Why is a tubular photobioreactor with glass tubing significantly better than one with plastic tubing?
Tubular photobioreactors (PBR) exist in laboratory and production sizes, as they are the only systems that are able to produce high quality biomass for cost efficient production of high-end algae products such as DHA, astaxanthin and spirulina. The containers of tubular reactors can be made of different materials. Either disposable polymer or sustainable glass can be used.
In industrial operation, plastic tube systems offer no significant advantages over glass tubular PBRs. There are however, serious disadvantages. This explains the extreme rarity of plastic reactors production on a larger industrial scale.
Below you will find the most important reasons why a tubular PBR with glass tubing is significantly better than one with plastic tubing:
Reduced biofilm build up
Glass and polymer tubing show differences in the surface conditions of the material. The effect is that algae settle much faster on the inside of plastic than glass tubes. This biofilm reduces light transmission and therefore the productivity of the tube. It also increases the risk that bacteria will settle in and contaminate the algae solution.
Easier to clean
In connection with increased biofilm formation in plastic tubes, it is made even more difficult as tubes made of polymer are more difficult to clean. Mechanical cleaning methods such as pigging or using plastic pellets are out of the question because they would scratch the plastic tubes. These scratches would then again be places where biofilm formation could be more pronounced.
Stable to temperature effects
Glass is relatively stable to temperature effects. Considering typical PBR-level tube lengths of 50 meters, a temperature fluctuation of 20 degrees Celsius brings about a mere 2.2 mm length variance over these 50 meters. For plastic solutions, the situation is very different: length variance is 10 to 20 times greater than that of glass, depending on the type of plastic used. This complicates the construction of a plastic system, as the tubes must be able to move throughout the day must also be accounted for.
No sagging due to higher rigidity
Tubing made of different materials also exhibits different stiffness factors. Glass tubing is significantly more rigid than polymer tubes. Lower rigidity or stiffness of the polymer tubes leads to sagging of the tubes when they are installed horizontally. Sagging of the tubes leads to problematic consequences: if you want to drain the cultivation solution from the PBR to harvest algae or clean the system, pools will be left behind in the sagging areas.
5 to 50 times longer service life
Three main factors influence the service life of a PBR tube in outdoor operation:
- How stable is the material against UV radiation and chemical influences?
- How easily does the material scratch?
- How stable is the material in its resistance to deforming?
Glass tubes are hardly affected by these influences. As a result, they are able to offer consistent productivity without restrictions for up to 50 years of use. Plastics, on the other hand, are not very resistant to these factors. In comparison they age relatively quickly under environmental conditions and have to be replaced frequently: PMMA tubes after 8-10 years, and often much earlier. PVC tubes after 1-3 years.
Lower total cost of ownership
Plastic tube PBRs have a significantly higher total cost of ownership than glass PBRs. The main reasons for this are high replacement costs for degraded plastic tubes along with higher costs for cleaning.
Due to their comparable light transmission, PBRs made of PMMA and glass have similarly excellent productivity when new. However, PVC produce at least 25% less algae than glass due to the absorption properties of plastic. In contrast to glass systems, plastic systems also have to contend with factors that reduce their productivity as they continue to operate. In the case of plastics, light transmission typically deteriorates over time. This is partly due to UV radiation (especially in PVC) and partly because more and more biofilm builds up, that increasingly becomes more resistant to being removed by cleaning. In addition, plastic systems must always be completely stopped from operation during cleaning, which also has a negative impact on the system’s productivity.
This article is an extract from our e-book “Algae cultivation in plastic tubes compared to tubular glass PBRs“. If you would like to inform yourself in detail about the advantages and disadvantages of plastic tubing in algae cultivation, you can download the e-book for free.