Recaust Reminder: Pressure Filter Operation

McFarlen Engineering Ltd. has prepared this “Recaust Reminder” to provide operational suggestions to its customers. Please contact us should you have any questions or require further information.

Many kraft pulp mills in Canada and the USA operate white liquor and/or lime mud washing pressure filters. Filtration is accomplished by passing the liquid through filter socks that are installed on perforated stainless steel support tubes. Lime solids collect on the outside surface of the socks, which are periodically cleaned by a back-wash. Although these filters were supplied by several OEMs (Eimco, Ahlstrom, Dorr-Oliver, Goslin-Birmingham, Enso-Gutzeit), they all are based on very similar design and operation principles.

The filters socks usually are made of polypropylene felt, with the best quality socks also consisting of the Gore®membrane (which provides a reduction in acid washing frequency, improved clarity and a longer period of service).

When operating with Gore®socks, better performance is often achieved by adjusting the timing sequence in the WLPF. The suggested liquid level drop for backwash is approximately 16 to 18 inches which will occur quickly because of a short timer setting and fast operating valves. If level drops and backwash volumes are excessive, higher pressures will be observed in the filter as a result of three compounding factors, namely:

Figure 1: Contributing Factors to Higher Filter Pressure

First, a larger backwash volume increases the level in the feed tank beyond that expected during suggested operating parameters. This greater differential to the set point causes an increase in pump speed producing a more flow and in turn higher pressure in the filter.

Second, during filtration, lime mud accumulates on the socks. If backwash volumes are excessive some of the discharged mud is swept back into the feed tank. Over time a new equilibrium of higher suspended solids is reached. If the liquor being pumped to the filter contains higher solids, an increase in pressure is experienced.

Third, as feed liquor enters the filter it encounters a rapid expansion and a corresponding loss in velocity. This causes a separation of heavier particles determined by the flow and diameter of the vessel or, in other words, the rise-rate of the liquid. Larger particles settle into the cone and fines collect on the socks. As the liquor rushes from the filter during the abnormally long backwash it is fines which are recycled. Therefore, the higher feed consistency is driven up by a much heavier fines loading. More fines create higher pressure.

It is believed that reduced operating pressure would be observed if the settings were changed to:

  • 3 minutes for filtration
  • 2-4 seconds for backwash
  • 25 seconds for sedimentation

Before any changes are made it would be useful to actually observe the current settings and record them. Please contact McFarlen Engineering to discuss any aspect of this possible change.

Acid washes should be less frequent and more white liquor may be produced due to an increase in the number of cycles per day, each maintaining high flow instead of trailing off as it does at the end of five minute cycles.

Note: In the WLPF, “Slippage,” the drop in level which occurs when both valves are open as the vessel returns to the filtration cycle, should be minimized to less than an inch or two. To accomplish this, the timers and valves must be operated in the appropriate strategy. It may be necessary program the DCS to initiate backwash valve closure and/or a small increase in pump speed to provide more holdup pressure to support the liquid in the feed line and feed nozzle.

A 10% increase in underflow consistency from the WLPF translates into 55% more liquor in the mud. As shown in a typical material balance, Figure 2 below, this can be viewed as an opportunity to turn down the front end of the process or one may wish to increase white liquor production. Samples should be taken, for density determination, from White Liquor and Weak Wash pressure filter underflows. Adjustments should then be made to ensure that target values of 42-43% and 45-46%, respectively be met.






Figure 2: Typical White Liquor Clarification Material Balance at 30% & 40% Underflow Densities