Application case 4: Electrolysis flue gas purification

Electrolytic flue gas purification process, not only can remove aluminium harmful substances of the process, protect environmental, but also recycle the aluminium fluoride generated in the purification process during the production process, reducing the cost of aluminium. So with the growing awareness of environmental protection, flue gas purification treatment, utilization, is becoming an integral part of the processes of electrolytic aluminium production and so on.

The system is specially designed for the electrolytic flue gas purification; according to the size of the production flexible configuration. The system has been successfully applied in many electrolytic flue gas purification control systems of Shandong Nanshan Aluminium, Tokai Aluminium etc.

一. Process：

   Gas:

   Flue gas temperature produced by electrolytic aluminium is between 80 ~ 100℃, so do not need cooled, directly go into venturi reactor.

   Harmful substances in the flue gas after reacting with fresh alumina, flue gas through the bag filter, remove the solid fluorine aluminium contained in the gas; obtain purification flue gas in accordance with the emission standards, emission to the atmosphere by induced draft fan.

Alumina:

fresh alumina from fresh alumina silo, by fresh alumina chute convey to the Venturi reactor, and react with harmful substances in the flue gas , to obtain a fluorine containing alumina; in the bag-type dust collector, the carrier oxy-fluoride aluminium separated from gas separation, one part as the cycle alumina, through the chute flow into venturi reactor, continue reacting with the gas, recycled; another portion is returned as alumina, along the alumina chute returned to the pneumatic hoist, transported to fluorine- containing alumina silo for aluminium use.

   Blowback:bag dust collector needs blowback, in order to ensure the normal use of dust collector. Blowback has three ways:

   Differential pressure mode: based on differential pressure value between bag dust collector import and export, control start and stop of blowback.

   Timing modes: blowback cycle in a certain period of time.

   Manual mode: Manual control once blowback of a solenoid valve stage dust collector.

二. Control System

System structure:

In order to ensure control systems advanced and reliable, the system uses a centralized monitoring, decentralized control structures.

Purification subsystems in each sub-control station set up a separate, each has its own purification PLC, control cabinet, and PC is optional and can be configured as required. To ensure system reliability, system equipment has the three operations: the field level, hard manual control room, computer control. Sub-station is unattended.

Also, set a total operating station, you can monitor and control multiple sub-stations, you can save human resources.

For system openness and scalability, system network architecture used Industrial Ethernet.

   System Configuration:

   PLC: according to user requirements, system size, selects and uses the famous brands of AB, SIEMENS and so on.

   PC: use Advantech IPC.

   Control Network: according to the actual distance between sites, communication media can choose five lines or fiber.

   According to the number of nodes configured the number of Ethernet switches, fiber optic transceivers.

   System features:

Controller contains a PLC, PC and Ethernet.

1.     Control

PLC completes data collection, processing, according to the program executes control functions.

System has automatic controls, interlocking, alarm and other functions, and can regulate the pressure of induced draft fan and so on.

Be able to complete the automatic start, automatic shutdown and automatically filter blowback.

The third electrolytic aluminium gas purification of East China Sea illustrates blowback process.

Dust removal system has eight precipitators; each precipitator has 10 solenoid valves.

Ø       Manual mode:

Manually start one precipitator for blowback.

Dust each solenoid valve action is: On 5 seconds, off 2 seconds (the on/off time can be modified online).

Action order: 1 # Valves -> 4 # Valves -> 5 # Valves -> 8 # Valves -> 9 # Valves -> 2 # valve -> 3 # Valves -> 6 # Valves -> 7 # valves -> 10 # valve

By the operator to manually activate a precipitator blowback, automatically starts blowback fan, after the 10 valves actions again according to the above sequence, stop blowback fan, automatically end blow backing.

Ø       Differential pressure mode:When the differential pressure between import and export of a dust precipitators reaches the upper limit, it will automatic start blowback, the blowback solenoid valve operating time and sequence, in the same way as the manual mode; the different is differential pressure mode, blowback dust collector do not limited to only one.

After 10 Valves cycles in the order of the above sequence, if the differential pressure between import and export is still higher than the lower limit, the solenoid valve of this dust precipitator will continue to cycle until the differential pressure back to the lower limit so far.

Ø       Timing mode:The actions of each solenoid valve of dust precipitator is in the same way with manual operation, the differential pressure: On 5 seconds, off 2 seconds; but action is in a different order: not a small cycles moving across internal dust according to 1,4,5,8,9,2,3,6,7,10 order, but after all the valves in the same position of all dust precipitators opening and closing it again, and then moves to the next valve, is a large cycle moving across the various dust.

Action sequence is: (SVmn denotes the n valve of m precipitator)

SV101→> SV201→>… SV801→> SV104→> SV204→> … SV804→>… …SV110→> SV210→>…SV810→>.

These are a major cycle after cycle three times, stopped for 30 minutes, then start the next three major cycles, then stop 30 points,…….repeated cycles. When a filter is stationing, the corresponding solenoid valve 10 out of a cycle.

2.     Position monitoring

   PC is the man-machine interface, you can monitor the process parameters, equipment operation; also can switch the mode of operation, issue control commands, view alarm history, historical curve, and print statements.

     The upper picture as follows:

    Process screen:

    Display temperature, pressure, flow, pressure and other process parameters, as well as equipment operation / stop valve on / off status.

   Operating window can be opened, for system start, stop, operation mode switch, manual operation etc.

   Alarm summary:

   A chronological list all alarm points, Including alumina drying, level high, low, temperature, pressure abnormalities.

   Safe operation:

   For different operations, you can assign different operating authority, to ensure system security.