Industrial projects are much larger in scale than a demonstra- tion project。 Larger filter units than the one considered here could be deployed to treat a much larger daily feed。 Larger filters have a proportionally smaller footprint within a process。 Savings in capi- tal investment can be achieved with larger filters because they have a net lower price per unit of filtering   area。

1。60

1。50

100

90

1。40 80

70

1。30

60

1。20 50

1。10 40

30

1。00

20

0。90

0 20 40 60 80 100 120 140

Filter rotation cycle (sec。)

10

0

0     20    40    60    80   100  120  140  160  180  200  220  240  260  280  300

Fig。 7。 Hourly dewatering cost using the rotary filter under different operating conditions。 The processing duty is 100 m3/d, the price of electricity is 0。05 US$/kWh, the unit price of the filter is 27。5 kUS$/pc and the life-span of the filter is 20 years, and the  labor cost  for the specified  dewatering  duty is 0。342  US$/h。

Cycle time (sec。)

Fig。 9。 Hourly dewatering cost contours on the domain of operating conditions。 The processing duty is 100 m3/d algae feed containing 20 w/w% dry algal biomass。

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

5。4。 Process  energy  and  capital investments

The process capital investment was plotted in Fig。 6 against the process energy demand, including the energy required by the fil- trate water pump, vacuum pumps, and by rotating the drum filter in the feed tank。 Across the range of operating conditions, differ- ences greater than a factor of 3 arise in both the process capital investment and energy demand。 For example, operating the filter at a higher rotation speed of 10 s per cycle, the resulting capital investment can be more than 3 times less compared to the case with a rotation cycle of 120 s, while the process energy demand is nearly 4 times as much。 For a given processing duty, a large vari- ety of options are available through balancing the trade-off between the process capital and energy investment: the process capital investment can be saved by using more energy or vice versa。 From the viewpoint of process economics, this interplay of factors can be engineered to minimize the overall dewatering cost, including the capital and the operating   costs。

5。5。 Dewatering  cost  and  process optimization

The dewatering cost (Eq。 (36)) under various operating condi- tions is shown in Fig。 7。 A conic curve with a minimum is obtained for each of process。 The higher dewatering cost, on the left side of the curve is due to the high energy consumption by the filter, which is operated at a high rotational speed with a cycle time less than 20 s。 It is also shown in Fig。 7 that operating the filter at too slow a speed (with a cycle time greater than 40 s) also gives rise to a higher dewatering cost。 In this case, capital costs are higher with lower throughput rates。 Fig。 7 shows that the hourly dewater- ing cost is strongly dependent upon the operating conditions, and can vary widely。 Process simulation enables a comprehensive over- view of the impact of operating conditions on the dewatering cost, and a means to select process parameters for reliable and econom- ical operation。