Promession is an ecologically-conscious method for disposing of human remains by freeze drying. It was invented and patented in 1999 by the Swedish biologist Susanne Wiigh-Mäsak.
The method is based on three steps:
- Reducing the body of the deceased to a fine powder, thereby allowing subsequent decomposition to be aerobic. This is achieved by submerging the body in liquid nitrogen, making the remains so brittle that they shatter into a powder as the result of slight vibrations. The powder is then dried, reducing the deceased remains to around 30% of their original body weight.
- Removing and recycling metals within the powdered remains.
- Shallow-burying the powder in a biodegradable casket.
The first facilities for promession-based funerals, known as Promators, are due to be ready in 2008. They will be located in Sweden, Germany, Great Britain, South Korea and South Africa.
The volume of remains left by this procedure is about three times that left by a cremation, but the advantages claimed include avoiding the release of pollutants into the atmosphere (for instance, mercury vapour from dental fillings) and the rapid degradation of the remains after the procedure (within 6 to 12 months). The procedure meets the requirements of new European Union pollution laws.
The terms "promession" and "Promator" are neologisms. "Promession" is derived from the Italian word for "promise" (promessa), the promise being the good environmental management of the Earth.
A complete ecological study should include the energy requirements of cryogenic freezing compared to cremation. Depending on size, the human body
is between 55 and 78 percent
Much of the energy required for cremation is the heat needed to convert water to steam.
To heat water from 20 °C
to 100 °C requires 334.5 Joules
(J/g) of water
. To convert this into steam requires 2258 J/g, meaning the total heat required is 2593 J/g.
Much of the energy required for cryogenic freezing
is the latent heat
needed to freeze the water used and to cool a mass
to the temperature
of liquid nitrogen
- To cool water from 20 °C to 0 °C (273 K) requires 83.6 J/g.
- To freeze water requires 334 J/g.
- To cool ice from 273 K to 77 K requires 457 J/g.
- The total heat removed is 874 J/g.
Comparison of cremation and cryogenic freezing
The ratio of the heat moved for the two methods is almost exactly a factor of 3. However, the effect of the second law of thermodynamics
must also be considered. To heat a mass requires heat energy; and to cool something below ambient temperature
requires work energy or electricity that must be generated from heat energy. Typically, electricity is generated from heat at roughly 33% efficiency. A common (non-cryogenic) freezer
, operating on the Rankine cycle
, might have a coefficient of performance
of about 3, thus removing 3 units of heat for every 1 unit of work input. So overall, it takes at least one unit of heat energy burned in an electric power
generator to run a freezer that will remove one unit of heat energy at 0 °C. It should be considered whether this is cold enough to make tissue brittle. To cool even further, a good cryogenic cooler will only remove 1 unit of heat at 77 K for every 20 to 25 units of work, so the coefficient of performance is only 4 to .
More accurately, the coefficient of performance (CoP) is a function of temperature. For a Stirling engine cryocooler, this is about , where Tc is the cooler temperature.
The average CoP of the Stirling cryocooler operating between 273 K and 77 K is 0.2 (20%).
- The electricity needed to cool water from 293 K to 77 K using a Stirling cryocooler is 4424 J/g.
- To cool water from 293 K to 273 K with a Rankine cooler is 27.9 J/g.
To cool ice from 273 K to 77 K with a Stirling cryocooler is 2312 J/g.
To cool water from 293 K to 77 K with a Rankine cooler and then Stirling cryocooler is 2340 J/g.
Hence it is most efficient to use a Rankine freezer to freeze the water and then a Stirling cryocooler to reduce the temperature of the resulting ice to 77 K. If we assume electricity is generated from heat at 33% efficiency, then it takes 7091 J of heat to create 2340 J of electricity.
The energy efficiency of the freezing and boiling processes involved can be made very high. It takes an estimated 2.75 times more primary energy to cryocool water than it does to turn it to steam; much of this energy, however, can be recycled, though this may require additional equipment. A Stirling cryocooler can run in reverse, efficiently moving latent heat from cryocooled ice to warm water. This can be made to generate electricity, thus replacing some of the electricity used in the initial cooling process. The use of energy recuperation and counter-flow heat exchangers also significantly improves the quantities of energy consumed.