Simple Dew Point Control HYSYS v8.6

Transcription

1 Simple Dew Point Control HYSYS v8.6 Steps to set up a simulation in HYSYS v8.6 to model a simple dew point control system consisting of: Gas chiller Flash separator Liquid stabilizer with gas recycle & compression Product gas compression Simple propane refrigeration loop When the simulation is set up the overall PFD should look like the following figure. Basis A gas plant is processing 100 MMscfd (dry basis) to produce a spec pipeline gas as well as a pipeline raw mix liquid product. The following are known conditions for the feedstock and specification for the products: The composition of the feed gas is shown in the following table. The gas enters the plant at 400 psia & 120 F. The gas has a small amount of water, 2 lb water per MMscf dry gas. The produced pipeline gas should have a gross heating value between 905 to 1050 Btu/scf 1 & a hydrocarbon dew point no higher than 15 F. The produced pipeline gas should be delivered to the pipeline at 1000 psia and no higher than 120 F. The produced liquids shall be exported via pipeline & stabilized to have a TVP (true vapor 100 F no greater than 103 psia. A simple propane loop will be used to provide the chilling duty. The condenser will operate at 120 F. The minimum approach temperature within the chiller will be 10 F. 1 If the gross heating value spec cannot be achieved set the chilled separator to the lowest reasonable temperature when using a simple propane chilling loop, -30 F. Rev March 13, 2016

2 Air coolers will be used to cool gases & liquids to 120 F. Component Mol% N CO C C C i-c n-c i-c n-c n-c n-c n-c n-c Create new simulation file Start the program from Start, All Programs, Aspen Tech, Process Modeling V8.6, Aspen HYSYS, Aspen HYSYS V8.6. When the program opens choose the New button. Define the Components & the Property Models Specify components, fluid property packages, & crude oil assays The first step is to add a set of pure chemical species to represent the gas & water phases. With Component Lists highlighted click on the Add button. From the list of pure components pick: H2O, Nitrogen, CO2, Methane, Ethane, Propane, i-butane, n-butane, i-pentane, n-pentane, n-hexane, n-heptane, n-octane, & n-nonane. Rev March 13, 2016

3 The next step is to pick a fluid property package. From the Fluid Packages screen click the Add button. Choose the Peng-Robinson option and make sure it is associated with Component List 1. It would be a good idea to save this file. Click the File tab & select Save As. Choose an appropriate name & location. Set up & Solve the Flowsheet Gas Chilling & Separation When you activate the Simulation & you ll see a blank flowsheet. We will want to create a feed stream & attach it to an LNG Exchanger. The outlet will be attached to a flash separator. Rev March 13, 2016

4 Ensure that the model Palette is visible. If it is not, press the View tab & click Model Palette. Place the following items on the flowsheet: A Material Stream, Dry Feed A Material Stream, Feed Water A Mixer, Combine An LNG Exchanger, Chiller A 3-Phase Separator, DPC Separator. Double-click on the Dry Feed stream to open up the entry forms for this stream. Enter the 100 MMscfd flowrate in the Molar Flow box. Note that once you enter the flowrate it changes the units to lbmole/hr. Now we need to specify the composition. Select Composition under Worksheet in the left-hand column. Click the Edit button to bring up a form to enter the composition of this stream. Enter the values from the table in the Basis section as Mole Fractions. Note that these add up to approximately 100, not 1. Select the Normalize button. Click OK. Now you should see that the form associated with the stream is in green, meaning that all values for the stream have been calculated. Rev March 13, 2016

5 We want to do the same thing for the water portion of the feed represented by the stream Feed Water. Double-click on the Feed Water stream to open up the entry forms for this stream. Enter Rev March 13, 2016

6 200 lb/hr in the Mass Flow box (to represent the 2 lb/mmscf water content). Enter the pressure but do not enter the temperature. Select Composition under Worksheet in the left-hand column. Click the Edit button to bring up a form to enter the composition of this stream. Enter a 1 for the H2O mole fraction. Select the Normalize button. Click OK. Now you should see that the form associated with the stream is still yellow because the temperature has not been specified. That is OK, we re going to back-calculate the final condition so that the total feed gas is 120 F. Rev March 13, 2016

7 For most of the unit operations we ll define connections & create new streams using the operation s Design form. Double-click on Mixer. Define the 2 Inlets as Dry Feed & Feed Water. Define a new Outlet stream as Total Feed. Select the Worksheet tab. Note that the flowrate & pressure of the Total Feed stream are calculated. But we still have to specify some type of temperature information to fully calculate Total Feed. Specify the temperature as 120 F. Note that not only have all properties been calculated for Total Feed but also the final conditions for Feed Water have been determined. A Mixer is an isenthalpic operation, so the enthalpy for Feed Water (and hence its temperature & quality) became specified once we fully specified Total Feed. Rev March 13, 2016

8 We now want to model the gas side of the Chiller. We could use a Cooler operation, but since we ll ultimately want to calculate approach temperatures between the gas & the propane in the chilling loop an LNG Exchanger is more appropriate. Double-click on Chiller. Specify the 1 st Inlet Stream as Total Feed & define the Outlet Stream as Chilled Gas. For now specify the Pressure Drop as 0. Make sure that specification for Hot/Cold is Hot. Rev March 13, 2016

9 We now want to specify the cold separator & determine the properties of the produced gas. Doubleclick on DPC Separator. Specify the Inlet as Chilled Gas. Create new streams, Cold Vapor, Cold Liquid, & Cold Water as the Vapour, Light Liquid, & Heavy Liquid, respectively. Let s estimate the needed temperature for the cold separator Click on the Worksheet tab & specify -10 F for the temperature of Chilled Gas. Notice that all values are calculated for Chilled Gas, Cold Liquid, & Cold Vapor. Rev March 13, 2016

10 Is this temperature for the cold separator appropriate to make spec pipeline gas? The primary variable that we can control with the temperature is the dew point of the produced gas at the pipeline conditions. We ll look at the P-T diagram for Cold Vapor to get an indication of whether we ve come close to the dew point spec. Up in the ribbon under the Home tab, click on Stream Analysis & choose Envelope. In the pop-up form choose Cold Vapor as the Object & click OK. Rev March 13, 2016

11 The results in the Design tab show that the Cricondentherm is F, much colder than it needs to be to meet the 15 F pipeline dew point spec. We could reduce Chiller duty (and ultimately power required for the propane cooling loop) by allowing this temperature to be higher. Note from the PT diagram that the dew point at the pipeline inlet pressure, 1000 psia, is about -10 F, less than the cricondentherm. However, since the gas in the pipeline will experience pressures lower than the inlet s 1000 psia, it is more appropriate to use the cricondentherm as the controlling value for this spec. For now we ll use trial-and-error to determine an appropriate temperature for the cold separator. Note that if we specify the temperature of Chilled Gas as 9.5 F we get a cricondentherm of Cold Vapor of just over 15 F. Rev March 13, 2016

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13 Have we met the heating value spec? We can determine this from additional properties calculated for Cold Vapor. Double-click on Cold Vapor & select Properties under the Worksheet in the lefthand column. Notice that an HHV has been calculated of 1,175 Btu/scf. This is too high & will require more heavy hydrocarbons be removed. But before we modify the cold separator s operation we will add the liquid stabilizer section. Liquid Stabilization The next step is to determine if the produced liquid will make the TVP spec of 103 psia. Doubleclick on Cold Liquid & select Properties under the Worksheet heading in the left-hand column. At the bottom of the list there is an item for True VP at 37.8 C [psia]. The value is psia, much higher than our spec. We can look at the composition to see the problem it has 16% methane. This is much too high to try to have in a raw mix NGL. Rev March 13, 2016

14 We can process the high-pressure liquid in a low pressure stripping column to remove these light ends. Let s add two more units: A Control Valve, VLV-001 A Reboiled Absorber, Stabilizer. Double-click on VLV-100. Specify the Inlet as Cold Liquid and define a new stream Flashed Liquid as the Outlet. Let s define the stabilizing column as a 10-stage column with a kettle reboiler. Double-click on Stabilizer. Set the Top Stage Inlet feed as Flashed Liquid. Define new streams Recovered Gas for the Ovhd Vapour Outlet and Stabilized Liquid for the Bottoms Liquid Outlet. Define the stream Q-Reboiler for the Reboiler Energy Stream. Set the # Stages as 10. Press the Next> button to continue the definition for this tower. Rev March 13, 2016

15 Accept the default Once-Through reboiler configuration. This will model a kettle reboiler. Press the Next > button to continue the tower s definition. Let s look running the tower at 200 psia. Specify 200 for both Top Stage Pressure & the Reboiler Pressure. Press the Next > button to continue the tower s definition. We re able to specify temperatures on this next form. Ultimately we will want to run the reboiler in such as way as to produce a liquid with a 103 psia vapor pressure at 100 F. If we were running the tower at 103 psia then we could set the reboiler temperature as 100 F. However, since we re running the tower at a higher pressure the reboiler temperature must be higher; for now let s set an estimate of 200 F. Press the Next > button to continue the tower s definition. Rev March 13, 2016

16 We will not have to specify a boil-up ratio since we re going to use a TVP spec on the reboiler. Leave this blank & press Done The tower does not run automatically because the specifications have not been fully defined. Select Specs Summary item in the left-hand column. Notice that the default spec on the column is to produce an overhead product rate (whose value has not been specified). But this is not how we want to run this column. Before we enter the true spec click on the Active box for Ovhd Prod Rate to turn it off. Rev March 13, 2016

17 Let s add the reboiler temperature as the operating spec. Select Specs item in the left-hand column. Press the Add button for column specifications. On the list select Column Temperature & press Add Spec(s) Select Reboiler as the Stage & enter 200 for the Spec Value. Close this form. Even though we have fully specified the tower the feed coming from VLV-100 has not been fully specified, so the tower will not run. Go to the Worksheet tab and enter 200 for the pressure of Flashed Liquid. Now that this feed is fully specified the tower will quickly calculate & converge. Rev March 13, 2016

18 How close are we to creating a stabilized liquid with the correct TVP? Let s create a new spec for this but don t make it active; we can then see how close we are. Select the Design tab and then the Specs item in the left-hand column. Press the Add button for column specifications. On the list select Column Stream Property Spec near the bottom of the list & press Add Spec(s) Select the Stabilized You ll have to go to another form to actually pick the type of stream property. Click the Select Property button. On the next form select the tree structure under Standard & choose True 37.8 C; press Select. Enter the value 103. Close this form. Rev March 13, 2016

19 Now let s go back to the Design tab & Specs selection. Highlight the Stream Property Spec & you can see that the calculated TVP is actually psia, lower than the desired 103 psia. We ll have to decrease the temperature in the reboiler. Select the Active check box; now the tower becomes unconverged (because we have overspecified the unit with both the TVP spec & the reboiler temperature spec). Select the Temperature column specification & uncheck its Active checkbox. Now the tower will converge again, now with a reboiler temperature of F. Rev March 13, 2016

20 What does the stabilized liquid look like? Double-click on Stabilized Liquid & select Composition under the Worksheet tab. Note that there is essentially no methane & very little ethane all of this material has been stripped out into the overhead vapor stream. Let s look at how much has been stripped out. Double-click on Recovered Gas. Select Composition under the Worksheet tab. Notice that this gas has very high concentrations of methane & ethane. But could this be directly produced as pipeline gas? Select Properties. Note that the HHV is too high, 1449 Btu/scf. Rev March 13, 2016

21 Recycle of Recovered Gas One might ask we didn t include a condenser on the stabilizer column. We can effectively get this effect by reconfiguring the process to recycle the recovered gas from the stabilizing column upstream of the chiller & cold separator. However, since the recovered gas is produced at a lower pressure it must be compressed to a higher pressure consistent with the original feed gas. Rev March 13, 2016

22 Let s add three units: A Compressor, Recycle Gas Compressor A Mixer, Recycle Mixer. A Recycle, RCY-1. Note that some of the items have been flipped on the PFD shown above. This was done by selecting the item on the Flowsheet, selecting Flowsheet/Modify in the ribbon, & then selecting Flip Horizontal. Double-click Recycle Gas Compressor. Set the Inlet as the Recovered Gas stream. Create an Outlet stream HP Recycle Gas & a work Energy stream W-Recycle Compressor. Select the Worksheet tab. Set the outlet pressure of the HP Recycle Gas to 400 psia. Note the calculations are completed using the default adiabatic efficiency, 75%, and gives an outlet temperature of 225 F. Rev March 13, 2016

23 Now let s combine the HP Recycle Gas with the Total Feed & introduce it into the Chiller. Doubleclick on Chiller & delete Total Feed as an Inlet Stream. Instead, create a new stream, Process Feed, as the Inlet Stream. Double-click on RCY-1. Select HP Recycle Gas as the Inlet. Create a stream Recycled Gas as the Outlet. Double-click on the Mixer Recycle Mixer. Select Process Feed as the Outlet. For now, only select Total Feed as the Inlet. Rev March 13, 2016

24 At this point the simulation has converged but without the Recycled Gas being mixed with the fresh feed. But the stream has been initialized and the recycle calculation can proceed. Now, double-click on Recycle Mixer & add Recycled Gas as the second Inlet stream. Now the simulation should converge including this recycle back to the fresh feed. How has adding the recycle gas affected the final results? There is not a great deal of Recycled Gas being mixed with the fresh feed so the composition of the Cold Vapor does not change by much. The cricondentherm increases only slightly to F. The produced gas also still has a higher heating value that is too high, 1176 Btu/scf. We can try to decrease the HHV by reducing the temperature of the Chilled Gas. Let s lower this temperature to the lowest limit reasonable for a simple propane chilling loop, -30 F. Reducing this temperature does shift more of the heavy ends out of the produced gas & the HHV is lower. However, the HHV of Chilled Gas is still too high, 1145 Btu/scf. Unfortunately this is pretty much the best we can do when using a chilled single-stage flash separation unit. Prevention of Freezing in DPC Separator Even though there is not a great deal of water in the fresh feed there will still be water drop-out & potential freezing in the DPC Separator. The typical technique is to inject a high concentration of ethylene glycol (EG) upstream of the Chiller to prevent ice or hydrate formation. Rev March 13, 2016

25 Return to the Properties section. Select Component List -1 to view the active component list. Use the search term glycol. Select EGlycol from the databank list & press Add. The component EGlycol will be placed at the bottom of the list. Return to the Simulation section. Let s add a stream for the ethylene glycol, EG, into the Recycle Mixer. Double-click on the stream EG. Select Composition & set it to 85 wt% ethylene glycol & 15 wt% water. Select Conditions; set the pressure to 400 psia & its temperature to 120 F (typical for air cooling after regenerating in a small packed column). For now set the mass flow rate to 370 lb/hr (this should make the Cold Water stream about 55 wt% glycol). Rev March 13, 2016

26 Nearly all of the glycol will be part of the Cold Water stream. This stream will be about 55 wt% glycol. Is this enough to protect from freezing? We can check the hydrate formation temperature for the Cold Water stream to determine if we ve added enough glycol (the hydrate calculation will also determine ice conditions, too). Right-click on the Cold Water stream & choose Create Stream Analysis, Hydrate Formation. Click on the Design tab & select Connections. Note that solids will form at these conditions. You can get more information form the Performance tab. The freezing point temperature for this stream is -8.2 F. So, we do not have enough glycol added to provide protection from solid formation. Rev March 13, 2016

27 Let s manually add more EG to bring the hydrate formation temperature down to -40 F (colder than the actual DPC Separator conditions but will match what will be the coldest temperature of the refrigerant in the Chiller). Making manual adjustments to the flowrate of EG shows that you need to add 2285 lb/hr of the 85 wt% glycol solution to provide protection from solid formation down to -40 F. Note that the glycol concentration in Cold Water will be 78 wt%. Rev March 13, 2016

28 Propane Refrigeration Loop We need to add a refrigeration loop to be able to cool the feed & recycle gases to the DPC Separator temperature. Add the following equipment to the flowsheet: A Compressor, C3 Compressor A Cooler, C3 Condenser. A Control Valve, C3 Valve. Let s create the streams for the refrigeration loop starting at the Chiller. Double-click on Chiller. Create new inlet & outlet streams Refrig Liquid & Refrig Vapor, respectively. Make sure that these streams are associated with the Cold side. Specify a zero pressure drop. Under the Worksheet tab specify the conditions for the outlet stream Refrig Vapor (1 vapor fraction & -40 F). Rev March 13, 2016

29 Double-click on the stream Refrig Vapor. Set the composition to 100% propane. (The temperature & the vapor fraction should already have been set to -40 F & 1, respectively). Rev March 13, 2016

30 Next let s connect the cold liquid to the let-down valve. Double-click on C3 Valve. Set the Outlet as Refrig Liquid. Create a new stream Condensed Liquid as the Inlet. Select the Worksheet tab; set the temperature of Condensed Liquid to 120 F & the Vapour / Phase Fraction to 0 (i.e., saturated liquid). Note that the flowrate can now be calculated (276,972 lb/hr) to ensure an energy balance in Chiller. Rev March 13, 2016

31 Now let s complete the refrigeration loop. Double-click on C3 Compressor. Select Refrig Vapor as the Inlet & create HP Vapor as the Outlet; create W-C3 Compressor as the Energy stream. Double-click on C3 Condenser. Select HP Vapor as the Inlet & Condensed Liquid as the Outlet; create Q-C3 Condenser as the Energy stream. Under Parameters set the Delta P as 0. Now the refrigeration loop calculations are completed. Product Compression The final step is to add compression for the final product gas. Add to the flowsheet the unit: A Compressor, Product Gas Compressor Rev March 13, 2016

32 Double-click on Product Gas Compressor. Select Cold Vapor as the Inlet & create HP Product Gas as the Outlet; create W-Product Compressor as the Energy stream. Select the Worksheet tab; set the outlet pressure as 1000 psia. Note that outlet temperature is less than 120 F, so a final cooler is not needed to be able to introduce this gas into the pipeline. Optimizing Process The basic process has now been set up. Note that there are three major power users: Product Gas Compressor 4,027 hp Recycle Gas Compressor 111 hp Refrigeration Compressor 7,982 hp In addition there are two major heat users: Stabilizer s reboiler MMBtu/hr Reboiler to regenerate the ethylene glycol (not included in this simulation). The major step for optimization is to recover the refrigeration from the cold streams from the DPC Separator. By doing so there would be less refrigeration duty needed, reducing the power requirement for the C3 Compressor. Also, by warming the Cold Liquid before going to the Stabilizer the amount of reboiler duty will also be reduced. As an example, if we send the Cold Vapor to the Chiller to recover some its refrigeration (heating to -14 F so that the outlet of the Product Gas Compressor is limited to 120 F) then we reduce the power to the C3 Compressor (7,327 hp) but increase the power to the Product Gas Compressor (4,275 hp). Overall the power needed for these two compressors decreases by 3.4%. Rev March 13, 2016