Two-Phase Expanders Replace Joule-Thomson Valves at Nitrogen Rejection Plants

Transcription

1 Two-Phase Expanders Replace Joule-Thomson Valves at Nitrogen Rejection Plants 5 th World LNG Summit, 1 st to 3 rd December 2004 Katarzyna Cholast and Andrzej Kociemba Process Advisors Ostrów Wielkopolski Poland John Heath Special Projects Research & Development Ebara International Corporation Sparks, Nv, USA. 2PhaseExpander@btconnect.com INTRODUCTION A problem often encountered in the production of natural gas from underground sources is nitrogen contamination. The nitrogen may be naturally occurring and/or may have been injected into the reservoirs as part of an enhanced oil recovery or enhanced gas recovery operation. Natural gases which contain a significant amount of nitrogen may not be saleable since they do not meet minimum heating value requirements. As a result the feed gas will generally undergo processing, wherein heavier components such as heavy hydrocarbons or carbon dioxide are initially removed and the remaining stream containing nitrogen and methane, and also possibly containing lower boiling or more volatile components such as helium, hydrogen and/or neon, is separated cryogenically when passing through a nitrogen rejection unit. The nitrogen rejection unit (NRU) comprises cryogenic rectification columns and Joule-Thomson (J-T) valves. J-T valves are applied to reduce the pressure of streams entering the rectification columns in order to decrease the stream temperature below the temperature of condensation. Low concentration of the more volatile components in the cryogenic separation of nitrogen and methane hinders the efficiency of the cryogenic rectification as it reduces the amount and quality of the available nitrogen reflux and thus the separation of the nitrogen and methane is carried out to lesser extent than is desirable. This results in the loss of some methane with the nitrogen overhead from the nitrogen rejection unit. There are three main aspects associated with the loss of methane : - The environmental impact. - The economic loss. - With depleting gas sources the pressure drop of the feed gas may fall below the limit which assures efficient operation of the NRU and specifically adequate temperature drop before entering cryogenic columns. "Two-Phase Expanders Replace Joule-Thomson Valves at Nitrogen Rejection Plants" 5 th World LNG Summit, 1 st to 3 rd December Cholast, Kociemba & Heath 1 of 1

2 With the depleting gas source the feed gas pressure may require expensive investment in a pre-compression step. It would be desirable to improve the efficiency of the NRU operation so that even with the drop of the feed gas pressure there will be still enough energy in the process to run it. BRIEF DESCRIPTION OF THE PROCESS Currently all energy needed for natural gas separation in low temperature units is provided by pressure reduction of the natural gas across Joule-Thomson valves. In the process described, the Joule-Thomson valves are replaced with liquid to two-phase expansion turbines (TPExp), which use the more efficient thermodynamic isentropic depressurisation cycle instead of isenthalpic depressurisation across a Joule-Thomson valve. Turbines take energy out of the process bringing about greater cooling of the streams passing through and increasing efficiency. Figure 1. Simplified Partial NRU Process Schematic "Two-Phase Expanders Replace Joule-Thomson Valves at Nitrogen Rejection Plants" 5 th World LNG Summit, 1 st to 3 rd December Cholast, Kociemba & Heath 2 of 2

3 The processing of the feed gas for the separation of nitrogen and methane employing twophase expansion turbines would involve then the following steps : (a) Pre-cooling the feed gas stream and, as liquid, entering the first two-phase expansion turbine (TPExp) (b) Exiting the TPExp as two-phase feed stream (at reduced pressure and cooled down) to enter the bottom part of high-pressure column (HPC) (c) Within the HPC separation into streams : liquid enriched with methane ( rich liquid ) and liquid enriched with nitrogen ( poor liquid ) (d) Subcooling of the rich liquid leaving the HPC and passing it through the second TPExp; the outgoing two-phase stream at reduced pressure enters low pressure column (LPC) (e) Subcooling of the poor liquid and passing it as reflux to the LPC (f) Within the LPC, separation of streams : LNG with the outlet in the bottom and waste gas with the outlet in the top of the LPC With the reflux greatly cooled down the separation of methane from the waste gas stream is considerably more effective compared to the previous J-T arrangement. "Two-Phase Expanders Replace Joule-Thomson Valves at Nitrogen Rejection Plants" 5 th World LNG Summit, 1 st to 3 rd December Cholast, Kociemba & Heath 3 of 3

4 TWO-PHASE EXPANDER DESIGN CONCEPT Figure 2. Ebara Two-Phase Expander Cross Section "Two-Phase Expanders Replace Joule-Thomson Valves at Nitrogen Rejection Plants" 5 th World LNG Summit, 1 st to 3 rd December Cholast, Kociemba & Heath 4 of 4

5 Two-phase expander design concepts fundamentally follow existing single-phase turbine and expander technology. The hydraulic energy of the pressurized fluid is converted by first transforming it into kinetic energy, then into mechanical shaft power and finally to electrical energy through the use of an electrical power generator. The generator is submerged in the cryogenic liquid and mounted integrally with the expander on a common shaft. The cryogenic induction generator uses insulation systems specifically developed for cryogenic service giving submerged windings significantly superior dielectric and life properties. Figure 3. Two-Phase Hydraulic Runner Assembly Figure 2 portrays the cross section of a typical Ebara International Corporation cryogenic two-phase submerged expander. The expander consists of a nozzle ring generating the rotational fluid flow, a radial inflow reaction turbine runner and a two-phase jet exducer. Figure 3 illustrates an enlarged cross section of the two-phase hydraulic runner assembly with inlet nozzle ring. "Two-Phase Expanders Replace Joule-Thomson Valves at Nitrogen Rejection Plants" 5 th World LNG Summit, 1 st to 3 rd December Cholast, Kociemba & Heath 5 of 5

6 Symmetrical flow is achieved in the two-phase expander by utilising a vertical rotational axis to stabilize the flow and to minimize flow induced vibrations, with the direction of flow being upward to take advantage of the buoyant forces of the vapour bubbles. ( Expanders with horizontal rotational axis generate asymmetric flow conditions which can result in higher vibration levels. ) The hydraulic assembly is designed for continuously decreasing pressure to avoid any cavitation along the two-phase flow passage. FIELD EXPERIENCE USING TWO-PHASE EXPANDERS To upgrade low-methane natural gas by extracting undesired nitrogen, two Ebara two-phase expanders (TPExps) were installed at the Polish Nitrogen Rejection Unit shown in Figure 4 Figure 4 Nitrogen Rejection Plant in Poland "Two-Phase Expanders Replace Joule-Thomson Valves at Nitrogen Rejection Plants" 5 th World LNG Summit, 1 st to 3 rd December Cholast, Kociemba & Heath 6 of 6

7 Figure 5 Assembling the Expander Figure 6 Expander Installation Briefly, based upon the operational experience the following statements may be made : - The expanders required limited modification of the existing equipment and consequently their installation was easy and quick. - The two-phase expanders have been in stable operation for more than hours now. Throughout that period regular inspections have shown no incipient failures in bearings or materials, vibration levels have been less than 20% of API 610 allowable limits. - The expanders operate surprisingly quietly; they are not heard while working with neighbouring equipment of average noise level below 80dB. - The employed expanders have made the process really flexible in terms of its adjustment to changing mass flows, varying even by 100%. Even with such considerable changes they assure easy and precise regulation of levels in the cryogenic columns, which is of fundamental value for stable running of the process. - Due to the greater obtained temperature difference the heat exchangers operate in a more efficient and flexible way minimizing the danger of so called cold leaving out of cold-box. - The use of two-phase expanders allows the LNG product from the NRU to be at considerably higher outlet pressure, increasing to approximately 2 bars. The benefits have "Two-Phase Expanders Replace Joule-Thomson Valves at Nitrogen Rejection Plants" 5 th World LNG Summit, 1 st to 3 rd December Cholast, Kociemba & Heath 7 of 7

8 appeared as bigger capacity and lower required compression, thus with lower fuel gas consumption per compressed unit at the product s compression stage. - With the expanders operating there is a significant increase in LNG output from the NRU of upto 250% compared to when Joule-Thomson valves were in operation. Of great significance is that the higher pressure of the LNG product from the NRU and the increase of LNG output take place concurrently. Figure 7. Two-Phase Hydraulic Performance. Differential Pressure & Efficiency vs Mass Flow for a Range of Rotational Speeds The two-phase expanders operate at variable speeds in order to adjust to the changing mass flows and pressure conditions of the plant. Figure 7 presents the hydraulic performance of the two-phase expanders as a scatter graph. Efficiency is defined as the ratio of electrical power generated divided by the hydraulic power input. Hydraulic power input is the product of mass flow and differential pressure. The solid vertical red line depicts rated mass flow and the solid horizontal red line indicates rated differential pressure. "Two-Phase Expanders Replace Joule-Thomson Valves at Nitrogen Rejection Plants" 5 th World LNG Summit, 1 st to 3 rd December Cholast, Kociemba & Heath 8 of 8

9 Figure 8. Two-Phase Hydraulic Performance. Differential Pressure & Power vs Mass Flow for a Range of Rotational Speeds Figure 8 presents the same data as Figure 7, but plotted as a line chart for differential pressure and power versus mass flow. The solid red lines in Figure 8 indicate the rated differential pressure and the rated mass flow. The volumetric flow increases with increasing differential pressure due to the expansion of the two-phase fluid and is seen here as reducing mass flow. Cooling the LNG stream is significantly more efficient using two-phase expanders rather than single-phase expanders or other devices. Figure 9 presents the LNG temperature drop versus the power output for the previously described two-phase expander and the cooling effect on the LNG stream is seen to be directly related to the power output. "Two-Phase Expanders Replace Joule-Thomson Valves at Nitrogen Rejection Plants" 5 th World LNG Summit, 1 st to 3 rd December Cholast, Kociemba & Heath 9 of 9

10 Figure 9. Cooling Effect of Two-Phase Expansion SUMMARY and BENEFITS ANALYSIS - Because of the higher efficiency of the described process employing Ebara liquid twophase expansion turbines the reflux is of better quality (in terms of lower temperature) as well as the other streams being deeply cooled which will compensate for lower concentration of nitrogen in feed gas. Thus the NRU can operate now with lower nitrogen concentration in the feed gas whilst keeping such parameters of the process as the loss of methane at the optimal level. - By the use of the presented method one can run the process of nitrogen and methane separation even with short-term carbon-dioxide increases without having to prepare expensive and extensive additional carbon dioxide removal steps. The employed liquid two-phase expansion turbine can accept short term higher carbon dioxide concentration with no danger of plugging or consequent shut-down of the whole NRU. - The described process, being very efficient, allows for running it at a lower feed gas pressure. In case of reducing pressure of the feed gas from depleting sources one can postpone the decision to install an expensive pre-compression step. "Two-Phase Expanders Replace Joule-Thomson Valves at Nitrogen Rejection Plants" 5 th World LNG Summit, 1 st to 3 rd December Cholast, Kociemba & Heath 10 of 10

11 - Due to the high efficiency of the process presented above there is a possibility of taking out of the process considerable amounts of low-pressure or high-pressure liquefied natural gas (LNG) or a liquid nitrogen stream, running the nitrogen methane separation at the same time. The possibility of producing LNG may be useful for the plants where the Peak Shaving concept is going to be applied. If taking out liquid nitrogen is considered, one should be aware of the increased methane content in waste gas and the associated cost of that. - Employing liquid two-phase expansion turbines in the separation of nitrogen and methane will allow generation of energy that can be used in different forms. - The more efficient process employing liquid two-phase expansion turbines requires less energy to be provided to the separation unit to obtain the same final results as with Joule- Thomson valves or alternatively has got more cooling capacity with the same inlet parameters. This makes the process more flexible, easy to operate and controllable with no danger of shut-down even with considerable changes of feed gas parameters.. For more information please 2PhaseExpander@btconnect.com "Two-Phase Expanders Replace Joule-Thomson Valves at Nitrogen Rejection Plants" 5 th World LNG Summit, 1 st to 3 rd December Cholast, Kociemba & Heath 11 of 11

12 BIBLIOGRAPHY and REFERENCES - Ross, Greg; Davies, Simon; Vislie, Geirmund; Hays, Lance; "Reductions of Greenhouse Gas Emissions in Oil and Gas Production and Processing by Application of Biphase Turbines", 1996, - Hays, Lance, "History and Overview of Two-Phase Turbines", International Conference on Compressors and Their Systems", Institution of Mechanical Engineers, London, Bond, Ted, "Replacement of Joule-Thomson Valves by Two-Phase Flow Turbines in Industrial Refrigeration Application", 2000, - Chiu, Chen-Hwa; Kikkawa, Yoshitsugi; Kimmel, Hans E.; Liu, Yu-Nan; "New Cryogenic Two-Phase Expanders in LNG Production", Third Topical Conference on Natural Gas Utilization, AIChE 2003 Spring National Meeting, New Orleans, Louisiana, USA - Shively, R.A. and Miller, H., Development of a Submerged Winding Induction Generator for Cryogenic Applications, in Proceedings of the IEEE Electrical Insulation Conference, Anaheim, California, Gebhart, Benjamin et al.; "Buoyancy-Induced Flows and Transport" Hemisphere Publishing Corporation, New York, 1988, ISBN Hsu, Peter; Evrensel, Cahit A.; Kimmel, Hans E.; "Cavitation-Free Cryogenic Two- Phase Expanders", CAV 2003, Fifth International Symposium on Cavitation, Osaka, Japan, November Boom, R.W. et al.; "Experimental Investigation of the Helium Two Phase Flow Pressure Drop Characteristics in Vertical Tubes", Proc. ICEC 7, pg , Elliott, D.G.; Weinberg, E; "Acceleration of Liquids in Two-Phase Nozzles", Technical Report no , Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA, Filina, N.N.; Weisend II, J.G.; "Cryogenic Two-Phase Flow: Applications to largescale systems", Cambridge University Press, 1996, ISBN Vislie, Geirmund; Davies, Simon; Hays, Lance; "Further Developments of Biphase Rotary Separator Turbine", Paper presented at IBC Separation Systems Conference, May 1997, Oslo, Norway. - Perlmutter, M.J.; Kimmel, H.E.; Chiu, C.H.; Paradowski, H.: "Economic and Environmental Benefits of Two-Phase Expanders", Proceedings LNG 14, March 2004, Doha, Qatar. - Fischer, C; Kimmel, H.E; "Improved LNG Production Process Using Two Phase Expanders", Proceedings 5 th World LNG Summit, 1-3 December 2004, Rome, Italy. "Two-Phase Expanders Replace Joule-Thomson Valves at Nitrogen Rejection Plants" 5 th World LNG Summit, 1 st to 3 rd December Cholast, Kociemba & Heath 12 of 12

13 Two-Phase Expanders Replace Joule-Thomson Valves at Nitrogen Rejection Plants Katarzyna Cholast Process Advisor Ostrów w Wielkopolski, Poland Andrzej Kociemba Process Advisor Ostrów w Wielkopolski, Poland John Heath Ebara International Corporation, Sparks, Nevada, USA 2PhaseExpander@btconnect.com Fifth World LNG Summit 1 st st 3rd December 2004, Rome, Italy

14 Natural gas entering Nitrogen Rejection Units obviously contains an undesirable amount of nitrogen. The two-phase cryogenic expander evaporates the nitrogen thus improving the quality and the quantity of the liquefied methane, significantly improving the thermodynamic efficiency.

15 This paper presents the operation of two- phase cryogenic turbine expanders for cryogenic gases, operating at a natural gas liquefaction plant in Poland. Technology is available from Ebara to manufacture and operate reliable cryogenic expanders to expand liquefied gases partially into the vapour phase. The design of the two-phase expander allows operation free of cavitation and with low vibration levels.

16 Krio Polish Oil & Gas Odolanów, Poland Nitrogen Rejection Plant Installation Site for Two-Phase Exducer Turbine 2003

17 Process Technology Diagram

18 Simplified NRU Process Schematic

19 Cryogenic Distillation Columns

20 Hydraulic Assembly Design Concept Expansion across a jet exducer with helical fluid passages for vapour formation.

21 Radial and Axial Converging Nozzle Ring Axial Convergence of Nozzle Ring

22 January 2003 Installation of Two-Phase Expander at Krio Polish Oil & Gas Odolanów, Poland

23 Ebara Two-Phase Expander during assembly at the Krio Polish Oil & Gas Site

24 Screen display

25 Two-Phase Expander Performance

26 Two-Phase Expander Performance

27 Isentropic Temperature Reduction vs. Power Output

28 General View

29 LNG Road Tanker Loading

30 Thank You Katarzyna Cholast Ostrów w Wielkopolski, Poland Andrzej Kociemba Ostrów w Wielkopolski, Poland John Heath Ebara International Corporation