two-phase expanders replace joule-thomson valves at nitrogen
TRANSCRIPT
"Two-Phase Expanders Replace Joule-Thomson Valves at Nitrogen Rejection Plants"
5th World LNG Summit, 1st to 3rd December 2004. Cholast, Kociemba & Heath 1 of 1
Two-Phase Expanders Replace Joule-Thomson
Valves at Nitrogen Rejection Plants 5th World LNG Summit, 1st to 3rd December 2004
Katarzyna Cholast and Andrzej Kociemba Process Advisors Ostrów Wielkopolski Poland
John Heath Special Projects Research & Development Ebara International Corporation Sparks, Nv, USA. [email protected]
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.
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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"
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The processing of the feed gas for the separation of nitrogen and methane employing two-
phase 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.
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TWO-PHASE EXPANDER DESIGN CONCEPT
Figure 2. Ebara Two-Phase Expander Cross Section
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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 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.
Figure 3. Two-Phase Hydraulic Runner Assembly
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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
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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 10 000 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
Figure 5 Assembling the Expander
Figure 6 Expander Installation
"Two-Phase Expanders Replace Joule-Thomson Valves at Nitrogen Rejection Plants"
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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.
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.
Figure 7. Two-Phase Hydraulic Performance. Differential Pressure & Efficiency vs Mass Flow for a Range of Rotational Speeds
"Two-Phase Expanders Replace Joule-Thomson Valves at Nitrogen Rejection Plants"
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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.
Figure 8. Two-Phase Hydraulic Performance.
Differential Pressure & Power vs Mass Flow for a Range of Rotational Speeds
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SUMMARY and BENEFITS ANALYSIS
- Because of the higher efficiency of the described process employing Ebara liquid two-
phase 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.
Figure 9. Cooling Effect of Two-Phase Expansion
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- 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 email [email protected]
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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, www.mpptech.com/techpp/tech_home.htm
- Hays, Lance, "History and Overview of Two-Phase Turbines", International Conference on Compressors and Their Systems", Institution of Mechanical Engineers, London, 1999.
- Bond, Ted, "Replacement of Joule-Thomson Valves by Two-Phase Flow Turbines in Industrial Refrigeration Application", 2000, www.mpptech.com/techpp/tech_home.htm
- 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, 2000.
- Gebhart, Benjamin et al.; "Buoyancy-Induced Flows and Transport" Hemisphere Publishing Corporation, New York, 1988, ISBN 0-89116-728-5
- Hsu, Peter; Evrensel, Cahit A.; Kimmel, Hans E.; "Cavitation-Free Cryogenic Two-Phase Expanders", CAV 2003, Fifth International Symposium on Cavitation, Osaka, Japan, November 2003
- Boom, R.W. et al.; "Experimental Investigation of the Helium Two Phase Flow Pressure Drop Characteristics in Vertical Tubes", Proc. ICEC 7, pg 468-473, 1978
- Elliott, D.G.; Weinberg, E; "Acceleration of Liquids in Two-Phase Nozzles", Technical Report no.32-987, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA, 1968
- Filina, N.N.; Weisend II, J.G.; "Cryogenic Two-Phase Flow: Applications to large-scale systems", Cambridge University Press, 1996, ISBN 0-521-48192-9
- 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, 21-24 March 2004, Doha, Qatar.
- Fischer, C; Kimmel, H.E; "Improved LNG Production Process Using Two Phase Expanders" , Proceedings 5th World LNG Summit, 1-3 December 2004, Rome, Italy.
Katarzyna CholastKatarzyna CholastProcess AdvisorProcess Advisor
OstrOstróów Wielkopolskiw Wielkopolski, , PolandPoland
Andrzej KociembaAndrzej KociembaProcess AdvisorProcess Advisor
OstrOstróów Wielkopolskiw Wielkopolski, , PolandPoland
John HeathJohn HeathEbara International Corporation, Sparks, Nevada, USAEbara International Corporation, Sparks, Nevada, USA
EE--mail: [email protected] mail: [email protected]
Fifth World LNG SummitFifth World LNG Summit11stst –– 3rd December 2004,3rd December 2004, Rome, ItalyRome, Italy
TwoTwo--Phase Expanders Replace JoulePhase Expanders Replace Joule--Thomson Thomson Valves at Nitrogen Rejection PlantsValves at Nitrogen Rejection Plants
The twoThe two--phase cryogenic expander phase cryogenic expander evaporates the nitrogen thus improving evaporates the nitrogen thus improving
the quality and the quantity of the the quality and the quantity of the liquefied methane, significantly improving liquefied methane, significantly improving
the thermodynamic efficiency.the thermodynamic efficiency.
Natural gas entering Nitrogen Rejection Natural gas entering Nitrogen Rejection Units obviouslyUnits obviously contains an undesirable contains an undesirable
amount of nitrogen.amount of nitrogen.
Technology is available from Ebara to Technology is available from Ebara to manufacture and operate reliable manufacture and operate reliable
cryogenic expanders to expand liquefied cryogenic expanders to expand liquefied gases partially into the vapour phase. gases partially into the vapour phase.
This paper presents the operation of twoThis paper presents the operation of two--phase cryogenic turbine expanders for phase cryogenic turbine expanders for
cryogenic gases, operating at a natural gas cryogenic gases, operating at a natural gas liquefaction plant in Poland.liquefaction plant in Poland.
The design of the twoThe design of the two--phase expander phase expander allows operation free of cavitation and allows operation free of cavitation and
with low vibration levels.with low vibration levels.
Installation Site for Installation Site for TwoTwo--Phase Exducer Phase Exducer
TurbineTurbine20032003
Krio Polish Oil & GasKrio Polish Oil & GasOdolanOdolanóów, Polandw, PolandNitrogen Rejection Nitrogen Rejection
PlantPlant
Process Technology DiagramProcess Technology Diagram
Simplified NRU Process SchematicSimplified NRU Process Schematic
Cryogenic Cryogenic Distillation Distillation ColumnsColumns
Hydraulic Assembly Hydraulic Assembly Design ConceptDesign Concept
Expansion across a jet exducer with helical Expansion across a jet exducer with helical fluid passages for vapour formation.fluid passages for vapour formation.
Axial Convergence of Nozzle RingAxial Convergence of Nozzle Ring
Radial and Axial Converging Radial and Axial Converging Nozzle RingNozzle Ring
January 2003January 2003
Installation of Installation of TwoTwo--Phase Phase
Expander at Krio Expander at Krio Polish Oil & Gas Polish Oil & Gas
OdolanOdolanóów, w, PolandPoland
Ebara TwoEbara Two--Phase Phase Expander during Expander during
assembly aassembly at the t the Krio Polish Oil & Krio Polish Oil &
Gas SiteGas Site
Screen displayScreen display
TwoTwo--Phase Expander PerformancePhase Expander Performance
TwoTwo--Phase Expander PerformancePhase Expander Performance
Isentropic Temperature Reduction Isentropic Temperature Reduction vs. Power Outputvs. Power Output
General General ViewView
LNG Road Tanker LoadingLNG Road Tanker Loading
Katarzyna CholastKatarzyna CholastOstrOstróów Wielkopolskiw Wielkopolski, , PolandPoland
Andrzej KociembaAndrzej KociembaOstrOstróów Wielkopolskiw Wielkopolski, , PolandPoland
John HeathJohn HeathEbara International CorporationEbara International Corporation
Thank YouThank You