Overcoming the challenges of high dry solids evaporation
Black liquor is a complex solution of water, organic, and inorganic components. This composition varies from mill to mill, so the evaporation system must be flexible in design over a wide range of operating parameters. The physical properties of black liquor change significantly when it is concentrated. Concentrated black liquor has high viscosity, which can cause significant problems with fouling if these characteristics are not accounted for in system design.
The systems employ crystallization technology to produce the highest dry solids product with minimum downtime for washing and cleaning of the equipment internals. As an option, systems for treating the liquor to reduce its viscosity, and systems for de-activating the calcium in the black liquor to prevent scaling, can be integrated directly into the evaporation plant.
Falling Film (FF) Evaporators
This evaporator design relies either on tubes or plates as heat transfer surfaces. Liquor is processed on the inside of the s intubular units but on the outside of the heat transfer surface in plate designs.
FF evaporators consist of a liquor sump from which a defined volume of liquor is continuously recirculated to the top of the heating element
A distribution device,typically a tray or a spray nozzle in some designs, then distributes the flow of liquor over the entire heating surface. Holes in tubular units or slots for plate units are positioned to allow the liquor to fall onto the tubesheet or the plates.Even distribution ofliquor is a critical consideration for this type of designs and both the tray and the tubesheet(or plate element) must all be level.
Following the distribution device,a thin film of liquor is established on the heating surfaces and flows downward back to the liquor sump while being partally evaporated. Heat-transfer rates are considerably better,especially at higher concentrations, when using falling film designs over rising film designs since the liquor falls turbulently over the heating surface. Any liquor preheating requirement is also efficiently accomplished in the falling film design.
Energy-efficiency for reduced costs
Highest dry solids liquors with maximum availability using crystallization technology
Cleanest reusable condensates using the minimum amount of steam
Lowest steam and electricity consumption
Tolerate even non-soluble scaling
Specification limit of single sheet:
1. The width is less than 2m and the length is less than 9m
2. The plate group is assembled by welding each single plate, and the size can be according to the design requirements.
3. The spacing between plates is recommended to be greater than 25 mm
4. Clean medium (gas / liquid state) can be used inside the plate, and unconventional medium can be used outside the plate.
5. Plate material selection: carbon steel / stainless steel / titanium alloy.
6. Pressure bearing of plate: the pressure bearing is determined according to the arrangement of plate points, which is 20 kg in general and 40-50 kg in maximum.
|item||5 effect station||combined evaporation process|
|MVR pre concentration||5-effect evaporation station|
|Evaporation water (t/h)||100||64.28||35.72|
|RMB/T water evaporation capacity||45||25.93|
Note:In operation cost estimation:steam 150rmb/t,electricity 0.6 rmb/kWh,water 0.5rmb/t.
The investment of combined evaporation process equipment increased: evaporator (2500 m2) 375x10000 RMB; MVR compressor 400x10000rmb, total 775x10000 RMB
Annual operating cost reduction of combined evaporation process: 3672-2115 = 1557 (10000RMB)
Investment increase payback period of combined evaporation process: 755 ÷ 1557=0.5year
It can be seen that taking the scale of 100t / h as an example, the combined evaporation process can recover the increased investment in half a year, and save 1557 (10000 RMB) every year in the future, with considerable economic benefits.