Aspen Plus Cogeneration Model

Aspen Plus

Copyright 2008-2011 by Aspen Technology, Inc. All rights reserved.

Aspen Plus, Aspen Properties, the aspen leaf logo and Plantelligence and Enterprise Optimization are trademarksor registered trademarks of Aspen Technology, Inc., Burlington, MA.

All other brand and product names are trademarks or registered trademarks of their respective companies.

This document is intended as a guide to using AspenTech's software. This documentation contains AspenTechproprietary and confidential information and may not be disclosed, used, or copied without the prior consent ofAspenTech or as set forth in the applicable license agreement. Users are solely responsible for the proper use ofthe software and the application of the results obtained.

Although AspenTech has tested the software and reviewed the documentation, the sole warranty for the softwaremay be found in the applicable license agreement between AspenTech and the user. ASPENTECH MAKES NOWARRANTY OR REPRESENTATION, EITHER EXPRESSED OR IMPLIED, WITH RESPECT TO THIS DOCUMENTATION,ITS QUALITY, PERFORMANCE, MERCHANTABILITY, OR FITNESS FOR A PARTICULAR PURPOSE.

Aspen Technology, Inc.200 Wheeler RoadBurlington, MA 01803-5501USAPhone: (1) (781) 221-6400Toll Free: (1) (888) 996-7100URL: http://www.aspentech.com

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