Sometimes assuming the technology is at our doorstep and determining if it is viable in a given application is a good approach. So let's look at very large capacity carriers (VLCCs) which can carry between 1.40 - 2.24 million gallons. How much salt would be required to match the equivalent ocean crossing of 3,000 miles. We need to work backwards.
VLCC horsepower for a 1.82 million gallon tanker is around 42,500 HP. If we can assume graphene membranes are in a tight form factor with 100x flux of today's membranes, then we would have approximately 40 m2 per membrane or about 40 Watts x 100 = 4,000 W ~ 5 HP. Dividing 42,500 by 5 would equal = 8,500 membranes (quite cumbersome). Therefore, graphene membranes need to be manufactured with large surface areas, two orders of magnitude much larger than current designs. If this were the case then only 85 membranes would be required to produce 500 HP each and the surface area would have to contain 4000 sq meters. This would take up a large footprint, but the goal would be to generate electricity and many large ships have externally-mounted electric propulsion motors that pivot, eliminating rudders and also allowing higher maneuverability as well.
If it were possible to propel the ship with PROP, then how much salt would need to be loaded to "fuel" the journey of 3,000 miles? One would have to determine a concentration based on the water the ship is traversing. If the ship was in brackish water, it would require less salt than if it were traversing though saltier seas. More math to follow...
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