Stock Vessels for Improved Quality and Energy Savings

Transcription

1 Innovative Mixing of 6%-20% Consistency Stock Vessels for Improved Quality and Energy Savings Marc R. Moseley Manager, Global Pulp & Paper Industry Philadelphia Mixing Solutions, Ltd. Dr. Wojtek Wyczalkowski Research Fellow Philadelphia Mixing Solutions, Ltd. World s Largest Fluid Mixer

2 Objective At present, most stock chests are only partially mixed with side entry mixers. Maximum consistency is restricted to less than 6.5% due to mixing limitations. However, proper application of modified impeller technology combined with zonal mixing results in uniform mixing of the entire chest using only 50% of the power required by traditional side entry mixers. This technology will allow for uniform mixing of paper pulp up to 20% consistency. Independent estimates of savings due to improved sheet profiles, increases production efficiencies i i and energy savings varies, but can exceed $100, per month (U.S.).

3 In this Presentation 1. Introduction 2. Measurement of Rheological Properties of Paper Stock Calibrated Impeller Test 3. Modified Impeller Design 4. The Physics of Mixing Yield Stress fluids 5. Mixing of High-Concentration Paper Pulp in Machine Chests 6. 50% Energy Savings: Application Example 7. Paper Pulp Mixing Lab Quantification 8. Conclusion 9. References

4 Introduction Long term consistency fluctuations are well handled by automatic control system. Short term fluctuations are not Typical machine chest mixing is limited only to the chest bottom. The solution to paper stock consistency fluctuations is to create multiple areas of zonal mixing. Long held perception that stock chest mixing equipment sufficient enough to mix entire volumes required an unmanageable capital investment. Due to the complex rheology of paper pulp, numerical simulations and small scale experiments have not been fully persuasive Total chest mixing can be accomplished by utilizing correctly sized modified impeller designs. Energy savings of 50% are possible compared to standard side entry mixers

5 Rheology Measurement The quantification of fluid properties is the single the most important part of the mixer design process Test setup m (3 ) diameter vessel holding m3 (160 gal) of pulp. Mixer has variable speed drive with 3 modified impellers. The shaft has strain gauges for torque and bending moment measurement.

6 Rheology Measurement Fluid rheological properties is the basic information that is needed for mixer sizing. For mixing applications it is sufficient to quantify the relation between fluid viscosity and the impeller Shear Rate. (Power Law) Impeller(s) is placed in to Newtonian fluids of known viscosity While mixing, the power data vs. RPM is collected for different viscosities. (Re) As the result the following graph is produced

7 Rheology Measurement By doing a similar test in a known non-newtonian fluid the impeller Shear Rate constant t can be calculated l as well Once the calibration is completed the system (vessel+impeller(s)) it is ready for measurement of the properties of the unknown fluid. We call it (CIT) Calibrated a Impeller e Test From the power measurement of mixing in unknown fluid the apparent fluid viscosity is calculated. By running the test at different RPM the Viscosity vs. Shear Rate (RPM) can be calculated and presented in the Power Law form. This data can be used for scale up of mixer of any size. No Yield Stress can be measured. PMSL test setup used 3 (0.91 m) diameter vessel 1000 Apparent Viscosity Comparison -s] Viscosity [Pa Shear Rate [1/s]

8 Rheology Measurement Calibrated Impeller Test 1. Provides reliable data for mixing 2. Applicability is limited to Laminar and Transitional regime of mixing 3. Impeller size must be adequate for the process 4. Data can be presented in form of Power Law and used for scaleup of mixers 5. Yield Stress can not be measured

9 Modified Impeller Design The Counter Flow provides a higher pumping rate than conventional impellers because it can efficiently operate at much higher impeller-to-tank diameter ratios. This is a greater advantage where stagnancy can be a problem. Conventional turbines induce air entrainment, require significantly greater power and produce less flow. As shown by Computational Fluid Dynamics (CFD) modeling, laboratory validation trials and current mill operations mixing up to 20% paper stock, by pumping both upwards and downwards, the Counter Flow Impeller will degas stock and provide uniform consistency throughout stock vessels. CounterFlow Impeller Vectors

10 Modified Impeller Design Utilizing multiple Counter Flow s will provide multiple areas of zonal mixing to maximize residence time and minimize the short circuiting potential from new stock flow. Additionally, the slower rotating speed of the Counter Flow has the potential to provide energy savings, minimize any potential fiber shear damage, and increase the life cycle of standard mixing equipment. CounterFlow Impeller Vectors

11 Yield Stress Fluids Cavern formation in yield stress fluids (Nienow at al)

12 Paper Pulp Mixing Paper pulp mixing concept with several mixing zones. Each impeller p p p g p g p creates its own mixing zone which is not connected to the adjacent one. The upper zone is open to help the release of air.

13 Paper Pulp Mixing Thus, the three major requirements for proper high-concentration h ti pulp mixing are: Thorough, uniform mixing within each mixing zone No short circuiting of fresh material into vessel outlet Degassing of pulp at surface

14 Application Example 6.1 m (20 ) diameter vessel configuration example for CFD simulation A Two 56 kw (75 Hp) side entry mixers and B Three modified impellers on one 56 kw (75 Hp) top entry mixer.

15 Application Example Residence time distribution for side entry mixers (SE) and modified impeller mixer (CF) Mixing time calculated from CFD is as follow: 185s for side entry mixers and 225 s for modified impeller mixer. The mixing efficiencies are 44% for side entry and 54% for modified impeller. The modified impellers are using only 50% of the power required by The modified impellers are using only 50% of the power required by side entry mixers.

16 Startup Torque The start-up torque for mixed pulp does not exceed the mixer running torque. The start-up torque for fresh pulp exceeds running torque by factor of 2.7. This application can be handled by a standard motor.

17 Viscosity Data Apparent Viscosity Data for different types of paper pulp and consistencies.

18 Paper Pulp Mixing A = side entry mixers B = top entry mixing with modified impeller A = side entry mixers, B = top entry mixing with modified impeller technology. For this purpose, velocity iso-plane of 0.5 m/s was selected. Simulation was performed for 2000 Pa Yield Stress Bingham fluid.

19 Paper Pulp Mixing Lab Test 4% 20% stock mixing, 0.91 m (3 ) Vessel, 3 CF Impellers CF Zonal Mixing 4%-20% Consistency Stock with Tomography.wmv.wmv

20 Conclusion Using large D/T modified impellers to mix paper pulp chests delivers significant advantages over traditional side-entry mixing methods. Mixing a 6.1 m diameter pulp chest with high fiber concentration exceeding 6.5%, with modified impellers offers 50% of power savings compared with traditional side-entry mixers 56 kw vs. 112 kw. This new approach mixes the entire vessel content with less power than required by side entry mixers. CFD simulation indicated that mixing quality and consistency fluctuations are much improved over side entry mixing. This leads to additional significant savings on pulp quantity that goes in to process and paper quality. Independent estimation of savings due to the lower paper consistency fluctuations varies, but can exceed $100, per month (U.S.).

21 Conclusion The most significant part of testing is the measurement of pulp properties. It was demonstrated that the Calibrated Impeller Test is a preferred method of collecting data for scale up. Apparent viscosity of pulp does not follow Power Law or Hershel Bulkley Law. Using both models for power calculations will lead to over sizing the mixer. The type of pulp and pulp consistency are not the only parameters that affect apparent viscosity. Additives and fiber length distribution may change the rheology requiring more statistical data.

22 References Amanullah A., Hjorth S.A., Nienow A.W., A New Mathematical Model to Predict Cavern Diameters in Highly Shear Thinning, Power Law Liquids Using Axial Flow Impellers, Chemical Engineering Science, Vol. 53, No3, pp , Bennington C. P. J., Kerekes R. J., Grace J. R., The Yield Stress of Fibre Suspensions, The Canadian Journal of Chemical Engineering, Vol 68, October 1990, p. 748 Wyczalkowski W., How Fluid Properties Affect Mixer & Impeller Design, Chemical Engineering, June 2004, p. 43.