Recently I’ve been researching mechanical vapor recompression (MVR) which is an energy saving heat recovery process used in evaporation and distillation.
As with a lot of other processes in BHO extraction, this is actually an old technology that is widely known and implemented in various other industrial processes. It really just needed to be adapted and properly scaled down for our uses.
The basic idea involves recovering and recycling the latent heat of vaporization carried by the solvent vapor, rather than rejecting that heat during condensation.
A useful example I’ve used to envision this is to think about a pot of boiling water on the stovetop. If you put your hand above the liquid surface, you’ll notice the vapor is hot, at essentially the same temperature as the boiling liquid, about 100C for water at atmospheric pressure. Normally that heat is simply rejected to the atmosphere as the water boils, the same basic thing a recovery chiller or dry ice does on the recovery side in BHO while you continue adding fresh heat to the pot to maintain boiling.
So naturally you can ask, “why not just redirect that heat back into the pot?” The problem is temperature gradient. The vapor leaving the boiling liquid contains the large amount of latent heat used to generate it, but it’s at essentially the same temperature as the liquid, so there is no useful temperature difference to drive that heat back into it. This is where the compressor comes into play.
The compressor takes the vapor and raises its pressure. When you increase the vapor pressure, you also raise its boiling/condensation temperature, a.k.a. saturation temperature. The act of compression also raises the actual temperature of the vapor, so it will generally leave the compressor somewhat superheated relative to its new saturation temperature.
Imagine the steam from the boiling pot is at 100C and the liquid is also at 100C. You can’t effectively use one to heat the other. If you compress the steam to a pressure where it will condense at, say, 110C, you now have vapor on one side of the heat exchanger that can condense at 110C and boiling liquid on the other side at 100C. The compressed vapor first gives up any superheat, then begins to condense at roughly 110C, releasing its latent heat through the heat exchanger and back into the boiling liquid. That heat boils off another fraction of the liquid, which enters the compressor, gets recompressed, and the cycle continues.
The easiest way I’ve found to understand why this latent heat recycling is so powerful is to picture it frame by frame. Say an MVR setup has a 10 kW heater and, just for an easy example, is able to recover 90% of the heat used for evaporation. Of the first 10 kW put into generating vapor, roughly 9 kW gets transferred back into the liquid when that vapor condenses, while the heater continues adding another 10 kW. That recycled heat continues circulating and essentially snowballs until the heat being lost from the system balances the fresh energy being added.
Obviously that’s a simplified example that ignores compressor work, sensible heat, heat loses, etc. but it helped me understand that it’s not creating energy, it’s repeatedly recycling most of the latent heat instead of rejecting it every cycle.
