1 July 2005
Making more of the mill
Paper mills save with analytical.
By Dave Joseph
It's no news the U.S. pulp and paper industry has hit hard times. With the escalation of personnel costs, international competition, and environmental demands, making a profit in the pulp and paper industry is increasingly difficult. So is there any good news? Yes, a little. In an industry recession, pulp and paper mills are leaner and meaner. The industry in the U.S. has moved away from making commodity products and is concentrating more on niche products like tobacco paper, fine papers, and high quality magazine papers to stay competitive internationally. Yet, the industry still has bridges to cross in dealing with pulping processes, environmental standards, measurement techniques, and waste treatment, to name a few.
The EPA-mandated cluster rule regulated all forms of pollutant, whether it is in the gas, liquid, or solid phase. While historically kraft mills used chlorine to bleach paper, the small quantities of carcinogen dioxin produced in pulp mills were unacceptable. The switch to chlorine dioxide, which doesn't produce dioxin, led to big changes in operating procedures and cost the industry a lot of money. Now it's become important to check the effluent coming from the mill. One of the biggest problems is effluent in the water phase. Some mills have closed rather than go through with changes to meet regulations.
Without the money or manpower to make sweeping changes that might cut costs, mills must make small incremental changes and improvements that can add up to big savings.
One such opportunity arises in using liquid analytical instrumentation. Something as simple as making an additional electrical conductivity measurement can help optimize the pulp yield or increase recovery of expensive chemicals. Or the addition of a new measurement like turbidity may reduce the loss of wood fiber in wastewater. Each mill needs to find measurements that optimize yield in every step of the process.
In the kraft process, measuring white liquor (strongly basic solution for chemical digestion) alkali concentration allows optimization of cooking time and product pulp properties. Direct measurement of the alkali concentration is possible using automatic titrators. However, due to the difficulty of extractive sampling, many mills opt to use electrical conductivity to provide real-time control feedback. This instrument is best installed in the clarified white liquor feed to the digester, and retractable instruments are preferred for safe, easy cleaning. Measuring the alkali concentration entering the digester allows fine control of the liquor flow rate to match the wood loading, improving throughput and minimizing variability.
Bleaching and wet end
Chlorine dioxide bleaching (now the standard of the industry) allows efficient bleaching at lower pH (3.5 to 4) without degrading the cellulose component of the pulp. The extraction stages occur at elevated pH (10 to 11) to maximize color removal. pH measurement is very cost-effective at this stage of the process because the chemicals used to manufacture chlorine dioxide represent a significant operating cost. Stable pH control increases net bleaching efficiency and lowers cleanup costs since less bleach is required. The headbox location is also a critical pH measurement since an upset condition will immediately result in an offspec product. Advanced diagnostics can alert the operator to common modes of failure, such as broken electrodes or coated sensors.
You can use conductivity measurement to good effect throughout the chemical recovery process. Since the steam used in the evaporator comes from the boiler, a leak of black liquor into the steam condensate can cause substantial damage to the boiler tubes. Conductivity measurement can detect even a trace of black liquor in condensate and is low-cost insurance for the recovery boiler tubes.
Pulp and paper mills produce a myriad of wastes, including wood shavings, spent pulping liquors, collected spills, and washings from dregs, lime mud, and bleach residue, all which you must remove from the water before releasing it into the environment. One critical area is color removal, which may require multiple stages of clarification using lime, alum, or ferric chloride. These chemicals capture contaminants by forming a solid floc that you can separate from the effluent stream. The flocculation chemicals are most efficient (and cost-effective) at controlled pH levels, measuring pH in heavy solids can increase frequent sensor cleaning. If you choose tough, long-life pH sensors, however, the cost savings in chemicals will outweigh the cost of measurement.
Boiler, steam plant operations
Controlling water chemistry to prevent scaling and corrosion inside boiler tubes requires the use of expensive chemicals such as ammonia or hydrazine. Conductivity measurements monitor the salt content of the condensate and determine when you need to add pure (makeup) water to the circulating water to prevent fouling. pH measurements help control corrosion in the boiler since such damage sharply increases below pH 7, while operating above pH 9 increases the precipitation of solids on metal surfaces that decreases heat exchange. Typically, you maintain levels around pH 8.5. Dissolved oxygen measurement also plays a role since continuous dissolved oxygen levels above 7ppb have been implicated in premature failures due to pitting and metal fatigue. Measuring trace dissolved oxygen following the deaeration stage assures oxygen scavengers are doing their job and not damaging metal.
For every cost-saving liquid analytical measurement, there are many more for mill managers to consider for reducing costs. These small changes can add up to big differences in profitability.
Behind the Byline
Dave Joseph is a senior industry manager at Emerson Process Management, Rosemount Analytical Liquid Division, in Irvine, Calif.
SAGD bitumen slurry with fist-sized rocks
Oil Sands, Fort McMurray
The largest single hydrocarbon deposit in the world is the Athabasca oil sands of northeastern Alberta, which contain more than 1.7 trillion barrels of oil. Unlike a conventional oil reserve, oil sands (also called tar sands) contain oil in sand or carbonate suspension with such high viscosity it is immobile under normal temperatures and pressures. Traditionally, you remove bitumen by open pit mining, followed by transport and heating with solvents, and separation of the oil and other products. In recent years, the adoption of steam assisted gravity drainage (SAGD) has significantly improved efficiency. During SAGD, horizontal well drilling occurs near the base of the bitumen deposit. Injected steam into the wells heats the bitumen, reducing its viscosity and allowing it to flow under the force of gravity to the lower producer well. From there, you pump it to the surface, and it goes on to the upgrader, which extracts the heavy oil for further refining. The most efficient means of moving the bitumen slurry to the upgrader is hydro-transport, pumping it with water. Water added for transport reduces pipeline capacity and productivity. Also, you must remove all water during upgrading, increasing energy consumption.
In the past, process noise due to high solids content overwhelmed magmeters that measured the flow of the bitumen slurry. Further complicating reliable measurement is the varying size of the sand, up to fist-sized rocks, and the uneven mixing and stratification of the heavy oil and water. So, instead of mags, users used wedge meters. Unfortunately, these meters suffered high wear from the entrained rocks, affecting accuracy and requiring frequent rebuilds. Now, users have started to adopt high-signal magnetic flowmeters in these applications. They designed a special high-durability liner in collaboration with oil sands producers for bitumen applications.
Not only does the high-signal magmeter provide a more accurate, reliable signal and require less maintenance than the wedge meter, but it does not obstruct the flow. This allows an increase in flowrate, using the same pipe and pumps and with no increase in pumping cost. Since the production of most upgraders is limited by the bitumen input, this increase in flowrate directly increases plant production rate.
SOURCE: Terry McLean, Spartan Controls, Edmonton Alberta
Optimizing flowrate of 84% solids paste
Falconbridge Mine, Timmins, Ontario
As part of its Kidd Creek Deep Mine project, Falconbridge successfully piloted an underground reticulation system for a tailings/sand/ binder paste to back-fill stops in the mine. The key advantage of this new system is it operates with an 82-84% solids paste, instead of the more typical 60-70% slurry.
In an application where no other flowmeter would work, the high-signal magmeter provided high accuracy, ±1% of flowrate, verified by drop tests, with good stability and fast response.
Using the high-density paste minimizes water drainage and the associated costs of underground pumping and slimes disposal, while minimizing the amount of binder needed to achieve comparable strength. In addition, stable and accurate flow and pressure measurements allow the user to predict system flow parameters, such as rheology, and optimize system operation to reduce the risk of freefall, water hammer, and other conditions that accelerate system wear.
SOURCE: Dave Counter, Falconbridge Inc., Timmins, Ontario
Increasing throughput, reducing chemical consumption
Abitibi Paper Mill, Fort Frances
Most pulp mills operate their bleach plants with medium-consistency stock, also called mid-brights, which is ~15% consistency. Operating with high-consistency stock, or high brights, 35%+ allows the user to increase production rate through the same equipment. Also, the bleaching process is more efficient with high-consistency stock since it contains less water. This significantly reduces bleaching chemical consumption.
After the bleach tower, dilute the stock in two stages for pumping to the storage tanks. First, dilute the stock to ~16% consistency as it exits the bleach tower. Additional dilution in this first stage is not practical, as the chips will simply float in the water. After the first stage, fully absorb dilution, extract air, and further dilute stock in standpipes to ~4% for storage.
Accurate and reliable flow measurement of the 16% stock and 4% stock is critical for accurate water and chemical dilution. While measurement of 4% stock is routine, Abitibi relies on a high-signal magmeter to measure the 16% stock, which actually varies from 12-20% consistency.
Using the high-consistency stock has allowed Abitibi to increase production through the bleach plant by 50% over the original design, in addition to reducing chemical consumption per ton of production.
SOURCE: Dane Lowey, Abitibi Fort Frances
Diagnose and treat magnetic flowmeters, noisy flows
By Mark Menezes
Magnetic flowmeters, also called magmeters, see wide use in industries to measure flows of water and water solutions. They offer significant advantages, such as cost effectiveness, reliability, and accuracy when you compare them with other flow technologies. Most liquid applications fall within the magmeter limitations of low-to-medium pressures and temperatures, conductive liquids, and slurries.
In any magmeter application, the objective is to maximize signal, while minimizing extraneous noise. The most common sources of noise in magmeter applications are faulty grounding, high-process noise, and intermittent electrode failure. Unfortunately, from the flow signal alone, it is impossible for a user to distinguish between noises from these three causes. More importantly, it is also impossible to distinguish between a noisy flow signal and a truly noisy flowrate. So, extraneous noise can mask a genuine increase or decrease in flow variability. Severe noise in control applications can lead to unnecessary valve travel and wear, so the user may be tempted to apply excessive damping, further masking true process variability and reducing control effectiveness.
What causes faulty grounding? Magnetic flowmeters use Faraday's Law, measuring the electric field generated by a conductive fluid moving in a fixed magnetic field. To ensure that any electric potential is due solely to the Faraday Effect, you must ground the fluid to ensure it has zero electric potential upon entering the flowmeter. To do this, you'll need a grounding strap, grounding rings, or a dedicated grounding electrode. Unfortunately, it is common to compromise the integrity of the ground over time—a wire can break, or you can have a corroded or coated connection. When this happens, power supply hum can enter the system, resulting in a noisy flow signal.
In any magmeter, the two electrodes used in the flowtube should provide a signal with equal amplitude, yet opposite polarity, since they are located directly across from each other. Unfortunately, leakage of process fluid or moisture into an electrode terminal compartment will cause the electrode to short intermittently, causing a noisy signal. Eventually, the electrode will fail entirely, causing a consistently low reading. Similarly, some process conditions can cause intermittent or permanent coating of the electrodes. Again, without a diagnostic or some independent reference, the user is not aware of this problem, which is especially serious in safety, quality, or environmental applications.
The electrode failure diagnostic compares the signals from the two electrodes and ensures they are of equal amplitude. The online remedial action is useful, since it is very specific and unlikely to be familiar only to an expert maintenance technician.
Process noise occurs when:
Noisy flows are becoming more common in the process industries for two reasons. First, users are striving to reduce water content in their flows to ease environmental compliance and reduce energy consumption. This lowers pumping costs; plus you also need to remove added water later. Less water means higher solids content and higher noise. Second, users are upgrading AC mag transmitters to DC. AC mags drift, and are becoming obsolete and harder to obtain from suppliers. However, a mag flowtube capable of tolerating a very strong AC current is not as tolerant of DC, just as a person can momentarily tolerate 15 Amps of AC without injury but not 15 Amps of DC. So, applications that worked well with AC mags become noisy when upgraded to the more stable, accurate, and widely available DC mags.
While the user cannot distinguish a noisy flow signal from a truly noisy flowrate, the microprocessor in the smart magmeter transmitter can by analyzing the frequency of the noise. DC mags drive their coils at a fixed frequency, such as 6 Hz. While the microprocessor will primarily scan this drive frequency, it can also periodically check for noise at other frequencies. While a faulty ground will cause a signal at a frequency of 50-60 Hz, process noise typically causes any signal at frequencies lower than the drive frequency.
The remedial action for a faulty ground is straightforward—fix the ground. The fix for high-process noise is not so obvious since it is the process itself causing the noise. In some applications, the user can try to operate at a higher drive frequency, such as 30 Hz, to avoid the noise. However, if noise remains high, the best solution to achieving an acceptable signal-to-noise ratio is to increase the signal itself by replacing the existing magmeter with a high-signal magmeter. These devices incorporate much heavier windings in the flowtube, and as a result they can handle a much higher coil current from the transmitter—as much as ten times higher. High-signal mags provide a strong signal, requiring minimal damping, even in applications with high noise.