Special Issue on “Advanced in Dewatering and Drying Processes”

Drying and dewatering processes have many industrial applications such as in the agricultural, energy, food, chemical, pharmaceutical, paper, and textile industries. In most drying applications, there is a need to control or improve the quality of the output product [1,2]. At the same time, since the drying process is an energy-intensive operation, any improvements to the existing dryer design are desirable in order to save energy [2,3]. This is a motivation for further research on process intensification and energy consumption reduction, while ensuring a high-quality final product [3]. Therefore, it is important to have a good understanding of the process and the underlying mechanisms.

In this Special Issue, current knowledge and new trends in drying and dewatering techniques have been described, both in terms of experimental and theoretical approaches to the design of new drying and dewatering systems. A total of 11 submissions with different focuses were published.

The focus of the papers can be divided according to the used drying method, the application, and the material dried (see

Table 1. Analysis of the contributions in the Special Issue.

No.	Drying Type	Application	Material
1	freeze drying	food	porous materials
2	convection, fluidized bed and solar drying	food	curcuma longa
3	indirect drying	fuel	bark
4	droplets natural evaporation	biological	water droplet
5	gas–liquid separation	fuel	NG
6	microwaves and freeze drying	food	tomato
7	conventional air drying	food	beans
8	microwave/hot air drying + ultrasound treatment	food	hawthorn
9	conventional air drying—pretreatment	fuel	lignin
10	droplet natural evaporation	biological	blood droplet
11	convective drying + pulse electric field treatment	food	mushrooms

), with most papers relating to food drying. Energy (fuel drying) and biological applications are also represented.

Drying in the food industry is traditionally the most widespread application. Eight contributions have been published on this topic for drying different materials. Different drying methods were investigated, ranging from conventional hot-air drying to freeze drying or microwave drying. A crucial parameter of drying in the food industry is the preservation of the quality of the dried product. Ndisanze and Koca investigated the product quality and fruit characteristics of tree tomatoes using a combination of microwave and freeze drying. A study by Levin et al. presents a prediction of the heat and mass transfer behaviour during freeze drying of porous food particles with the aim of optimising the process, concluding that an increase in porosity is associated with a reduction in mass transfer resistance, but at the expense of reduced heat transfer through the dried particle layer. Llano et al. studied the effect of different drying methods (convection drying, fluid bed drying, and traditional sun drying) on the quality of Curcuma longa (turmeric) powder. The results showed that convection drying and fluidized bed drying, unlike solar drying, did not have a significant effect on the quality of turmeric. Dadan et al. analysed the use of a combination of convection drying with pulsed electric field for drying mushrooms. Kaveh et al. discussed the effect of ultrasonic pretreatment and microwave hot-air drying on drying time, energy requirement, and quality characteristics of hawthorn fruits.

Unlike food applications, where the aim is to maintain product quality, energy applications are all about improving the material properties for maximum energy gain in subsequent use. Three papers have been published on this topic. Havlík and Dlouhý deal with indirect drying of biomass in energy systems and analyse the influence of the degree of final drying of wet biomass on the required size of the dryer. They conclude that for drying wet bark, the optimum degree of drying in indirect dryers is between 31 wt% and 13 wt%. Ji et al. proposed a new design of combined gas–liquid separator for natural gas transportation and storage. Saadon and Osman investigate the effect of drying pretreatment on the hydrolysis of lignin from Napier grass when using conventional air drying.

Drying of biological materials is a rather peripheral part of the application of drying, on which two papers have been published. Here, it involves the dewatering of droplet solutions of various biological materials, using natural evaporation to subsequently biologically analyse the resulting product. Pal et al. investigate the drying kinetics of droplets containing globular protein, phosphate-buffered saline and thermotropic liquid crystals. Ancheyta-Palacios et al. present an experimental study that investigates pattern formation in dry blood droplets with different concentrations of ultrapure water.

In general, drying has a wide range of applications. The research in all published papers focused mainly on describing the kinetics of the drying process. The theoretical description of the process is relatively complex and is often limited by several boundary conditions; therefore, all research papers are solved experimentally. In particular, the current trends in research topics are the combination of different drying methods [2,4,5,6,7], the use of material pretreatment to modify the material properties before drying [1,3,8], and the reduction in the energy consumption of the process [4,9]. A major challenge for the future is the development of methods that, while maintaining the highest possible quality of the dried product, will at the same time achieve the lowest possible energy consumption, also taking into account the economic aspect of the process [1] and the topics on the use of renewable energy sources and their environmental impact [9], which are nowadays very intensively addressed.