Foam Contol Application
Feed points: As mentioned above, when replacing oil based or concentrated silicone products a redistribution of feed locations and amounts will be necessary. This is also true of the more insoluble competitive emulsion products, but generally less modification is required. These recommended feed points will be covered in more detail in the individual equipment sections. However, there are some common rules to use for all systems to improve performance.
- Do not place the feed point for the emulsion too far upstream of the location to be controlled, as this will often reduce effectiveness.
- Avoid applying the emulsion to the suction side of a pump, as the high shear may damage the product.
- When locating feed bins or routing feed lines avoid high or low temperature areas to prevent product damage prior to reaching the feed location – Insure feed points are not plugged and use check valves when feeding to pressurized lines.
- Set up capability to measure individual feed point flows to ease optimization.
Feed Equipment: The pumping system used for oil based products will have too much capacity for application of emulsion products. Concentrated silicones may have similar or slightly lower feed volumes, but generally will not have enough pump feed heads to deliver all the necessary emulsion application points. Competitive silicone emulsions may have similar volumes and feed points, but as mentioned earlier additional feed may be used in latter washing stages to optimize the application. In the case of replacement of oil or concentrated silicone systems a separate feed equipment setup is generally required. The current emulsion feed setup can usually be used if all feed points are covered, but compatibility with the current product should be checked to avoid gelling or other undesirable effects.
Before application or trailing is started, the number and estimated capacity of the necessary feed points need to be considered. Mill personnel should be surveyed to determine problem areas and availability of feed points to the system determined and set up. General rules of thumb for necessary emulsion volumes are:
- 20 to 50% of the current oil based volume. The higher end of the range would be used at startup/transition, the lower end at full optimization.
- Similar to 25% higher volume of the current concentrated silicone product. In some cases usage may be slightly less.
- Similar to 25% less than the current silicone emulsion product volume.
- A rough distribution of volumes to the system is 60 to 75% to the initial washing stage, 20 to 40% to the intermediate stages, and 10 to 15% to the latter washing stages. This will be very dependent on the system design and operating parameters.
Transition Strategy: In no case should the current product feed be completely discontinued and the other started. Due to the differences in product functionality mentioned earlier, this type of transition can often create system upsets resulting in premature termination of the application. The recommended strategy is to replace the current feed locations by stage or individual application one at a time beginning with the latter washing stages. Since the free rinsing emulsions travel more with the liquor system rather than the pulp, this allows for the product to be gradually introduced to the earlier system stages prior to full feed point replacement. Additionally, the latter stage applications are generally less critical or sensitive so system control and confidence in the product can be established with process operators. The feed points to the remaining washing stages can then be replaced in step-wise fashion until the entire system is transitioned.
Sufficient time must be allowed at each transition point. This will be dependent on system size and other parameters, but generally a minimum of 30 minutes for low risk feed locations to one hour or more for critical feed points should be allowed prior to additional transitions. It should be noted that until the entire filtrate system is turned over after full transition, which may require 8 hours or longer, no significant optimization reductions should be attempted. There is often a synergistic effect when both products are being fed to the system, particularly when replacing oil or concentrated silicone based products, so a false positive impact may be attributed to the new application until the current product is fully purged from the system.
The following are specific application recommendations for individual washing system types.
Key operating parameters to these systems are vat inlet consistency, vacuum developed, and amount of shower water applied/filtrate tank levels. Control parameters are typically conductivity of the liquor in the final washed pulp mat and solids content of the filtrate removed from the first washing stage.
Air entrained with the pulp stock in the vat will reduce drainage rate and lead to loss of developed vacuum and/or reduced throughput capacity and potential vat overflows. Finely entrained air in the formed pulp mat will impede displacement of the dirtier vat liquor by the cleaner shower water resulting in poor washing efficiency with higher carryover of dirty liquor to the next stage. This can result in higher than desired mat conductivity which leads to production throughput reductions or higher fresh water shower usage resulting in lower than desired filtrate solids percentage.
Defoamer application points are designed to remove the air from the stock and formed mat to prevent the above problems. A graphic overview is shown in the following figure, with the common feed points designated in red and less typical points shown in green.
- Initial washing stage/stock approach: A feed point to the stock flow to the first washing stage is common, with the actual location being the primary knotter accepts line or dilution to the knotter accepts/washer prior to the vat. In addition, a feed point to the entire shower header is used to help deaerate the mat and provide some control for the second stage washer vat. In the past oil bases were fed to the lower shower bar or bars of the washer to help control the vat. This should be avoided if possible, and the whole header fed to improve coverage and maximize the emulsion performance. Often a small feed is used at the knotter rejects screen or tank to hold down surface foam. This is often a spot where over usage can occur as it is not critical to the process and is often not given attention.
- Intermediate stages: Feed will depend on how an individual system performs. A feed to the shower header of the intermediate stages is used, and at times a feed to the inlet stock is also used, particularly for the second washing stage. This feed point is generally to the vat dilution or the repulper dilution of the previous stage. This point is not typically used for subsequent stages unless it is a stage following screening.
- Latter washing stages: A feed to the shower header is typically the only feed point used. However, if the stage is one following a screening operation (often termed a decker), a feed to the inlet stock may be needed. A feed point to the fresh water shower on the last washing stage is generally not used.
Key operating parameters are the inlet flow and consistency, blower amperage loading, liquor transfer rate among the stages, shower water applied, and loading on the wire transfer rolls and drives. The usual key control measurement is the conductivity of the pulp traveling off the end of the washer. Since the mat is only formed once, poor mat formation due to air entrainment or other factors is a major issue. Also, the applied showers must drain sufficiently to avoid carryover to the next section (known as “flooding”) which unbalances the liquor transfer among the stages and can lead to high carryover off the end of the washer along with high loading on the belt and tension roll drives. Also, many times the foam will be generated by the action of the blowers and sucked into the units causing high amperage or kickout which will cause loss of overpressure and cause poor washing/flooding.
- Initial washing stage/stock approach: These feed points are similar to those of the vacuum system, with feed to the stock inlet provided by an addition point to the primary knotter accepts or dilution line. Inlet consistencies are higher (3-4% vs 1-3%) than vacuum washers, so at times there is no dilution line after the knotter.
- Washing stages: Locations are variable based on each individual washer, but generally the first, third, and next to last showers will be fed. Other stages may require feed points so the option to feed any of the stages needs to be available. In addition a small feed to one of the blower feed silos will be needed to prevent entraining foam into the blower. This is usually the first, i.e. at the front of the washer, but at times can be the second depending on configuration.
As with the belt washer, inlet consistencies are higher than vacuum systems. Due to the system being under pressure, higher inlet temperatures may be seen and this is the reason this type unit is used in conjunction with oxygen delignification systems, as no intermediate cooling is necessary. A key difference in this system is that since much of it is under pressure, there are few areas to allow the entrained air to escape except for filtrate tanks and the intermediate stock feed chests. This means aerated stock sent to the washer will drain poorly and can not be addresses at the washer itself. The filtrate tanks in the system are also generally much smaller than those in the vacuum systems and turn over faster, but allow less time for the air to separate out and may not provide adequate soap separation in softwood systems. Finally, if these washers drain poorly they will cause high drum rpm levels which will overload the drive motor causing a kickout. This will create plugging in the washer, often requiring a shutdown to clean it out. Cooling time is required prior to cleanout for high temperature stages, causing significant production loss.
Key operating parameters include stock flow rate, drum rpm, drive motor loading, pond/shower water flow and level, and stock tank levels. Control parameters include final pulp conductivity and first stage solids percentage.
- Initial washing stage/stock approach: As with all systems the stock feed is treated via a feed point to the primary knotter accepts or dilution prior to the first washer. The other feed point is typically the washer repulper to help control the second stage. At times a feed may be necessary to the crum sluice, as this spot generates a lot of foam which is returned to the system. Some systems also use a feed point to one of the filtrate tanks, particularly if soap levels are high in the system. Showers or ponds are not typically fed.
- Intermediate stages: A feed point to the stock feed tank or washer inlet may be used to supplement the repulper feed from the previous stage. As with the first stage, the repulper and crumb sluice are common feed points.
- Final stage: Often no feed point is used on this stage. A feed to the stock feed tank or washer inlet may be used to supplement the repulper feed on the previous stage, or a crumb sluice feed may also be needed.
- Common feed point for this unit is to the filtrate from the unit.
- Typical feed points are to the primary screen accepts or dilution line prior to the final washer. Older systems may use a feed prior to the screens to minimize foaming in the screen itself.
- Feed points are usually to the blow tank dilution or stock feed line to the first post oxygen washer. As with other pressure systems, the repulper of this washer or feed to the following stage are also common feed points.