MECHA'NICAL STIRRING IN A CONTINUOUS EVAPORATING CRYSTALLIZER: THE NEW FeB CONTINUOUS VACUUM PAN

Transcription

1 MECHA'NICAL STIRRING IN A CONTINUOUS EVAPORATING CRYSTALLIZER: THE NEW FeB CONTINUOUS VACUUM PAN by Gerard Joumet, FeB Sugar Division, Bd de I'usine, Lille Cedex. France Otto Bultas, Swenson Process Equipment Inc., Lathrop Ave., Harvey, IL F ran~is Rousset, Applexion Inc., Lathrop Ave., Harvey, IL Presented at the 29th Biennial Meeting of the ASSBT in Phoenix, AZ :.., SWENSON ~ "'"...," - Telephone Fax 'PPLIIIOI 193 Telephone: FAX:

2 MECHANICAL STIRRING IN A CONTINUOUS EVAPORATING CRYSTALLIZER THE NEW FCB CONTINUOUS VACUUM PAN by Gerard Journet, FCB Sugar Division, Bd de I'usine, Lille Cedex, France Otto Bultas, Swenson Process Equipment Inc., Lathrop Ave., Harvey, IL Franc;ois Rousset, Applexion Inc., Lathrop Ave., Harvey, IL ABSTRACT In an FCB continuous evaporating crystallizer (vacuum pan), the massecuite is stirred only by natural convection due to evaporatio.n. Energy savings can be achieved by reducing the quantity of water evaporated during the crystallization operation by feeding such crystallizers with syrups of higher dry substance content. As a consequence, agitation in the evaporating crystallizer will be insufficient and have a detrimental effect on the heat exchange coefficients and the crystallization rate, as well as increasing the fouling of the calandria. One way of solving these problems and always ensuring adequate massecuite agitation is to produce agitation by external means, that is,. mechanical stirring. An experiment conducted on the second half of an FCB continuous evaporating crystallizer at the Colleville sugar factory resulted in a gain of about 2 C on the total 6t and 1 to 1.5 points in the crystallization exhaustion. Extrapolations from these results have led to an upgrading of the design of the FCB continuous vacuum pan to one with a new shape of calandria which makes possible the installation of mechanical stirrers. The expected thermal gains with this new design are 5 to 6 C minimum on white beet sugar (total Lit). 1. BACKGROUND The FCB continuous evaporating crystallizer (vacuum pan) was first installed in 1968 at the Nassandres sugar factory to process white sugar massecuite. This first evaporating crystallizer was equipped with a longitudinal plate calandria. Commencing in 1976/1978, FCB's continuous evaporating crystallizers were equipped with calandrias consisting of rows of horizontal steam tubes arranged in vertical layers (see Figure 1). Since then, the qualities of this type of calandria have been amply demonstrated : good heat exchange coefficients, low head pressure loss, limited scaling, excellent long-term reliability and cost-effectiveness. As with all existing continuous evaporating crystallizers, the FCB continuous evaporating crystallizer does have a drawback: it is difficult to increase its capacity. For this reason, the authors sought a simple, efficient and inexpensive way of increasing the capacity of the FCB continuous evaporating crystallizer in terms of its massecuite flow rate and evaporating capacity, without thereby reducing the obtainable exhaustion. 194

3 Meanwhile, an 6 'J ol~ tion tovvards an ever lower evaporatior. requirement In an evaporating crystallizer has appeared progressively over the last 10/1 5 years. Indeed, for the obvious reasons of energy economy, sugar manufacturers want to process an increasingly concentrated feed syrup (SL 1). Dry substance contents (OS) were on the order of 70%-73% some 10/15 years ago, whereas they now range more commonly between 74% and 78%. Similarly, syrup run-offs 1 and 2 are far less dilute than in the past and the dry substance feed syrups are more on the order of % than 77-79%. The reduction in the water evaporation requirement is on the order of 20-35%. 2. BASIC IDEA In a continuous evaporating crystallizer, the agitation and movement in the massecuite are directly dependent on the quantity of water evaporated in the evaporating crystallizer. If less water is evaporated, both the movement and the agitation are reduced. Consequently, we were quite naturally led to consider fitting the continuous evaporating crystallizers with mechanical stirring. 3. REALIZATION OF THE IDEA After examining several possible arrangements for mechanical stirrers in a continuous evaporating crystallizer, the layout shown in Figure 2 was chosen. This allows a stirrer to be installed with sufficient room on the feed side, and it does not require the removal of any tubes On the other hand, the diameter of the stirrer is necessarily limited and the flow from it has little effect on the lowest part of the calandria. As a matter of fact, this installation seems to be the best compromise sol~tion. Thanks to the prior agreement and kind cooperation of the Colleville sugar factory, the mechanical stirring system was tested in an FeB continuous evaporating crystallizer during the 1995 sugar campaign (see Figure 3). The main specifications of the Colleville CCTR continuous evaporating crystallizer for B sugar were as follows: Volume 64 (2260 fe) Heating Surface 61 7 (6640 tf) ~iameter 3500 mm (1 1'-6") Length mm (38'-11 ") Compartments 13 m 3 m 2 195

4 The second half of the evaporating crysta!lizer (compartments 9 to 13) was et9d with si v stirrers. This was a logical choice, because the last compartments are those where the least water is evaporated, the dry substance content is the highest and natural agitation is the least. Compartments 9 to 12 were each provided with one stirrer; and compartment 13, whose volume was twice as large as the previous ones, was provided with two stirrers. Obviously, steam agitation was eliminated from the compartments fitted with stirrers. Moreover, the usual feed syrup introduction device is replaced by a feed pipe directed toward the axis of each stirrer. 4. Results The stirrers were started at the beginning of the campaign, at the same time as was the. evaporating crystallizer, and they worked satisfactorily through the last day of the campaign, without causing any particular mechanical or electrical problems. Qualitatively speaking, the stirring has an unquestionable effect. Viewed from above, the swirl of the massecuite mass was plain to see. Observing the surface of the massecuite in a compartment during a change in the operating conditions of the stirrer from full stop, or vice versa, a significant change in the behaviour of the massecuite could be noted. As a rule, the Colleville sugar factory cleans the crystallizer twice during a campaign. For safety reasons, this frequency was observed during the 1995 campaign. The crystallizer showed less encrustation, however, on both occasions. In order to demonstrate the quantitative impact of mechanical stirring, a series of measurements and analyses was made during the 1994 campaign in order to determine the representative operational parameters of the FCB evaporating crystallizer when it was functioning without stirrers. During the 1995 campaign, the same measurements and analyses were carried out, and the relevant operational parameters determined. Of course, every effort was made to keep the operating conditions as close as possible to those of the 1994 campaign. In particular, it was the aim, insofar as possible, to work with the same massecuite flow rate, the same water evaporation rate and the same massecuite dry substance content. The measurements were even made approximately 10 days after cleaning the crystallizer, just as was done ir;t The comparison between the 1994 and 1995 operational parameters is given in Table

5 Operational balances in 1994 and 1995 (In customary U.S. units of measurement.) balance 1 (68 h) balance 2 (16 h) balance (48 h) Condensed water flow rate cu.ft./h B feed syrup flow rate cu.ft./h B massecuite flow rate stlh OS content of B feed syrup % OS content of B massecuite % B massecuite purity % B mother liquor purity % Exhaustion % abs. ~Pty Calandria pressure psi Vapor space vacuum psi Calandria steam temperature of Vapor space vacuum temp. of Total t1t of Operational balances in 1994 and 1995 (In metric units of measurement.) balance 1 (68 h) balance 2 (16 h) balance 48 h Condensed water flow rate m 3 /h B feed syrup flow rate ml/h B massecuite flow rate tih OS content of B feed syrup % OS content of B massecuite % B massecuite purity % " B mother liquor purity % Exhaustion % abs. ~Pty Calandria pressure abs bar Vapor space vacuum abs bar Calandria steam temperature C Vapor space vacuum temp, C Total t1t OK '

6 A gain of about 2 C on the total ~t of the crystallizer should be noted, as well as a rather marked gain of about 1 to 1.5 points of purity in crystallization exhaustion. Tests were made to determine whether this gain in exhaustion might be attributable to a difference in sucrose solubility between 1994 and Figure 4 illustrates the 1994 and 1995 Colleville solubility curves (Ksat as a function of the NSNI ratio) : a difference in solubility was found, but it does not affect the B sugar zone in which NSNI ranges between 0.8 and 1.6. Therefore, there can be no doubt that the increase in exhaustion was due to the mechanical stirring. The 1994 and 1995 operational conditions were simulated using FCB's continuous evaporating crystallizer simulation software, CRISCONT. Figure 5 illustrates the good correlation between the massecuite OS content measurements and the calculated values. This simulation software uses modulation coefficients for heat exchange and mass exchange, which make it possible to adjust the simulation to particular operational conditions. In the present case, with regard to the 1995 balance, we distinguished between compartments 1 to 8 without stirrers and compartments 9 to 13 with stirrers. Table 2 gives the coefficients used for the simulation : modulation coefficient heat exchange modulation coefficient mass exchange gain in modulation cpts 1 to 13 cpts 1 to 8 cpts 9 to 13 coefficient % % - Table 2 Table 2 shows a quite marked gain in favor of compartments equipped with stirrers in comparison with those which were not so equipped. The effect of the stirring was also demonstrated by the transient reaction upon stopping or restarting all of the stirrers. This transient reaction effects in a reproducible and appreciable way both the instantaneous pressure in the calandria (at constant pressure above the massecuite) and the condensate flow rate (Table 3). 198

7 Stopping the stirrers Starting the stirrers Relative time pressure condensate pressure condensate in calandria flow rate in calandria flow rate (seconds) (psi) (cu.ft.lh) (psi) (cu. ft.lh) 30" before " after " after " after " after Variation (-22%) (+19%) Equivalent t.t OF OF - Table 3 The electrical power consumption was on the order of 0.30 to 0.35 kw/m 3, which are low values. This depends on the compartments and on the state (viscosity) of the massecuite. Because of lack of reference measurements in 1994, it is not possible to show directjy the contribution of stirring on the quality of grain size. However, two positive indications should be noted: IRIS made crystal size measurements during the 1995 campaign while the evaporating crystallizer was running at a low tightening value (massecuite OS content about 89%). These measurements show that stirring improved the coefficient of variation (CV) of the seed magma (remixed C sugar), which was reduced from 39% to 33% at the exit of the crystallizer. The centrifugation performance of the B sugar massec~ite was better than usual during the 1995 campaign. We are convinced of the positive effect of mechanical stirring on the crystal quality, both in terms of crystal size distribution (CV) and in terms of twins and agglomerates (crystal regularity according to Hill mark). 199

8 5. CONCLUSIONS 5.1 IMPROVEMENT OF EXISTING VACUUM PANS The performance gains measured on Colleville's B sugar evaporating crystallizer were significant, even th ough only the second half of this crystallizer was equipped with stirrers. The overall improvement in performance is directly related to the increase in massecuite circulation through the calandria which is induced by the stirrer. This results in an increase in heat exchange coefficients, an increase in the crystallization rate and a better massecuite mixing in the compartments, which consequently reduces the residence time distribution, thereby improving crystal size quality. It is, therefore, possible to boost an existing crystallizer to the point of gaining a lower ~t by several degrees, with an unchanged capacity, or of obtaining a 20 to 40% increase in production, as the case may be. 5.2 NEW PAN DESIGN The excellent performances registered at the Colleville sugar plant led us to go further and to redesign our continuous evaporating crystallizer in order to obtain a better adaptation for mechanical stirrers. Thus, after 20 years of operation, the FCB continuous evaporating crystallizer, CCTR type ("Cuite Continue atubes Ronde" = Round Continuous Vacuum Pan with horizontal Tubes), is replaced by a new design, an evolution of the first one: CCTW type ("Cuite Continue atubes en W ' =W Shape Continuous Vacuum Pan with horizontal Tubes). The specifications imposed for the new pan were evident: possibility of installation of mechanical stirri ng under the calandria that speeds up the massecuite flow across the tube nest. To which we added: decreasing the hydrostatic head above the calandria and improving homogeneity of the tube nest, increasing section of lateral channel for massecuite recirculation. We kept the principle of a calandria with horizontal tubes fitted in vertical rows. We showed by simulation that this type of tube nest presents the best compromise between head loss and heat exchange coefficients (2 opposite characteristics). 200

9 In order to optimize the efficiency of the mechanical stirring and to minimize the maximal hydrostatic head, the bottom shape of the tube nest is horizontal. In order to optimize the heat exchange surface, to improve the lateral recirculation of massecuite and to increase the lateral overilow height (that is linked to the recirculation f1owrate), the upper shape of the tube nest is sloped towards the lateral channel. Consequently, the shell shape is naturally placed around the tube nest. This creates a large lateral width of recirculation channel that is progressively reduced downwards (head increasing) and is progressively closed centerwards under the tube nest for avoiding dead zones (see Figure 6). Figure 7 shows the gain versus a CCTR continuous vacuum pan. The new pan. like the former one, can operate for all products in a raw sugar plant as well as refinery, cane and beet. Mechanical stirring is an option that makes it possible to achieve very substantial improvement in ~t and exhaustion, as was proven at the Colleville sugar plant. Like the former design, the CCTW continuous evaporating crystallizer range includes 5 diameters and 9 len~ths (see Table 4). The nominal volume doubles every 5 pans. The SN ratio is 10 m 2 /m (3.05 sq.ft.lcu.ft.). CCTW Continuous Vacuum Pan RANGE I Length of Pan (mm) Table 4 (In metric units of measurement.)

10 CCTW Continuous Vacuum Pan - RANGE Length of Pan (ft.) Nominal Diameter (ft.) Number of Tubes Nominal Volume (cu.ft.) m n Table 4 (In customary U.S. units of measurement.) The massecuite circulation through the pan always follows a U shape. There are 13 compartments without mechanical stirring and 11 with mechanical stirring (see Figure 8). With 11 compartments, the number of stirrers is 12. The impeller diameters vary from 400 to 900 mm (1.31 to 2.95 ft.). For example, a pan of 100 m (3531 cu.ft.) will be fitted with impellers of 750 mm diameter (2.46 ft.). The enhancements of the CCTW when compared with the CCTR are the following: easy installation of mechanical stirring, larger section of recirculation (+ 25 % in average), lower maximal and mean hydrostatic head, lower massecuite height, better homogeneity of the tube nest, less space requirement, shorter and lower pan. 202

11 These enhancements make possible the following gains: D.t decrease of about 1 to 2 C without mechanical stirring, D.t decrease at a minimum of 4 a 6 C with mechanical stirring (white and high raw massecuite), improved rate of exhaustion, improved grain size quality, complete replacement of the jigger steam by the mechanical stirring, decreased encrustation. Needless to say, all these advantages fully justify the decision of FeB to upgrade the continuous vacuum pan to the new design with mechanical stirring, a feature now being offered to the sugar industry in their new standard pan. 2 03

12 N o "'" ~ to C nj ~ :1:1- Z:I- "';:Z _IS 8. Steam boxes and doors 9. Steam Inlet 1. Shell ~. Catchall 10. Condensed water oullet 2. longitudinal partlllon 5. Masseculte extracllon pipe 11. Non-condensable gas outlet 2'. Transversal partitions 6. Oulck-dralnage pipes 12. Tube nest 3. Calandrla Shields 7. Reinforced ends 1.3. Magma Inlet

13 Figure 2 CI) c: ~ ~ I CD :l o (.) CD "l:j CIt en 0 c ~ -0- ~ 5 c: ~ (.) CD c > 0.,!. - CD "t: :::I u CD 15 CIt C en "'0 :l ~ t: J Figure 3 205

14 .. Figure 4.. COMPARISON OF SOLUBILITIES nd strike 1.5 zone ~ til ~ 1.3, :.-.-~ t-----j-i , NSIVV Figure o ~ 90 CD 89 o, ~ A,~ SIMULATIONS ~--~-----r _+----~--~~ compartments

15 Figure 6 wllh oul mechanical stirrer with mechanical stirrers ~ ~! ~ \ 0.: I \0 : I 0: I 0: I ~ :.&1 0 I' ~ I il ol --_._- - :~ 00000: -i-o}o.- -.-! :1:0000 1: 0.000: g : : : o 0 0 \ ( goo \0 :: :: ~: : o : g :.0 o " ooooooooeoooooo masseculte level OOl r ~ : 1 ' ~ &, ISoo ~ '\ : I.:::.! : "\ : I q ~o \ ~.-.o-.- o1;,- i :;;; i o ) o ::~ 00: :: 0\0 00.0:: ) o 00.00! : :: & o o 0 0 ~ 0 0t '0 0 "-... '. / C \ i \ - Figure 7 CCTR o 4000 CCTW o 3910 masseculte level goooo oo : o 0 o 0 :: : : :: 0000 :0 00: I:! 00 0 S : o. 0 o o 0 0 0". o. : : 0 f:. : 01. :.0 0 O mossecuite level o o goooo ooz : : & g : : :-: I Q o o o " 0 o 0 : : o : Ii i : 0 II 0 :.."~ I~ 3 o )( 3' c 3 ~ [ n' J 207

16 - Figure 8 CCTW WITHOUT MECHANICAL STIRRING I BUB Upper Bottom U B ". \ '.. J. ' 8. massecuite. Ol:ltput \ - '. -:,.-- ' -.' '~,MaSseculte, -:,''''"":": " draln~ge U B U B y. u CCTW WITH MECHANICAL STIRRING I B u B U B /~.'.,,i\ I I. \., -'... ~-.- _..._... f / " \ i ".~ - t I \... /...' \ '" I./.- :-.... ~, I..., 1!. '.. 1. ~ /. '..,. \.. i j., J :...,--._..,6..\......,/ " I \ i ; '......_-_... \ - ' U B U B U: B : Upper passage between compartments Bottom passage between compartment 208

17 COLLEVILLE SUGAR FACTORY CONTINUOUS EVAPORATING CRYSTALLIZER (Continuous Vacuum Pan) tv 0 U) Strike Volume Surface Diameter Length Compartments 2nd 3 64 m m 3500 mm mm cu.ft sq.ft ft ft.