(79a) Investigation of Crystallization Fouling on Column Internals

Fouling is the formation and deposition of solids on surfaces in mass and heat transfer applications. Due to the impairment of equipment, such as columns, fouling is a highly undesirable process in industry.

Fouling represents one of the main causes of malfunctions in distillation columns. However, only a few studies on fouling in columns are available so far. To characterize the fouling tolerance of different column internals, a test rig has been set up and put into operation at the Institute of Plant and Process Engineering at the Technical University of Munich. A column with a diameter of 150 mm is used to investigate random and structured packings, and a column with a diameter of 300 mm is used for the investigation of multiple trays.

Since fouling is mostly a slow, gradual process, which can last for years without malfunctions, an acceleration of the experiments is necessary to generate results in a reasonable time. Crystallization fouling is a suitable mechanism for these investigations since the mechanism can be accelerated, and the crystallization fouling is responsible for a significant number of malfunctions in columns in the industry.

The crystallization fouling is forced in the test rig using a saturated sodium chloride water solution and air. Due to evaporation in the column, the solution gets supersaturated and causes the salt to crystallize, which then partly deposits on the internals. The fouling on the internals narrows the free cross-sectional area for the gas flow, which leads to an increase in the pressure drop over the column.

Since the different internals have different separation efficiencies, the fouling tolerance of the internals is difficult to compare directly. To consider the different separation efficiencies, the pressure drop increase is related to the evaporated water mass, which acts as a measure of the actual mass transfer. The fouling tolerance of the internals can be compared according to the amount of evaporated water until a specific pressure drop is reached. In our investigations, this limit is 1.5 times the initial pressure drop. The more water is evaporated, the higher the fouling tolerance of the internals. In addition, the pressure drop can be measured over several levels in the column so that a local detection of the fouling is possible.

These investigations can quantify the fouling tolerance of different internals and operating conditions to identify the most suitable trays and packings for processes prone to crystallization fouling. In this contribution, the test rig and first measurement results will be presented.