[IEEE 2013 IEEE Industry Applications Society Annual Meeting - Lake Buena Vista, FL, USA...

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Mobile Application Tool for the Optimal Operation of Electrolytic Tinning Lines

Eduardo H. Enrique Senior Electrical Engineer

Stantec Consulting Ltd. 49 Frederick Street

Kitchener, ON, N2H 6M7, Canada [email protected]

Abstract - A mobile application for the optimal operation of

electrolytic tinning lines has been developed. Microsoft Excel

was the software of choice for the development of the user

friendly operator interface and the optimization engine. The

mobile application tool is available online through Google Sites

and can be used for the optimal operation of any electrolytic

tinning line whether it is halogen or Ferrostan ™ type. The tool

is also seamlessly adaptable to optimize tin-free lines. Allowance

is made for the adjustments of constraints due to a variety of

process conditions; like limits on the maximum line speed,

reduced number of anodes and changes in the optimal current

density. The mobile application has been successfully used in the

optimization and operation of a halogen-based electrolytic

tinning line.

Index Terms-Anodes, computer applications, continuous

production, cost function, electrochemical processes, electrodes,

online services, optimized production technology, process

control.

I. INTRODUCTION

The control of the coating gage is the main concern for the operator of an electrolytic tinning line. This coating gage is proportional to the plating current delivered by the anodes located in the plating tanks. The steel strip moves along these tanks, where the top and bottom surfaces of the material are progressively plated, until the specified coating gage is reached.

In addition to the control of the coating gage, the electrolytic tinning line operator is concerned with plating the quality. There is a direct correlation between the plating quality and the density of the plating current as described in [1] and discussed later in this paper. For this reason, the plating of the steel strip is spread along several electrolytic tanks, such that the plating current density can be kept within some predetermined limits.

Finally, the electrolytic tinning line operator is concerned with minimizing operational costs by maximizing the process throughput. This last concern is common to any industrial process.

The optimal operation of the electrolytic tinning process takes into account all the concerns listed above by operating the plating line at the conditions recommended by the designer of the equipment. The plating current is adjusted proportionally to the line speed to track the fluctuations in the material throughput. The plating current density is

Pablo D. Enrique Student - Nanotechnology Department

University of Waterloo 200 University Avenue West

Waterloo, ON, N2L 3Gl, Canada [email protected]

maintained constant by enabling and disabling the anodes such that the plating quality remains at an optimum.

The challenge arises when two different coating gages are specified for the top and bottom side of the steel strip. Under these conditions, a balance should be reached to achieve the target coating gages, the plating quality and the maximum line throughput as described in [2].

The mobile application tool presented in this paper offers a simple, cost effective solution for the optimal operation of electrolytic tinning lines. The tool is applicable for any combination of coating gages for the top and bottom side of the steel strip. This tool incorporates a set of optionally adjustable electrolytic tinning line parameters, such as maximum line speed, total number of plating anodes (for the top and bottom side of the steel strip) and optimal current density.

11. DESCRIPTION OF THE ELECTROLYTIC TINNING UNE

The electrolytic tinning plating process takes place in the plating tanks also referred as plating cells. In these tanks, a series of anodes are immersed in an electrolyte, as shown in Fig. 1. The anodes are connected to a DC power source, made of controlled rectifiers or, for older systems, a motorgenerator (MG) set. Each tank has one or two plating anodes per strip side, depending on the design. In Fig. 1, there are two anodes for the top side and two for the bottom side of the steel strip in each tank. Each anode can be connected or disconnected independently from the DC power source. The selection of the number of active (connected) anodes is directly related to the plating current density.

Fig. 1. A Ferrostan electrolytic tinning line with five plating tanks.

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III. MATHEMATICAL MODEL FOR TlNNING LINES

In a typical electrolytic tinning line, the coating gage (also referred as coating weight) deposited on the steel strip surface is proportional to the plating current and inversely proportional to the travelling speed of the material. Furthermore, the wider the steel strip, the lighter the coating weight per unit of surface. A simplified expression that relates these four variables is described in [3], [4] and shown in (1), where CIV is the coating weight in pounds per base-box or lb/BB (where base-box is a unit of area), I is the plating current in amperes, kp is a plating constant, N is the tinning line speed in feet per minutes and W is the strip width in inches. The mathematical model (1) is a linear approximation of the plating process in the proximity of the optimal operating conditions.

I C = --

W k NW p

(1)

The plating constant kp is the result of the Faraday constant described in [5], the plating process efficiency and the units of measurement of the process variables shown in (1). This constant is given by the designer of the electrolytic tinning line. For operating conditions that deviate from those specified the designer of the plating line, the plating constant kp needs to be adjusted. Therefore, changes in the line due to upgrades or equipment ageing affect the mathematical model (1).

A. Plating Current per Anode

The plating tanks are connected in parallel to the DC plating current source. Given that all the anodes have similar geometry, it is reasonable to assume that they share the same current. The plating current I shown in (1) is expressed in (2) as a function of the current in each anode, where the current per anode is designated by 1a and the number of active (connected) anodes is designated as na.

(2)

B. Plating Current Density

The current density is calculated by dividing the plating current given in (2) by the area defined by the steel strip width and the length of the active anodes, measured in the direction of the travelling strip. Considering that a typical electrolytic tinning line has multiple plating tanks and assuming that the anodes in these tanks have similar geometry, the plating current density is given by (3), where D is the plating current density in amperes per square foot or ASF and a, is the length of a single anode in inches.

1441 D= ---

1441a or D= --

al W (3)

IV. OPTIMAL OPERATION OF TINNING LINES

As described in the introduction, the optimal operation of an electrolytic tinning line is achieved when the target coating gage, the best plating quality and the maximum throughput are reached simultaneously. The coating gage is controlled by the plating current, as shown in (1). The plating quality is closely related to the plating current density. Finally, the maximum plating throughput is limited by the design characteristics of the electrolytic tinning line.

As described in [6], we defme a cost function J that becomes a minimum when the plating current densities on the top and bottom side of the steel strip are as close as possible to optimal current density and the tinning line speed is as close as possible to the maximum plating throughput. This cost function J, shown in (4), is a function of the maximum throughput NT in fpm, the optimal current density Do in ASF, the top and bottom side plating current densities D, and D2 in ASF and the coefficients CN and C, that can be adjusted according to a desired performance, with sub index i = 1, 2 corresponding to the top and bottom side of the steel strip.

V. OPERATIONAL CONSTRAINTS OF TlNNING LINES

There are operational limits that determine the maximum throughput of the electrolytic tinning process. These limits are the maximum mechanical line speed and the maximum current capacity of the anodes.

A. Maximum Mechanical Line Speed

The maximum mechanical line speed NM is given by the designer of the electrolytic plating line. This speed limit may change from time to time due to plating line upgrades (where the maximum speed limit increases) or plating line aging (where the maximum speed limit decreases). There are situations where reduced line speed limits are adopted due to temporary conditions like maintenance or commissioning of new equipment. Therefore, the maximum mechanical speed limit NM needs to be considered as a constraint during the minimization of the cost function J.

B. Maximum current per anode

The conductive capacity of the anodes is limited by design. This limited capacity is not an important factor for light coating gages. For heavy coating gages, however, the limited current capacity of the anodes becomes relevant factor.

The analysis of the electrolytic tinning line model (1) shows that the coating weight CIV can be increased until the current capacity limit of the anodes is reached. At that point, for heavier coating gages, the tinning line speed needs to be reduced to compensate for the limited plating current. The maximum plating speed Ne in fpm shown in (5) derives from the combination of (1) and (2), where the maximum current capacity of the anodes 1aM in expressed in amperes.

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VI. OPERATIONAL TARGETS OF TINNING LINES

(5)

As shown in (4), there are two targets required for the optimal operation of the plating line. One target is the maximum tinning line throughput NT and the other is the optimal current density Da.

A. Maximum Tinning Line Throughput

As indicated earlier, there are two speed constraints for the tinning line; one is the speed limit NM, given by the mechanical limitations of the tinning line and the other is the speed limit Ne, given by the maximum current capacity of the anodes. In order to define the throughput limit of the tinning line, it is necessary to determine which of these two line speed constraints is predominant. By comparing NM with Ne, the smaller of the two is chosen as the maximum tinning line throughput NT, as shown in (6). This is the target used for the optimization of the plating process.

(6)

B. Optimal Plating Current Density

There is a range of current densities that produce acceptable plating quality. This current density range is a design characteristic of the tinning line. Within this range, some current densities produce better plating quality than others. Although an approximation, it is safe to consider that the middle point of this current density range will produce a better outcome than the current densities at the limits of the range. This middle point is designated as the optimal current density Da.

VII. MINIMIZATION OF THE COST FUNCTION

Minimizing the cost function (4) for electrolytic tinning lines presents some challenges. The process variable na representing the number of anodes is discrete, i.e. an anode can be either active or inactive. Hence, the plating current density given by (3) is a nonlinear function of the number of active anodes. There are some techniques described in this section that help to overcome this challenge whilst simplifying the optimization process.

A. Optimal Number 0/ Active Anodes

The first step in the minimization of the cost function J is the search for the optimal combination of top and bottom number of active anodes nao, and nao 2 subject to the target coating weight CF' the maximum tinning line throughput NT and the optimal current density Da. This is a finite search routine that uses a lookup table with all the possible combinations of active anodes, computing the current density using (1), (2) and (3). The cost function (4) is solved for a tinning line speed N equal to the maximum throughput NT.

B. Optimal Line Speed

The selection of the optimal number of active anodes described earlier may result in current densities that deviate from the optimal Da. This is important for the particular case of unequal coating weight targets for the top and bottom side of the steel strip.

From the analysis of (4), it is c1ear that the closer the top and bottom plating current densities are to the optimal Da, the smaller the cost function J, due to the presence of the difference Da - Dj in the cost function. This condition is visualized in Fig. 2 and mathematically expressed in (7).

(7)

Current densities Dj and D2 are computed from (3) as shown in (8). Additionally, the plating currents per anode Ja, and Ja2 for the top and bottom side of the steel strip are computed using (1) and (2) as shown in in (9).

(8)

(9)

Combining (8) and (9), we arrive to the line speed NJ (10) in fpm that makes the top and bottom plating current densities equidistant from the optimal current density Da.

(10)

Finally, the optimal line speed Na is the smaller of the plating line throughput NT and the line speed NJ, as expressed in (l1).

(11)

1-D2-Dl

I DO-Dl D2-DO

1-"I" "1

I I I -

Dl DO D2

CUrrent density D CASF)

Fig. 2. Distribution of target current densities around the optimal point.

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VIII. DEVELOPMENT OF THE MOBILE APPLTCATION TOOL

The optimization of the electrolytic tinning process described in the previous sections was made available to the line operators by the development of an on-line tool. This tool consists of a computational engine, invisible to the user, and an operator interface. A snapshot of the operator interface is shown in Fig. 3.

A. User Account

The application was designed and hosted with Google Sites. The application was password protected and access was given to specific Google Accounts, giving only certain individuals the ability to use the tool.

The primary language used for the development of the mobile application was html. This allowed for the creation of pages, links and the embedding of the main application within the website, as weH as access from mobile and traditional desktop operating systems without the need for plug-ins.

Optimal Coating Calculator

Enter coil data

Coating

Weight

Top:

Bottom:

Steel Strip Width:

No coating:

Minimum:

Maximum:

o CTG 5 CTG

270 CTG

� I Limits: 20" to 42"

OPTIONAL Plating Line Limits and Targets

Available top cells � Valid range: 1 to 14

Available bottom cells � Valid range: 1 to 14

B. Computational Engine

The computational engine, holding the calculations required for the optimization of the electrolytic tinning line, was developed in Microsoft Excel. The Excel tables contain line specific information, like process efficiencies and plating constants, as weH as general information common to any electrolytic tinning line. The search for the optimal plating parameters is made in a table that contains all possible combinations of the number of active anodes for the top and bottom side of the steel strip.

The computational engine has an optional learning tool for those lines that have changed over the years or have been modified and no longer follow the original mathematical model. This learning tool searches for the best plating constants for the line (one constant for the top and another for the bottom side of the steel strip), that results in the highest plating quality.

Graphical Representation

Current Oensity in A.S.F. "'Top oO-Sotlom -Optimal 1000

900

800

700

600

600 400

300 () �.

200 100

Target current density DQQ:J A.S.F. Typical range 300 to 500 0 20 22 24 26 28 30 32 34 36 38 40 42

Max line speed

Results

Number of Active Cells

Top: I 7

Bottom: I 11

Plating Current Setpoints

Top: I 29,222

Bottom: I 61,627

�

I I

I

I AMPS

I AMPS

F.P. M . Valid range 290 to 1750

Optimal Line Speed:

912 I F.P.M.

Coating weight on steel strip

Top: � Lb/BB

Bottom: I 0.300 I Lb/BB

REFRESH INSTRucnONS ABOUT

Steel strip width in inches

Current per Cell in Amps ..... Top

10000

8000

6000

4000

2000

0 0 1 2 3

CHANGELOG DOWNLOAD

4 5 6 7 8 9 10 # of active cell.

Fig. 3. Operator's interface was designed and developed within Google Sites.

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C. Operator Interface

As previously indicated the operator interface, shown in Fig, 3, was designed and developed within Google Sites. The operator has the ability to select the top side and the bottom side coating weights independently. A message wams the operator when the selected coating weight or strip width is outside of the capabilities of the electrolytic tinning line. When these targets are exceeded the optimization tool is disabled.

All the electrolytic tinning line design limits, such as maximum mechanical line speed, minimum and maximum coating weights and minimum and maximum steel strip width, are entered in a password protected screen.

An area designated as Optional Plating Line Limits and Targets is provided for line conditions that deviate from those recommended by design. For instance, the operator has the option of indicating the maximum desired line speed during the commissioning of new equipment. Based on this new limit, the optimization tool calculates the best combination of anodes and optimal line speed to obtain the best plating quality for the top and bottom side of the steel strip. Likewise, the number of available plating anodes can be adjusted for those instances where some of the tanks are being serviced. Finally, the operator has the option of adjusting the target current density. All optional adjustments are restored to default values when the refresh pushbutton, shown at the bottom of Fig. 3, is depressed.

On the bottom left-side of the operator interface, the application displays the parameters required by the user for the optimal operation of the electrolytic tinning line. These parameters are the optimal number of top and bottom anodes, the optimailine speed and the total plating current for the top and bottom anodes.

An optional box provides the calculated coating weight as shown in Fig. 3. This is a requirement in cases where there are discrepancies between the production and lab standards for the measuring of the coating weight.

D. Graphical representation

A graphical representation of the process variables is integrated into the operator interface. The plating current density and the plating current per anode are distinctively represented for the top and bottom side of the steel strip. The predicted plating current density values are easily compared to the optimal current density, as shown in Fig. 3. The equidistance of the plating current densities for the top and bottom side of the steel strip discussed earlier and shown in Fig. 2 are easily verified by the position of the circle and tri angle markers at both sides of the target current density. The red line representing the target current density is dynamically adjusted by the change of the target current density in the Optional Plating Line Limits and Targets box on the left side of the screen.

At the bottom right -side of the operator interface, there is a graphic display of the predicted plating current per anode for

the top and bottom side of the steel strip. These plating currents can be compared to the maximum current per anode. This graphic also shows the optimal number of active cells in the abscissa axis.

IX. IMPLEMENTATION

The mobile application tool described in this paper has been successfully applied for the optimal operation of a halogen-type electrolytic tinning line. The learning tool option was activated in the initial phase of the implementation stage to reverse-engineer the mathematical model for this particular plating line, taking into ac count similar line types as described in [4], [7]. Once the efficiencies and plating constants of the line were obtained, the learning tool option was deactivated.

In a second phase, the mobile application tool was used to schedule regular production, with satisfactory results. During this regular production period, lab test results were used to further refine the values of line efficiency and plating constants.

Due to the successful application of the application, the plant management decided to incorporate the algorithm into a process controller.

X. PLATFORMS

The application has been deployed in multiple platforms as shown in Figs. 4, 5 and 6. In Fig. 4, the application is shown running on a desktop with Microsoft Windows 7 as the operating system (OS). In Fig. 5, the application is shown running on a Blackberry device with a QNX based Playbook 2.1 OS. In Fig. 6, the application is shown running on a smartphone with Android as the OS.

The application can run multiple instances, independently from each other. It can also run from a standalone Excel file without the need of an internet connection.

"- __ .. 4>.1 ... ... __

Fig. 4. Application deployed on a desktop with Windows 7 OS.

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,..,..--\ _ lo.&.fl_"'"'V"_ .. >t

"I0Il ....... __ �T� . .. .. . . . . .. . .. . ..

--_ .. -

Fig. 5. Applieation deployed on a Playbook with the QNX based Playbook 2.1 OS.

XI. CONCLUSIONS

The optimal operation of an electrolytic tinning line requires the control of the plating current, the line speed and the number of active plating anodes. Although the acquisition of the hardware required for a real-time optimal controller is not a major expenditure, it is a complex endeavor that requires an extended commissioning as described in [8]. The cost-benefit of such effort needs to be evaluated before the upgrade is carried out. Additionally, there could be unknown limitations of the tinning line capabilities that need to be investigated before the investment is committed.

The mobile application tool described in this paper offers a cost effective alternative to evaluate the advantages of operating the electrolytic tinning line at its optimal operating point without the need of a large capital investment. Limitations of the plating line can be discovered and dealt with before the real-time optimal controller is implemented. Additionally, the mobile application tool can be used to reverse-engineer the plating line mathematical model, when this is not available.

The multiple platforms capabilities and the limitless access make the application described in this paper a versatile and portable tool that is ideal for the scheduling and monitoring of tinning lines production.

Fig. 6. Applieation deployed on a smartphone with Android OS.

REFERENCES

Periodicals: [I] G. S. Mare, D. Groot, (2010, Apr. ), "lmproving the quality of tinplated

steel using a novel teehnique to study the effeet of industrial proeess parameters. " Journal of The Southern Afriean Institute of Mining and Metallurgy.

[2] E. H. Enrique, B. M. Moffatt, (1999, Oe!. ), "Simplified optimal eontrol for tinplate proeess," AlSE Publieation fron and Steel Engineer, vol. 76, No 10, pp. 40-43.

[3] Canadian General Eleetrie Company Limited, instruction Bookfor No. 3 TlN/TFS Line, vol. l. & H, 1972.

[4] W. E. Hoare. (1951 ). "The development, produetion and manufaeture of eleetro-tinplate," lET Journals of the institution of Production Engineers, vol. 30, pp. 104-133.

[5] F. W. Sears, M. W. Zemansky, College Physics, Addison - Wesley, 1966, pp. 587-591.

[6] V. H. Quintana, Numerical techniques for the optimal power system operation. Leeture notes, University of Waterloo, 1992.

[7] D. L. B. Pollnow (1977, Jan. ), 'Tinplate, and eleetrolytie tinning at the Iseor works, Vanderbijlpark. " Journal of The Southern Afriean Institute ofMining and Metallurgy.

[8] D. F. Unsworth, B. M. Moffatt, M. Barbon, Gary Lee, (1999, Oe!. ), "Automation of Dofaseo's No. 3 eleetroplating tinning line," AlSE Publieation iron and Steel Engineer, vol. 76, No 10, pp. 35-39.

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