Drying is Cinderella of process engineering. For many years it was considered rather art than science. When insight into the process deepened, what seemed simple turned out to be complex and difficult to grasp. At about that moment 20 years ago Prof. Arun S. Mujumdar of McGill University realised that the discipline had grown enough to deserve a specialised international symposium. In consequence the 1st International Drying Symposium was organised in 1978. Over the years IDS has grown into a mature international symposium holding three parallel sessions for three days and accompanied by numerous additional events. Undoubtedly the advancement in drying research observed over these two decades can be largely attributed to IDS. The IDS proceedings besides Drying Technology (journal) provide a major source of information on drying process. Over the last 20 years drying became a significant scientific discipline with its structure, theories, methodology, literature. It is difficult to judge to what extent the accompanying advancement of drying technology can be also attributed to the existence of IDS, the creative role of IDS in this respect cannot yet be neglected.

One of the major advantages of IDS, as noticed by Prof. Mujumdar in his plenary lecture (IDS'98 proceedings – paper C2105), is exchange of ideas between participants of different disciplines, countries and industries. The participants could notice, sometimes to their great surprise, that drying is not exclusive to their discipline but is shared by many people throughout the world. This allowed them to raise the level of their own work, avoid unnecessary duplication of research and sometimes develop fruitful cooperation. In the following paragraphs we will have a closer look on how well IDS'98 continued the tradition of IDS.

IDS'98 clearly is the largest IDS ever held. It is difficult to judge what was the reason of such high participation, possibly the location in Greece, the country that invented the idea of a symposium (translating from ancient Greek: sym – co, posion – drinking). It may also be true that since most of the drying research is generated in Europe, any European location would also attract participants.

Statistical distribution of all 292 participants by country is shown in Figure 1. It is interesting to observe that some previous trends were discontinued. First of all it was not the organising country which had the largest participation but France which, taking into consideration their large presence at IDS'96, has become a drying superpower. Second, the combined participation of USA and Canada almost equalled that of France unlike at IDS'96 where North America had rather weak representation. An increase in German participation is also noticeable. Brazil and Japan as usual, take a high score. Certainly the presence of former Soviet-block countries including Poland, was enhanced by a generous reduction of the registration fee offered by the Organising Committee. Sadly enough the Czech Republic, where drying research once flourished (IDS'90 was held in Prague) was only represented by two participants.

Figure 1. Distribution of IDS'98 participants by country

It was pleasure to welcome participants from countries that probably have never before participated in IDS like Cameroon, Macao and Kuwait. It is especially advantageous to see representation of Central Africa where drying is probably the major method of food conservation.

Unfortunately, only 21 (7%) participants came from industry, among them very few represented dryer manufacturers. This detrimental trend is noticeable (Coumans, 1996) and more effort should be undertaken in future to break it.

Together 280 papers were presented (1 plenary, 7 keynote, 100 oral, 172 poster). The structure observed by W.J. Coumans in his report from IDS'96 (W.J. Coumans, 1997, Some impressions of IDS'96, Drying Technology, 15(3&4) pp. 1243-1250) applies equally well to papers presented at IDS'98. The classification is as follows:

For comparison data from IDS'96 were also added. As one can see the overall trend is preserved with a small tendency of IDS to become heavier on agricultural & food side.
Figure 2. shows a detailed distribution in sub-categories. Papers on osmotic dehydration were so numerous that OD is extracted as a separate category from Agricultural & Food Products.

Figure 2. Distribution of IDS'98 papers by subject

It can be observed that IDS is a conglomerate of two groups of interest: agricultural and food drying and everything else. Owing to a countless number of agricultural and food products it is always possible to write a paper according to the following scenario: take a new product, perform a drying test, plot the results, fit a curve to it, write some conclusions and send it to IDS. Occasionally some additional measured properties are also thrown in e.g. sorption isotherms or water diffusivity unless they are published in a separate paper.

An ideal paper describing a new product, in my opinion, should use the following scenario: take a new product, describe its size and form, measure its sorption isotherms at various T, fit  a_w = f(X,T) equation to them, measure drying kinetics of a thin layer, fit a CDC to it, estimate effective water diffusivity from drying kinetics and fit an equation to it, plot shrinkage of your product and fit an equation to it, estimate specific heat of bone dry solid, possibly dependent on T and fit an equation to it, write conclusion and publish.

Only such data could become a valid entry of a product database and a basis for dryer design. Otherwise, searching for pieces of fragmentary information in literature becomes a nightmare and it is easier to measure everything anew. A paper by Azzouz et. al. (B988) on drying of grapes could be used as an example to follow.

At future IDS papers of the first kind should be discouraged and papers of the second kind encouraged, besides, it is a role of international bodies to set up standards for methods of such measurements and presentation forms of their results.

It is interesting to find out what are the dryer types or equipment oriented problems the most often dealt with in the papers presented (of all categories). Figure 3 shows such a distribution.

Figure 3. Distribution of IDS'98 papers according to practical aspects

It is of no surprise that various forms of fluid bed drying are on the rise. Superheated steam drying and electric drying (by microwave, RF, infrared radiation etc.) have a high share too. However, solar drying and heat pumps both related to energy savings have a low count together with all energy aspects as shown in Fig. 2. It is a fact that energy issues in drying become less significant and most attention is concentrated on product quality interaction.

For academics is it equally interesting to know which are the hot theoretical subjects in drying research. Figure 3 identifies several main subjects present in papers of theoretical or semi-theoretical nature. It is somewhat surprising to see many works on the external heat and mass transfer in drying (including the keynote lecture by Prof. Konovalov (A23)). The subject relatively well described in textbooks has secrets that still need to be investigated as shown by an impressive experimental work by Berg and Karlsson (A770). Luikov's equations of coupled internal heat and mass transfer have many supporters and parameters in these equations are identified more often. One would only long for a paper that would explain quantitatively how and in which conditions this approach is better than the old Fick equation dominant in research papers on diffusion in solids.

Although Luikov's equation stems from irreversible thermodynamics this subject itself is poorly represented although drying is purely dissipative i.e. irreversible in nature. Surprisingly enough analysis of drying stresses, once a hot subject, was now represented only by two papers. On the other hand, more papers now present sophisticated microscopic models of internal heat and mass transfer in which stresses and deformation are taken care of. The idea of rigorous description of heat and mass transfer in a porous structure elaborated by S. Whitaker was mastered by the team P. Perré & I. Turner which again presented new spectacular results and a tool for numerical wood drying calculations (A365).

Figure 4. Distribution of IDS'98 papers by research subjects

Their example shows how deep you can go with analysing a single drying process these days. You only need a computer and people like Perré and Turner.

A closer inspection of 3 volumes of IDS'98 proceedings (altogether 2192 pages) indicates that the situation in drying research is rather stable. No dramatically new subject emerged at IDS'98. However, since IDS'96 the NMR imaging technique had found its way to drying labs and measurements of moisture content profiles can now be made in situ (A264, A676). This technique will definitely improve the existing drying theories and will allow for more accurate parameter estimation. Flow imaging techniques will be used more frequently to measure flow fields in dryers. Neural network applications matured and are now a standard technique.

Computer models of the drying process, including micro-scale internal heat and mass transfer will become more trustworthy and practical. Combining these directly or by a trained neural network with a CFD model of the dryer itself will in future provide the ultimate tool for design of dryers.

Little can be concluded about future trends in drying applications. Dryer manufacturers (except two) did not present papers at IDS'98. Dryer types investigated in laboratories were mostly fluid bed, spray and spouted bed as far as dispersed solids are concerned. This, however, may be as well attributed to experimental ease and existing facilities. Very few papers presented experimental or theoretical work turned into an industrial scale dryer.

Superheated steam drying will probably find more industrial applications as Fig. 3 illustrates. Drying by electromagnetic irradiation (MW, RF, IR) will also remain a subject of industrial interest in the nearest future.

IDS'98 was not only lecture and poster sessions. Two Working Parties of EFCE assembled: the Working Party on Drying and the Food Working Party. Panel discussion entitled "Applications of Drying Equipment" and an open forum "University-Industry R&D Transfer" took place. A software presentation session was also organised. The IDS Advisory Panel assembled and discussed subjects relevant to future of IDS. The sites of next IDS were announced: IDS'2000 – The Netherlands, IDS'2002 – Beijing, China. Last but not least the traditional award ceremony took place during the conference dinner where 7 researchers were awarded 6 prestigious awards awarded to them by an international jury of referees presided by Prof. M. Roques.

An unusual highlight of the conference dinner was a poem dedicated to Prof. A.S. Mujumdar on completion of 20 years of IDS'98. The poem by C. Kavafis, distributed to all participants, was printed on parchment and skillfully illustrated by Amit Mujumdar. It was certainly a pleasant surprise to his father.

20 years is the end of adolescence. In the age of maturity it is time to look rather for quality than quantity. IDS'98 established probably a critical mass for IDS series which equals to a number of papers that can be presented in 3 days of 3 parallel sessions and 2 poster sessions. In future it will be probably necessary to accept papers more selectively enforcing the quality criterion.

A typical imperfection of IDS was an inadequate representation of industrial sector. To me it is not a deficiency of IDS alone but of many similar symposia which do not attract end-users of a particular product. In our case the end user is dryer operator and not dryer manufacturer. Manufacturers will show up if their customers show up too. The latter may be attracted to IDS by additional events like drying courses or exhibitions of equipment and better advertising in the end-user circles. If not we will end up presenting more semi-scientific papers at the "Applications of Drying Equipment" panel discussion as it was the case at IDS'98. The open forum "University-Industry R&D Transfer" even noticed that perhaps researchers should seek contact with industry on their own instead of passively waiting for being contacted by industry. Inviting keynote speakers from industrial sector could also help. In fact the keynote lecture on drying difficult products in a fluid bed by P. Filka and I. Filkova (A1) was a good example. In general, keynote lectures should identify and signal new areas and trends rather than present a historical perspective.

As we all know conferences are not only a place to listen to lectures but also to meet old friends and make new ones, establish cooperation, exchange ideas, get inspiration and as a result upgrade the quality of our own research and, at extreme end, the quality of life. To reach these goals the location of IDS'98 in Porto Carras at Sithonia part of Halkidiki peninsula, flawless weather and Greek wine, food and music was just perfect.

For this and all their efforts to make IDS'98 a memorable one the drying community would like to thank Prof. C. Akritidis and Prof. G. Saravacos and their teams of Thessaloniki and Athens.