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Processes in the adsorber
An adsorption zone h is formed in the adsorber itselfz within which the concentration of the adsorptive depends on the inlet concentration ce to a minimum exit concentration thereafter ca falls off. At the same time, the saturation loading X is reduced along the adsorption zones of the adsorbent. The saturation load corresponds to the capacity of an adsorbent to take up an adsorptive, it depends on the respective partial pressure of the adsorptive.
The length of the adsorption zone is determined by the respective adsorption isotherm. The setting of the adsorption equilibrium takes a finite time. The slower the establishment of equilibrium, the longer the adsorption zone becomes.
The rate of adsorption mainly depends on the surface diffusion. In order to reach the active sites of the adsorbent, the gas molecules have to diffuse into the pores of the adsorbent. This is associated with a delay in equilibrium. The rate of diffusion depends on the type of molecule, the temperature and the partial pressure.
The length of the adsorption zone is technically very important, since its relationship to the bed height of the adsorbent determines the adsorption capacity of an adsorber. A large adsorption zone means a small breakthrough loading compared to the saturation loading, which is technically undesirable.
During continuous operation, the adsorption zone migrates through the adsorber to the exit. The adsorptive then occurs in measurable quantities in the exiting gas. The point in time at which the adsorptive can be detected at the exit is referred to as the "breakthrough". The time from the start of operation to the breakthrough of the adsorptive is called the breakthrough time td designated.
The outlet concentration of the adsorptive increases in further operation and finally reaches the value of the inlet concentration ce(ca/ ce= 1). The adsorbent is now saturated, the time to complete saturation is called the saturation time ts designated. The time increase in the adsorptive concentration in the outflowing gas based on the adsorptive concentration at the inlet into the adsorber is called the "breakthrough curve".