Do Your Plant a Good Turn

By Guy Weismantel and Ernie DeMartino

In response to pressure on the bottom line, many chemical companies have virtually eliminated personnel training at the plant level. As a consequence, expertise about centrifuge technology has dwindled, if not disappeared, at many sites. The outsourcing of maintenance tasks has added to the problem because most contract operators have virtually no centrifuge knowledge. The situation is not likely to change in the near future. This loss is all the more debilitating because engineering schools do not include centrifugation as part of their normal curriculum.

It is not unusual to enter a facility and see one or more centrifuges lying idle. Oftentimes they are not functioning properly and no one on staff has the ability to repair them. There are no easy answers, but a better understanding of the factors that affect centrifuges is essential to getting the most out of these units.

Effective centrifuge control requires online monitoring of each infeed and outflow to regulate machine operating parameters, which are tied to product quality and production requirements. This can require a control model that involves measurement of both liquid and solid characteristics , or, in the case of three-phase centrifuges, even more sophisticated hardware. A process that experiences viscosity changes adds to control complexities.

With a sophisticated piece of equipment like a centrifuge, adequate mechanical monitoring also is essential to spot incipient problems and to maximize a machine's mean time between failures.

You must have an appropriate centrifuge to begin with. One of the world's experts on fluid-particle separation technology, Frank Tiller, professor emeritus of chemical engineering at the University of Houston, says, "Even with today's computer models, it is no easy task to properly choose a centrifuge. Liquids and particulates are different in every process, and when particles are 'on the move' they act strangely. Particles [or liquids] have a velocity that is both axial and radial; therefore, motion is complex."

Add to this the variables of concentration, viscosity, particle size and shape and it is not surprising that an end user often needs to do some experimental work to properly define the size and type of machine needed. "While this effort can be supported by some crude theoretical models based on Stokes Law," says Li Wenping, Tiller's associate at the University of Houston, "you never have all the things you need to know. And, the centrifuge does not act as a single black box."

What Wenping means is that in today's units, the centrifuge is almost always part of a system. It may be essential to add polyelectrolytes to the feed or to heat it prior to entering the unit. The use of a polyelectrolyte (for example, in sludge dewatering) may dramatically affect the moisture content of sludge exiting from the centrifuge, thereby saving millions of Btus in a subsequent drying operation. Similarly, heating the feed (for instance, in a crude oil/water mixture) may enable a separation that would not happen otherwise.

Types of units
Centrifuges fall into two main categories: sedimentary and filtration. Sedimentary machines include solid and screen bowl decanter, high-speed disc and tubular units. Filtration units include basket, peeler, pusher and worm screen (sometimes called oscillating worm screen). Special types and modifications of centrifuges are found in both major categories. Table 1 provides some broad selection guidance, and Table 2 details typical solids loadings in feed streams for units.

Despite the many types of centrifuges available, they share some common aspects. Feed always enters a rotating cylinder that is spinning on either a vertical or horizontal axis. Gravitational force produces the separation.

Centrifugal action forces particles or heavier liquids to the wall of the cylinder, while the lighter material, called the centrate, moves toward the center of the bowl where it can be removed. The action can be batch or continuous. Many units now are designed to be self-cleaning.

A continuous solid-bowl centrifuge consists of a cylindrical-conical solid bowl shell with an internal contoured scroll conveyor. The scroll rotates at a small differential speed to the bowl. The feed (often a slurry) enters through a hollow shaft in the helical conveyor and discharges into the pool inside the machine. The centrifugal forces exerted by the spinning unit move the heavier solids to the bowl wall. The difference in rotational speed between the bowl and the helical conveyor causes the solids to be moved along the bowl walls to the discharge port. The clarified liquid pools and overflows through a weir and exits at the large diameter of the bowl.

Developments
The chemical industry is benefiting from centrifuge research projects related to crude oil exploration and production (E&P) that have received significant funding from the U.S. Department of Energy (DOE). Many E&P operations (such as handling drilling muds, cleaning produced water and taking the dirt out of crude oil) involve significant fluid-particle separation work.

DOE project accomplishments include:

* Pinpointing proper feed temperature;

* Identifying the correct polymer to effect separations;

* Working through sensor and control problems;

* Defining proper bowl speeds;

* Developing feed-forward controllers; and