Study of the absorption unit in production of hydrochloric acid

Transcription

1 Study of the absorption unit in production of hydrochloric acid Johan Haglind Department of Chemical Engineering, Lund Institute of Technology A study of the absorption unit in the hydrochloric acid plant at Kemira Kemi AB in Helsingborg has shown that if you want to raise the production or to get a higher concentration of the hydrochloric stream, the gas flow and hydrochloric acid concentration into the absorption unit should be raised. A more effective cooling of the circulations streams to the absorption towers and an increasing cooling of the gas stream into the absorption unit, will affect the absorption positive. Introduction Kemira Kemi AB is located in Ramlösa south of Helsingborg. Here different chemicals are produced such as sulfuric acid, hydrogen peroxide and hydrochloric acid. The hydrochloric acid that is produced here has a concentration of w- %, and is principally used internal for the production of water chemicals, but some is for sale to external customers. In the future Kemira would like to raise their production and concentration of the produced hydrochloric acid and because of that, they wish to the following tropics. Losses of hydrochloric acid you can expect from the absorption unit at different kinds of production conditions. Connection between saltwater flow at different incoming temperatures and the content of hydrochloric acid in the production stream and the remaining acid after the absorption unit. Alternatives to reduce the sulfuric acid concentration in the gas stream. Which are the most cost-effective methods to increase the hydrochloric acid concentration in the production stream. The hydrochloric production is based on the Mannheimerprocess, in which a reaction between potassium chloride and sulfuric acid takes place in ovens up heated with natural gas. H SO + KCl K SO HCl The hydrochloric gas is then passed to a cooling unit so that it s temperature falls to about 60 C. After this the gas is led to an absorption unit, which contains 5 absorption towers, in which the gas is absorbed in counter flowing water to a concentrated hydrochloric acid. The produced acid, about 7,1 ton/h, which is tapped off in tower 1, contains a concentration between 0- w-% HCl. The absorption is strongly exothermic, and because of this, the circulation streams in the first three towers are equipped with heat exchangers. Each absorption tower is packed with some kind of packing: Tower 1 and 4 - ceramic Pall- and Raschig- rings and tower, and 5 Pall rings made of C-PVC Figure 1 Overview of the absorption unit

2 Theory During the absorption of HCl in water a lot of energy is produced, and the amount of heat that is produced is influenced on and influences several factors. The amount of HCl that is absorbed affect on how much hydrochloric acid that has been absorbed earlier in the system, and also the temperature of the solution. A temperature profile will appear in the tower. A more concentrated solution leads to lower solution enthalpy, and lesser heat will be emitted. The temperature of the solution affects how much hydrochloric acid that will be solved in the solution, because of the hydrochloric acid equilibrium curve is temperature dependent. To be able to describe the absorption of hydrochloric acid in water, the absorption towers are split up into differential elements. A material balance and a energy balance is put up for each differential element, to describe the connection between the absorbed amount HCl and the temperature in each element. The enthalpy of solution for hydrochloric acid varies depending on which concentration of hydrochloric acid the solution contains. A low concentration results in high enthalpy of solution. The equilibrium curve for HCl/water is not linear over the whole absorption interval, which depends on high concentration in the solution and the exothermic reaction. Because of this, the equilibrium curve will get steeper at high temperatures. An equation for the equilibrium curve, which depends on both the concentration in the solution and the temperature, has been calculated in Matlab. The equation is: ( 7,78+ 0,0447 T + (1,7498 0,115 T x y = 10 (1. At low concentrations x 0,08 and low temperatures the equilibrium curve is linear and Henry s law is used, to describe the equilibrium curve. Calculations For the calculations some assumptions about the effectiveness of the system have been made. Through experience the effectiveness is assumed to 99, %. Every morning during the production of hydrochloric acid the cooling unit for the gas stream is flushed with water. The solution from this flushing is then led in between absorption tower and, and at the same time, the incoming water flow to the absorption unit, is reduced. The solution contains a quite high concentration hydrochloric acid causing a shifting in the absorption. During all the calculations the stream from the flushing has been assumed to be zero. It has also been assumed that the temperature profiles and concentration profiles are linear in all towers. A model for the system has been made to describe the streams in from of magnitude and concentration. Material balances and energy balances have been calculated for the absorption unit in Matlab. K G a-values have also been calculated for each tower. To describe the equilibrium curve at a certain concentration and temperature, the calculated equation (1 is used. K G a-values To be able to calculate the K G a-values, following equations have been used: y1 z V dy = KG a S P (1 y( y y y where: z = tower height V = gasflow in tower S = cross - section P = pressure, atm N OG of the tower y = y - value on the equilibrium curve Figure Operating line and equilibrium line for tower 1 Figure Equilibrium curve at different temperatures and concentrations As you can see in figure the equilibrium curve is not linear for tower 1 and the N OG -value is calculated by plotting y against 1 (1 y( y y The integration under the curve gives the N OG -value for tower 1. Same procedure is made for tower and..

3 Table 1 Calculated K G a-values and K G a-values from supplier Tower K G a, mol/sm atm K G a, mol/sm atm 1 8,7 19, 5,8 4,1 5,8 4 4,7 5,8 10,1 Figure 4 N OG -value for tower 1 When calculating tower 4 and 5 the equilibrium curve is linear and can be described by following 6 equation: y = 8,59 10 x The N OG -value is here calculated through: y1 y ( y1 y1 ( y y N OG = där ( y y ln = ( y y ln y ln 1 y1 y y The calculated K G a-values for the different towers give a hint, how effective every tower is working. A high K G a-value shows that there is a big driving force in the tower. These values have then been compared with values from the supplier for the packing material in tower, and 5. What you can see is that, there are differences between the values, but that they are in the same range, table 1. Pressure drop To get an idea of how well the absorption towers are running, some pressure drops have been measured. The results have then been compared with calculated values from suppliers and literature. Pressure drop arise when the gas streams are passed through the towers. The magnitude of the pressure drop depends on type of packing in the towers, and the magnitude of the gas streams and liquid streams. The total pressure drop for each absorption tower is the sum of pressure drop losses due to type of packing and losses in pipes, fittings and tower exit and entrance. The calculated pressure drops and the measured values can be seen in table. Table Calculated and measured pressure drops Tower Calculated P, 1,1 4,7,4 4,5, mmvp Measured P, ,5 18 mmvp As you can see, the measured pressure drop is considerable higher, than the calculated value for tower 5. This indicates that the packing is worn and jamming the tower. To get an idea of how close we are to the flooding point a Lobo-diagram has been used Figure 5 Lobo-diagram

4 The calculated values are low and as you can see in the figure 5 they are far from the flooding line. For that reason you should raise the gas flow in each tower alternative raise the liquid flow so that you will get closer to the flooding line and get a better absorption. Wetting rate The wetting rate is a suitable magnitude to estimate how effective the liquid is distributed over, and wetting the surface of the packing in the towers. Calculations show that the wetting rate is low in all towers and therefore an increase in the liquid flow would favor the absorption of hydrochloric acid. Results The efficiency of the absorption unit to day, absorbed quantity hydrochloric acid of total incoming quantity, is quite good, about 99,4 %. The calculated Kga-values are relative low, probably because of the low gas flows. Higher gas flows give a better contact between gas and liquid, which in term leads to higher Kga-values and a more efficient absorption. Another option to further raise the efficiency is to raise the concentration of hydrochloric acid in the incoming gas stream to the absorption unit. Using an IR-equipment to seek for air leakage from the ovens to the absorption unit, and reduce these, is a further way to get a higher concentration. Results from pressure drops calculations also indicate that, the liquid flow is low and can be raised. The values in the Lobo-diagram lie far from the flooding line and a higher flow will displace the values further to the right. As you can see from the measured pressure drops over tower 5, the packing in this tower is not in a good condition. An exchange of the packing to a packing with similar properties is therefore recommended. As earlier mentioned the temperature in the towers has an essential affect on the absorption and this make the cooling of the recirculation streams of big importance. To be able to increase the concentration a better cooling is recommended. An investment in more heat exchangers alternatively improving the existing heat exchangers for tower 1 and, would probably affect the absorption positive. During the present production the gas stream into the absorption unit is cold, but a further cooling would make the absorption even better. At the top of every absorption tower there are four nozzles spreading the circulated liquid smoothly over the packing material in the towers. The nozzles that are used to day, lead probably to that liquid is not distributed over the tower s whole crosssectional area. Exchange of these nozzles to other types of nozzles is therefore recommended. The incoming gas stream to the absorption unit contains, apart from hydrochloric acid, small amounts of impurities. These impurities form some kind of tar, which can jam the nozzles. Another problem that can happen with the nozzles is that they brake, and that the liquid will not be distributed over the tower s whole cross-sectional area and instead run straight down the tower. Therefore it s important to receive indications of how well the nozzles are running. One way to solve this is to procure some kind of flow meter. The best suitable flow meter would be a variable area flowmeter. Conclusions All suggestions that have been brought up to raise the efficiency, the hydrochloric acid concentration in the product stream and the production of hydrochloric acid, require some kind of investment. The suggestions for investments are as follow: New packing for tower 5 Better cooling of the circulations streams and incoming gas stream to the absorption unit Variable area flowmeter for tower 1, and New nozzles for all towers Consider the most cost-effective investments it would be an exchange of the packing, better cooling and new nozzles. Nomenclature L' = Liquid G' = Gas F = Packing g = 9,81 µ L G w w = Gas flow = Liquid m/s = Viscosity = Density flow density density factor of of (m water water s s / m at 0 at 0 o o C (cp C References Hasse, B., Koch H. (1968, Adiabatic hydrochloric acid absorption, Chemische Technik, vol. 0, p Kantyka, T.A., Hlincklieff, H.R. (1954, Adiabatic absorption of hydrochloric acid, Trans. Inst. Chem. Engrs vol., p. 6-4.

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