Material: Polymer versus glass

Material: Polymer versus glass

The central components of a closed PBR are the containers, in which the algae cultivation takes place. These containers can be made of either transparent plastic or glass. Both materials have advantages and disadvantages that have an influence on how often the production has to be interrupted and how high the PBRs total cost of ownership is. These factors then have an impact on the production costs of the algae.

 

Materials, from which PBRs are constructed

 

Polyethylene (PE)

Polyethylene as a polymer variety is commonly used during algae cultivation in plastic bags. Bags of PE are particularly cost efficient at procurement. They typically however, need to be replaced at the latest, after one season or a year. This is due to them being easily covered by algae and their poor clean ability. The replacement is linked to material costs and lots of manual labor, and is therefore expensive.
 

Polyvinylchloride (PVC)

Polyvinylchloride is typically used for flat panel and tubular reactors. The procurement costs of these systems at a given volume are cheaper than those systems made of glass. Due to PVC not being able to transmit the entire light spectrum, algae cultivation in these systems is less productive than with other materials. Furthermore, PVC degrades very quickly under UV radiation that it needs to replaced, when being used outside, every 2-4 years.

 

Polymethylmethacrylate (PMMA)

A plastic alternative for the manufacturing of tubular reactors is the use of polymethylmethacrylate (PMMA or Perspex). Compared to PVC, PMMA has the advantage of transmitting the entire light spectrum and hardly degrading. The lifespan of PMMA under solar lighting is approximately 10 years. However, a PMMA reactor of the same volume costs more than a glass reactor, which lasts 50 years. Additionally, the issue of biofilm formation is common for plastics such as PMMA.

 

Conclusion regarding PE, PVC and PMMA

Generally speaking, to ensure that photobioreactors made of plastic can produce at the same degree of efficiency, the plastic elements must be replaced within operation comparably often. When regarding longer operation periods of the reactor, this results in an unfavourable total cost of ownership. Moreover, some polymer varieties emit substances into the algae solution, resulting in algae cultivation at food grade not being possible.


Borosilicate glass

Tubular PBRs are almost always made of borosilicate glass. This glass offers numerous advantages compared to other polymer varieties. It allows the entire light spectrum to reach the algae inside the tube. It is resistant against UV radiation, chemicals and salt water. This is the reason why tubes made of borosilicate glass are just as productive after 50 years as they are at delivery. They are also not very susceptible to biofilm formation. If this should occur at all, they can be easily cleaned.

The comparison of glass and different polymer varieties

Polymer versus glass
For further understanding: The table presents how well or poorly the alternatives perform regarding important evaluation criteria; from “very well“ (++) to “very poorly“ (--).
Fundamentally, PBRs made of glass and PMMA are comparable in their productivity, due to the light transmission being similar when new. The productivity of PMMA however, decreases more and more during its lifespan, whereas it stays consistent with glass. Due to its poorer light transmission, PVC is less productive from initial use and continuously decreases in transmission and productivity due to material yellowing. PE bags transmit light well when new and are therefore initially productive in operation, however they end up covered in algae. Moreover, the productivity suffers due to frequent production interruptions.

Learn more about this topic: How to choose the correct tubing material for photobioreactors

Benefits of borosilicate glass

 

Light transmission

  • Excellent light transmission
  • No solarisation or browning effect
  • No UV-protective additive or coating necessary to secure material properties
  • Lifetime of borosilicate glass > 50 years

Fire protection

  • Glass does not burn or give off toxic fumes

Leaching

  • Glass is a chemically highly resistant material. With plastic, depending on the polymer type, monomers or oligomers of hazardous substances such as bisphenol-molecules can be leached into the algae culture.

Cleaning

  • Mechanical stability allows continuous in-line cleaning with polymer pellets
  • Chemical stability allows cleaning in place (CIP)
  • Lower material and maintenance costs compared to quality polymer

Thermal stability

  • Tubes only: No need for expansion loops due to low thermal expansion
    Example: for 5.5 m long tubes and a temperature increase of 20 °C/ 36 °F the expansion of Borosilicate glass is only 0.36 mm/0.01’’ while polymers expand from 3.3-8.8 mm/0.13’’-0.35’’ depending on polymer type

Cost saving

  • Glass tubes can last fifty years and longer
  • Tubes only: Reduced number of racks due to high mechanical stability, which allows increased tube support distances without sagging of tubes
    Example: double distance compared to PMMA tubes
  • Tubes only: Reduced number of connections due to long tube lengths of 5.5 m

Sagging (Tubes only)

  • No permanent deformation of glass tubes in contrast to polymer tubes
  • No remaining puddles in tubes when emptying the system
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