Why bag filters may not be the best option for you.

Why bag filters may not be the best option for you.

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. Another method is a body feed, which we use on our filter trolleys and filter presses, where a filter aide is mixed into the fluid 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 original micron sizes in decreasing order, 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 caught down the line. The process stops when the first filter with coarse pores gets clogged.

We can use carbon in column in bags or cylinders, but again 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. 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.

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.

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.

Another application is when you need sub-micron (0.45 micron) filtration where a press or trolley is not an option. We typically connect a bag filter housing on the exit for of a filter trolley for polishing, especially to ensure small particles of carbon, etc… did not make it through our filtrate. Note that a cartridge filter would be better in this scenario.