Accurate assessment of the Hirakud smelter aluminium reduction cell thermal balance using only temperature measurements

Transcription

1 Accurate assessment o the Hirakud smelter aluminium reduction cell thermal balance using only temperature measurements M. Dupuis Géniim Inc Alger t. Jonquière, Québec, Canada G7 M9 marc.dupuis@genisim.com A. Koshie, V. Janakiraman Indian Aluminium Company, Limited Hirakud , Dist. ambalpur rissa, India a.koshie@indal.co.in. Karthikeyan, D. aravanan Conederation o Indian Industry 35/1, Ahiramapuram, 3 rd treet, Alwarpet, Chennai , India s.karthikeyan@ciionline.org ABTRACT In the present study, the heat balance o the Hirakud smelter aluminium reduction cell has been accurately assessed using only a surace thermocouple and a pyrometer. As it is well known that using a single linear or quadratic relationship to estimate the heat lux rom the measured surace temperature does not provide the accuracy required to close the cell heat balance, this approach was not used in this study. Instead, the undamental natural convection and radiation heat lux equations were used systematically or each measurement point. Practically, this means that the ambient air temperature and at least one ambient radiative temperature must also be known in addition to the surace temperature or each measurement point. ometimes, the radiative heat lux could not be accurately estimated using a single ambient radiative temperature. In those cases, a more accurate estimation based on two ambient radiative temperatures and corresponding view actors has been used instead. It turns out that although at least three temperature measurements are required to estimate a single heat lux, it takes a lot less time to make those temperature measurements using a pyrometer than directly measuring the heat lux using an expensive heat lux meter. Furthermore, no loss in accuracy was detected when using this approach as the cell heat balance could be closed within 5%, the level o accuracy typical o cell heat balance assessment using heat lux meter.

2 INTRDUCTIN In his paper Pot Heat Balance Fundamentals, Bruggeman [1] wrote as introduction: The heat balance is not only a ocal point or the economics o production, but also or the science o aluminum production. He also added: By the end o the 1960 s, correlation and simple mathematical models harnessing the power o early computers were available to improve cell design. ince then, both measurement methods and models have advanced urther to the point where, new designs are not considered without rigorous testing and screening using these sophisticated tools. o obviously, the irst priorities o the Hirakud smelter cell retroit program has been to measure the cell heat balance and to develop reliable mathematical models. The present paper ocuses on the work done to assess the thermal balance o the cell. THE CELL HEAT BALANCE The concept o the cell heat balance is quite simple. the total electrical power ed to the cell, less than hal is actually used to produce aluminium. The remaining part must be dissipated as heat losses by the cell in order or it to maintain its thermal equilibrium. Experimentally, a cell voltage break down is required in order to calculate the cell internal heat i.e. the heat that the cell needs to dissipate to maintain its thermal equilibrium. In turn, this can be experimentally conirmed by directly measuring the cell heat losses. I the cell heat losses correspond to the calculated cell internal heat, those measurements can be used with conidence in order to calibrate the mathematical models o the cell. HEAT LE MEAUREMENT METHD In general, cells loose heat by natural convection and by radiation. The equations that describe the physics o those two heat transer mechanisms are well known. In the early cell heat balance measurement campaigns, those equations were indirectly used to compute the heat luxes on the dierent cell suraces and hence to compute the total cell heat losses []. They were only used indirectly probably because o the limitation o the computing power at that time: the undamental equations were used to correlate the dierent cell surace temperatures to the heat luxes (see Figure 1 extracted rom []). Those correlations were established in preparation or the actual ield measurement campaign in order to reduce it to the measurement o the cell surace temperatures only. This approach turned out not to produce very accurate results.

3 Figure 1: Relation between surace temperature and heat dispersion (Fig 6 in []) Very recently [3], those same undamental equations have been presented again as background theoretical knowledge, but the authors careully speciied that: Due to geometry and other conditions o an electrolysis cell, (those) equations cannot be used directly or (calculating) the heat low rom shell wall to air. In [1], Bruggeman clearly expressed the current conventional wisdom o the industry by speciying that heat lux transducer must be used to carry out cell heat loss measurement campaigns. He even pointed out that: Haupin developed a heat lux transducer especially or pot measurements. Apart rom Alcoa, most o the rest o the industry is rather using commercially available heat lux transducers (see Figure ). Figure : Commercially available heat lux transducers (

4 Unortunately, commercially available heat lux transducers are airly expensive and relatively ragile. They are also characterized by a airly long response time o around 10 minutes. This means that cell heat balance measurement campaigns using heat lux transducers are airly long and expensive to carry out. For that reason, the challenge posed to the authors was to ind an accurate way to assess the Hirakud cell thermal balance using only temperature measurements. FUDAMENTAL HEAT FLUX EQUATIN The general orm o the heat transer equations have been published multiple times, reerences [,3] being two examples. But as the natural convection heat loss equations are semi-empirical, there exact orm varie rom author to author. In the present work, we used the ollowing equations to compute the heat luxes []: q ( W / m ) = q + q tot c r (1) k qc ( W / m ) = Nu ( T TA) () L ( T + 73 ) ( T 73 ) ) q εσ (3) r ( W / m ) = + For vertical suraces, we have: 1 Nu = 059. Ra, or 10 Ra 10 9 () 1 3 Nu = Ra, or 10 9 Ra 10 1 (5) For horizontal suraces acing up we have: Nu = 05. Ra, or 10 Ra 10 (6) 1 3 Nu = 011. Ra, or 10 7 Ra (7) And inally, or horizontal suraces acing down we have: Nu = 07. Ra, or 3 10 Ra 3 10 (8)

5 Where: 3 gβl ( T ) = T A Ra Pr (9) υ In order to be able to evaluate equations () and (9), we need to know the value o k, υ and P r at T, the air ilm temperature: T ( T + T ) A = (10) Those air properties are presented in Table 1 []: Table 1 Property values o air at atmospheric pressure T ( C) k (W/m C) ν (m/s) Pr E E E E E E E E E E E E E E E E E E Finally, by itting the data o Table 1 with order polynomials, we can establish the ollowing equations or the air thermal conductivity, the air cinematic viscosity and the air Prandtl number respectively:

6 3 k =.01E 15 T E 11 T.118E 8 T E 5 T (11) 3 υ = 1.38E 17 T 3.5E 1 T E 11 T E 8 T + 1.3E 5 (1) 3 Pr = 1.937E 13 T 6.581E 10 T E 7 T.788E T (13) Equations (1) to (13) deine a close orm unction that can be summarized as: q tot ( W / m ) = F ( T, T, T, ε, L, ) (1) q A Where: T (ºC) T A (ºC) T (ºC) ε L (m) is the measured surace temperature is the measured air temperature close to the surace is the measured acing radiative background temperature is the surace emissivity is the surace typical length is the surace orientation (V, H or D) MEAUREMENT CAMPAIGN In order to calculate the global cell heat losses, approximately 00 suraces must be established around the cell. The area o each o those suraces must be calculated in order to be able to, in turn, calculate the heat dissipated by each o them: Q ( W ) = A F ( T, T, T, ε, L, ) (15) i i q N A Q ( kw ) = /1000 (16) cell Q i 1 i

7 The actual implementation o this approach is not as complex as it may look. It is quite easy to evaluate ahead o time A, ε, L and or each surace. For a given cell design, once established, the value o those items will not change. This leaves only three temperatures to be measured per surace T, T A and T during the measurement campaign. In comparison, in a standard measurement campaign using heat lux transducers, only q tot is measured or each surace. Yet, measuring T and T with a small hand held pyrometer and T A with a thermocouple and a small hand held multimeter is ar less cumbersome and requires ar less time than using slow to response heat lux transducers connected by wires to an expensive and delicate heat lux meter! ANALYI F THE REULT Despite all the theoretical and advance preparation work, there were no guarantees that this new measurement approach would produce accurate results. In act, our irst attempt did not; the measured heat losses signiicantly exceeded the calculated cell internal heat! Analysis o the results revealed that we were overestimating the heat lux o some very hot suraces or which the radiation term became very large. course, we knew that the radiative exchanges around a cell are very complex and that considering that each surace is only seeing one background radiative temperature could well turn out to be an unrealistic over-simpliication. ur irst results conirmed that this was the case, at least or some critical suraces. This led us to consider that those suraces are seeing two background radiative objects that are not at the same temperature. course, this in turn introduce the need to calculate the view actors or each object: q ' [ F ( T + 73) ( T + 73) ) + (1 F) ( T + 73) ( T 73) )] r ( W / m ) = εσ + In a second attempt, we use this new equation to evaluate the radiative heat transer o the ew regions o the cell were hot suraces are partially seeing another hot surace. This second time, the percentage o closure was in the acceptable rage as we can see in Table II. A third attempt, which was also successul, on a dierent cell conirmed that we had developed an accurate method to assess the thermal balance o the Hirakud cell using only temperature measurements. (17)

8 Table II

9 CNCLUIN The heat balance o the Hirakud smelter aluminium reduction cell has been accurately assessed using only a surace thermocouple and a pyrometer. No loss in accuracy was detected when using this approach as the cell heat balance could be closed within 5%, the level o accuracy typical o cell heat balance assessment using heat lux meter. Furthermore, it turns out that although at least three temperature measurements are required to estimate a single heat lux, it takes a lot less time to make those temperature measurements using a pyrometer than directly measuring the heat lux using an expensive heat lux meter. REFERENCE (1) BRUGGEMAN, J.N., Pot heat balance undamentals, Proc 6 th Aust Al melting Workshop, p () ARAI, K. and YAMAZAKI, K., Heat balance and thermal losses in advanced prebaked anode cells, TM Light Metals, p (3) HAUGLAND, E., BRET, H., GIKLING, H. and HIE, H., 003. Eects o ambient temperature and ventilation on shell temperature heat balance and side ledge o an alumina reduction cell, TM Light Metals, p () DUPUI, M., Computation o heat transer coeicient tables establishing boundary conditions between hot suraces and their surroundings, internal report.