GB2504127A

GB2504127A - A method for monitoring the heat flux through walls of industrial reactors via thermoelectric device(s)

Info

Publication number: GB2504127A
Application number: GB1212910.2A
Authority: GB
Inventors: Trygve Eidet; Kai Johansen
Original assignee: TEGMA AS
Current assignee: TEGMA AS
Priority date: 2012-07-20
Filing date: 2012-07-20
Publication date: 2014-01-22
Also published as: GB201212910D0; WO2014012857A1

Abstract

A method and device for monitoring the heat flux through walls of industrial reactors (1)is defined, where the method comprises providing a thermal reservoir at a first temperature T1, where T1 is lower than the wall temperature T2 of the reactor (1) is described. One or more of the thermoelectric device(s) (TEG) (3) has two dissimilar electric conductors, where at least one of the dissimilar electric conductors has thermal contact with the thermal reservoir and the other electric conductor has thermal contact with a portion of the outer wall of the reactor (2). The electric current is connected from the one or more thermoelectric devices (3) to a monitoring device which is able to record and/or display the electric current from the one or more thermoelectric devices (3). The TEG (3) may comprise magnesium silicides or skutterudites

Description

Method and device for monitoring the heat flux through walls of industrial reactors The present invention relates to a method and device for monitoring the heat flux through walls of industrial reactors.

Background

Mctallurgical processcs are generally run with huge amounts of cxccss cncrgy which is lost as heat. The energy efficiency of the process may be improved if heat loss is regenerated as electric energy. Energy recovery systcms in usc in metallurgical industry today, such as Rankine cycle, maybe limited by system issues such as safety, reliability, heat flux impedance, maintenance cost, investment cost etc. Location of and magnitude of heat fluxes from reactors and furnaces arc not often being measured and utilized as leading indicators for process control purposes. Such information may be utilized in process optimizationicontrol, and in safety systems.

Today's lack of this information is compensated by running the technology further from the maximum efficiency than required.

Passive cooling of reactor exterior does not provide countermeasures when a higher hcat flux than the natural convection provides is needed. In some cases this leads to catastrophic failure of thc furnace/reactor. For safcty rcasons, thc only activc cooling available often times is compressed air blowing which is not very efficient, and causes work environment issues.

Prior art

The thermoelectric effect is a well known effect of a direct conversion of temperature differences to electric voltage and vice-versa. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when a voltage is applied to it, it creates a temperature difference. The thcrmo-clcctric cffcct cncompasscs thrcc separately idcntificd cffccts; thc Sccbcck-cffcct, Peltier-effect and Thomson-effect. Of these, the Seebeck effect is the conversion of temperature differences directly into electricity arising when a temperature difference exists between two dissimilar electrical conductors or semiconductors.

From CN 101 615 870 B it is known a method for utilising waste heat from an aluminium cell air duct by providing a thermoelectric module on the outer wall of a vent discharge duct on the upper part of an aluminium cell. The hot end of the thermoelectric module is connected with the outer wall of the vent discharge duct, while the cold end is cooled through air cooling or by a cyclic heat-conducting medium; and the heat energy in the vent discharge duct is converted into electric cnergy through the thermoelectric module, and the electric energy is output through a lead-out end of the vent discharge duct.

From RU 2 180 367 it is known a method for detecting local zones of breakage of hearth of an aluminium cell, by measuring with use of instruments physical parameters of construction members of hearth for detecting zones of local breakage according to fluctuation of those parameters relative to standard values of production process; measuring electric current load of all cathode rods and determining zones and degree of hearth breakage according to value of decreasing electric current load from standard one for cathode rod or group of cathode rods; and measuring in addition temperature of all cathode rods for more accurately defining zones and degree of hearth breakage according to lowered temperature of cathode rod or group of cathode rods relative to standard value of technological process.

Objective of the invention The main objective of the invention is to provide a method and device for monitoring the heat flux through walls of industrial reactors.

Description of the invention

The present invention is based on the realisation that the thermoelectric effect, more precise the Sccbcck effect, may be utiliscd in industrial reactors for a simple and retrofit-able monitoring of the temperature and thus indirectly the heat flux (i.e. the heat loss) through the reactor wall, since the voltage provided by a thermoelectric device according to the Seebeck effect is directly dependent on the temperature difference between the two dissimilar electric conductors. Thus, by providing a thermoelectric device having one of two dissimilar electric conductors in thermal contact with a low temperature reservoir with a steady temperature and the other electric conductor in thermal contact with the outer reactor wall, the produced electric voltage by the thermoelectric device provides a direct measurement of the temperature and thus the heat flux out of the reactor wall at the location of the reactor wall in contact with the thermoelectric device.

Thus in a first aspect, the invention relates to a method for monitoring the heat flux through a wall of an industrial reactor, wherein the method comprises: -providing a thermal reservoir at a first temperature Ti, where Ti is lower than the wall temperature T2 of the reactor, -providing one or more thermoelectric devices having two dissimilar electric conductors, where one of the dissimilar electric conductors are made to have thermal contact with the thermal reservoir and the other electric conductor is made to have thermal contact with a portion of the outer wall of the reactor, and -connecting the electric current from the one or more thermoelectric devices to a monitoring device able to record and/or display the clcctric current signal from the one or more thermoelectric devises as a measure of the heat flux through the reactor wall.

The thermoelectric device may advantageously be able to utilise the Peltier effect in order to provide the opportunity of active cooling or heating of the reactor wall. The thermoelectric device may thus be a thermoelectric generator which may either convert thermal energy to electricity by the Seebeck effect or make use of electricity to extract thermal energy by the Peltier effect. Such a thermoelectric generator (TEG) may advantageously consist of modules with a cold side and a hot side with the thermoelectric materials in between. They may advantageously be planar plate-resembling components that can be implemented in the sidewall, top or bottom of industrial reactors, such as i.e. metallurgical reactors. Since thermo-electric materials are generally poor thermal conductors, they may alternatively be integrated as part of the reactor insulation in such systems, provided that they are protected from the direct contact with the content of such reactors and that they are integrated in such a way that the operational temperature design limits of the TEG components, such as for instance the maximal operational temperature on the hot side of the TEll, are not exceeded during operation.

In case of having metallurgical reactor where the TElls arc implemented in the reactor design, or added onto an existing design, these generators will provide the following benefits: 1. Due to the temperature gradient and the heat flow through the thermo electric generators, electricity will be generated from the thermal waste energy. This electricity may be utilized to lower the total electricity consumption of the plant.

2. The electricity production of a TEG is related to the temperature difference between the hot and cold side of the generator which again is dependant on the energy flow through the reactor embodiment where the TEG is mounted. Therefore the electricity production of the TEll is providing in situ information about the thermal conditions in the reactor and can be used as a local sensor in the reactor. In-situ information about local thermal conditions in the reactor can be used one of several input parameters for continuous adjustment of the operational parameters of the reactor, like local feed rate, slag-! or electrolyte composition, power input, electrode position, tapping settings, etc. 3. In case the temperatures in the reactor embodiment is higher than design criteria, this can be registered by a thermo electric generator and the generator may also be used as a heat pump under these conditions by sending current through the generator. This will revert the function of the thermo electric generator such that heat will be extracted from one side to the other and thus cool the overheated area of the reactor embodiment.

Another advantage of employing TEGs for monitoring the heat flux through reactor walls is that some of the heat loss is converted into useful energy (electric current) and thus constitute a waste energy recovery.

In a second aspect, the invention relates to an arrangement for monitoring the heat flux through reactor walls, wherein the arrangement comprises: -one or more thermoelectric devices each having a first electric conductor and a second electric conductor, where the first and second conductor are dissimilar, and -a monitoring device electrically connected to the one or more thermoelectric devices and which is able to record and/or display the electric current signal from the one or more thermoelectric devises as a measure of the heat flux through the reactor wall, and where -either the first or second electric conductor of each of the one or more thermoelectric devices is/are made in thermal contact with a first thermal reservoir at temperature Ti, where Ti is lower than the wall temperature T2 of the reactor, and -the other electric conductor of the either first or second electric conductor in thermal contact with the thermal reservoir of each thermoelectric device is made in thermal contact with a portion of the outer wall of the reactor.

The thermoelectric device may advantageously be adapted to enable producing an electric current due to the Seebeck effect when subject to a temperature difference between the first and second electric conductors and able to produce a temperature difference between the first and second electric conductors due to the Pclticr-cffcct when being subject to an imposed electric potential. This feature has the advantage of enabling active temperature adjustments of the reactor wall in contact with the thermoelectric device. In this case, the thermoelectric device is a thermoelectric generator and should in addition comprise means for imposing an electric current through the thermoelectric generator.

Generally, the thermoelectric generator or device may be implemented in reactor embodiments or vessels as simple stand alone units with electrical contacts for harvesting or adding electrical current. In some cases it may be desired to introduce cooling circuits to the cold side to provide the ideal conditions for optimizing the efficiency of the thermoelectric generator when used as a waste heat energy recovery system.

In the case of the aluminium electrolysis cell technology in use today a good example of use for this invention is sectioned thermoelectric generators mounted on the sidewall of the cells. Here, the surface temperature, heat flux and control need is well suited for the function of thermoelectric generators, as well as thc occasional need for local excessive cooling.

The heat reservoir may be any conceivable mass with sufficient thermal inertia and! or specific heat capacity to enable holding the electric conductors of the thermoelectric generators in contact with the thermal reservoir at a substantially constant temperature Ti. Examples of suited thermal reservoirs may simply be the ambient air or systems for active cooling such as i.e. heat exchangers, cooling fluid circuits etc. The thermoelectric generators installed on an aluminium cell side wall would provide several benefits: Reduce energy consumption by the value of the thermoelectrically generated energy.

ii. Increase production by the increased control accuracy. Control accuracy will increase the efficiency limit of the technology caused by the ability to operate closer to the maximal current load.

iii. Reduce energy consumption by the value of the increased efficiency.

iv. Expand pot life because out of control situations may be omitted. This is a huge cost driver in aluminium production today.

These effects are economically desirable, and will also reduce the carbon footprint of aluminium production.

Example embodiment of the invention The example embodiment is related to usc of the thermoelectric generator as combined waste energy recovery system and sensor in metallurgical reactors or furnaces associated with aluminium cells.

The temperature of the electrolyte is one of the key process variables in an aluminium reduction electrolysis cell. The temperature variance in an aluminium cell is typically higher than desired, and has a strong influence on the current efficiency. Another important process variable is the electrolyte composition. Both the temperature and the composition of the electrolyte need to be controlled in order to control the superheat of the electrolyte.

The sidewalls of an aluminium cells are designed to provide conditions for a ledge of frozen electrolyte to be formed on the inside of the cathode materials to provide corrosion protection of the sidewall materials. Depending on the temperature and superheat of the electrolyte, the ledge thickness will be reduced or increased. As a consequence the heat flux through the sidewall will be affected. Measurement of the local (and global) temperature and superheat in the electrolyte is challenging due to the corrosive environment and the semi-continuous nature of the process. Too high bath temperature leads to decreased current efficiency, reduced crust thickness and increased energy losses. It also accelerates the losses of volatile gases. Too low bath temperature on the other side, leads to reduced solubility of alumina and increased resistivity of electrolyte. It also leads to more difficult routine operations, such as for instance anode replacements, due to the increased presence of lumps and harder crust. There are several operational control actions that can be used to impact the process temperature and superheat of the electrolysis bath to stabilize the process.

Better and faster information about the localized thermal conditions in the electrolysis bath is of great importance to improve the selection and timing of such actions.

Thermoelectric generators have the ability to produce electrical current when exposed to a thermal gradient. The output of such thermoelectric generators is influenced by the hot and cold side temperature of the generator as well as the heat flux through the generator. By implementing thermoelectric generators at the sidewalls of an electrolysis cell, local in-situ information about the heat flux through sidewalls can thus be collected and used for controlling actions.

Thermoelectric generators can be attached, or retrofitted, to existing electrolysis cells on the outside of the cathode structure. Localization of generators on the cells can be on the cell sidewalls, and/or on the freeboard coverand/or on the bottom. An alternative configuration is to implement the thermoelectric generators inside the cathode, as part of the sidewall or bottom design of the cell. Implementation of a thermoelectric generator will influence the thermal balance ofthe aluminium cell, and adjustments to the sidewall and/or bottom design is necessary to optimize the thermal gradients in the cell after implementation of the thermoelectric generator.

The preferred thermoelectric materials for the generators would allow for operation at temperatures in the range of 300-600 degrees Celsius (hot side of the thermoelectric generator), which means that the installation of the thermoelectric generator should be installed such that the waste heat from the electrolysis cell where the generator is mounted will be sufficient to maintain the temperature of the hot side of the generator within the temperature range of 300-600 degrees Celsius under normal operational conditions.

Suitable thermoelectric materials may be Skutterudites or Silicides, which offers good thermoelectric performance and ZT values in this temperature region, but also other thermoelectric materials with similar properties may be used.

Figures 1,2 and 3 illustrate examples of how the invention may be applied. In Figure 1 it is shown schematically, seen from above, a metallurgical reactor 1 with four side walls 2 which all leak heat energy marked as arrows HF! to HF4. On each reactor side wall 2 there is placed a thermoelectric device 3 which monitors the heat flux through the corresponding reactor side wall 2. A similar arrangement is shown in Figure 2, except that there is applied two thermoelectric devices 3a on one reactor side wall 2. These thermoelectric devices 3a provides more detailed information of the heat loss through this reactor side wall, as indicated by arrows HF2a and HF2b. In Figure 3, one of the thermoelectric devices is a thermoelectric generator 4 which is in this example engaged to actively extract more heat through the reactor side wall 2, as indicated by arrow HF2a(total) which is the total heat loss over that wall section and arrows HF2a which is the conductive heat loss through the reactor side wall and arrow marked "Peltier" which is the heat flow resulting from the engagement of the thermodectric generator 4. Thus, HF2a(total) = HF2a + Pelt icr.

Claims

CLAIMSI. A method for monitoring the heat flux through a wall of an industrial reactor, wherein the method comprises: -providing a thermal reservoir at a first temperature T1, where T1 is lower than the wall temperature T2 of the reactor, -providing one or more thermoelectric devices having two dissimilar electric conductots, where one of the dissimilar electric conductors are made to have thermal contact with the thermal reservoir and the other electric conductor is made to have thermal contact with a portion of the outer wall of the reactor, and -connecting the electric current from the one or more thermoelectric devices to a monitoring device able to record and/or display the electric current signal from the one or more thermoelectric devises as a measure of the heat flux through the reactor wall.
2. A method according to claim 1, whercin the method further compriscs recovering a part of the energy loss from the reactor by collecting the electric current produced by the one or more thermoelectric devices.
3. A method according to claim 1 or 2, wherein the method further comprises applying the one or more thermoelectric devices for active temperature regulation of the reactor wall by imposing an electric current thorough the one or more thermoelectric devices.
4. A method according to claim 1, 2 or 3, wherein the method further comprises utilising the electric current from the one or more thermoelectric devices to monitor and regulate thc rcaction kinetics of the reactions raking place in the industrial reactor.
5. A method according to any preceding claim, wherein the method is applied to metallurgical reactors.
6. A method according to claim 5, wherein the metallurgical reactor is an electrolysis cell.
7. An arrangement for monitoring the heat flux through reactor walls, wherein the arrangement comprises: -one or more thermoelectric devices each having a first electric conductor and a second electric conductor, where the first and second conductor are dissimilar, and -a monitoring device electrically connected to the one or more thermoccctric devices and which is able to record and/or display the electric current signal from the one or more thermoelectric devises as a measure of the heat flux through the reactor wall, and where -eithcr the first or second electric conductor of each of the one or more thermoelectric devices is/are made in thermal contact with a first thermal reservoir at temperature T1, where T1 is lower than the wall temperature T2 of the reactor, and -the other electric conductor of the either first or second electric conductor in thermal contact with the thermal reservoir of each thermoelectric device is made in thermal contact with a portion of the outer wall of the reactor.
K An arrangement according to claim 7, wherein the arrangement further comprises an electric generator adapted to impose an electric current through the thermoelectric device.
9. An arrangement according to claim 7 or 8, wherein the one or more thermoelectric devices consist of modules with a cold side and a hot side with the thermoelectric materials in between, and which are shaped into planar plate-resembling components that can be implemented in the sidewall, top or bottom of industrial reactors.
10. An arrangement according to claim 7, 8 or 9, wherein the one or more thermoelectric devices are integrated as part of the reactor insulation of the industrial reactor.
11. An arrangement according to any of claims 7 to 10, wherein the one or more thermoelectric devices comprises one or more thermoelectric materials chosen from; Skutterudites or Magnesium Silicides.