A fresh look at design of spray gas quench systems using the Theory of Inventive Problem Solving (TIPS)

Transcription

1 ILASS Americas, 21 st Annual Conference on Liquid Atomization and Spray Systems, Orlando, FL, May 2008 A fresh look at design of spray gas quench systems using the Theory of Inventive Problem Solving (TIPS) Charles W. Lipp* Thomas J. Kling C. P. Christenson Engineering & Process Sciences The Dow Chemical Company Freeport, TX USA Abstract Rapid quenching or cooling of gas streams with sprays is critical in a number of applications including flue gas conditioning, incineration, acid gas processing, and pyrolysis gas processing. Many approaches have been taken to design spray quenches. The art and science of these designs is scattered throughout the academic and patent literature. Analysis of a specific design performance is a routinely solved engineering problem because of the availability of spray measurements and computational tools. Using systematic innovation tools, such as Theory of Inventive Problem Solving (TIPS), can transform the design process by reducing complexity to the lowest level while achieving robust results. This method, also known by the Russian acronym TRIZ, enables rapid development and exploration of innovative design space. The methodology, which includes ideality, function analysis, substance field analysis, and innovation algorithm will be illustrated with an industrial scale spray quench design. This paper is a condensed introduction to this method. TIPS has been applied to increase the quality and yield of innovative solutions to difficult problems. It is more than theory and latest buzzword; it is a practical means of understanding and resolving complex innovative problems.

2 2/9 Introduction Sprays are used for gas quenching because of the extraordinary high cooling rate that can be achieved in an industrial scale. Several systems such as humidification and gas conditioning have similar design requirements and complexity. Many spray quench design concepts have been published in the academic and patent literature. A review in this area shows a number of approaches that have been used over time to achieve effective results and to resolve design conflicts. Spray quenches must be highly reliable; however, they are costly to build and complex to design. Innovation Process The process of innovation is often viewed as having a chaotic or random nature. A flash of innovation, a light bulb turning on is the perception of how new ideas develop. For example, a team brainstorming can typically yield 300 to 600 ideas with perhaps 5% actionable potential solutions. Using TIPS can transform the design process by reducing complexity to the lowest level while achieving robust results (1). Systematic innovation approaches produce three to five times the number of actionable potential solutions than brainstorming, with far fewer non-viable ideas and a dramatically higher signal to noise ratio. The potential solutions developed with TIPS have different characteristics because the fundamental conflicts of the design are being directly addressed. TIPS, also known by the Russian acronym TRIZ, enables rapid development and exploration of innovative design space. The TRIZ methodology was developed in the former Soviet Union by Genrikh S. Altshuller (2) who recognized similar concepts being applied in patents in different technology areas. TIPS (2, 3) uses several key concepts and methods including ideality, function analysis, substance field analysis, and innovation algorithm which will be described in terms of spray technology. A spray quenching problem will be used to demonstrate the application of the TIPS in order to describe fundamental issues developing better designs of spray systems. A typical introductory training for this method is 16 to 32 hours of instruction. Spray Quench Design A quench system shown schematically in Figure 1 is provided to define the context of the problem. Hot gas is cooled from 1300 to 500 K in a 1.5 m diameter refractory lined pipe by evaporating liquid water. Three critical design requirements exist: 1. Protect downstream equipment from excessive temperature, 2. Prevent refractory damage by avoiding quench liquid contact with the walls, and 3. Operate over a range of 20 to 100% of design gas flowrate. Evaluation of a proposed design often requires Lagrangian calculations of trajectory, heat transfer, and evaporation Lefebvre (4). Sh = + Sc Equation Re D g Nu = Re Pr Equation D g These models describe the physics of the system (5) based on a gas heat and mass transfer limitations. Current CFD models such as Fluent 6.3(6) have capabilities to rapidly model all of these aspects of a spray quench. Modeling challenges often center on the range of scales involved, from the droplet diameter to the hardware size which can easily span six orders of magnitude. Conjugate heat and mass transfer have been included the models by others including Elperin (7). In many applications, these complex design cases are readily solvable with current tools. Analysis of droplet trajectory depends on accurate inputs of drop size, velocity, and starting trajectory. The models required need a sub-model of the drop drag coefficient to predict the drop trajectory. This is necessary to calculate the overall heat transfer coefficient for each drop over the drop trajectory. Both heat transfer and mass transfer must be considered to determine evaporation rate. The size distribution of the spray in a specific design must be incorporated in the model to gain a complete representation of the system. Sprays used for quenching can be generated with a number of technologies including single fluid and two fluid atomizing nozzles and a number of configurations. Pagcatipunan and Schick have described a methodology for design (8). The challenge is to determining the best quench design for a specific system. Design methodology Application of the TIPS enables a comprehensive analysis of the design issues and considerations. A full description of all potential tools is provided by several authors (4, 8). This spray quench application is a worked example to illustrate some of the concepts. Rapidly cooling hot gas is the primary useful function of the spray quench. There are several engineering parameters used to describe rate of gas cooling. A functional description does not have engineering parameters to represent the system. Often, the inventive process is a team effort. Problem solvers with experience, successes and failures, are advantaged by their

3 experience but it limits the space of search to the familiar. To avoid being mentally trapped in the familiar ruts and by egos and vocal team members, the use of TIPS abstracts the problem into function space, then applies known solutions and decision matrices to narrow the area of search. 3/9 Ideality The concept of ideality is a foundational concept of TIPS, as defined by Equation 3. Solutions over time tend toward higher states of ideality. A subtle, but important distinction is to define the characteristics of the ideal solution rather than a better solution. The ideal solution characteristics serve as the ultimate benchmark, against which to measure potential solutions. The functions are the most basic level rather than the engineering parameters of design. For this example, the primary useful function is to cool the gas. There are several harmful actions, including the energy consumption to create sprays, liquid that contacts the wall, many failure points and costliness to fabricate. The ideal solution has only useful functions and no harmful actions. For example, a spray quench requires no energy to create the liquid spray, drops never contact the wall, and equipment never fails. As a team works through a group exercise to define the ideal final result, a more complete understanding of the design problem develops. Ideality + UsefulFunctions = HarmfulActions Cost Equation 3 Understanding Design Contradictions The function diagram is the critical tool used to describe the problem at the most fundamental level. Expressing the functions in the most basic form along with the tradeoffs, known as contradictions in TIPS terminology, provides the means to identify the area of the design where major improvements will yield the greatest benefits. Clearly expressing the technical contradictions in the design is challenging because a complete understanding of most problems spans the experience and knowledge of more than one person. Accurately describing the interactions of components and actions is difficult because the basic function must be stated in terms of physics. The engineering parameters are not needed in a functional description; however, they can be useful to establish the direction of the action. When one element acts on another, it changes a parameter, e.g. location, temperature, phase, etc. A function map of this spray quench is shown in Figure 2 and visualizes the system objects, tools and the interactions. The circles represent objects and lines represent actions. For example, the spray nozzle disperses fluid. Action verbs are required in describing the actions. Another example of a function is: the feed header distributes quench liquid. Tools are defined as a main tool or auxiliary tools according to their function and importance. Harmful actions in functional diagram notations are shown in red. The main tool to achieve the quench is the spray or subdivision of liquid into drops. Several adverse actions are shown in this diagram including drops striking the walls and drops cooling only a portion of the gas. Some of the advantageous and adverse actions require several objects interacting for the function. Quenching requires several operations to be achieved simultaneously. An example of a technical design contradiction is that small drop size is required to achieve the surface area and larger drop size is required to penetrate the gas. Adverse actions indicated in the function diagram help design teams better understand the fundamental issues of the design and identify such trade offs. A completed function map compactly provides a visual representation of the interaction of components without excessive attention to the engineering parameters of the design. For a design to be effective, the main function(s) must be sufficient. Sufficient drop surface area and evaporation are essential for all designs. Often, the focus of designers is only on the required function enabled by the collection of drops. As spray technology practitioners, part of our domain expertise is in the design and selection of spray nozzles to achieve this essential function of creating a collection of drops which will evaporate to produce the desired cooling rate. The required science and methods of calculation of the heat transfer coefficient of individual droplets is well established. Obviously, a spray quench with inadequate drop properties will not achieve the primary useful function of rapidly quenching the gas. The harmful actions, highlighted in the function diagram, shows several issues - - droplets that strike the wall due to the drop trajectory and the influence of the hot gas; secondly, gas that is cooled and mixes with the droplets, reducing or inhibiting the droplet evaporation; and lastly, incomplete mixing of the hot and locally cooled gas. An effective improved design must address all conflicts. One design conflict is the mixing of the gas that has been cooled by evaporation with the hot gas to assure that all of the gas is below the required temperature. Industrial scale systems often involve ducts which are greater than 1 meter in diameter. In large ducts, spray penetration limits the quenching. A commonly used

4 solution is to segment the liquid into smaller streams (requiring smaller nozzles) and distributing these nozzles over the cross-section. Spray quench issues with the gasification quench were resolved by just such a method, where a number of spray nozzles were used on three radial headers to distribute the flow over the cross section as described in Figure 4. At low operating rates, one of the three headers was be used. As the quench water flow was increased to handle greater hot gas input, additional headers were be brought into service. One of the classic TIPS tools is the 40 inventive principles shown in Table 1. These principles identified by Altshuller are common to solve a wide variety of engineering problems. Table 2 lists selected US patents in the quenching and gas conditioning area, along with the inventive principles used. Segmentation or subdividing the flow is the most commonly applied principle in these patents. This principle was used as the foundation to resolve the example quench problem. These patents can be categorized into dry or wet wall designs. Wetted wall designs are advantageous to control fouling or deposition of solids or other materials. The wetted wall designs establish the wet-dry interface at a fixed point, adding liquid to assure a controlled amount of liquid on the wall. The technical conflict of wetted wall designs is resolved by evaporation a small fraction of the liquid which allows design with larger drops. In addition, excess quench liquid enables a wide range of gas flows to be quenched with little need for complex controls. Generating new solutions The major strength of the TIPS tool set is the ability to focus the inventive energy to the most productive regions. The conflict zone which is identified from a function diagram can be represented as a substance field diagram. This diagram shows the connectivity or relationships of the substances and energy fields in the conflict zone. The spray quench system substance field (SuField) diagram is shown in Figure 3. Based on the structure of the SuField diagram, a collection of 76 design heuristics or standard solutions (9) are available. For this example, the applicable areas are listed in Table 3. The standard solutions describe changes to resolve the conflict. For example, the first row in Table 3 the action which is difficult to control within the constraints of the problem is clearly an issue for spray quenches. A standard solution suggested is to use a more intense action and remove excess material. Often, spray quenches utilize a large excess of liquid followed by a removal step, for example US Patent 3,842,615. Specific solution concepts are developed from the focus areas listed in Table 3. Describing this process is beyond the scope of this paper. The solution generated in resolving the prototype gasification facility issue is the standard solution listed in row 3. This solution the field should have a maximum or minimum intensity can yield the design principle found in a number of the patents, that of segmentation. A number of low capacity spray nozzles mounted on lateral headers achieve the desired results as shown in Figure 4. Generating potential solutions based on all eight solution areas outlined in Table 3 provides the designer an inclusive set of approaches. This methodology enables a more complete set of solutions to be readily developed by a knowledgeable team, by utilizing the function analysis and SuField diagrams. A precise characterization of the problem results from such work. Narrowing the search space maintains the relevant physics and promotes the creativity of a design team. One might say that the creativity flow has been enhanced because of a focus, thus avoiding the distractions inherent in open brainstorming. The criteria used for narrowing and validation are readily developed from the characteristics in the ideal final result and implementation constraints. Practical experience applying these methods in teams with diverse skills provides a significant number of very unique solutions. By identifying very specific areas of focus, the level of engagement of individuals tends to be very high. The strong ownership of the concepts by the team is critical to develop and implement solutions. Conclusions TIPS based tools are practical, productive and cost effective, not just theory. The tools allow subject matter experts to rapidly and more exhaustively develop inventive solutions. The outcome of using TIPS methodologies in complex problem solving is a more complete and viable set of solutions. One of the most useful tools is the function analysis that helps describe and visualize design issues, enabling a deeper understanding of the core design questions. Applying design heuristics narrows the search space for solutions, enhancing the creative process of even the most creative person. Nomenclature Nu Nusselt number Pr Prandtl number Re Reynolds number based on drop diameter and gas properties Sc Schmidt number based on gas properties 4/9

5 Sh Shirwood number Subscripts D Drop g gas l liquid References 1. Reynard, S., Good Chemistry: Dow Pairs Six Sigma and Innovation, July/August, isixsigma Magazine, G. Altshuller, H. Altov, Lev Shulyak, And Suddenly the Inventor Appeared: TRIZ, the Theory of Inventive Problem Solving, Technical Innovation Center, Inc.; 2nd edition (May 1996) 3. Rantanen, K., Domb, E., Simplified TRIZ, CRC Press Lefebvre, A.H., Atomization and Sprays, Hemisphere Publishing Corporation, 1989, pp Sjenitzer, F., The evaporation of a liquid spray injected into a stream of gas, Chemical Engineering Science, 17, , Ansys.com(fluent) 7. Elperin, T., Fominykh, A., Conjugate mass transfer during gas absorption by a filling liquid drop with internal circulation, Atmospheric Environment, 39 (2005) Pagcatipunan, C., Schick, R., Maximize the Performance of Spray Nozzle Systems, Chemical Engineering Progress, December Fey, V., Rivin, E., Innovation on Demand, Cam bridge University Press, TRIZ Journal Gasification Proto Plant Quench Refractory lined inlet 1300K inlet 1.25 m diameter inlet piping Outlet temp 450 K metal piping Quench Water Inlet Hot gas inlet Cooled gas outlet Figure 1. Schematic of example system 5/9

6 Feed header Distributes Conveys Quench Liquid Cools Disperses X Inhibits Evaporation Accelerates Forms Drops Spray Nozzle Properties Surface area and relative velocity Mixes Gas Evaporates X Strikes wall Walls Figure 2. Function diagram of quench system showing objects and functions. Adverse functions are shown in red. 6/9

7 Figure 3. Substance Field Diagram, SuField, showing the conflict zone interactions Header 1 Header 2 Header 3 Figure 4. End view of multiple nozzle solution used for gas quench 7/9

8 Table principles of TRIZ 40 Inventive Principles 1. SEGMENTATION 2. TAKEOUT 3. LOCAL QUALITY 4. ASYMMETRY 5. MERGING 6. UNIVERSALITY 7. NESTED DOLL 8. ANTI WEIGHT 9. PRELIMINARY ANTI ACTION 10. PRELIMINARY ACTION 11. BEFOREHAND CUSHIONING 12. EQUIPOTENTIALITY 13. OTHER WAY ROUND 14. CRUVATURE 15. VARIABILITY or DYNAMICISM 16. PARTIAL or EXCESSIVE ACTION 17. ANOTHER DIMENSION 18. MECHANICAL VIBRATIONS 19. PERIODIC ACTIONS 20. CONTINUITY OF USEFUL ACTION 21. HURRY THROUGH 22. BLESSING IN DISGUISE 23. FEEDBACK 24. INTERMEDIARY 25. SELF SERVICE 26. COPYING 27. SERVICE LIFE cheap/short vs. expensive/long 28. MECHANICS SUBSTITUTION 29. PNEUMATIC or HYDRAULIC CONSTRUCTIONS 30. FLEXIBLE SHELLS and THIN FILMS 31. POROUS MATERIALS 32. CHANGE OF COLOR 33. HOMOGENEITY 34. DISCARD and RECOVER 35. CHANGE PHYSICAL or CHEMICAL PARAMETERS 36. PHASE TRANSITIONS 37. THERMAL EXPANSION 38. STRONG OXIDANTS 39. INERT ATMOSPHERE 40. COMPOSITE MATERIALS Table 2. Quench related patents and inventive principles used Patent number Year Title Content Principles Used US Apparatus for separating and quenching Combination cyclone and spray quench Preliminary actionremoving solids US Spray cooling apparatus and Spray cooling removing Segmentation method solid product US Rapid cooling for hightemperature gas streams Wetted wall spray quench for pyrolysis gas Local quality- wetting wall, segmentation US Cooling device Gas cooling for corrosive Segmentation, steams US Nozzle assembly Gas condition with removable nozzles and headers US Evaporative gas cooling system and method US Automatic gas conditioning method US Method and device for temperature reduction of exhaust gas by making use of thermal water Gas flow use to prevent drops from reaching wall Control method for gas conditioning tower Control method using superheated water asymmetry Segmentation, preliminary action Segmentation, local quality Segmentation Segmentation, preliminary action 8/9

9 ARIZ Standard Solution number (9) Table 3. Summary of ARIZ solution area for gas quenching Description of standard solution Applicability/comments Action that is difficult to control within the constraints of the problem use most intense action and excess removed Yes quench systems often use great excess of liquid to assure complete action Add another substance connected to the first substance Yes, potential Field should have maximum or minimum intensity Yes, potential Add another field to system- if existing substances and fields Yes, electric field must be maintained If involving magnetic fields use ferromagnetic NA Enhance the effectiveness and controllability of a system with an incomplete SuField by introducing missing (or new elements) Potential- very broad question The effectiveness can be enhanced by tuning or detuning of the frequency of a field action of the object or the tool The effectiveness can be enhanced by tuning or detuning of the frequency of a field action of the object or the tool When two actions are incompatible -- one action is performed during pauses on the other action Yes, highly turbulent systems have broad spectrum of frequencies Yes, potential NA 9/9