SIMULATIONBy

GAUTHAM SProcess Engineer

SimulationIt is the process of examination of a problem not by subjecting a direct experimentation but by a software capable of producing the results under certain caveats.

SOFTWARES AVAILABLEAspen Hysys, Aspen Plus, Unisim, Pro II, Flarenet, Pipesim, Pipenet, etc

Basis of Process Scheme Simulation

• Fluid Characteristics• Predictions of the well behavior along the life of the well.• Selection of Simulator and thermodynamic package• Consideration of safety margins

Fluid Characteristics• Fluid Composition-Assay/Hypo/Pure Component• P-V-T analysis• Other properties of fluid such as wax appearance

temperatures

Fluid Composition- Pure Component

PVT- Analysis

Predictions of the Well Behavior

• PVTPRO or PVTsim can be used to predict the well behavior.

• Well behavior includes Temperature profile, pressure profile, Flow regime etc., throughout the well

• Sometimes the client may give information on this regard. Otherwise, it is obtained using the software.

Selection of Simulator • The different simulator available are Aspen HYSIS, Aspen Plus, Unisim,

PRO II, etc.,• VB, the liquid molar volume at the boiling point TB, for Water is wrong

in the Aspen Plus databank PURE10. In PURE10, the value of VB in CC/MOL is 1401.38; however, the value should be 18.5064 CC/MOL. This property VB used in some transport properties, viscosity, diffusion coefficient like Wilke Chang model. It will not effect liquid density calculations

• The knowledge of limitations of different simulators is necessary to choose the effective simulator.

Selection of Thermodynamic Package

• The thermodynamic packages are chosen based on the nature of the process and composition of the substance.

• The different property packages available are Equation of States Activity Models Chao Seader Models Vapor pressure Models Miscellaneous

• In petroleum upstream industry, Peng Robinson Equation, Glycol Pkg and Amine Pkg are commonly used.

Thermodynamics• To do simulation, an understanding of the

thermodynamics is requisite.• Vapor Liquid Equilibria helps us to understand the fluid

behavior• So let us refresh the basics.

Vapor Liquid Equilibria

B=? C=? B=?B= BUBBLE POINT & C= DEWPOINT B= CRITICAL POINT

Vapor Liquid Equilibria

B=? C=? A=? B=? C=?B= BUBBLE POINT & C= DEWPOINT A= CRICONDENBAR, B= CRITICAL POINT &C=CRICONDENTHERM

Retrograde CondensationRetrograde Condensation is the process of Compression leading to vaporization (and not condensation) at a certain temperature.At T2 , retrograde condensation occurs.C is the critical point in the figure

Equation of States(EOS)• An equation that describes the relationship between

pressure, temperature, and molar volume for any homogenous fluid at equilibrium.

ExampleIdeal Gas Equation, PV = nRTVanderwaals Equation

Frequently Used EOS• Soave-Redlich-Kwong (SRK) equation• Peng-Robinson (PR) equation

Soave-Redlich-Kwong (SRK) Equation

• The prediction of liquid density is very poor. Ideal for heat and mass balances where water phase accuracy is not essential. It is ideal for simple heat and mass balances.

• For polar systems, SRK always makes a better prediction but in the petroleum engineering business we do not usually deal with those.

Peng-Robinson (PR) Equation• Same as SRK but prediction of liquid density is better than

for any of the SRK derivatives, none the less it is still poor for heavier hydrocarbons (>C3). Pressure range is extended (up to1000 bar). It is ideal for simple heat and mass balances.

• PR does a better job (slightly) for gas and condensate systems than SRK

• PR calculates the Universal compressibility factor(Zc) at critical point better than other EOS.

Consideration of Safety Margins

• Safety margins are considered in the flow rate to include an overall safety factor to the process. It is usually suggested by the client.

Rated flow = Normal flow + ( flow margin * normal flow)

Example of a Process Scheme

Mass Transfer• Generalized mass transfer equation

Mass in = Mass out + Mass accumulated + Mass disappeared

• Law of conservation of mass states the following.At steady state,

Mass in = Mass out

Heat Transfer• Generalized heat transfer equation

Heat in = Heat out + Heat accumulated + Heat disappeared

• Law of conservation of energy states the following.At steady state,

Heat in = Heat out

Outcomes of Process Scheme Simulation

• Fluid properties and HMB• Equipment sizing can be done but we use in-house excel

files to size• Line sizing can be done but we use in-house excel files to

size

Process Simulation Development Methodology

• Reservoir fluid behavior and fluid composition• GOR, inlet pressure and temperature at the well head• Formation of wax or water• Pressure profile• Client specification on product quality• Selection of simulator and thermodynamic method• Selection of margin• Process Flow Scheme

Process Simulation Report• Introduction

- Background & Objectives• Basis of Simulation Study

- Scope of the study & tools employed

- Reference documents- Product specifications- Test run review

• Results of the simulation study- Quality of test run

simulations- Alternative

thermodynamic results- Robustness of the design- Additional investigations

• Conclusion

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