two-phase expanders replace joule-thomson valves at nitrogen

"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.

John Heath

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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



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.



TWO-PHASE EXPANDER DESIGN CONCEPT

Figure 2. Ebara Two-Phase Expander Cross Section



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



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



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



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



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



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



- 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]

John Heath

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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

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

two-phase expanders replace joule-thomson valves at nitrogen

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