Part 1: Analysis of Existing Dynamic Pipeline Mixers or Line Blenders at Post Mixing

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This is a continuation of the Process Intensifier - Optimization with CFD: Part 1 paper.

Analysis of Existing Dynamic Pipeline Mixers or Line Blenders

Pete Csiszar, Black & Baird Ltd., North Vancouver, B.C.

Keith Johnson, Independent Consultant, North Canton, Oh

Thomas Post, Post Mixing Optimization and Solutions, Pittsford, NY

Abstract

More and more processes require intense agitation to achieve micromixing to minimize unwanted byproducts and improve selectivity. If this were done in conventional mixing tanks, the size of the agitation equipment would be prohibitively large in regard to size and cost. Most processes requiring process intensification require this high degree of agitation only for a short period of time.

An ideal application is therefore mixing in a horizontal pipe. The disadvantage of static mixers is the lack of degrees of freedom. It is often impractical to alter the volumetric flow rate to achieve various degrees of agitation or the addition or subtraction of mixing elements. Dynamic pipe mixers allow for the alteration of impeller speed and therefore the degree of mixing can be accomplished without altering the volumetric flow rate through the pipe. Another advantage is that it is relatively simple to remove a dynamic mixer and change the impeller type. The length of the pipe section is neither increased nor decreased, as it would be for the static mixer.

An in-depth Internet search yields only three manufactures of line blenders or in-line mixers: Lightnin, Hayward Gordon, and Plenty. Of these three, Plenty does not elaborate what is inside the pipe. From brochures and Internet descriptions, we were able to construct four commercially available dynamic pipe mixers, which we will call forthwith Process Intensifiers. They had both radial and axial impellers.

In Part 1, we describe the design of these four Process Intensifiers. Using CFD as our experimental laboratory, we put each of these to the test by studying how well they can agitate the fluid in a nominal 10" Schedule 40 pipe flowing at 1100 GPM or 250 m3/hr. Velocity profiles, flow patterns, tracer studies, pressure distributions, shear rates, power numbers, power, and resident time distributions are determined. Investigation of the CFD outputs shows "flow-flooded" impellers, dead-zones, excessive recirculation and poor residence time distributions. Suggestions on improving these designs are made.

In Part 2, we show how we took this knowledge and created an optimized Process Intensifier by focusing on minimizing the dead-zones, achieving a good residence time distribution, and creating new internals and a new impeller design which controls the flow in the locally intense mixing volume.

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