Suspension by side entering agitators

233

Suspension by Side Entering Agitators KLAUSDIETER KIPKE EKATO Riihr- und Mischtechnik GmbH, 7860 Schopfleim (Received January 26, 1984)

(F.R.G.)

Abstract Most of the processes for desulphurization of flue gases are wet methods based on lime. Thus, in nearly all process stages suspension of solids has to be maintained by agitators. Special mixing problems are only present in the surge tank for the washing liquid below the scrubber. Owing to space reasons only side entering agitators may be used. Based on different pilot-scale tests, investigations have been made on the influence on the suspension capability of the agitator position (number of agitators, horizontal and vertical inclination), the diameter ratio, and the impeller type.

Kunfassung Die meisten Rauchgasentschwefelungsmethoden basieren auf Nass-Methoden mit Kalk. Dadurch muss die Suspension der Feststoffe in fast allen Prozessstufen durch Rtihrer aufrecht erhalten werden. Spezielle Mischprobleme gibt es nur im Absorber fur die Waschfliissigkeit unterhalb des Waschturmes. Aus Platzgriinden kbrmen nur seitlich eingebaute Rtihrer benutzt werden. Mit verschiedenen Versuchen in Piiotanlagen wurde der Einfluss der Rtihreranordnung (Anzahl der Rtihrer, horizontale und vertikale Neigrmg), sowie des Riihrertyps auf die Suspen- _, des Durchmesserverhaltnisses sionsfiihigkeit untersucht. Synopse Die Mehrzahl der Verfahren zur Rauchgasentschwefelung sind Nasswiischen auf Kalkbasis. Dabei mtissen in fast allen Verfahrensstufen mit Hilfe von Rtihrwerken Suspensionen aufrechterhalten werden. Aussergewtihnliche ni’hrtechnische Probleme ergeben sich lediglich in dem Auffangbehcilter fiir die Waschflussigkeit unterhalb des Absorbertunns. Aus P&zgninden konnen Rti’hrwerke hier nur seitlich unter Niveau eingebaut werden. Es wurde an geomem’sch rihnlich verkleinerten Behiiltern der Einfluss der Riihreranordnung am Behrilterumfang, der vertikalen und horizontalen Anstellung der Rtihrwelie, des Riihrerdurchmessers und des Riihrorganform auf das Suspendiervenniigen untersucht. Fur diesen speziellen Anwendungsfall der Unterniveaueinbaus wurde eine Gleitringdichtung entwickelt, die neben ki’ngeren Wartungsintervallen den Vorteil bietet, dass ein Ausbau zur Inspektion oder zum Austasch von Verschleissteilen bei gefii’lltem Behalter moglich ist. Alarmierende Nachrichten ilber zunehmende Umweltschdden durch den S02-Gehalt der Luft zwingen immer mehr Betreiber von Grossfaterungsanlagen, ihre Abgase wirksam zu entschwefen. Als Antwort auf diese Forderung, der such vom Gesetzgeber Nachdruck verliehen wird, wurden weltweit bisher mehr als 80 Verfahren zur Rauchgasentschwefelung entwickelt. Die Mehrzahl hiervon sind Nasswdschen auf Kalkbasis. 02552701/84/$3.00

Chem.

Eng.

Process.,

18

(1984)

Sie haben den Vorteil, dass das Absorptionsmittel preislich gUnstig und stciitdig ver,f%gbar ist. Ausserdem i&n in der Baustoffindustrie venvertbarer Gips erzeugt werden, so dass Deponieprobleme entfallen. In allen Verfahrensschritten dieser Nassw&hen ist der Einsatz von Riihnverken zum Aufrechterhalten von Suspensionen erforderlich, wie die stark vereinfach te Beschreibung des Prozessablaufes zeigt: Zunachst wird das Absorptionsmittel, z.B. Kalkmilch, angesetzt und aus einem Vorlagebetilter mit einer definierten gleichbleibenden Feststoffkonzentration in den Absorber eingesprilht. Dort wird es im Gegenstrom zu dem Rauchgas gefihrt, wobei das Schwefeldioxid als Calciumsulfit gebunden wird. Die Waschsuspension wird im Absorberunterteil aufgefangen und zum Oxidator gej%hrt. Bei einigen Verfahren findet die Oxidation zu Gips such direkt im Absorber durch Einblasen von Luft statt. Anschliessend wird die Gipssuspension ilber Zwischenbehalter zur Entwrisserung auf Trommelfilter oder Zentn’figen geflhrt. Die gesamten Vorlage- und Zwischenbehalter sind in der Regel mit zenm.sch von oben eingebauten Riihrwerken bestuckt, auf deren Auslegung nicht mehr eingegangen zu werden braucht. Vollig andere Verhdltnisse herrschen im Auffangbehrilter unterhalb des Absorbers. Bei Betilterdurchmessem his zu 20 m sir& die Fiilltihen klein, das FiillhiihenlDurchmesserverhiiltnis hlldl betrcigt ilblicherweise 0,4. Die dort vorliegenden Sulfitl 233-238

0 Elsevier Sequoia/Printed

in The Netherlands

234

Sulfa&Suspensionen haben Feststoffkonzentrationen bis 20 Gew. %. Die Komgrossen liegen in der Regel unter 0,06 mm. Da der Raum oberhalb dieses Behalters durch den Wascher verbaut ist, bleibt meist keine andere Wahl, als den Rtihrer seitlich unter Niveau einzubauen. Diese Einbaulage wurde bisher fast ausschliesslich beim Homogenisieren grosser Lagertanks in der olindustrie gewahlt, wobei ein oder hochstens zwei Ruhrwerke mit Propellern pro Behrilter eingebaut wurden. Die hieriiber vorliegenden Erkenntnisse reichen jedoch bei weitem nicht aus, urn sichere Aussagen Faber das Suspendierverhalten bei dieser Bauweise zu treffen. Daher wurden grundsatzliche Untersuchungen zum SUSpendieren mit seitlich eingebauten Riihrem durchgefihrt. Ziel der Untersuchungen war es zuniichst, die fiir die Auslegung des Betriebsbehalters notwendigen ‘Scale-up ‘-Regeln festzulegen. Aus diesem Grund wurden zwei geometrisch ahnlich verkleinerte Auffangbehiilter mit 100 und 700 1 Inhalt gebau t. Des weiteren wurden der Einfluss der Rtihreranordnung, der Anstellung der Rtihrwelle, des Ruhrerdurchmessers und der Riihrorganform auf das Suspendiervermogen untersucht. Rtihrgu t waren SulfitlSulfat-Suspensionen aus Waschern verschiedener Verfahren. Als optimal erwiesen sich drei oder vier Riihrwerke gleichmasig am Behiilterumfang angeordnet. Weniger als drei Ruhrwerke oder eine ungleichmassige Anordnung bringen keine brauchbaren Resultate. Urn den flachen Behalterboden vollstandig von Ablagerungen freizuhalten, ist eine Anstellung der Ruhrerachse sowohl nach unten urn den Winkel OLals such aus der Mitte urn den Winkel fl erforderlich. Wird der Riihrer waagerecht eingebaut (0~= 0) so bilden sich meistens Ablagemngen in der Behiiltermitte aus, die selbst bei Erhiihung der Riihrerdrehzahl und somit such der Leistung nicht mehr abgetragen werden kiinnten. Die seitliche Anstellung g versetzt den Behalterinhalt in eine leichte Rotation und darf nur in sehr engen Grenzen variiert werden. Ist fi zu klein, setzt sich Feststoff in WandrUhe ab (Bereich II). Bei zu grossem p und dadurch zu grosser Rotation ergeben sich trotz der Neigung nach unten erneut zentrale Ablagerungen im Bereich I. Dies ist auf den Grenzschichteinfluss am Boden zuriickzu#ihren, der eine nach innen gerichtete Sekundarbodenstromung erzeugt (Teetassen-Effekt). Dieser Sachverhalt wird zusatzlich dadurch kompliziert, dass die optimalen Winkelstellungen von der Behaltergrosse abhiingen, such wenn die Massstabsveriinderungen geometrisch rihnlich vorgenominen werden. was iiber eine Energiebilanz zwischen der Rtihrerenergie und den Reibungsverlusten an der Wand nachgewiesen werden kann. Die Suspendieruntersuchungen wurden mit unterschiedlichen Rtihrorganen durchgefiihrt. Van allen Formen schnitt immer der 3-fltigelige Propeller energetisch am @nst&sten ab. RauchgasentHaup tstichlich in amerikanischen schwefelungsanlagen wird zum Aufrech terhalten der Waschsuspension in Absorbem iiberlicherweise der 4oder 6-blattnge Schrd’gblattni’hrer eingesetz t. Die Untersuchungen haben jedoch gezeigt, dass selbst eine Steigerung der Riihrerleistung auf den 4-fachen Wert, den ein Propeller zum einwandfreien L&en der Aufgabe be& tigt, nicht den gewti’nschten Erfolg bringt.

Ohne diese Kenntnis ist eine Auslegung der Ruhrwerke ftir die Waschsuspensionen, die den Anforderungen mit minimalem Aufwand an Investitions- und Betriebskosten genugt, nicht miiglich. The fact that the death of forests observed nowadays is in some way connected with acid rain appears to be undisputed on the basis of current knowledge. As is well known, SO2 emissions are, amongst other emissions from power stations which burn fossil fuels, responsible for the formation of acid rain. In consequence, legal authorities have lately been demanding that power stations should drastically reduce the amount of S02. This is being achieved through the introduction of additional desulphurization plants. Flue gas desulphurization processes have already been in use for many years, particularly in the United States and Japan, since the coal there has a considerably higher sulphur content than German coal. This is the reason why most German contractors who build flue gas desulphurization plants rely on Japanese and American licences. In most flue gas desulphurization processes which operate on the wet principle, an interesting new agitation problem has occurred, namely the suspension of solids by side entering stirrers. The flow diagram of a flue gas desulphurization plant is shown in Fig. 1 using the KRC process as an example. Flue gas is introduced into the scrubber 1 where SO2 reacts with lime, quicklime or slaked lime. Subsequently, it is oxidized via sulphite to calcium sulphate (gypsum) through introduction of air into the lower part of the absorption tower. The solids content of the wash suspension is approximately co = 16 wt.%. The agitation problem thus consists of keeping the gypsum in suspension by means of I I rm

Process water Lrrne stcoe Lime stcne/Sulfite .Gypsum Fi1trote

Gypsum Fii.

1 Scrubber 2 Hydrocyclone 2 #z;rn tarding -

Pro&

tank

5 Scrubber feed tank 6 Silo t. Limestone 8’ l$w&one stcmge tar& --

water

1. Flow diagram for flue gas desulphurization.

235 side entering agitators or, expressed in a different way, of ensuring that the bottom remains free from solids. The diameter of the scrubber can be as large as 20 m; however, the liquid level is relatively low, around 5 m, for example. No similarly formulated problems seem to have appeared in the literature, except for information on the arrangement of side entering agitators for the purposes of homogenization, for instance in refineries or storage tanks. It can be seen from Fig. 2 that in the case of horizontally entering stirrers (ol = 0) the angle of displacement from the radial direction, /3, plays an important role.

Sp?d variation by Thyristor

h

dr

L

Fig. 3. Test arrangement: vessel diameter dl= 700 mm, 1400 mm; diameter ratio d&t = 0.04-0.08; filling height ratio h&l = 0.3-0.5; clearance h3/dl = 0.065.

p=-100

p=2..10-

p400

Fig. 2. Blending with side entering agitator.

If p > lo”, the whole tank contents will rotate without producing any mixing in the axial direction. At a negative fl the tank contents rotate In the opposite direction, again without axial mixing. It was found experimentally in large storage tanks that the optimum angle for homogenization is fl= 7”- 10”. Based on these values, pilot plant experiments were conducted by EKATO which demonstrated that the above conditions are not optimal for the solution of the suspension problem. For this reason, systematic and comprehensive experiments had to be carried out in order to determine the position and dimensions of the stirring device regarded optimal from an energetic standpoint. In addition, an attempt was made to find a preliminary basis for a scale-up law. Initially, it was assumed that u = constant would be a logical scale-up criterion on account of the jet action In the tank. However, the fact that gypsum was to be suspended, which has already frequently led to unusual conclusions in the past concerning scale-up laws, was a further reason to investigate at least two different tank sizes. The experimental set-up is represented diagrammatically in Fig. 3. Three stirrers were used, displaced by 120”. The tank diameters were d = 700 and 1400 mm. Stirrers investigated were an EKATO propeller, a hydrofoil marine propeller (Fig. 4), and a six-bladed pitchedblade turbine. Pitched-blade turbines are usually recommended by the Japanese and American licensers. The solids investigated and their characteristic data are listed in Table 1. It was soon concluded from the experiments that the setting angle could not be equal to zero, but instead

Fig. 4. Constant pitch propeller.

TABLE 1. Industrial products investigated. Product

Company

Particle size

Concentration

Gypsum Gypsum Lime Gypsum

K S S B

90% < 63 Nrn 90%<56rm 84% < 100 pm 90% < 100 pm

16 8.8 13.8 12

(wt.%)

236 the shaft had to be inclined slightly downwards, whereupon there were practically no differences in the range (Y= So--15’. However, angle fi exerts a much stronger influence on the suspension capability. Figure 5 illustrates the observations concerning change in this suspension capability on varying the stirrer inclination.

Tip speed

‘ktrifU@

Urcmst.

fWCe

dZ =Q.dV.

ru*

=gdV+* PreSsUreforcedP.g.g-Ah-dP 1)Outside boundary layer dZ = dP 2lWithin bwndary -Inwards

Fig. 6. Rotational

~~1

p-Radial

ds”,atio”

>

Fig. 5. Suspending with side entering agitator.

At an optimum setting the complete bottom is solid free, both in the centre and at the corners and, in part, the solids are suspended right to the top. However, when the angle is smaller than Popt it is observed that, while the centre is free, depositions still occur in the tank corners and they partly move as wandering dunes. If angle p is too large, the corners remain solid free but the solids accumulate at the centre. It is also seen clearly that the whole of the tank contents is subject to a strong rotational motion. These observations apply to a given propeller configuration and a constant tip speed. The fact that accumulation at the centre takes place when the rotational motion in the tank is too strong, as a result of a larger setting of l3, bears a strong resemblance to the so-called ‘teacup effect’. This can easily be observed when a spoon is used to stir a cup containing tea leaves. Tea leaves which arrive near the bottom of the cup move in a remarkable way towards the centre. Finally, it is seen that they all accumulate at the centre of the cup bottom. The physical explanation of this phenomenon is relatively simple when the relation of forces acting on a volume element is considered, as shown in Fig. 6. As is well known, the rotational motion of the liquid results In the formation of a vortex, ie. the liquid level in the centre is lower and, near the tank walls, higher than under non-strirred conditions. A volume element dV which circulates In the tank with an angular velocity wU is subject to a centrifugal force Z. For equilibrium to exist, a pressure force P must act towards the centre. This force appears as the result of different static pressures on the outer and inner side of the volume element.

layer dZ
directed secondary flow wr

flow above futed ground (teacup effect).

The pressure on its outer side is higher than that on the inner side so that the pressure difference produces a pressure force acting towards the centre. The external pressure is higher since, as a result of the vortex formation, the liquid level there is higher by Ah. This equilibrium applies to the whole tank, to all radii at all levels and layers, with only one exception, namely near the bottom. At the bottom, as a result of wall friction, the rotational velocity is reduced to w, = 0 (no-slip condition). In this boundary layer, which in large tanks is approximately SO mm thick, the peripheral velocity or the rotational velocity w, is smaller than that based on the total tank contents. This implies that the centrifugal force must also be smaller. The pressure force, however, is retained in the boundary layer, so that the previous equilibrium of forces is disturbed and P becomes greater than Z. This results in the particles moving towards the centre. The flow pattern which develops in the tank depends, therefore, very strongly on the rotation of the tank contents, which, in turn, can be represented by the circumferential velocity w, of the rotating liquid. If this W, , produced by the agitator as a result of its tip velocity, diameter and position, is excessively large, the teacup effect results. If it is too small, however, it produces accumulation in the corners. In the course of the experiments a very remarkable effect was observed. It was concluded that in the tank of diameter 700 mm the optimum angle was approximately /I = 14”. Within the framework of the scale-up investigations it was fast observed in the 1400 mm tank that, when retaining geometrical similarity and hence the angles (Y and /I, the scale-up criterion was found to be n = constant, which was not acceptable. However, by varying in the large tank the original optimum angle obtained from the 700 mm experiments, it was observed that the optimum angle in the large tank was around fi = 10”. With this new optimum angle the circumferential velocity in the large tank was slightly higher than in the smaller tank. A re-examination of the optimum angle in

237 the 700 mm tank showed conclusively, however, that the optimum angle there was greater and the difference was not due to errors in measurement. Hence the optimum P-angle depends on the size of the tank. Such a behaviour has not been observed until now. If, in addition, the flow patterns in tanks at constant angle are considered, the impression is very strong that at practically constant stirrer tip speed U, the liquid in larger tanks rotates faster than in smaller ones. More rotation means, however, a greater influence of the teacup effect. For this reason, an energy balance was set up so that such a behaviour could be supported theoretically. The basic idea originates from the fact that with increase in scale, the volume, and hence the mass, increases with the cube of the scale factor while the surface at which friction occurs increases only quadratically. An energy balance was set up for the steady state conditions represented in Fig. 7. It was assumed here that the power introduced was lost through friction at

w=wu.Y

h

Couette -flow

To.- p

Tangential stress

W- To. d,’

Drag force

Ez- W.wu

Power of friction

Balance

:

Impeller power/Power of friction: wu2.d, -u3.sin31).

d ’

Fig. 8. Velocities with side entering agitators.

Assumption for balance: 1)Entering energy flow G [knpeller powerlu 2)Leaving energy flow P Friction at wall E,-w3.sin3P.d 0 E,- T,

d,’

22 - u”.ein3fl.d,Z

wu

Fig. 9, the agreement with the theoretical assumption is quite good for different tip velocities and angle settings (EKATO propeller). For the two tank sizes, the dependence on the angle is represented in Fig. 10. This shows quite clearly that the plotting of w,/u at inclinations to the horizontal of Q!= 5” and 10’ results in two different curves. The optimal angle of /3= 14” found in the 700 mm tank corresponds in this diagram to w,/u of around 0.06. The same value is obtained in the large tank of dr = 1400 mm at an optimal angle of p = 10”. Therefore, it follows from this representation that the main parameter in the design is the velocity ratio w,/u. However, on account of friction, this w/u does depend on the setting angle.

Fig. 7. Energy balance.

the walls. This is shown in Fig. 7 by the relationship between the energy fluxes El and E, entering and leaving. For a rough representation of the friction at the walls, the simplest velocity profile was chosen, ie. the so-called Couette flow (Fig. 8) which permits the shear stress at the wall to be calculated in a relatively simple manner. The energy losses by friction can then be determined relatively simply, and thus the relationship given in Fig. 8 between the circumferential velocity in the tank, w,, the tip velocity of the stirrer U, the angle /3, the tank diameter dr and the diameter ratio da/d,. This relatively simple theoretical relation was confumed experimentally by producing a visible circumferential velocity w, in the tank with the aid of floating cork and wood particles for different tip velocities U, diameter ratios, tank diameters and angles a! and f3. According to the relation given in Fig. 8 the ratio is w,/u - ul” . The proportionality factor depends on the type and shape of the impeller. As can be seen from

1.5 wulu (wu/ula 1.0 as 08 0.7 0.6 3 Propeller d2/d, =0.05?

Fig. 9. Influence of tip speed on rotational speed.

238 In the determination of the scale-up and the setting of the optimum angle applicable to a particular tank, it was confirmed that, irrespective of the stirring device, a scale-up criterion u = constant is not sufficient. Instead, according to Fig. 12, the scale-up criterion varies between u = constant and the well-known Einenkel straight line. Therefore, the dependence on the tank size should be accounted for by Qul~l~madel = @Pk4dX withx <0.31. Fig. 10. Influence of angle of mounting and vessel size on rotation (three-bladed EKATO propeller; tilling height ratio hl/dl= 0.3; clearance h3/dl= 0.065).

If the dependence on tank size and on angle fi is deduced from the equation given in Fig. 8, the following relation results: (~,/urcf,-“~

- sin3’2fi

The comparison between theory and measurement shown in Fig. 11, and the agreement is quite good.

is

The tanks used in the experiments were not equipped with baffles or internal apparatus which could hinder the rotational motion. It follows, therefore, that if industrial tanks are equipped with baffles at the bottom or at the wall, the above relationship can no longer be ap plied indiscrim~ately.

UM_

Fig. 11. Correlation of test results for a three-bladed EKATO propeller with filling height ratio hl/dl= 0.3.

10

1

dpfdl4

*

Fig. 12. Scale-up for a three-bladed EKATO propeller with hl/dg = 0.3; condition: no solids settled.

Conclusions The results of the investigations can be summarized as follows: (1) No difference exists with regard to power demand between the EKATO propeller and the hydrofoil type of propeller. (2) The six-bladed pitched-blade turbine requires about four times the power of the EKATO propeller to fulfil the same task. This is due to the fact that the jet of the pitched-blade turbine spreads out considerably more than with the propeller. Thus it results in considerably smaller speeds on the basis of the continuity equation, which are insufficient for suspending the solids. (3) The optimum angle fl depends on the tank size. (4) The scale-up rule is below the Einenlcel criterion but above tip speed u = constant. (5) Larger diameter ratios lead to lower power and tip speeds,

which are favourable

with regard to erosion.