INVESTIGATION OF PARAMETERS AFFECTING A SELECTIVE FLUE GAS DESULFURIZATION WITH AN AMINE BASE ABSORBENT.
ABSTRACT: A novel selective absorption of SO2 from a produced stream containing SO2, CO2, N2 and steam mixture with amine solution was investigated using an absorption/stripping columns in laboratory scale. SO2 concentration, stripper temperature, gas to liquid ratio and solvent pH were considered as working parameters to investigate their effect on SO2 removal efficiency from the flue gas. According to the experiments, sulfur dioxide which was in the range of 4000 ppm to 8000 ppm in gas mixture was reduced to less than 200 ppm and 1000 ppm respectively.Furthermore the selectivity of the absorption is tested by performing an8 days test which showed the selectivity of more than 99.8 %. Observation showed the effectiveness of parameters ranked to the least as pH, stripper temperature, inlet SO2 concentration and gas to the liquid ratio.
Keywords SO2 absorption, regeneration, amine solution,parameters
As sulphur dioxide is a major atmospheric pollutant generating acid deposits, its abatement in flue gases is of important industrial concern[1,2].Fossil fuels,combustion in power plants, boilers, incinerators, roasting of sulphide ore in metallurgy and sulphuric acid industry are the major sources of sulphur oxide. Although SO2 is a dangerous gas to humanhealth, it's destructive to environment. Large part of sulphur oxides is also added in air due to volcanic eruption. These amount of SO2 in atmosphere leads to air pollution and acid rain [3,4]. Due to presence of moisture in atmospheric air these oxides react with moisture and forms sulphurous acid. The oxides react in the atmosphere forming sulphuric acid, which leads to acid rain. Because of the lowering of the permissible emission limits, there has been a rising interest in new treatment processes for absorbing gaseous SO2, using various reactants.
Many processes emissions to air while providing a new tool to solve the recycling puzzle.The selectivity of this process with high performance separate this process from others, which can have lots of usage in industries. In this process Sulphur, in the form of sulphur dioxide (SO2) is recovered, which can be used in bleaching as a feed chemical, hydrosulphite manufacture, pH adjustment and residual peroxide destruction.
1. Absorption mechanism
In water solution, SO2dissolve reversiblyin a hydration and ionization equilibrium reaction. In the first reaction SO2 molecules are covered with water molecules. Then the result reacts with water to form sulphurous acid according to reaction (2). Then sulphurous acid appears to contribute in an equilibrium to release proton as indicated in reaction (3). SO2 (g) -SO2 (aq) (1)
SO2 (aq) + H2O (L) - H2SO3(L) (2) - + + have been developed, so far. Among them limestone, H2SO3 (L) -HSO3 +H -SO2+2H (3) calcium hydroxide and magnesium hydroxide slurries,sodium hydroxide solutions, and some organic solvents are used as absorbent in those processes[6,7,8].The most well- known flue gas desulfurization processes are mainly based on scrubbing with limestone slurries of the flue gas. These are known as non-regenerative processes [9,10]. Typically, the by-product is either discarded in a landfill or converted into gypsum for use in wallboard and cement manufacturing. Disposal in a landfill requires a large initial capital investment as well as significant resources to maintain the landfill throughout the life of the plant.
There are also regenerative SO2 removal processes like Wellmann-Lord and the citrate processes [9,12] In these processes, SO2 is absorbed by aqueous solution, then recovered from the SO2 rich solution either as SO2 by steam stripping or as elemental sulphur by reacting to H2S[12,13,14].
The recovery and recycling of sulphur are being aided by the commercial introduction of generable selective SO2 scrubbing from gas streams. This new process can reduce emissions to air while providing a new tool to solve the recycling puzzle.
In order to prepare the solvent solution, two kilograms of the prepared amino compound absorber were added to 40 litters of water. So the concentration of 43 grams per litter is prepared, in this case the volumetric percent of 6.6 is reached with a pH of 11.
According to previous studies, the pH should be in a range of 5-6 for the best SO2 absorption [16,17]. Therefore Sulphuric acid was chosenand added to the solvent to reduce pH and help SO2 absorption due to its features. Thus, 2.5 litters of sulphuric acid, with a concentration of 99 percent were added to the solvent solution to reach the desired pH. The pH of the overall solution then reached 6.01.
Two elements each with the power of 2000 Watts are used in the stripper column that heatsupthe absorber solution to approximately 120 degC. Each element is connected to temperature controllers to be set as desired. Heating up to 120 degC will cause the pressure to increase up to 0.8-0.9 (bar). At the top of this tower, a thermocouple is placed to measure the temperature and set the top temperature and pressure at a desired point.
Solvent flow rate varies from 100 to 400 millilitres per minute at the entrance of absorber column. After solvent entrance, it is sent to desorption column via a 750 (w) pump. Then the absorber flow is preheated in a shell and tube heat exchanger illustrated in figure 1, prior to entering the desorption tower.
Feed gas is produced by using three gas cylinders, N2, CO2 and SO2 and a boiler to produce steam. The CO2 and N2 gases were commercial grades with a purity of 99.99%, and the SO2 gas was used with a high purity of 99.9%. All these components are mixed in a collector designed for this purpose. Figure 1 shows a scheme of the pilot plant employed in the experiments. The employed absorber and regenerator were made of 316 stainless steel with an internal diameter of 15 cm, and a height of 1.5 (m). The absorbent flows through the pipes to the absorber and set by using the rotameter to the desired point. All the experiment conditions
are achieved and set by using equipment designed in the pilot. The inlet produced flue gas is analysed by using TESTO 350 containing sensors which can report the concentration of SOX, CO2 and NOX.
3. RESULTS AND DISCUSSIONS
As SO2 is an acidic gas, the increase of efficiency by increasing pH of absorption solution is expected to be favourable to absorption. Aqueous amine compound has base properties so its pH affects the absorption process significantly. Figure 2 indicates the absorption variation with pH at a certain operational condition. Conditions chosen here due to better efficiency were desorption temperature of 120 oC, G/L ratio of 375 and SO2 concentration of 6200 ppm.
By changing the proportion between the amount of sulphuric acid and amine, the pH value of the absorption solvent is adjusted. The desulfurization efficiency increases asymptotically with the increase in pH. It is observed that the SO2 concentration has strongly changed in the outlet of absorption tower. In the other word the absorption efficiency highly depends on pH variation. As it is shown in Figure 2 by increasing absorbent pH, absorption efficiencysharply increased due to the increase in SO2 absorption affinity. The highly tendency of solvent to absorbed can beinterpret along of proton reduction and needs of more SO2 to be absorbed. However in low PH value a decrease in the number of active groups for absorption are seen, while a pH value that is too high leads to an increase of volatile loss, so a suitable PH value must be found.
The absorption variation with gas to liquid ratio. Absorption behaviour shows the increase of gas to liquid ratio, cause a sharply decrease in efficiency. At constant liquid flow rate, a decrease in the gas film thickness occurs by increasing the inlet gas flow rate, and gas-liquid mass transfer area and coefficient increase a little, which is favourable to absorb but on the other hand, an increase in the inlet gas rate decreases the gas-liquid contact time, which reduce the absorption efficiency and effects dominantly on efficiency decrease.
The effect of SO2 concentration on absorption is also investigated. From figure 4 it can be concluded that the efficiency of absorption into amine solution decrease with an increasing in SO2 concentration of the inlet gas. By increasing the SO2 concentration the capability of solvent to absorb SO2 decrease and this is due to absorbent saturation at higher concentration To justify the effect of solvent on absorption the SO2 inlet concentration was increased from 2400 ppm to 9000 ppm. The concentration of SO2 in the outlet of absorption tower showed that the SO2 absorption efficiency decreased from 94 % to 55%. When the SO2 concentration exceeds 7000 ppm, the buffer capacity declines and influence hardly on SO2 removal efficiency.This shows that SO2 concentration is one of the most important parameters affecting on desulfurization while a small change in SO2 concentration in produced flue gas would result in a high change inabsorption efficiency.
Indicates that at 110 degC, the best absorptionefficiency is reached.Absorption variation with desorption temperature shows that by increasing temprature from 100 to 110 an increasing in effiency is observed which is due to approximately complete bounds recovory between SO2 and solvent molecules. By more increasing temperature a decrease in efficiency is observed. Experiment shows that by increasing temperature to 120 degC absorption highly decrease from 79% to 63%. While the temperature increased to 120 degC, the pressure increase from 0.3 bar to about 0.85 bar and does not permit the absorbent to desorb the dissolved SO2.
4. Analysis of the Safety and Economy
In the explained process a novel solvent with a little toxicity,inexpensive (about 2 $ per kg), high selectivity and absorption efficiency is emplayed. The process is preformed at atmospheric pressure and a temperature of lower than 110 oC which reduce the dangerous condition due to low pressure and temperature. In addition, the pH range of using absorbent is near 6 and obtained by addition of sulphuric acid to decreases the volatility of amine and increase pH. Therefore, the damage of the absorbent on air can be nearly ignored due to approximately neutral pH an less evaporation. Furthermore, corrosion of equipment at this acidic pH should be considered.
5. Selectivity investigation.
To produce a real flue gas with the desired composition, a mixture of diesel and mercaptan is burned in a furnace to produce flue gas with SO2 concentration of 2400 ppm.The increase in CO2 concentration is due to the decrease in air oxygen by burning diesel in indoor air. The selectivity of absorption to SO2 is presented in figure 6 and figure 7 respectively. Figure 6 shows the concentration variation ofCO2in the inlet and outlet of absorption column. As it can be seen, the difference between the outlet and inlet is about 0.04 % which shows the selectivity of about 99.8% for SO2. Figure 7indicates the comparision of SO2 concentration variation in the outlet and inlet of absorption tower. the SO2 concentration in the outlet of the desorption tower is approximately constant on the average of 270 ppm, which shows the performance of about 90%.Table 1 presents the operating conditionFor this observation. In this table any parameter that may affect the absorption and regeneration are presented.
It can be noted from this table that the performance for the absorbent with the concentration of 0.05% (V/V) is good enough to introduce the absorbentas a selective high efficient desulfurization process.
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