Simple Dew Point Control HYSYS v10. When the simulation is set up the overall PFD should look like the following figure.

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1 Simple Dew Point Control HYSYS v10 Steps to set up a simulation in HYSYS v10 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 (Y Grade). The following are known conditions for the feedstock and specifications for the products: The composition of the feed gas is shown in the following Component Mol% table. The gas enters the plant at 400 psia & 120 F. The gas is nearly saturated with water at the inlet conditions, 48 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. N CO C C C i C n C i C n C n C n C n C n C 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 January 9, 2018

2 A propane refrigeration 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. Air coolers will be used to cool gases & liquids to 120 F. Create new simulation file Start HYSYS. One way to do this from Windows 10 is to press Start, go down the alphabetical list to Aspen HYSYS, then Aspen HYSYS V10. 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 January 9, 2018

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 dry feed stream, add the appropriate amount of water, & attach the wet feed to an LNG Exchanger. The outlet will be attached to a flash separator. Rev January 9, 2018

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 temperature & pressure. Enter the 100 MMscfd flowrate in the Molar Flow box. Note that depending upon your default set of units the values entered may change. The figure on the right shows pressure in psig (even though we d really like psia) & molar flow in lbmole/hr (though we d really like MMscf/day). We ll look at how to change these after setting up this initial part of the process. 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 January 9, 2018

5 We want to do the same thing for the water portion of the feed represented by the stream Feed Water. Doubleclick on the Feed Water stream to open up the entry forms for this stream. Enter 4,800 lb/day in the Mass Flow box (to represent the 48 lb/mmscf water content). Enter the pressure but do not enter the temperature. Note that for the set of units currently in use HYSYS automatically replaces the mass rate with the equivalent amount in lb/hr. Rev January 9, 2018

6 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. For most of the unit operations we ll define connections and create new streams using the operations Design forms. Double click on Mixer. Define the 2 Inlets as Dry Feed & Feed Water (by pulling down the stream names from the lists). Define a new Outlet stream as Total Feed by typing in the new name. 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 conditions 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 1. 1 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 January 9, 2018

7 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 the previously defined Total Feed & define the Outlet Stream as a new stream Chilled Gas. For now specify the Pressure Drop as 0. Make sure that specification for Hot/Cold is Hot. We now want to specify the cold separator & determine the properties of the produced gas. Double click on DPC Separator. Specify the Inlet as the existing stream Chilled Gas. Create new streams, Cold Vapor, Cold Liquid, & Cold Water as the Vapour, Light Liquid, & Heavy Liquid, respectively. Rev January 9, 2018

8 Let s estimate the needed temperature for the cold separator. Click on the Worksheet tab & specify 15 F for the temperature of Chilled Gas. Notice that all values are calculated for Chilled Gas, Cold Liquid, & Cold Vapor at 15 F. This means that the vapor out of the separator is at its dew point at 15 F. This makes the pipeline s dew point spec, right? No, not really. But how would we know this? We can look at the phase envelope for Cold Vapor to determine if the vapor will have a minimum dew point temperature at all pressures it is likely to experience in the pipeline. We ll do this using a Stream Analysis. 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. The results in the Design tab show that the Cricondentherm is 20 F, warmer than the temperature needed to meet the 15 F pipeline dew point spec. At what pressure does this occur? We can view the PT diagram by selecting the Performance tab & the Plots option; now we can see that the maximum temperature for the phase envelope is at 600 psig, very much in the possible range of pipeline operating pressures. 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. And since the temperature is 20 F, this gas does not make this spec. Rev January 9, 2018

9 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 January 9, 2018

10 Now that we ve met the dew point spec we can determine if we ve 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 left hand column. Notice that an HHV has been calculated as Btu/lb.mole. Is this within the acceptable range? Since the units are different than those in the spec ( Btu/scf) we have to do a unit conversion. Instead, let s reset the units used for reporting the results & come back to this. Setting the Units Used by HYSYS The preference for this example is to use U.S. Customary units typically used in the gas processing industry: temperature in F, pressure in psia, molar flow in MMscf/day, heating values in Btu/scf, liquid flow in gal/min, & mass flow in lb/hr. The default that was in place when this simulation was started was pretty close. But let s do better. Under the Home tab there is a section for Units that show the current set being used & a button to change options within a unit set. For this problem the default is a set called Refining US1 (for new files the starting set is dependent on what was previously used on the particular computer being used). To control the actual set being used let s create a new one which we will call Gas Rev January 9, 2018

11 Processing US. Click on the Unit Sets button to bring up the form to examine the available unit sets & change (if desired). In the lower section let s start with the Field unit set. Select Field & press Copy. A new unit set will be created (here called NewUser). Double click on this name & enter Gas Processing US. In the upper section we can set the units used for various properties in the simulation. We can see that most units used are very reasonable. For example, temperature is in F, actual liquid volume flow as USGPM, & pressure is in psia. (Great! We don t have to do anything for this.). Rev January 9, 2018

12 Let s change the units for molar flow & heating value to meet out purposes. Go to the Display Units list & change these units to MMSCFD & Btu/SCF, respectively. Now we can close this form by clicking OK. Now that we ve changed the units on heating value we can go back to the properties for Cold Vapor to see if we ve made the HHV (higher heating value) spec. Now the value is clearly reported as 1175 Btu/scf & we see that it is too high. This will require more heavy hydrocarbons be removed. But before we focus on this let s add additional processing to stabilize the liquid formed (since this will involve recycling back some evolved gas). Liquid Stabilization Before change conditions to lean out the gas let s determine if the produced liquid will make the TVP spec of 103 psia. Double click 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 NGL mix. Rev January 9, 2018

13 We can process the high pressure liquid in a lower pressure stripping column to remove these light ends. Let s add two more units: A Control Valve, VLV 001 A Reboiled Absorber, Stabilizer. Rev January 9, 2018

14 Double click on VLV 100. Specify the Inlet as Cold Liquid and define a new stream Flashed Liquid as the Outlet. There is a message that the pressre drop across valve is unknown, but that is OK, we ll set that in a bit. Let s define the stabilizing column as a 10 stage column with a kettle reboiler. Double click on the Reboiled Absorber. Rename it 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. Accept the default Once Through reboiler configuration. This will model a kettle reboiler. Press the Next > button to continue the tower s definition. Rev January 9, 2018

15 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 a 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 should be higher; for now let s set an estimate of 200 F. Press the Next > button to continue the tower s definition. 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 Rev January 9, 2018

16 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. Let s add the reboiler temperature as the operating spec. Select Specs item in the lefthand 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. Rev January 9, 2018

17 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. 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 January 9, 2018

18 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 can decrease the reboiler temperature to allow the vapor pressure to increase. 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 reconverge; the calculated reboiler temperature is F. Rev January 9, 2018

19 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. Just enough is allowed to remain (by changing the reboiler temperature) to have the desired vapor pressure. Let s look at how much Gas 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. More than likely it won t make the dew point spec either. Recycle of Recovered Gas One might ask why we didn t include a condenser on the stabilizer column. A condenser would allow us to wash the propane & heavier (C3+) back down the column & out with the Stabilized Liquid. We can effectively get this effect by reconfiguring the process to recycle the recovered gas from the stabilizing column upstream of Chiller. 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 January 9, 2018

20 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 F. Rev January 9, 2018

21 Double click on RCY 1. Select HP Recycle Gas as the Inlet. Create a stream Recycled Gas as the Outlet. 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 the Mixer Recycle Mixer. Select Process Feed as the Outlet. For now, only select Total Feed as the Inlet. 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. Rev January 9, 2018

22 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 The inlet feed gas is nearly water saturated at the entry to the process. When the water drops out of the gas phase when it is cooled there is a potential freezing in the Chiller & DPC Separator. A typical technique to prevent ice or hydrate formation is to inject ethylene glycol (EG) upstream of the Chiller. An aqueous solution of EG has the ability to suppress the formation of ice. In it s pure state EG has a freezing point of 8 F, but aqueous solutions have freezing points that are lower. Notice from the chart on the right 1 one may get freezing protection to 30 F or lower by maintaining a EG concentration in water of 85 wt% to 50 wt%. What are the appropriate concentrations to consider for our process? We would like to make sure that there is freezing protection for the entire concentration range before & after the water is absorbed. We want protection not only at the process temperature but also the coldest temperature at the tube wall. This means we have to protect below the 30 F process temperature but to the coolant temperature of 40 F or lower. Based on these considerations we will want a concentrated EG solution of 83 wt% (protection to 40 F, the coldest tube temperature expected in Chiller). This should be injected at a sufficient rate so that it will be diluted to no lower than 80 wt% (protection to 50 F) 2. 1 Engineering and Operating Guide for DOWTHERM SR 1 and DOWTHERM 4000 Inhibited Ethylene Glycol based Heat Transfer Fluids, Dow Chemical technical publication, ttrans/pdfs/noreg/ pdf&fromPage=GetDoc 2 Note that even though we could try to operate in the region of lower glycol concentrations (60 wt% diluted to 55 wt%) the normal practice is to operate in the higher concentration range; if excess water comes in with the gas then the higher concentrations actually get better freeze protection, not worse. Rev January 9, 2018

23 To be able to add an EG solution we must add ethylene glycol to the component list. Return to the Properties section. Select Component List 1 to view the active component list. Highlight H2O in the selected components. Use the search term egly. Select EGlycol from the databank list & press Add. The component EGlycol will be placed right after H2O, second in the list. Return to the Simulation section. Let s add a stream for the ethylene glycol, EG, into the Recycle Mixer. Rev January 9, 2018

24 Double click on the stream EG. Select Composition & set it to 83 wt% ethylene glycol & 17 wt% water. Select Conditions; set the pressure to 400 psia & its temperature to 60 F (typical for underground storage; we ll find out a more reasonable temperature later). For now set the mass flow rate to 5,333 lb/hr (this should make the Cold Water stream about 80 wt% glycol). Propane Refrigeration Loop The next detail we can is a refrigeration loop to be able to cool the feed & recycle gases to DPC Separator. 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 January 9, 2018

25 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). Do not specify the pressure drop across the valve this will be determined automatically when the high pressure (for condensation) and low pressure (for vaporization) are determined. Rev January 9, 2018

26 You can specify the composition in almost any of the streams in this loop. It is most convenient to do so at the stream out of the condenser. (Maybe not for a single stage of compression, but definitely most convenient when going to multiple stages.) Double click on the Condensed Liquid stream. Select the Composition item & press the Edit button. Enter a 1 for Propane, press Normalize, then OK. Note that the calculations have been performed for this stream, including the determination of the flowrate (277,990 lb/hr); this flowrate has been calculated to ensure an energy balance in Chiller. 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. Normally we would want to specify the outlet pressure, but we re going to let HYSYS use the condenser s pressure for this. Rev January 9, 2018

27 Now let s complete the refrigeration loop. Double click on C3 Condenser. Select HP Vapor as the Inlet & Condensed Liquid as the Outlet; create Q C3 Condenser as the Energy stream. At this point the condenser has not yet been fully defined. Under Parameters set the Delta P as 0. Now the status is green & the refrigeration loop calculations are completed. Product Compression The final step in this simple simulation is to add compression for the final product gas. Add to the flowsheet the unit: A Compressor, Product Gas Compressor 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 F (less than the spec of 120 F minimum), so a final cooler is not needed to be able to introduce this gas into the pipeline. Rev January 9, 2018

28 Additional detail to the Flowsheet There many details that can be added to this flowsheet. When done with these additions the flowsheet will look like the following. Ethylene Glycol Regeneration The initial flowsheet assumes that 83 wt% ethylene glycol (EG) can be made available to the process. In an actual process this EG is not a fresh feed, but rather it is recirculated after the water picked up in the DPC Separator is stripped out. We can add the following major operations to regenerate the EG: a stripping column with a reboiler & partial condenser a cross exchanger to recover heat from the stripped EG a pump to bring the lean EG up to the injection pressure a recycle operation. Rev January 9, 2018

29 Let s create the streams while creating the unit operations. Create the stripping column using the Distillation Column Sub flowsheet module from the Columns tab of the model Palette. Double click on this module; on this first screen: Name the column EG Stripper. Set the number of stages to 2 Set the condenser type to Full Rflx. Create the stream Hot Rich EG as the Inlet Stream to stage 2. Set the Ovhd Vapour Outlet as Water Vapor, the Bottoms Liquid Outlet as Hot Lean EG, the Condenser Energy Stream as Q EG Condenser, and the Reboiler Energy Stream as Q EG Reboiler. When ready press the Next > key. We ll define the reboiler as a kettle reboiler. Keep the default option of Once through & Regular Hysys reboiler and press Next >. Rev January 9, 2018

30 EG strippers operate near atmospheric condition to keep the reboiler temperatures as low as possible. We ll first assume a zero pressure drop across the column. Set the Condenser Pressure and the Reboiler Pressure to 1 atm. (Note that the pressure will be converted to units of psia.) Press Next >. The product off the top of the column should be essentially water vapor at 1 atm, so we can set a temperature estimate for this as 212 F. Press Next > when done. For now let s estimate the reflux ratio as Press Done Rev January 9, 2018

31 Let s define the cross exchanger that will preheat the cold water/eg feed and recover heat from the hot stripper bottoms. Use the LNG Exchanger module to create EG Cross Exchanger (you may want to flip the exchanger horizontally depending on how you place it on your flowsheet). Specify Cold Water as an inlet stream & its outlet as Hot Rich EG; specify this as a Cold stream. Specify Hot Lean EG as an inlet stream & create LP Lean EG as its outlet; specify this as a Hot stream. Set both pressure drops as 0. We d like to start the calculations without creating a heat based recycle loop. So, let s specify the outlet temperature in Hot Rich EG as 200 F. Now the hot side streams should be calculated. (The tower hasn t executed yet so these streams are still unavailable.) Let s go back & run the column. Double click on EG stripper. We have made a specification on the condenser but not on the reboiler. Select the Specs item. Click the Add button for column specifications. Select Column Component Fraction & click Add Spec(s) Name this spec Bottoms Mass Fraction; set the Mass Fraction value to 0.83 for EGlycol for the Liquid coming from the Reboiler. Close this window. Rev January 9, 2018

32 Select Specs Summary. The only two active specs should be Reflux Ratio & Bottoms Mass Fraction. Select Run (you may not even have to press this button). It should converge very quickly. Select the Performance tab & the Column Profiles item. You can see that our estimate for the top temperature was pretty close. The bottoms temperature is F. Rev January 9, 2018

33 The 214 F condenser temperature has the implication that there us a fair amount of glycol in the overhead vapor. The more glycol that leaves with the vapor, the more makeup that must be added. Let s minimize these glycol losses by setting the temperature to 212 F. Do this by adding a temperature spec on the condenser (go to the Design tab, select Specs, click Add, select item Column Temperature & click Add Spec(s) ; then set the spec value for Condenser as 212 F). Now when you make this active the column will reconverge with this as the condenser s temperature. You can click on the Worksheet tab & see that there is only a small amount of glycol being lost in the overhead. Rev January 9, 2018

34 We can go back to the flowsheet & see that EG Cross Exchanger has also converged. We can now finish up the return of the lean EG stream. The LP Lean EG stream needs to be pumped up to the delivery pressure & tied in to the EG feed stream. Add a pump Glycol Pump (you may want to flip horizontal depending on how you place it on your flowsheet). Set the Inlet as LP Lean EG, create the Outlet as EG to Recycle, and create the Energy stream as W EG Pump. Go to the Worksheet tab & set the pressure for EG to Recycle as 400 psia (to match the EG stream). Notice that the pump outlet is 30.1 F. This is notable for two reasons: This is lower than the initial spec that the ethylene glycol would be entering at 60 F. The EG Cross Exchanger actually allows us to get fairly cold by recovering refrigeration in the Cold Water stream. In fact, this temperature may actually be too low. Typical return temperatures should be 40 to 55 F. This higher temperature cannot be directly specified in EG Cross Exchanger; as soon as you change the spec from one on the outlet of the hot side to one on the cold side you set up a recycle loop and this module cannot automatically solve this. But you can manually reduce the temperature of Hot Rich EG until the temperature of LP Lean EG rises above 40 F. Reducing the spec from 200 F to 191 F will do this. Rev January 9, 2018

35 Finally, let s close off the recycle. Double click on RCY 2 & set the Inlet as EG to Recycle & the Outlet as EG. The calculation will quickly converge. Optimizing the 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 7,988 hp In addition there are two major heat users: Stabilizer s reboiler 3. 3 MMBtu/hr EG stripper s reboiler 0.5 MMBtu/hr. A question for optimization can any of these streams be reduced to reduce the operating expense for the process? Some thoughts: Most of these values are dependent on the operating conditions of DPC Separator. This sets the amount of gas that needs to be recompressed, the amount of light ends to the Stabilizer that need to be stripped off, compressed, & recycled back, and the amount of water absorbed & regenerated in EG Stripper. The big operating cost and one that can be addressed with further design is the power needed for the refrigeration loop. There are two ways that this could be done: o We could try 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. However, note that by increasing the temperature of the gas before the Product Gas Compressor the required power in this compressor will increase, negating the majority of the power savings. o We could increase the number of refrigeration stages of compression with associate recycle of the intermediate gases from the intermediate stage economizers. It is typical that a two stage system can save about 20% of the power required by the refrigeration system. Rev January 9, 2018