I have been assigned to manufacture an evaporator for liquid phase of digestate originating from biogas plant. My inputs are that liquid digestate contains around 7.7% of dispersed solids and the biggest particle is maximum 500 micron in diameter (size of perforation on a screw press sieve that performs primary solid-liquid separation). My goal is to try and decrease percentage of solids entering the heat exchanger as much as possible or economically viable so to diminish heat transfer surface fouling problems. Which filtration options do you find most adequate for aforementioned task? Before mechanical screw press separation I have from 12% to 18% of solids and after mechanical screw press separator I get from minimum 4 to maximum 7.7% of solids in liquid phase. Solid phase after separation has 27%-33% of solids and I would like to evaporate enough water from the liquid phase so to achieve similar percentage of solids (27%-33%) in the liquid phase as well. I want to do it through evaporation. Since my goal is to obtain an end product (solid and liquid phase mixed after evaporation) with around 30% of solids, and I can not do that directly through evaporator (due to fouling of inner heat exchange surfaces of evaporator), I decided to raise percentage of solids entering the evaporator only to around 14% which proved to be maximum practical limit. Because I want to evaporate as much water as possible and I'm limited with 14% solids (we have enough thermal energy though) it is very important to maximally lower percentage of solids prior entering the evaporator. This way when I reach maximum of 14% of solids in the evaporator I will have great portion of water already evaporated and only small amount will remain. The lower percentage of solids entering the evaporator the smaller portion of water remains when 14% of solids is reached after evaporation. This is OK for me since this evaporation product being concentrated to 14% of solids will be mixed with the mechanically separated solid phase (27%-33% solids) afterward. Weight ratio between solid phase (30% solids) and liquid phase (14% solids) will be tipped heavily towards solid phase so adding this 14% liquid phase should get me only slight decrease of percentage of solids in solid phase (from 30 to around 29%). Therefore I need as effective filtering of solids as possible prior to entering the evaporator. Just to point out that we did try to raise percentage of solids in the evaporator but it could have only been done in scraped surface evaporators which are costly to produce and costly to operate so I can not use them. What can I do to lower these maximum 7.7% of solids to lets say 0.1% (if I do that, then my math of lowering 30% of solids in the end product to 29% is OK)?
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Ernest Mayer Forum Moderator 52 Posts
Re: How do I decrease percentage of solids entering the heat exchanger?22 November 2011 at 1:29pmIf the screw press only reduces solids from 12%-18% to 4%-7.7% solids, which is terrible, I would suggest replacing it with a filter press.