Pulp & Paper Manufacturing - Seeing the Forest for the Trees
For those of us who enjoy reading a good hardcopy book, part of the experience is feeling the physical paper in our hands. We know what "good" book pages feel like - thicker than thin, crisp, with a slightly rough texture that grabs our fingers, and dense enough that it feels like each page deserves to carry the weight of the words printed on it. For us to enjoy this experience as much as we do, a great many things must happen correctly across the paper manufacturing process.
Transforming mature trees into paper stock that will later become the pages in our favorite books (and so many other products) is the domain of the Pulp & Paper Industry. In this article, we’re discussing the concept of viewing Pulp & Paper manufacturing integrity through a macro lens, using automated instrumentation as one tool to avoid large-scale upsets.
Given the sheer scale of paper manufacturing supply chains, we tend to look at individual production systems for process improvement opportunities (since we're an instrumentation engineering company at heart), but have to remind ourselves that even larger opportunities exist when we zoom out to look at paper plants and supply chains in aggregate. That is to say, automation engineers serving the Pulp & Paper sector should remember to see the forest for the trees!
What does this mean for avoiding process upsets and driving reliability in paper production? Three things:
- Total Cost - Over an entire supply chain, there are dozens of discrete cost centers that all roll up into the unit price of a pound of paper. Defending against process upsets at each stage can actively limit knock-down effects, keeping the end product's total cost competitive and consistent.
- Predictive Performance - The more data we can acquire from our processes, the more we can model, test, and analyze predictive outcomes ahead of time. This foresight helps improve efficiency by adjusting operations to match expected conditions. In addition, this helps identify areas of production that need extra focal support. Both of these benefits directly serve to solve potential disruptions before they occur.
- Long Term Sustainability - In a finite sense, a certain number of trees in the ground equates to a certain yield of paper products (at a prescribed price). Process disruptions impact these yields, where issues drive up waste and yields fall too low, leading to an imbalance between tree harvest and paper output. Avoiding disruptions and minimizing waste helps keep us on a sustainable path (especially when considering the massive chemical, energy, water, and manpower resources consumed too).
Ensuring Process Integrity over Extended Paper Runs
Extended, continuous runs are commonplace in the Pulp & Paper industry, a factor which makes uptime a critical metric in managing production. As with most continuous operations, process integrity becomes paramount - that is, the degree to which operational parameters and control loop functions hold true over the course of the run.
The largest threat to process integrity over continuous runs is found in mechanical fouling and control loop drift, both of which describe the steady trend away from a setpoint that occurs when equipment conditions naturally change over time. In Pulp & Paper, what common condition changes lead to integrity lapses?
- Feedstock Changes - as chip plants work through their feedstock piles, the consistency of wood type, age, condition, and physical properties naturally change.
- External Influences - some process steps are subject to external influences that may impact overall process integrity, such as pulp material absorbing excess atmospheric moisture while staged for too long before milling.
- Equipment Performance - most notably at thermal process steps, process media often builds up over time, "fouling" heat exchangers, coating pump heads, plugging sensor ports, and generally slowly degrading equipment performance over the course of the run.
- Hysteresis Changes - hysteresis is defined as the lag between an input conditions' measurement and the requisite output action from the control system. As systems foul and conditions change, hysteresis configurations can result in longer lags between inputs and outputs, degrading performance.
- Utility Conditions - water quality is particularly crucial to pulp and paper production, and normal changes in water treatment conditions can impact process performance (such as with turbidity and temperature changes occurring across the day). Similar ebbs and flows across all utilities can directly influence process integrity.
Applying Instrumentation towards Total Paper Manufacturing Reliability
Since paper manufacturing is certainly more the scale of forests and less that of single trees, we'll share three high-level applications within which advanced instrumentation can drive reliability over the entire course of production. These concepts are meant to embody the three managerial metrics we introduced above: solving for total cost, predictive performance, and long-term sustainability.
As the primary utility in the production of pulp and paper products, water treatment is an incredibly crucial process necessary to ensure that as much water as possible can be reused instead of discharged. Chemical, biological, and filtration treatment processes all work to reduce water consumption overall, allowing every gallon of fresh water to be used multiple times for log wash, pulp rinse, pulp cook, mill headbox feed, non-contact cooling, and more. In these systems, we have found through recent projects that adding pressure and temperature instruments - and their associated control and alarm loops - helps mills catch fouling-induced pressure losses and over-setpoint temperatures that were previously leading to upsets.
Compressed Air & Vacuum
Most compressed air and vacuum systems are controlled to primary pressure setpoints at the main receiver, but few systems have online visibility across their distribution piping or measurement at key points-of-use. Interestingly enough, we find that these loud, high temperature, large scale mill environments do not lend themselves well to audited and catching compressed air leaks, which lead to pressure and volume losses, and ultimately system upsets. Adding air pressure and flow monitoring at the far ends of distribution piping schemes, and right at key connections (especially right after pressure reducing valves that can themselves drift) can combat disruptions as well as cut energy costs wasted through leaks.
Paper manufacturing can be quite chemical-intensive, namely in the areas of pulping, bleaching, washing, wet end preparation, starch addition, and coating application. Chemicals used throughout paper production are costly, sensitive to adulteration, and hazardous in nature. For these reasons, it's quite beneficial to ensure proper inventory and metering control, which is achievable through the use of advanced level, pressure, temperature, and flow instruments. Control loops should be configured for very high accuracy and repeatability in metered discharges, as well as for high sensitivity to unintended flows that would indicate leaks. Especially for high-hazard chemicals, automatic reaction logic should be included that can contain leaks and notify personnel of dangerous conditions.
Countless other single-task applications exist that can benefit from instrumentation and control improvements, beyond the scope of this article. We hope that the above insight has been useful in helping readers spot key application improvements that can be evaluated throughout your Pulp & Paper Manufacturing operation. More broadly, we hope that the overriding goals of managing total costs, predictive performance, and long-term sustainability can serve as motivation and justification for your future reliability projects. Even if you can only solve one potential disruption source at a time, you’re starting a meaningful process of continuous improvement that can only pay off in spades!
As a veteran-owned small business, Whitman Controls is dedicated to supplying premium quality, reliable, technologically advanced instrumentation for use in nearly any application. Our Bristol, CT manufacturing facility embodies over 40 years of engineering, fabrication, and customer service expertise, serving both end-user and manufacturing customers nationwide through direct and distribution channels.
Our values drive us to provide the highest level of servant partnership that you can find. To discuss your applications or to learn more about our capabilities, please contact us at (800) 233-4401, via email at [email protected], or online at www.whitmancontrols.com.