Ind. Eng. Chem. Process Des. Dev. 1985,24,223-226
Subscripts
0 = refers to the inlet of a module or to outside 1 = refers to the solvent or to the brine compartment or to the first module or to the first section 2 = refers to the solute b = refers to the brine F = refers to the feed i = refers to the ith series module or volume element, or to inside j = refers to the jth parallel module lz = refers to the kth section of a tapered plant m = refers to membrane M = refers to module p = refers to portion of modules or to parallel modules P = refers to permeate R = refers to reject s = refers to series modules or to sed of a hollow fiber module t = refers to tubes T = total w = refers to the membrane wall on the high pressure side z = refers to the zth section of a tapered plant Superscripts
o = overall quantities
- = averaged quantities
Chiolle, A.; Gianotti, 0.; Gramondo, M.: Parrini, 0. Desallnaflon 1978, 24, 3. Dandavati, M. S.;Doshi, M. R.; Gill, W. N. Chem. Eng. Sci. 1975, 30, 877. Drioii, E.; Lonsdale, H. K.; Pusch, W. Proceedings of the First International Congress on Environment and Energy Crisis, Torino, Italy, 1974. Gill, W. N.; Bansai, B. AIChE J. 1973, 19, 623. Hermans, J. J. DesaHnation 1978, 26, 45. Hung, C.; Tien, C. Desallnatlon 1976, 18, 173. Johnson, J. S.;Dresner, L.; Kraus, K. A. "Principles of Desalination", Spiegter, K. s., Ed.; Academic Press Inc.: New York and London, 1966;pp
345. 439.
Johnson, J.
S.;Bennlon, D. N. Chem. Eng. Prog. Symp. Ser. 1968,
64.
270. Jonsson, G.; Boesen, 0. E. Desalination 1975, 17, 145. Kedem, 0;Spiegler, K. S. Desalination 1966, 1, 311. Khedr. M. G. A. Indian J. Chem. 1980, 19A, 967. Lonsdale, H. K.; Merten, U.; Riley, R. L. J. Appl. Polym. Sci. 1965, 9 , 1341. Mavrov, V.; Mann, J. 6th Int. Congr. of Chem. Eng. Chem. Equip. Des. and Automation, Praha, 1978, 11.5. McCutchan, J. W.; (3081. V. Desalination 1974, 14, 57. Murakami, H.; Igarashi, N. Ind. Eng. Chem. Prod. Res. Dev. 1981, 2 0 ,
501. Pusch, W. Ber. Bunsenges. Phys. Chem. 1977, 8 1 , 054. Pusch, W. "Ultrafiitration Membranes and Applications", Cooper, A. R., Ed.; Plenum Press: New York and London, 1979;p 129. Saltonstall, C. W.; Lawrence, R. W. Desallnatlon 1982, 42, 247. Sherwocd, T. K.; Brian, P. L. T.; Fisher, R. E.; Dresner, L. Ind. Eng. Chem. Fundam. 1965, 4 . 115. Slrkar, K. K.; Rao, G. H. Ind. Eng. Chem. Process Des. Dev. 1981, 2 0 ,
116. Sirkar, K. K.; Dang, P. T.; Rao, G. H. Ind. Eng. Chem. Process Des. Dev. 1982. 21. 517. Sourirajan, S. "Reverse Osmosis"; Logos Press: London, 1970. Sourirajan, S.; Matsuura, T.; Hsieh, F. "Ultrafiltration Membranes and Application", Cooper, A. R., Ed.; Plenum Press: New York and London, 1979;p 21. Trevbal, R. E. "Mass Transfer Ooeration". 3rd ed.: McGraw-HiiI: New York. i980;p 128. Tweddle, T. A,; Thayer, W. C.; Matsuura, T.; Hsieh, F.; Sourirajan, S. Desaiination 1980, 32,181.
Literature Cited Bansal. 6.; Gill, W. N. AIChESymp. Ser. 1974, 7 0 , 136. Bird, R. 6.; Stewart, W. E.; Lighfoot, E. N. "Transport Phenomena"; Wiiey: New York, 1960;p 216. Carberry, J. J. AIChE J. 1960. 6 , 460. Channabasappa, K. C. Desalinatlon 1977, 23, 495. Channabasappa, K. C. Chem. Eng. Prog. Symp. Ser. 1970, 6 7 , 107.
223
Received for review June 14, 1983 Accepted January 20, 1984
This study was supported by a grant from the Italian Minister0 della Pubblica Istruzione.
COMMUNICATIONS Thermochemical Splitting Cycles of Sodium Chloride Two thermochemical splitting cycles, cycle 1 and cycle 2, of sodium chloride have been developed. Water and oxygen are consumed in cycle 1, and carbon dioxide and oxygen are consumed in cycle 2. Chlorine is manufactured by both cycles. The byproducts are sodium hydroxide in cycle 1 and sodium carbonate in cycle 2. The mass and heat flows of these cycles were determined and the heat requirements were estimated for each cycle. I f thermal regeneration is included in these cycles at levels of, for instance, more than 60 % , the thermochemical processes of this work might become economically and energetically competitive with current electrolytic processes of chlorine manufacture.
Introduction A considerable amount of information is available in the literature on various aspects of past and current chlorine generation methods (Versar, Inc., 1979). However, there is very limited information in the literature on the process to be less energy consuming than current electrolytic methods of chlorine manufacture. In this work two thermochemical processes of chlorine manufacture are proposed which use NaCl without employing any electrolytic processes. The processes reported
in this paper differ from previous nonelectrolytic processes in that the byproducts, Le., NaOH or Na2C03,are marketable, every product synthesized in each reaction step is reused in one of the other steps except Clz and the byproducts, and therefore the only chemicals which must be supplied in the processes are NaC1, H20,02, and COz. Related Nonelectrolytic Processes Already Reported C12Generation Process. A chlorine generation process consisting of eq 1 and 2 was proposed in the 1930's. As 0 1984 American Chemical Society
224
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 1, 1985
NaCl
+ 4/3HN03
-
NaN03 + 1/3NOCl + '/3Cl2
'/,NOCl
+ 2/3HN03
-
NO2 + '/&12
+ '/3HzO (1) + 1/3Hz0 (2)
early as that, the Allied Chemical Co. utilized a process which produced C12and NaN03 as byproducts. A facility was built before World War I1 at Hopewell, VA, and it was operated until the mid-l960's, when it was shut down due to a lack of demand for NaN03 (Versar, Inc., 1979). The preferable reaction temperature of eq 1is as high as the boiling point of the reaction mixture itself (Beekhuis, 1942). The generated Clz with eq 1 is fractionated from NOC1, which is then oxidized repeatedly a t about 373 K by an aqueous solution of HN03, about 65 wt %, to be converted almost completely to C12and NOz as shown in eq 2 (Beekhuis, 1940; Spealman, 1965). NaOH Manufacturing Process. A process related to this work is the Lowig process (Chem. Trade J. Chem. Eng., 1917) for the manufacture of caustic soda from sodium carbonate. This process is shown by eq 3 and 4. The 1/zNazC03+ '/zFe2O3 NaFeOz + '/2C02 (3)
-
NaFe02 + '/2Hz0 NaOH + '/2Fe2O3 (4) hydrolytic reaction of NaFeOz shown in eq 4 is almost completed a t 343-353 K and produces about 30 wt % NaOH aqueous solution (Wise, 1882; Matsui and Yasuda, 1923). NO2 Absorption Process. An absorption process of NOz by water is well-known and presently used in the manufacture of nitric acid. The reaction of this process is known as eq 5. NOz + l/zHzO+ ' / 4 0 2 HN03 (5)
-
NOz is more efficiently absorbed by water a t reduced temperatures (Burdick and Freed, 1921; Spealman, 1965). In the current nitric acid manufacturing process, absorption of NO2takes place at or above atmospheric pressure. The pressurized absorption process of NO2is superior, both in absorption efficiency (98-99%) and in the concentration of the final nitric acid, to the atmospheric pressure process (Strelzoff, 1956). Proposed Processes Manufacturing Process of C12 and NaOH. A promising process of manufacturing Clz and NaOH was developed in this work. This process has been constructed by combining those reactions shown by eq 1, 2,4, 5, and an available reaction of eq 6 found for this work. This cyclic process of the chemical reactions is named cycle 1. cycle 1: NaCl
-+
+ 4/3HN03
NaNO,
+ '/3C12 + 2/3H20 (1) NO2 + 1/sC12 + '/3H20 (2)
-l/,NOCl
+ '/&No3 NaFe02 + 1/2H20 NaOH + 1/zFe203 (4) 2NO2 + 1/20z + H 2 0 2HN03 (5) NaFeO, + NO2 + 1/40z(6) NaN03 + '/2FeZO3
'/,NOCl
According to cycle 1, only 1/2 mol of H 2 0 and ' / 4 mol of O2 are consumed to produce 1 / 2 mol of C12 and 1 mol of NaOH from 1 mol of NaC1. As already stated, eq 1, 2, 4, and 5 have been applied or are being applied in the chemical industry. However, eq 6 had never been investigated and was found to be characteristic by this work. The chemical reactions, in general, are conducted at one particular temperature even when they have more than one phase. Equation 6 differs
from general reactions a t this point. The condensed and gaseous phases of eq 6 are controlled by each other at several different temperatures in this work. The condensed phase is kept a t higher temperature than the gaseous phase. When eq 6 is at a temperature higher than 800 K through the reaction system, the components of the gaseous phase are determined to be NO and O2 as shown in eq 7 by a calculation on the basis of the thermodynamic
>800 K
NaN03(1) + '/,Fe203(s) N*e02(s) + NO(g) + 3/40z(g) (7) data (Barin and Knacke, 1973). The gaseous phase consisted of NO and O2 also has an independent equilibrium as shown by eq 8. NO + '/zOz F? NOz (8) The equilibrium constant, KP(!), of eq 8 is shown a t 298-1300 K by eq 9, which is derived from the Gibbs free log Kp(8) = log P N O ~ / P N O P O ~ ' i z = 2875/T - 0.875 log T + 0.00025T - 1.420 (9) energy change of eq 8 on the basis of the thermodynamic data (Barin and Knacke, 1973). As is obvious from eq 9, the reaction of eq 8 proceeds to the right at almost 100% below 400 K. Therefore, when the temperatures are kept a t above 800 K in the condensed phase and at below 400 K in the gaseous phase on eq 6, the reaction spontaneously proceeds to the right irrespective of the equilibrium constant of eq 7 in a homogeneous temperature system. That is the reason why the heterogeneous temperature distribution in the system of eq 6 has the effect which is equivalent to the artificial removing of the generation gases on the condensed phase. Additionally, the equilibrium constants of eq 7, KP(,)= pNOp0,3J4, calculated for homogeneous temperature system, are 3.890 X lo4 at 800 K, 3.232 X at 900 K, 1.010 X at 1000 K, 0.1577 at 1100 K, and 1.486 a t 1200 K on the basis of the thermodynamic data (Barin and Knacke, 1973). Practical reaction temperature in the condensed and gaseous phases in eq 6 must be decided by considering its reaction rate. This work adopted the respective preferable temperatures of 1050 K in the condensed phase and 298 K in the gaseous phase. When the temperatures were controlled at 1050 K in the condensed phase and at 298 K in the gaseous phase in eq 6, NaN03 was converted to NaFeO, at almost 100% in about 30 min. The other observations concerning cycle 1in this work are as follows. NaCl decomposed at almost 100% with eq 1to generate Clz and NOCl at 120 O C in about 20 min with air flowing a t 0.2 L/min, and NaFeO, produced with eq 6 was hydrolyzed a t more than 98% with eq 4 to form an aqueous solution of NaOH, about 28 wt %, at 70 "C in about 30 min. Manufacturing Process of C12 and Na2C03. A promising process of manufacturing C12and NaZCO3was also developed by this work. This process was constructed by combining those reactions shown by eq 1,2,11, and an available reaction of eq 10 found for this work. This cyclic process of the chemical reactions is named cycle 2. cycle 2: NaCl + 4/3HN03 NaN03 + '/,NOCl + 1/3Clz+ 2/3H20(1) l/,NOCl
+ + + + + + + + 2/3HN03
NOz
'/&lZ
1/3H20 (2)
NaN03 + '/&02 '/zNazC03 NO 3/40z NO2 NO Oz HzO 2HN03
(10) (11)
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 1, 1985 225
Qp=
7.71
-
Qr=
&=
30.8
Figure 1. The mass and heat flows of the thermochemical splitting cycles of NaCl: (a) cycle 1; (b) cycle 2. The amounts of the chemicals were represented in moles.
According to cycle 2, only ' I 2mol of C 0 2 and mol of Oz are consumed to produce ' I 2mol of ClZand 'Iz mol of Na2C03from 1 mol of NaC1. As already mentioned, if NO and O2 exist together a t the stoichiometric ratio, those chemical species are almost completely converted to NO2 below 400 K as is obvious from eq 9, and NO2 is more efficiently absorbed by water a t reduced temperatures. Thus, eq 5 and 11 are identical when NO was converted to NOz by O2before absorption of NOz by water. Equations 1, 2, and 11 of cycle 2 are regarded to be common to cycle 1. On the other hand, eq 10 was investigated experimentally and by thermodynamic calculations. The practical reaction temperature of eq 10 should be above 1100 K. The AGO of eq 10 is -3.09 kcal
at 1100 K and the reaction spontaneously proceeds to the right a t temperatures higher than 1100 K. The reaction rate of eq 10 is relatively fast. For instance, about 97% of the initial NaN03 (6.320 X mol) reacted with C02 to form Na2C03after only 5 min at about 1100 K with C02 flowing a t 0.63 L/min under atmospheric pressure.
Discussion Mass and Heat Flows of the Cycles. The mass and heat flows for cycles 1 and 2 were estimated and were based on the previously mentioned thermodynamic data and the experimental results. The flow sheet for this work was determined by applying the method developed for estimating the thermal efficiency of the thermochemical
226
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 1, 1985
water splitting cycles (Funk and Reinstrom, 1966; Knoche and Funk, 1977; Dokiya et al., 1979). The estimated mass and heat flows for these cycles are shown in Figure 1. The concentration of nitric acid used in the manufacturing process of NaN03 (eq 1) was assumed to be 66 wt % for both cycles. The following assumptions were made in the construction of the flow sheets. Cycle 1. The conversion of NaCl was assumed to be 100% for an initial mole ratio of HN03/NaC1 = 4. When three times the stoichiometric amount of 70 w t % HN03 aqueous solution was added, the conversion of NOCl was assumed to be 100%. NaN03 was assumed to be completely converted to NaFeOz for the stoichiometric ratio of NaN03to Fez03. The nitric acid manufacturing process is assumed for the nitrogen dioxide absorption step (eq 5). The conversion ratio for nitrogen dioxide was assumed to be 100% under about 2 atm pressure of nitrogen dioxide at 298 K. The conversion of NaFe02 was assumed to be 100% for an initial mole ratio of H20/NaFe02 = 11.4 to give a 30 w t % NaOH aqueous solution. Cycle 2. The formation step of NaN03, the oxidation step of NOC1, and the absorption step of nitrogen dioxide are the same as in cycle 1. On the other hand, a carbonation step for NaN03 decomposition is adopted in this cycle. NaN03 is assumed to be completely converted to Na2C03for a mole ratio of C02/NaN03 = 3/2. Heat Requirement i n Chlorine Manufacture. The heat requirement for splitting NaCl in these cycles was calculated by using eq 12 (Funk and Reinstrom, 1966; Qreq.
=
C ( ~ T O+ Qr) - C Q p
X
Ri
(12)
Knoche and Funk, 1977; Dokiya et al., 1979) as shown on the base of the flow sheets in Figure 1. C(AHTO + Q,)and CQ, X R . for cycle 1 are respectively 4.24 X lo9 cal and 2.44 X 10Q cal/ton of chlorine with 1.126 tons of sodium hydroxide, and each of them for cycle 2 are 3.66 X lo9 cal and 2.46 X lo9 cal/ton of chlorine with 1.493 tons of sodium carbonate, if Riis assumed to be l in both cases. Therefore Q, is readily determined a t Ri= 0-1 from eq 12. For instance, if Ri= 0.8, the heat requirement for manufacturing 1 ton of chlorine for cycle 1 is 2.29 X lo9 cal, and for cycle 2 it is 1.69 X lo9cal. If Ri= 0.6, it is 2.78 X lo9 cal for cycle 1 and 2.18 X lo9 cal for cycle 2. The heat requirements for this work do not include the energy for separation and transport of the process streams. The heat requirements for this work can be compared roughly with the heat requirement currently used in the manufacture of chlorine. Currently the theoretical heat requirement is calculated from the thermodynamic data (Rossini et al., 1952; Parsons, 1959) as 3.28 X lo9 cal for the production of 1 ton of chlorine with 1.126 tons of sodium hydroxide for all types of electrolytic chlorine cells: namely, mercury, diaphragm, and membrane cells. The practical heat requirement is 7.2 X lo9 to 7.3 X lo9 cal for the same products for the current electrolytic cells (Versar, Inc., 1979). In calculation of the theoretical and practical heat requirements, this work assumed that the fossil fuel equivalent of electrical energy was three times the amount of electrical energy delivered to the current chlorine process. In calculating the theoretical heat requirement, the energy efficiency of the electrical process was assumed to be 100%. Since hydrogen is produced as a byproduct by the current methods, the heat of the combustion of hy-
drogen is subtracted from the total theoretical and practical heat requirements to derive the heat requirements presented here. When substantial thermal regeneration is conducted in these thermochemical cycles of this work, for instance at more than 0.6 of Ri,not only cycle 2 but cycle 1 might become economically and energetically competitive with current electrolytic processes of chlorine manufacture. Even though the energy required for separation and transport of the chemicals must be added to the heat requirements of these thermochemical cycles, the methods used here may result in financial and energy savings. Nomenclature AGTO = Gibbs free energy at T K, kcal A H T O = enthalpy of reaction at T K, kcal Kp(i)= equilibrium constant of the ith reaction Q, = A H T O - A H 0 2 9 8 ~ heat , output (exothermic)for cooling the reaction products, kcal Q, = A H T O - AH0298K, heat input (endothermic) for heating the reactants, kcal Q., = heat requirement for the splitting cycles of NaCl, kcal Ri = ratio for thermal regeneration T = temperature, K p = pressure, atm E = heat exchanger R = reactor S = separator Subscripts s = solid state 1 = liquid state g = gaseous state Registry No. NaCl, 7647-14-5; NaFeOz, 12062-85-0;HN03, 7697-37-2;COz, 124389;02,7782-44-7;NaOH, 1310-73-2;Na2C03, 497-19-8; C12, 7782-50-5;HzO, 7732-18-5;NO, 10102-43-9;NOz, 10102-44-0.
Literature Cited Barin, I.; Knacke, 0. "Thermochemical Propertles of Inorganic Substances"; Springer-Verlag. Berlln. 1973. Beekhuls, H. A. (assigned to the Solvay Process Co.), U.S. Patent 2 208 112, 1940. Beekhuls, H.A. (assigned to the Solvay Process Co.), U S . Patent 2 269 000, 1942. Burdick, C. L.; Freed, E. S. J . Am. Chem. SOC. 1921, 4 3 , 518. Chem. Trade J . Chem. Eng. 1917. 6 1 , 322. Chem. Ztg. 1926, 50, 68. Doklya, M.; Yokokawa, H.; Kameyama, T.; Fukuda, K. J . Nectrochem. SOC. Jpn. 1979, 4 7 , 156. Funk, J. E.; Reinstrom, R. M. Ind. Eng. Chem. Process D e s , D e v . 1966, 5 , 336. Knoche. K. F.; Funk, J. E. Int. J . nydrogen Energy 1977, 2 , 377. Matsui, G.; Yasuda, Y. J . Chem. SOC. Jpn. Ind. Chem. Sec. 1923, 26, 827. Parsons. R. "Handbook of Electrochemical Constants"; Butterworths Scientific Publication, London, 1959. Rossini, F. D.; Wagman, D. D.; Evans, W. H.; Levin, S.; Jaffe, 1. "Selected Values of Chemlcal Thermodynamic Propertles"; US. Government Rlnting Office: Washington. DC, 1952. Spealman, M. L. Chem. Eng. 1965, 7 2 . 198. Strelzoff, S. Chem. Eng. 1856, 63, 170. Versar, Inc. "A Survey of Potentlel Chlorlne Production Processes"; National Technical Information Service, Springfield, VA, 1979. Wise, W. L. English Patent 4364, 1882.
Department of Energy Chemistry National Chemical Laboratory for Industry Tsukuba Research Center Yatabe, Ibaraki 305, Japan
Norio Takeuchi
Received for review December 23, 1982 Revised manuscript received March 6, 1984 Accepted April 9, 1984
