Biodiesel production

Introduction

 

Production of biodiesel has been gaining the attention during past decades. The synthesis of biodiesel involves the reaction of vegetable or animal fats and oils with short-chain alcohols. The product of this transesterification reaction is biodiesel with glycerol as a by-product. A typical biodiesel process has the following steps:

 

  • Pretreatment of the feedstock
  • Mixing of alcohol and acid-catalyst
  • Transesterification reaction
  • Separation of glycerol
  • Purification by washing
  • Water removal.

 

 

Separation challenges

 

There are two steps in this process, which have potential benefit of applying the low shear technology. These are separation of glycerol and water removal. In both processes biodiesel requires the separation from these phases. All three liquids are immiscible with each other (except biodiesel can be slightly miscible with water) and have following properties:

 

Table 1. – Liquid properties.

LiquidDensity, kg/m3Dynamic viscosity @ 20 °C, cp
Biodiesel≈ 880≈ 1,67-5,28
Glycerol12611408
Water10001,002

 

In the second step of biodiesel production, oil feedstock is mixed with alcohol (typically methanol or ethanol) and, possibly, an acid-catalyst. Intense mixing during this step ensures a more effective reaction rate between the phases. Methyl-ester (biodiesel) and glycerol are the products of this reaction. Under the influence of intense shear forces present during the mixing, the glycerol can be dispersed in very fine droplets throughout the fluid mixture.

 

A similar separation challenge is encountered in the final treatment stage of the biodiesel, where it is water washed in order to remove byproducts and residues. These residues can be glycerol, unreacted methanol, and remaining catalyst. To meet required specification of water content in biodiesel, residual water has to be separated.

 

Small-scale production units typically have a batch process of biodiesel production. Therefore, a long time settling procedure, where glycerol can coalesce and settle can be satisfactory. A simple decanter system can be used in these batch processes. Industrial scale production involves continuous process and requires advanced methods of liquid-liquid separation.  The reported set-ups in a big scale plants mainly utilize centrifuges for the separation process. Though this type of equipment can remove even smallest dispersed droplets down to sizes 1-10 μm, it is generally energy demanding with high maintenance requirements and costs.

 

In addition, centrifuges require certain inlet pressure in order to reach highest operational efficiency. Centrifugal feed pumps used for that application may cause high shear forces to the fluid flow, thus creating smaller droplets and reducing the separation efficiency of the downstream treatment equipment.

 

 

Use of hydrocyclones for biodiesel separation

 

Although there is a scarce amount of research done on using hydrocyclones for the treatment of biodiesel, ones that can be found in the literature show promising results. The development and industrial implementation of liquid-liquid hydrocyclones in the oil and gas sector happened during 80’s and 90’s of the last century. The simplicity of this equipment type, no power consumption and the low  maintenance requirements made it superior to the centrifuges.

 

Similar to the petroleum sector, the combination of low shear centrifugal pumps together with liquid-liquid hydrocyclones can be an effective and cost-efficient solution for the biodiesel production process.

 

Low shear centrifugal pumps would manage to transfer the fluid mixture without breaking-up droplets, thus ensuring high efficiency ratio for the hydrocyclones in separating the biodiesel from the impurities.

 

This technical solution might help to lower the overall expenses required for biodiesel production, which, in turn, can make this type of fuel more competitive compared to the petrodiesel.

 

 

Additional reading

 

Sawangpon, W., 2012. “Liquid-Liquid Hydrocyclone Design for Purification of Palm biodiesel”. Master Thesis, Prince of Songkla University, Hat Yai City, Thailand.

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