W ALLEV AND, JOSEPH H. *, DAVID R. GROOM, TONY A. HEINBAUGH, and LARRY T. CARLSON, American Crystal Sugar Co., PO Box 1227, 1700 North 11th St.

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1 W ALLEV AND, JOSEPH H. *, DAVID R. GROOM, TONY A. HEINBAUGH, and LARRY T. CARLSON, American Crystal Sugar Co., PO Box 1227, 1700 North 11th St., Moorhead, l'vln An on-line autotitrator for measuring juice hardness. The requirement to provide soft molasses to chromatographic separators has placed additional demands upon operational control and has heightened the analytical load on the factory lab. During periods when degraded beets are being processed and lime usage is increased, rapid swings in juice hardness necessitate tight management of the process in order to minimize chemical use (NaOH, HC!, and Na 2 C0 3 ) and to achieve optimum softener performance. Hence, frequent sampling of juice before and after softening is needed to capture changes in hardness levels. At the Moorhead (MN) factory, hardness samples are taken only twice during an 8-hour shift leaving long periods of operation without timely information. Typical hardness titrations (involving difficult to detect color changes) tend to be operator dependent. To provide more timely feedback to the beet end process operators, we installed an Iotronic Aquacon SH-20 on-line hardness autotitrator. Cost ofthe instrument (1999) was $3000 (US). Originally designed for the water treatment industry, the manufacturer modified this instrument for use on filtered second carbonation juice. In use throughout three campaigns, it gives near real-time data using photometric determination of titration end points on hardness levels of 60PPM to 400PPM CaO at a sampling rate of four times an hour. With this information, operators can control soda ash addition during periods of degraded beet processing to within 118 pound per ton of beet compared to 1 pound per ton ofbeet previously. Additionally, with the prediction of softener regeneration cycles tightened up, instances of column overloading and under-regeneration have been reduced, promoting increased resin life, diminished chemical use, and insuring soft molasses production. 249

2 Introduction In 1993, American Crystal Sugar began using ion exclusion chromatography to recover sugar from molasses. Since molasses desugarization (MDS) needs molasses of low calcium content (softened), it is essential to limit the calcium and magnesium levels in molasses fed to ion exclusion plants. Juice containing elevated levels of divalent ions has a detrimental impact on the process. Of the softening options available, Crystal opted to iristall NRS thin juice softening systems in our factories. Thinjuice softening requires monitoring of the softening columns' performance. Without proper control, performance can vary as much as the beets being processed. Beet processing uses large amounts of lime (calcium oxide) in purifying the beet juice. It is from the processing of the beets that the juice acquires most of its hardness. As long as the raw material is consistent and the system is under control, softening is a fairly routine operation. Once an exhausting column of our NRS softeners has approached its maximum capacity for calcium removal, it must be taken off line and the resin regenerated. The polishing column is placed into exhausting service immediately with the standby column going on line to perform the polishing function. Note in Figure I that the capacity of the system increases as the feed hardness increases. In practice, it is assumed that the service columns will be regenerated with just enough caustic to acmeve desired leakage levels. Ifhardness loading exceeds capacity, the system will not be regenerated properly. There will be an increase in calcium leakage into the juice comes out of the columns. Continued overloading will produce a rise in calcium leakage and, eventually, hard molasses will be produced. NRS Operating Isotherm System Resin Capacity 25, ~ ~~ r ~~=--=-~ a" ~ J.. / )0' soo Feed Hardness ppm CaO Fig.] If, on the other hand, the service cycle is too short for the hardness of the juice, excessive regenerations will take place. Increasing the number of regeneration cycles throughout a campaign increases caustic use. Leakage can be very low but it is not required; extra caustic is used for no benefit. Hence, it is important to have softeners under control to soften the juice effectively while minimizing chemical use and insuring resin integrity

3 Another method, which provides a crude softening, is to add chemicals such as soda ash. The properly-controlled addition of soda ash, containing sodium, is a useful adjunct in managing NRS softening, particularly when thin juice hardness is high. Soda ash offers the advantage ofdelivering a more consistent as well as lower hardness to the columns. Putting soda ash into second carb juice cuts down on, at times, more expensive caustic u~ed for regeneration offering a monetary benefit plus reducing hydroxyl. alkalinity from regenerant recycle to the beet end. Likewise, utilizing soda ash helps reduce the load on the resin in addition to increasing the effective alkalinity of the juice when the lime salt contribution to alkalinity exceeds that due to sodium and potassium salts. The correct addition of soda ash and the regeneration of the resin with caustic are manageable but only if correct, timely information is available. Moreover, soda ash is highly melassigenic, so minimizing excessive addition lowers sugar loss to molasses. To strike the balance needed to assure hardness removal, while minimizing regeneration and soda ash addition, it is necessary to monitor the hardness levels of the thin juice, before and after softening. Discussion In order to monitor the thin juice hardness levels, Crystal tests at three points in the softening stream along with any molasses intended for use in the ion exclusion plants. From the inception ofmds, the procedure for calcium testing at Crystal has been the standard EDTA titration; samples are run two to four times a shift. The titration ofthis darkly colored juice is not particularly easy to perform because it depends upon the detection ofa slowly responding endpoint that is multiple and ambiguous (red disappearing and a blue-green appearing) within. It is highly operator-dependent. Besides the difficulties in achieving consistent titrations there is an inherent weakness in the current analytical routine used to monitor hardness levels. The lab measures the hardness ofthe various streams at regularly spaced intervals ofas long as four hours each. Hardness may change unpredictably at intervals of varying times. Unless the softening system is under firm control, radical changes in the hardness of the incoming juice could require a more frequent sampling routine, necessitating additional testing by the factory lab. Thus, the analytical scheme for managing juice softening and molasses hardness leaves some room for improvement. Given the above, it was desirable to find an on-line titration system for measuring the hardness entering softening station. Iotronic [USA] manufactures a line of analytical instruments that produce real-time data on a variety of analyses, employing spectrometric determination of titration end points. Specifically, Iotronic had an on-line autotitrator available that was set up to measure water quality hardness. The Iotronic AQUACON SH is an easily installed, unattended wall-mounted analyzer which gives accurate, reproducible, and timely data useful for operating juice softening and is relatively inexpensive. The principle of operation is quite simple. The sample is titrated inside the measuring chamber by means of a high precision peristaltic pump. As the color of the solution changes, the 251

4 stoichiometric titration endpoint is determined photometrically by the monochromatic detector. The hardness concentration of the juice is calculated from the number of revolutions the pump requires to complete the titration, and displayed in PPM CaC03, convertible to other units. During the initial test-out of the instrument at American Crystal Sugar's Tech Services Center in 1999, the existing configuration was found to be incompatible with the requirements for analyzing beet juices. This was due to masking of the titrametric color changes by the juice's high coloration. Iotronic satisfactorily reconfigured the water analyzer to specifications suitable for beet juices. We ran juice from the Moorhead plant several times on the reconfigured instrument. The results compared favorably with the conventional Nalco test. Various combinations of the thin juice and some high-color thick juice from the previous campaign were run with similar results. Most noteworthy was a sample of thick juice diluted 1: 1 with water. This diluted juice (high-color, high-brix, and high viscosity) measured at better than 95% the value attained manually (~300PPM). Though this specific sample was not run for color, another sample of 1 part thick juice to 3 parts thin juice had a color of4800 RBU, implying a 9,600 RBU color for the 1: 1 dilution. This was thought to be a significant test of the AQUACON's ability to provide an accurate hardness value over a wide range of hard thin juice colors. The instrument was placed into the Moorhead factory for on-line testing and installed into a customized NEMA enclosure. A cooling coil and thermostatically controlled solenoid valves were added to control temperature. Installation was near the second carb filter station adjacent to the thin juice sample points used by the lab. The instrument had access to 120 VAC, filtered second carb juice, and waste at that location. A 4-20rnA output line was run to the control room for display on Rosemount which was linked to PIMS for off-site display of the readings. Limit and alarm values were set at various levels. Fig. 2 After some trial and error, a 15 minute sampling interval was chosen, giving 4 readings per hour or.32 per shift. The rationale for that sampling rate was based upon the need to provide data at 252

5 regular intervals (titration times being dependent upon the amount of hardness), and the needto minimize reagent usage. Following installation, the analyzer began operating late February, After a period of testing, we began to receive acceptable data and did so for about three weeks. Early on we had been getting transient spikes which seemed to have no connection to juice hardness, due to a variety of causes: two, in particular, a Leuconostoc infection plugging the lines with dextrans, and a worn-out pump tube. Toward the close of the campaign, we were able to get nearly a month's worth of good data. After replacing a pump tube, most of the mystery spikes disappeared. Though we have not been able to realize the goal of real-time data, one-hour aggregates (four data points) have been attainable. We have been encouraged by the excellent tracking exemplified on the 90-minute average seen in Figure 3, a plot from the campaign exhibiting four months of operation. " Aspen Process Explorer - Hord Thin JUice apxl II!I~ E2 2S4.42!J 0 Go 300 A.",_ IPH IF G C O, 30 M... ""egol ffi.oooooo 0 Go BOO Actual PPM IF G C Ol 90 Minute Go ISO AVeJ&ge GPM IF G C O l SO Minute Integal n.s12 0 Go 400 AduaI. PPM IF G =0' 90 Minu" SOOAASHACAISONFEEOAAIE Go JOOAve<""" LBSIIF G =0 I GOMinul. InJ"",oI Fig. 3 In terms of accuracy, a truly analytical approach was deemed unnecessary. As long as the instrument could track the laboratory values for hardness, staying fairly close to the lab values, we felt that optimum control could be maintained. A single lab analysis of the hard thin juice no more than once a shift should be adequate for checking the instrument's performance. Even from the beginning, trending was quite favorable. As long as the agreement between the lab and instrument is close and the trends from the instrument are rational, the system can be run from the analyzer data. During this period, the beet end personnel were able to adjust soda ash addition to obtain appropriate hardness. 253

6 In Figure 4, we see a ten-day display showing soda ash addition (faint, near the bottom of the chart), slice (faint, toward the top of the chart), lab analysis (small, dark dots, six per day), and thirty-minute running averages of the instrument measurements (seen as a broad gray trace). Notice how closely the lab and instrument values track. During the eighth day displayed (04127/01), a rise in hardness witnessed by both lab and instrument is accompanied by the carbonator increasing the soda ash addition rate. In response to this increase, hardness was seen to decrease. About the tenth day, the operator saw a sharp spike in hardness and again countered by raising the rate of soda ash addition. Fig. 4 Excursions to "zero" by the analyzer are fictitious and are due to hardness outside the range of the instrument, the low end being around 30-50PPM. These misleading zeros were problematic for optimum control. In order to decrease the chance ofan operator acting upon a spurious excursion to zero, we designed an algorithm which takes any analyzer value less than 40 and sets it equal to 40. The purpose of smoothing the analyzer output is simply to make the data appear more reasonable to the operators so they do not make unnecessary adjustments to the process. Figure 5 shows the final nineteen days of the campaign. The 15-minute instrument values appear as very noisy dips to zero and the 4-hour laboratory values represented by the bolder line. A view of the same time frame, seen in Figure 6, shows the laboratory values (identical to the graph of the lab value seen in Figure 5), 90-minute aggregates of the normal instrument display (the darkest and, generally, lowest trace), and the "filtered" instrument display (seen weaving between the other two lines). The low end noise has been satisfactorily reduced, even though the impending end of campaign had added its own share of noise four days prior to the end of slice. 254

7 Fig. 5 Fig.6 A later view (Figure 7), taken from the campaign, shows soda ash addition as a lighter-hued graphing rising and falling above the lab and analyzer values. The lab (small diamond markers) and analyzer track fairly well as ninety minute aggregates. Notice how the operators have responded to the upswings in hardness by putting on more soda ash, and how both the lab and instrument values respond to the reduced hardness resulting from adding soda ash. Fig. 7 Some difficulties with the current year's ( ) operation have involved maintenance of this instrument. There is also a further need to massage the data as received by the operators, to the extent that statistical process control (SPC) conditions need to be imposed onto the data stream. A degree of variability not seen with the laboratory analyses tends to confuse the amount of control required for smooth operation of the system. To that end we need to revise the way the control room views the data. 255

8 Conclusions The on-line hardness analyzer can assist in optimizing NRS operation. It has the potential to do this in four ways: ensuring proper regeneration; minimizing Ulll1eCessary regeneration; reducing re-cycle of spent regenerant and rinse, which are highly alkaline; and aiding in the efficient use of soda ash. To be able to realize the full capability of this instrument there are some tasks remaining which we need to address including resolving maintena:p.ce issues and proving a more rigorous filtering to the signal received by the control room. Accordingly, the instrmnent offers another means for enhancing performance and revenues ofthe process; Acknowledgements I wish to thank the following people for the help and support they gave me in completing this project and its report: Mr. Ronald Kehler, Iotronic [USA], and the following personnel from American Crystal Sugar Company: Mr. Tony Heinbaugh, Mr. Larry Carlson, Mr. David Groom, Mr. Terry McGillivray, and Mr. leffmoritz. 256