三甘醇再生

TRIETHYLENE GLYCOL REGENERATION IN NATURAL GASDEHYDRATION PLANTS: A STUDY ON THE COLDFINGERPROCESSF. Gironi, M. Maschietti, V. Piemonte, Università degli Studi di Roma “LaSapienza”, D. Diba, S. Gallegati, S. Schiavo, Comart SpA, Ravenna, ItalyThis paper was presented at the Offshore Mediterranean Conference and Exhibition in Ravenna, Italy, March 28-30, 2007. It was selected for presentation by the OMC 2007 Programme Committee following review of information contained in the abstract submitted by the authors. The Paper as presented at OMC 2007 has not been reviewed by the Programme Committee. ABSTRACTNatural gas pipeline transportation requires very low water content in the gas stream in order to avoid condensation or hydrate formation. To reach this goal, when triethylene glycol (TEG) is used to dehydrate natural gas, after the absorption step TEG must be regenerated to levels substantially above 98.5-99.0 % by weight available from atmospheric distillation of glycol-water mixtures. In order to regenerate TEG to higher purity levels some of the methods used require a stripping gas, a solvent or to perform the distillation under vacuum. A simpler method to perform a further dehydration of TEG is the use of a water exhauster, known as Coldfinger, where the vapour in equilibrium with the liquid to be dehydrated is continuously condensed and removed. In this work, the Coldfinger apparatus was modelled and a study on the most relevant operating parameters was carried out. A process simulation of a natural gas dehydration plant, provided with a Coldfinger water exhauster for TEG regeneration, was performed on a case study. It was shown that the dehydration process with Coldfinger unit is capable of reaching current water content specifications in a simple and economic way.INTRODUCTIONNatural gas at the producing well contains significant quantities of water vapour. Typically, the gas is water-saturated at the condition of pressure and temperature of the well and a dehydration process is required. In fact, water content must be reduced in order to prevent liquid water condensation and hydrate formation in the pipeline transportation system. Nowadays, typical values of allowable water content in the gas transmission lines range from 70 to 120 mg/Nm3 [1].Among methods available for natural gas dehydration, absorption by means of triethylene glycol (TEG) is one of the most common. Water removal from the gas stream takes place by means of countercurrent contact between the gas, fed to the bottom of a contactor tower, and TEG, which is a liquid with a great affinity for water, fed at the top of it.The crucial part of the process is represented by TEG regeneration. If the water-rich TEG is distilled in a simple atmospheric column, TEG can not be regenerated to levels above 98.8-98.9 % by weight. This is caused by the reboiler operating temperature, which can not be fixed at temperature above 204 °C. In fact, this tempe rature must be regarded as an upper limit for TEG processing, because of thermal degradation at higher values [2,3].In the past these regeneration levels were sufficient because values of allowable water content in the lean gas were higher and regeneration was commonly performed in a simple atmospheric still column. On the other hand, in order to reach current water content specifications, it is necessary to regenerate TEG up to levels substantially above 99.0 % by weight.Fig 1: Water content in the dehydrated gas as a function of liquid to gas ratio in the absorption column. Case study process parameters: contactor theoretical stages 4, contactor pressure 50 bar, wet gas temperature 40 °C, TEG temperature at the inletport 100 °C.As an example referred to a typical natural gas dehydration case study, Fig. 1 shows water content in the lean gas as a function of the liquid to gas ratio in the absorption column. The curves, obtained by process simulation, refer to four different values of regenerated TEG mass fraction. This case study clearly shows that it is necessary to obtain TEG regeneration levels in the range 99.5-99.9 % by weight in order to dehydrate natural gas to current specifications. Fig. 1 also shows that for low regeneration levels (98.5-99.0 % by weight) a further increase in the liquid to feed ratio in the absorption column has no effect and can not allow to reach water content specifications. At low regeneration levels, also increasing the number of theoretical stages of the contactor does not allow to reach water content specifications.For these reasons, several alternative regeneration processes (vacuum distillation, stripping gas, azeotropic distillation, etc.) have been proposed in order to enhance TEG regeneration levels. If TEG-water distillation is performed under vacuum, TEG can be regenerated up to 99.9 % by weight, but high operating costs and plant control complexity are serious drawbacks. Another possibility is represented by using some of the dehydrated natural gas as a stripping gas in order to further dehydrate TEG. The contact with lean TEG exiting the still column can take place in the surge drum where TEG make-up is introduced or in an appropriate stripping column. Operating in this way lean TEG purity can be increased above 99.9 % by weight, but the consumption of some of the dehydrated gas represents an economic loss. Furthermore, the use of stripping gas causes a considerable increase of the gas send to flare, unless an expensive recovery system is arranged. Purity of lean TEG exiting the still column can also be enhanced by using a circulating solvent (such as heptane or octane) to perform an azeotropic distillation instead of the simple TEG-water distillation. In this way lean TEG concentration can be increased up to 99.99 % by weight but plant complexity and costs are increased because of the need of a three-phase separator at the top of the distillator, the needs of treatments for the oily water discharged from this separator and devices for the solvent circulation line (such as a circulation pump and a heater) [4].If the required purity of the lean TEG is in the range 99.0–99.9 % by weight, Coldfinger process [5,6] is a valuable alternative. This process is similar to the conventional dehydration process, except for the use of an apparatus, also called water exhauster, where the liquid stream exiting the reboiler of the still column is further dehydrated. Coldfinger process is advantageous because no stripping gas, solvent or further energy consumption are required and because of the simplicity of the water exhauster which can be easily installed as an addition to a conventional plant.In this work, a study on the performance of the Coldfinger regeneration process, in the context of natural gas dehydration, is carried out and a discussion on the influence of the most relevant process parameters is presented.THE COLDFINGER WATER EXHAUSTERIn Fig. 2 a schematic representation of the Coldfinger water exhauster is reported. The liquid stream exiting the reboiler of the still column is conveyed to the Coldfinger apparatus where a further dehydration takes place. The liquid flows horizontally through the lower portion of the apparatus, which is provided with some buffles to maintain the surface of the liquid agitated and turbulent in order to increase the vaporization rate. The liquid can be considered in equilibrium with its vapour, which occupies the remaining part of the vessel. In the upper part of the apparatus, an elongated horizontally disposed condenser (the “cold finger”) occupies a little part of the vapour space and is in contact with the vapour. The cold U tubes of the condenser cause a local condensation of the vapour in close proximity. Because of free convection and local condensation on the tubes, new vapour is drawn from the bulk to the proximity of the condenser, whereas the uncondensed and cooled vapour comes back and mixes again with the bulk. Droplets of condensed liquid gravitate to a collector shaped in a way to allow the liquid to flow and to be removed from the vessel. Removed liquid, which is a TEG-water mixture, can be refluxed to the still column or to its reboiler in order to be distilled again. Since the vapour is richer in water with respect to the liquid, as a result a further dehydration of the liquid flowing in the lower part of the apparatus is obtained. The pressure inside the vessel can be conveniently maintained at values nearly atmospheric by means of a gas regulation line, which can be fed by splitting minimal quantities of lean gas produced in the same dehydration unit.If the Coldfinger apparatus is fed by a liquid mixture about 98.8-98.9 % TEG by weight, which is the typical concentration of the liquid exiting the still column reboiler at 204 °C and 1.1 bar, TEG purity can be increased up to 99.5-99.9 % by weight operating at atmospheric pressure. Dehydration levels which is possible to obtain in the Coldfinger apparatus basically depend on the rate of heat removal by the condenser and on the rate of hot vapour rising to the proximity of the cold tubes.Fig 2: Schematic representation of the Coldfinger apparatus.These parameters depend on the extension of the cold surface of the condenser, the temperature of the refrigerating fluid and on the fluid dynamics of the two phases inside the apparatus.GAS DEHYDRATION PROCESS WITH COLDFINGER TEG REGENERATIONA typical flow scheme of the natural gas dehydration process, with TEG regeneration enhanced by the Coldfinger apparatus, is reported in Fig. 3.Wet gas is fed to the bottom of the absorption column (A), whereas regenerated TEG (13) is fed to the top of it. As a consequence of the contact between gas and TEG, a stream of dry gas is obtained from the top, whereas a stream of water-rich TEG (1) is withdrawn from the bottom of the column and is conveyed to the regeneration section. Since the absorption column operates at high pressure (typically 40-80 bar) and low temperature (typically 20-60 °C), whereas the regeneration section operates at press ure around 1 bar and high temperature (up to 204 °C), water-rich TEG is expande d and heated before entering the still column (S). In order to heat water-rich TEG, heat can be conveniently recovered from the regeneration section. In particular, water-rich TEG after expansion (2) can be pre-heated while removing heat at the Coldfinger tubes and then (3) at the condenser of the still column. Because of heating and reducing pressure, some flash gas is produced and separated from the liquid stream in a flash drum (D). After filtration (F) to remove impurities, water-rich TEG undergoes another pre-heating step in a heat-exchanger (H), recovering heat from lean regenerated TEG (11), and then is fed to the still column.Water-rich TEG is distilled in the still column. The reboiler (R) can be heated by exhaust gases of the combustion of a small fraction of natural gas produced in the unit and used as a fuel. Reboiler temperature must be limited at 204 °C to avoid TEG thermal degradation. In this condition, TEG mass fraction at the reboiler will be around 98.8-98.9 % by weight. In order to increase TEG mass fraction, this liquid stream is conveyed to the Coldfinger apparatus (C), which can be realized in a separate vessel or in a vessel integrated with the reboiler itself, as in the case of Fig. 3. Heat removal at the condenser placed in the upper part of the Coldfinger apparatus can be conveniently provided by water-rich TEG as explained above.Fig 3: Typical flow scheme of natural gas dehydration with Coldfinger regenerationprocess.As an alternative, if a greater amount of heat removal is required in order to reach higher regeneration levels, cooling water can be employed to give a lower temperature of the cold tubes. Condensed liquid on the “cold fingers” is reintroduced inside the column. Because of the small quantity of this liquid stream, its inlet point has negligible effect and so it can be conveniently reintroduced inside the reboiler. Regenerated TEG exiting the Coldfinger apparatus is cooled (H) as far as possible before being pumped (P) back to the absorption column.PROCESS SIMULATIONThe process represented in Fig. 3 was studied by means of the process simulator Hysys. Phase equilibria conditions were represented by means of a thermodynamic model based on the Peng-Robinson equation of state. Correct representation of phase equilibria conditions of the binary system TEG-water, especially at high temperature and for mixture composition close to water infinite dilution, proved to be an essential requirement for regeneration process design. A thermodynamic study on TEG-water system was carried out on published data in order to optimize equation parameters. Therefore, to represent binary TEG-water system, simulator data bank was not used. Details on thermodynamic modelling are provided elsewhere [7].Since commercial process simulators do not provide Coldfinger apparatus by default, an user-defined external routine (Coldfinger routine) was generated in order to include the water exhauster in the whole process scheme. At first, Coldfinger routine was tested separately by considering an inlet liquid at typical reboiler conditions (around 200 °C, 1.1 bar and TEG 98.9 % by weight) and varying the heat removal at the condenser, for three different liquid feed flow rates.Fig. 4 shows TEG mass fraction in the liquid stream exiting the bottom of the water exhauster as a function of the rate of heat removal. As it was expected, increasing heat removal leads to an increase of liquid dehydration. Furthermore, it can be noticed that the shape of the curves resembles the behaviour of condensation degree, as a function of heat removal, in an isobaric cooling of a vapour mixture with some uncondensable gas. In the first region of the curves, that is for small amounts of heat removal, the slope is quite low, which is to say dehydration is not very effective. Increasing the rate of heat removal a sudden change in the slope of the curves is found. In this region, condensation degree at the cold tubes increases considerably. As a result, a great amount of water is removed from the system, causing a significant liquid dehydration. At very high heat removal rates, the slope of the curves suddenly decreases again.Fig 4: Regenerated TEG mass fraction obtained by means of the Coldfinger water exhauster as a function of the heat removal. Inlet liquid conditions: 204 °C, 1.1 bar,TEG mass fraction 98.9 %This is because in this region an almost complete condensation of the vapour in the proximityof the cold tubes is obtained. Since the system contains also small quantities of gas, the uncondensed gas is strongly cooled. When it mixes back with the bulk, it starts to cool the whole system slowing down new vapourization from the liquid. As a consequence, further dehydration is strongly reduced.After testing the water exhauster alone, the Coldfinger routine was included in the whole process scheme of simulation. Thus, an analysis on the performance of the natural gas dehydration with Coldfinger regeneration process was carried out on a case study. A typical wet gas composition was selected, as reported in Table 1. In some cases, depending on wet gas temperature, a pre-dehydration was obtained by separation of a liquid phase before entering the absorption column. Other process parameters were fixed as follows:- Total inlet gas flow rate: 100000 kg/h;- Absorption column pressure: 50 bar;- Absorption column theoretical stages: 4- Still column theoretical stages: 3- Still column reflux ratio: 0.10The number of theoretical stages of the absorption column was not increased above the reported value because this parameter has a poor effect on gas dehydration, especially when compared with regenerated TEG purity, which is the key factor of the process. Similarly, the number of theoretical stages at the still column was kept at the reported low value because no further effect on TEG dehydration can be achieved by increasing this parameter. Still column reflux ratio was fixed at the minimal value which allowed to reduce TEG losses around zero (0.001 % by weight on the circulating glycol). As for the absorption column, TEG losses were less than 0.1 % by weight in all simulations.The process was studied for different wet gas temperatures (from 20 to 60 °C), varying the liquid to gas ratio (L/G) at the absorber column in the range 0.03-0.13, which is to say that circulating TEG was varied from 3000 to 13000 kg/h. Heat removal at the Coldfinger was realized exchanging with TEG from the absorption column, which is the more convenient solution. Continuous lines in Fig. 5 show the results obtained by these simulations. As it was expected, process performance is strongly influenced by the temperature of the wet gas entering the absorption column. In fact, the temperature profile of the absorption column depends primarily on the wet gas temperature, because of the low (L/G) ratio. Furthermore,at higher wet gas temperature, the ratio (L/G) has a greater influence on the dehydration level that is possible to obtain.Tab. 1: Natural (wet) gas composition selected for the case study.Total inlet gas flow rate: 100000 kg/h.Component Weight fractionMethane 0.3853Ethane 0.2408Propane 0.1948n-Butane 0.0687iso-Butane 0.0325C5 and superior 0.0592Water 0.0044Nitrogen 0.0122Carbon Dioxide 0.0021Fig 5: Water content in the lean gas as a function of the liquid to feed ratio in the absorption column referred to four different wet gas temperature. Continuous lines refer to Coldfinger regeneration process. Dashed lines refer to regeneration bymeans of stripping gas.In the case study under consideration, the use of the Coldfinger apparatus allows to reach typical water content specifications for wet gas temperature up to 50 °C. At lower gas temperatures, the process is more efficient for two reasons: favourable thermodynamic conditions at the absorption column and improvement of Coldfinger regeneration because of the lower temperature of water-rich TEG, which can provide a larger heat removal rate at the “cold fingers”. However, even at 60 °C is possible to str ongly reduce water content in the dehydrated gas stream, down to 200 mg/Nm3approximately. In this case, a further improvement can be obtained refrigerating the Coldfinger with cold water from an external circuit, in order to reach higher levels of regenerated TEG purity.Continuous lines in Fig. 6 report natural gas consumption for the dehydration process with Coldifnger regeneration enhancement. Reported values refer to two different wet gasC.temperatures: 20 and 60 °lines) for two gas temperatures.Gas is consumed at the burner of the still column reboiler in order to produce the vapour rising into the column. Values range from around 20 to 70 kg/h, depending on the circulating liquid flow rate. At 20 °C gas consumption is lower b ecause some water is separated before entering the absorption column, thus leading to a smaller boiling up at the reboiler.A classical dehydration process scheme, with TEG regeneration enhanced by using some of the dehydrated gas, was simulated in order to provide a comparison with the Coldfinger process. Process flow scheme is the same as Fig. 3, except for the absence of the Coldfinger, which is substituted by a stripping column fed by some gas splitted from dehydrated gas stream. The number of theoretical stages of the stripping column was fixed at 2, which is a typical value for this process. All the other process parameters were kept equal to the values reported above. In a preliminary set of simulations, stripping gas flow rate was varied until similar water content specifications, with respect to Coldfinger process, were obtained. This similarity was found operating the stripping column with a gas flow rate equal to 100 kg/h. The dependence of water content on liquid to gas ratio for this process is represented by the dashed lines shown in Fig. 5. Operating the stripping column at a constant gas flow rate, a minimum water content was found at high liquid to gas ratio. This is caused by two contrasting effects: increasing the circulating liquid improves the dehydration at the absorption column but regeneration becomes more difficult. This behaviour does not appear with Codfinger process because, when circulating TEG increases, heat removal rate at the “cold fingers” increases as well. A correct design of the Coldfinger can lead to a constant regeneration level while circulating liquid increases.Dashed lines in Fig. 6 show the consumption of dehydrated gas, with respect to the process with regeneration enhanced by stripping gas. Comparing the two processes, it can be seen that the Coldfinger process is capable of dehydrating natural gas consuming less dry gas, with respect to similar water content specifications. In particular, the gas consumption of the process with stripping gas regeneration resulted to be from 2.5 to 6 times greater than the Coldfinger process.CONCLUSIONSA study on the Coldfinger apparatus was carried out in order to underline its capability of further dehydrating TEG from atmospheric distillation. It was demonstrated that Coldfinger unit is capable of increasing TEG mass fraction up to 99.5 –99.8 % by weight, operating at atmospheric pressure. An analysis on the performance of the natural gas dehydration, including Coldfinger regeneration process, was carried out on a case study by means of the process simulator Hysys. It was demonstrated that current water specifications in the dehydrated gas can be reached without adding stripping gas, solvent or increasing energy consumption.REFERENCES[1] Huffmaster M.A., “Gas Dehydration Fundamentals”, Laurance Reid Gas Conditioning Conference, 2004[2] Steele W.V., Chirico R.D., Knipmeyer S.E., Nguyen A., “Vapor Pressure of Acetophenone, 1,2-Butanediol, 1,3-Butanediol, Diethylene Glycol Monopropyl Ether, 1,3-Dimethyladamantane, 2-Ethoxyethyl Acetate, Ethyl Octyl Sulfide, and Pentyl Acetate”, J.Chem.Eng.Data 41 (1996) 1255-1268[3] Rajeh A.O., Szirtes L., “Thermal Decomposition of Organic Derivatives of Crystalline Zirconium Phosphate – III. Thermal decomposition of diethylene glycol, benzyl alcohol and benzylamine intercalates of zirconium phosphate”, J. 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