4. MATERIALS AND METHODS
5.7 Pilot scale trials with a disc stack centrifuge
5.7.1 Scale down with a defined medium
5.7.1.1 Fully active disc stack
The grade efficiency curves obtained for the fully active stack at a range of flow rates are shown in Figure 5.7.1. The adapted Rosin-Rambler-Sperling- Bennet (RRSB) fitted curve given by Mannweiler (1990) (section 3.3.1) for the scaled down BSB centrifuge is shown for comparison. The centrifuge grade efficiency curves are S-shaped and deviate from Stoke's law due to flow vortices around the spacer ribs and uneven particle distribution at the disc openings as described in section 1.3.2.
A major part of the grade efficiency curve at each flow rate in the SAOOH overlies the curve reported by Mannweiler (1990) for the larger centrifuge, indicating that separation characteristics are independent of scale for dilute suspensions of PVAc. As in Mannweiler's work, critical particle diameter dç. was calculated using the radius at the inner edge of the channel riser (ie. R^ = 0.043 for the SAOOH). Separation is restricted to the area above the channel risers where feed enters the discs.
-o H 1.0
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0.8 w 0.6 0.4 0.2 0.0 d/dcFigure 5.7.1: Grade efficiency curves to show the separation o f 0.055% (w/v) PVAc in a disc stack separator (W estfalia SAOOH-205) fitted with a fully active stack.
O
V
□ Feed flow rate 554 L h ' \ d^ 1.6 pm
Feed flow rate 302 L h'*, d^ 1.2 pm Feed flow rate 793 L h'*, d^ 1.9 pm
Standard PVAc curve for scaled down BSB disc stack cen trifu g e(M an n w eiler 1992)
The curve for a flow rate of 793 L h*^ shows carryover of large particles into the supernatant. Above 580 L h'^ the total flow capacity of the centrifuge is exceeded and feed flows into the overflow section together with supernatant. As the flow rate increases, the flow pattern inside the centrifuge becomes more turbulent and particle re-entrainment into the supernatant can occur as shown in the curves for 554 L h '\ Mannweiler (1994) has suggested that this is a common problem in small machines which tend to have poorly defined flow patterns.
5.7.1.2 Scaled down disc stack
Figures 5.12-3 show separation efficiency results using the Westfalia SAOOH-205 with only 1/4 of the separation area active as described in section 4.5.5.2.
Configuration (a) with the active discs positioned 1/4 of the way up the stack clearly shows an improved separation (curve lies further to the left) not at all representative of the full stack operation. This is due to pre-settling of particles in the sediment holding space, so that the centrifuge behaves partly like a tubular bowl.
Configuration (b) is where the active discs positioned 1/10 of the way up the stack are supported firom the bottom by blanking discs with identical channel risers to those in the active discs. At 74 L h’* the grade efficiency curve overlies that for configuration (a) at the same flow rate, suggesting that pre-settling occurred. The marked difference in separation efficiency between high and low flow rates with this arrangement is not representative of full stack operation where all flow rates yield the same curve.
Configuration (c) has the active discs positioned 1/10 of the way up the stack supported by blanking discs without channel risers. The separation showm by this arrangement closely follows the standard PVAc curve given by Mannweiler (1992) for the larger BSB centrifuge with well defined flow patterns. It shows a slightly better separation than in the SAOOH-205 full stack curve wdth respect to carryover of large particles at higher flow rates. The
0.8 - 0.6 - 0.4 - 0.2 - 0 . 0 1 2 3 0 4 d/d^
Figure 5.7.2: Grade efficiency curves to show the separation o f 0.055% (w/v) PVAc in a disc stack separator (W estfalia SAOOH-205) fitted with a scaled down 25% active stack. Active discs situated 1/4 o ff base, with no channel risers in supporting discs.
B , □ Feed flow rate 137 L h ' \ d^ 1.6 pm
O Feed flow rate 74 L h ' \ d^ 1.2 pm
Feed flow rate 199 L h'*, d^ 1.9 pm
Standard PVAc curve for scaled down BSB disc stack sep arato r (M an n w eiler 1992)
73 1 . 0 0.8 0 . 6 0.4 0 . 2 0.0 d/d
Figure 5.7.3: Grade efficiency curves to show the separation o f 0.055%
(w /v) PVAc in a disc stack separator (W estfalia SAOOH-205) fitted with a scaled down 25% active stack. Active discs located 1/10 o ff base, with channel risers in the supporting discs.
B , □ Feed flow rate 137 L h ' \ d^ 1.6 pm
O Feed flow rate 74 L h'% d^ 1.2 pm
^ Feed flow rate 199 L h '\ d^ 1.9 pm
o D o D Q Q
po
□ □ 0 . 8 - 0 . 6 - -a H 0.4 - 0 . 2 - 0 . 0 d/dF ig u r e 5.7.4: G rade efficien cy cu rv e to sh o w the se p aratio n o f 0 .055% (w /v) P V A c in a disc stack se p ara to r (W e stfa lia SAOOH-205) fitted w ith a scaled dow n 25% active stack. A c tiv e discs situated 1/10 o f f base, w ithout channel risers in s u p p o rtin g discs.
g , □ Feed flow rate 137 L h'*, d^ 1.6 p m O F eed flow rate 74 L h ' \ d^ 1.2 p m
F eed flow rate 199 L h ' \ d^ 1.9 p m
separation area to flow rate ratio is the same in both full and quarter active stacks. However, the flow rate has decreased relative to the bowl volume in the scaled down stack and the flow capacity is not exceeded, resulting in less particle re-entrainment.
The differences in separation efficiency shown by the scale down configurations (a-c) can be explained by flow path considerations.
In the fully active SAOOH-205 stack, the discs are fitted with channel risers, ie. holes which when aligned form a vertical channel through which the bulk of process material flows at a relatively small centrifugal pressure compared to that at the disc tip as illustrated in Figure 1.3.2. The original intention of this design was to reduce the re-entrainment of settled solids into the incoming feed stream. However, riser channels can create flow vortices which disturb particle settling. Moreover, channel risers 1/3 of the way up the slope of the discs severely reduce the potential separation area since separation begins at the inner edge of the channel riser. More modem designs such as the CSAl have semi-circular channel risers at the edges of the discs, which prevent solids re-entrainment without compromising settling area.
In scale down configuration (a), (Figure 5.7.2), the lack of channel risers forces feed to the outer edge of the discs and pre-settling of solids occurs in the central part of the sediment holding space before feed enters the active part of the stack.
At the mid flow rate used in the full stack experiments significant numbers of large particles remain in the supernatant. This is due to large vortices forming around the channel risers, thereby reducing active separation area.
Similar effects were observed in the scale down version of the full stack with channel risers (configuration b. Figure 5.7.3). Particle re-entrainment associated with a turbulent flow pattern occurred at the higher flow rates whereas at the lower flow rate of 74 L h'^ re-entrainment of large particles was not observed. However, the flow pattern at 74 L h'* where pre-settling of
particles appears to occur (curve lies towards the left of the standard graph) was not duplicated at the same specific throughput in the fully active stack.
In the case of configuration (c), (Figure 5.7.4), separation was slightly better than for the full stack. This was due to the incoming feed being forced into the sediment holding space before entering the active part of the stack. Pre settling of solids does not occur in this lower part of the bowl due to turbulence (Willus and Fitch 1973) but performance may have improved due to the extra separation area made available by the longer flow path. In addition, particle re- entrainment would not occur where the feed meets the channel risers in the active disc section in this case, because the feed is already evenly distributed between the discs.