NO20181482A1

NO20181482A1 - Method and system for controlling suction of off-gases from electrolysis cells

Info

Publication number: NO20181482A1
Application number: NO20181482A
Authority: NO
Inventors: Morten Karlsen; Are Dyrøy; Eirik Manger
Original assignee: Norsk Hydro As
Priority date: 2018-11-20
Filing date: 2018-11-20
Publication date: 2020-05-21
Also published as: EP3884083A1; EA202191402A1; CA3115415A1; WO2020104343A1; BR112021006307A2; AU2019382770A1; ZA202102193B; NZ774481A; AU2019382770B2

Description

Method and system for controlling suction of off-gases from electrolysis cells

The present invention relates to a method and a system for controlling the suction of off gases from electrolysis cells for production of aluminium, the cells can be of Hall-Héroult type, preferably with prebaked anodes.

The Hall-Héroult process, named after its inventors, is the most used method by which aluminium is produced industrially today. Liquid aluminium is produced by the electrolytic reduction of alumina (Al2O3) dissolved in an electrolyte, referred to as bath, which mainly consists of cryolite (Na3AlF6).

A sketch of a prior art alumina reduction cell is shown in Figure 1.

In an alumina reduction cell, hereafter referred to as the cell, several prebaked carbon anodes are dipped into the bath. The alumina is consumed electrochemically at the anode.

As can be seen from Equation (1), the carbon anode is consumed during the process (theoretically 333 kg C/t Al).

The lower part of the cell, the cathode, consists of a steel shell lined with refractory and thermal insulation. A pool of liquid aluminium is formed on top of the carbon bottom. The cathode, in the electrochemically sense, is the interface between the liquid aluminium and the bath, described by

and the total cell reaction becomes

Pure bath (Na3AIF6) has a melting point of 1011 °C. To lower the melting point, the liquidus temperature, aluminium fluoride (AIF3) and calcium fluoride (CaF2), to mention the most important ones, are added to the bath. The bath composition in a cell may typically be 6-13 [wt%\ AlF34-6, [wt%] CaF2 and 2-4 [wt%] Al2O3. Lowering the liquidus temperature makes it possible to operate the cell at a lower bath temperature, but at the expense of reduced solubility of Al2O3 in the bath, demanding good Al2O3 control. It should be mentioned that if the concentration o Al2O3 gets too low (less than approx. 1.8 wt%), the cell enters a state called anode effect. During anode effects, the cell voltage increases from the normal 4-4.5V up to 20-50V. Anode effect is a highly unwanted state, not only because it represents a waste of energy and a disturbance of the energy balance, but also because greenhouse gases (CF4 and C2F6) are produced at the anode. Very often the anode effect requires a manual intervention of an operator.

The bath temperature during normal cell operation is between 940 °C and 970 °C. The bath is not consumed during the electrolytic process, but some is lost, mainly due to vaporization. The vapour mainly consists o Na AlF4. In addition, some bath is lost by entrainment of small droplets, and water present in the alumina feed reacts to form HF.

In order to protect the environment, the gas is collected by a hooding and a gas suction system and further cleaned in a gas scrubbing system. More than 98% of the AIF3 is recovered in the scrubbing system and recycled back to the cells. In addition, the content of sodium oxide (Na2O) and calcium fluoride (Ca2F) in the fed Al2O3 neutralize AlF3. The neutralized amount is also a function of the penetration of sodium into the cathode, and hence the cell age. As an example, one 170 kA cell emits about 60 equivalent kg AIF3 pr. 24 hours and uses approximately 2500 k AgI2O3 pr. 24 hours. The amount A oIfF3 due to neutralization for one 170 kA cell is between 0 and 20 kg per 24 hours (dependent of cell age). However, since most of the AIF3 is recycled, the real consumption of AIF3 is very small compared to the consumption A oIf2O3.

At the sidewalls of the cathode there is a frozen layer, called side ledge, which protects the carbon sidewall from erosion. The thickness of the side ledge is a function of the heat flow through the sides, which is a function of the difference in bath temperature and liquidus temperature.

The challenge is thereby to ensure stable cell operations resulting in a stable protective side ledge, while minimizing energy input and maximizing production.

Given reasonable operational targets, it is an established operational practice that minimizing the process variations around target values results in good process operations in the sense of minimum pollution to the environment, maximum production and minimum expenditure. Used in the context of the alumina reduction cell the focus should be on achieving low anode effect frequency, good gas scrubbing efficiency and low deviation from target when it comes to alumina concentration, bath temperature and acidity. If the control of the alumina concentration is reasonably good, one has to focus on the bath temperature control and the AlF3 control.

In controlling an electrolysis cell, there are, up till now, typically three main controlled variables: bath temperature, concentration of AlF3 and concentration o Al2O3, and three control inputs: anode beam adjustments (controlling energy input), addition o AlF3 and addition of Al2O3, and this is well documented in the prior art.

For instance, the applicant’s own W02009/067019 relates to a method for controlling the mass and energy balance of a cell by using a non-linear predictive model.

The present invention relates to a method and a system for controlling the suction of off gases from electrolysis cells or cells for production of aluminium where the cells being of Hall-Hérolt type with anodes, preferably prebaked and carbon-based, and further provided with a hooding connected via gas duct to a main ducting that transports the gas to a Gas Treatment Centre GTC. The gas flow in the gas duct can be controlled by a valve or more specific a damper, where one or more process variable/s such as pressure (P) and temperature (T) in the cell hooding (CH) and/or the gas duct (GD) are measured and used as input signals to a controller comprising a calculator and an algorithm. The controller can be a Programmable Logic Controller (PLC) that calculates the actual mass flow in the channel based upon a pre-defined algorithm and produces an output set signal corresponding to a wanted flow rate, the signal is further transmitted to an actuator that regulates the position of the damper, and followingly the gas flow in the gas duct.

To further optimize the energy control of Hall-Héroult cells for production of aluminium and also to be able to further contribute to the stabilization of the thermal balance of the cells, the inventors have found that the suction of off-gases from the hooding can be controlled in a new and inventive manner. Further benefits of the invention are that the off-gases from individual cells can be collected more efficient, in particular where the hooding is less gastight than designed, for instance due to wear, damages or the like.

The benefits of the present invention are in particular to control and optimise the amount of gas sucked off from electrolysis cell in a way that process variations in the cells can be reduced and emissions to the environment avoided/limited. By that, the electrolysis cells can be operated closer to operational targets and process limits, and it will be possible to achieve lower amount of emissions to the surroundings and lower energy consume per kg aluminium produced combined with more stable and efficient production process.

The control of the amount of gas sucked off involves a method and system for in-line measurements of pressure and temperature of the process gas, where these signals are used as input to a controller which produces an output signal to a damper controlled by an actuator that regulates the flow of said gas being sucked off individual cells.

The above mentioned advantages and further advantages can be obtained by the invention as defined in the attached claims.

The invention shall be described further by examples and figures, where:

Fig. 1 discloses a sketch of the main features of a prior art alumina reduction cell (Prebake) with its hooding,

Fig. 2 discloses schematically one hooding of a cell and its connection to a main gas duct of a suction system where pressure and temperature is measured in a gas duct,

Fig. 3 discloses similar details as shown in Fig. 2, where in addition pressure inside the hooding of the cell is measured,

Fig. 4 discloses plural cells monitored according to the principles of Fig. 2, preferably a group of cells connected to a common forced suction string, and similarly as above connected to a main gas duct,

Fig. 5 discloses two rows of cells monitored according the principles of Fig. 3, and further being connected to a GTC via a “H” formation.

In general, the invention as shown in Fig. 2 is based on utilising local measurements to balance and control the suction rates for each individual electrolysis cell (EC) in a pot line. By using clever and customised measurements/sensors, known relations between static pressure, temperature, and suctions rates as well as leakage break points, the emission/suction state and condition of each cell can be monitored. With automated dampers, commonly butterfly valves, controlled by a programmable logic controller (PLC), the rates can also be tuned and tailored so that cells with low hooding efficiency, i.e. high chances for leakage, receives more attention (i.e. increased gas suction flow rates) compared to cells with good hooding efficiency. In this manner, the total suction rate for the actual cell and also the plant might be reduced depending on specific needs and hooding handling.

In Fig. 2 the gas duct (GD) connecting the hooding of the cell (CH) and the main gas duct (MGD) and further transporting gas from the electrolysis cell, comprises a pressure transmitter (PT) and a temperature transmitter (TT), where both transmit signals to a programmable logic controller (PLC). The PLC controls the position of a gas duct damper (GDD) and thereby the flow rates through the gas duct in accordance to a predefined algorithm. In one embodiment, the controller can have an additional software enabling it to operate as a digital twin.

The static pressure and the temperature in the off-gas duct of the cells can be measured. These measurements can then be used to determine the net suction rates from the cells, either by known correlations or from model results. Calibration measurements might be needed to verify the function of the equipment.

The equipment collecting the measurements must be maintained regularly due to risk of fouling.

Monitoring of individual suction rates in combination with pressure, preferably static pressure, inside the cells’ superstructure as shown in Fig. 3 can even give warnings and act on low/bad hooding efficiency. By collecting pressure data from this area, measures or actions for limiting leakages of process gas to the surroundings can be taken, for instance by adjusting the damper to allow for a higher suction rate.

The status of individual cells can be monitored on a screen as a process sheet having different colour codes.

An efficient Automated Cell Suction (APS) system according to the invention may also supplement or even substitute a forced suction system, see Fig. 4. A forced suction system is commonly known as a separate forced suction string (FSS) connected to a group of cells, typically 8-10 and further having a suction channel with a booster fan blowing the extra suction into the main gas duct (MGD) of the plant’s gas extraction system. It can be arranged parallel to the ordinary gas suction channels between the cells and the main ducting and operated when there is a need for extra suction from at least one cell in the actual group of cells connected with the string. An example of a forced gas suction system is disclosed in Applicant’s own EP 1252373 A1.

The forced suction system is operated for instance when anode change takes place and lids of a cell is removed and thus the hooding is punctured. The forced suction will be sufficiently strong to avoid substantial amounts of process gas entering out of the opening in the hooding.

In some arrangements it would be possible to eliminate the necessity of a separate forced suction system completely, by the implementation of the present invention. Such elimination should be verified by modelling and simulations before implementation.

Operating an APS system, the static pressure inside the cell’s hooding can be monitored as shown in Fig. 3 by one or more pressure transmitters (PT’). This is one of the most challenging parts of the system, as the suction pressure here is quite low, of the order 5-8 Pa. In addition, scaling or fouling of pressure sensors can cause errors in the measurements over time. To reduce measurement errors, it is suggested to have at least two, preferably three to four pressure measurements (sensors / Pressure Transmitters) in the cell hooding. Trending averages can be used for control and calculating minimum and maximum values.

The pressure measuring points should preferably be placed on the same level as the gas skirt since this is where leakages will occur at too low suction rates. There is also a need to develop good control algorithms around the measurements, filtering out noise and disturbances. Additional flow measurements may be done by application of wing anemometers in the gas duct channels as these are believed to give a rather stable signal.

It is important to avoid deposits in the channels and on the dampers. To overcome this, it is suggested to have an automated procedure that completely opens and partially closes the dampers on a regular basis and running on one cell at a time. Adjusting the damper position on one cell will change this cells’ suction rate significantly, but the rest of the cells will not be affected. This also shows how a forced suction mode to some extent can be minimized and obtained by adjusting the damper.

In the final stage, see Fig. 5, the damper control of the dampers of all cells can be interlinked with fan control on the Gas Treatment Centre (CTC) to make up a full APS system. With such a connection, the fan power can be adjusted to account for e.g. sessional variations or overall changes in hooding efficiency. In this manner power can be saved and the gas treatment systems can potentially be used in a more optimal manner.

Preferably, (see Fig. 3) the pressure (P) and temperature (T) at the gas duct (GD) connecting the hooding of the cell (CH) and the main gas duct (MGD) is monitored to keep the changes or variation under surveillance by use of a wireless pressure state or a pressure controller (pressure transmitter, PT) and a wireless temperature probe (temperature transmitter, TT) which is connected to an Ethernet system including one or more PLC’s (Programmable Logic Controllers), and further including a central PLC (CPLC) covering the entire pot line, see Fig. 4.

In certain situations, the APS system can be utilized to control and correct the cells’ heat balance. The energy evacuated with the off gas is directly proportional to the mass flow, and the control valve provides a measure for swiftly changing the heat flux out of the cells.

The Idea is to be able to monitor the suction status from each individual cell, which is defined by the relationship between static pressure and temperature, to:

1. Preserve the flow from each cell at a stable and constant value, or as commonly known as flow-balance of the suction system for the whole pot line (series of cells) or part of a pot line depending of the system size. The flow, expressed by pressure and temperature, should be as constant as possible to be able to maintain the emission level at a minimum. The flow from each cell should preferably be kept at a rate that captures substantially all process gases from the inside the hooding, even despite the hooding may be leaking or even punctured. In addition, during start-up conditions or during anode change, the suction rate must be adjusted accordingly by the PLC to achieve this.

2. Identify any decreasing pressure / increasing temperature at the relevant cell number or position. If such a case is occurring, it means that something is wrong with the suction system or cell control system and some actions have to be taken by the PLC to mitigate the changes.

3. Make a code for maintenance actions by use of different colours on actions level and type to be performed, which should be displayed/visualized for operators on one or more screens in the pot line control centre.

4. Set Start-up cells at the correct suction value/level via the PLC and monitor and set the control damper of the cell in the right position as the temperature of the cell rapidly decreases as it stabilizes after start-up to minimize the impact on the pot line emission and hence the environment. In practice this means, to use the tools of pressure and temperature which is displayed, to change the position of the damper as the pressure and temperature changes as the cell operation will be normalized. This can be done by the PLC.

5. Another aspect of the system, is to implement wireless surveillance of cell’s suction to automate the regulation and hence balancing of the suction system by use of motor controlled / pneumatic controlled regulation for the outlet duct dampers.

6. By applying the registered outlet pressure and temperature values as a state of heat losses to the heat balance control or surveillance system of the cells one will have a new tool to combine with existing system to control the energy input to the cell.

An appropriate APS systems will:

• Reduce emissions

• Reduce operating costs by lowering the gas suction rates for individual cells towards a minimum suction depending on precise measurements of the sub pressure inside the hooding compared with other information that can indicate the condition of the cell, for instance supported by models of the cell, such as a digital twin

• Reduce the total gas volume to be handled by the GTC

• Make the individual cells more autonomous

• Utilise the available gas suction rate better, for instance to create an enhanced suction on individual cells

• Allow sessional variations in gas suctions rates from GTC

• Provide possibilities for quantifying cell hooding efficiency

• Reduce thermal heat losses

• Reduce energy consume

• Give possibilities to control and quickly correct the energy flux with the off gas

Claims

1. A method for controlling the suction of off-gases from electrolysis cells (EC) or cell in a plant for production of aluminium, the cells being of Hall-Hérolt type, provi with a hooding (CH) connected via one gas duct (GD) to a main gas ducting (M that transports the gas to a gas treatment centre (GTC) with suction means, whe the gas flow in the gas duct (GD) can be controlled by a gas duct damper (GDD) c h a r a c t e r i s e d in t h a t

one or more process variable/s such as pressure and temperature in the gas (GD) are measured and used as input signals to a controller (PLC) comprising calculator where the controller calculates the actual mass flow in the gas duct (G based upon a pre-defined algorithm and produces an output set si corresponding to a wanted flow rate, the signal is transmitted to an actuator (A) t regulates the position of the gas duct damper (GDD), and followingly the gas flo the gas duct (GD) from individual cells.

2. A method in accordance with claim 1,

c h a r a c t e r i s e d in t h a t

in addition, the pressure in the hooding of the cell (CH) is measured by at least pressure transmitter (PT’) and an input signal is accordingly transmitted to th controller (PLC).

3. A method in accordance with claim 1 or 2,

c h a r a c t e r i s e d in t h a t

the flow from each cell is optimised to a minimum where the onset of leakage is individual property of each cell, varying with the hooding efficiency, and diffe suction rates are applied to each cell for keeping the cells sealed and avoid lea to cell’s surroundings, while still keeping the suction rate as low as possible.

4. A method in accordance with claim 1 - 3,

c h a r a c t e r i s e d in t h a t

the flow in the gas ducts (GD) of the individual cells in the plant are regulat towards an optimized minimum flow, where the gas flow generated by the sucti means of the gas treatment centre (GTC) is adjusted correspondingly to optimi the amount of gas to be treated and the energy consume.

5. A method in accordance with claim 1 - 4,

c h a r a c t e r i s e d in t h a t

the flow in the gas ducts (GD) of the individual cells can be controlled and chan to quickly adjust the heat balance of the cells.

6. A method in accordance with claim 1, that keeps the said automatic damper syst free from deposits and scaling

c h a r a c t e r i s e d in t h a t

the damper opens and closes partially at regular intervals so that deposits o damper blade is exposed to different airflows and cleaned.

7. System for controlling the suction of off-gases from electrolysis cells (EC) in a pla for production of aluminium, the cells being of Hall-Hérolt type and further provi with a hooding (CH) connected via a gas duct (GD) to a main gas ducting (MG having means for generating a suction that transports the gas to a gas treat centre (GTC), where the gas flow in the gas duct (GD) is controlled by a damper, c h a r a c t e r i s e d in t h a t

the system further comprises sensor/s for measurement of one or more proce variable/s such as pressure (P) and temperature (T) in the gas duct (GD) where t measured value/s are represented by input signals to a controller (PLC) comprisi a calculator, the controller calculates the actual mass flow based upon a pre-def algorithm and produces an output set signal corresponding to a wanted flow r an actuator (A) that regulates the position of the gas duct damper (GDD) a followingly the flow in the gas duct (GD).

8. System in accordance with claim 7,

c h a r a c t e r i s e d in t h a t

the system comprises in addition sensor/s (PT’) for measurement of static pressur inside the cell hooding, where the measured value/s are represented by addi input signals to the controller (PLC) that produces an output set signal for control the flow in the gas duct (GD).

9. System in accordance with claims 7-8,

c h a r a c t e r i s e d in t h a t

the flow in the gas ducts (GD) of the individual electrolysis cells (EC) in the plant a regulated towards an optimized minimum flow, where the gas flow generated by suction means is adjusted correspondingly to optimize the flow.

10. System in accordance with claim 7,

c h a r a c t e r i s e d in t h a t

the algorithm is based upon known relations between static pressure, temperat and suctions rates of the gas.

11. System in accordance with claim 7,

c h a r a c t e r i s e d in t h a t

the controller (PLC) is an integrated part of a local cell controller.

12. System in accordance with claim 7,

c h a r a c t e r i s e d in t h a t

the controller (PLC) is an integrated part of a central controller (CPLC).

13. System in accordance with claim 7,

c h a r a c t e r i s e d in t h a t

the signals are transmitted either wireless or by cable.

14. System in accordance with claim 7,

c h a r a c t e r i s e d in t h a t

the controller (PLC) is able to control the electrolysis cell (EC) as an autonomo cell, maintaining the relative pressure inside the hooding (CH) and outside constant by controlling the gas duct damper (GDD), and further that a minim suction of the cell is always maintained by controlling the gas duct damper (GD and that changes in cell condition with regard to pressure and/or temperatu detected and compared with historical data stored in the controller (PLC), wh appropriate actions regarding control of the gas duct damper (GDD) are taken.

15. System in accordance with claim 7,

c h a r a c t e r i s e d in t h a t

the controller (PLC) comprises a digital twin model of the cell.