19 The Israel Chemist and Chemical Engineer Issue 8 · November 2021 · Kislev 5782 History of Chemistry Articles Figure 1. Prof. Loeb at home in Omer in his study on his 91st birthday. On the back wall to his left is a copy of Prof. Loeb’s favorite piece of art, Waterfall, by M. C. Escher. Loeb marked the image with the words Free energy, and the man in the picture at the bottom he labelled Gibbs. Loeb appreciated the pun on the term “free energy,” its thermodynamic meaning, and that the Escher machine can produce energy for “free.” Loeb told the author that the Escher illusion of a water-powered perpetual motion machine demonstrates the ideal engineering feat that he strived for his entire life. (Photograph by author) Definition of PRO Loeb et al. (1975): “Pressure-Retarded Osmosis (PRO) is defined as a process in which solvent (water herein) permeates osmotically against a hydraulic pressure gradient. The process is thus capable of producing energy, providing that a high osmotic pressure solution (concentrated brine herein) and a low osmotic pressure solution are simultaneously available. “The Dead Sea appears to be an ideal and perpetual source of both concentrated brine and low osmotic pressure solution, the latter to be supplied either by the River Jordan or by brackish springs. Similar possibilities should exist at Great Salt Lake and other saline bodies of water in the world. As another possibility, saturated brine formed from the dissolution of salt mountains could possibly be used in conjunction with sea water as the low osmotic pressure solution” [3]. Request for an article on how the idea for PRO came about In 2003, Statkraft, the Norwegian government energy company, was working on PRO and asked Sid if he would prepare an article describing how the idea came about. Sid wrote this up for them twice but each time they asked him for something less technical. Sid asked if I would prepare this, which I did, and after Sid’s editing, we sent the article to Statkraft (personal communications, Nov. 2003 to Feb. 2004). That article is given below. I recall a conversation with Sid at the time that Stafkraft first asked if he would be willing to come to Norway for a visit to advise on PRO. He told me that he was considering not going. Sid was a person who held himself up to the highest ethical standards. His recent calculations had convinced him that PRO was not economically feasible and therefore he could not in good conscience advise on the project. He said that he did not have the right to waste Statkraft’s money on his expenses. I recall saying to Sid that he should not kill the PRO idea; that in 10 or 20 years another “Sidney Loeb” might come along and improve the technology and make PRO viable. Sid expressed to Statkraft his hesitation about the trip. They told him that PRO was one of several energy sources that they were studying to see which offered the best possibilities. They wanted him to advise them no matter what. Sid then decided to assist them in any way possible. After ten years work on PRO, Statkraft in 2013 announced the decision to terminate their PRO project [4]. According to the trade publication, ForwardOsmosisTech, the reasoning behind this decision was as follows: “…there are no high performing AND CHEAP [capitals in original text] bulk PRO membranes available for purchasing today – and this situation is not expected to change within the next 5 years. Add on top of this, the huge scale of operation needed in order to drive down the actual OPEX costs of producing electricity in an osmotic power plant, and you’re left with a rather bleak outlook. It is very likely this [is the] kind of reasoning that has led Statkraft to the conclusion that economical viability for osmotic power production lies too far in the future to justify further funding of their project [5].” Advances over the past decade in membrane and other technologies have led to renewed interest in PRO. Chung and Wan (2020): “In summary, membrane technologies for PRO have come a long way since the concept was proposed in the 1950’s. There have been significant progress in fabrication of novel PRO membranes, understanding of PRO mass transport, design, optimization, operation, and maintenance of various PRO processes. With several PRO processes in pilot
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