Unseen Power - Hydrocyclone

Hydrocyclone

What is a Hydrocyclone?
A hydrocyclone is a static device that applies centrifugal force to a liquid mixture so as to promote the separation of heavy and light components.

The hydrocyclone is a closed vessel designed to convert incoming liquid velocity into rotary motion. It does this by directing inflow tangentially near the top of a vertical cylinder. This spins the entire contents of the cylinder, creating centrifugal force in the liquid. Heavy components move outward toward the wall of the cylinder where they agglomerate and spiral down the wall to the outlet at the bottom of the vessel. Light components move toward the axis of the hydrocyclone where they move up toward the outlet at the top of the vessel.

Hydrocyclones are also related to centrifuges in that both are intended to separate heavies and lights by application of centrifugal force to liquids. The key difference is that hydrocyclones are passive separators capable of applying modest amounts of centrifugal force, whereas centrifuges are dynamic separators that are generally able to apply much more centrifugal force than hydrocyclones. Another key difference between hydroclones and centrifuges is cost. Centrifuges are expensive precision rotating machines that often need sophisticated control, whereas hydrocyclones have no moving parts and usually no controls at all so they are lower cost devices.

Hydrocyclones and centrifuges are complementary rather than competing devices. If gravity alone will settle a significant portion of your solids in a minute or two using a quick bottle test, you should investigate hydrocyclone separation. If settling takes much longer than this, then you may need a centrifuge or other separation method.

Hydrocyclone Features

Hydrocyclones offer a simple, cost-effective way to separate solids or gases from liquids or to separate liquid mixtures. Like a centrifuge, a hydrocyclone separates mixtures based on the differential densities of the mixture fractions.

However, UNLIKE centrifuges, hydrocyclones...

have no moving parts
require no separate power source
require less maintenance - none at all in some applications
have modest prices that make them affordable for many industrial processes

How They Work - a simplified explanation

A pump or other external pressure source causes the liquid mixture to flow through the hydrocyclone.

The mixture enters the hydroclone tangentially and spins in the round interior of the hydrocyclone.

Spinning causes the heavier fraction to be thrown toward the wall of the chamber. This fraction continues on a downward spiral path at the tapered wall to an outlet at the bottom apex. This outlet is known as the underflow.

The less dense fraction moves in the opposite direction, spiraling upward on the axis of the hydroclone toward an outlet at the hydroclone's top center. This outlet is known as the overflow.

Construction

ChemIndustrial makes hydrocyclones in both 316 stainless steel and chemical-resistant polypropylene. ChemIndustrial's clamped flange modularity allows great flexibility in hydrocyclone configuration, including...

several inlet geometry choices
extended body lengths to increase separation contact time
right and left-handed models
simple integration of heavies handling options such as accumulation chambers and automatic valves.

Skid pipework is available in polypro or 316. Clamps are 304 Stainless while gaskets are either EPDM or Viton®. Skid frames usually are built from 304 Stainless tube.

Other materials may be available on special order.

Hydrocyclone Arrays

Multiple hydrocyclones can be manifolded into...

parallel arrays for higher flow rates
valved parallel arrays to allow manual or automatic rate adjustment
series arrays for multistage or multiphase recovery

Limitations

Hydrocyclones are surprisingly effective at separating mixtures into heavy and light fractions. However, they are not perfect separators. Here is a discussion of some of the limitations and some ways of dealing with them:

Hydrocyclones don't work for all separations.
Even when hydrocyclones work well, it is rare for them to achieve perfect separation. Expect some heavies in the overflow and always some lights in the underflow.
Hydrocyclones are not good at handling variable flow rates. The reason is that flow variations cause the spin rate in the hydrocyclone to vary. This leads to variations in the g-force that drives the separation.
The best way to combat this problem is to run each hydrocyclone at a constant flow rate which is also the optimum flow rate for the separation task.
If the rate must vary, the proper strategy is to install multiple smaller valved hydroclones instead of a single larger one and turn the small ones on and off as required to satisfy the current flow.
ChemIndustrial's staged valved parallel arrays of hydroclones provide a way to address this problem.
Underflow issues:
- Variation in the percentage of heavies leads to less efficient separation. This is because the underflow orifice must be big enough to flow all of the separated heavy material at maximum heavies concentration. When the heavies concentration is less, the orifice still passes the same volume with extra light material making up the difference.
  ChemIndustrial may be able to provide a process control solution to this problem where cost is justified.
- Low concentration of solid heavies requires the underflow orifice to be very small. This can lead to bridging and plugging of the orifice.
  An accumulation chamber addresses this problem by providing an engineered semi-isolated holding zone for "heavies". The accumulation chamber can be discharged periodically by a timed blowdown cycle, or under the control of a turbidimeter or other sensor.