Product Design

ELSEVIER Computers and Chemical Engineering 24 (2000) 959-966

Computers & Chemical Engineering

www.elsevier.com/locate/compchemeng

Product design Arthur W. Westerberg *, Eswaran Subrahmanian

Department of Chemical Engineering, Institute for Complex Engineered Systems, Carnegie Mellon, Pittsburgh, PA 15213, USA

Abstract

We respond to a request to examine the area of product design and its impact on chemical engineering. We focus on how we are educating our undergraduates and deliberately adopt a controversial stand. Examples of products requiring chemical engineering concepts are new drugs, a new tape that one can remove without peeling off paint, computer chips and an oxygen-enriching device. We find more of our students involved in product design, both in the traditional chemical industry, where the move to high value added chemicals is occurring, and in start-up companies. There are teaching materials on product design from other disciplines, and Cussler and Moggridge have an upcoming text on chemical product design. Product design requires we spend considerable effort in defining what is to be designed and how we shall measure success of the design, activities we as a discipline de-emphasize in our current process design course. We find a need for very diverse views when designing products, often requiring we team up with people having backgrounds in such fields as fine arts, business, social sciences and other engineering disciplines. A more detailed look at product design activities allows us to point at some of the tools that are useful for this activity. One of our suggestions is that product design may be a better capstone activity as it brings together the students' entire educational experiences and not just their chemical engineering educational experiences. 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Product design; Controversial stand; Traditional chemical industry

1. Introduction

For that past few years, we have been involved in teaching general engineering design courses at Carnegie Mellon. The latest among these courses is one that invites any students on campus in their junior or later year to participate in the design of engineered products. We suspect it is for this reason, we were invited to prepare this paper.

Our goal for this paper is to look at the opportunities for chemical engineers in the product design area, with the intended outcome to influence much more interest in this area in our teaching. The interest is already rampant in the industry. Few, if any, chemical engineering curricula and research efforts concern themselves explicitly with product design. Most of us in the process systems area concentrate on process (rather than product) design and operation.

We start this paper with many examples of products

* Corresponding author.

that chemical engineers would have a distinct advantage in designing. We then make a case for why product design is important for our discipline and should not be ignored. We can easily note that industry has to be and is very actively looking at chemical product design. We next extract the general characteristics of product design that distinguishes it from process design. We hope to make evident areas for research and teaching by making this comparison.

Other engineering disciplines have for some time emphasized product design, and we find in their text- books guidelines for a design process for products (e.g. Pugh, 1990). We outline such a process based on these books, along with approaches we have also developed while teaching our interdisciplinary engineering design courses. We draw specifically from the aforementioned product design course and on another we have taught on design problem formulation for the last decade.

We point out many of the tools we now use in process operation that would help in the design of products. Finally we offer suggestions for changes to the chemical engineering curriculum to make product design a more serious part of it.

0098-1354/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S0098-13 54(00)00400-2

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2. Examples of product design

Everything is a product but some things are more so. In universities, we generally teach chemical process (rather than product) design. In process design, the product the students create is the design, while the ultimate product is the designed plant. Our teaching emphasis is on supporting the generation and analysis of process alternatives for the manufacturing of commodity chemicals. We typically ask the students to discover the most economic process they can devise. Thus we also present to our students the primary goal for their designs: minimum cost and/or maximum profit.

Our goal in this paper is another type of product in which chemical engineers should have a distinct advantage when designing. Examples of these types of products are: a portable device to deliver enriched oxygen to am-

bulatory heart patients a tape that sticks to a painted surface for a year and

then can be removed without pulling off the paint a drug to combat Parkinson's disease a nonfat replacement for cooking oil when frying

food a fuel for race cars a disposable diaper a hand warmer for cold winter football games or for

use on the ski slopes

You can see the contrast with process design by considering the following problems, which are much more typical of what we use as examples when teaching our current chemical engineering design courses: a plant to deliver 1000 tonnes/day of ammonia a manufacturing facility to produce 30% of the cur-

rent demand for vitamin C a process to produce the above new fuel for race

cars, once what it is has been determined

We find today that chemists are much more in the front line of discovering new specialty chemicals. Chemical engineers tend to take over when someone wants such a new chemical manufactured. We would like to argue here that chemical engineers can and should move earlier and into the product design activity, even for specialty chemicals.

3. Some key references

There are three primary references we have used for this paper. The first is the textbook by Dym and Little (2000). The authors produced this book specifically to support student team-based product design activities. We are currently using it in the multidisciplinary engi-

neering product design course we just mentioned above. We have found this book to be particularly useful in teaching students a number of simple but very useful tools to use to attack product design problems. We have also used the previous edition of the text by Ulrich and Eppinger (2000) in this same course. Finally we have found web pages indicating that Cussler and Moggridge (1999) are well along in preparing a text on chemical product design. It appears this last book will be specifically for the design of chemically related products.

4. Why chemical product design?

In a conference paper posted on the web, Cussler and Moggridge (1999) make a very strong case that we should become interested in chemical product design. They state that over half of the BS chemical engineers are now entering product-oriented companies, a large increase over the past. We have witnessed a similar shift at CMU.

A second argument is that our traditional oil and chemical companies are undergoing major changes. To- day, anyone anywhere can manufacture a commodity chemical. If a major company or a country makes it a policy to manufacture styrene, it can buy a turnkey manufacturing plant and, without too much effort, be in business. When one can purchase process technology for commodities so easily, the only advantage a company can have in the long run is to produce the commodity cheaper and/or better, hopefully in a man- ner recently protected by patents. To be cheaper suggests optimizing operations and/or finding improved routes that use less expensive raw materials and/or less fuel. To concentrate on and optimize what a company produces best is the adopted business strategy. Opti- mization technology is also readily available so it alone allows a company only to keep up with what others are doing. Interestingly, such things as being close to the raw materials will win if the final products are cheap to ship relative to the raw materials. Of course, very large tankers and pipelines change the economics of trans- porting commodities and their raw materials.

We have all witnessed a shift by several chemical companies to high value added chemical manufacture. We can see a move to producing such things as specialty chemicals, perhaps as biotech chemicals and pharmaceuticals. However, these are the products of which we are speaking, so these companies are already in the product design business.

We should note that some firms - - notably 3M and Kodak - - have seemingly always directed a considerable part of their attention to the design and manufacture of chemical products.

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A last observation is that the US at this time is a hotbed for new start-up companies. Start-up companies are almost always in the product business and not the commodity business. If we want our chemical engineering students to participate in this exciting activity, they too should learn about product design.

5. Process versus product design

We do teach design in our traditional curriculum. We need to examine how product design differs from what we now cover in our traditional capstone process design course. In this section, we shall contrast process and product design.

5. I. Characteristics of process design

The design effort in process design is to design a process to manufacture what is often a commodity chemical, such as vinyl chloride, ammonia or styrene, or we may dealkylate toluene. We typically tell the students what we want to be made and how much we want of it per year. We also ask them to discover a minimum cost or maximum profit process, i.e. we tell them the primary goal is economics. Of course we also tell them to consider safety, environmental impact (for example, as constraints on producing wastes), and perhaps control. The approach is to ask the students to invent a process built of unit operations to arrive at their design.

The stress in this activity is on large-scale manufacture. The lifetime of the process designed is often measured in decades. The processes are typically con- tinuous ones.

This course is excellent in making the students use all that they have learned in their previous courses on thermodynamics, transport, unit operations and kinet- ics. It is a great unifier of concepts for them, imparting a deeper appreciation of the technical chemical engineering subjects they have learned. It is, in short, an invaluable synthesis of technical concepts they have previously studied.

Students start often by looking for existing processes described in the literature. These they use to develop a base case. We ask them to invent alternatives for their separation process, to heat integrate everything, and so forth. The analysis involves carrying out heat and material balances, leading finally to a simple economic analysis based typically on estimating the prices for equipment, utilities, raw materials and products.

5.2. Character&tics of product design

Product design is different. Perhaps the most important difference is that a key step in product design is

deciding what the product is to be. One would be robbing the students of the excitement of designing a product if one were to tell them all the functionality it should have, suggest to them how they might create the product to get that functionality, and then tell them how they should judge it to be a successful product.

In product design, one typically starts with a word description that outlines what might - - and we stress might - - be wanted. An example already given earlier is to invent a tape that sticks to a painted surface for a year and then can be removed without pulling off the paint. Design specifications are for new product functionality; they are not just purity specs for given molecules.

A goal in product design is to meet customer needs - - often referred to as the 'voice of the customer.' Students have to establish what the product should do, they also have to decide how to invent alternative products capable of delivering this need, and they have to decide how to evaluate the worth of what they have invented. They could easily be involved in developing a business plan for their product as a part of establishing its worth.

Products are different from chemical processes in the following key ways. A product typically has a short life measured in months, compared to perhaps 1 or 2 years. Only if it can be protected by a fairly all encompassing patent and a long development process - - such as a pharmaceutical - - a company can hope it will last much longer.

The company that brings a really new product to the market has a distinct advantage by being first. Thus the design effort may stress getting the product out the door over creating the most economic solution. One hears numbers that suggest being first, leads to captur- ing and often holding onto 70% of the market.

6. Product design is a mix of many talents

Chemical product design is a mixture of many talents, including those from business, fine arts, social science, other engineering, as well as chemistry and chemical engineering.

6.1. Business

Even for commodity chemical manufacturing, business issues prove to be extremely important. Unfortu- nately, we seldom convey this message to our students. For example, an oil company will make or lose much more on the purchase of the right tanker of crude oil than it ever will by optimizing the running of a refinery (see Davis, Subrahmanian and Westerberg (1999) for a discussion of this problem). Technology will exist in a few years that will support making this decision much

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better than we can do today. For example, to decide which to buy, if any, of the tankers of crude that are available, one would like to know the following kinds of information: the amount and composition of the crude in the

tanker (this one is not easy) the amount and compositions for all crude and

products currently in the company's inventory in which tanks at which sites are all those crudes and

products the predicted prices at which they can sell the prod-

ucts vs. time the possible production capacities for all facilities for

all products where geographically are the customers for the

products

The enormous optimization problem you probably see forming here would then be something like searching for that combination of purchases that maximizes the probable present worth of the company, subject to accounting in some fashion for the risks involved. To our knowledge, no one can do this calculation yet. Today much of the buying is based on very intelligent intuition and rough heuristics.

Businesses today are analyzing their supply and value chains. They are looking both upstream and down- stream of their own manufacturing to see if they can reduce costs overall. They may ask cooperating suppli- ers to meet much more stringent standards. They may also evaluate the advantages of consolidating steps along the chain. These business decisions are very important to the well being of the company.

Another example of where business decisions signifi- cantly impact company profits is as follows. Suppose Company A manufactures a high quality monomer at a low cost that it knows others are unlikely to match. However, others may manufacture an inferior monomer at similar costs by simply purchasing existing off-the-shelf technology. The company has to decide whether to sell at higher prices and allow others to get into the market or sell at prices that make the inferior monomer not worth the price needed by the other companies to make a profit. These decisions are likely much more important than all sorts of optimization of processes. However, once made, then optimization offers added profits.

For product design in start-up companies, business decisions are extremely important. Venture capitalists will generally not support a company that does not have a good and believable business plan, laying out its strategy to make money, ultimately. These business decisions often dictate survival, not just profit.

Such decisions can include among other things, the decision that the company will sell low, build up market share, and then increase prices only after it has a

dominant position. These decisions directly affect the profits that we would use to make our decisions when optimizing economic benefits.

6.2. Fine arts

Products have to appeal to consumers. The look and feel of the interface one creates for the product often is the key issue for this appeal. We are risking a lot when we design technical programs, including the user interface, without using the services of people who know which colors and shapes suggest technical competence, and so forth. Non-intuitive interfaces can be a night- mare for users. Non-reassuring interfaces can give users a lack of confidence in the tools.

In our course on product design at CMU, one group designed a laser tape measure. Fine arts design department students spent their entire time designing the color, what sorts of buttons to have, the functionality of the buttons, etc. They did this while the electrical engineers designed the analog chip and circuit board needed inside to do the measurements and to respond to the interface. The difference in customer appeal of their first and their final design was enormous. Few people would have bought the first; the last was very attractive, intuitive and just looked like it was a durable and technically superior product.

Another project we had was to design what became a web-based teaching module to learn about industrial processes for air separation. The goal was a module that would appeal to both high school students and bachelor level chemical engineers. Fine arts students helped their chemical engineering team members to develop the look and feel of the presentation, worrying about the impact of color, organization of the pages, strategic use of animation, and so forth. As we intro- duce computer tools into the workplace, issues of information design, presentation, style and aesthetics become an integral part of their design.

6.3. Social sciences

Fields of sociology, anthropology, psychology, lin- guistics and history of technology provide a rich set of methods to enhance the understanding and execution of product design. These fields provide methods that con- tribute to identifying customer needs, for qualitative methods for decision making and for evaluation of the design process through such instruments as questionnaires, interviews, focus studies and historical analysis. We should see social sciences as our partner in creating engineers who understand consequences and effects of such social processes on decision making and priority setting.

Social science can also play a significant role in helping create a product design environment to support

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collaborating teams having members with diverse backgrounds. As engineers, we should welcome this interface to adapt the methods of social sciences for product and design process design.

6.4. Excellent chemical and chemical eng&eering technology

Finally, for chemical product design, there is no escaping the need for superior science and technology. For example, discovering a mixture of components that will glow when put into high shear can make or break a product. Or finding ways to deliver a drug over 5 h when ingested is required technology for many products.

6.5. Therefore, product design is a mix of talents

One generally does not do product design as we are discussing it here with a team comprising of only chemical engineers. A valid experience requires much more than the technology to be considered to make a successful product. In our course, one of the electrical engineering students understood this point only at the end of the course when he stated that he could be the best circuit designer in the whole world, but, if he produced products no on wanted to buy, he would be a failure.

7. A list of the characteristics of chemical products

We summarize here many of the characteristics we see for chemical products. The characteristics suggest issues we must then consider when designing them.

7.1. Classes of products

We see at least the following three classes of products as being ones for which chemical engineering talents would prove useful. I. Products which are chemicals such as pharmaceuti-

cals, proteins, insecticides, cleaning fluids, lubricants;

2. products that require chemistry to manufacture them such as computer chips and layered manufacturing products; and

3. physical devices that involve chemistry in their delivered functionality, such as a portable oxygen generator and lens that change color in the sunlight.

7.2. General characteristics

As noted earlier, they tend to be high value added

products. They have a lifetime measured in months (before a

better competitive product appears). Small volume and high value suggests one will often

use an existing multipurpose batch plant for specialty chemicals, a general purpose billion dollar fabrication plant for chips, or an existing manufacturing line for physical products. Design of the manufacturing process is often, therefore, a retrofit design or the design of how to operate an existing process differently.

If one is designing the manufacturing process - - that is, what is often the process design part of the design activity, one is carrying out an activity that is often more like designing a kitchen. Based on the range of recipes produced in the past, one knows one wants a few pots of this size, a few more of that and so forth.

Chemical products are often solids. Strangely, we in academia in the US do not emphasize the handling of solids.

8. The product design process

We shall outline here the steps involved in product design. Dym and Little illustrate these steps in figure 2.3 on p. 31 of their text (Dym & Little, 2000). This material as we present it here, is also based on ideas in Ulrich and Eppinger (2000) and on our own experiences in teaching our engineering product design courses.

8.1. Client statement (word statement of need)

The first activity is to discover the goals for the design - - i.e. what it must deliver for all the stakeholders involved to be happy about it. Stakeholders include the designers, the manufacturers, the distributors and the customers. An example of a customer need for the tape above is that it must stick to paint and it must come off a year later without stripping off the paint. If we were manufacturing it, we would also want it to be easy to manufacture and safe. Persons who sell it to the customer may also want it to be available in a familiar form. Dym and Little suggest the construction of an object tree that organizes the desired objectives in a hierarchy, where we may, for example, put safety as a top node. Under safety we might say we want it to be harmless if touched, possibly ingestible and so forth.

In our problem formulation course, we called this first step, the establishing of the goals for the design. We break goals into objectives and constraints, noting that the same goal could be either, depending on the

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effort we may be willing to expend to meet it. For example, we could say we want the product to have a minimum manufacturing cost or we could simply state we want it to cost no more than $0.25 to manufacture it per 10 m of length. The former implies more work to discover the better design.

Products always have many objectives they must meet. By making them concentrate on establishing their goals, of which they usually generate a dozen, students quickly realize that product design must be a tradeoff activity. To this end, our formulation course discusses pareto optimization concepts.

Another difficulty with goals is that in the early stages most of them are very subjective.

Dym and Little discuss different strategies to elicit the needs of all the shareholders, including brainstorm- ing and questionnaires.

A next step once one has a number of goals listed, is to decide the relative importance of each. Dym and Little propose using a weighting scheme applied to the objective tree.

8.2. Managing the design process

Dym and Little outline some useful management tools that can help organize student projects. The first is a 'work breakdown structure,' which organizes the tasks the students decide they will need to develop a design that meets all their goals. One can use a 'linear responsibility chart' to assign these tasks to individuals. Assignments may be as primary responsibility, consul- tant, and so forth. Microsoft's Project allows the development of activity networks that partially order the tasks, calendars and Gantt charts.

8.3. Establishing design function

This step is hard. One has to convert the previously defined objectives into an appreciation of all functions the product must deliver. These are often in the form of an action verb and a noun. For example, a tape must deliver physical strength (it cannot itself disintegrate as one is sticking it on the wall). It must deliver holding power (e.g. stick solidly enough that it can hold a picture onto a wall), and so forth. Dym and Little suggest a host of tools for this activity also.

8.4. Estimate desired levels of performance

One should look at each goal and establish the limits that one expects for it. For example, suppose one is trying to design a product that is similar to that of a competitor but is cheaper to manufacture. We should assess how big a savings we want. We might be barely interested in a savings of $1/unit. Any savings beyond $5/unit would guarantee we are interested ($5/unit).

This understanding helps to reduce the number of options we have to examine by establishing just where in the space of outcomes that we are really interested.

8.5. Develop tests

In our course on problem formulation, we emphasize the creation of tests that allow one to pick a design alternative and assess for it how well it does in meeting the stated goals. For example, we need to state clearly how we intend to assess the manufacturing cost. If a pill is to dispense a medicine over 8 h when ingested, how do we predict that the designed pill meets that goal. Are we going to model it, prototype it, ask a panel of experts, or what? If we prototype it, how many tests will we run and under what conditions? Will humans be involved? Tests can often be very time consuming and often felt to be not worth the effort. Students have to make that decision also. For example, mechanical engineers shudder when someone suggests creating a finite element model for a part they cannot model in their favorite FEM program. For this case they would often rather build a physical prototype and test it.

8. 6. And so forth

There are many more steps that we shall not describe at length. These include establishing the space in which one is willing to look for design alternatives, generating and evaluating alternatives, etc. One must also decide which parts we shall purchase and which we shall manufacture in-house. The team has to decide how to assemble the final product. It must also think seriously about distributing, servicing, and ultimately disposing of the product.

By this time, you should have the idea that product design is fascinating and complex. Hopefully you can see that design teams that have diverse backgrounds involved in them are generally needed.

9. Ties to current chemical engineering process design technology

One can make a case to tie almost all the research chemical engineers are now doing to the design of products. For example, research on tribology asks about how surfaces are lubricated. This science is at the heart of the design of the reading heads for disk memories. So also are computational fluid calculations that permit the modeling of how these heads fly above the disk. Here we shall point at a select few that are in our current process systems engineering methodologies and are directly relevant. From these examples, one can readily see many more.

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9.1. Searching for chemicals with desired functionality - - an inverse problem

There is an interesting inverse problem in chemistry that relates directly to product design. Today we are more and more skilled at using first principles to com- pute the properties of a given molecule. We migrate up from quantum mechanical calculations ultimately to the estimation of mixture properties, using the more detailed computations at each step to give us experiments we can use to fix parameters at the less detailed levels. The inverse problem we need to solve for the design of a specialty chemical is to select the properties and find the molecule or mixture of molecules that give us those properties. Papers in the PSE area, for example by Stephanopoulos and Venkatasubramanian and their research students, have discussed the use of group contri- bution methods to find combinations of groups to give a prescribed behavior. Given the groups, one then attempts to construct molecules having them.

In chemical product design, one recent development that is going to have a significant impact is combinato- rial chemistry. With our ability rapidly improving in the area of fabricating chemical processes on a chip through the use of micro- and nano-technology manufacturing techniques, we can imagine using mass screening techniques to discover chemical products with desired properties. This screening is Edisonian, but, when one can do 100 000 experiments in a day instead of perhaps 5000 in a week, it become a viable screening approach. Quiram, Jensen, Schmidt, Massachusetts, Mills, Ryley et al. (1999) presented a nice review paper on doing chemistry on a chip last year at FOCAPD99 in neighboring Breckenridge.

9.2. Ties to process design and operation methodologies

Many of the tools we have developed for the design and operation of chemical processes are very useful for product design. For example, we can and are extending the process simulators that we use for commodity chemicals so they can handle specialty chemicals. There are now extensions to aid in the design of bio related processes. We also see simulators appearing for the design of batch processes (see work by Harmon Ray and his group at Wisconsin and again by Stephanopou- los and his group at MIT). These are for the design of products that are themselves chemicals. Electrical engineering is the origin for simulators for the design of chip fabrication processes. The very significant chemical and transport aspects of these simulators are where we are making chemical engineering contributions.

We often elect to manufacture chemical products in existing facilities. When the product requires the manufacture of a chemical, we use chemical batch processes. All the techniques we have developed for scheduling

and the expansion of batch processes are central to this activity. These technologies often involve the setting up and solving of mixed integer nonlinear programs, many of which are the themes of papers at this conference. See extensive work by the research groups of Gross- mann at Carnegie Mellon, Sargent and Pantelides at Imperial College and Reklaitis and Pekny at Purdue for examples.

Since we will typically reuse equipment, retrofitting it for new products is a significant part of the design activities for products. This problem is combinatorially extremely difficult. We really should dedicate more activity in PSE to this problem and directly relate our efforts to product design.

The manufacture of pharmaceuticals offers a very interesting engineering problem. The FDA approves both a drug and the exact process by which one manufactures it. This approval takes years and is very costly. In this case, design is directed at not changing anything about the manufacture that could be interpreted as changing the manufacturing process. Scale-up is often the creating of parallel identical processes rather than creating a larger single process. It seems evident also that design of the process must go hand-in-hand with the design of the first process by which one produces enough for testing. It cannot really come later.

10. In closing, some recommendations

A main goal for this article is to make us think seriously about specifically redirecting our efforts in chemical engineering toward the design of products for which our background is important. The move by many chemical companies to high value added products and by many start-up companies to product design in the US would seem to dictate this redirection. These are the employers of more and more of our students. Since much of what we do is already directly relevant, redirection may be simply making students aware of how these courses impact product design. We also attempted here to make the case that we must sensitize our students to the need for them to interact with others from business, fine arts, social sciences, and so forth. This interaction is crucial to the success of product design activities in which they will almost certainly participate.

Based on the discussion we have undertaken in this paper, we offer the following recommendations: Continue with course material on the design of

chemical processes. However, this could be less than the capstone design course currently is. It should include some experience on the design of batch chemical processes as well as on the design of large commodity chemical processes.

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Add a product design course in which there are projects that require chemical engineering expertise but also the expertise of many other disciplines. This course may be better labeled the capstone course as it really brings together an appreciation of their entire educational experiences and not just their chemical engineering educational experiences.

As a part of this last item, have students learn to work on multidisciplinary teams. They should work with students from business, fine arts, humanities, and so forth. In product design these different view- points are essential.

The following two recommendations are for adding a better understanding of chemical engineering technologies that they will almost certainly need in product design. Teach more about solids handling as many chemical

products are delivered as solids. Add more material on batch processes in the unit

operations course.

In thermodynamics, occasionally ask students to find chemicals that have prescribed properties.

References

Cussler, E. L., & Moggridge, G. (1999). An introduction to chemical product design (access either of these WWW pages as they contain the same material) http://www.aidic.it/CONGRESSI/ icheap4/papersicheap4/paperhtm/moggridge.htm; http:// www.chang.cam.ac.uk/news/abstract 15april 1999.html.

Davis, J. G., Subrahmanian, E., & Westerberg, A. W. (1999). SCOPE: a blackboard model-based decision support system for crude-oil trading. International Journal of Intelligent Systems in Finance and Management, 8.

Dym, C. L., & Little, P. (2000). Engineering design: a project-based introduction. New York: Wiley.

Pugh, S. (1990). Total design: methods for successful product engineering. Wokingham, UK: Addison-Wesley.

Quiram, D. J., Jensen, K. F., Schmidt, M. A., Massachusetts, P., Mills, L., Ryley, J. F., & Wetzel, M. D. (1999). Integrated microchemical systems: opportunities for process design. Brecken- ridge, CO: FOCAPD99.

Ulrich, K. T., & Eppinger, S. D. (2000). Product design and development (2nd ed.). New York: McGraw-Hill.

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