PAPER CHEMISTRY LABORATORY, INC. Summer 2005 Newsletter
CLOSED LOOP NANOTECHNOLOGY CONTROL
How to Maximize Quality and Productivity at Minimum Cost
John Penniman
Introduction
Over a period of 30 years, chemists first used microelectrophoresis to measure zeta potential and later used titration to measure cationic demand. We were doing the best we could with the tools available, but all we really learned is that a small negative ‘charge’ contributes to better runnability.
Toward the end of the period, the author’s lab did a series of more than 5000 experiments, in which the wet end parameters zeta potential and drainage were measured. Handsheets were made of the prepared stock; Scott Bond, sizing and sheet ash were additionally assessed.
Simultaneous improvement in both process parameters and physical properties was obtained by operation close to zero zeta potential. In some cases the benefit was so great as to defy conventional explanation.
Further investigation has led to an understanding of the significance of intermolecular attraction, the importance of stock homogeneity and the necessity of neutralizing the residual repulsive surface charge in order to permanently maximize (instead of impair) strength properties.
Technical Background and Objectives
The principal scientific objective is to increase intermolecular contact, and maximize all the available attractive forces. They are extremely powerful, and they are free! Included are van der Waals Force, hydrogen bonding, preferential wetting, electrostatic attraction and interparticulate coupling. Detailed discussion is beyond the scope of this review.
The first step is to use a mono-functional cationic chemical, such as cationic starch. It should be cost-effective in charge neutralization. Sufficient quantity is added to make the stock positive, with a zeta potential in the range +5 to +10mV.
The second step is to mix the cationic component thoroughly, so that stock and chemical are homogeneous. In the absence of homogeneity, it is impossible to maximize intermolecular contact and realize its many benefits.
To emphasize this point: stock is often not well mixed, with a high zeta potential standard deviation (SD) of 4-5mv, and 6-8 breaks per day. On the other hand, stock mixed by preferential wetting requires 2 orders of magnitude less size, and has a low SD of 0.6mV, and excellent runnability.
Third, the anionic nanoparticle is added at the precise addition rate to fully neutralize the electrostatic charge of the cationic stock. It is mixed to homogeneity. The active chemicals form a 2-dimensional structure, and provide the best possible balance between formation and retention.
The result is that individual particles have been nanoflocculated in a two-dimensional structure. Positive and negative charges have been balanced; residual repulsive charge has been completely eliminated. Intermolecular contact is enhanced, with the counter-intuitive effect of making possible a sheet ash increase of 5-10% accompanied by greater strength.
Current practice contrasts sharply with that described above. Lacking the ability to make precise measurement of electrostatic charge, a cationic polyacrylamide retention aid is customarily added. Macroflocculation results, automatically impairing strength and end-use performance.
Implementation
Well executed papermaking nanotechnology has the capability of optimizing end use performance at a high level of uniformity. Value-in-use of the product is significantly increased.
Drainage is measured, and increased or decreased over a wide range in order to maximize productivity and machine operating efficiency. Increase in product strength resulting from inter-molecular proximity is substantial, enabling use of less expensive raw materials, such as filler and recycle fiber.
Raising the bar on quality and performance while simultaneously reducing both operating and raw material cost requires new sensors, greatly enhanced thoroughness of mixing, and continuous closed loop control of chemical feed rates.
Under this modus operandi, the chemical supplier continuously maintains certain supply vessels at prescribed minimum levels of specified product, sets up the control parameters, and applies touch-screen control to chemical feed rates. Effective execution eliminates conventional stock prep labor. Closed loop control minimizes dedicated presence of supplier personnel.
Nanotechnology data is combined with raw material costs, usage rates, and the computerized Product Information (PI) information. A running calculation of actual cost is published. Maximum productivity and the most cost-effective operation are thereby sustained.
Mixing and Charge Neutralization
Jouni Matula was awarded the 2003 Finnish Prize for Innovation, for his accomplishments in developing the hardware for providing greatly improved chemical additive mixing with paper stock on the wet end. The clever use of booster pumps is described on www.wetend.com While the quality of mixing thoroughness remains to be scientifically defined, the technology is undeniably highly beneficial and cost effective.
The undersigned has added additional sensors to an on-line zeta potential instrument in order to quantify the quality of homogeneity at the critical two points on the machine, www.papermaking-chemistry.com We anticipate this will enable fine tuning of the chemical additive mixing parameters, so as to enable maximum homogeneity.
Depiction of the wet end begins with a screen of red rectangles, one for each initial ingredient, including provision for entering control algorithms. Automatic metering and mixing are controlled by a touch-screen. A screen of pink circles follows, to enable specification of physical property limits, including the mixing thoroughness of cationic component with stock.
A third screen contains a single yellow square for the anionic microparticle, plus provision for specifying quantity. A fourth screen of pale yellow circles specifies physical performance properties including homogeneity and surface charge neutralization; with set-points that specify quality level; other set points control chemical feed rates, so as to optimally balance actual production output and machine capacity.
The data is analyzed by neural network, fuzzy logic software, thereby ensuring that even the most obscure beneficial relationships are exposed and exploited.
Conclusions
Our recommendations for optimizing wet end performance do not correspond to the conventional wisdom, and sound extreme to some papermakers. However, they rest on the following fundamental principles:
Simultaneously maximizing retention, drainage and physical properties is ONLY accomplished by closed loop control of nanoflocculation.
Process chemistry efficiency and uniform product quality are ONLY maximized by mixing stock to homogeneity.
Strength is ONLY maximized by neutralizing the residual repulsive surface charge. Otherwise, the product is forever weakened.
The new technology maximizes cost-efficiency and clearly signals an exciting new opportunity. Papermakers willing to confront a disruptive new paradigm can look forward to unparalleled quality while maximizing productivity at minimum cost.
| John G. Penniman |
| www.papermaking-chemistry.com
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