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Ind. Eng. Chem. Res. 2003, 42, 1426-1432
Experimental Investigations of Foaming in a Packed Tower for Sour Water Stripping Robin Thiele,*,† Ole Brettschneider,† Jens-Uwe Repke,† Holger Thielert,‡ and Gu 1 nter Wozny† Institute of Process and Plant Technology, Technical University of Berlin, Berlin, Germany, and ThyssenKrupp EnCoke GmbH, Bochum, Germany
A foaming problem in an industrial-size deacidifier (packed column) within the ammoniahydrogen sulfide circulation scrubbing process (AS process) on a specific site is experimentally investigated. The foamability of solutions is successfully assessed in a small test cell, and the Bikermann coefficient is calculated to characterize the foaminess behavior. The investigations revealed that filtration of the real sour water taken from the specific site does not alter foaming characteristics. Impurities, especially phenol and its derivatives, cresols, are found to highly contribute to foaminess behavior. Experiments in a pilot plant revealed higher pressure drop and lower flood point data. Critical spots of foam formation are identified, and design suggestions for column internals are given. 1. Introduction Foaming can be a serious problem in the process industry, reducing throughput and separation performance or even causing contamination of products due to takeover of foam from other vessels. The starting point of this work is a foaming problem in an industrial-size packed tower (deacidifier) within the ammonia-hydrogen sulfide circulation scrubbing process (AS process). The pressure drop of this tower is observed to rise from steady-state conditions for no apparent reason. Similar deacidifiers on other sites do not show that behavior, possibly because of different sour water compositions, caused by different coal compositions or carbonization conditions. Impurities formed during the carbonization process can cause foaming, and their contribution to foamability is so far unknown. The deacidifier is supposed to be redesigned, and maximum operating parameters (i.e., flood point data) using this particular sour water with its specific impurities are requested by the engineers. Studies about foaming in deacidifiers of the AS process cannot be found in the literature; an experimental investigation is therefore necessary. Because of the difficult toxic system and the required complex equipment, the experimental results achieved in this academic research are of rare value. The following chapter gives an overview of foam in general, foam stabilization due to different forces and mechanisms, and, finally, foaming in columns. After that, the determined composition of the sour water will be presented and its foamability will be assessed in a small test cell, resulting in a coefficient characterizing foamability. The contribution of the impurities to foaming behavior will be roughly estimated by experiments with synthetic solutions. Pilot-plant experiments with the packing used in the redesigned tower are carried out to obtain pressure drop and flood point data. The transferability of test-cell * To whom correspondence should be addressed. Phone: +49 30 314 21634. Fax: +49 30 314 26915. E-mail:
[email protected]. † Technical University of Berlin. ‡ ThyssenKrupp EnCoke GmbH.
Figure 1. Characterization of foam on the basis of gas fraction.1
experiments to the real column is evaluated. The hydrodynamics inside the column are observed and presented in different pictures taken from movies. After the identification of critical spots of foam formation, design suggestions for column internals are given. 2. Foam Theory Two foam types are distinguishable. Kugelschaum lies directly on the surface of the liquid (Figure 1) where, because of a high liquid fraction (1 - φ), different bubbles do not interact. The surface forces are responsible for the formation of spherical bubbles. Age of foam rises with height, and gravity forces lead to draining of the liquid. Thus, the liquid films (lamellae) dividing the different bubbles are thinned and the Kugelschaum is converted to polyederfoam. In this area coalescence (film rupturing) prevails. In polyederfoam, three lamellae intercept in a Plateau border with an angle of 120° between the lamellae (Figure 2). Most foams having any significant persistence contain gas, liquid, and a foaming agent. The foaming agent may consist of one or more of the following: surfactants, macromolecules, or finely dispersed solids. The foaming agent is needed to reduce surface tension and thereby lower the required energy to generate the increased surface area in a foam.2
10.1021/ie020676y CCC: $25.00 © 2003 American Chemical Society Published on Web 02/19/2003
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Figure 2. Foam lamella and plateau borders in polyederfoam.2
It is well agreed in the literature that pure liquids do not foam. If single bubbles persist at the surface of a pure liquid, the sample surely is contaminated. Unimolecular films with a surface concentration of 10-10 mol/cm2 can already influence foaminess behavior; thus, special surface cleaning methods need to be applied if a clean surface is necessary for the measurement of foamability.3 The stability of foams is considered against two different processes: film thinning and coalescence (film rupturing). Coalescence reduces the total surface area and therefore surface energy, signifying that foams are thermodynamically unstable; the term stable is used to mean stable in a kinetic sense. The life of foams can vary over many orders of magnitude, from seconds to years.4 Kister4 summarized the forces acting on a liquid film surrounding a bubble in a foam structure and common mechanisms causing foam stability (Table 1). In the following subchapters, the most important forces and mechanisms in foams that require further explanation will be described. 2.1. Surface Tension. Droplets of liquids and bubbles of gas tend to adopt a spherical shape to minimize total surface free energy. The responsible force is surface tension. Surface tension γ° causes a pressure difference across a curved surface, with a higher pressure on the concave side (inside the bubble). Because the pressure inside the bubble is uniform, the different radii at the Plateau border and the lamella cause different pressures inside the liquid (∆P ) 2γ°/R) and consequently a capillary flow (Laplace flow) to the Plateau borders leading to thinning and rupture of the lamellae and causing foam collapse.2 A restoring force can be presented by the Marangoni effect. 2.2. Marangoni Effect. By throwing an oil-drenched sponge into a lake, Marangoni discovered in 1871 that
a liquid with low surface tension spreads itself on a liquid with high surface tension.5 In a surfactantcontaining solution, the surfactants concentrate at the liquid surface. When liquid drains from a film, the film is thinned at this area, causing an increase in the surface area. The additional area is supplied by liquid from the bulk, which is leaner in surfactant concentration. Because of a lower surfactant concentration at the surface, the surface tension rises. The Marangoni effect provokes a surface flow from the nonthinned (low surface tension) area to the thinned (high surface tension) area, which works against drainage and restores the film. The mass-transfer-induced Marangoni effect does not need surface-active components. Here the Marangoni stresses, surface tension gradients, are caused by variation in the liquid composition or temperature at the surface, stabilizing or destabilizing the lamellae.6 In distillation systems, the enhanced mass transfer in thin films leads to a concentration of the less volatile component. If the less volatile component has a higher surface tension (Marangoni-positive system), the thin film is served with liquid from the Plateau borders, thus restoring the film. Foams stabilized by the masstransfer-induced Marangoni effect can be stable and often lead to severe foaming in columns.4 An example for a positive distillation system is methanol/water. The Marangoni effects also have an influence on heatand mass-transfer processes. The induced convection in the liquid increases gradients and therefore enhances transfer rates. Reviews on this subject are available.7 2.3. Foaming in Columns. Numerous foaming case histories of columns have been reported in the literature (see work by Kister4). These suggest the following symptoms for foaming problems: (1) premature flooding and massive entrainment [a foaming condition or aggravation of it is indicated by (a) a sudden increase in the differential pressure, (b) a differential pressure exceeding 50% of the tray spacing, or exceeding 8-10 mbar/m for packed beds, (c) an erratic differential pressure]; (2) nonreproducible pressure drop measurements or flood points; (3) beginning of flooding from steady-state conditions for no apparent reason; (4) flood problem sensitive to temperature; (5) antifoam addition leading to an increase in throughput; (6) abnormal temperature profiles due to reactions occurring on higher stages of the column (e.g., in amine absorbers). Important for this work is the fact that foams can be wall-stabilized (cellular foams) as reported by Kister4 for small and pilot-size columns as used in Chapter 4. If this type of foam occurs, the transferability on industrial-scale columns is questionable. The reasons for foaming can be numerous and sometimes astonishing. A few chemicals that can be responsible for foaming are given by Kister:4 high-molecularweight organic solvents, corrosion inhibitors, reaction
Table 1. Forces and Mechanisms in Foams4 forces on a liquid film gravity: liquid drains from the lamellae interfacial tension: coalescence and disappearance of bubbles capillary forces: tend to stabilize the bubble surface viscosity: opposes drainage of liquid from the film
mechanisms causing foam stabilization the Marangoni effect: surface flow from the nonthinned (low-surface-tension) surface to the thinned (high-surface-tension) surface, counteracting drainage the mass-transfer-induced Marangoni effect: when surface tension increases due to mass transfer, the Marangoni effect can occur Ross-type foaming: a weak solvent-solute interaction may cause surface activity and therefore foaminess gelatinous surface layer: such a layer immobilizes the liquid inside the film; formed by chemical or intermolecular interactions
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Ind. Eng. Chem. Res., Vol. 42, No. 7, 2003 Table 2. Feed Composition chemical analysis
component
c [g/L]
component
hydrogen sulfide carbon dioxide
3.00 10.50
ammonia hydrocyanic acid sodium (Na+)
14.00 0.04 3.00
phenol 2-methylpyridine + 3-methylpyridine 3-methylphenol (m-cresol) 4-methylphenol (p-cresol) dimethylphenol
a
Figure 3. Experimental setup for foaminess assessment.
products of solvents and materials of filters or similar equipment, finely suspended solids, oils or greases, and even leached-off additives from plastic packings. 3. Experimental Assessment of Foam Stability Assessment of foamability in a small and easy to use test cell can be done prior to experiments in the pilot plant to obtain a general rough estimate of the likeliness of foaming in the column. Different test methods are reported in the literature, which have been applied with various success. To obtain significant results, the test should be made at operating conditions (pressure, temperature, and composition). However, this will always be a compromise between effort and detailed replication of operating conditions. Two different types of methods are given in the literature: dynamic methods and static methods. In dynamic methods, the foam is in a state of dynamic equilibrium between rates of formation and collapse. In static methods, the foam is once formed and then allowed to collapse. No regeneration by input of mechanical energy is done. Static methods are used for foams of high stability, whereas dynamic methods are applicable for foams with a low foamability.8 A simple and easy to apply method is the bottle shake test. A closed bottle is strongly shaken up and down and then set on a table. The foam height and the time taken for the foam to collapse are measured. Foaminess4 is indicated with settling times greater 5 s. 3.1. Experimental Procedure. A preliminary assessment of the questioned solution is done with the “bottle shake test”. Short settling times in the range of 10 s are observed, suggesting that the solution has only small foaminess behavior. A more detailed dynamic method is chosen to obtain hydrodynamic mechanisms similar to those of the real column and because of the low foamability of the chemical system of the AScirculation scrubbing. In this case a method proposed by Bikerman3 is used. In this pneumatic method, the foam height is measured while nitrogen flows through a sintered frit producing small bubbles in the liquid (Figure 3). In this work 100 mL of solution is filled into a glass cylinder with an inner diameter of 4.5 cm and kept at a constant temperature by a thermostatic bath. Because of evaporation, the composition of the liquid changes during the experiment. When solutions of components with high volatility (e.g., 3-methylpyridine)
GC-MS screening (impurities) signal intensity, c [g/L] 1.210 n.d.a 0.280 0.190 0.060
n.d.: not detectable (
