Pressurization with single phase equipment<\/h2>\n\n\n\n
In\ncertain situations, after compression and cooling, the acid gases\n(CO2 and H2S) are mixed with water. Depending on the ratio of the\nmass flow rate of water and acid gases, these gases are dissolved in\nwhole or in part in water. This method of mixing with total or\npartial dissolution presents several advantages:<\/p>\n\n\n\n
–\nIn the total dissolution case, the volume flow rate of the\nre-injected gases is considerably reduced since the total volume flow\nactually re injected is similar to that of the water.<\/p>\n\n\n\n
–\n In the partial dissolution case, the re-injected pressure is\nintermediate to that of the individual re-injection pressures\ncorresponding to the liquid and gas phases. This takes into\nconsideration the difference in hydrostatic head which is determined\nby the length of the re-injection column and the densities of each\ncomponent (CO2, H2S and water). However, considering that the\ncompression of a gas phase is considerably more energy demanding to\nthat of a liquid phase, this dissolution case upstream the\nre-injection well is less energy demanding compared to individual\nre-injection. \n<\/p>\n\n\n\n
Conventionally, this operation is carried out using a pumping unit for the water, a compression unit for the acid gases and a mixer at the pumping and compression outlet.<\/strong> Compression of gases requires a lot of equipment, given the heating of gases during compression: in addition to rotating machines and their drive, heat exchangers as well as water-gas separators are required to avoid entrainment of water in the compression stage located downstream. See below a sketch of the installation.<\/p>\n\n\n\n When\nthe mass flow rate of the acid gases is low, the gases can be\ncompletely dissolved into the water before reaching the re-injection\npressure therefore saving a lot of compression energy. Downstream of\nthe mixing point, the liquid mixture is raised in pressure using a\nmonophasic pumping stage in order to reach the re-injection pressure.\nConcerning the equipment, when the volume flow rate of acid gases is\nlow, compression takes place using positive displacement compressors.\nOn the other hand, when it is high, the use of radial rotodynamic\nmachines is preferable.<\/p>\n\n\n\n This equipment is particularly heavy and bulky<\/strong>. Furthermore, they require costly materials to take into account the aggressiveness of acid gases.<\/p>\n\n\n\n An alternative solution: a two-phase rotodynamic pump using helico axial hydraulic cells<\/strong>. Several arrangements may be considered:<\/p>\n\n\n\n At this stage, a very small fraction of the acid gases is dissolved into the water due to the low inlet pressure (this pressure level depends on the gas treatment technology \u2013 Chemical versus physical dissolving agents). During compression, part of the acid gases gradually dissolve into water while the undissolved gases heat up in a compression mode intermediate between a polytropic (only gas) and an isothermal compression (continuous cooling at constant temperature). In the present situation, the temperature rise of the gases is moderate taking into account the constant cooling of the gases by water whose calorific capacity is higher than that of the gases. The cooling of the gas by water is facilitated by a very large contact surface (presence of numerous bubbles and pockets of gas as well as droplets of liquid) taking into account the permanent mixing of the two phases during compression. Of course, the cooling of the gas is accompanied by some heating of the liquid. See below a sketch of the installation. <\/p>\n\n\n\n This is carried out in order to facilitate the dissolution of the gas phase into the water (reaching a dissolution equilibrium) in an external mixer<\/strong> before their introducion into the pump to finalize the two phase compression. At the end of compression, a last mixer may be used to reach the dissolution equilibrium condition. These external mixtures can be combined with cooling in order to enhance the dissolution and to facilitate the compression of the gas phase. Two comments must be made: a) It should be noted that dissolving a gas into a liquid is dependent on numerous parameters and takes some time even when the conditions for dissolution are met. This is called “Kinetics of dissolution”. b) cooling of CO2 and H2S must be made according to the dew point formation of hydrates.<\/p>\n\n\n\n A first section comprises one or several large diameter radial impellers for the compression of a strictly gaseous phase allowing, at the same time, to substantially reduce the gaseous volume and to reach a significant dissolution fraction into the liquid phase. The liquid and gaseous phases are circulated into a mixer after this pre-compression stage before being introduced into a second two-phase pumping section composed of helico axial impellers. This back to back arrangement makes it possible to significantly reduce the input volume at the two-phase section and to raise both the pressure and the density of the two-phase mixture, two elements enhancing the \u201ctwo-phase efficacy\u201d.<\/p>\n\n\n\n See in the “Two-Phase” Chapter and the “Two Phase Pumps” section to see a) The definition of the \u201cTwo phase efficacy\u201d (different to the Two Phase Efficiency) b) and how the two-phase efficacy varies with the GLR (Gas-Liquid Ratio) and the density ratio, GLDR.<\/p>\n\n\n\n http:\/\/yvcharron.com\/index.php\/two-phase-flow-pumps\/<\/a> <\/p>\n\n\n\n It should be noted that regarding these two-phase compressions, the two phase pump presents characteristics which are different from that corresponding to the pumping of a mixture of hydrocarbons. <\/p>\n\n\n\n For more details see the document below:<\/p>\n\n\n\n – In water-acid gas compression, the inlet volume flow rate (therefore the impeller geometry) is determined by the volume of the gas phase while the outlet volume flow rate (therefore the impeller geometry) is determined by that of the liquid phase which contributes to give a conical shape to the two-phase pump rotor unlike a pump dedicated to the pumping of two-phase hydrocarbons with little intermediate dissolution provides a cylindrical shape to the rotor.<\/p>\n\n\n\n –\nIn water-acid gas compression, the compression of the mixture is\nrather ineffective at the inlet of the pump given the large volume of\ngas and the low density of the mixture while this compression is very\neffective at the outlet, the mixture consisting essentially of liquid\nwith a very high density.<\/p>\n\n\n\n These two comments explain the advantage of dividing the multiphase pump into two sections mounted back to back<\/strong> (a single-phase gas section and a two-phase section) with a dissolution mixer in between these two sections.<\/strong><\/p>\n\n\n\n This work was carried out in 2000 at IFP. For more information use form:<\/p>\n\n\n\n http:\/\/yvcharron.com\/index.php\/contact\/<\/a> or<\/p>\n\n\n\n![\"Injection](\"http:\/\/yvcharron.com\/wp-content\/uploads\/2020\/03\/Compression-Mono-Sour-GasWater.png\")
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Pressurization with two phase equipment <\/h2>\n\n\n\nBasic arrangement: acid gases and water are mixed before their admission into a multiphase pump<\/strong>. <\/h4>\n\n\n\n
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<\/figure><\/div>\n\n\n\nSecond arrangement: the two-phase gas – water mixture is extracted at an intermediate pumping stage<\/strong><\/h4>\n\n\n\n
Third arrangement: the two phase compression consists of two sections mounted back to back.<\/strong><\/h4>\n\n\n\n
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