## 1 Introduction  
The separation of components in gas-liquid systems, where the liquid is the dispersed phase, is important in industrial practice. There are many reasons for the need to separate such systems. These include the purification of gas products (being final products with given parameters meeting the specification or components for further processes), ensuring the protection of compressors, catalytic, or sorption beds against the negative impact of drops constituting the liquid phase, reducing the emission of aerosol drops into the natural environment or surroundings, e.g., the workplace, etc. An effective, easy-to-implement, and economically beneficial method of eliminating dispersed liquid pollutants from gas streams is the use of gas-liquid coalescing filters. Separation technologies using this type of filter have been used for many years. The process principles comprising gas-liquid and liquid-liquid coalescers are quite similar--both systems have been studied experimentally and theoretically. Understanding the mechanisms of the process is supported by various models describing its course, taking into account the deposition of drops on the structure's fibers, the connection of drops with ones deposited on the fibers or with each other in the flow, the transport of drops through the porous structure to the outlet, and finally the detachment of droplets or drainage of liquid from the outlet surface filter [1]. The operational parameters as well as the structural and surface properties of the fibrous bed have a key impact on the nature of the two-phase flow, which determines the effectiveness of capturing and combining droplets carried by the flow and the effective drainage of the liquid at the outlet, where the gravitational separation takes place [2; 3; 4]. In recent years, the film-channel model was developed, which describes the process principles and, what is very important, clarifies the effect of material wettability on the saturation profiles across the media [5; 6; 7]. The model was validated in numerous research studies; therefore, it accounts for a well-established knowledge regarding the operation of coalescing filters. Few other research studies focus on the effect of the drainage layer in the coalescing filters, including its effects on the saturation profiles, the pressure drop, and the separation performance [8; 9; 10; 11]. Referring to the drainage phenomena, a numerical analysis coupled with AFM measurements for various types of fiber and droplet conformation was published in the literature [12; 13]. These works were focused on the measurements of force required to detach a droplet from wettable and non-wetted fibers (droplets on single filaments or on fiber intersections). Many research studies refer to the effect of fiber wettability on the performance of coalescing filters [14; 15; 16; 17], including the influence of mesh spacers' location on their operation [17]. Moreover, literature studies on the influence of the presence of liquid retained inside the filter on geometric changes in its structure as a result of surface tension forces were published in recent years [18]. This effect may be important for structures with loose fiber packing (i.e., high porosity) and low dynamic stability of the structure. In the case of multi-layer structures composed of separately produced layers, the presence of liquid may affect the spaces between them in a sandwich, which then translates into the overall effectiveness of the filter [19]. There are many studies related to gas-liquid coalescer operation improvement by modification of fibrous material. However, the papers usually refer to typical testing procedures, including saturation at a steady flow rate. In such flow conditions, the possible detrimental effects of interrupted operation are not revealed, which can be of high importance in industrial practice.