Fouling Rate Of Calcium Carbonate On Sieve Trays

Fouling can be defined as the unwanted formation and accumulation of deposits on solid surfaces which is often prevalent phenomenon in many industrial facilities. Distillation and absorption are being widely used in engineering practice due to its technical maturity. Fouling on the column internals will obstruct natural operation when the column encounters the material which deposits easily, it can reduce flux of the column, heighten pressure drop, increase energy consumption. It can also block up the sieve tray, endanger safety in production. Sieve tray column is used widely in distillation and absorption. However, very little study has been devoted to fouling process in sieve tray column.Fouling process is usually slow, it may take days or even months to build up in industrial production. To simulate precipitation fouling, the concept of accelerating fouling is adopted in this work. It allows significant fouling to occur within several hours or several days. Tests were carried out in the system of Ca(OH)2-H2O-CO2-Air system. Fouling caused by the precipitation of calcium carbonate on the surfaces of sieve trays made of four kinds of materials(SS304, PTFE, titanium, low carbon steel) was studied in a 100 mm diameter column. Process of the fouling growing-up was recorded by digital camera. Component and morphology of the fouing were analysied by FT-IR, XRD and SEM. The effect of suspension concentration, temperature, CO2-inlet concentration, F factor, time and sieve tray material on the fouling rate was determined. The experiments were performed in the following conditions:mass concentration of Ca(OH)2 suspension (c1) from 1.01 to 7.08 wt%, temperature (T1) from 15 to 55 ℃, a circulating solution of a constant volume (0.025 m3), and the CO2-inlet concentration(c2) from 300 to 1,000 ppm, gas inlet temperature (T2) ranging from 49 to 51 ℃, F-factor for gas phase loading factor from 1.61 to 2.71 m·s-1(kg·m-3)0.5. Three different crystal forms which are vaterite, aragonite and calcite respectively are found in the proceeding of fouling, crystal tends to form stable calcite thermodynamically in the end. The process of calcium carbonate precipitation on the surface of sieve trays both inside the sieve pores and beside the sieve pores is described.Experiments indicated that the whole fouling process is mainly consisted of three rate stages which can be called as "stimulating stage-the first stage", "transition stage-the second stage" and "sustained and steady stage-the third stage", respectively. During the first stage, there is a distinguished prompt increase in the fouling rate. It can rise to the maximum value dramatically under the operation conditions. When the maximum rate appears, the second stage will follow. Then the rate obviously declines to a stable value, the second stage can also be called as transition period. In the third period the fouling rate curves almost keep as horizontal lines. Compared with the first and the second stages, the fouling rate in the third stage is much smaller.On the base of experimental data, factors’ affecting on the fouling rate can be summarized with the equation:u= Ae -E/RTca1cb2qcv,lqcv,gτe, fouling kinetic parameters of calcium carbonate on the surface of the four sieve trays are obtained. The conformity between the experimental data and the simulated ones permits us to know that the fouling rate of calcium carbonate on the surface of sieve trays under certain conditions can be predicated and calculated. The average value of relative error between the measured data and the simulated values is less than 15.0%. The fouling activation energy on the third stage is the biggest one among the all stages, and the fouling activation energy on the second stage is a litter bigger than that of the first stage. The bigger the activation energy value is, the lower the fouling rate is.From the runs in the experiment, the affinity sequence of factors expect for the surface material affecting the fouling rate can be expressed as following order:1) during the first and the second stage:qv,1>qv,g> τ> c1> c22) during the third stage:qv,1> qv,g> c1> c2The study shows that among the four types of sieve trays, titanium-made sieve tray has the best potential of anti-fouling, low carbon steel-made sieve tray has the least potential of anti-fouling. Surface free energy of PTFE is the lowest among the materials selected in the runs, PTFE has the characteristics of anti-fouling. However, PTFE’s potential of anti-fouling is no better than that of SS304 and titaniumt because of the pure PTFE’s low mechanical strength and low wear resistance. PTFE does not possess a high sense of utilization value if it is selected as the material for anti-fouling in column.The fouling crystal is calcite when the value of [Mg2+]/[Ca2+] is low comparatively, this may be attributed to inhibition on the crystal from [Mg2+] is very poor. The main fouling crystal will be vaterite when the value of [Mg2+]/[Ca2+] is high, just because inhibition on the crystal from [Mg2+] is power obviously.According to experimental phenomenon and analysis of the result, effective measure and research direction for anti-fouling in sieve tray column are obtained as following:1) The fouling can be controlled in the induction period if the operation parameters are adjusted in the appropriate range, so the fouling formation can be postponed.2) The fouling both incompact and deep-set exists in the experimental runs, CaCO3 crystal grain deposits on the interface, the topic that adhering strength’s evolving according to time which is worth to be examined in depth from the viewpoint of engineering.3) The fouling on sieve trays in industrial columns is of more complex and is a poorly understood problem. In order to understand the process and mechanism of fouling, there is a need to carry out a thorough research based on complex circumstances of industrial production, and cooperation between chemists, chemical engineers and material scientists is expected.