PIONEERING ETHANE TECHNOLOGY
Reprinted from Hydrocarbon Engineering, August 2015
Christopher Campos, Ebara International Corporation, USA, discusses the development of ethane technology, providing a logistical view of natural gas liquid products.
Michael Faraday is considered by the electrical engineering community to be one of the fore-fathers of induction. Faraday performed years of distinguished research in electrical systems, magnetism, material properties and equations. He is most famous for his theory of induction which has become part of the now famous Maxwell Equations. Faraday stated, “The induced electromotive force in any closed circuit is equal to the negative of the time rate of change of the magnetic flux enclosed by the circuit.” This one statement and associated equation, resulted in multiple research opportunities and spawned a new segment for what would become the field of Electo-Motive Force or EMF.
As an electrical engineer, I never considered how much of an impact Michael Faraday had on the world of electro-magnetism until I started researching transformers, reactors, motors, generators and even hydrocarbons such as ethane (more on ethane later). Faraday experimented with his theory of induction to the greatest extent possible in 1831 and the general concept of his theory can be simply represented with Picture 1.
Since alternating current (AC) was not discovered yet, Faraday had to use the only captured source of electricity at the time, the direct current (DC) battery (or voltaic cell). The DC battery supplies a constant stream of current if connected by a closed conductor loop. Inherently, magnetism is not associated with it, only an electrical force from the battery (potential energy). Faraday found that if he used a common iron object and wrapped the conduit around half of it and then wrapped a separate closed conduit around the other half, current would flow through the second closed loop. Of course, this would appear as magic back in the 1830’s, but Faraday (with independent credit to Joseph Henry as well) discovered that there was a second force taking place and that this second force could induce the electrons in the second closed circuit to move. This force would be known as induction (Picture 2).
James Clerk Maxwell would take the work of Michael Faraday, Joseph Henry, Carl Friedrich Gauss, André-Marie Ampère, and other well noted scientists to complete the set of famous Maxwell Equations which provide the fundamental understandings of the Electro-magnetic Force (Picture 3).
How does Michael Faraday relate to ethane? In 1834, Faraday was applying his electrolysis process (method of using electrical current through a beaker solution to determine the resulting outcome) on a potassium acetate solution. A current applies an electrical force to the solution which essentially tears apart the molecules. This resulted in a hydrocarbon that he initially took for methane. Independently, research in organic chemistry was testing ethane by reductions with ethyl cyanide, ethyl iodide with potassium metal creating aqueous acetates. It was 30 years later that Carl Schorlemmer correctly concluded that Michael Faraday’s by-product was actually ethane, or C2H6.
Ethane via Ship
Today, ethane is commonly produced both naturally and as a by-product of Natural Gas Liquids (NGL’s) from the liquefaction process of natural gas. Although ethane is abundant at any natural gas deposit, few LNG processes actually harvest it for sale. Most ethane is often flared off as a by-product. However, the U.S. shale gas boom has led to the separation and production of large quantities of ethane during the low temperature liquefaction process from the NGL’s.
Ebara International Corporation, Cryodynamics Division (EIC Cryo) is no stranger to supplying submerged motor pumps for liquid ethane and other hydrocarbon liquids. EIC Cryo has a 40 year history of propane, butane, ethane, ethylene, and propylene pumps. Most of these applications have been for land-based petrochemical plants. Recently, EIC Cryo has seen an interest for these hydrocarbon applications for marine applications. LPG has commonly been traded via LPG marine carrier around the world, similar to LNG carrier trading. However, the market for international ethane trading has been relatively small. Typically gas carriers have tank volumes under 30,000 cubic meters.
Recent investment in larger gas carriers was spurred on by the low cost U.S. shale gas with petrochemical plants in Europe and Asia hedging their future feedstocks on this supply. This is the case of Reliance Industries who invested in the world’s largest gas carrier being constructed by Samsung Heavy Industries (SHI) in Goeje, Korea. This Very Large Gas Carrier (VLGC) will have a capacity of 87,000 cubic meters and will haul mostly liquid ethane feedstock from the U.S.A. to the Jamnagar Refining and Petrochemical Complex in Gujarat, India. EIC Cryo has been awarded the contract to design and manufacture a submerged motor cargo pump for offloading the liquefied ethane. Ethane feedstock is somewhat unique in that it is very inconsistent. Unlike typical methane LNG Carriers, the temperature and density of the ethane feedstock can vary widely based on the refinery process. EIC Cryo had to develop a pump that could handle ethane’s coldest range (-100°C) to a potential butane laden range of (+0°C). Additionally, densities of the product could range from 550 kg/m3 to 610 kg/m3.
In 2008, EIC Cryo performed a technical evaluation that resulted in installing test fluid transfer pumps in Ebara’s test facility to transfer LNG or LPG from the test system to the storage system. This meant that the transfer pumps would have to be designed for two different hydrocarbons. Operating temperature range was -168°C to -34°C.
As part of the independent evaluation, EIC Cryo chose the reliable 2EC-092 pump model that is installed on hundreds of LNG Carriers and LPG Carriers. These pumps have proven reliable and efficient and have generally run for more than 16,000 hours between overhauls. As EIC Cryo performs tests every day, the pumps would be subjected to multiple starting and stopping sequences, a long maintenance interval and a high reliability requirement.
The submerged motor cargo pump, relatively speaking, is a very simple design (Picture 4). It consists of a motor designed to be submersed in the pumping fluid, a single piece construction shaft/rotor assembly, impeller, diffuser and inducer for low NPSHR. The pump is held together by the pump and motor housings. The final piece is an end bell housing which accommodates the electrical power cables for the motor, upper motor bearing and discharge nozzle.
As these pumps are designed for low temperature service, all of the components have to be rated for low temperature service as well. Additionally, fluid density will affect the pump’s performance and fluid velocities, requiring more power for denser fluids. The bearings were specifically designed by EIC Cryo for low temperature and low viscosity service. The bearing designs are proprietary of EIC Cryo and designed to work specifically with submerged motor pumps. As with the rest of the pump components, the bearing races and liners have to be sized for the specific fluid temperature and density it is being designed for (Picture 5).
Over the years, EIC Cryo has developed a reputation for innovative and ground breaking technology. EIC Cryo truly is an engineer-to-order based company; however, manufacturing costs for this type of business model can be expensive and sometimes price is the driving factor for turn-key projects. This leaves advancements in technology to corporations who are willing to invest in the next generation of gas products.
This is the case for Wärtsilä who has recently developed and completed testing on their ethane rated 50DF duel fuel engine. The idea of using ethane as a reliable fuel source is a novel concept, which could allow the use of Boil-off Gas (BOG) from the ethane tanks to power a ship’s engine. This is similar to conventional LNG Carrier designs. EIC Cryo has already invested in this concept with adaptions of current stripping/spray pumps and dedicated fuel gas pumps to pump liquid ethane.
Other recent areas of development include high pressure ethane pumps for regasification to a vaporizer, which are already in operation. EIC Cryo has received interest from several parties about taking this process to a marine application and designing an Ethane FSRU.
EIC Cryo is also exploring the development of an Ethane Expander to help increase the production efficiency of liquefying the ethane product. LNG liquefaction plants have utilized cryogenic expanders for years to improve their overall efficiencies. By reducing a portion of the pressure of the LNG and mixed refrigerant streams across an expander instead of using a Joule-Thomson valve, the enthalpy of the stream is reduced, resulting in lower boil-off losses and cooler outlet temperatures.
Submerged motor pumps incorporate detailed design and physical understanding of hazardous gas areas. Without the understanding of hazardous area zones, the concept of the submerged motor pump could not operate. Inside a sealed tank environment would be classified as a Zone 0 (using typical IEC nomenclature) or non-hazardous, as there is no presence of oxygen. Within a few meters outside of the sealed flanged, due to the presence of oxygen, a hazardous area exists. Should any leakage occur, then any spark, fire or flame must be contained per the requirements of the project (NEC, IEC, IECEx, etc.). Beyond this hazardous zone, the likelihood of explosion or fire is reduced, but electrical equipment must still meet requirements of the project (NEC, IEC, IECEx, etc.).
Since submerged motor pumps are completely installed inside of the sealed tank environment and the motors are rated for low temperature service, the pumps can operate without the potential for creating a spark, fire or explosion. Additionally, submerged motor pumps don’t require a mechanical seal or any other sealing device that could be a source for gas leakage.
The major component of the submerged motor pump is the motor itself (Picture 5). The motors are of 3-phase, squirrel cage, induction design. The stator is fabricated from silicon iron laminations and form- or random-wound copper windings. The windings are protected with an epoxy insulation applied via a vacuum impregnation process to remove air pockets during the curing process. The rotor is also made from silicon iron laminations and then completed with aluminum bars between the two aluminum end caps. The end caps can be cast in place or pre-fabricated. The major consideration is the tensile stress induced in the bars as the rotor is cooled to cryogenic temperatures. The differential thermal contraction properties of the aluminum bars and the silicon iron laminations must be carefully considered by the motor designer.
Finally, the submerged motor owes its birth-right to Michael Faraday and his theory of induction leading to the concept of electrically induced magnetism. Although one could not know if Faraday had the fore-thought to think about using his theory on an induction motor application (this would come from another fore-father of electricity, Nikola Tesla 1887), the evolution of this induction technology continues to advance. In 1929, Pleuger Pumps developed submersible turbine pumps used in the oil industry. The 1960’s saw the first deep-well submersible water pumps. In 1958, the first submerged motor LNG pump was used on the Methane Pioneer.
“Frequently Asked Questions About Ethane Crackers”, http://www.alleghenyfront.org/story/frequently-asked-questions-about-ethane-crackers .
“Michael Faraday’s Law of Induction”, http://en.wikipedia.org/wiki/Faraday%27s_law_of_induction .
“The History of Submerged Motor Pumps in the LNG Industry”, Michael Cords, February 24, 2011. http://www.ebaraintl.com/technical-papers/the-history-of-submerged-motor-pumps-in-the-lng-industry.com
“Expansive Thinking”, Christopher Finley, Hydrocarbon Engineering, May 2013. http://www.ebaraintl.com/news/expansive-thinking-two-phase-liquified-gas-expanders-for-improving-lng-liquefaction-plant-efficiency.com
Platts. “South Korea Samsung Heavy Industries to build world’s first very large ethane carriers for Reliance.” Ramthan Hussain, Pardeep Rajan, Philip Vahn, E. Shailaja Nair, http://www.platts.com/latest-news/petrochemicals/singapore/south-korean-samsung-heavy-to-build-worlds-first-27535836, 15 August 2014
Tradewindnews.com. “Samsung secures $720 VLEC contract from Reliance.” Iren Ang Singapore, http://www.tradewindsnews.com/weekly/341643/Samsung-secures-720m-VLEC-contract-from-Reliance , 25 July 2014
“Wärtsilä 50DF engine successfully demonstrates its capability to operate on ethane gas”, Wärtsilä Corporation Press Release, 4 May 2015. http://www.wartsila.com/media/news/04-05-2015-wartsila-50df-engine-successfully-demonstrates-its-capability-to-operate-on-ethane-gas