PC-APR23-PG22-23.1_Layout 1 12/04/2023 14:02 Page 22
HEAT TRANSFER
A SUSTAINABLE ALTERNATIVE The increasing interest in ZLD is driven
I
n 2021 the global market for zero liquid discharge (ZLD) technology was estimated at $1 billion and is forecast to grow at almost 12% over the next ten years1
. The rise is
being driven in particular by an increase in adoption of the technology by the food and drink and textile industries as a growing world population puts greater pressure on fresh water supplies.
What is ZLD?
Zero liquid discharge (ZLD) is a liquid waste stream treatment which involves transforming liquid waste streams into clean water (which can be reused) and a minimum volume of solid residues. A well-designed ZLD system should minimise or even eliminate liquid waste streams, resulting in clean water for reuse or environmentally-friendly discharge, and a solid residue suitable for further processing (often to recover valuable components for use elsewhere) or for safe disposal.
ZLD is being implemented across a wide range of industries, including chemical and petrochemical production, food and drink production, textiles, energy and power, and pharmaceutical manufacturing. These industries are being driven to adopt the technology due to growing environmental awareness of the hazards of toxic wastewater and increasing environmental regulation. In turn this has increased the costs of handling and disposing of such waste streams, and in some cases has made such
22 APRIL 2023 | PROCESS & CONTROL
by cost saving and environmental protection goals. Matt Hale, International Sales & Marketing Director, HRS Heat Exchangers, explains the crucial role that heat transfer technologies play
disposal impossible. As a result, companies are looking for more sustainable alternatives, and ZLD is one of the leading technologies in this area.
As water scarcity and environmental pollution around the world intensifies, ZLD becomes more feasible and widespread, and the relative costs of ZLD technology versus the alternatives (assuming alternatives even exist) are lowered.
Separating all of the water out of the product requires large amounts of energy. It takes roughly 6 times more energy to evaporate water (latent heat) at its boiling point then the energy needed to actually bring it to that boiling point (sensible heat). For that reason, ZLD processes often start with a separation process based on (reverse osmosis) membranes. However, to achieve complete separation, evaporation / crystallization processes are needed for completing the process. As explained before, evaporation (due to the latent heat) is highly energy consuming. Therefore, it is wise to
choose an evaporation process that involves ways of energy optimisation, the most popular being: Multistage evaporation: using the latent heat of the evaporated water as an energy source in a next evaporation stage reduces the overall consumption of the boiler to the evaporation plant. Thermal Vapour Recompression (TVR): evaporated steam is mixed with boiler steam. The reuse of the evaporate steam reduces the energy demand. Mechanical Vapour Recompression
(MVR): An MVR compressor (driven by an electrical motor) can be used to compress the evaporated steam, thus increasing its pressure, and use this steam as the energy input for the process. MVR compression is very efficient in terms of energy consumption. (Multistage) vapour compression plants remain the main method employed for ZLD processing globally, with evaporation typically recovering around 95 per cent of wastewater as distillate. Any remaining concentrate is
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70
