The Solvay Process for Sodium Carbonate - Research Paper Example

The Solvay Process for Sodium Carbonate The Solvay process for Sodium Carbonate Introduction The Solvey process is an important industrial process that is widely used to produce sodium carbonate (Na2CO3) from Sodium chloride and Carbon dioxide under a complex chain of reactions with Ammonia gas(NH3) being produced as an intermediary. The resulting, ammonium carbonate (NH4HCO3) then reacts with NaCl to produce NaHCO3 and ammonium chloride (NH4Cl). The overall reaction can be given as shown below: i. NaCl(aq) + CO2(g) +NH3(aq) + H2O(l) NaHCO3 + NH4Cl ii. CaCO3 CaO + CO2 (both the CaO and the CO2 are needed in the process) iii. NH4Cl(aq) + 0.5 CaO(s) NH3(aq) + 0.5 CaCl2(aq) + 0.5 H2O  The final important part of the reaction normally involves the decomposition sodium bicarbonate NaHCO3 through heating at high temperatures (calcination) whereby sodium bicarbonate NaHCO3 precipitates and converts to sodium carbonate (Na2CO3) while both carbon dioxide (CO2) and water (H2O) are liberated as gases. This paper critically investigates the thermodynamics of final reaction of the Solvay process (calcination step) during the industrial production of sodium carbonate. Calcination of Sodium Bicarbonate (NaHCO3) The thermal decomposition of sodium bicarbonate NaHCO3 (commonly known as calcination reaction) normally involves heating Sodium hydrogen carbonate to temperatures above 80°C. Although the calcinations reaction normally takes place at temperatures of or above the thermal decomposition of the reactant, it occurs below the melting point of the product material. Calcination is the last step in the production of soda ash. The process begins with an aqueous solution of sodium chloride which is introduced into ammonia followed by carbonization. The ammonium carbonate produced (NH4HCO3) then reacts with NaCl to produce Sodium carbonate (NaHCO3) and ammonium chloride (NH4Cl). The result is heated to release carbon dioxide (CO2) which is the recycled. The calcinations process as mentioned before receives NaHCO3 and produce Na2CO3 carbon dioxide and water (Barner & Scheuerman, 2012).The bicarbonate then undergoes a dehydration reaction in which it gradually breaks down into sodium carbonate, water and carbon dioxide as shown below: This reaction is made up of three side reactions which accompany the substances. NH4HCO3 →NH3+ CO2+ H2O, (NH4)2CO3 →NH3+ CO2+ H2O and NaHCO3+ NH4Cl →NH3+ CO2+ H2O + NaCl Thermodynamics The decomposition of NaHCO3 depends on a number of factors. The pressures at which the carbon dioxide and water are taken back play a big role. It has been reported that the decomposition occurs at 270oC on the higher side and as low as 38oC in one research. The decomposition is crucial in pore formation of sodium carbonate. In most researches the decomposition temperature was set between 121oC and 149oC for optimized conditions. The largest specific area was reported to occur between the temperatures of 204oC and 316oC in a draft oven. The activation energy is higher than 3kcal/mole in diffusion controlled processes meaning that the decomposition is controlled kinetically but may not hold at higher temperatures where the surface area is lost (Barner & Scheuerman, 2012). The thermal decomposition involves the use of heat energy to break the compound(sodium bicarbonate into simpler compounds namely sodium carbonate, water and carbon dioxide. As a result, the reaction is endothermic and the enthalpy changes can be calculated using the underlying equation of: ΔG = ΔH – TΔS  Where ΔG=the reaction’s standard free energy change for the reaction ΔH and ΔS= the enthalpy & entropy changes of the reaction respectively For this reaction: Fig.1: The solubilities of the salts at various temperatures The calcination reaction is an example of a cyclic process of a chemical industry production whereby green=reactants, black=intermediates and red=products. The product of thermal decomposition of sodium carbonate is primarily sodium carbonate (Na2CO3) from brine which is a source of sodium chloride (NaCl); and from limestone as a source of calcium carbonate (CaCo3). The overall process is: 2NaCl + CaCO3→ Na2CO3+CaCl2. The actual thermal decomposition of the sodium carbonate is intricate (Galswey & Brown 2009). A simplistic and simplified chemical description of this process can be given by four differently interacting chemical reactions can be illustrated as follows: in the first step, carbon dioxide (CO2) passes through a concentrated aqueous sodium chloride solution tablet salt (NaCl) and ammonia (NH3). NaCl + CO2+HN3+H2O→ NaHCO3 + NH4Cl (l). In the industrial based practices, this reaction is carried out by passing concentrated brine (salt water) in the two towers and is absorbed by the carbonate salt. In the first tower, the ammonia bubbles are passed through the brine (salt water) and absorbed by it. In the second tower, carbon dioxide bubbles are passed up through the already ammoniated brine; and the bicarbonate salt (sodium bicarbonate/baking soda) which in turn precipitates the solution. In a basic decomposed solution, NaHCO3 is basically of a lesser water solubility than the sodium chloride. The ammonia (NH3) buffers the sodium carbonate solution at a basic pH; this buffering process if done without the ammonia in the basic pH, a hydrochloric acid by-product would make the whole solution acidic and shun the precipitation expected in the calcination process (Clugston & Flemming 2000). The important ammonia catalyst in the Solvay process for the first phase of the reaction (I) is reclaimed in a later reaction step; and a very little ammonia is consumed. The carbon dioxide required in the reaction (I) is produced by heating (calcination) of the limestone at a relative temperature of 950-1100 degrees Celsius. The calcium carbonate CaCO3 in the limestone is relatively converted into an active quicklime (calcium oxide (CaO)) and carbon dioxide respectively: CaCo3→CO2+CaO (II). The sodium bicarbonate (NaHCO3) formed as a result of precipitation in step (I) is filtered out from the thermally decomposed ammonium chloride (NH4Cl) solution and the solution is again reacted with quicklime (calcium oxide (CaO) left over from heating the limestone in step (II). 2NH4Cl + CaO→2NH3+CaCl2+H2O (III). In conclusion, CaO always forms a strong basic solution. The ammonia from reaction (III) is recycled back to the initially used brine solution of reaction (I). The Sodium bicarbonate salt (NaHCO3) precipitate from reaction (I) is converted to the final product that is sodium carbonate (washing soda: (Na2CO3) through calcination process at 160-230 degrees Celsius producing water and carbon dioxide as byproducts. 2NaHCO3→Na2CO3+H2O+CO2 (IV). The carbon dioxide produced in the (IV) step is recovered for re-use in the step (I) of Solvay process. Finally the overall product of the decomposition is: 2NaHCO3→Na2CO3+H2O+CO2+H2O300-500K. Uses and Applications of Sodium Carbonate Sodium Carbonate is one of the most important chemicals with a wide range of applications. Industrial applications of sodium carbonate. Sodium carbonate is produced in all continents of the world and the demand is upto 33 million tons a year. Sodium carbonate has received applications in the industry. These uses include glass containers, flat glass, chemicals, detergents and soaps, other glass, pulp and paper, metals and mining and others.The glass industry takes a larger percentage of sodium carbonate application accounting for more than 50% of the worlds soda ash produced. In the chemical sector sodium carbonate is used in the production of chemicals such as sodium chromate, sodium silicate and sodium bicarbonate. The chemical sector accounts for about 18% of the total production. In production of soaps and detergents sodium carbonate is used as builder in the formulation for detergent chemicals such as sodium tripolyphosphate. In industrial waste treatment, sodium carbonate is used as a neutraliser for acid wastes. In the pulp and paper industry, soda ash is used as a neutraliser just like in the textile industry in brine purification. In the manufacture of cosmetics, sodium carbonate plays a big role. Since sodium carbonate belongs to the GRAS (Generally Regarded As Safe) family it has received industrial application in the food sector The Solvay process may some of the example of industrial thermal decomposition of sodium carbonate (calcination reaction of NaHCO3). The ammonia-soda process is one of the majorly chemical processes used in the production of sodium carbonate (soda-ash). The ammonia-soda process was developed into a modern industrial form by Ernest Solvay during the 1860s. The ingredients used in the manufacture of sodium carbonate thermal decomposition are inexpensive and readily available: limestone from mines and salt brines from the inland sources or the sea. The worldwide production of soda-ash through the calcination reaction of NaHCO3 has been estimated to have risen to about 42 billion kilograms (92 billion pounds); this is averagely six kilograms per year (13lb) for each individual using soda-ash on the earth. The calcination-based chemical plant reaction process roughly produces three-quarters of the overall sodium carbonate supply. References Barner. H.E. & Scheuerman, R.V. (2008). Illustration of Thermochemical Data fur Compounds and Aqueous Species. New York: John Wiley and Sons. Barner. H.E., and Scheuerman, R.V. (2012) Thermochemical Data for Compounds and Aqueous Species, John Wiley and Sons, New York. Clugston, M., & Flemming, R. (2000). Advanced chemistry. Oxford: Univ. Press. Galwey, A. K., & Brown, M. E. (2009). Thermal decomposition of ionic solids. Amsterdam: Elsevier. Read More