外文翻譯--三種天然氣脫水方法的比較（英文）

1、Journal of Natural Gas Chemistry 20(2011)471–476Comparison of three methods for natural gas dehydrationMichal Netusil?, Pavel DitlDepartment of Process Engineering, Czech Technical University, Prague 6, 166 07, Czech Rep

2、ublic[ Manuscript received April 6, 2011; revised May 23, 2011 ]Abstract This paper compares three methods for natural gas dehydration that are widely applied in industry: (1) absorption by triethylene glycol, (2) adsorp

3、tion on solid desiccants and (3) condensation. A comparison is made according to their energy demand and suitability for use. The energy calculations are performed on a model where 105 Nm3/h water saturated natural gas i

4、s processed at 30 ?C. The pressure of the gas varies from 7 to 20 MPa. The required outlet concentration of water in natural gas is equivalent to the dew point temperature of ?10 ?C at gas pressure of 4 MPa.Key words gas

5、 reservoir; underground gas storage; natural gas; gas dehydration1. IntroductionThe theme of natural gas (NG) dehydration is closely con- nected with the storage of NG. There are two basic reasons why storing NG is an in

6、teresting idea. First, it can decrease the dependency on supply. Second, it can exploit the maximum capacity of distribution lines. NG is stored in summer periods when there is lower demand for it, and is withdrawn in wi

7、nter periods when significant amounts of NG are used for heating. Underground Gas Storages (UGSs) are the most advantageous option for storing large volumes of gas. Nowadays there are approximately 130 UGSs inside the Eu

8、ropean Union. Their total maximum technical storage capacity is around 95 bcm. According to the latest update, over 70 bcm of additional stor- age capacity will come on stream in Europe till 2020 [1]. There are three typ

9、es of UGSs: (1) aquifers, (2) depleted oil/gas fields and (3) salt cavern reservoirs. Each of these types possesses its own physical characteristics. Generally, the allowable pressure of stored gas inside a UGS is up to

10、20 MPa. The inside pressure increases as the gas is injected and decreases when there is a withdrawal. The output gas pressure depends on further pipeline distribution. Distribution sites normally begin at 7 MPa. The tem

11、perature of the gas usually ranges from 20?35 ?C. The exact temperature varies with the location of UGS and with the time of year. A disad- vantage of UGSs is that the gas becomes saturated by water vapors during the sto

12、rage. In the case of depleted oil fieldUGSs, vapors of higher hydrocarbons also contaminate the stored gas. The distribution specification sets the allowable water concentration in NG by specifying a dew point temper- at

13、ure (Tdew) of NG. Tdew is usually taken to be ?7 ?C for NG at 4 MPa [2]. This value is equivalent to roughly 5 gH2O/m3NG at 4 MPa. The water content in NG at saturation is depen- dent on the temperature and pressure with

14、in the UGS. This is well presented in Figure No.20, Chapter 20, in the GPSA Data Book (12th Edition). The average value of H2O in NG is five times higher than that of required. A dehydration step of NG from UGS is theref

15、ore essential before the gas is distributed. This paper compares industrially applied dehydration meth- ods according to their energy demand and suitability for use.2. Dehydration methods2.1. AbsorptionFirst dehydration

16、method is absorption of H2O. Absorp- tion is usually performed using triethylene glycol (TEG). Ab- sorption proceeds in a glycol contactor (a tray column or packet bed) with countercurrent flows of wet NG and TEG. During

17、 the contact, TEG is enriched by H2O and flows out of the bottom part of the contactor. Enriched TEG then continues flowing into the internal heat exchanger, which is incorporated at the top of the still column. It then

18、flows into the flash drum,? Corresponding author. Tel: +420-2243522714; Fax: +420-224310292; E-mail: netusil.michal@gmail.com This work was supported by the Inovation and Optimalization of Technologies for Natural Gas De

19、hydration (No. FR-TI1/173).Copyright©2011, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(10)60218-6Journal of Natural Gas Chemistry Vol. 20 No. 5 2011

20、 4732.2. AdsorptionThe second dehydration method is adsorption of H2O by a solid desiccant. In this method, H2O is usually adsorbed on a molecular sieve, silica gel or alumina. A comparison of the physical properties of

21、each desiccant is shown in Table 1 [5,6].Table 1. Comparison of the physical properties of desiccants used for NG dehydrationProperties Silica gel Alumina Molecular sieveSpecific area (m2/g) 750?830 210 650?800Pore volum

22、e (cm3/g) 0.4?0.45 0.21 0.27Pore diameter ( ? A) 22 26 4?5Design capacity 7?9 4?7 9?12(kgH2O/100 kgdesiccant)Density (kg/m3) 721 800?880 690?720Heat capacity (J·kg?1·K?1) 920 240 200Regeneration temperature (?C

23、) 230 240 290Heat of desorption (J) 3256 4183 3718Source: Tagliabue (2009), Gandhidasan (2001)The amount of adsorbed H2O molecules increases with the gas pressure and decreases with its temperature, which are taken into

24、account when the process parameters are de- signed. Adsorption dehydration columns always work period- ically. Minima of two bed systems are used. Typically, one bed dries the gas while the other is being regenerated. Re

25、gen- eration is performed by preheated gas, as depicted in Figure 3.Figure 3. Scheme of the temperature swing adsorption dehydration processThe heater for TSA can be realized as an ordinary burner or as a shell and tube

26、heat exchanger warmed by steam or hot oil. The regeneration gas flows through the adsorbent into a cooler (usually using cold air) and then further into the sep- arator. Most of the desorbed humidity from the adsorbent i

27、s removed there. A downstream flow of wet NG through the adsorption column is usually applied. In this way, floating and channeling of an adsorbent is avoided. The regenera- tion is performed by countercurrent flow in or

28、der to provide complete regeneration from the bottom of the column, where the last contact of the dried NG with the adsorbent proceeds. The typical temperature course for regeneration of molecular sieves is presented by

29、Kumar (1987) [7]. The shape of the curve representing the course of the outlet regeneration gas temperature is typically composed of four regions. They arespecified by time borders A, B, C and D with appropriate bor- der

30、 temperatures TA, TB, TC and TD. Regeneration starts at point A. The inlet regeneration gas warms the column and the adsorbent. Around a temperature of 120 ?C (TB), the sorbed humidity starts to evaporate from the pores.

31、 The adsorbent continues warming more slowly, because a considerable part of the heat is consumed by water evaporation. From point C, around the temperature of 140 ?C (TC), it can be assumed that all water has been desor

32、bed. Adsorbent is further heated to desorb C5+ and other contaminants till point D. The regenera- tion is completed when the outlet temperature of the regener- ation gas reaches 180–190 ?C (TD). Finally, cooling proceeds

33、 from point D to E. The temperature of the cooling gas should not decrease below 50 ?C, in order to prevent any water con- densation from the cooling gas [7]. Part of the processed NG is sometimes used as the regeneratio

34、n gas. Then it is cooled, and water condensed when it is separated. After H2O sepa- ration, the regeneration gas is added back into the processed stream. So-called LBTSA (Layered Bed Temperature-Swing Ad- sorption) proce

35、sses are an upgrade of TSA method. Here, the adsorption column is composed of several layers of different adsorbents. Hence, the properties of the separate adsorbents are combined in one column. For example, a combinatio

36、n of silica gel with alumina is used in NG dehydration. Alumina has better resistance to liquid water, so it is put in the first place to contact the wet NG. This ordering prolongs the life- time of the silica gel, which

37、 is placed below the alumina layer.2.3. CondensationThe third dehydration method employs gas cooling to turn H2O molecules into the liquid phase and then removes them from the stream. Natural gas liquids (NGLs) and conde

38、nsed higher hydrocarbons can also be recovered from NG by cool- ing. The condensation method is therefore usually applied for simultaneous dehydration and NGL recovery. NG can be advantageously cooled using the Joule-Tho

39、mpson effect (JT effect). The JT effect describes how the temperature of a gas changes with pressure adjustment. For NG, owing to expan- sion, the average distance between its molecules increases, leading to an increase

40、in their potential energy (Van der Waals forces). During expansion, there is no heat exchange with the environment or work creation. Therefore, according to the conservation law, the increase in potential energy leads to

41、 a decrease in kinetic energy and thus a temperature decrease of NG. However, there is another phenomenon connected with the cooling of wet NG. Attention should be paid to the forma- tion of methane hydrate. Methane hydr

42、ate is a solid in which a large amount of methane is trapped within the crystal structure of water, forming a solid similar to ice. The hydrate produc- tion from a unit amount of water is higher than the ice forma- tion.

43、 Hydrates formed by cooling may plug the flow. This is usually prevented by injecting methanol or monoethylengly- col (MEG) hydrate inhibitors before each cooling. Figure 4 depicts a dehydration method utilizing the JT e