Difference between revisions of "Water prewash"

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<metadesc>Water prewashing biodiesel to help with water washing stages</metadesc>
 
==Introduction==
 
==Introduction==
A water prewash is a way to give your biodiesel a preliminary wash after the reaction is finished but '''BEFORE''' draining off the glycerol byproduct. Performing a water prewash stops the transesterification reaction dead in its tracks by drawing much of the water-soluble methanol and catalyst out of the biodiesel and into the glycerol layer. This can help reduce soap, reduce the chance of emulsion formation, and make further washing easier.
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[[File:7 percent wash comparison.JPG|400px|thumb|right|Comparison between water pre-wash and no pre-wash.  Both samples are from the same, soapy batch about two weeks after production. The one on the left was taken before water pre-wash and the one on the right, after. Note the soap layer on the glycerol and the gelled soap in suspension in the left hand sample.]]
  
After experimentation using 7% water for the prewash was found to give the best result, but it will still work well with 5%.
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A water prewash is a way to stop the biodiesel transesterification reaction and reduce the soap content of the processed biodiesel. This makes further washing easier as it reduces the chance of forming emulsions.
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==Advantages of water prewashing==
 
Washing has traditionally been thought of as one of the more challenging, time-consuming aspects of making biodiesel. For people who use the 7% water prewash method, this is no longer the case. Most people who have used the 7% water prewash method report that their biodiesel is much easier to wash afterwards.
 
  
Although this technique may help you save wash water overall, be aware that it will add water to your glycerol byproduct and therefore may make it harder to recover the methanol from it. If you don't recover methanol from your glycerol, carry on.
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Water is added to the processor and mixed while the glycerol byproduct is still present in the mix. When the glycerol is drained, the water comes with it leaving relatively dry biodiesel.
  
Water prewashing can also be used prior to [[Dry Washing]] the fuel.
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==Advantages and disadvantages of water prewashing==
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Washing has traditionally been thought of as one of the more challenging, time-consuming aspects of making biodiesel. Water prewashing makes biodiesel much easier to wash afterwards, and reduces the overall amount of wash water required.
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The disadvantage is that the drained glycerol will contain most of the water, which makes methanol recovery difficult - it is unlikely that the resulting methanol will be pure enough to reuse.  In addition the remaining biodiesel will be slightly damp, so methanol recovery from that will also produce methanol that may not be pure enough to reuse.  Neither of these disadvantages are a concern, however, if you don't plan to recover methanol as part of your process.
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The more water added, the more soap is removed - but also more byproduct is produced.  Beyond 7% (of the original oil volume) little extra soap is removed.  Many homebrewers use 5% water washes.
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Water prewashing can also be used prior to settling and [[dry washing]] the fuel, though the benefit of doing this is not so clear.
  
 
==How to water prewash==
 
==How to water prewash==
Start by running the biodiesel reaction in your processor as normal. Process as usual until your batch is finished, but '''BEFORE''' turning off your pump or mixer, add in your 7% water prewash and continue mixing. Here are the details for the two most common base reactions:
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Start by running the biodiesel reaction in your processor as normal. Process as usual until your batch is finished, but before turning off your pump or mixer, add in the 7% water prewash and continue mixing. Here are the details for the two most common base reactions:
  
 
===Single stage base reaction===
 
===Single stage base reaction===
 
Process as usual until your batch is finished but keep your pump or mixer running.
 
Process as usual until your batch is finished but keep your pump or mixer running.
  
With your pump or mixer still running, add 7% water (that's 70ml per litre of water for every litre of liquid in  the processor---oil plus methoxide).
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With your pump or mixer still running, add 7% water (that's 70ml of water for every litre of oil being processed - not including methoxide).
  
 
After you have added in the 7% water, continue pumping/mixing for an additional 15 to 20 minutes.
 
After you have added in the 7% water, continue pumping/mixing for an additional 15 to 20 minutes.
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===Two-stage 80/20 base reaction===
 
===Two-stage 80/20 base reaction===
 
Process as usual until your batch is finished but keep your pump or mixer running.
 
Process as usual until your batch is finished but keep your pump or mixer running.
With your pump or mixer still running, re-introduce the glycerol you drew off at the end of Stage 1 back into your processor PLUS 7% water (that's 70ml per litre of water for every litre of liquid in the processor---oil plus methoxide.)
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With your pump or mixer still running, re-introduce the glycerol you drew off at the end of Stage 1 back into your processor PLUS 7% water (that's 70ml of water for every litre of oil being processed - not including methoxide).
  
 
After you have added in the Stage 1 glycerol PLUS the 7% water, continue pumping/mixing for an additional 15 to 20 minutes.
 
After you have added in the Stage 1 glycerol PLUS the 7% water, continue pumping/mixing for an additional 15 to 20 minutes.
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Using your usual technique, wash your biodiesel until the wash water is clear. With the 7% water prewash you may find that you can use a faster, more vigorous washing technique with less risk of causing an emulsion. You may also find you need to wash your biodiesel just once or twice instead of three, four or five times it previously needed. This technique can save you a lot of time, water and aggravation!
 
Using your usual technique, wash your biodiesel until the wash water is clear. With the 7% water prewash you may find that you can use a faster, more vigorous washing technique with less risk of causing an emulsion. You may also find you need to wash your biodiesel just once or twice instead of three, four or five times it previously needed. This technique can save you a lot of time, water and aggravation!
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''Credit to Alistair (aka HCII) for determining the effects of using different water wash quantities''
 
[[Category:Biodiesel]]
 
[[Category:Biodiesel]]
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[[Category:Processing methods]]

Latest revision as of 19:57, 27 February 2013

Introduction

Comparison between water pre-wash and no pre-wash. Both samples are from the same, soapy batch about two weeks after production. The one on the left was taken before water pre-wash and the one on the right, after. Note the soap layer on the glycerol and the gelled soap in suspension in the left hand sample.

A water prewash is a way to stop the biodiesel transesterification reaction and reduce the soap content of the processed biodiesel. This makes further washing easier as it reduces the chance of forming emulsions.

Water is added to the processor and mixed while the glycerol byproduct is still present in the mix. When the glycerol is drained, the water comes with it leaving relatively dry biodiesel.








Advantages and disadvantages of water prewashing

Washing has traditionally been thought of as one of the more challenging, time-consuming aspects of making biodiesel. Water prewashing makes biodiesel much easier to wash afterwards, and reduces the overall amount of wash water required.

The disadvantage is that the drained glycerol will contain most of the water, which makes methanol recovery difficult - it is unlikely that the resulting methanol will be pure enough to reuse. In addition the remaining biodiesel will be slightly damp, so methanol recovery from that will also produce methanol that may not be pure enough to reuse. Neither of these disadvantages are a concern, however, if you don't plan to recover methanol as part of your process.

The more water added, the more soap is removed - but also more byproduct is produced. Beyond 7% (of the original oil volume) little extra soap is removed. Many homebrewers use 5% water washes.

Water prewashing can also be used prior to settling and dry washing the fuel, though the benefit of doing this is not so clear.

How to water prewash

Start by running the biodiesel reaction in your processor as normal. Process as usual until your batch is finished, but before turning off your pump or mixer, add in the 7% water prewash and continue mixing. Here are the details for the two most common base reactions:

Single stage base reaction

Process as usual until your batch is finished but keep your pump or mixer running.

With your pump or mixer still running, add 7% water (that's 70ml of water for every litre of oil being processed - not including methoxide).

After you have added in the 7% water, continue pumping/mixing for an additional 15 to 20 minutes.

Two-stage 80/20 base reaction

Process as usual until your batch is finished but keep your pump or mixer running. With your pump or mixer still running, re-introduce the glycerol you drew off at the end of Stage 1 back into your processor PLUS 7% water (that's 70ml of water for every litre of oil being processed - not including methoxide).

After you have added in the Stage 1 glycerol PLUS the 7% water, continue pumping/mixing for an additional 15 to 20 minutes.

When you are finished mixing the prewash, settle the batch as usual in your processor or after pumping into your settling tank or wash tank.

After the batch has settled, drain off the glycerol byproduct. Now that it has 7% water mixed in with it, your glycerol byproduct will be much thinner than usual and there will be more of it.

Using your usual technique, wash your biodiesel until the wash water is clear. With the 7% water prewash you may find that you can use a faster, more vigorous washing technique with less risk of causing an emulsion. You may also find you need to wash your biodiesel just once or twice instead of three, four or five times it previously needed. This technique can save you a lot of time, water and aggravation!

Credit to Alistair (aka HCII) for determining the effects of using different water wash quantities