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https://plus.google.com/communities/103583759337363889002 Steam Cycle Simulation – Aspen Plus v8.6
The attached gives steps to set up a simulation in Aspen Plus v8.6 to model a simple Rankine steam cycle for electricity production. The system consisting of: Fuel side with natural gas feed, air blower, combustion chamber, & fuel side of the steam boiler. Steam side with steam turbine, steam condenser, condensate pump, & steam side of the boiler. The simulation will be set up assuming isentropic steps for the rotating equipment. When the simulation is set up the overall PFD should look like the following figure.
Create new simulation file Start the program from Start , All Programs All Programs,, Aspen Tech, Tech, Process Modeling V8.6, V8.6, Aspen Plus, Plus, Aspen button. Choose the Gas Processing then Plus V8.6. V8.6. When the program opens choose the New button. Processing then the Gas Processing with Metric Units template. Units template. Click the Create button. Create button.
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Define the Components & the Property Models Specify components, Specify components, fluid fluid property property packages, packages, & crude oil oil assays assays
The first step is to define a set of pure chemical species to represent: Steam as modeled by pure water & using property correlations consistent with the ASME Steam Tables. The natural gas fuel, air, & combustion exhaust as pure light components modeled by the Peng‐Robinson equation of state (EOS). Now let’s add components to model the fuel side of the system. Go back to the Component Lists item Component Lists item & click on the Add the Add button button to create Component List Component List ‐ 2. We need components for the following: ‐ 2. Steam. For now we’ll model as pure water. Natural gas. For now let’s model this as a possible mixture of methane, ethane, & propane. Air. For now we’ll model this as a mixture of oxygen & nitrogen. Combustion gases. At the minimum we’ll also need carbon dioxide and water (which we also need for modeling the steam). However, we’ll also want to take into account incomplete combustion (forming carbon monoxide) as well as NOx formation (for now just as NO, NO 2, & N2O). Click the Find button button to bring up the databank search form. You can enter either the entire formula, part of a name, or several other possible search items to find all of the desired chemical species. When the proper compound is found, select it in the list & click Add selected compounds. The following figure shows a search for H 2O. As you are adding compounds you may be asked whether to add or replace the compound already in the list; choose the Add option. option.
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Define the Components & the Property Models Specify components, Specify components, fluid fluid property property packages, packages, & crude oil oil assays assays
The first step is to define a set of pure chemical species to represent: Steam as modeled by pure water & using property correlations consistent with the ASME Steam Tables. The natural gas fuel, air, & combustion exhaust as pure light components modeled by the Peng‐Robinson equation of state (EOS). Now let’s add components to model the fuel side of the system. Go back to the Component Lists item Component Lists item & click on the Add the Add button button to create Component List Component List ‐ 2. We need components for the following: ‐ 2. Steam. For now we’ll model as pure water. Natural gas. For now let’s model this as a possible mixture of methane, ethane, & propane. Air. For now we’ll model this as a mixture of oxygen & nitrogen. Combustion gases. At the minimum we’ll also need carbon dioxide and water (which we also need for modeling the steam). However, we’ll also want to take into account incomplete combustion (forming carbon monoxide) as well as NOx formation (for now just as NO, NO 2, & N2O). Click the Find button button to bring up the databank search form. You can enter either the entire formula, part of a name, or several other possible search items to find all of the desired chemical species. When the proper compound is found, select it in the list & click Add selected compounds. The following figure shows a search for H 2O. As you are adding compounds you may be asked whether to add or replace the compound already in the list; choose the Add option. option.
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Below is an example of components retrieved from the Aspen databanks. There are two issues with default manner in which this list is presented. One, the Component ID Component IDss are not very descriptive of the compound (especially as compared to the Alias the Alias values). values). Two, the order does not group the compounds in a convenient manner. We can address both of these issues before proceeding much further.
Let’s change the Component ID values to mostly match the Alias values. Select the Component ID value either by double‐clicking on it or by clicking & then pressing the F2 key. Once selected, type in the new ID & press the Enter key. key. Aspen Plus will ask what you really want to do by making this change; click the Rename button. Rename button. Change all IDs. Rev 0.0
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Now press the Reorder button. A form pops up that will allow you to move selected compounds up or down so that they in a convenient order. Press Close when done.
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The next step is to assure that an appropriate fluid property package has been chosen for these compounds. Click on Methods in the All Items list on the left. From here we see that the Peng‐ Robinson EOS has been specified as the base method (per the choice of template originally chose). Also the ASME steam table option has been specified for cases when only water is present in the stream. These are the desired options so we can continue on.
Now is a good time to save the file before we start setting up the process simulation. Click the File tab & then the Save As item. Choose the Aspen Plus Backup option.
Set up & Solve the Flowsheet Working Units Activate the Simulation option. Note that you’ll see a blank flowsheet. We would like to show the calculations with a modified set of SI units, in particular: Temperature as °C. Pressure as bar (absolute). Mass flow as kg/sec. Molar flow as kg.mol/sec. Heat duty as kJ/sec. Power as kW.
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Under the Home tab click the Unit Sets button. In the list of unit sets click on the row for SI ‐CBAR & press Edit . Proceeding through the various tabs allows you to determine what will be used for the display of the results as well as the default units for the input. Most of the units are what we desire, but not all. For example, you can see that Mass Flow will be reported in kg/hr, not quite what we want.
Let’s pull down the lists associated for the Flow related values & pick options that are in terms of seconds, not hours. Go into the Heat tab. Change Heat related values from J to kJ and power related values from W to kW.
Steam Cycle We will want to create a simple Rankine cycle with the following process conditions: Saturated steam production at 125 bar. Final condensation to 20°C. Steam turbine operating at ideal reversible conditions. Condensate pump operating at ideal reversible conditions. No extra pressure drop through heat exchangers or piping.
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Let’s place the following units from the Model Palette to the flowsheet 1: 2 Heater s, a turbine (as Pressure Changers, Compr , ICON3), & Pump. Ultimately it will be depicted as follows (with rotation of the pump icon).
Connect the units with the following streams: In the Model Palette click on the Material stream button. Draw as follows: Draw a stream into the red arrow of PUMP; call it CONDNSAT. o Draw a stream from the red arrow out of PUMP & into the red arrow of BOILER; call it o HP‐STEAM. Draw a stream from the red arrow out of BOILER & into the red arrow of STMTURBN; o call it AIR‐2. Draw a stream from the RED arrow out of STMTURBN & into the red arrow of o CONDNSR; call it EXHAUST. Draw a stream from the red arrow out of CONDNSR; call it COND‐2. o In the Model Palette click on the Heat stream button. Draw as follows: Draw a stream out of the blue arrow of BOILER; call it Q‐BOILER. o Draw a stream out of the blue arrow of CONDNSR; call it Q‐ CONDSR. o In the Model Palette click on the Work stream button. Draw as follows: Draw a stream out of the blue arrow of PUMP; call it W‐PUMP. o Draw a stream out of the blue arrow of STMTURBN; call it W‐TURBN . o Let’s start to initialize the water circulating through the steam loop.
1 If
the Model Palette is not visible choose the View tab & click on the Model Palette button or press the F10 key.
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Double‐click on the CONDNSAT stream. Select Temperature & Vapor Fraction for the Flash Type. Enter 20 C for the Temperature & 0 for the Vapor Fraction (i.e., a saturated liquid). Specify the Composition as pure water; enter a 1 for H2O in the list with the Mole‐Frac option. Let’s use a flow basis of 1 kg/s.
Now let’s set the operating parameters for the various units. Double click on the PUMP icon to open the input sheet. Make sure the Pump option is specified under Model . Specify the Discharge Pressure as 125 bar. Finally, to define this as an ideal pump specify a 1 for both the Pump & Driver Efficiencies.
Now let’s define the operating conditions for the steam side of the boiler. Double click on the BOILER icon to open its input form. We want to specify the outlet steam as saturated vapor. We could specify the vapor fraction. Instead we’ll define 0°C of superheat. We also want to specify a zero pressure drop. We can do this by specifying a zero value for the pressure.
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Now let’s define operating conditions for the turbine. Double click on the STMTRBN icon to open the input sheet. Make sure the Turbine option is specified under Model . We want to define this as an ideal turbine so specify a 1 for both the Isentropic & Mechanical Efficiencies. We need to specify something about the Discharge Pressure even though we don’t really know what it is, only that is corresponds to the water vapor pressure at 20°C. For now specify a value of 0.1 bar; we’ll fix it later. We know that this is a condensing steam turbine. The default for the Turbine model is that only vapor will exit, so this will have to be changed. Click on the Convergence tab. Pull down the list for Valid phases & change to Vapor ‐Liquid ‐ FreeWater .
Now let’s define the operating conditions for the steam condenser. Double click on the exchanger icon to open its input form. We want to specify the outlet as saturated liquid. We want to make sure that one of the Flash Type options is Vapor fraction & set the appropriate value as 0 (i.e., saturated liquid). We also want to specify a zero pressure drop (i.e., let the discharge pressure setting from the turbine control this). Make sure that one of the Flash Type options is Pressure & set the appropriate value as 0 (i.e., zero pressure drop). We now have enough settings to be able to run the simulation. Open the control panel (item under the Home tab) & press Run. Some warnings may come up but they will be addressed later. We can summarize the results on the flowsheet by modifying the Stream Results settings. Select the Main Flowsheet . Select the Modify tab & select the Temperature, Pressure, & Vapor Fraction items. Click in the lower left‐hand corner of the Stream Results section. On the pop‐up form select the
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Heat/Work item. Also, change the format for the Pressure value to show three decimal places (i.e., as “%.3f”). Do the same for the Vapor fraction format. Click OK .
We can now see a summary of the results on the flowsheet below. One thing to note is that our guess for the turbine’s discharge pressure was in error. The pressure should actually be 0.019 bar to correspond with a condenser outlet temperature of 20°C. We could go back and change the value manually. However, we’ll use one of Aspen Plus’s operations to automatically set it to match the condenser outlet.
We will use the CALCULATOR operation to “feed forward” the Condenser’s pressure to the turbine’s discharge pressure. Even though we could use a calculator without seeing any indication on the flowsheet we’ll instead put an icon on the flowsheet to give an indication that it is there.
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From the Model Palette, choose Manipulators, Calculator , & select ICON1. Place it on the flowsheet near the exhaust stream. (For the flowsheet shown it is rotated vertically so it can “hang” below the line.) Rename it SET‐P. Double‐click on the icon to open up its input form. Define the variable PCNDSR as the pressure calculated for stream CONDNSAT (i.e., the saturation pressure at 20°C). Specify this as an Import variable (i.e., the value has to be calculated by Aspen Plus & will be “read” by the calculator operation).
Now define the variable PTURBN as the steam turbine’s discharge pressure. Specify this as an Export variable (i.e., the value be “written” as a parameter in a downstream operation).
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Finally, define the relationship in the Calculate tab. Using the Fortran method enter the statement “PTURBN = PCNDSR” (starting in column 6).
Now we can rerun the simulation and get the results summarized below. We can see that the CALCULATOR has done its job; the outlet pressure from the steam turbine is now the same as the inlet to the condenser. However, the temperatures are different. What’s wrong?
The problem is that most of the calculations are done with the default PENG‐ROB properties, not the desired STEAM ‐TA properties. The exception is the outlet of the turbine which recognizes the liquid formed as FreeWater & uses the STEAM ‐TA option for its properties. Hence, the inconsistency. We can force the units to use the STEAM ‐TA option to do the calculations by making modifications to each unit’s Block Options settings. In the Simulation tree structure for each operation select Block
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Options. Under the pull‐down list for Property method select STEAM ‐TA. The form for STMTRBN is shown as an example.
Now when we rerun the simulation we get consistent results. Notice that there are subtle changes to the heat & work streams. For example, the boiler heat is now 2,581 kJ/sec; it was 2,808 kJ/sec when calculated by the PENG‐ROB method (a difference of 9%).
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In preparation for additional changes we need to modify the settings for STMTRBN. On its input form click on the Convergence tab. Change the Valid phases to Vapor‐Liquid‐Liquid. When we rerun we get a minor warning that the outlet is below its dew point (which we already know since this is a condensing turbine).
When dealing with the positive & negative values for the heat & work streams remember the two conventions used by Aspen Plus: If the heat or power stream is an outlet of a unit then Aspen Plus has calculated the value to make other operating specifications (such as the outlet temperature in an exchanger). If it is an inlet to a unit then Aspen Plus uses the value to determine the outlet conditions. Heat represents energy to or from the unit operation; it is in the direction of the arrow if the heat is positive or in the opposite direction if it is negative. Work, on the other hand, represents energy to or from the universe; the energy flow is in the opposite direction as that for heat. Since Q‐BOILER is negative for a heat stream pointing away from the BOILER, then the energy flows into the boiler’s fluid. Since W‐TURBN is negative for a work stream pointing away from the STMTURBN, then the energy flows out of the turbine’s fluid. From the results shown we can calculate the thermal efficiency of this steam cycle. We should always make use of the absolute values for the heat & work streams. For this steam cycle:
th
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W‐TURBN W‐PUMP
Q‐BOILER
1078 13 2581
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Fuel & Combustion System We will want to create a simple natural gas burner/boiler with the following process conditions: Natural gas is available at industrial delivery pressure, 20 bar‐g & 15°C. We will characterize the natural gas as 100% methane. Air is available at 25°C. We will characterize the air as a 21/79 O 2/N2 molar mixture and bone dry (i.e., no water). We want to add enough air so that there is 20% excess oxygen based on complete combustion of the natural gas. The combustion process occurs near atmospheric conditions so the natural gas must be let down in pressure. However, a blower is needed to push the air into the combustion chamber. The pressure drop through the burner/boiler/flue combination is 0.3 bar. The flue gas is emitted at 120°C to prevent any liquid dropout & subsequent corrosion problems. Let’s place the following units from the Model Palette to the flowsheet: Valve, Compressor 2, RGibbs Reactor, & Heater 3. Ultimately it will be depicted as follows. (We’ll discuss the SET calculators as we go.)
Connect the units with the following streams: In the Model Palette click on the Material stream button. Draw as follows: Draw a stream into the blue arrow of the LETDOWN valve; call it FUELGAS. o Draw a stream from the blue arrow out of the LETDOWN valve & into the blue arrow of o the COMBSTN reactor; call it LP‐GAS. Draw a stream into the blue arrow of the AIRBLWR compressor; call it AIR. o Draw a stream from the blue arrow out of the AIRBLWR compressor & into the blue o arrow of the COMBSTN reactor; call it AIR‐2. Draw a stream from the blue arrow out of the COMBSTN reactor & into the blue arrow o of the HRSG exchanger; call it COMBGAS. Draw a stream from the red arrow out of the HRSG exchanger; call it FLUEGAS. o In the Model Palette click on the Heat stream button. Draw as follows:
2 Note
that the compressor has been rotated vertically to get the inlet stream below the compressor & the outlet stream above. 3 Note that the heat exchanger has been rotated vertically to get the heat stream below the exchanger.
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Draw a stream out of the blue arrow of the HRSG exchanger; call it Q‐HRSG. In the Model Palette click on the Work stream button. Draw as follows: Draw a stream out of the blue arrow of the AIRBLWR compressor; call it W‐BLOWER. o o
Let’s start setting parameters for the inlet streams. Let’s initialize the natural gas stream first. Double‐click on the FUELGAS stream. Select Temperature & Pressure for the Flash Type. Enter 15 C for the Temperature & 20 barg for the Pressure. Enter 1 for the C1 value as Mole‐ Frac. Let’s use a flow basis of 1 kg.mol/sec. Now let’s initialize the AIR stream. Double‐click on the AIR stream. Select Temperature & Pressure for the Flash Type. Enter 25 C for the Temperature & 0 barg for the Pressure. Enter 0.21 for the O2 & 0.79 for the N2 values as Mole‐Frac. As a starting point let’s define the flowrate as 12 kg.mol/hr. Let’s specify the outlet pressure of 0.3 bar‐g after the let‐down valve. Double‐click on LETDOWN. Specify 0.3 barg as the Outlet pressure.
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We want to make the air blower an ideal reversible compressor. Double‐click on AIRBLWR. Select the Compressor as Model . Pull down the Type list & choose Isentropic. Specify 1 for the Isentropic & Mechanical Efficiencies.
Now it’s time to model the combustion portion of the fuel gas burner. There are various options for doing this. One of the simplest (and would normally be done for hand calculations) would be to define all combustion reactions & specify the extent of conversion for each. Instead, we’re going to take advantage of the full thermodynamic capabilities of Aspen Plus & use a reactor that will minimize the Gibb’s free energy. All we have to do is list the expected products & Aspen Plus will calculate the resulting product distribution that honors the material & energy balances as well as any chemical equilibrium limitations. Double click on the RGibbs Reactor icon. Set Pressure to 0 (to represent a zero pressure drop) & specify 0 for the Heat Duty (to signify adiabatic operation). That’s pretty much it. The default is to include all species in the component list as potential products.
Now let’s see how much heat can be transferred out of the combustion gases by specifying the combustion gas side of the boiler. Double click on the heater icon. Set the conditions to the outlet conditions out the stack: 120 C & 0 barg.
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We haven’t addressed the calculator operations yet but we can still run the simulation. The results are summarized on the Flowsheet show that the combustion temperature will be 1734°C. We can double‐click on the COMBGAS stream & see that there will be some CO & NOx formed at these conditions.
There are still a couple items to be done to “clean up” the simulation & format of the results. The first is for a matter of convenience – how should we specify the pressure of the AIR‐ 2 stream out of the air blower? Right now the pressure into the COMBSTN operation is set separately for the two inlet streams (LP‐GAS & AIR‐2). If a study was to be performed & the pressure were to change then having the specifications in two separate locations could lead to them being changed differently. It sure would be nice to set it only in one location & then have the other location update automatically. We can do this with a CALCULATOR operation. From the Model Palette, choose Manipulators, Calculator , & select ICON1. Place it on the flowsheet near the AIR‐2 stream. (For the flowsheet shown it is rotated to the left so it can “hang” off the line.) Rename it SET‐AP.
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Double‐click on the icon to open up its input form. Define the variable PFUEL as the pressure for stream LP‐GAS (i.e., the pressure out of the let‐down valve). Specify this as an Import variable (i.e., the value has to be calculated by Aspen Plus & will be “read” by the calculator operation).
Now define the variable PAIR as the air blower’s discharge pressure. Specify this as an Export variable (i.e., the value be “written” as a parameter in a downstream operation).
Finally, define the relationship in the Calculate tab. Using the Fortran method enter the statement “PAIR =PFUEL” (starting in column 6).
The second change involves a convenient way to make sure that the correct amount of air is added to match the “excess oxygen” spec. The amount of stoichiometric oxygen is determined from the combustion reactions. For methane, ethane, & propane the reactions are, respectively: Rev 0.0
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CH4 + 2 O2 CO2 + 2 H2O C2H6 + 3.5 O 2 2 CO2 + 3 H2O C3H8 + 5 O2 3 CO2 + 4 H2O This shows that we need to know the composition of the fuel gas (in molar amounts) to determine the stoichiometric amount of oxygen needed. The “excess” part is additional oxygen (as a multiplier) that is added. The final consideration is that the specification in Aspen Plus is not just for the rate of oxygen but rather of the air; so we have to take into account the composition of the air account for the large amount of nitrogen also be introduced into the COMBSTN operation. Since we have set the composition of the fuel gas to be pure methane & the basis flow rate to 1 kg.mol/sec then the stoichiometric oxygen flowrate is twice this, 2 kg.mol/sec. We also need to increase this by 20% to include the desired excess. And we need to take into account the oxygen content in the air to determine the air rate. So overall:
nair
n O2
stoich
1 f excess
y O2
2 1 0.2 0.21
11.43 kg.mol/sec .
We could do these calculations prior to running Aspen Plus and enter the air rate. Or we could do the calculations within Aspen Plus. From the Model Palette, choose Manipulators, Calculator , & select ICON1. Place it on the flowsheet near the AIR stream. (For the flowsheet shown it is rotated to the right so it can “hang” off the line.) Rename it SET‐AFLO. Double‐click on the icon to open up its input form. Define the variable AIRFLO as the molar flow of the stream AIR (i.e., the pressure out of the let‐down valve). Specify this as an Export variable.
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Now let’s start defining Import variables. First define the variable YO2 as the O2 mole fraction in the air. Specify this as an Import variable.
Next let’s define Import variables for the combustible portions of the fuel gas (even though we’ve only used methane we have included the possibility for ethane & propane, too). Define the variables C1FLO, C2FLO, & C3FLO. Make sure these are specified as Import variables.
Finally, define the relationship in the Calculate tab. Using the Fortran method enter the statement: AIRFLO = 2. * C1FLO + 3.5 * C2FLO + 5. * C3FLO AIRFLO = AIRFLO * (1. + 0.2) AIRFLO = AIRFLO / YO2 (starting in column 6).
The simulation can be rerun giving the results summarized below. Note that correct air flow has been calculated, 11.43 kg.mol/sec. Rev 0.0
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One more modification, that to directly show the mole fractions of all of the streams (since right now the results are only shown as molar flows). Expand the Setup options in the left‐hand Simulation tree structure. Select Report Options. Note that Mole Flow basis option is specified but none of the Fraction basis options. Select the Mole option.
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Rerun the simulation. Now when you double‐click on the COMBGAS stream you will not only see the composition out of COMBSTN in molar flows but also as mole fractions.
Tying the Two Systems Together Even though the steam cycle & fuel gas systems are in the same Aspen Plus flowsheet they are really modeled separately. The steam cycle has converged with a basis of 1 kg/sec water circulation rate & the fuel system has converged with a basis of 1 kg.mol/sec fuel gas. We will tie the systems together by “pushing” the duty from the fuel side of the boiler to the steam side & adjusting the water circulation rate in the steam cycle to ensure this is the only heat needed for the steam cycle. Now let’s connect the two systems. Rename the stream Q‐BOILER to Q‐RESID (for “residual”). Rename the stream Q‐HRSG to Q‐BOILER. Right‐click on Q‐BOILER, select Reconnect , Reconnect Destination, & attach to blue inlet arrow on the BOILER exchanger.
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Run the simulation. Notice that for the combination of fuel gas rate & water circulation rate there is too much generated from the combustion side of the boiler to be absorbed by the steam. We can see this because the “residual” heat from the steam side, Q‐RESID, is 763,426 kJ/sec. Since 766,007 kJ/sec was generated from the combustion side then only 2,581 kJ/sec was needed in the steam side.
The results show that we really need 296.8 kg/sec water circulating in the steam cycle to absorb all of the heat from the combustion side. We could enter this value manually but then we would have to do the hand calculation over again if any conditions were to change. Instead we will let Aspen Plus calculate the proper flowrate. From the Model Palette select a Design Spec (the one shown in the PFD is the Design option & rotated to the right). Change the name to ADJ‐WFLO. Double‐click on ADJ‐WFLO to get its input forms. Create a variable RESIDUAL to represent the residual heat around the boiler (i.e., the difference between the heat generated on the combustion side & the heat needed on the steam side).
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Select the Spec tab. Designate the RESIDUAL variable as the Spec variable, set its Target value to 0 (I.E., to match up the combustion side & steam side requirements). Set its convergence Tolerance to 0.5. Select the Vary tab. To adjust the mass flow of the CONDENSAT stream first choose Stream‐Var as the Type 4. Let’s assume that the Upper limit is less than 500 kg/sec; we’ll set the Lower limit as 0.1 (a slightly positive number). Set the Step size as 0.01 & the Maximum step size as 0.1.
We can look at the results & see that the anticipated water circulation rate has been found, 296.8 kg/sec.
4 Do
not choose Mass‐Flow as the Type; this will point to the flow of an individual component, not the entire stream.
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Additional Stream & Unit Analyses There are additional analyses that we may want to perform for this simulation. Since the goal of the process is to create power we should be very interested to determine the various thermal efficiencies of the systems. To calculate the efficiency of the boiler we need to determine the heating value of the fuel gas used. To do this we will make use of the built‐in net & gross heating values (lower & higher, respectively). Expand the Setup item in the left‐hand tree structure of the Simulation items. Under Property Sets create a New set called HEATVALS. Edit that property set & add the properties QVALNET & QVALGRS.
Next we want to add these properties to the simulation report. Under Setup in the left‐hand tree structure choose Report Options. Go to the Stream tab & click on Property Sets.
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Select HEATVALS in the Available property sets list & press >. This will move HEATVALS to the Selected property sets list. Click on Close.
Now we can rerun the simulation. Now when we look at the Results for a stream we will see the net & gross heating values at the bottom of the list.
We can now start to calculate various efficiencies for the combined fuel/steam system. Boiler efficiency. This will be the amount of heat that is transferred out of the combustion section of the system into the steam system. This can be based either on the lower (net) heating value but more normally on the higher (gross) heating value:
HHV
HHV m
766,007 kJ/sec 0.8601 . 55515.1 kJ/kg 16.043 kg/sec
Wturbine W pump Qboiler
319,838 kW 3,716 kW 0.4127 . 766,007 kJ/sec
Overall efficiency. This is normally calculated as the product of the combustion side’s efficiency & the steam cycle’s efficiency: H HV t h
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Steam cycle thermal efficiency. This has already been calculated as the ratio of the net work produced by the steam cycle to the boiler heat in:
th
Qboiler
0.86010.4127 0.3550 .
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However, this does not take into account the energy needed to run the air blower. Instead, we should use the ratio of the net work produced to the entering heating value (again, in terms of HHV):
total,HHV
Wturbine Wpump
W blower
HHV m
319,838 kW 3,716 kW 7622 kW 0.3464 . 55515.1 kJ/kg 16.043 kg/sec
Let’s set up an Excel spreadsheet to do these calculations. You can start with a spreadsheet with labels that look like below. Note that values that will be determined from the Aspen Plus simulation (either as an input or a calculated value) are in a blue font & will have a light green background.
The information we want to put into this table & use for calculations will come from stream results (Material, Heat, & Work) as well as equipment information (i.e., model results). We could copy & paste individual data values between the Aspen Plus simulation and the spreadsheet; it is more flexible to copy entire tables of results to the spreadsheet & then pick out the values desired. Perform the following steps: In your spreadsheet create three news tabs & call them Material Table, Heat Table, & Work Table. In your Aspen Plus simulation select the Streams option under Results Summary in the left‐ hand tree structure. The default shows the Material tab selected. Click the Copy All button. Go to the Material Table tab in your spreadsheet & select cell A1. Right‐click & select Paste. You may want to adjust column widths so you can more readily read all of the values.
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In your Aspen Plus simulation select the Heat tab. Select the square in the upper left part of the table & click (you should see the entire table highlighted). Right‐click this upper left square of the table & select Copy . Go to the Heat Table tab in your spreadsheet & select cell A1. Right‐click & select Paste. You may want to adjust column widths so you can more readily read all of the values.
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In your Aspen Plus simulation select the Work tab. Select the square in the upper left part of the table & click (you should see the entire table highlighted). Right‐click this upper left square of the table & select Copy . Go to the Heat Table tab in your spreadsheet & select cell A1. Right‐click & select Paste. You may want to adjust column widths so you can more readily read all of the values.
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January 1, 2015
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In your Aspen Plus simulation select the Models option under Results Summary in the left‐ hand tree structure. The default shows a summary report with the Heater tab selected. Click the Send to Excel button. Use the default form of One table per Excel worksheet . Select the option to Add tables to existing workbook ; click the Browse button & find the spreadsheet that you’ve created. Click on the Export tables to Excel button. When done click OK for Open Excel File. You should see tabs for the various types of equipment in your simulation.
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January 1, 2015
Now that we have the results in the spreadsheet let’s start to connecting the cell values in the Summary page. Many of the values can be referenced to a single cell, e.g., the mass flow rate of the fuel gas as “='Material Table'!I6”, the steam turbine power as “='Work Table'!D2”, or the steam turbine mechanical efficiency as “=Compr!E17”. The total molar flow rate of the fuel gas is a little more complicated since the total value is not reported in the material table; it can be determined as the sum of all the molar flow rates of the individual components, “=SUM('Material Table'!I11:I21)”. Note that even though the units on the values could be extracted from the row description in column A of the sheets it is easier to enter them as text values.
Some additional cleanup: It is convenient to format the numbers larger than 1,000 to a number with no decimal places & comma separators. The signs on the heat & work terms are dependent on whether the values are transferring in or out of a particular unit. Only the absolute values should be reported here (important here only for the power term associated with the steam turbine).
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January 1, 2015
Now we want to add formulas to calculate the efficiency values: Cell E2, “=B3 * B4” Cell E3, “=B3 * B5” Cell H2, “=E4 / E2” Cell H3, “=E4 / E3” Cell H5, “=(E8 ‐ E7) / E4” Cell H7, “=(E8 ‐ E7 ‐ E6) / E2” Cell H8, “=(E8 ‐ E7 ‐ E6) / E3”
We now have a spreadsheet created with a fairly flexible format that allows us to calculate new efficiencies for modifications to the Aspen Plus simulation. All we would have to do is copy in the new stream tables & model results. For example, we can get derive new efficiency values for the following changes in operating parameters: Pressure drop through the fuel gas system is 0.2 bar (not 0.3 bar). The isentropic efficiencies of all rotating equipment is 85% (not 100%) & the mechanical efficiencies are 95% (not 100%). 150°C of superheat supplied to the steam.
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January 1, 2015
