A continuous blending system is like a very long batch in that they both have a startup, a run time and a shutdown. Benefits over batch blending include elimination of sequential addition, in-line dilution and a smaller footprint for reactions.
Less intermediate and final product storage is required, reducing in-process inventories. Higher-concentration raw materials can be diluted as part of the continuous blending process, resulting in smaller storage and delivery systems for those materials.
Quick changeovers and just-in-time product manufacturing are additional benefits. However, these benefits will be difficult to realize or keep consistent without proper design and maintenance of the continuous blending system.
This article addresses maintenance issues for the supply, metering, mixing, analysis, general layout and control components of continuous blending systems. See the figure.
Raw material delivery systems can vary greatly and must be addressed for each material. Recirculation-type supply systems use pumping and backpressure control, introducing mechanical energy into a system. This energy can turn into heat. Some materials can be heat sensitive and lose potency. Other materials might require heating to keep from freezing.
Positive displacement pumping is used when a raw material should not be agitated or mixed too much. Some materials require low flow rates to keep particles in suspension or solutions from settling out.
Material can precipitate and build up at recirculation points in storage tanks. Chunks then can fall back into the flow stream and cause blockage in meters and valves. Periodic cleaning could be required. Strainers can prevent the blockage.
More than one liquid blending system can share these delivery systems. The supply systems should be sized larger than the anticipated demand of the connected systems. The delivery system can absorb flow and pressure disturbances more easily as connected blending systems are started up and shut down.
A good rule of thumb for a recirculation-type supply is four times the demand. The idea here for a supply system is very much like a kitchen faucet: Open the valve and raw material is available. Starting flow to a blending system should not cause a large bump in delivery system operation. Fluctuations in the supply will cause variability in the metering to all connected running systems and will adversely affect product quality.
Delivery system flow or pressure control should be tuned relatively slow so compensatory control moves are slow and do not upset the metering loops. Leave the higher-speed control to the metering loops.
Production demands for formulations can over-draw from a supply system, causing delivery problems. Systems and formulations should be timed to avoid these situations. The alternative to scheduling is to increase the capacity of the system to meet the demands.
Liquid metering typically involves a flowmeter and flow control valve combination. This step is the "make" or "break" for continuous blending. Meter calibration can drift not only as a result of wear and age, but also as a result of raw material buildup. Very small metering valves can become partially blocked by contaminants or raw material precipitates,creating annoying control problems and product variability.
Valve performance can deteriorate if the equipment is not properly serviced and maintained. A sticky valve stem can cause the controller to cycle around its setpoint because the valve cannot achieve the position required for the desired setpoint.
This valve stiction can cause the desired flow to be a very difficult target to hit. It can make the problem look like loop tuning. A tuner will probably slow the loop down to an unacceptable level to try to minimize overshooting problems, lengthening the loop cycle time while still not resolving the problem.
To identify this situation, place the control loop in manual mode, and move the valve in small (1 percent or less) increments. If the valve does not move with each small increment but does with one larger movement after enough adjustments are made, this valve should be serviced. An acceptable level of valve stiction typically should be 1 percent or less.
The flexible operation of continuous blending systems comes with a price. Formulations can show up after a system has been running that require metering or some other part of the system to run outside of its design range. Valves can be required to support too large of an operating dynamic range for the various formulations. Former good quality might not be met.
Many processes, including blending, are installed with equal percentage valves as a default trim type. An equal percentage valve has an initially slow opening characteristic, and the further it opens the faster it opens. This feature provides good control under increasing backpressure conditions as a result of the increased flow. But it also gives equal percentage valves a large gain change over its range that can affect loop tuning.
Loop tuning of product addition should be optimized for operational conditions. Lambda tuning techniques are a good fit here because they make many loops with dissimilar physical response characteristics behave the same. If all metering runs respond the same to process upsets, the benefits to product quality are obvious. See the sidebar for more information on this tuning method.
The mixing manifold sets the order of addition. Pipe lengths and flow rates can be set to allow residence times or reactions to occur before the next materials are added.
Once the materials are in the same pipe, the introduction of turbulence by static mixers or powered dynamic mixers can homogenize the ingredients or promote reactions. Mixers typically are sized for minimum effective flow rates. Running below this range can cause quality problems. Also, maintenance of these mechanical components can be a challenge if the product is prone to buildup or the components cannot be cleaned efficiently.