Rapid estimation of equilibrium water dew point of natural gas in TEG

Transcription

1 From the SelectedWorks of ali ali 2009 Rapid estimation of equilibrium water dew point of natural gas in TEG ali ali Available at:

2 Journal of Natural Gas Science and Engineering 1 (2009) Contents lists available at ScienceDirect Journal of Natural Gas Science and Engineering journal homepage: Rapid estimation of equilibrium water dew point of natural gas in TEG dehydration systems Alireza Bahadori *, Hari B. Vuthaluru Department of Chemical Engineering, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia article info abstract Article history: Received May 2009 Received in revised form 6 August 2009 Accepted 6 August 2009 Available online 2 September 2009 Keywords: Correlation Natural gas Dew point Triethylene glycol Gas dehydration Evaluation of a triethylene glycol (TEG) system involves first establishing the minimum triethylene glycol (TEG) concentration required to meet the outlet gas water dew point specification. In the present work, simple-to-use correlation, which is simpler than currently available models involving a large number of parameters, requiring more complicated and longer computations, has been developed for the rapid estimation of the water dew point of a natural gas stream in equilibrium with a TEG solution at various temperatures and TEG concentrations. This correlation can be used to estimate the required TEG concentration for a particular application or the theoretical dew point depression for a given TEG concentration and contactor temperature. Actual outlet dewpoints depend on the TEG circulation rate and number of equilibrium stages, but typical approaches to equilibrium are 6 11 C. Equilibrium dewpoints are relatively insensitive to pressure and this correlation may be used up to 300 kpa (abs) with little error. The proposed correlation covers VLE data for TEG water system for contactor temperatures between Cand80 C and TEG concentrations ranging from to wt%. The average absolute deviation percent from the data reported in the literature is 0.5% which shows the excellent performance of proposed correlation. This simple-to-use correlation can be of immense practical value for the gas engineers to have a quick check on equilibrium water dew point of natural gas at various temperatures and TEG weight percents. In particular, personnel dealing with natural gas dehydration and processing would find the proposed approach to be user friendly involving no complex expressions with transparent calculations. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction The natural gas industry has recognized that dehydration is necessary to ensure smooth operation of gas transmission lines. Dehydration prevents the formation of gas hydrates and reduces corrosion. Natural gas is dehydrated using either a liquid desiccant (i.e. glycols) or a solid desiccant. But economics frequently favor liquid desiccant dehydration when it meets the required dehydration specification (Mokhatab et al., 2006). Glycols are typically used for applications, where dew point depressions of the order of C are required (Gas Processors and Suppliers Association Engineering Book, 2004). Triethylene glycol (TEG), the most common for natural gas dehydration, is used in a countercurrent mass transfer operation inside a contractor to establish the required water content in the outlet gas (Bahadori, 2007). Following the process flow in Fig. 1, * Corresponding author. Tel.: þ ; fax: þ address: alireza.bahadori@postgrad.curtin.edu.au (A. Bahadori). the regenerated glycol is pumped to the top tray of the contactor (absorber). The glycol absorbs water as it flows down through the contactor countercurrent to the gas flow. Water-rich glycol is removed from the bottom of the contactor, passes through the reflux condenser coil, flashes off most of the soluble gas in the flash tank, and flows through the rich-lean heat exchanger to the regenerator. In the regenerator, absorbed water is distilled from the glycol at near atmospheric pressure by application of heat. The regenerated lean glycol exits the surge drum, is partly cooled in the lean-rich exchanger and is pumped through the glycol cooler before being recirculated to the contactor (Bahadori, 2009). Evaluation of a triethylene glycol (TEG) system involves first establishing the minimum triethylene glycol (TEG) concentration required to meet the outlet gas water dew point specification (Bahadori et al., 2008). Several equilibrium correlations for predicting water dew point of natural gas in equilibrium with a TEG dehydration system have been presented since However, all methods are limited by the ability to measure accurately the equilibrium concentration of water in the vapor phase above /$ see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:.16/j.jngse

3 A. Bahadori, H.B. Vuthaluru / Journal of Natural Gas Science and Engineering 1 (2009) Nomenclature A B C D T T d W Temperature, K Water dewpoint temperature, K the TEG purity in water (weight percent) triethylene glycol (TEG) solutions (Twu et al., 2005). In the correlations developed by Parrish et al. (1986) and Won (1994), the equilibrium water concentrations in the vapor phase were determined at infinite dilution (essentially 0% TEG). The other correlations use extrapolations of data at lower concentrations to estimate equilibrium in the infinite dilution region (Parrish et al., 1986; Won, 1994). Herskowitz and Gottlieb (1984) measured the activity coefficients of water in TEG at two temperatures, and K. The lowest mole fraction of water for which measured activities were and at K and K, respectively. These fit the measured activity coefficients to the van Laar equation. They did not measure data in the infinite dilution region. In order to predict the equilibrium behavior in the infinite dilution region, most researchers simply extrapolated the measured data at low water concentrations to infinite dilution using an activity coefficient model such as van Laar. However, extrapolating the van Laar or any other activity coefficient model will yield erroneous results for the infinite dilution activity coefficients. The GPSA data book presented an equilibrium correlation based on the work of Worley (1967). In general, the correlations of Worley (1967), Rosman (1973) and Parrish et al. (1986) agree reasonably well and are adequate for most TEG system designs. All are limited by the ability to measure accurately the equilibrium concentration of water in the vapor phase above TEG solutions. In view of the above, there is an essential need to develop an easyto-use method for rapid and accurate prediction of equilibrium water dew point of natural gas in TEG dehydration system. 2. Methodology to develop simple correlation The required data to develop this correlation includes the reported data (Parrish et al., 1986; Herskowitz and Gottlieb, 1984) for the rapid estimation of water dew point (Td) of a natural gas stream in equilibrium with a triethylene glycol (TEG) solution at various contactor temperatures (T) and TEG concentrations (W) in percent. In this work, water dew point (Td) of a natural gas stream in equilibrium with a triethylene glycol (TEG) solution is predicted rapidly by proposing a simple correlation. The following methodology has been applied to develop this correlation: Firstly, water dewpoints (Td) of a natural gas stream in equilibrium with a triethylene glycol (TEG) solution are correlated as a function of contactor temperatures for different TEG concentrations. Then, the calculated coefficients for these polynomials are correlated as a function of TEG concentrations. The derived polynomials are applied to calculate new coefficients for equation (1) to predict the water dew point (Td) of a natural gas stream in equilibrium with a triethylene glycol (TEG) solution. Table 1 shows the tuned coefficients for equations (2) (5). In brief, the following steps are repeated to tune the correlation s coefficients. 1. Correlate the water dew point (Td) of a natural gas stream in equilibrium with a triethylene glycol (TEG) solution as a function of contactor temperature for a given TEG concentration 2. Repeat step 1 for other TEG concentrations. 3. Correlate corresponding polynomial coefficients, which are obtained in previous steps versus TEG concentrations, a ¼ f(w), b ¼ f(w), c ¼ f(w), d ¼ f(w) [see equations (2) (5)]. So, equation (1) represents the proposed governing equation in which four coefficients are used to correlate the water dew point (Td) of a natural gas stream in equilibrium with a triethylene glycol (TEG) solution as a function of contactor temperature and TEG concentration where the relevant coefficients have been reported in Table 1. T d ¼ a þ bt þ ct 2 þ dt 3 (1) Where: a ¼ A 1 þ B 1 W þ C 1 W 2 þ D 1 W 3 (2) b ¼ A 2 þ B 2 W þ C 2 W 2 þ D 2 W 3 (3) c ¼ A 3 þ B 3 W þ C 3 W 2 þ D 3 W 3 (4) d ¼ A 4 þ B 4 W þ C 4 W 2 þ D 4 W 3 (5) In the above equations, Td and T are the water dew point temperature and the contactor temperature in K, respectively and W is the triethylene glycol (TEG) purity in water (in weight percent). The tuned coefficients in equations (2) (5) are reported in Table 1. These coefficients help to cover the reported data provided by Herskowitz and Gottlieb (1984) and Parrish et al. (1986) for the contactor temperature variations of 80 C, and TEG purity of weight percent. 3. Results and discussions Fig. 1. Typical TEG-Natural Gas Dehydration System (Bahadori, 2007). Figs. 2 4 show the water dew point of a natural gas stream in equilibrium with a TEG solution at various TEG concentrations and contactor temperature between C and 80 C. As can be

4 70 A. Bahadori, H.B. Vuthaluru / Journal of Natural Gas Science and Engineering 1 (2009) Table 1 s used in Equations (2) (5). Coefficient 90% < TEG < 99% 99% < TEG < 99.9% 99.9% < TEG < % A B C D A B C D A B C D A B C D seen, there is a good agreement between predicted results and the reported values. Table 2 shows the average absolute deviation percent from the literature reported data in is 0.5% which proves the excellent accuracy of the proposed correlation. Since the TEG dehydrators usually operate at temperatures of less than 70 C, there was no practical need to include temperatures higher than 70 C in the graphs of this work. The equilibrium water dewpoints calculated by this correlation are based on this fact that the condensed water phase is considered as a metastable liquid. At low dewpoints the true condensed phase will be a hydrate. The equilibrium dew point temperature above a hydrate is higher than that above a metastable liquid. Therefore, this correlation predicts dewpoints which are colder than those which can actually be achieved. The difference is a function of temperature, pressure and gas composition but can be as much as 8 11 C. When dehydrating to very low dewpoints, such as those required upstream of a refrigeration process, the TEG concentration must be sufficient to dry the gas to the hydrate dew point. This correlation can be used to estimate the required TEG concentration for a particular application or theoretical dew point depression for a given TEG concentration and contactor temperature. Actual outlet dewpoints depend on the TEG circulation rate and the number of equilibrium stages, but typical approaches to equilibrium are 6 11 C. Table 2 shows the average absolute deviation percent from the literature reported data in is 0.5% which proves the excellent performance of this simple proposed correlation. Typical example is given below to illustrate the simplicity associated with the use of proposed correlation for rapid estimating dew point of natural gas at various temperatures and TEG weight percents Example 0.85 million Sm 3 /day of a natural gas enters a TEG contactor at 38 C and 40 kpa (abs). The target H 2 O dew point is 4 C ( K). Calculate the lean TEG concentration in mass percent at this given temperature (38 C). Assume a 6 C approach to equilibrium: Solution: a) Assume glycol concentration ¼ 98 percent a ¼ (from equation (2)) b ¼ (from equation (3)) c ¼ (from equation (4)) d ¼ (from equation (5)) Equilibrium water dew point (K) ¼ K (from equation (1)) Calculated water dew point þ 6 ¼ K Correlation, 90% TEG Correlation, 95% TEG Correlation, 97% TEG Correlation, 98% TEG Correlation, 99% TEG Correlation, TEG=99% Correlation, TEG=99.5% Correlation, TEG=99.8% Correlation, TEG=99.9% Fig. 2. Water dew point of a natural gas stream in equilibrium with a TEG solution at various contactor temperatures and TEG concentrations ranging from 90% to 99%. Fig. 3. Water dew point of a natural gas stream in equilibrium with a TEG solution at various contactor temperatures and TEG concentrations ranging from 99% to 99.9%.

5 A. Bahadori, H.B. Vuthaluru / Journal of Natural Gas Science and Engineering 1 (2009) Correlation, TEG=99.95% Correlation, TEG=99.98% Correlation, TEG=99.99 Correlation, TEG=99.995% Correlation, TEG=99.998% Correlation, TEG=99.999% Fig. 4. Water dew point of a natural gas stream in equilibrium with a TEG solution at various contactor temperatures and TEG concentrations ranging from 99.9% to %. b) Assume glycol concentration ¼ 99 percent a ¼ (from equation (2)) b ¼ (from equation (3)) c ¼ (from equation (4)) d ¼ (from equation (5)) Equilibrium water dew point (K) ¼ K (from equation (1)) Calculated water dew point þ 6 ¼ K. c) Assume glycol concentration ¼ percent a ¼ (from equation (2)) b ¼ (from equation (3)) c ¼ (from equation (4)) d ¼ (from equation (5)) Equilibriumwater dew point (K) ¼ K (from equation (1)) Calculated water dew point þ6 ¼ K. The calculated result ( K) has good agreement with water dew point ( K). So glycol purity meets targeted water dew point. We have suggested 6 C of approach because is a usual standard practice. The closeness of the result to the prediction could vary if we takes a different approach to equilibrium. Table 2 Prediction water dew point of a natural gas stream in equilibrium with a TEG solution at various contactor temperatures and TEG concentrations in comparison with the reported data (Parrish et al., 1986; Herskowitz and Gottlieb, 1984). TEG Temperature, weight K percent Proposed correlation results K Reported data (Gas Processors and Suppliers Association Engineering Book, 2004; Parrish et al., 1986; Herskowitz and Gottlieb, 1984) K Absolute deviation percent Average absolute deviation percent (AADP) Conclusions In the present work, simple-to-use correlation, which is much simpler than currently available models involving a large number of parameters, requiring more complicated and longer computations, has been developed for the rapid estimation of equilibrium water dew point of a natural gas stream in equilibrium with a triethylene glycol (TEG) solution at various contactor temperatures and TEG concentrations. The correlation covers VLE data for TEG water system for contactor temperatures between C and 80 C and TEG concentrations ranging from to wt%. This correlation can be used to estimate the required TEG concentration for the theoretical dew point depression for a given TEG concentration and contactor temperature. Equilibrium dewpoints are relatively insensitive to pressure and this correlation may be used up to 300 kpa (abs) with little error. The average absolute deviation percent from the data reported in the literature is 0.5% which shows the excellent performance of proposed correlation. The correlation proposed in the present work is novel and unique expression which is non-existent in the literature. Simple-to-use approach can be of immense practical value for the gas engineers to have a quick check on water dew point of natural gas at various temperatures and TEG weight percents without performing any experimental measurements. In particular, personnel dealing with natural gas dehydration and processing would find the proposed approach to be user friendly involving no complex expressions with transparent calculations. Acknowledgements The lead author acknowledges the Australian Department of Education, Science and Training for Endeavour International Postgraduate Research Scholarship (EIPRS), the Office of Research & Development at Curtin University of Technology, Perth, Western Australia for providing Curtin University Postgraduate Research Scholarship and the State Government of Western Australia for providing top up scholarship through the Western Australian Energy Research Alliance (WA:ERA). The authors also acknowledge anonymous reviewers and the editor for their useful comments to improve the original version of paper. References Bahadori, A., Hajizadeh, Y., Vuthaluru, H.B., Tade, M.O., Mokhatab, S., Novel approaches for the prediction of density of glycol solutions. Journal of Natural Gas Chemistry 17, Bahadori, A., New model predicts solubility in glycols. Oil & Gas Journal 5 (8), Bahadori, A., New model calculates solubility of light alkanes in triethylene gycol. Petroleum Chemistry 49, Gas Processors and Suppliers Association Engineering Book, 12th ed., 2004 Gas Processors & Suppliers Association (GPSA), Tulsa, OK, USA. Herskowitz, M., Gottlieb, M., Vapor liquid equilibrium in aqueous solutions of various glycols and polyethylene glycols. Journal of Chemical & Engineering 29, 173. Mokhatab, S., Poe, W.A., Speight, J.G., Handbook of Natural Gas Transmission & Processing, first ed. Gulf Professional Publishing, Burlington, MA, USA. Parrish, W.R., Won, K.W., Baltatu, M.E., Paper presented at the 65th GPA Annual Convention, San Antonio, TX, USA. Rosman, A., Water equilibrium in the dehydration of natural gas with triethylene glycol. Transactions of the AIME 255, 297. Twu, C.H., Tassoneb, V., Simb, W.D., Watansiri, S., Advanced equation of state method for modeling TEG water for glycol gas dehydration. Fluid Phase Equilibria 228, Won, K.W., Paper presented at the 73rd GPA Annual Convention, New Orleans, LA, USA. Worley, S., Proceedings Gas Conditioning Conference, University of Oklahoma, Norman, OK.