Pumps: Reducing Energy Consumption in Pulp and Paper Processing by Pumping Higher Densities
September 1, 2002 By Pulp & Paper Canada
New pumping technology can reduce energy and other resource consumption by processing pulp at higher consistencies, eliminating the need to pump as much slurry, and to remove the additional water.In t…
New pumping technology can reduce energy and other resource consumption by processing pulp at higher consistencies, eliminating the need to pump as much slurry, and to remove the additional water.
In the process of making paper, wood fibre is reduced to a slurry form which can be pumped, facilitating the paper forming process. Traditionally, these slurries are very highly diluted — to the extent that they are 97% water. In this diluted form, the stock also becomes very pumpable which becomes a convenient means of transporting the stock through. The pumping is an integral part of the various processes that are required to convert this pulp, which is generally an off color, into something that is white and can be manufactured formed into paper.
What’s happening today, with 97% water and 3% stock, is that getting rid of that water is an expensive proposition. Quite often, much of that water is stripped off with a vacuum or drainage or even steam.
And because 97% of the stock you are pumping is water, there is a cost involved there as well. If you could double the consistency of the pulp to 6%, the amount of water you have to pump and remove is cut in half and the amount of energy needed to remove that water will be reduced by a proportional amount. So the game plan is to try to increase that consistency and still provide for efficient pumping.
We have found that if you can get this stock into a pump, you can generally pump it. The main problem is getting it to move efficiently into the pump, especially at the 6% and greater level — and to get it in there as homogeneous stock without the stock becoming segregated segregation.
If you have ever looked at pulp at higher consistencies, like at the 6% level, you could probably walk on it with snowshoes — the stuff does not want to flow very well. So, generally, the stock will have to be agitated. Another issue is that the fibres in the stock have a tremendous affinity for air. So the fibres will trap air that is entrained in the process of making this pulp. There are plenty of ways and places upstream that this can happen.
A centrifugal pump is a by nature a centrifugal device. The air, being of much lower density than the stock, will tend to collect near the center of the pump and form a blockage or air pocket at the inlet to the pump. This air pocket prevents the stock from entering into the pump and the pump ceases to work after a short period of time.
It is very common for a centrifugal pump in this situation to experience this — a phenomena so common that it is given a name, air-binding. The typical size pump we are talking about here has a 10-inch suction, and delivers up to 600 feet of head. The head requirement necessitates high rotation speed, 1800 rpm being typical. The high speed, and large suction size further encourage air-binding. The trick, then, is to prevent that air from forming that blockage and to prevent centrifuging from taking place or, alternatively — to actively encourage centrifuging and then to extract that air from the inside of the pump.
The technology that we are bringing to market today is the latter of the two approaches — to deliberately encourage centrifuging and let the air collect at the inlet of the pump and along the center of the shaft.
The typical size of pump we are talking about here is a 10 or 12 inch suction pump with horsepower in the 500 range. This type of pump will deliver up to 600 feet of head, a requirement that is needed for this type of process.
The higher consistency of the products being pumped, the higher the friction losses in the piping. So the pump needs to generate higher pressure and more head in the to effectively overcome this friction.
Our solution to the problem of air binding is to extend the impeller out into the suction passage of the pump where we add two devices to this extension at the inlet to the impeller.
One is a device, similar to an auger, which acts as a fluidizer. It actually sticks out of the pump inlet into the storage tank that the pump takes its suction from. Usually, these pumps are bolted right to the side of the storage tank. This fluidizer is intended to agitate the stock, and convert it into a more fluid form so that it flows much easier and becomes thixotropic — a fluid that becomes more liquid as you agitate it. Much the way latex paint thins as it is agitated.
As the stock is fluidized by this auger, it allows the stock to move into the pump. The next device that is in line, added just before the impeller, is a centrifuge. That device causes the stock to rotate at the speed of the pump, which deliberately makes the pump a centrifuge. This now causes the air trapped in the stock to move towards the center of the shaft. Through the back end of the impeller, we have a vacuum port, connected to a mechanical vacuum pump which extracts that air from the suction of the pump and this allows the pump to operate normally.
The traditional pump, without the fluidizer or the added centrifuge has been able to pump up to about a 6 percent consistency with reasonable success. With these added devices, we have been able to pump up to a 15% consistency. With a 15% consistency, let me give you some idea of the type of savings that a typical pulp and paper mill will encounter.
From the pumping perspective, if you went from 6% to 15%, the amount of water that you would have to pump, would be cut by a factor of two and a half. Just by virtue of that, you would have cut your power requirements for pumping by about 60%.
Not only are you reducing the amount of pulp that you have to pump, but further on downstream, you are reducing the amount of water that you have to remove from the pulp stock. So you are using less energy from auxiliary sources. Plus, the amount of water that needs to be recycled is less, which means less pumping in other areas of the plant and lower energy requirements.
Another area I would like to touch on is that most processes usually control the process with a control valve. The way they work is to create a bigger pressure drop across the control valve in order to restrict the amount of flow involved.
There are opportunities to control flow and other process parameters by other means other than using a control valve. Variable frequency drives can be used to do that. To many people, that may sound like a no brainer, but the difficulty is that employing a variable speed drive to control processes in industry has not been very successful in the past because there has not been a lot of integration between the pumps and the electronic controls for the variable frequency drives.
In other parts of ITT Industries, we are now bringing to market, especially in the water management area, electronic speed controllers that can be applied to existing centrifugal pump. And, there are opportunities to do the same thing in the industrial and process area, providing flow, level, or consistency control from the pump itself.
In another area, mixing technology — one of the parameters that determines the value of the paper is the brightness of the paper. The way that brightness has been attained in the past is through bleaching, using elemental chlorine. Last year, the cluster rules that were implemented in the pulp and paper industry looked at all of the types of emissions coming from a plant. One of the chemicals that contributes to this waste stream is the chlorine, which in its reactive form, makes dioxins.
The chlorine was a very effective bleaching agent. But with the cluster rules, that is not a favored technique any longer. So plants have been using chlorine dioxide, which does not form dioxins in the process. Chlorine dioxide is a very expensive chemical and it is important to minimize its use in the process. One way to do that is to mix it effectively with the stock. Traditionally, the way chlorine was mixed was to throw it into the stock and leave it to the action of the pump to do the mixing — which does a mediocre job. So one of the other opportunities in the pulp and paper arena is to develop new mixing technologies that allow plants to
mix these bleaching agents much more effectively, reducing the amount of cleanup that is necessary afterwards. Remember, these remediation processes are not adding any value to the manufacturing process and in fact are increasing the cost of production. What we are developing here are in-line mixers, which are based on pumping technology. These mixers, installed in the stock pipeline, are designed to highly agitate the stock as it is going through the pipeline itself, providing for thorough mixing.
In summary, applying fairly simple, new technology solutions to traditional pumping and mixing applications in the pulp and paper industries will have a beneficial effect on the consumption of energy, reducing the use of expensive water and chemical resources, reducing the non-value added remediation processes and improving the overall efficiency of the process of pulp and paper manufacturing. These results will be positive for the manufacturer, the communities in which they operate, the global environment and ultimately, the consumers of the product.
Barry Erickson is the vice president of technology at ITT Industries’ Goulds Pump unit. This speech was originally given at the US Department of Energy’s Office of Industrial Technology Exposition.
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