Investigation of Foam In Distillation and Absorption Columns

Investigation of foam in distillation and absorption columns

Foam in process engineering can cause severe problems in distillation and absorption towers which may result in pressure loss, therefore reduced capacity, reduced separation efficiency, contaminated products in affiliated processes through foam transfer and to a change in reaction behavior by disadvantageous retention times [1] 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003200300033003800310035003400310035000000 . Those observations were verified in preliminary works at the department of Process Dynamic and Operation of Technical University Berlin [2-4].

The effects lead to economic inefficiency and - due to a higher energy demand - environmental pollution. Highly affected by foam are processes as crude oil, alcohol and biodiesel distillation, (sour) gas scrubbing and CO₂-absorption. In a field study of 31 malfunctions in distillation towers, foaming ranges with 51 of more than 900 cases on 11^th place [1] 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003200300033003800310035003400310035000000 .

Presently, foaming behavior cannot be described analytically or thermodynamically based on physical properties (density, viscosity, etc.) and therefore, hydrodynamic models cannot consider foaming in respect of process dimensioning.

In food and detergent industry, the Bikerman-index is known for decades as an indicator for foaming tendency and stability of mixtures of materials. As a result of the complex hydrodynamics and geometries in distillation towers, the shear forces induced by the tower internals lead to massive foam destruction and generation rates. Therefore, this index is not reliable transferable to distillation towers. Besides, the existing foam stability test methods are bound to ambient conditions.

Since there are no prediction possibilities for foaming in distillation towers available, the industrial solutions to face this problem are on the one hand oversized dimensioning, or on the other hand the search and application of chemical foam inhibitors, which might contaminate the product or pollute the environment. Packed towers are considered more advantageous under foaming conditions [2] 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003200300033003800310035003500300033000000 , [5] 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003100390032003500300030003200320033000000 compared to tray columns.

Beside the works at the department [2-4] and one specialized examination [6] 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003100390032003500300036003700300032000000 , there haven't been any scientific methodical investigations on the hydrodynamics of foaming in distillation towers, especially packed towers.

The main problem of actual fundamental investigations is the uncertain upscale ability to distillation towers, since physical properties as well as geometrical (internals) and process related parameters (hydrodynamics, loads, temperature, etc.) are highly influencing factors. As known from the food industry, more or less complex foam stability testing devices for “static” foams were developed [7] 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003100380035003600310031003800360035000000 .

Therefore, the development of a foam test cell adapted to distillation towers, which considers the internal geometries / shear forces, could help dealing those problems. The new adapted test cell is capable of generating foam out of a liquid sample by aeration and/or adjustable sprinkling (Figure 1). It has a wide operation range of up to 200°C and vacuum conditions. The insertion of packing elements into the 80 mm diameter test cell should provide information about their respective behavior to foam.

For the reason of transferability, a pilot plant with a glass column of 300 mm diameter and 2,000 mm packing height was built. 7 commercial packings (random and structured) are compared according to their respective sensitivity to foaming with several foaming material systems. The operation experiences should provide hints concerning advantageous packing parameters and are used for the upscale of an adapted foam test cell. Until now, the hydrodynamics of 2 mass-% Butanol-water / air was examined with all packings (Figure 2).

Examinations on a “classic” test cell according to Bikerman and on the adapted test cell were carried out. At the example of (pure) MDEA-water solution (CO2 absorption) it has been found, that a foam tendency assessment by glass frit aeration (Bikerman test cell) is not significant enough for an adequate transferability to packed towers. Although MDEA-water solution was foaming in the aerated Bikerman test cell, a problem-free pilot plant operation was possible.

In the adapted test cell, foam generation by sprinkling showed a better transferability to packed towers. It appears that an assessment by sprinkling in terms of packed columns is significant and will be reviewed in more detail.

In the lecture details of the new test facility and about investigations of different column internals and fluids will be given.

Figure 1: Adapted test cell Ø 80 mm with adjustable sprinkling

Figure 2: Foaming in a structured packing Montz B1-350M, Ø 300 mm, packing height 2,000 mm;
L = 50 m³/m²h, F-factor 0.5 Pa^0.5, 2 mass-% butanol-water / air

Literature

[1]         Kister, H.Z.: What Caused Tower Malfunctions in the Last 50 Years?, Chem. Eng. Res. Des. 80 (1), 5-26, 2003

[2]         Thiele, R.; Brettschneider, O.; Repke, J.-U.; Thielert, H.; Wozny, G.: Experimental Investigations of Foaming in a Packed Tower for Sour Water Stripping, Ind. Eng. Chem. Res. 7 (42), 1426-1432, 2003

[3]         Thiele, R.; Wiehler, H.; Repke, J.-U.; Thielert, H.; Wozny, G.: Hydrodynamics of Foaming Systems in Packed Towers, Presentation at AIChE Annual 2004, Austin, 2004

[4]         Thiele, R.; Repke, J.-U.; Diekjakobs, B.; Thielert, H.; Wozny, G.: A General Rate-Based Model for Industrial Reactive Absorption and Desorption Processes in Sour Gas Treatment, AIChE Spring Distillation: Topical Conference Proceedings, 197-208, 2005

[5]         Lebedev, N.; Vladimirov, A. I.; Kos'min, V. D.: Effect of Foaming on Hydrodynamics of Mass-Transfer Contact Devices, Chemistry and Technology of Fuels and Oils 33 (6), 332-334, 1997

[6]         Chen, G. X.; Cai, T. J.; Chuang, K. T.; Afacan, A.: Foaming Effect On Random Packing Performance, Symposium Series No. 152, IChemE, 392-399, 2006

[7]         Potreck, M.: Optimierte Messung der Bierschaumstabilität in Abhängigkeit von Milieubedingungen und fluiddynamischen Kennwerten, Dissertation, TU Berlin, 2004