Over the years, the meaning of the term ''process intensification'' has changed and today, there is still no precise and generally accepted definition. At least there is a general consensus about the fact that process intensification requires a holistic view on the process, considering the process as a whole system. But the term itself is essentially used as a collective term for diverse measures that aim at a ''significant'' improvement of a process with regard to its efficiency. The problem with that definition is how one can distinguish process intensification from classical process optimization and to what extent an improvement qualifies as ''significant'' (or ''drastic''). In addition, the measure for the efficiency of a process can still be quite diverse (e.g., energy efficiency vs. time efficiency) which makes things even more difficult.

Often, process intensification is associated with striking but vague catch phrases like ''cheaper, smaller, cleaner'' or ''making more with less''. This may be appropriate regarding current discussions in the context of sustainable development. For example, the possibly enhanced corporate image might be one reason for companies to implement process intensification solutions.

A New Concept

However, all these purely descriptive and qualitative attributes do not contribute to specify the term ''process intensification'' more precisely. In the academic community, the lack of an appropriate and exact definition has led to controversial discussions about whether it is necessary at all to propagate process intensification as a new discipline. Hence, there is a clear need of a distinct definition. In a recent contribution a new approach to process intensification has been proposed, based on a concept which encourages a more fundamental view on the individual process steps by analyzing process routes in the thermodynamic state space. This allows for the identification of optimal process routes and provides a basis for a systematic and rigorous classification of process intensification options.

In the following, possibilities for process intensification measures will be outlined for which improvements in productivity have been reported in the literature. These improvements can concern, e.g., a reduction of the plant size by reducing the size of single apparatuses or by reducing the number of the latter by integrating two or more unit operations into a multifunctional unit. Other intensification potentials concern, e.g. the specific energy consumption, the amount of reactants used, and of waste products being produced for a specified production height (i.e., the feedstock efficiency). In many cases, the measures to perform the process improvements have been found incidentally or as an empirical result of a series of experimental studies. To summarize, there is currently neither a theoretical basis nor are there scientific guidelines for process intensification available that can be generalized and thus help to identify process intensification options when analyzing a chemical process as to its efficiency.

Classification Of Components

Process intensification can be achieved by an enhancement of processes which leads to smaller apparatuses and/or by process integration which leads to a reduced number of process steps. Systematic process intensification, however, can only be performed with the help of a suitable methodology that enables us to understand the process under investigation in more detail. For this, one currently still relies to a great extent on experiments.

To get an overview of the options available for process intensification, suitable systematic categories in order to classify the measures have to be introduced. In this regard, Stankiewicz and Moulijn propose to divide the field of process intensification into two areas, namely ''process-intensifying equipment'' and ''process-intensifying methods''. This scheme allows a rough classification of the different measures, but still it is only one of several possible classification schemes one could imagine.

From a mechanistic point of view, it is worth considering a differentiation between (drastic) quantitative improvements on the one hand and improvements as a result of qualitative changes on the other hand. Microreaction technology, monolithic reactors, and reactive separations are examples of the first category, since they allow for a significantly improved mass and heat transfer, while in principle the same physicochemical mechanisms still apply as in conventional apparatuses. Of course, in the case of, e.g., microreactors, the significance of the individual mechanisms (e.g., diffusion processes) that contribute to the overall performance can be completely different. The second category comprises, e.g. alternative forms of energy supply for chemical reactions such as microwaves and ultrasound, the use of new reaction media such as ionic liquids, microemulsions, and supercritical phases, or the use of new auxiliary agents such as phase-transfer catalysts. All these measures involve substantial qualitative changes in the process since here new (additional) mechanisms come into play.

Hierarchical Levels

Finally, another possibility for the classification of process intensification options is to analyze the production process at different hierarchical levels and to gather the intensification measures in superordinate groups at the respective levels. This classification scheme will be explained in the following.

In the chemical and process engineering community most articles on process intensification are based on the unit operation approach and tackle the process at the level of apparatuses, thus ignoring the more detailed and fundamental levels below. Taking into account a more flux-oriented approach which decomposes the process into functions rather than considering the apparatuses, an extended and more fundamental perspective on the process is possible. Following this new concept, a chemical production process can be decomposed into a multi-scale structure of four hierarchical levels. The most detailed level is the molecular level, at which phenomena on the scale of individual molecules are regarded. At the next level one considers molecule populations that build up a thermodynamical phase (phase level). In the process, the thermodynamical phase(s) are embedded into apparatuses, or - more abstract - into individual process spaces. This is the process unit level. Usually, the process consists of several such process units. The interconnection between the individual process units and thus the overall process flowsheet can finally be analyzed at the superordinated plant level.

It is obvious that some of the measures for process intensification that are discussed at the individual levels seem to fit to more than just one level. In particular, changes at a certain level have in most cases a strong impact on the levels ''above'' (e.g., changes at the phase level imply alterations at the process unit level and the plant level respectively). For the classification of process intensification measures into the individual levels it is essential to identify the level at which the main influence on the process is caused.

References and figures are available from the authors.

Read more about this topic in Ullmann's

This article is an excerpt from the Ullmann's Encyclopedia of Industrial Chemistry (wileyonlinelibrary.com/ref/ullmanns) which celebrates its 100^th anniversary in 2014. More about the topic can be found in the encyclopedia article on Process Intensification, 1. Fundamentals and Molecular Level. More concept articles on general interest topics in industrial chemistry and chemical engineering can be found on the Ullmann's Academy homepage