Ethanol Mixing or Agitation Specification Mistakes

Fluid and slurry agitators are some of the key items of process equipment required in ethanol manufacturing. Most plant engineers, engineering firms and operators must rely on specifications written by others on similar projects. Many of these specifications are grievously in error. For example:

• Impeller diameters and speeds specified that would not result in even minimal solids suspension.
• Combinations of motor size, impeller system and speed specified that result in less than 1% of motor power being invested. (Yes, you read that right; 1% is not a typo!)
• Bizarre impeller designs, with poorly understood power, performance and hydraulic/mechanical characteristics.
• Higher power specified than needed in some applications.
• Motor power specified without regard to impeller type or shaft speed.
• Inefficient impeller designs specified, with poor hydraulic stability.
• Radial turbines called out with up-and-down mixing specified.
• Expensive horizontal ribbon blenders specified where turbines will suffice.
• Shaft diameters specified that would break if full motor power were applied.
• Smaller shafts specified through the seal than the extension shaft, resulting in a built-in weak point in an expensive location to repair.
• Seal type left up to the equipment vendor, when fugitive emissions regulations may dictate seal type.
• Grain slurry makedown at temperatures greater than 150F/66C with no consideration given to breaking up the pasted starch balls that form.
• No differentiation of sizing based on stage in fermentation.
• Two impellers specified where one will suffice.
• One impeller specified where two are required.
• Upper impeller located above normal liquid level
• Impeller so high off bottom that flow reversal occurs, resulting in no solids suspension
• Pumping capacity specified without defining equations to be used, making it open to liberal interpretation.
• Steady bearing materials specified which are subject to rapid wear in slurry service.
• No thought given to energy costs or maintenance costs.