Linde Kryotechnik AG, Daettlikonerstrasse 5, CH-8422 Pfungen, Switzerland

Transcription

1 LARGE SCALE HELIU LIQUEFACTION AND CONSIDERATIONS FOR SITE SERVICES FOR A PLANT LOCATED IN ALGERIA Froehlich P., Clausen J.J. Linde Kryotechnik AG, Daettlikonerstrasse 5, CH-8422 Pfungen, Switzerland ABSTRACT The large scale liquefaction of Helium extracted from natural gas is depicted. Based on a block diagram the process chain, starting with the pipeline downstream of the natural gas plant to the final storage of liquid Helium is explained. Information will be provided about the recent experiences during installation and start-up of a bulk Helium liquefaction plant located in Skikda, Algeria, including part load operation based on a reduced feed gas supply. The local working and ambient conditions are described including challenging logistic problems, like shipping and receiving of parts, qualified and semi-qualified subcontractors, basic provisions and tools at site, and precautions to sea water and ambient conditions. Finally the differences in commissioning (technically and evaluation of time and work packages) to a European location and standards will be discussed. INTRODUCTION Skikda (Arabic: ةدكيكس ) is a city in the north eastern part of Algeria having a port on the Gulf of Stora in the editerranean sea. Natural gas, oil refining, and petrochemical industries were developed in Algeria in the last decades and pipelines have been built for their transportation to the coast. SONATRACH is an Algerian government-owned company formed to exploit the hydrocarbon resources of the country. Its diversified activities include all: exploration, extraction, transport, and refining. In arch 2003 SONATRACH formed the joint venture Helison with Germany's Linde AG. The venture's plan was to construct and operate a Helium plant in Skikda, Algeria. Helium is a product, which will be used in different industrial applications and the worldwide demand of Helium is constantly increasing. This presented paper describes how Helium is extracted from the existing liquid natural gas plants (LNG-trains), and liquefied and stored for transportation to final users. Destination of the Helium product liquefied in Skikda is mainly Europe.

2 Natural Gas (NG) Sour Gas NG Treatment Pipeline Transport CH 4 CH 4 + N 2 + He NG Liquefaction He Extraction Cryogenic Gas Separation N 2 + He CH 4 + C 2+ He Purification PSA He He Liquefaction LNG Ship Transport LHe Container Transport LIN FIGURE 1. General large-scale Helium extraction HELIU EXTRACTION FRO LNG The Oil & Gas company SONATRCH operates natural gas facilities at Hassi R el, Arzew, and Skikda. For gas transportation pipelines are used. In Skikda, SONATRACH operates a six train natural gas liquefaction plant, consisting of four older liquefaction trains situated on one side of the LNG storage area and two newer ones, located on the opposite side of the compound. On January 19th 2004, an explosion destroyed one of the older trains which was built differently to the newer, using boilers to make high-pressure steam to drive turbines reducing the capacity of the gas stream. Linde AG, Germany, and SONATRACH are having build up a plant at Skikda's LNG complex designed for 600 SCF/y liquid Helium talking in gas business units, which is equivalent to 2233 Nm 3 /h or a liquefaction rate of 3400 Liter/h. Helium is extracted by fractional distillation from natural gas, which contains at the Skikda site up to 6.4 % Helium. Since Helium has a lower boiling point than any other element, low temperature and high pressure purification processes are used to remove nearly all other gases (mostly nitrogen). The LIN extracted from the gas will be used later for various shield- or pre-cooling applications within the plant. Additional LIN, required for different process requirements apart from the LNG / Helium plant, will be delivered by truck trailers. The raw Helium gas is further purified by pressure swing adsorbtion (PSA) and residual contaminants are removed in the liquefaction coldbox by low temperature adsorbers (refer to process flow shown in Figure 1). Activated charcoal within the Helium Coldbox is used as a final purification step to remove Hydrogen, resulting in % pure Helium. The crude Helium from the natural gas liquefaction (LNG-Trains operated by SONATRACH) will be feed to the Helium plant by a low and high pressure stream with following composition: Feed Gas (LP): CH4: 53 mol %, N2: 43%, He: 4%, Ar: 0.1%, H2: 0.01% Feed Gas (HP): CH4: 58 mol %, N2: 35%, He: 6,4%, Ar: 0.2%, H2: 0.02% Figure 2 shows the process flow diagram of the Skikda Helium plant, which is designed for a product mix of: SCF/y or 2233 Nm³/h liquid Helium (= 9.5 t/d or 3400 Liter/h) Nm³/h liquid Nitrogen (= 100 t/d) Nm³/h gaseous Nitrogen (= 40 t/d)

3 GAN Outlet Feed Gas Compression Raw Helium Separation + LIN production raw Helium (83% He) Helium Purification pure Helium Helium Liquefaction + Storage N 2 to AT HP Feed (6% He) LP Feed (4% He) LIN (99.9% N2) Coldbox LP recycle HP recycle Coldbox LIN N 2 storage tank PSA Oil removal system LHe storage tank LHe Tailgas Buffer FIGURE 2. Process flow diagram of Helium plant in Skikda The Skikda Helium plant consists of four major units: Unit 1: Unit 2: Unit 3: Unit 4: Feed gas compression Unit Three stage compression of both feed gases (Feed 1 and Feed 2) as well as the pressurisation of the tail gas from the He Purification PSA Unit and the LP and HP recycle gases from the Crude Helium Recovery and LIN Production Unit. Raw Helium Separation and LIN Production Unit (NRU Coldbox) In this unit the raw Helium separation, crude Helium recovery and LIN production takes place. The gas phase leaving the NRU coldbox contains 83.5 mol % of Helium when it is sent to the PSA-Unit for purification. Pressure Swing Adsorbtion Unit (PSA-Unit) This PSA process is based on the physical adsorption phenomena, that highly volatile compounds with low polarity, as represented by hydrogen or Helium, are practically not-adsorbeb compared to molecules such as Nitrogen and Hydrocarbons. The pressure swing adsorption process depends on two pressure conditions: 1) Adsorption of Nitrogen and Hydrocarbons is carried out at high pressure to increase the partial pressure and, therefore, the loading of the impurities on the adsorbent material. 2) Desorption or regeneration takes place at low pressure to reduce the residual loading of the impurities as much as possible, in order to achieve a high product purity, high delta loading adsorption / desorption and subsequently a high Helium recovery. Hence most impurities in the raw Helium stream are selectively adsorbed and a high-purity Helium product is obtainable (pure Helium of mol %). Approximatley 2800 ppm Hydrogen remain in the pure Helium stream discharged by the PSA Unit, which have to be removed by the 20 K adsorber(s) within the liquefier cold box. Helium Liquefaction Unit, which will be described in more detail.

4 HELIU LIQUEFACTION UNIT The Helium Liquefaction Unit itself consists of three sub systems: 1. Helium Compression (one low stage, two high stage compressors) and Oil Removal 2. Helium Liquefier Coldbox 3. Liquid Helium Storage & Dual Filling Stations Figure 3 shows a modified Brayton cycle based on three pressure levels, two stages of compression and five stages of expansion. Liquid nitrogen from the NRU coldbox is used for pre-cooling of the Helium to roughly 80 Kelvin and for shield-cooling of the two storage tanks. The choice of a five-expander-cycle allows to provide the refrigeration as close as possible to the required temperature levels, resulting in an efficient liquefaction. The LHe storage tank pressure is kept as low as possible however sub-atmospheric pressure at the suction side of the compressor is avoided. The outlet pressure for turbines T2, T3 and T4 (approx 3.8 bar a) is resulting from the compressor frame size selection. Liquid Nitrogen pre-cooling of the Helium down to the 80 K level (at all operation modes) assures perfect adsorption of impurities. LIN T1 T2 T3 T4 T5 ORS HP 80K Adsorption 20K Adsorption P LP HX1 HX2 HX3 HX4 HX5 He Feedgas from PSA-Unit LHe Tank JT Valve LHe Tank JT Valve He He FIGURE 3. Process flow of Helium liquefier unit General Process Description of Helium Liquefaction, Storage and Compression The HP cycle stream discharged from the high pressure stage screw compressors (oil lubricated) is routed via the on-skid bulk oil separators to the oil removal skid. The oil removal skid consists of three coalescing stages where the oil is removed down to the ppm level and one final charcoal adsorber. Downstream of the adsorber the oil content in the

5 Helium is less than 10 ppb by weight. A particular filter (not shown in figure 3) prevents the carry-over of particulate matter in the HP Helium stream entering the coldbox. The pure Helium stream coming from the PSA Unit enters the coldbox at a pressure of 20 bar. Both the feed gas and the recycle Helium streams are cooled down to approximately 80 K in the first plate-fin heat exchanger HX1 by evaporation of liquid Nitrogen (used in a thermosyphon mode) and the counter current P and LP Helium streams. The superheated Nitrogen leaves the cold box at ambient temperature. A dual switchable molecular sieve adsorber system at the 80K level is provided for the feed stream and a single adsorber with bypass is included to remove Nitrogen trace impurities from the HP cycle stream. The feed stream flows through the second HX2 and third heat exchanger HX3 to the 20 K adsorber. In this dual bed charcoal adsorber the Neon and Hydrogen trace impurities are removed. Provisions are made to regenerate the adsorbers without affecting any liquefier operation. The HP cycle stream is split after purification in the 80K adsorber and cooled down in the second heat exchanger HX2. The mayor part of the flow continues through the colder heat exchangers. The slip stream is expanded in two turbines T1 and T2 to approximately 4 bar a and 38 K. The turbine outlet stream forms part of the P cycle stream. Inside exchanger HX3 the HP stream is split once more. Part of the stream is expanded in turbine T3 and returned to the P cycle stream. The remainder of the HP cycle stream is further cooled and combined with the feed stream downstream of HX3. The resulting HP mixture is processed in the 20K adsorbers and cooled down further. Downstream of the fourth heat exchanger HX4, this HP cycle stream is split and part of the flow is expanded in turbine T4. The discharged cold gas (about 11 K) is returned via the heat exchangers HX4 through HX1 to the warm end of the cold box as P cycle stream. The remaining HP cycle stream is further cooled in a fifth heat exchanger HX5, expanded in turbine T5, further cooled down in HX5 (second core) and expanded through an automatically controlled Joule-Thomson valve where it partially liquefies. The resulting two-phase stream is transferred to the liquid Helium storage tanks (dewars). During special operating modes such as cool-down, warm-up or idling the dewars can be by-passed automatically. During normal operation the LP flash and boil off gas from the dewars is warmed up in all five heat exchangers to ambient temperature, leaves the cold box and is fed to the suction side of the low stage cycle compressor. Each turbine train can be stopped, isolated, warmed-up and serviced while the liquefier remains in operation. General Process Description of Helium Loading Liquid Helium product is stored in two liquid Helium storage tanks (dewars) at a temperature of -269 C (4 K) and at a pressure of about 0.2 barg. The Helium vapour return gas from the dewars is warmed up in heat exchangers HX5, HX4, HX3, HX2 and HX1 to ambient temperature and routed to the LP-Helium compressor. The suction pressure of LP- Helium compressor is 0.05 barg. Two dual loading stations with weight-scales for Helium containers are located underneath the liquid Helium storage tanks. The Helium containers could be filled with

6 Power [kw] spec. Power [kwh / kg_lhe] Feed [%] isotherm. Pow er Shaft Pow er (2 Compr.) Shaft Pow er (1 Compr.) spec. Pow er [kwh/kg_lhe] Linear (isotherm. Pow er) FIGURE 4. Part Load Operation liquid Helium from both tanks. The Helium vapour return gas from the containers are routed back to the heat exchangers HX2, HX3, HX5 or an external evaporator dependent on the Helium vapour return temperature (piping not shown in Figure 3). The Helium vapour return gas from the containers is also routed to the LP-Helium compressor. Warm or polluted containers are treated outside the Helium loading house at the container conditioning station. PART LOAD OPERATION As a result of the mentioned explosion in the LNG plant in 2004 three out of six gas liquefaction trains were destroyed and two, which were not operating at the time, were damaged. After the devastating explosion the original design of the entire Helium facility plant had to be reviewed. After a first survey of the remaining LNG-trains, a reduced feed gas quantity of max 55% was calculated. For the Helium liquefier the turndown operation and capacity was reinvestigated as follows (refer to Figure 4): 1. With a minimum feed gas flow down to 0%, the liquefier still could be operated in a steady state condition. 2. This curve shows the characteristic of the isothermal power at the inlet of the Helium liquefier coldbox. The characteristic of the isothermal power is matched to a straight line. The two data spots at a feed gas mass flow of 30% are equivalent to the isothermal power of twin- or single operation of the two high stage compressors (HPcompression). 3. This curve indicates the required effective shaft power of the entire compression system - operation in twin mode. The bend within the characteristic below 50% of feed gas results from a decreasing efficiency factor. Because of

7 increasing internal losses of the compressors and increasing mass flow over the bypass. 4. This curve shows the shaft power if one high stage compressor is turned down. 5. This curve shows the required specific power per weight of liquefied Helium. This characteristic is based on a single HP compressor operation. For an improved power balance during reduced feed gas flow one of the HP compressors can be shut down. This new operation mode had to be implemented later into the plant control. The design of the process is based on a floating medium pressure of the second stage compressors. The floating medium pressure enables reasonable operating efficiencies, allows the Helium liquefier to operate stable between 0% and 100% feed gas flow. INSTALLATION & COISSIONING The four major challenges during installation and commissioning at site are the following: Logistics All skids were fabricated by Linde or their sub-suppliers in Europe or the U.S.A. Direct shipment from the States to Algeria was very difficult if not impossible. Therefore all devices first had to be shipped to a centralized interim buffer stock in Europe for transshipment collection. Urgent needed commissioning spare parts for instance were always delivered by accompanied air-fright, because any available courier service could not handle the just in time delivery to the site. Bulk aterial Apart from the prefabricated skids all bulk material, like stainless steel bolts or nuts, were subsequent just in sequence imported to Algeria. Despite of local retailers, to fulfil our high quality standards, even SWAGELOK fittings or spare SS-tubing were shipped at site. Installation Tools Standard cryogenic tools for installation (welding rods approved for low temperature applications), puller and torques for shaft seal refurbishment of our compressors, lapping equipment, etc. had to planed detailed in advance. Each vendor packages at site was equipped by separated specialized selected tool boxes. Salty & Sandy Ambient Conditions Due to the off-shore conditions at site (saltwater atmosphere combined with sandy and dusty winds), in particular thorough corrosion protection and proper pipe cleaning was essential for a successful installation and commissioning. Apart from Linde AG's standard installation procedures for sandblasting, purging and flashing for pipe cleaning, Aqua illing was additionally contracted for high pressure pipe cleaning. High pressure water at a level of 1000 bar was used to remove any remaining weld slag, corrosion or sand intake.

8 CONCLUSION By using high efficiency Linde Kryotechnik turbines and optimisation of the Helium gas management for the compression system, the plant is running stable conditions with feed gas available between 10% and 50%. The plant is designed for full load operation to achieve 600 illion Standard Cubic Feet per year (2233 Nm 3 /h) of liquid Helium. Currently the plant will produce approx. 250 to 300 SCF/y to the market. The product quality is pure liquid Helium of % purity. Even so a content of 2800 ppm(v) of hydrogen have to be removed from the Helium feed gas stream inside the He-coldbox. A Helium-Recovery rate of the entire Helium plant with two supplying LNG units of 99,6% was achieved (He-Recovery Rate with one LNG-Unit is: 95.5%) On schedule 40 month after the effective date of contract, the first filling of a 11'000 Gallon container took place in June The plant runs properly in liquefaction mode from then. FIGURE 5. NRU-Coldbox, LIN-Tank and PSA-Unit FIGURE 6. Valve-Carriage for PSA-Unit FIGURE 7. Dual Helium Filling Bays and Helium Liquefier FIGURE 8. Liquid Helium Storage and Filling Area REFERENCES n. a.