The In’s and Out’s of Pillow Bags
A lot of the downstream process work that I do involves 0.5l through 50l scale product batch working volumes. The typical tool I first reach for to hold the bulk product is a hanging pillow - many vendor’s make them in this size.
A standard EVA bag has two 3/8" ID tubes and a 1/8"ID sampling port. All three connections are located within 1-2" from the center of the bottom seam. One thought that always concerns people is whether the bag will leak at this point. I give kudos to the suppliers that at least I have never had this issue.
Sampling Ports
In subsequent posts, I’ll discuss sampling in more detail. Needless sampling septums do exist (I think Charter Medical is one supplier); however using a needle and syringe with latex gloves does impose significant risk to accidental puncturing of the bag. For this reason the injection port septum is attached to 1/8”ID tubing. The tubing certainly helps make the process less risky as the needle will normally rub again the tubing without puncturing. However, differing suppliers provide varying solutions to this so look at this element carefully. To be candid, I don’t like any sampling port solution that I have seen to date - at all! The product designers seem to assume you will insert and retract your sample from close to the septum – however this is typically a dead-flow area and the fluid in this region may not be representative of the bulk. The dead-end is typically 2” in length and it is hard to find needles that are sufficiently long to sample from the bulk.Dead-Ends and Corners
Discussed above was the observation that the injection port is often not a very good design solution for doing what it is meant for – sampling. But if you need to do any mixing or blending the length of the injection port means that material in the port cannot be mixed without using a syringe to repeatedly draw and expel liquid from the tube. The designers also make it convenient for themselves by locating the tubing sets at the middle of the bag. For many applications it would be better to have the inlet and outlet located at opposite corners. This would help in bag mixing and reduce the likelihood of the corner being areas of reduced mixing.
Foam
Many process solutions apart from buffers require quiet a lot of care in pumping around to avoid entrainment of air and the inadvertent production of foam. Denaturation of protein at foam liquid/air interfaces is well understood. Bags score well in this respect as their transparency makes operational errors – well transparent. But I also find that apart from where gas has been introduced, the absence of liquid/air reduces the ability of creating foam. This is clearly different from any rigid open vessel. In addition, due to the flexibility of the bag it is often possible to manual manipulate the bag to help contact the foam against the wall to help the foam get reincorporated in the bulk. For process operations with denaturing gas/liquid issues or very stable, slow draining foam – this should be a serious practical reason to consider bags.
Drainability
And another thing! Often I want to drain and recover every milliliter of fluid from the bag. In my experience disposable hanging bags are great for this. Sure I generally lose whatever is in the sampling tube, but I can possibly use a syringe for this (– often I might not want to add this to the bulk anyway). In comparison to stainless steel and most rigid plastic vessels the fluid drainability is really good. Since the disposable bags is basically two pieces of EVA or other plastic with a seam weld around the perimeter, the seam works in out favor acting as a drainage channel directing nearly every last drop towards the outlet port. If you look carefully, most bags inlet and outlet port tubing inserts extend approximately 2-3mm into the bag, so you do loose a little.
So in an ideal world a 10l hanging bag would look like this…<
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