Natural gas processing using mixtures of glycols and alcohols for removal of water, heavy hydrocarbons and carbon dioxide
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Developing new gas fields in cold and harsh environment requires cost effective technology for removal of water, heavy hydrocarbons (HHC) and carbon dioxide. Operating at such low temperatures requires a technology that do not experience freeze out and prevents hydrate formation. Mono- Ethylene Glycol (MEG) and methanol (MeOH) are both used as hydrate inhibitors in the industry today. Freezing point of MEG and MeOH is -13°C and -98°, respectively. By mixing MeOH and MEG together, a freeze out temperature lower than the freezing point of the components themselves will be experienced. In addition the mixture will function as a hydrate inhibitor, preventing hydrates from forming. A process utilizing this mixture, operating at low temperature, is of great interest since it can eventually simplify the pretreatment process of sales gas and/or LNG.A literature review of different dehydration methods for natural gas was performed in thesis, together with different hydrate inhibition technology. Physical absorption as a method for removal of CO2 is also presented. A collection of experimental data for freeze out temperature, hydrate formation temperature and solubility in mixtures of water, MEG and MeOH, was performed and used as a basis for evaluating accuracy of the Cubic Plus Association- Equation of state (CPA-EoS) to calculate freeze out, hydrate formation and solubility in such mixtures. Two different CPA-EoS was evaluated for solubility calculations: CPA-DTU and CPA-NeqSim. CPA-NeqSim was used for calculations of freeze out and hydrate formation in mixtures of water, MEG and MeOH, and showed good accuracy. CPA-DTU was chosen for simulations performed in HYSYS.Four different process solutions has been performed and evaluated in HYSYS using the CPA-DTU. Two processes were designed for production of sales gas, where one processes had an initial CO2 content of 2 mole %, and the other 15 mole % CO2. Flow rate of gas was 20 MSm3/d. The first process included an extraction process for dew point control of the gas, with injection of a MeOH-MEG mixture to avoid freeze out and hydrate formation. A fractionation process was also included, making liquid propane, butane and naphta. The second process was expanded with an absorption process downstream the extraction process for removal of CO2 from 15 mole % to 2,5 mole %. A MeOH-MEG-water mixture was used as physical solvent in the absorption process. A case study was performed on the two cases, to see if it was possible to achieve LNG-specifications for water, CO2 and HHC by using a MeOH-MEG mixture. A cascade process was implemented for liquefaction of the natural gas and for process integration with the extraction process and the absorption process. Results from the simulations performed in HYSYS show that the mixture achieves LNG specifications for water and HHC of 1 ppm and 1000 ppm, repectively. This is achieved in an extraction process using an expander, where injection of MeOH-MEG is used for prevention of freeze out and hydrate formation. An absorption process is installed downstream the extraction process for removal of CO2 from 15 mole % to 2,5 mole %. A case study was performed to evaluate if a MeOH-MEG-water mixture could reach CO2 specifications of 50 ppm in a physical absorption process. It was clearly stated that unfeasible circulation rates was necessary with the operating temperatures and pressures chosen in this thesis. An adsorption process was therefore installed downstream the absorption process for deep removal of CO2 to 50 ppm. After have met all specifications regarding water, CO2 and HHC, the natural gas was liquefied in a convential cascade process using pure refrigerants. The cascade process was integrated with the extraction and absorption process and proved to reduce the power consumption of the absorption process with 50 %, for the case with 15 mole % CO2.This thesis indicates that the biggest potential for a MeOH-MEG mixture lays in processing of gas for sales gas. For a CO2 content of 2 mole %, a simple extraction process with injection of MeOH-MEG was used to achieve low water and HC dew point of the gas. For a CO2 content of 15 mole % an absorption process using a MeOH-MEG-water mixture as physical solvent was used to achieve a CO2 content of 2,5 mole % in the sales gas. However; high losses of MeOH are indicated for the extraction process, and as much as 0,53 Sm3/h is lost. The loss of MeOH in the absorption process is calculated to 0,02%, and make up of MeOH is therefore needed for both the extraction and absorption process. The absorption process is operating with a circulation of 5047 Sm3/h, consisting of a MeOH-MEG-water mixture. Loss of methane is calculated to 1,4% and total power consumption is 20 MW. Over 40 % of the MeOH lost in the extraction process follows the propane product in the fractionation process. Specification for MeOH in propane is therefore not achived, and also CO2 content in propane exceeds the specifications. This indicates that the process needs further optimization to achieve all specifications related to NGL and condensate for natural gas. Evaluating other mixtures like MEG-TEG could be interesting to reduce the loss of solvent in the extraction process, and achieve these specifications.