New technology of black liquid concentration by vapor compressor heating
At present, multi effect evaporation is widely used in pulping alkali recovery evaporation. In the multi effect evaporation system, the more effective the evaporator is, the better the economy is, but the investment cost is also increased simultaneously. According to different requirements of evaporation water, 5-7-effect evaporation system is generally used at present, which has good economy.
In recent years, with the maturity of mechanical vapor recompression (MVR) technology. This technology has been widely used in wastewater treatment (including chemical pulp black liquor, APMP & BCTMP and other chemical mechanical pulp wastewater), food industry, seawater desalination and other fields.
1. MVR Technology
MVR technology is to compress the secondary steam of the evaporator by mechanical method, increase its pressure and temperature, increase its enthalpy, and then send it back to the heating chamber of the evaporator to be used as heating steam, so as to keep the feed liquid in boiling state, while the heating steam itself condenses into water. Compared with the multi effect evaporation technology, MVR technology compresses and recycles all the secondary steam and recovers the latent heat, so it is more energy-saving than the multi effect evaporation technology.
Application of MVR Technology:
When the concentration of evaporated liquid is raised and its boiling point is raised within a certain limit, for example, when the concentration of evaporated black liquid is below 20%, the boiling point is raised no more than 1.5 ℃.
It is used in the areas or enterprises where the electricity price is cheap and the heat energy is expensive.
2. Application of technology in black liquor evaporation of pulp
Taking the evaporation of wheat straw pulp black liquor as an example, the combination evaporation process of MVR technology and multi effect evaporation was compared with the multi effect evaporation process. The details are as follows.
2.1 main parameters and evaporation scheme
The results show that the flow rate of dilute black liquor is 128.6t/h, the temperature of dilute black liquor is 70 ℃, the concentration of dilute black liquor is 10%, the concentration of concentrated black liquor is 45%, and the evaporation water is 100t / h. The multi effect evaporation process is 5-effect evaporation station, and the combined evaporation process is MVR pre concentration + 5-effect evaporation station.
The operation of the two evaporation processes for the treatment of wheat straw pulp black liquor is shown in the table.
Operation of two evaporation processes for treating paper pulp black liquor
|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.
Advantages of MVR Technology:
After start-up, there is no need for fresh steam, or a small amount of fresh steam is consumed, so the operation cost is low.
Simple structure, single effect operation, simplify the pipeline, instrument, electrical system.
Easy to start, simple operation, stable operation, less maintenance.
It occupies a small space and has few utilities. In general, there is no need for condenser and cooling tower