HYSYS TEG脱水例子Natural Gas Dehydration with TEG

Natural Gas Dehydration with TEG 1 1 Natural Gas Dehydration with TEG © 2001 Hyprotech Ltd. - All Rights Reserved. 1.1.9 Natural Gas Dehydration with TEG_4.pdf 2 Natural Gas Dehydration with TEG 2 Workshop At the wellhead, reservoir fluids generally are saturated with water. The water in the gas can present some problems: • formation of solid hydrates can plug valves, fittings or pipes • the presence of water along with H2S or CO2 can cause corrosion problems • water can condense in the pipeline causing erosion or corrosion problems Generally, a dehydration unit is used in gas plants to meet a pipeline specification. There are several different processes available for dehydration: glycols, silica gel, or molecular sieves. The natural gas industry commonly uses tri-ethylene glycol (TEG) for gas dehydration where low gas dew point temperatures are required, such as in the design of offshore platforms in the Arctic or North Sea regions or for other cryogenic processes. In this example, the water dewpoint spec for the dry gas is -20°C (-4°F) at 6200 kPa (900 psia). Learning Objectives Once you have completed this section, you will be able to: • Model a typical TEG dehydration unit • Determine water dewpoint for a gas. Prerequisites Before beginning this section you need to be able to: • Add streams, operations and columns. Process Overview 4 Natural Gas Dehydration with TEG 4 Column Overview TEG Contactor TEG Regenerator Natural Gas Dehydration with TEG 5 5 Building the Simulation Defining the Simulation Basis For this case, you will be using the Peng Robinson EOS with the following components: N2, H2S, CO2, C1, C2, C3, i-C4, n-C4, i-C5, n-C5, H2O, and TEG. Starting the Simulation Adding the feed streams 1. Add a Material stream for the inlet gas with the following values: In this cell... Enter... Name Inlet Gas Temperature 30°C (85°F) Pressure 6200 kPa (900 psia) Molar Flow 500 kgmole/h (10 MMSCFD) Component Mole Fraction N2 0.0010 H2S 0.0155 CO2 0.0284 C1 0.8989 C2 0.0310 C3 0.0148 i-C4 0.0059 n-C4 0.0030 i-C5 0.0010 n-C5 0.0005 H2O 0.0000 TEG 0.0000 6 Natural Gas Dehydration with TEG 6 2. Add a second Material stream for the TEG feed to the TEG Contactor with the listed values. The values for the Stream TEG Feed will be updated once the Recycle operation is installed and has calculated. Mixer Operation The composition of the natural gas stream has been provided on a water-free basis. To ensure water saturation, the gas is mixed with water prior to entering the Contactor. Add a Mixer to mix the Inlet Gas and Water to Saturate streams. In this cell... Enter... Name TEG Feed Temperature 50°C (120°F) Pressure 6200 kPa (900 psia) LiqVol Flow 0.5 m3/h (2 USGPM) Component Mass Fraction H2O 0.01 TEG 0.99 In this cell... Enter Connections Name Saturate Inlets Inlet Gas Water to Saturate Outlet Gas + H2O Parameters Pressure Assignment Equalize All Work Sheet Water to Saturate, Flowrate 0.5 kgmole/h (1.1 lbmole/hr) Water to Saturate, Composition 100% Water Water to Saturate, Temperature 30°C (85°F) Natural Gas Dehydration with TEG 7 7 Separator Operation Any free water carried with the gas is first removed in a separator operation, FWKO. Add a Separator and provide the following information: What is the vapour fraction of the stream Gas+H20? (It should be less than 1.0 to ensure saturation) _________ In this cell... Enter... Connections Name FWKO TK Feed Gas + H2O Vapour Outlet Gas to Contactor Liquid Outlet FWKO How much water is removed by the Separator? __________ What is the hydrate temperature of Gas to Contactor? __________ 8 Natural Gas Dehydration with TEG 8 Contactor Operation The TEG Contactor can now be simulated. Add an Absorber column operation with the following specifications and Run the column. Valve Operation The Rich TEG stream is flashed across the valve, VLV-100. The outlet pressure will be back calculated. Add a Valve with the following values: In this cell... Enter... Connections Name TEG Contactor No. of Stages 8 Top Stage Feed TEG Feed Bottom Stage Feed Gas to Contactor Ovhd Vapour Dry Gas Bottoms Liquid Rich TEG Pressures Top 6190 kPa (897 psia) Bottom 6200 kPa (900 psia) In this cell... Enter... Connections Inlet Rich TEG Outlet LP TEG Natural Gas Dehydration with TEG 9 9 Heat Exchanger Operation Regen Feed is heated to 105°C (220°F) in the lean/rich exchanger, L/R HEX, before entering the Regenerator Add a Heat Exchanger with the following values: In this cell... Enter... Connections Name L/R HEX Tube Side Inlet Regen Bttms Tube Side Outlet Lean from L/R Shell Side Inlet LP TEG Shell Side Outlet Regen Feed Parameters Tubeside Delta P 70 kPa (10 psi) Shellside Delta P 70 kPa (10 psi) Work Sheet Regen Feed, Temperature 105°C (220°F) Regen Feed, Pressure 110 kPa (16 psia) 10 Natural Gas Dehydration with TEG 10 Regenerator Operation The TEG Regenerator is simulated as a Distillation Column. The TEG Regenerator consists of a condenser, a reboiler and one ideal stage. 1. Add a Distillation Column to the case. In this cell... Enter... Connections Name TEG Regenerator No. of Stages 1 Feed Regen Feed Condenser Type Full Reflux Ovhd Vapour Sour Gas Bottoms Liquid Regen Bttms Condenser Energy Cond Q Reboiler Energy Reb Q Pressures Condenser 101 kPa (14 psia) Condenser Delta P 2 kPa (1 psi) Reboiler 103 kPa (15 psia) Specs First Spec - Tray Temperature Stage Condenser Spec Value 102°C (215°F) Status Active Second Spec - Tray Temperature Stage Reboiler Spec Value 205°C (400°F) Status Active Natural Gas Dehydration with TEG 11 11 2. Set the Damping Factor (on the Solver page) to Adaptive. This will result in much faster convergance for this column. Mixer Operation TEG is lost in small quantities, so a makeup stream is required to ensure that the material balance is maintained. 1. Add a Material Stream. Third Spec - Reflux Ratio Spec Value 1.0 Molar Status Estimate Fourth Spec - Draw Rate Draw Sour Gas Spec Value 1 kgmole/h (0.02 MMSCFD) Status Estimate In this cell... Enter... In this cell... Enter... Connections Name Makeup TEG Temperature 15°C (60°F) Component Mass Fraction H2O 0.01 TEG 0.99 12 Natural Gas Dehydration with TEG 12 2. Add a Mixer with the following information: Pump Operation A pump is installed to raise the pressure of the TEG before it enters the Contactor. Add a Pump with the following information: In this cell... Enter... Connections Inlets Makeup TEG Lean from L/R Outlet TEG to Pump Parameters Pressure Assignment Equalize All Work Sheet Liquid Vol. Flowrate of TEG to Pump 0.5 m3/h (2 USGPM) What is the flowrate of Makeup TEG? __________ In this cell... Enter... Connections Inlet TEG to Pump Outlet Pump Out Energy Pump Q Work Sheet Pressure of Pump Out 6275 kPa (910 psia) Natural Gas Dehydration with TEG 13 13 Heat Exchanger A second heat exchanger is added to cool the TEG returning to the Contactor. Add a Heat Exchanger with the following information. Recycle Operation The Recycle installs a theoretical block in the process stream. The feed into the block is termed the calculated recycle stream, and the product is the assumed recycle stream. The following steps take place during the convergence process: • HYSYS uses the conditions of the assumed stream and solves the Flowsheet up to the calculated stream. • HYSYS then compares the values of the calculated stream to those of the assumed stream. • Based on the difference between the values, HYSYS modifies the values in the calculated stream and passes the modified values to the assumed stream. • The calculation process repeats until the values in the calculated stream match those in the assumed stream within specified tolerances. In this case, the lean TEG (TEG Feed) stream which was originally estimated will be replaced with the new calculated lean TEG (TEG to Recycle) stream and the Contactor and Regenerator will be run until In this cell... Enter... Connections Tube Side Inlet Pump Out Tube Side Outlet TEG to Recycle Shell Side Inlet Dry Gas Shell Side Outlet Sales Gas Parameters Tube Side Delta P 70 kPa (10 psi) Shell Side Delta P 35 kPa (5 psi) Work Sheet TEG to Recycle, Temperature 50°C (120°F) 14 Natural Gas Dehydration with TEG 14 the recycle loop converges. 1. Double click on the Recycle button. On the Connections tab enter the following information: 2. Switch to the Tolerance page on the Parameters tab. Complete the page as shown in the figure below. The tolerances for Flow, Enthalpy and Composition need to be tightened. What is the hydrate temperature of Sales Gas? _________ How does this compare with the hydrate temperature of Gas to Contactor? Recycle Button The TEG concentration is very high so it is necessary to tighten the tolerances, especially on composition, to ensure accurate solutions. Natural Gas Dehydration with TEG 15 15 Analyzing the Results One of the criteria used to determine the efficiency of a dehydration facility is the water dewpoint of the dry gas. This can easily be checked by finding the temperature at which water will just begin to condense. First, all traces of TEG must be removed from the stream being tested because TEG affects the H2O dewpoint. This is accomplished by the use of a Component Splitter. The resulting stream is then cooled and its outlet temperature is varied by an Adjust operation to find the point at which water just forms. 1. Add a Component Splitter with the following values: Save your case! The Component Splitter does not do a flash to separate components. The separation is specified by the user. Component Splitter Button In this cell... Enter... Connections Name Remove TEG Inlet Sales Gas Overhead Outlet TEG Only Bottoms Outlet Water Dewpoint Energy Split Q Parameters Bottoms Pressure 6155 kPa (893 psia) Overhead Pressure 6155 kPa (893 psia) Work Sheet Water Dewpoint temperature -20°C (-4°F) TEG Only Temperature 10°C (50°F) Splits TEG Fraction in Overhead 1.0 All Other Fractions 0.0 16 Natural Gas Dehydration with TEG 16 2. Add a Separator to remove the condensed water. An Adjust operation will vary the temperature of Water Dewpoint until the dewpoint specification is met for the stream Gas Out. Add an Adjust operation to manipulate the temperature of the Water Dewpoint stream until the flow of the XS H2O stream is just greater than 0, a value of 0.01 kg/h works well here. The resultant temperature of the Water Dewpoint stream will then be the dewpoint of that stream. In this cell... Enter... Connections Feed Water Dewpoint Vapour Outlet Gas Out Liquid Outlet XS H2O Adjust Button In this cell... Enter... Connections Adjusted Variable Water Dewpoint Variable Temperature Target Variable Object XS H2O Variable Comp. Mass Flow - H2O Target Value Source User Specified Target Value 0.01 kg/h (0.022 lb/hr) Parameters Method Secant Tolerance 0.005 kg/h (0.01 lb/hr) Step Size 5 oC (10 oF) The tolerance must be small here as the target value is close to 0, but an XS H2O flow of 0 means that the dewpoint has not been reached yet. Natural Gas Dehydration with TEG 17 17 Exploring with the Simulation Exercise 1 The addition of stripping gas (slipstream from Sales Gas) will enhance the ability of the Regenerator to remove water from the rich TEG. A Tee operation is used to split Sales Gas into 2 streams. • Strip Gas flow = 50 kgmole/h (110 lbmole/hr) The stream pressure is 6155 kPa which is too high for the Regenerator. Use a recycle, a cooler and a valve to transfer the flow and composition of Strip Gas to stream SG to Regen at the following conditions: • T = 70°C (160 oF) • P = 110 kPa (15 psia) SG to Regen enters as a feed to the Regenerator Reboiler. Does the TEG concentration in Regen Bttms increase? 18 Natural Gas Dehydration with TEG 18