INTERNATIONAL DESIGN CONFERENCE - DESIGN 2000 Dubrovnik, May 23 - 26, 2000.
STRATEGIES FOR THE DESIGN OF PROCESS ENGINEERING MACHINES UNDER SPECIAL MECHANICAL, THERMAL AND CHEMICAL CONDITIONS G. Schäfer and P. Dietz Keywords: design, product development, process engineering machines Abstract: Process engineering machines (e.g. shredders, extruders, separators) were designed in the past from a special processing point of view, no generall design theory for that kind of machines are available. The information exchange between mechanical and process engineering was restricted by different point of views. In order to change this situation, a special research programm has been established in 1986 at the Technical University of Clausthal under multidisciplinary design aspects. During the run of the programm, a design methodology for process engineering machines under multidisciplinary design aspects (special mechanical, thermical and chemical conditions) has been developed. Out of the use of this metholody, new designs of these machines with better process and economical parameters could be realised.
1. Development processes in mechanical engineering and process engineering All process engineering processes are linked with the utilisation of machines, equipment and apparatus that are obtained from mechanical engineering fields or are developed specifically for the process concerned. This is the basis for the idea behind research into a special field which has been in progress for over 12 years at the Technical University in Clausthal, and which - as well as object-oriented development of processes and machines - is also looking into the question of whether and how new methods spanning several different disciplines lead to a more focused and simultaneous development of process and system [Dietz 1993, Dietz 1996]. The objective is clear: The principle of a parallel development of products and processes, which has already been implemented in some disciplines and which is also referred to as “concurrent engineering”, is to be applied to process engineering [Dietz 1996]. Figure 1 illustrates by the conventional course of a process-engineering development how a detailed flowchart is produced from the basic process engineering flowchart, and then an installation flowchart from that; the design elements of the system can be chosen from the latter - the developer or supplier of the elements knows nothing whatsoever about the process until the installation plan is published. A further development in the interplay between process and process elements is thus not possible in both mechanical engineering and process engineering terms, because only familiar standard elements can be included in the considerations. It is abundantly clear precisely from developments in process engineering that concurrent engineering brings considerable potential for development, especially since - as is well known - innovations are largely indicated by developments from other specialised fields.
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Figure 1. Traditional development of processing technology process
Figure 2. Design methodology for process engineering machines
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2. The procedure for developing process engineering machines Endeavouring to treat the development process as a general procedural model and to optimise it with the help of theoretical system observations is nothing new. The well-known systematic design procedures such as VDI guidelines 2221 [VDI 2221] and 2222 [VDI 2222], as well as the procedure described by Blaß [Blaß 1989] for developing process-engineering processes, involve the repetition of an operational step. This step consists of transforming abstract task-defining modules into equipment and machine designs that - in relation to process engineering - are combined to form a complete process engineering system or machine. VDI guideline 2222 [VDI 2222] can provide support with making this procedure concrete; this guideline is complemented by elements of the process engineering procedures, which are discussed below. The procedure plan shown in Figure 2 comprises three columns. The left-hand column contains the utilities incorporated, e.g. DIN 28004, or creativity techniques, while the column on the right contains a summary of the operational results that are to be obtained. The middle column is based in accordance with VDI guideline 2222 on a subdivision into four phases: the planning phase, the conception phase, the design phase and the implementation phase. The procedures shown in the margin indicate that, in accordance with [Kruse 1995], checking must be constantly carried out against the list of requirements, and that the interactive optimisation of process and machine requires a "loop" as specified in VDI 2221. 2.1 Planning phase The origin of the demand which led to the definition of the problem or task can, according to Kruse [Kruse 1995], be traced back to two separate sources: Real customer: A person or an organisation issuing the request or demand. Imaginary customer: A set of requirements for a new system or for a modification to an existing system produced in-house, generated through market research, questionnaires, maintenance reports or other sources. In both cases, once the task has been thoroughly specified it must then be clarified and specified with greater precision in order that a consistent "language" can be found that is even relevant across a range of technical fields for the implementation of a technical product. This consistent "language" is the basic prerequisite for a correct solution of the problem, since the person(s) requesting the development and the project team working on it do not represent a physical unit; the representatives of process-, mechanical- and control engineering etc. who make up the problem-solving team do not share the same training or background, and thus do not use the same language. With the information gained in the process of clarifying the task (patent enquiries, analysis of the competition, market analysis etc.), a detailed analysis is conducted - i.e. the task definition is subdivided and/or allocated. The "list of requirements" which is produced by the work in the planning phase is constantly expanded, updated, modified and implemented in tandem with development up to the point at which the process-engineering machine is produced. 2.2 Conception phase The conception phase starts with the "subdivision into system modules", including the assistance offered by system engineering, the DIN 28004 standard, the VDI 2222 guideline and also the procedure as specified by Blaß [Blaß 1989]. In terms of system engineering, in this operational stage the complex system of task definition is broken down into less complex partial systems [VDI 2221]. Each function can here be represented in the form of a black box [VDI 2222]. Figure 3 illustrates this type of representation for the function "convey material" with the inputs and outputs "energy, material, signal". In contrast with a flowchart, it is important that this function does not yet have any corresponding entity in the design, and is therefore still open to new solution principles, combinations of principles, and the function can even be questioned as a whole. Unlike conventional development in mechanical engineering, in the development of a process-engineering machine the flow of material forms the primary function while the signal flow and energy flow are of lesser significance.
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Figure 3. Black box description of a pump The representation of a partial system through a functional structure and the breakdown into other basic functions described in [VDI 2222] is similar to the representation in the form of a flowchart. In contrast to this, however, the functions do not represent unit operations with corresponding system components, but rather they allow a system to be traced back to basic scientific operations, some of which have never been brought to a technical solution. This enables innovative solutions to be found which are ideally suited to the process-engineering process that is to be realised in the context of developing a machine and/or system. 2.3 Design phase Within the design phase, which in mechanical engineering is characterised by the dimensioning of the power flows, the choice of materials and the specification of the effective movements, the dimensioning of the material and energy flows and thus the specification of the individual modules of the system or process-engineering machine is carried out. This process is supported by calculations and simulations, and creative techniques (brainstorming, synectics etc.) or discursive solution methods (classification systems, design catalogues etc.) can also be employed - the information presented in DIN 28004 may also prove useful in the sense of a catalogue of solutions. As in the previous step, the opportunity for innovation lies in the rejection of standard solutions. In the case of processengineering machines, extremely intensive contact is likely to take place here between process engineers and mechanical engineers. The information required to produce the process-engineering flowchart is made available through the technical/economic optimisation of the design implementation of the active structures produced in the sense of operational contents of the initial subsidiary points of the design phase (Figure 2): All the equipment and machines required for the process, and the main flowlines (main pipelines, primary transport routes) Naming and flows / quantities of the incoming and outgoing materials Naming of energy and energy carriers Characteristic operating conditions 2.4 Implementation phase When the process-engineering flowchart is produced in the context of the design phase, a representation is produced of the process-engineering machine under development that embraces all the structural components in the equipment/machine. After further evaluation and modification using the updated list of requirements, this flowchart forms the basis for creating the machine and system, as the following examples should clarify.
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3. Application of methodology in the development of process-engineering machines 3.1 Example 1: Development of a reaction ball mill In this example, particular attention should be paid to the conception and dimensioning phases. Process engineering generates the task of finding a mechanical implementation for the process of converting iron silicide to trichlorohydrosilicon by hydrochlorination. The mechanical implementation must include a grinding zone in which the iron silicide is ground up to provide it with reactive surfaces, and a heated reactor in which the iron silicide is converted to trichlorohydrosilicon by feeding in HCl in gaseous form. The problem is expressed in a process-engineering flowchart (Figure 4 top) in accordance with the procedures outlined in DIN 28004 and DIN 30600, and thus, based on the catalogue of illustrations provided in DIN 30600, a preliminary decision is made to link a grinding station that conforms to the requirements in series with a suitable reactor. A functional structure (Figure 4, bottom) of the entire function "grind up and convert FeSi" is set up. This makes it clear that the partial functions "grind up FeSi", "mix educts" and "feed in activation energy" can be combined after they have been converted with the help of the solution methods for the corresponding action principles as presented in [VDI 2222]. The partial functions are converted to a common action structure of a reaction mill with integrated grinding functions and chemical conversion in a reaction chamber [Dietz et al. 1992].
Figure 4. Function analysis of hydrochlorination of iron silicide to trichlorohydrosilicon; top: flow chart with main and additional information, bottom: functional structure of the entire function "grind up and convert FeSi"
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Although the example presented concerns only an extract of a process-engineering process and concerns the development of a machine from the very outset, it nevertheless becomes clear in the procedure that the methods developed in the area of product application form an innovation potential also for development of the process. The collaboration needed between machine engineering and process engineering in the areas of task definition, development of functional structure, and evaluation and selection of solutions (which is not shown here) can also be recognised. The joint and detailed working out of the solution principle, including a process-engineering analysis of the principles, has also enabled the swinging system as shown in Figure 5 to be optimised.
Figure 5. Principal scheme of swinging mill for hydrochlorination of iron silicide to trichlorohydrosilicon 3.2 Example 2: Developing a reaction compressor for recycling plastics In contrast to the example above, this example concerns the development of a new type of processengineering process for chemical recycling of plastics with the aim of producing new gases, oils or waxes. The idea behind this project is based on the degradation of polymers in supercritical water, when a thermal and reactive comminution occurs, with a resulting reduction in molecular weight. The result of the process-engineering considerations is the basic flowchart (Figure 6), which serves as a basis and leads to a clarification of the task definition and to an initial functional analysis. One point that can be derived from this is that, for example, the plastics must be changed to a state in which they can be handled under the conditions listed above. The analysis of batch tests and the implementation in a team of representatives of mechanical engineering has led to a detailed representation of the process in the form of a basic flowchart - working with flowcharts has proved useful in this context.
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Figure 6. Flow chart with additional information for the demands on a reaction compressor
Using the details from the basic flow chart and an additional list of requirements, a functional analysis can be carried out. One should bear in mind that the flowchart in Figure 6 contains specifications of unit operations, which are abandoned again in the functional structure in order to open the way to innovations - this step was accompanied by discussions of some seriousness in the development team. It can thus be seen quite clearly that the function-oriented analysis allows the opportunity for an innovation that goes far beyond the familiar elements, e.g. pumps, valves etc., and that most importantly of all it gives the opportunity for a solution that can fulfil several functions in one functional area. Here, too, a solution for a process-engineering machine has developed through the incorporation of methods from machine engineering, most importantly during the design phase. The third party involved in this project deserves special mention: the operating conditions and the intensified corrosive attack by the substances involved in the reaction required that a special material should be developed for the reaction compressor.
4. Summary Using the example of process-engineering machines, a systematic and methodical procedure is presented for developing design components and systems for process engineering with its complex task definitions. This procedure supports development work across a range of different technical fields. The methodology for developing and designing technical systems described in VDI guideline 2221, the design methodology described in VDI guideline 2222 - from examples to design system approaches in process engineering - the system engineering used as a problem-solving methodology in process engineering, and DIN 28004 have all helped to demonstrate that all these procedures contain interesting and innovative elements, but that because of their field-specific orientation and the fact that they contain such single-faceted examples they are poorly suited for supporting work on tasks that span different technical fields. Combining the procedures led to a recommendation which makes the most of including innovative problem-solving methods from mechanical engineering, whereas process-engineering development was conducted essentially through flowcharts. The result is that development of both process and machine is structured more flexibly. Some examples show the successes achieved with this methodology in the context of a special research field and clearly show, amongst other points, that the considerable potential for innovation can also lead to new challenges on the process side as well as the mechanical side.
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References Blaß, E.: Entwicklung verfahrenstechnischer Prozesse. Otto Salle Verlag, Frankfurt am Main, 1989 Dietz, P.: Konstruktionssystematische Überlegungen und beanspruchungsgerechtes Gestalten von Maschinen in der Verfahrenstechnik. Konstruktion 45 (1993), No. 1, p. 17 - 24 Dietz, P.: Der SFB 180: Konstruktion verfahrenstechnischer Maschinen - eine Übersicht. Berichte und Ergebnisse aus dem Sonderforschungsbereich 180. Kolloquium am 15./16. Februar 1996. Clausthal Dietz, P.; Bock, U.: Bestimmung des Leistungseintrags in einer Schwingmühle. Mitteilung aus dem Institut für Maschinenwesen der TU Clausthal No. 17 (1992) Kruse, P.: Anforderungen in der interdisziplinären Systementwicklung: Erfassung, Aufbereitung, Bereitstellung. Dissertation; TU Clausthal 1995 Richtlinie VDI 2221: Methodik zum Entwickeln und Konstruieren technischer Systeme und Produkte. VDI Gesellschaft Entwicklung Konstruktion Vertrieb, Ausschuß Methodisches Konstruieren. VDI Handbuch Konstruktion 1983 Richtlinie VDI 2222: Konstruktionsmethodik; Konzipieren technischer Produkte. VDI Gesellschaft Konstruktion und Entwicklung, Ausschuß Konstruktionsmethodik. VDI Handbuch Konstruktion 1977 Dr.-Ing. Günter Schäfer Fritz-Süchting-Institut für Maschinenwesen Technische Universität Clausthal Robert-Koch-Str. 32 38678 Clausthal-Zellerfeld/Germany Tel.: +49 5323 722270; Fax.: +49 5323 723501; E-Mail:
[email protected]
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