-purple corrosion products, mainly consisting of CuSand Cu2S. The H2S present in LPG oxidize elementalsulphur (W. Sun 2009). For carbonyl sulphide(COS), which does not cause corrosion of test copper strips even atconcentrations of up to 100 ppm, but in the presence of water, it hydrolyses toH2S and CO2 , which may accelerate the corrosionprocesses (Smith and Joosten).
Figure (1) Corrosion of oil and gas equipment under the influence of H2S.1.4.
2CO2 corrosionIn LPG pipelines andequipment internal general corrosion begins with CO2 corrosion, thereaction of iron from the pipe with aqueous bicarbonate to produce scale (ironcarbonate), water and carbon dioxide(Rennie 2006).Carbon dioxide corrosion is also known as sweet corrosion and it is one of themost important problems in the oil and gas industries. The severity ofcorrosion depends on many factors such as the concentration of CO2,temperature, pressure and velocity in the solution(M. Nordsveen 2003). In CO2 corrosion of carbonsteel, when the concentrations of Fe2+ and CO32-ions exceed the solubility limit, they can precipitate to form solid ioncarbonate according to the reaction below The metal anodicdissolution reaction takes as follows The reaction takesplace through intermediate reactions involving hydroxyl ions (OH-)and its individual rate decreases with decreasing pH and a time will reachwhere cathodic reaction becomes predominant and it acts as a rate controllingstep on the metal surface. In the CO2medium, the rate of cathodic reaction is mainly affected by the partialpressure of CO2. Thus dissolved CO2 have the tendency to form weak carbonicacid (H2CO3) in the solution which increases the cathodicreaction kinetics by dissociation to bicarbonate and hydrogen ions. Figure (2) Carbon dioxide corrosion of carbon steel1.
5 Handing Off-SpecLPGOff-spec production for any type of product such as gasoline, jet,diesel, chemical and LPG and this is termed as slop. Slop is created whenstream fails refinery product specification. Off-spec (slop) production occursduring start-up, shutdown, transportation and flushing(Poeand Mokhatab 2016).
Disadvantage of slop is mainly due to its high cost. Refineries eitherlose money because the slop production is unrecoverable or it costs significantamount of money to recover the value. Unlike other products, handling off-specLPG having limited options for correction by blending, reprocessing anddisposal. On some occasions refineries forced to adopt non-recovery optionssuch as flaring or use as fuel gas which leads to significant value degradation(Jonesand 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 heatcontent of the LPG gas is significantly lower than the value of the liquidfuel. 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 sweeteningis an important unit, needs attention in terms of quality consistency andimprovement.
From the literature survey it is observed that several publicationsavailable on Natural Gas, Fuel gas, flue gas sweetening but very fewliteratures available on LPG sweetening and optimization.Considering the above industrial, commercial valueand significant safety & environmental factors associated with LPG, theresearch work focused on · A thorough literature review on the LPG sweeteningprocess, amine absorption, solvent selection, process and performanceparameters, kinetic modeling and optimization.· Identified an industrial LPG absorber and collectedactual operating conditions, design details and feed and product quality information.· Amine absorption flow sheet model development usingamine and acid gas package.
· Identified the performance parameters of the sweeteningprocess.· Validating the model parameters and output data withactual industrial data.· Optimization of absorber parameters for maximumproduct quality. · Usage of the model in actual industrial application.1.6Research contributionKineticmodels 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 usedin data interpretation, process optimization and control.
It is also used topredict the behavior of reacting systems where conditions significantlydifferent from those that have already measured (ProMax,Petro-SIM,CHEMCAD,ASPEN Plus & ASPEN HYSYS. (CuiQing and ShaoMei 2004; Aylott and Vander Merwe 2008; Hanyak 2012; Schefflan 2016)1.6.
1Process performance and operationA process simulator is software used for themodeling of the behavior of a chemical process in steady state or dynamicconditions by means of pressures, temperatures and flows. The processsimulators are used to predict the behavior of process, identifying the processbehavior at different operating conditions, optimize the process for maximumefficiency, equipment sizing, CAPEX and OPEX. Process simulation as discipline usesmathematical models as basis for analysis, prediction, testing and detection ofa process performance. A model based engineering (MBE) approach appliesadvanced process models in combination with observed laboratory and plant data.
1.6.2Advanced process control applicationsSteady state and dynamic simulation models are usedin advanced process control projects. The first step is to develop a steadystate model of the process where APC is planned.
Then, the model is calibrated to reflect the realplant conditions and after adding all dynamic data (volumes, valve sizes, kfactors, controllers, etc) and setting up the right pressure-flow relations, itcan 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 newinstrumentation needs, to check the feasibility of the inferential and toestimate the potential benefits. It can also be used to detect ill-conditioningof the selected APC variables(Foss, Lohmann et al. 1998).Dynamic model that will be able to reproduce all non linearity and dead timesof the process when changing the process conditions or introducingperturbations.1.6.3Process design in FEED (Front End Engineering Design)Process simulation models are used for to comparethe various process scheme and select the optimum scheme with respect toproduct quality, energy consumption and equipment size and cost.
It provideinputs to engineering disciplines such as Mechanical, Piping, Instrument,Electrical, Pipeline, safety, etc. Simulation models used to generate heat andmaterial balance for different cases and it gives input different equipmentsuch as pumps, compressors, chillers, cooling system, utility system, safety,pressure relief, fire network sizing calculations. Simulation models providedinput for designing piping, instrumentation, ESD and process control systemrelated to the process.1.7Outline of the thesisThe thesis divided into seven chapters are providedin this Chapter11.0 Introduction1.
1 Commercial and market value1.2 Environmental Limit1.3 Safety and HAZARD1.
4 Pipeline and equipment corrosion1.4.1 H2Scorrosion1.4.
2 CO2corrosion1.5 Handing Off-Spec LPG1.6 Research contribution1.6.1 Process performance and operation1.
6.2 Advanced process control applications1.6.
3 Process design in FEED (Front End Engineering Design)Chapter2 2.0 Literature survey 2.1 Acid gas treating process2.2Absorption2.
2.1Physical Absorption2.2.1a. Selexol process2.2.1.b Rectisol process2.2.2Chemical Absorption2.3 Kinetic modeling2.4 Amine treatment process2.5 Amine selection2.6 Performance parameters2.7 Process simulation2.8 Optimization of LPG sweetening and similartreating processChapter-3LPG amine absorption processComparison of different models Process simulation models, Thermodynamic methods and selection criteria Chapter-4 Information related to industrial LPG amine absorptionprocess Simulation model development, Industrial absorber operating and design parametersQuality data on feed and products composition, amineconcentration, lean and rich amine loading, Thermodynamic method selection, flow sheet modelingand convergence.Chapter-5 Model validation, Input and output parameters of the modelChapter-6OptimizationOptimization of the flow sheet modelSingle objective optimization and case studyoptimization method Optimization of input and output parameters Model findings, results, model finding andconclusionsChapter-7 Results and discussionChapter-8 Applications of research are provided