Post on 09-Mar-2016
description
Aspen Plus Cogeneration Model
Aspen Plus
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Contents iii
Contents1 Introduction.........................................................................................................1
2 Components .........................................................................................................2
3 Process Description..............................................................................................3
4 Physical Properties...............................................................................................5
5 Chemical Reactions ..............................................................................................6
6 Simulation Approaches.........................................................................................7
7 Simulation Results ...............................................................................................8
8 Conclusions ........................................................................................................12
9 References .........................................................................................................13
1 Introduction 1
1 Introduction
This model simulates an integrated cogeneration process. It includes thefollowing features:
A set of conventional chemical species for this process.
Typical process areas including: burning, compression, heat exchange,power generation, and the main streams connecting these units.
Property methods and unit operation models used in this process.
2 2 Components
2 Components
The table below lists the components modeled in the simulation.
Component ID Type Component name Formula
H2O CONV WATER H2O
N2 CONV NITROGEN N2
O2 CONV OXYGEN O2
CO CONV CARBON-MONOXIDE CO
CO2 CONV CARBON-DIOXIDE CO2
ARGON CONV ARGON AR
METHANE CONV METHANE CH4
ETHANE CONV ETHANE C2H6
PROPANE CONV PROPANE C3H8
3 Process Description 3
3 Process Description
An outline of the cogeneration process which includes the letdown, GasTurbine and Steam Generation sections is shown in Figure 1.
Figure 1: Cogeneration Overall Process
The feedstock of this cogeneration process is natural gas, which containsMethane (83.62%wt), Ethane (7.33%wt), Propane (7.25%wt) and Argon(1.8%wt).
Firstly, a turbine is used in the letdown area to utilize the internal energy ofthe natural gas to generate electrical power. After expanding, the gaspressure drops from 19.5 bar to 8 bar while generating 0.60MW of power.
Secondly, mixed with steam (8 bar) and compressed air (1324000kg/hr), thegas is burned completely in the burner to produce hot gas at 979C. The hotgas is passed through a gas turbine to produce 103.4 MW of electrical power.As a result, its temperature drops to 551C and its pressure drops from 8 barto 1.1 bar.
Thirdly, the hot gas is passed to the steam generation area to recover heat.The gas runs through 5 heat exchangers and is cooled down by water orsteam as follows:
E100 - cooled from 551to 492C
E101 - cooled from 492 to 320C
HIERARCHY
GASTURB
HIERARCHY
LETDOWN
HIERARCHY
STMGENNATGAS2
AIR
NOXSTEAM
HOTGAS1
POWER2
NATGAS
POWER1
WATER1
WATER14POWER3X
STEAM-A
STEAM-B
STEAM-C
HOTGAS9
WATER24
RC
RC
W
MIXER
POWERMIXPOWEROUT
W
4 3 Process Description
E102 - cooled from 320 to 238C
E103 - cooled from 238 to 234C
E104 - cooled from 234 to 175C
Then the outlet stream HOTGAS6 from E104 is split into HOTGAS7A andHOTGAS7B. HOTGAS7A is cooled to 108C in E106 and HOTGAS7B is cooledto 131C in E105. Afterwards these two streams are mixed again and arevented out of the process. The BFW (boiler feed water) used in this areaincludes two pressure grades, one at 76.5 bar and the other at 6.9 bar.Heated by the hot gas, BFW turns to steam. Then the steam is let downthrough a turbine to produce electrical power. Finally, three steam products,each at different pressure grades, are obtained and 37.6MW of electricalpower is generated.
Process summaryArea Purpose
Let Down Uses the internal energy of the natural gas to generate electricalpower
Gas Turbine Burns the natural gas to generate electrical power using a gasturbine
Steam Generation Recovers the heat from the hot gas to generate steam andelectrical power using steam turbines
4 Physical Properties 5
4 Physical Properties
The PR-BM property method (Peng-Robinson equation of state with Boston-Mathias modifications) is used for the properties of the natural gas andcombustion products. For the steam system in the steam generation area theSTEAMNBS property method is used.
6 5 Chemical Reactions
5 Chemical Reactions
The only reactor unit in this process is the burner modeled with RGibbs whichuses the Gibbs free energy minimization method. This determines theequilibrium composition of the products resulting from the many reactionsthat can occur.
6 Simulation Approaches 7
6 Simulation Approaches
Unit Operations The major unit operations are represented by Aspen Plusmodels as shown in the following table:
Unit Operation Aspen Plus Model Comments / Specifications
Heat exchanger HeatX Simplified shortcut design calculations.
Flash Flash2 Rigorous simulation of gas-liquid equilibrium.
Compressor/Turbine Compr Calculates electric power required orproduced.
8 7 Simulation Results
7 Simulation Results
The Aspen Plus simulation flowsheet is shown in Figures 2, 3, and 4.
Figure 2: Flowsheet of Letdown area
NATGA SNATGA S(IN)
NATGAS2 NATGA S2(OUT)
POWER1 POWER1(OUT)
EXP1
7 Simulation Results 9
Figure 3: Flowsheet of Gas Turbine area
Figure 4: Flowsheet of Steam Generation area
No errors occur in the simulation. Key simulation results are shown in thefollowing tables:
Key Stream Simulation ResultsFlowsheet Variable Value Unit
NATGAS2NATGAS2(IN)
HOTGAS1
HOTGAS1(OUT)
POWER2 POWER2(OUT)
AIR1
AIR2
ACPOWER
NOXSTEAM MIXGASHOTGAS
POWER2A
A IRCOMP
MIX1 BURN1
EXP2
WORKMIX
HOTGAS1HOTGAS1(IN)
POWER3X
POWER3X(OUT)
STM6
HOTGAS2
STM7
WATER4HOTGAS3
STM5
WATER2
WATER3
HOTGAS4
STM19
STM20
HOTGAS5
HOTGAS6STM18
WATER4A
HOTGAS7B
HOTGAS8A HOTGAS8B
HOTGAS9
WATER1 WATER14
WATER15
WATER16
STM8
POWER3
STM9
STEAM-A(OUT)
STM10STM11
POWER4
STM21 STEAM-B(OUT)
STM22
STM12STM13
POWER5
STM23
STEAM-C(OUT)
WATER24
E100
E101
E102
E103
E104
V100
P101
SPLIT1
MIX1
E106 E105
V101
P103
K100
SPL102
K101
SPL103
MIX103
K102
V102
POWMIX
Water & Steam
Hot Gas
Power Generated
10 7 Simulation Results
Feed
NATGAS total 25000 kg/hr
NATGAS-Methane 20905 kg/hr
NATGAS-Ethane 1832.5 kg/hr
NATGAS-Propane 1812.5 kg/hr
NATGAS-Ar 450 kg/hr
Steam for Burner 45000 kg/hr
Boiler feed water (High Pressure) 180800 kg/hr
Boiler feed water (Low Pressure) 42600 kg/hr
Air for Burner 1324000 kg/hr
Product
Steam-A (24bar) 27120 kg/hr
Steam-B (5bar) 6390 kg/hr
Steam-C (1bar) 185659 kg/hr
Electrical Power 140189.6 kW
Waste
Water 4231 kg/hr
Exhaust Hot Gas 1394000 kg/hr
Key Process Simulation ResultsKey Process Variable Value Unit
Temperature of Burner 978 C
Pressure of Burner 8 bar
Discharge Pressure of the NATGAS Turbine 8 bar
Discharge Pressure of the HOTGAS Turbine 1.1 bar
Discharge Pressure of High Pressure SteamTurbine 24 bar
Discharge Pressure of Medium PressureSteam Turbine
5 bar
Discharge Pressure of Low Pressure SteamTurbine 1 bar
Heat Balance in Steam Generation AreaHeat Balance of Steam GenerationProcess Value Unit
Inlet Enthalpy of Hotgas(hotgas1) -309530 kW
Outlet Enthalpy of Hotgas(hotgas9) -495670 kW
Heat Energy Supply of Hotgas 186146 kW
Enthalpy of Inlet Water 1 -786876 kW
Enthalpy of Inlet Water 14 -185583 kW
Enthalpy of Outlet Water 24 -18290 kW
Enthalpy of Outlet Steam 9 -96704 kW
Enthalpy of Outlet Steam21 -23231 kW
Enthalpy of Outlet Steam 23 -686151 kW
Heat Energy Absorption of Water in total 148083 kW
Electrical Power Generated in STMGENProcess 36164 kW
7 Simulation Results 11
Steam and Power Generation per 1 kg of Natural GasProduct Name Product Quantity
Steam at 24bar pressure 1.085 kg
Steam at 5 bar pressure 0.256 kg
Steam at 1 bar pressure 7.426 kg
Electrical Power 20187 kJ
12 8 Conclusions
8 Conclusions
The Cogeneration model provides a useful description of the process. Thesimulation takes advantage of Aspen Pluss capabilities for modeling. Themodel may be used as a guide for understanding the process and theeconomics, and also as a starting point for more sophisticated models forplant design and process equipment specification and purchase.
9 References 13
9 References
1 V. I. Dlugoselskii, V. E. Belyaev, N. I. Mishustin and V. P. Rybakov, "Gas-turbine units for cogeneration", Thermal Engineering, 54:1000-1003,2007.
2 Ligang Zheng and Edward Furimsky, ASPEN simulation of cogenerationplants, Energy Conversion and Management, 44: 1845-1851, 2003
1 Introduction2 Components3 Process Description4 Physical Properties5 Chemical Reactions6 Simulation Approaches7 Simulation Results8 Conclusions9 References