1. Multiple Effect Distillation
Design of a Biomass Slow Pyrolyzer-Multiple Effect Distillation (MED) Prototype Ali Amiri , Catherine E. Brewer Department of Chemical & Materials Engineering, New Mexico State University
Water Desalination
Desalination is a promising solution for freshwater shortages that can provide large quantities of high quality, potable water.
50% of all desalination plants are located in the Middle East, 20% in the US, 18% in Europe, and 12% in Asia.
In 2000, the installed desalination capacity worldwide was about 22 million m3/day, requiring 8.5 x 1018 J per year of energy, equivalent to 203 million tons of crude oil.
Water quality is categorized in terms of total dissolved solids (TDS) with units of ppm (mg dissolved salts/L of water):
Membrane / no-phase-change systems:
Include reverse osmosis (RO) and electro dialysis (ED)
Are usually powered by only electricity.
Thermal systems:
Include multi-stage flash (MSF), vapor compression (VC), and multiple effect distillation (MED).
Require low-temperature heat as the main energy input and a small amounts of electricity.
Advantages of MED over other techniques:
Lower energy consumption than MSF
Fewer number of effects compared to MSF
Reduces TDS to less than 10 ppm
Why slow pyrolysis and MED?
13 % of renewable energy-powered systems worldwide are solar-MED
Biomass slow pyrolysis has the potential to provide energy for small-scale thermal desalination systems, in particular MED systems, while also producing ~0.25 kg biochar per kg of biomass for soil applications.
In soil, biochar can sequester ~45% of the biomass carbon, as well as improve soil fertility and plant-available water retention.
Aspen Figure
Multiple Effect Distillation
Common evaporator combinations
Vertical and horizontal tube evaporator (Sen et al., Desalination, 279 (2011) 15-26)
The feed water and steam may switch their position depending on the design
Feed arrangements :
Forward feed
Parallel feed
Backward feed : it has some drawbacks
Need pumps between stages
The highest salinity brine which enters the first effect may cause scaling
Forward feed flow arrangement with regenerative heat exchangers in a six effect, horizontal tube water spray MED Unit (M.A. Darwish, H.K. Abdulrahim, Desalination, 228 (2008) 30-54).
Pre-heaters are to increase the top brine temperature (TBT)
gradually in order to avoid huge heat transfer area within the first effect.
water
Fresh water
ppm
Brackish water
2,000-10,000 ppm
Seawater
30,000-60,000 ppm
Our design for lab-scale biomass pyrolyzer-MED system
A 3 effect MED system with forward feed arrangement and pre-heaters will be used.
Horizontal steam tube-side with falling water film increases the energy efficiency and reduces the chance of scaling.
Biomass: pecan orchard pruning and shells, cotton gin trash, dairy manure, yard waste, etc will be used which are abundant in New Mexico
The pyrolyzer-MED interface components: burner to convert low-energy bio-oil and syngas into superheated steam and non-condensable gases, and a gas turbine generator to produce electricity.
Steam and non-condensable gases exiting the turbine generator will be used as the thermal energy source for the first effect within the MED unit.