5。3。 Processing capacity of the RDVF   filter

Under different operating conditions, the capacity of the selected RDVF filter for processing 20 w/w% algae feed is depicted in Fig。 5。 It can be seen that filter rotation cycle time varies inver- sely with algae filtration  capacity。

This capacity reduction results from the formation of a thicker algal cake-layer, which creates a larger hydraulic resistance when the filter is allowed to have more time working in the filtration zone。 The impact of cake-layer compression on the processing capacity is also observed。 It is shown in Fig。 5 that operating the fil- ter at 20 kPa gives the highest processing capacity, while in Fig。 3 the largest thickness of algal cake-layer occurs at 10 kPa of pres- sure。 This difference is attributed to the compression of algal cake-layer: a looser and thus thicker cake-layer is built up at the lower working pressure of 10 kPa as shown in Fig。 3, but with greater cake compression at 20 kPa, a higher mass throughput is still achieved。 As indicated in Table 1, when moving from 10 to

Table 2

Number of the selected filter units required to accomplish the processing duty of

100 m3/d 20 w/w% algae feed coming from the algae-harvesting centrifuges。

10

Cycle Number  of filters

0 10 20 30 40 50 60 70

Filtration pressure (kPa)

Fig。 4。 Effects of filtration pressure on the pore radius of the algal cake-layer and the minimum dewatering vacuum pressure。 The assumed parameter values  are: algae radius equal to 5 lm; surface tension of water equal to72 lJ/m2; tortuosity of

the pore typically equal to 2 according to the geometry; contact angle equal to p/4。

P。 Shao et al。 / Chemical Engineering Journal 268 (2015) 67–75 73

160 1。4 120

Pressure difference (kPa)

Energy demand (kW)

Fig。 6。 Plot of capital investment versus process energy demand under different operating conditions with the cycle time varying from 10 to 120 s and the filtration pressure varied from 10, 20, 40, to 60 kPa。

20 kPa of pressure, the porosity of the cake-layer was reduced from

0。73 to 0。67, and also the equivalent pore size went from 8 to 6 lm as shown in Fig。 4。 The trade-off between cake compressibility and its increased resistance to water permeation produces no further benefits at pressures above 20 kPa。 At these higher pressures, algae throughput becomes progressively less。

It is also shown in Fig。 5 that by operating the filter at a higher rotation speed, more feed can be processed。 For example, at 20 kPa, the filter dewaters only 70 m3 algae feed per day at a rotation cycle of 120 s, and the capacity is doubled with a cycle time of 30 s。 High rotary speeds offer a large potential for increasing algae product rate albeit with greater energy  consumption。

Table 2 summarizes the number of the filters needed for a pro- ject with a planned capacity of processing 100 m3  20 w/w%   algae

1。70

Fig。 8。 Plot of the minimized hourly dewatering cost, and optimized cycle  time versus  the  filtration pressure。

feed per day as a function of operational parameters。 The full range of parameters in Table 2 produces a possible range of 0。6–2。31 for the number of required drum filter units。 This translates to a choice of 1, 2 or 3 units, which in turn encompass only some correspond- ing operational requires from Table 2。 Given that the present pro- cess in on the demonstration scale, it should be noted that a full industrial installation with large daily throughputs by one or two orders of magnitude, will regimes a much large number of filtra- tion units and offers commensurately more configuration flexibil- ity。 For the present process, one filter would be suitable for fulfilling the total daily dewatering duty, operated at a rotation cycle time between 30 and 40  s。