Purification of waste water streams through evaporation can provide industrial facilities the opportunity to recapture and reuse nearly all liquid waste streams.
Evaporator technology varies from tubes to plates, inside versus outside, and up, down or flooded. Systems may consist of only a single effect or involve multiple effects. Mechanical vapour re-compression (MVR), external product pre-heaters, and multi-stage condenser systems may be incorporated into the process.
AHL’s design features:
AHL offers single-effect falling film evaporators with mechanical vapour re-compression (MVR) to drive the evaporation process. MVR evaporators are used where low pressure steam is not continuously available. Re-compression and re-use of the steam/vapour produced from the liquor itself, is used as the heat source to drive the evaporation process. “Starting steam” is required to start the evaporation process but may be discontinued once the MVR evaporator is in stable operation. All MVR evaporators have high efficiency mist eliminators internal or external to each vapour body.
Available materials of construction include 316L stainless steel, high nickel austenitic stainless steels, duplex stainless steels, and reactive metals such as titanium.
Multiple Effect Evaporator
Where plant steam is available, AHL offers multiple effect evaporator systems for services such as kraft pulp mill black liquor concentration. The technology has been the preferred choice in industries such as pulp & paper, ethanol production, ethylene glycol concentration and sugar refining. The effective use of steam improves with the number of effects in the multiple effect evaporator system to perform the same service (evaporator economy), a carefull review of the system capital cost versus operating cost is required.
In comparison to single effect evaporators driven by mechanical vapour recompression, multiple effect evaporator systems are run at a substantially lower electrical load for the same evaporation service. Multiple effect evaporators can be designed either as Falling Film, Rising Film (also called Long Tube Vertical) or Flooded Forced Circulation.
Pre-evaporation System Using Multiple Effect Evaporator
Pre-evaporation systems utilize low-level waste heat to evaporate weak liquor, strip the bulk of the TRS and VOC’s from the condensate prior to the main evaporator train. Aside from using waste heat, one of the main advantages of a pre-evaporation system is condensate segregation.
Evaporator Modification and Optimization
AHL specializes in evaporator modifications to existing systems. Evaporator modifications are pursued to increase capacity, improve economy, increase run time between cleaning, and reduce liquor carry over. Some examples include:
Contact AHL to enquire what evaporator modifications are possible and of greatest benefit in your plant.
A.H. Lundberg’s preferred technology to concentrate or crystallize solids from liquor or brine is Flooded Forced Circulation Crystallizer.
Full de-mineralization of the waste water and zero liquid discharge is available through the additional step of evaporative crystallization of the dissolved contamination, providing the plant with the opportunity to dispose of solid waste salts only.
Crystallizers utilize separate vessels for the heater body and vapour head with a modified shell and tube heat exchanger for the heater, generally limited to single pass brine or liquor flow on the tube side of the crystallizer heater body. Brine (or liquor) at near the limit of saturation is delivered to the crystallizer heater and is forced through the tube side of a heater. Steam is fed to the shell side of the crystallizer heater to heat the brine to above the boiling point. As the superheated brine then flows to crystallizer vapour head, a portion of the liquid is flashed as clean steam and is recovered as clean condensate, consequently concentrating the brine to a slurry. A portion of the concentrated brine slurry is discharged from the crystallizer system to a filter press to recover the salt crystals. The press filtrate is circulated back to the crystallizer heater and additional brine is added to the slurry recirculation from the vapour head to the crystallizer heater in direct proportion to the amount of vapour flashed and slurry discharged, perpetuating the evaporation cycle. The crystallizer vapour head utilizes high-efficiency chevron type mist eliminators to assure high purity condensate is recovered and chemical concentration is maximized to the brine slurry.
Available materials of construction for the crystallizers includes 316L stainless steel, high nickel austenitic stainless steels, duplex stainless steels, and reactive metals such as titanium.