Università degli Studi di Roma “La Sapienza” Facoltà di Scienze Matematiche, Fisiche e Naturali

Corso di Laurea Magistrale in Biotecnologie Genomiche, Industriali e Ambientali

STUDY OF THE MASS TRANSFER AND MIXING IN AN OPEN

PHOTOBIOREACTOR WITH LOCAL RECIRCULATION

Relatore interno: Prof.ssa Francesca Pagnanelli Candidato: Augusto Piazza Relatore esterno: Prof. Marco Bravi Matricola:1628769

Commercial uses of microalgae

Microalgae: large-scale production

Open ponds

Tubular photobioreactor

Photobioreactors made with low-cost materials

The problem in large-scale cultivation

biomass > 1 g/lh* max =1 cm

h*

h*

I: Photoinibition -> photon excess has dissiped by heat = stress = no replication

II: Photolimitation -> under 30 uE light is not sufficient to substain the growth = no replication

The problems of photosynthesis in no autotrophic microalgae

Optimum light exposition region

How to work around the problem?

Microalgal productivity is strongly limited by the so called “light saturation effect”

Microalgal cultures have been shown to exhibit a highter photosyntetic activity when a luminous energy exceeding the saturation is supplied in hight frequency pulses rather than in a continous flow.(Kok, 1957)

Grobbelaar (2009) observed that photosynthetic rates increase exponentially with increasing light/dark alternation frequency

Photoinibition and photooxidation would be greatly reduced by a proper combination between cell density (making culture layers away from the surface progressively darker) and culture mixing (repeatedly circulating cells from brighter to darker zones).

light:dark frequency period: a possible solution

●Millisecond flashes of high intensity light, followed by an approximately five- to ten fold longer dark period could enhance photosynthetic efficiency.

LIGHT PERIOD

Fast reduction of e- acceptors QA and QB in the PSII

DARK PERIOD

Fast oxidation of QA and QB

MAXIMUM PHOTON ACCEPTING CAPACITY OF PS II

A new PBR geometry design proposed: what there is behind

0.5 s

Aim of the thesis1. To Investigate the mass transfer and mixing of the open

wavy photobioreactor;

2. To analyze the suitability of the open wavy photobioreactor for the production of spirulina biomass by using a semi-continuous culture;

3. To analyze compositional characteristics of the obtained biomass and of the residual exhausted medium, containing exopolysaccharides.

1. Evaluation of mass transfer

Operative conditions:

• Flow rate: 600 l/h e 900 l/h

• Temperature: 18°-25°C

• Degree of grade: 6° e 9°

• To calculate the Kla was used a stripping with N2 method then the water was oxygenated with air;

•Then was used the relantionship that there was between a semi-continual reactor in time with a continual reactor in stationary phase;

Evaluation of KLa

 

Grade Flow

rate

Cavity

Last First

° (degrees) (L/h) (mg/L)

6 600 I test 2,31 2,04

6 600 Replica 2,63 2,42

6 900 I test 2,7 1,69

6 900 Replica 2,46 2

9 600 I test 2,85 1,96

9 600 Replica 2,80 2,05

9 900 I test 2,69 1,92

9 900 Replica 2,77 1,99

Flow rate

(l/h)Residence time

(s)

600 9900 6

Results

When the installation angle is lower , the higher flow rate increases the value of KLa.

Grade Flow

rate

Kla

( 18°C)

Kla

( 25°C)

CV

(18°C)

CV

(25°C)

(°) (L/h) (s-1) (s-1)  

6 600 4,2E-03 12 E-03 2% 3%

6 900 1,6E-02 34 E-03 18% 30%

9 600 1,7E-02 23 E-03 12% 26%

9 900 1,8E-02 49 E-03 3% 12%

Results

•At 9 ° the effect of the increase in flow rate is negligible If the flow rate increases the change of the angle is negligible

Grade Flow

rate

Kla

(18°C)

Kla

(25°C)

CV

(18°C)

CV

(25°C)

(°) (L/h) (s-1) (s-1)  

6 600 4,2E-03 12 E-03 2% 3%

6 900 1,6E-02 34 E-03 18% 30%

9 600 1,7E-02 23 E-03 12% 26%

9 900 1,8E-02 49 E-03 3% 12%

Comparison with the kLa in the PBR actually used●If kLa for a particular system is small, the ability of the reactor to deliver CO2 to the

cells is limited.

PBR kLa (s-

1)References

Concentric tube airlift

2 ×10-2 Contreras et al. (1998)

Bubble-column

1.7-4.7×10-3

Merchuk et al. (2000)

Inclinated tubular

3 ×10-3 Ugwu et al. (2002)

Flat-plate 3×10-3 Zhang et al. (2002)

Split-cylinder internal loop

9×10-3 Vega-Estrada et al. (2005)

Grade Flow

rate

Kla

(18°C)

Kla

(25°C)

CV CV

(°) (L/h) (s-1) (s-1)  

6 600 4,2E-03 12 E-03 2% 3%

6 900 1,6E-02 34 E-03 18% 30%

9 600 1,7E-02 23 E-03 12% 26%

9 900 1,8E-02 49 E-03 3% 12%

Evaluation of mixing time●Mixing time is used to establish the time required to homogenise a

bioreactor to an assigned degree (e.g. 90%) ●We used a strong electrolite: KCl 0.01 M

Circulation time

Ultimate value of property

Evaluation of mixing time

*Entire circuit

The LRBR is an upstream- and downstream-open bioreactor element that needs to be put in a closed circuit to be characterised.

To determine the mixing time of the wavy element the effect of the recirculating circuit should be separately determined and discounted from the above

Evaluation of mixing time

2. Analyze the suitability OPBR for the production of spirulina

Reference was made to the base case of a Spirulina culture carried out in a cylindrical photobioreactor to discuss the obtained results

Vertical cylidrical photobioreactor

●Diameter: 16 cm

●Height: 58.5 cm

●Volume: 6.5 l

●Air flow rate 130x 103 Nm3/h

●The cylindrical PBR was stored in a cabin 60 (l) x 70 (b) x 145 (h) cm completely covered with aluminum

film.

Wavy bottomed open PBRTwo slides with the following characteristics :

Length of each cavity =7,23 cm

Total length of exchange = 7,23*12= 86,76 cm

Useful length of the photobioreactor = 14,5 cm

Total amount of surface of exchange S=1258 cm2

Volume of each cavity = 1,25*104m3

Total cavity volume l=1,25*103m3

Growing conditions in the two photobioreactors Medium: Zarrouk’s medium

Lighting: 4 fluorescent lamps to cool white light(400-700 nm, 865 K, 32 W, 80 mmol fotoni m2s-1 )Photoperiod of 16h light and 8 of darkness;

PH= 10.6Conductivity=16.27 mS

Temperature: Ambient temperature

Spectrophotometric assays

●To quantify the presence of carbohydrates and proteins in the mass and in the supernatant were used the following colorimetric assays:

●Dubois (1956) : The assay is based on the phenol-sulfuric acid method to quantify the presence of carbohydrates in the mass e in the supernatant.

● Lowry (1951): The method combines the reactions of copper ions with the peptide bonds under alkaline  conditions with the oxidation of aromatic protein  residues.

Results: Spirulina semi-continual growth in the PBR’S

Cylindrical PBRAbs

Time (Days)

°C

Time(days)

Abs

Wavy-bottomed PBR

OD 690 Temp (°C)

Statistic analysis Temperature difference between the two PBR

●Test F for the Variance

C PBR WbPBR

mean 25.83 23.82

var 4.15 4.04

obs 39 39

dof 38 38

f 1.027

pvalue 0.467

Critic f 1.72

C PBR WbPBR

mean 25.83 23.82

var 4.15 4.04

obs 39 39

tot var 4.1

dof 76

T stat 4.39

P value 1.8 E-05

Critic t 1.66

• T test

Spectrophotometric assays on the mass: results Cylindrical PBR Wavy bottomed PBR

Carbohydrates Proteins

021

628

845

655

262

476

884

093

610

0811

0411

760%

10%20%30%40%50%60%70%80%90%

100%

0

5

10

15

20

25

30Biomass analysis

gC

om

pu

on

d/g

bio

mas

s %

°

Time (hours)

028

855

276

893

611

040%

10%20%30%40%50%60%70%80%90%

100%

0

5

10

15

20

25

30

35Biomass analysis

gC

om

pu

on

d/g

bio

mas

s %

°C

Time (hours)

°c

Relation between composition and temperature

●Wavy-bottomed pbr Cylindrical

3 Exopolysaccharides production°C

°C

Wavy-bottomed PBR

Cylindrical PBR

0 200 400 600 800 1000 1200 14000

2

4

6

8

10

12

Productivity

(gEPS/gbiomass) u

Time( hour)

0 200 400 600 800 1000 1200 14000

2

4

6

8

10

12

Productivity

(gEPS/gbiomass) u

Time( hour)

Conclusions (1)

• The open local recirculating photobioreactor object of the thesis has been characterized in its geometry by studying the mass transfer and the mixing:

• The results obtained are comparable with those of photobioreactors currently in use.

• Through its peculiar characteristics of recirculation and its geometry feature it is possible to limit the effects due to photoinhibition and photolimitation .

• The open PBR is more sensitive at winter temperature but in summer this is a solution for the termal shock

Conclusions(2)

At low temperature, increasing the inclination brings the mass transfer

coefficient to an upper limit. At the same time:

●local recirculation is lost, and so is the photobiologic advantage

●for the same length of the photobioreactor, a higher elevation should be

climbed up by the external recirculation device, likely requiring a pumping

device which causes a higher stress to the culture.

Conclusions(3)

At low temperature, increasing the inclination brings the mass transfer coefficient to an

upper limit. At the same time:

●At medium temperature, increasing the inclination boosts the mass transfer

coefficient (x 2) and increasing the flow rate boosts it also (x 3). At the same time:

●steeper slope advantageous, but local recirculation is lost, and so is the photobiologic

advantage

.

Thanks for the attention

green walls and PBR with internal light a real solution???

Yes but…...