Filtration is a function of surface area; how many square feet of filtration surface are available. The more area, other variables being the same, the more you will filter and will filter at a faster rate. Eventually the pores in your filter membrane get clogged and filtration stops, this is true for all filters (I’m overgeneralizing). We alleviate this process with a pre-coat, which means placing a filter aid over the filtration surface to create depth of filtration. Another method is a body feed, which we use on our filter trolleys and filter presses, where a filter aide is mixed into the solution you are filtering.
Bag filters are a suitable filtration option because they are compact and are sealed systems that require little manipulation. Some users believe a bag filter is a better option than a trolley or a press. The answer depends on the size of the batch to filter and the percentage of solids in the crude. We believe bags are best used when the crude oil is of top quality with low waxes, like BHO with a de-waxing column.
Though you can connect several bag filters in line with three different micron sizes in descending porosity, this will not eliminate all lipids in your crude. You will capture crystallized fats in decreasing size as the solution moves through the filters. But you will find that if you winterize your solution again, more lipids will drop out of suspension and you must filter again. To a degree, this problem negates the benefits of using 3 filters in tandem and is why we would not recommend 3 trolleys in tandem. The benefit in this setup is perhaps faster filtration given a set filter area, the small particles may pass but get caught down the line. The process stops when the first filter with coarse pores gets clogged which is fairly quickly.
Filter bags are not absolute rated. A bag can be rated at 10 microns as long as it can retain some particles that are this size. Paper on the other hand will retain all particles up to the size stated. So even if you filter down to 1 micron in a bag, you will effectively have "leakage" in your filtrate.
Many users use carbon in column in bags or cylinders, but this is not the best solution. Read our thoughts about this and how we recommend CIP instead of a carbon column.
One drawback of bag filters is the inability to pre-coat. Pre-coating helps capture smaller particles and prevents the filter from clogging prematurely by creating depth of filtration layer. You might imagine how your filter aide would settle on the bottom of the filter sock, and the solution would overflow and exit the filter sock above the filter aide.
Some users like filter bags because they are cheap. If calculated by surface area, nothing is cheaper than a filter press. A 5” diameter bag filter that is 18” long has a surface area of about 2.0 square feet. The 630mm filter trolley we sell has 3.3 square feet of filter area. A #4 bag filter has a surface area of 1.6 square feet. At $2.00 a sheet versus about $8.00 for a bag, a bag will cost about 8 times as much as a sheet yet filter roughly the same solids less efficiently.
Pressure buildup is a huge drawback to these filters. You must maintain a constant and smooth flow for these to work well. If you are not careful and monitor your pressure, your filter bag will burst and contaminate your filtrate. This seems to happen way too often and is easily preventable. Typically these filters must be replaced at a differential pressure of just 10 PSI to avoid rupture.
Once the bag is full, pressure will build up and you will know it is time to clean the bags. Removal of the bags is simple, and thankfully the bags are disposable. Cleaning the bags themselves to reuse is messy and labor intensive yet less so than a trolley.
Though a bag filter may be a great option for your particular application, we do not recommend it for carbon adsorption (use CIP instead), high volume operations (use a filter press), or crude high in fats (like CO2).
We recommend bag filters for an inexpensive closed loop option for low lipid hydrocarbon crude. Bag filters shine as a rough pass filtration stage for ethanol extracts where the fundamental particles are plant matter (Centrifuge like Delta CUP). Bags are used to capture 25 micron or smaller particles (Delta uses 73 micron bags) on a stream leading to a depth filtration module like a lenticular.