Blend Plant Material Balance

[RES3001]

At one of our facilities We process millions of pounds of blended product per year. The product is a blend of different resins that we receive via rail and tank car. The resins are received hot via tank cars and simply pumped into heated storage tanks. The resins received by rail are heated and then pumped off into other heated storage tanks. Various amounts of each resin are pumped into stirred heated "kettles" and blended together. During this process serveral additives (depending on the product being made) are added to the kettle which melt and blend in with the resins. After a certain period of time the finished blended product are pumped to finishing lines and processed into pellets which are packaged into super sacks (2,000 pounds each) or poly bags (55 pounds each). It is a very basic process and the material balance is simple. After performing a material balance around the process, we find that there is actually a 2% loss (shrinkage). At first glance, it seems reasonable but 2% of millions of pounds adds up and when looking at dollars it adds up to over $400k per year. When the process was built, the resins were very inexpensive but since they are petroleum based, the cost has risen steadily. Also, when the line was fisrt installed, there was little competition and margins were much better. Competition and rising raw material costs has slowly eroded margins and 2% loss has become a focal point. Has anyone that has read this have experience with batch blending processes that can share how much loss or shrinkage they've experieinced and offer an opinion on what is "realistic and reasonable" to expect for such a process?

 

[25362]

I have no experience with resin blending, but have some on weighing. Questions: could the accuracy of bulk weighing be [±] 2%? Are the scales duely calibrated?

 

[astabell]

Hello RES3001,

When I worked in a resin production and paint facility our yields would vary from product to product, mostly dependent on solids loading and viscosity. It sounds like the resins you are blending are fairly viscous, as you are heating them before transfer. 2% is not unreasonable yield loss when dealing with unheated viscous >10,000cp resins. I also agree with what 25362 has to say about scale precision, however, scale resolution should average out, hovering around your actual yeild loss.

There are ways to reduce yield loss in viscous resin systems, by insulating and heat tracing lines or reducing the rough edges the products see. I've seen gains of 0.5-1% yield by eliminating NPT style fittings, which tend to cause dead zones where denser product may accumulate.

Another thing I would look into would be if vapor is being lost during your blending process. Many resins have a significant volatile component, which could be going up your stack when you blend. This effect would be increased by heating the blend tanks.

This may seem obvious, but I'm going to mention it anyway, because I've seen the mistake done before- If yield is being measured in any point in the process by volume instead of by weight (for example, by using a radar tank level gauge) significant error is introduced into the system due to temperature dependence of density.

Good luck!