2。74 kUS$。 To accomplish this processing duty, it needs 5。59 kW power  input,  corresponding  to  a  yearly  energy  consumption  of

176。3 GJ and a yearly energy cost of 2。4 kUS$。 The energy-efficient nature of this filtration dewatering process is highlighted by com- paring with an evaporation process which requires 66,000 GJ of energy per year。 The annual labor cost of this project is 3 kUS$ and the annual spending on  process  maintenance  is  0。25 kUS$。 As a total, the aggregate cost for this project is 8。23 kUS$/yr。 For simplicity, interest costs related to initial capital investment were not included in this study。 This is not an unreasonable assumption, as there exist many subsidy programs which assist the develop- ment of clean energy processes  [37]。

A breakdown of the annual dewatering cost is shown in Fig。 10。 The biggest share, 37%, comes from labor, which is 7% higher than the second biggest share from energy。 In this case, the abnormally high proportion of labor costs results from the small scale of the operation。 Currently in algal biofuel industry, typical daily lipids production capacity of plants are in the range of 90–300 ton/d。 [37], corresponding to a dewatering duty of 1500–5000 m3/d har- vested algae feed (20 w/w%)。 This scale is not large enough to  cut

Fig。 10。 Breakdown of the dewatering cost among labor, energy, equipment, infrastructure  and maintenance。

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

down the labor cost。 In this simulation, the assumed industrial scale for estimating the labor allocated to this 100 m3/d demon- stration project is 5000 m3/d。 Under this scenario, there is a great potential for scaling up the algae-dewatering process and thus reducing the dewatering unit cost。 Given this scale is, for example, doubled。 The labor cost allocated to this simulated project would be cut in  half。

6。 Conclusions

A mathematical model was developed for the algae-dewatering process by a rotary drum vacuum filter。 Algal cake-layer is consid- erably compressed as the filtration pressure is increased beyond a critical pressure, and the compressed algal cake-layer becomes less porous and develops smaller equivalent pore size。 As a result, the densified cake-layer presents a much higher resistance for the fil- tration process。

Simulations showed that the pressure required to displace the filtrate water out of the porous algal cake-layer is generally lower than the filtration pressure。 High pressure systems are not required for dewatering the algal cake-layer。 Consequently, the water con- tent in the collected algae can be reduced to 1–2%, and the energy required for the subsequent algal biomass drying which has a high thermal demand can be significantly  reduced。

For  processing  100 m3/d  20  w/w%  algae  stream,  the capital

investment for the project is 54。73 kUS$, and the power required to fulfill this processing duty is 5。59 kW and the yearly energy cost is 2。45 kUS$, is much less than needed by the evaporation approach。 Process optimization indicated that the vacuum pres- sure in the filter should be controlled at 20 kPa, and the rotation cycle of the filter should be around 40 s so that the dewatering cost can be reduced to its minimum of 0。93 US$/h。 Breakdown of this cost reveals that 37% goes to labor, which represents the biggest share。 This results from the relatively small scale of the algal bio- fuel project。 Analysis indicates that scaling up the project will bring significant unit cost  benefits。

Acknowledgement

This research was funded by the HDE4-SIM, Algae Carbon Conversion Flagship Program of National Research Council Canada (NRCC)。