Harvesting of Microalgae by Centrifugation for Biodiesel Production: A Review

Contents lists available atScienceDirect

Algal Research

journal homepage:www.elsevier.com/locate/algal

Review article

Harvesting of microalgae by centrifugation for biodiesel production: A review

Yousef S.H. Najjar

^a,⁎

, Amer Abu-Shamleh

^b

aMechanical Engineering Department, Jordan University of Science and Technology, Irbid, Jordan

bDepartment of Renewable Energy and Sustainable Development, Jordan University of Science and Technology, Irbid 22110, Jordan

A R T I C L E I N F O

Keywords:

Biodiesel Microalgae Centrifugation Algal biomass Harvesting Dewatering

A B S T R A C T

Biodiesel production from algal biomass is widely considered a sustainable alternative to petroleum fuels, especially in the transportation sector. However, the high energy consumption associated with the harvesting phase poses a significant impediment to the substantial commercialization of algal biodiesel. Centrifugation is among those harvesting methods that have a high efficiency close to 100% but can consume more energy than it produces. This review aims to address possible solutions for the issue of the high energy cost of centrifugation as an efficient harvesting technique. In addition, a detailed overview of the potential types of centrifugation that can be used to harvest microalgae with their pros and cons is provided. The most prevalent centrifugal devices that have been identified to be effectively useful to harvest algal biomass include disc-stack centrifuge, solid- bowl centrifuge, hydrocyclone, tubular centrifuge, and multi-chamber centrifuge. Overall, the analysis showed that centrifugation alone is not suitable for the harvesting of microalgae for biodiesel production. Nevertheless, coupling centrifugation with other harvesting technologies, e.g. withflocculation, as a primary pre-concentra- tion step, can increase the energy output significantly thus, reducing the production cost.

1. Introduction

The harvesting-dewatering process is considered the second stage after cultivation in the biodiesel from microalgae production process.

Fig. 1shows the process of biodiesel production from microalgae. The major techniques available in microalgae harvesting and recovery in- clude mechanical, chemical, biological, electrical, or a combination of two or more of these processes [1–5]. Among the several harvesting- dewatering techniques that have been investigated and proven to be worthy are centrifugation, coagulation, flocculation, flotation, sedi- mentation, screening, andfiltration [6–10].

Many argued that the major bottleneck in microalgal industry is the improvement of the harvesting-dewatering techniques [11–16]. Ac- cording to Singh et al. [17], the high cost of harvesting and the lack of a universal harvesting technique are the reasons behind thefinancial and sustainable obstructions preventing the utilization of microalgae. The work of Richardson et al. [18] on the economics of microalgae oil in- dicates that, based on laboratory experiments, the production of algal oil was feasible in 1998, however, due to the absence of an efficient harvesting technique, we have yet to see any full-scale algae. Allnutt [19] argued, maintaining a favorable impact on the costs of the port- folio of products requires a maintained balance to reduce harvesting

costs. Thus, the need for continuing research to reduce the harvesting- dewatering costs will contribute to the overall reduction of the com- mercial threshold of biodiesel from microalgae.

The harvesting-dewatering step is known to be an energy-intensive process that limits the sustainability of utilizing such techniques [20–23]. Around 90% of the cost of instruments for biomass production originated from harvesting [24]. Moreover, the harvesting-dewatering step is very expensive and comprises about 20–30% of the total cost of microalgae biomass production [25–29]. However, under certain cir- cumstances, like when post-production is needed, the cost might reach 60% of the total cost of the process [30]. For algal biodiesel production, studies have shown that harvesting costs can represent up to 50% of the total cost of production [31–33]. The reasons behind the high cost of harvesting are due to dilute algae solutions, the large volumes to be processed, and the small size of microalgal cells [34,35]. Hence, de- termining the most suitable harvesting technique is dependent on the properties of microalgae, such as size, and characteristics of algal strain like cell density [36–38]. In addition, the properties of the target pro- duct also play a role in the selection process [39].

To date, no cost-effective commercial-scale microalgal harvesting- dewatering method is applied to a large-scale [40]. In this perspective, it is crucial to improve the efficiency of the current harvesting-

https://doi.org/10.1016/j.algal.2020.102046

Received 14 March 2020; Received in revised form 18 July 2020; Accepted 6 August 2020

⁎Corresponding author at: P.O. Box 3030, Irbid 22110, Jordan.

E-mail address:y_najjar@hotmail.com(Y.S.H. Najjar).

Available online 21 August 2020

2211-9264/ © 2020 Elsevier B.V. All rights reserved.

T

Nomenclature

C Cost, $

Ci Initial concentration Cf Final concentration CO2 Carbon dioxide CV Calorific value, kWh/kg

Cx Concentration of slurry entering unit process x,kg/m³ D Distance of fall, m

d Particle diameter, m E Efficiency factor, %

E Energy cost per unit volume of algae, kWh/m³ Ecost Energy cost per kg oil, $/kg

Edisc Energy required by the disc-stack centrifuge, kWh/m³ EIx Energy input for unit process x, kWh/1000 kg Eprod Energy produced, kWh

Ereq Energy required, kWh

E_{t h&} Energy required by the tubular and helical centrifuges,

kWh/m³

Ex Volumetric energy consumption for unit process x, kWh/

m³

FU Functional unit, kg D Distance of fall, m d Particle diameter, m g Acceleration of gravity, m/s² GHGs Greenhouse gases

H2 Hydrogen

l The length of the bowl, m LC Lipid content

moil Mass of oil produced, kg mrec Total biomass recovered, kg n Number of disks

N2 Nitrogen

O2 Oxygen

P Power, Watts

Q Volumetricflow rate, m³/s

Qm Centrifugeflow rate taken from master curve, m³/s R Distance of the particle from the axis of rotation, m RE Recovery efficiency, %

Rr Biomass recovery rate r‵ Radius from axis of rotation, m r Radius of particle, m

r1 Radius to the inner surface, m r2 Radius to the outer surface, m

t Time required for the algal particle to free fall a given distance in a liquid medium, s

TEIx Total energy input for dry microalgae, kWh/kg Vf Final volume of the solution, m³

vg Terminal velocity under gravity m/s

vgm Settling velocity taken from master curve, m/s Vi Initial volume of the solution, m³

Vp Volume of the particle, m³ Vs Settling velocity, m/s

Vx Volume of slurry for (x = 1, 2), m³

x Unit process for one or two dewatering stages Greek letters

θ Angle of the disk μ Dynamic viscosity, Pa.s ρ Density, kg/m³

Σ Sigma factor of centrifuge, m² ω Angular velocity, rad/s Subscripts

g Gravity

out Output

p Particle

w Water

Fig. 1.Flow diagram of the microalgae-to-biodiesel process.

dewatering technologies for inexpensive biodiesel production.Table 1 shows the efficacy of the most commonly utilized microalgal harvesting method [41,42].

In general, the purpose of the harvesting-dewatering is to increase the overall solid matter to 10–25% of the overall dry matter or to a dry product [43]. This increase in the total solid content is generally limited by the technology. Centrifugation is proven harvesting-dewatering technology in several industries because of the high solid concentration that can achieve which reaches up to 10–20% [44]. However, cen- trifugation is an energy-intensive process [45] and the production of various microalgal products is affected by the high cost associated with this process.

Consequently, the core problem that prohibits the commercial large- scale production of biodiesel from microalgae could be attributed to the dilute biomass titer produced by known strains of microalgae [46]. This is mainly because the harvesting-dewatering step, which is essentially used to concentrate the dilute microalgal solution, is an energy-in- tensive process that affects the cost of the overall process. This issue has not been fully addressed in the literatures [47]. However, under- standing the concepts and techniques behind the harvesting-dewatering stage of microalgae will contribute to solving the problem of making the production of microalgae commercially viable and feasible.

In this perspective, the centrifugation method is well known to have the highest efficiency among all the other harvesting-dewatering methods [48]. Nonetheless, the technique itself faces the issue of high- energy consumption [49]. Numerous publications addressed this topic to a certain extent however, they are fragmented and not well-oriented.

Thus, the aim of this review article is to pool all these reports while providing insights into the centrifugation method in terms of technical development and technological advancement as a contribution to sub- stitute fossil fuels with more sustainable and economical ones. In ad- dition, the review outlines the advantages and disadvantages of the centrifugal devices that are mainly used for biodiesel production from microalgae and the potential area of improvements for each. Further- more, the cost-benefit of combining centrifugation with other methods is assessed. Altogether, the paper describes the centrifugation har- vesting-dewatering technique with its merits and demerits and the limitations that hinder its commercial expansion and the possibility to overcome them in the process of biodiesel production from microalgae.

2. Centrifugation 2.1. General overview

Centrifugation is a physical dewatering method that depends on the generation of a centrifugal force which acts radially and accelerates the movement and separation of particles based on the difference in density between the particle and the medium surrounding it [44]. In this case, the denser particle will move outwards while the less dense particles will move inwards.

Centrifugation is reliable in separating highly dilute solutions uti- lizing suitable rotational speeds. In this context, the separation process mainly relies on the particle size and density difference of the medium components [50,51]. By utilizing this method, an increased biomass concentration and a high harvest efficiency can be achieved within a short period of time. The efficiency of the overall centrifugation process

depends majorly on the settling characteristics of the cells, the retention time of the slurry in the centrifuge (which is controlled by theflow rate), and the settling depth which can be minimized by the design of the centrifuge [23,52,53]. According to Javed et al. [54], centrifugation can be used effectively to recover microalgal biomass with 80%–90% of biomass recovery within 2–5 min. Another main advantage of this method is that no chemical additives are required 77. Consequently, biomass storage for a long period of time while preserving quality is possible in this case [55]. Although this type of removal mechanism is mostly used in food and pharmaceutical industries, many newly de- signed centrifuges have been used recently for biodiesel production [56,57].

Heasman et al. [58] conducted a centrifugation test on 9 different microalgae strains and reported harvesting efficiencies of > 95%, 60%, and 40% at 3000 g, 6000 g, and 1300 g, respectively [59]. In another recent study conducted by Japar et al. [60] to compare centrifugation to sedimentation and magnetic separation, 4 rotational speeds of 1000, 3000, 5000 and 7000 rpm were used for 5 min each. The comparison study involved using three different microalgal species which are Chlorella sp. UKM2, Coelastrella sp. UKM4 and Chlamydomonas sp.

UKM6. The results indicate that a harvesting efficiency of 98%, 96%, and 90% ofChlorellasp.UKM2,Coelastrellasp.UKM4andChlamydo- monas sp.UKM6, can be achieved respectively, using 7000 rpm for 5 min. It has been concluded that centrifugation has the highest effi- ciency compared to sedimentation and magnetic separation.

Although centrifugation appears to be feasible for high-value pro- ducts, it is way costly for lower-value products that are processed in an integrated system, such as biodiesel from microalgal oils. The main drawback of centrifugation is that it is an energy-intensive process that is far from being cost-effective for biofuels production [61].

In addition, the capital investment cost of centrifugation is rela- tively high and form another burden [62–64]. According to Qi et al.

[65], the centrifuges total investment costs can reach up to 30% of the total investment for this industry based on the feasibility analysis which was conducted on microalgae for biofuels. From another perspective, the increase in the capital cost with scale, makes the use of centrifuges for large-scale production more problematic. The cost of labor also has a significant contribution to the total cost [42]. Hence, there's a high need for automation to overcome the labor cost despite the additional cost associated with it. Moreover, many argued that the high main- tenance cost required is the main barrier hindering the expansion of such technology for large-scale biodiesel production. Specifically, when centrifuges are required to operate in saline environments. According to Mondal et al. [66], the high maintenance requirements of the freely moving parts is the main reason behind the high cost of centrifugation.

Greenwell et al. [67] argued that this together along with the high strength and corrosion-free alloys needed as construction materials for the centrifuges, means that these separations are expensive.

On the other hand, the high gravitational force and shear stresses during the centrifugation process can lead to the destruction of the cell structure of algae [68,69]. But this is dependent on the species used, as there are certain species that are more sensitive to such forces than others. For example, a study was conducted by Michiels et al. [70] to test the tolerance ofTetraselmis suecica,Isochrysis galbana,Skeletonema costatum, andChaetoceros muellerito shear stress.Severe cell damage was observed on Isochrysis galbana, Skeletonema costatum, and Table 1

Comparison of the various harvesting-dewatering methods of microalgae [41,42].

Harvesting method Solid concentration (%) Biomass recovery rate (%) Energy consumption (kWh/m³)

Centrifugation < 20 > 90 1.43

Filtration/screening 5–18 20–87 1.22

Flocculation < 6 50–90 0.15

Sedimentation 0.5–3 10–50 0.10

Chaetoceros muelleri when exposed to shear stress in the range of 1.2–5.4 Pa. Thus, optimizing the rotational speed is crucial in main- taining the structure of the cells of microalgae.

Xu et al. [71] conducted an experimental study onDunaliella salina as a potential algal type for biodiesel production. The species was ex- posed to various centrifugal stresses of 1000, 2000, 3000, 5000, 7000, 9000, 11,000, 13,000, and 15,000 g for 10 min. The results indicate a cell disruption when the centrifugal force increased above 5000 with an approximate 40% loss of the glycerol yields and intact cells with a centrifugal force higher than 9000g.

In this context, it appears that there is a need for scientific research in this particular area to optimize the centrifugation process for the various types of microalgae that has the potential to be used for bio- diesel production.Table 2summarizes the positive and negative aspects of centrifugation.

2.2. Energy requirements and balance

Despite the very high recovery efficiency, which can exceed 95%, centrifugation is known to be energy-consuming [55]. Mohn [79] re- ported that in order to harvest 1 m³of microalgae by centrifugation, an approximate energy demand of 1 kWh is needed [80]. However, this may vary greatly based on the type of centrifuge and the algal strain used. It has been reported that an energy demand of 8 kWh/m³is re- quired to increase the concentration of dry algal biomass to 22% [81].

On a large-scale basis, Batan et al. [82] developed a model based on a dewateringflow rate of 45,000 l/h ofNannochloropsiswith an energy consumption of 45 kW. The results showed that an electricity require- ment of 30,788 kWh/ha was needed for centrifugation, achieving a net energy ratio of 0.93 MJ of energy consumed per MJ of energy produced.

Another important factor that highly affects the energy consump- tion of the centrifugation process is the flow rate in l/min. The re- lationship between theflow rate of the centrifuge in l/min and the ef- ficiency of harvesting was addressed in Dassey et al. work [83]. As shown inFig. 2, the results indicated slowerflow rates correlated with higher energy consumption directed to a smaller culture volume per minute. The harvesting cost correlated to the various flow rates was also addressed in this study as shown inFig. 3. With lower harvesting efficiency, greater process volumes associated with lower energy con- sumption were found to be economical in contrast to smaller process volumes associated with higher energy consumption. Point B inFig. 3 refers to the most convenientflow rate that yields the most cost-effec- tive harvesting strategy, while point A refers to an increase in the oil cost attributed to the poor harvesting efficiency of the centrifuge. At that point (Point A inFig. 2), costs increased again because of the in- creased volume processed at 23 l/min which was negated by the poor harvesting efficiency.

AlthoughFig. 3suggests that there is an optimum point (Point B) that yields the lowest oil cost in ($/l) in terms offlow rate, it does not simply mean that the production process is feasible. This is because, in order to achieve economic feasibility of biofuel production from mi- croalgae, the oil cost should not exceed 1 $/l [84] compared to ap- proximately 10 $/l for Point B. Consequently, there is a need for

continuing research in order to reduce the high cost of the harvesting- dewatering to the price of conventional petroleum oil.

The energy balance of the centrifugation process can be calculated using Eqs. (1)–(3) [85]. Assuming no losses occurred during the downstream process, the energy input for unit process (EIx) in kWh/

1000 kg dry microalgae can be calculated using the following equation:

=

EIx VxEx (1)

where,Vxis the volume of slurry for (x = 1, 2) in m3, andExis the energy input for unit process x in kWh/1000 kg.

Vxcan be calculated using Eq.(2).

=

Vx FU

CxRE (2)

where,FUis the functional unit in kg,Cxis the concentration of slurry entering a unit process x in kg/m3, andREis the recovery efficiency.

The total energy input in kWh/kg dry microalgae (TEIx) can be calculated using Eq.(3).

∑

=

TEIx EIx (3)

where,EIxis the energy input for unit process x in kWh/1000 kg.

Based on the previous equations, the total energy requirements for the centrifugation process in kJ/kg range from 2.86–32.73 [86].

Table 3shows the energy consumption of various types of centrifuges used to derive biofuels from microalgae. It can be seen from this table that the energy consumption reported in the literature in (kWh/m³) ranges from 0.3 to 9. It can be concluded that the energy consumption is highly dependant on thefinal slurry concentration, as higher slurry concentrations require higher energy consumption according to Table 3.

The microalgal species plays a significant role in overall energy consumption. This is because the particle size of microalgae affects the flow rate required by the centrifuge and the amount of energy required.

In addition, the proper selection of the centrifuge type influences en- ergy consumption, specifically since more efficient and optimized centrifuges designed for microalgal applications exist.

2.3. Efficiency calculation

Centrifugation is quite similar to gravity sedimentation in which the gravitational force is replaced by the acceleration of the centrifuge to increase the concentration of solids in a liquid [87]. As the gravitational force via centrifugation increases the sedimentation rate increases. For this reason, centrifugation can be considered as an extension of gravity sedimentation [88].

The main factor that affects the separation efficiency is the behavior of the smallest particles in the system which is described by Stokes' law (Eq.(4)). Stokes' law implies that the velocity of sedimentation is di- rectly proportional to the density difference between the algal cells and the medium and the square of the algal cell's radius (Stokes radius) [89]. Hence, the particle settling velocity of a centrifuge is given by the following equation [88]:

Table 2

Advantages and disadvantages of centrifugation for harvesting microalgae [40,72–78].

Advantages Disadvantages

•

An efficient, rapid, and reliable technique for small volumes of microalgal cultures

•

Easier than other methods

•

Strain independent

•

Suitable for high-value products

•

Produces biomass with very low water content

•

Suitable for research purposes

•

No chemical additives are required

•

Costly and time-consuming for large culture volumes

•

Large initial capital investment

•

High labor requirement

•

High operational cost

•

High energy requirements

•

Strong centrifugal force can cause cell damage

•

Can lead to thickerfloating layer in some algal species, resulting in a great biomass loss

•

Impractical for sensitive species at high rotational speeds

= − Vs d ω ρ ρ R

μ

( )

18

p w

2 2

(4) where,Vsis the settling velocity in m/s,ωis the angular velocity in rad/s,Ris the distance of the particle from the axis of rotation (i.e., the distance of the particle position in the test tube to the center of the centrifuge rotor axis, when the centrifuge buckets are horizontal to the axis) in m,dis the particle diameter in m,ρpandρware particle density and water density in kg/m³, respectively, andμis the dynamic viscosity in Pa.s.

From Eq.(4), it is obvious that the settling velocity can be increased with an increase in the centrifugal speed, the particle size, the density difference between the algal particle and water, and the separation radius, while it decreases with an increase in the dynamic viscosity.

Moreover, the time required for the algal particle to settle can be calculated based on the Stokes' law using (Eq.(5)) [90].

= −

t πrμD

V ω ρ ρ R 6

( )

p 2 p w

(5) where,tis the time required for the algal particle to free fall a given distance in a liquid medium in s,ris the radius of the particle in m,Dis the distance of fall in m, andVpis the volume of the particle in m³.

According to Fujita et al. [91], the total volumetricflow rateQin m³/s can be calculated based on the concept of sigma values, which developed by Ambler [92].Q can be expressed in terms of terminal velocity and the acceleration of the centrifuge. Thus,Q can be calcu- lated by the following equation:

Q=2v Σg (6)

where,Qis the total volumetricflow rate in m³/s,vgis the terminal velocity under gravity in m/s,Σis the sigma factor of centrifuge in m².

vgcan be calculated using (Eq.(7)) as follows:

= −

v ρ ρ d g μ

( )

g 18

p w 2

(7) where,dis the diameter of the algal particle in m, andgis the accel- eration of gravity and is equal to 9.8 m/s.

Σis considered as a physical characteristic of the centrifuge, not the fluid-particle system, and it is the equivalent area of a settling tank theoretically capable of doing the same amount of useful work.Σcan be calculated for a laboratory test tube or bottle centrifuge, tubular bowl centrifuge, and disc-type centrifuge using Eqs.(8), (9), and (10)re- spectively.

=

⎛

⎝ ⎞

+ ⎠

Σ Vω

4.6 log ^r

r r

2 2 ‵²

1 2 (8)

where,r‵is the radius from the axis of rotation in m,r1is the radius to the inner surface in m, andr2is the radius to the outer surface in m.

= −

(

+

)

Σ π lω g

r r

( )

ln ^r

r r

2 22

12 2₂² 12

22 (9)

where,lis the length of the bowl in m, andnis the number of disks

= −

Σ πnω r r

g θ

2 ( )

3 tan

2 23 13

(10) where,θis the angle of the disk.

Based on the above,Σis actually the area in m²of a gravity settler that shares the same feed rate as sedimentation [103]. For centrifugal device and angular velocity, there is a uniqueΣvalue [104]. To scale up from a laboratory test ofQ1andΣ1toQ2for (vg1=vg2) the following equation is used:

Q = Σ

Q Σ

1 1

2

2 (11)

Eq.(11)is only applicable for the same type centrifuge in terms of type and geometry and if the centrifugal forces are within a factor of two from each other [92]. However, for different types of centrifuges, the efficiency factor (E) should be used. The value ofEaccounts for deviations from the idealflow and can be estimated experimentally for each type of centrifuge. Hence, to scale up from a laboratory test for two different types of centrifuges, the following equation should be Fig. 2.The efficiency corresponding to variousflow rates and energy consumptions [83].

Fig. 3.Centrifugation cost as a result of variousflow rates (Point A refers to an increase in the oil cost attributed to the poor harvesting efficiency of the cen- trifuge; while Point B refers to the most convenientflow rate that yields the most cost-effective harvesting strategy) [83].

used:

Q = E Σ

Q E Σ

1

1 1

2

2 2 (12)

The settling capabilities of various centrifugal devices and their limiting flow rates, published by Ladislav [105], can be located in Fig. 4. These results are based on the standardflow rate application and the applicable particle sizes.

In addition, the power (P) in watts required for the whole cen- trifugation process can be calculated using the following equation [106]:

=

P ρ Qω r_p ²₂² (13)

However, the actual energy input required for the centrifugation process should take into account the various centrifuge types and the different characteristics of the algae. To address this, James [107] de- veloped a special model to evaluate the energy performance of the different types of centrifuges. The model describes a linear relationship betweenPandQto calculate the energy cost per unit volume of algae (E) in J/m³as shown in (Eq.(14)).

⎜ ⎟

= ⎛

⎝

⎞

⎠

=

E P

Q ρ ω r_p ²₂²

(14) The model also provides the energy requirement equations for various types of centrifuges based on an assumption curve considering algal separation properties, named the master curve. The curve was developed based on a set of data for three types of centrifuges namely, tubular, disc, and helical conveyor. Theflow rates obtained from these data range from 0.02 to 57 m³/h. The curve also assumes a particle settling velocity of 0.1μm/s. The settling velocity and the centrifuge flow rate taken from this master curve are called (vgm) and (Qm) re- spectively. The relationship between the actual terminal settling velo- city and the total volumetric rate of the centrifuge with those obtained from the curve is given using the following equation:

=

( )

Q Q

3600

m v v

g gm

(15) Based on the above equation, the energy requirements for disc centrifuges, and tubular and helical centrifuges are given in Eqs.(16) and (17)respectively.

= < < ⁻

E_disc 1.447(Q_m)^0.304[0.15 Q_m 14 m . hour ]^{3 1} (16)

= < < ⁻

E_t&h 2.15(Q_m)^0.405[0.06 Q_m 2 m . hour ]^{3 1} (17)

According to this model, the energy required in kWh/m³ for a specificflow rate in m³/h is shown inFig. 5[107]. The master curves shift horizontally to higher or lower actualflow rates for algae with different settling velocities according to Eq.(15). The estimated accu- racy of the master curve based on the extremeflow rate obtained from the data set is ( ± 20%) for all the centrifuges. The master curve Table 3

Energy requirement for centrifugation of microalgae.

Species Lipid Content

(%)

Final slurry concentration (%)

Energy consumption (kWh/m³)

Centrifuge type Comments Reference

Chlorellasp. – 15–20 3.30 Disc-stack centrifuge Cost is impacted by volume [19,93,94]

– – – 0.95 Evodos

centrifuge (spiral plate)

The total operating expense is 0.076

$/m³

[19]

Nannochlorissp. 26 4 1.3 – – [83]

Nannochlorissp. 26 22 8 – – [83]

Scenedesmussp.and C. proboscideum

– 12 1 Self-cleaning,

disc-stack centrifuge

– [79,95]

Scenedesmussp.and C.

proboscideum

– 2–15 0.9 Nozzle

discharge centrifuge

– [79,95]

Scenedesmussp. andC.

proboscideum

– 22 8 Decanter bowl

centrifuge

– [79,95]

C. proboscideum – 0.4 0.3 Hydrocyclone – [79,95]

Chlorella vulgaris – 22.2 8 Decanter centrifuges Assumed that

5 wt% of the algal cells would be lost

[93,96]

– – 10–20 0.8–9 – Approximately 0.1 to 2%

total solids concentration feed required.

[93,97]

Nannochloropsis salina 25 12 3.65 Solid bowl decanting

centrifuge

– [93,98]

Scenedesmus dimorphus 25 – 1 – – [93,99]

Scenedesmus acutus 27 20 1.35 Decanter bowl

centrifuge

– [93,100]

Tetraselmis suecica – – 1.2 Self-cleaning disc

centrifuges

Consumed 81.6 GJ. ha⁻¹year⁻¹for continuous daily use of 6–7 h

[93,101]

Haematococcus pluvialis 40 15–25 1 Disc-stack centrifuge – [102]

Fig. 4.Performance of various centrifugal devices [105].

approach allows the estimation of the centrifugation energy-flow re- lationship for harvesting algae that extend a range of properties.

2.4. Types of centrifuges

For the most part, centrifuges can be of different types and sizes depending on their intended use. This section aims to discuss the types of centrifuges that are used in the separation of algae biomass from the culture's water for various applications. According to Shelef et al. [88], several centrifuges that have the potential to be used in the microalgal separation process were examined [79,108–111]. The main centrifugal devices that possess the potential to be utilized in biomass harvesting as shown inFig. 6are:

•

Disc-stack centrifuge

•

Multi-chamber centrifuge

•

Tubular centrifuge

•

Decanter centrifuge (Solid-bowl centrifuge)

•

Hydrocyclone

It has been reported that some of these centrifuges were very effi- cient for the separation of algae biomass by one-step, however, others were found either inefficient or require certain slurry feed concentra- tion [88]. According to Ghosh et al. [112], other centrifuges that are used for various applications were examined for the purpose of separ- ating or concentrating algae biomass too [79,88,109,110]. However, these centrifugal devices have to be interrupted and cleaned periodi- cally, especially the ones that are based on batch-disc solid release.

Although some of the centrifugal devices have high reliability and ef- ficiency, the operational cost factor should be also kept in mind. The advantages and disadvantages of each centrifugal device are provided inTable 4.

2.4.1. Disc-stack centrifuge

The disc-stack centrifuge is the most common centrifuge used for separating algae biomass for various applications including algal bio- diesel in pilot plants [113,114]. It consists of a shallow cylindrical bowl spaced between metal discs and is suitable for separating particles with the size of 3–30μm with very low concentrations of 0.02%–0.05% of microalgae cultures up to 15% solids [113,115]. A disc-stack centrifuge has been used successfully not only to separate solids/liquids, but also liquids/liquids from each other by using very high centrifugal forces in a single continuous process [116]. This type of centrifuge has a very low separation time as a result of its ability to apply a centrifugal force 4000–14,000 times gravitational force [117]. The more dense solid particles that are subjected to such high centrifugal forces are forced outwards against the wall of the rotating bowl, while less dense liquid particles are displaced in the center. Materials of various densities are, thus, separated into thin layers, and the narrowflow channel by 0.

4–3 mm between the closely-spaced discs implies that the distance materials must travel for this separation to occur is small [115]. Disc- stack centrifuges come in several types based on the mechanism of discharge and whether the solids are discharged or retained.

The main drawback of the disc-stack centrifuge is that it exhibits higher energy consumption than the other types. Amaro et al. [118]

studied the energy consumption of Westfalia HSB400 disc-bowl cen- trifuge with a limitedflow rate of 35 m³/h and an approximate normal operating demand of 50 kW. Based on these specifications, the se- paration energy cost can reach up to 1.43 kWh/m³. In order to conduct an economic analysis for biodiesel from algae using the Westfalia HSB400, a set of assumptions was made. The disc-stack centrifuge is fed with 0.02% of the dry weight of microalgae suspension having an oil Fig. 5.The energy requirement based on differentflow rates for three types of

centrifugation (tubular, helical, and disk) [107].

Fig. 6.The various centrifugation equipment used in the biomass harvesting.

Table 4

Comparison of various centrifugal types used for harvesting of microalgae.

Centrifugal type Advantages Disadvantages Particle size separation

Final slurry concentration (%)

aRelative energy required

Remark Reference

Disc-stack

centrifuge -

High removal efficiency.

- Can work in steady

mode.

- Low dry substance

content in the discharge system.

- Mechanically

complex.

- Costly.

- Require frequent

cleaning.

- Low separation time.

3–30μm 2–15 Very high One step harvesting by

slurry feedback.

[80,88,117,120,140,141].

Multi-chamber

centrifuge -

High solids capacity.

- No loss of efficiency up to completefilling

of chambers.

- Solid recovery

laborious.

- Discontinues

operation.

- Require manual

cleaning.

- Costly.

0.1–200μm 5–20 Very high Due to its mechanical

design, the maximum-m rotation speed that can be

reached is 6500 rpm.

[88,121,142–144].

Tubular

centrifuge -

Most efficacious.

- The simplest type.

- High centrifugal

force.

- Easy to be cleaned.

- Limited solid capacity

due to its geometry.

- Intermittent cleaning

required.

Below 0.1μm N/A N/A Used mostly in laboratory

studies.

[73,88,143,145,146].

Decanter

centrifuge -

High feed solids concentration.

- Economical.

- Low centrifugal force.

- Turbulence created

by scroll.

N/A 22 Very high Requires a 2% slurry

feed.

[88,143,147].

Hydrocyclone

- Low capital cost

- Can handle large quantities of slurry.

- Relatively cheap.

- Does not have any

moving parts.

- Low effective in

reducing total suspended solids.

- Have poor reliability.

- Can be used for a

limited number of microalgal strains.

Above 30μm 0.4 Low – [88,148–150].

a For the values of energy consumption, refer toTable 3in Section 2.2.

content of 40%. This would produce 7 kg/h of dry algal material, thus, 1.6 kg of algal oil. Assuming the recovery efficiency of the centrifuge is 100%, the 1.6 kg of algal oil obtained would yield an energy density of 11.71 kWh if the calorific value is assumed to be 7.32 kWh/kg [119].

Considering a 35 m³of culture broth required to be centrifuged to obtain such an energy yield, corresponding energy consumption of 49 kWh is needed. This means that the energy consumed at the harvesting- dewatering step only is 4 times the energy produced from microalgae as biodiesel.

To improve the energy return by centrifugation, pre-concentration by separation techniques to 0.5% of the dry weight is recommended.

This would result in a dry algal material of 175 kg yielding 70 kg of algal oil, thus, 70 kg of algal biodiesel having a calorific value of 512.4 kWh. In this case, 9.6% of the energy in the biodiesel product would still be required to cover the energy consumption of centrifugation.

Other recommended steps to improve energy efficiency include the use of the entire biomass instead of just the lipid fraction for energy pro- duction, or the use of centrifuges to remove other energy-intensive unit operations in the production of algal biofuels [118].

Cell disruption and damage can occur due to the use of disc-stack centrifuges as it was reported by Milledge et al. [120]. This also can be coupled with a reduction in the overall efficiency of centrifugation and a reduced concentration of solids recovered as a result of the smaller solid particles. Based on the parameters of a disc stack manufacturer (Fig. 7) [120], a minimum size of 7μm for micro-eddies has been found to be suitable for microalgae. Hence, extensive further researches to modify the design of the disc-stack centrifuges is highly recommended.

2.4.2. Multi-chamber centrifuge

A multi-chamber centrifuge consists of tubular bowls arranged coaxially, causing each chamber to collect particles of a specific size due to the distinct centrifugal forces exerted in each chamber [121].

Theflow that passes through the tubular bowls is subjected to pro- gressively higher accelerations. The feed is presented to the tube with the smallest diameterfirst andflows through the other tubes with larger diameters. Typically, up to six chambers are connected. The solid concentration that can be achieved from this type of centrifuge can reach up to 20% [121]. However, one of the main disadvantages is that the cleaning of solids needs to be done manually, resulting in a time- consuming cleaning process [122]. Hence, this type of centrifuge does not seem to be the most convenient method for large-scale microalgae harvesting. A schematic cross-section view of the multi-chamber cen- trifuge is shown inFig. 8[105].

2.4.3. Tubular centrifuge

The tubular centrifuge is based on a simple geometry: it consists of a very long tube, several times its diameter, rotating at each end between bearings [123]. The liquid is introduced at the bottom and theflow is essentially axial except in areas immediately adjacent to the inlet and outlet. As a result of the high centrifugal force the suspension is sub- jected to, the solid particles will move outwards of the bowl while the liquid will move to the top of the centrifuge. The tubular centrifuge is considered as one of the most efficient centrifuges for particles with a cut size of 0.1μm or below at high speeds [88]. Tubular centrifuges are used primarily for difficult separations that require high centrifugal force [124]. In contrast, high separation efficiency was reported at high throughputs and low centrifugal force. For instance, Gerardo et al.

[125] reported a harvesting efficiency of 100% by using a tubular centrifuge that operates at 13,000g.

The tubular centrifuge is equipped with a long tube that has a small diameter capable of rotating at high speeds and generating high grav- itational force which can exceed 45,000 g [126]. Lars et al. [123]

carried out an experiment to harvest yeast using a tubular centrifuge with a maximum rotational speed of 45,000 rpm. The tubular

Fig. 7.Disc-stack centrifuge [120].

centrifuge is applicable in laboratory studies for bench scale algae harvesting and for the performance prediction of the disc centrifuge [88]. This could be attributed to its very simple configuration [127].

The main problem with this type of centrifuge is that it lacks a provision for solids rejection, meaning the machine needs to be stopped before the solids can be scraped orflushed out manually.Fig. 9shows a

cross-sectional view of a simple tubular centrifuge [105].

2.4.4. Decanter centrifuge

Decanter centrifuge, also known as a solid-bowl centrifuge, is a commonly used device in algal harvesting [128,129]. It consists of a horizontal conical bowl that contains a screw conveyor, which both rotates in the same direction, but at a slightly different speed [130]. The feed slurry introduced at the center of the rotor passes through open- ings within the screw conveyor and it is centrifuged against the wall of the bowl. A helical screw conveyor carries the deposited solids and discharges them out of the centrifuge at higher rotational speeds than the bowl. The decanter centrifuge is designed to handle significant solid concentration in the feed suspension [131]. Sim et al. [132] reported a dry solid content of 15% by using a decanter centrifuge to harvest sewage-grown algae. Moreover, Mohn [79] obtained a solid con- centration of 22% of separated algae from a suspension containing 2%

solids [88]. Huge energy consumption was reported to achieve this concentration. Shelef et al. [110] attempted to concentrate an algal feed of 5.5% using a solid-bowl decanter centrifugation system, however, the attempt was futile [133]. Thus, further investigations and studies are recommended to improve the decanter centrifuge. A schematic diagram that contains the main components of decanter centrifuge is shown inFig. 10(Modified) [134].

2.4.5. Hydrocyclone

A hydrocyclone consists of a cylindrical section joined to a conical base [135]. The feed is tangentially injected into the upper cylindrical portion at elevated velocity, resulting in a powerful fluid swiveling motion [135]. The fine particles contained by the fluid discharges through the overflow pipe, while the remaining coarse particle dis- charge through the underflow orifice at the cone tip [136]. The scheme and working principle of the hydrocyclone are depicted in Fig. 11 [137]. Mohn [79] studied the application of hydrocyclones for algae harvesting and reported a low solid concentration of algal slurry with incomplete separation [136]. In addition, the analysis indicates that only Coelastrum, which grows in large aggregates, can be harvested using this technology. Gregg et al. [138] reported a low harvesting efficiency for particles smaller than 400μm using hydrocyclones for the harvesting of green algae. In terms of cell damage, a hydrocyclone is predicted to cause a certain level of mechanical disruption, however, there is a lack of specialized studies on its effect for large colonial mi- croalgae [139].

2.5. The combination of centrifugation with other harvesting techniques Centrifugation can be combined with other harvesting technologies to reduce the processing cost [151]. These other technologies are used mainly to pre-concentrate the algal slurry prior to centrifugation. A significant increase in the total energy output can be achieved by combining centrifugation with other technologies as was analyzed in Section 2.4.1. Salim et al. [152] studied the combination of cen- trifugation with flocculation to harvest non-flocculating microalgae upon the addition of variousflocculating microalgae. The operational energy of centrifugation was significantly reduced from 13.8 to 0.24 MJ/kg DW upon employing the bio-flocculation/pre-concentra- tion step. Bilad et al. [153] combined centrifugation with microfiltra- tion to harvest both algae speciesChlorella vulgarisandPhaeodactylum tricornutum. The cost analysis revealed that by using submergedfiltra- tion as a pre-concentration step, the energy consumption in kWh/m³ decreased from 8 to 0.84 forC. vulgarisand from 8 to 0.91 forP. tri- cornutum. Due to its high capability of pre-concentrate, the submerged filtration method was found to be responsible for 93.3% of that low energy consumption, while centrifugation accounted for only 6.7%.

In a recent study conducted by Soomro et al. [154], 4 combinations of harvesting-dewatering techniques were proposed for comparison.

These combinations include (a) electro-coagulation with aluminum Fig. 8.Cross-section diagram of a multi-chamber centrifuge [105].

Fig. 9.Cross-section diagram of a simple tubular centrifuge [105].

anodes coupled with centrifugation, (b) alumflocculation coupled with centrifugation, (c) magnetic separation coupled with tangential flow filtration, and (d) bioflocculation coupled withfiltration. In addition, centrifugation as a single-step was used as a benchmark. Although the study aimed to evaluate the environmental impact assessment for these technologies, a significant decrease in energy consumption from these combinations was noted. The lowest energy requirements were ob- tained from the combinations (d), (c), (b), (a) respectively. However, based on the cost analysis combination (d) and (b) have the lowest cost

in $ with a total cost of $4.3 and $111.9. While the combination (a) and (c) have a total cost of $919.4 and $23,313. 83. This means that even though the combinations that contain centrifugation have a high energy consumption, they have also a relatively low cost.

2.6. Cost analysis of centrifugation

Here we assess the cost-benefit of combining centrifugation with other methods. A microalgal solution with an initial solid content of 0.02% was assumed. The volume required to increase the solid content from 0.02% to 3% was calculated as follows:

= V V C

f C

i f

i (18)

where,Vfis thefinal volume required in m³,Viis the initial volume of the solution in m³,Ciis the initial concentration (%), andCfis thefinal concentration (%).

Thus, if the initial volume of the solution that contains 0.02% solid content is assumed to be 1 m³, thefinal volume required to increase the concentration to 3% is 150 m³for all the harvesting methods. The total biomass recovered (mrec) in kg from this volume for each harvesting- dewatering method, assuming the density of the solution is 1100 kg/

m³, can be calculated using the following equation:

=

mrec V C ρRf i r (19)

where,Rris the biomass recovery rate.

The oil produced (moil) in kg assuming the microalgal species used is Scenedesmussp. with a lipid content (LC) of 21.1% [155] can be cal- culated as follows:

=

m_oil m_recLC (20)

The energy required (Ereq) in kWh for each harvesting-dewatering method can be calculated as follows:

=

E_req V E_f (21)

The energy produced (Eprod) in kWh from the oil extracted, as- suming the calorific value (CV) of the microalgal oil to be 7.32 kWh/kg Fig. 10.A schematic diagram of a decanter centrifuge. (Modified) [134].

Fig. 11.A schematic diagram of a hydrocyclone [137].

Table 5

Conventional energy requirements for the harvesting-dewatering of 150 m³of microalgal solution.

Harvesting-dewatering method mrecovered(kg) Oilproduced(kg) Erequired(kWh) Eproduced(kWh) C

($)

Ecost

($/kg)

Centrifugation 32.7 6.90 214.5 50.5 15.0 2.18

Filtration 28.7 6.06 183 44.3 12.8 2.11

Flocculation 29.7 6.27 22.5 4.57 1.58 0.25

Sedimentation 16.5 3.48 15.0 25.5 1.05 0.30

[119], for each harvesting-dewatering method can be calculated as follows:

=

E_prod m CV_oil (22)

The production cost (C) in $ for each harvesting-dewatering method, assuming the cost of energy required is 0.07$/kWh, can be calculated using the following equation:

=

C 0.07E_req (23)

Hence, energy cost per kg oil in $/kg can be calculated as follows:

=

E C

cost m

oil (24)

The values for the maximum concentration, mass recovery rate, and energy consumption for each harvesting-dewatering method is obtained from Table 1. Table 5summarizes the energy requirements for each harvesting-dewatering method in terms of the production cost.

The amount of the microalgal biomass recovered from the solution does not vary a lot for all the harvesting-dewatering methods except for the sedimentation method which appears to have the lowest value, as shown inTable 5. This is mainly because the sedimentation method possesses a very low efficiency compared to the other methods. The amount of microalgal oil produced is mainly a function of the lipid content of the microalgae species. However, the harvesting-dewatering also plays a significant role as the higher the amount of biomass re- covered, the higher the oil produced. Consequently, the amount of oil produced by centrifugation is higher than other methods due to its high efficiency which was taken as 99%.

FromTable 5, it is obvious that the energy required to harvest the microalgal solution is higher for the centrifugation method compared to the other methods due to its high energy consumption of 1.43 kWh/m³. Similarly, the energy produced from the harvested oil is also higher when using centrifugation compared to other methods because of the high amount of oil produced. The ratio between the energy required and the energy produced is 4.92 and 4.29 and for theflocculation and centrifugation respectively, whereas, it is 4.13 and 0.59 forfiltration and sedimentation respectively. This means that the sedimentation is 7 times better than the centrifugation in terms of the ratio between the energy required for the energy produced.

The cost analysis indicates that the centrifugation and filtration harvesting-dewatering methods require the highest cost as shown in Table 5. The cost required to harvest the chosen volume is 15.0$ for the centrifugation method which is very high. Contrary, theflocculation, and sedimentation cost less than the centrifugation by 9.5 times and 14 times, respectively. However, this is an improper way to compare these

methods, as this amount is associated with the corresponding oil yield.

Thus, the energy cost per kg of oil was used as afinal way to compare these methods.Fig. 12shows the energy cost for the different methods.

It can be seen, from Fig. 12, that centrifugation has the highest energy cost followed byfiltration. Flocculation has the lowest energy cost with only 0.25 $/kg followed by sedimentation with 0.30 $/kg.

Coupling centrifugation with other methods may seem the best way to improve the performance of this harvesting-dewatering technique.

The coupling of centrifugation with the other three conventional methods is studied next. In order to achieve this, it was assumed that the other three methods were used to raise the concentration of the microalgal solution to 0.3%, and then the centrifugation is used again to further increase the concentration to 5%. Based on Eq.(15), the corresponding volume required to achieve such a concentration is 2500 m³. Thus, the total amount of oil produced is 17,233.4 kg. The results and the energy cost for each combination are depicted inTable 6.

It can be seen fromTable 6, that in terms of energy cost, the three harvesting-dewatering combinations required almost the same energy cost. This is because the majority of this cost comes from the second harvesting step, which is the centrifugation that is mainly used to in- crease the concentration of the microalgal solution from 0.3% to 0.5%.

Thus, using sedimentation as afirst harvesting-dewatering step and centrifugation as a second harvesting-dewatering would produce sa- tisfactory results. Nevertheless, the sedimentation process is limited by its maximum solid concentration percentage, which is 3%. Flocculation also seems like a bad preconcentration method as their maximum solid concentration percentage reaches only 6%. Thus, for higher harvesting- dewatering concentrations, it is preferable to use thefiltration method.

This will improve the overall economy of centrifugation by approxi- mately 142%.

Although this simple analysis gives a possible solution to the high energy cost of centrifugation, it does not account for other associated costs like capital investment, maintenance, and labor. Moreover, this analysis does not account for the time required to harvest such large batches. Overall, combining centrifugation with other harvesting-

Fig. 12.Energy cost of the various harvesting-dewatering methods.

Table 6

Conventional energy requirements of various harvesting-dewatering methods combinations.

Combination C

($)

Ecost

($/kg)

Centrifugation +filtration 263.1 0.0152

Centrifugation +flocculation 251.8 0.0146

Centrifugation + sedimentation 251.3 0.0145