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Production of fuels and chemicals using solar photothermochemistry
Energy Vol. 12. No. 3/4, pp. 31 I-318. Printed in Great Britain
036&5442/87
1987
53.00
+O.OO
krgnmon JournalsLtd
PRODUCTION OF FUELS AND CHEMICALS SOLAR PHOTOTHERMOCHEMISTRY
USING
V. K. MATHURand E. H. WONG Department of Chemical Engineering, University of New Hampshire, Durham,
NH 03824, U.S.A.
Abstract-Use of solar energy for the production of fuels and chemicals by a photothennochcmical technique where the entire solar spectrum can be used is discussed.The solar radiation is split into long and short wavelengths, the long wavelengths are used for a thermal process and the short wavelengths arc utilized for a quantum prcess.
INTRODUCTION The U.S. Department of Energy has for a number of years supported the development of solar thermal energy in the generation of electricity, industrial process heat and production of fuels and chemicals. The most attractive feature of solar energy is that its cost is free of the vagaries of OPEC politics and world’s supply as is in the case of nuclear and fossil fuels. In addition, it is clean and inexhaustible. The research work for the utilization of sunlight for the production of fuels and chemicals has been mostly restricted to coal gasification, wood pyrolysis, shale oil retorting, methane reforming and hydrogen production. In all these processes, the sun’s energy has been used basically as a source of heat. There is a growing concern that the special characteristics of sunlight namely, photoeffective radiation and high heat flux have not received as much attention for the production of fuels and chemicals. The possibility of applying photochemical methods for the manufacture of various chemicals has excited the chemical engineers ever since these methods of increasing reaction rates and causing selected reactions to occur were reported. There is a vast amount of literature on this subject. Most of it concerns the role of photochemistry in plant growth or deals with mechanism or rate of photochemical reactions. Relatively little information is available on the technical and economic feasibility of the production of fuels and chemicals on an industrial scale. It is obvious to consider the feasibility of using a portion of sunlight as a source of photo-effective radiation for industrial photochemistry. Although this possibility appears to be very attractive, there are many important factors to be considered. The portion of the solar spectrum which is useful for organic photochemistry lies in the U.V.and visible range 300-700nm which corresponds to only about 35% of the total energy falling on the earth. Plants are able to use sunlight because chlorophyll absorbs most of the visible portion of sunlight (400-650 nm) and uses the energy obtained this way in photosynthesis. The visible light which is effective in photosynthesis is in a region where energy levels correspond to less than that involved in most chemical bonds. Since relatively few reactions of industrial interest other than chlorination are activated by visible light, extensive use of sunlight in industrial photochemistry will depend on the development of methods for rendering the visible wavelengths effective. Furthermore, since the total energy of sunlight falling on the United States averages about 1 Cal/cm2 min, it would be necessary to concentrate this in order to use it effectively. Methods of concentrating the energy of sunlight are rather well developed as a result of studies for the utilization of sunlight as a heat source. Most of the photochemical reactions including industrial operations are carried out using mercury lamps for the generation of U.V.radiations. These lamps to some extent can be custom-made to suit a photoreaction and reactor. The design of a photoreactor operated on sunlight will present a challenging problem because of the limited availability of radiations of desired wavelength. Variations in light intensity and absorption with position 311
312
V. K. MAIHIJR and E. H. WONG
can affect the rate of reaction, cause concentration gradients, and induce diffusion. Last, but not least, the problem of intermittent supply of sunlight would cause further technical as well as economic problems. However, some of these problems can be solved by the use of photosynthesizers, beam splitting, and proper choice of high-cost chemicals. There are excellent reviews on the industrial applications of photochemical synthesis.‘-’ Factors influencing solar energy conversion include spectral quality and intensity, reactor configuration, temperature, sensitizers, and inhibitors. The majority of endothermic photochemical reactions require U.V.light for activation. The solar spectrum, however, contains only 4% of such wavelengths, the shortest being about 300nm. This seriously limits the overall efficiency of photochemical solar energy conversion, even under optimal laboratory conditions to about 10%. By comparison, solar to thermal energy conversion is at 60-65% efficiency. On the other hand, chemical systems provide the advantage of indefinite storage. Recovery of absorbed energy is achieved by reversing the initial endothermic reaction. Photochemistry has other advantages over thermal and catalytic methods. These include selective activation and reactivity of reactants, low thermal load and exact control of radiation. In this paper technical and economic feasibility of the production of chemicals using solar energy is discussed. PHOTOCHEMISTRY
IN CHEMICAL
PRODUCTION
Most practical applications of photochemistry lie in areas in which the effect of a photochemical primary process is amplified by a large number of thermal consecutive reactions. This is particularly true of photography, but also applies to photopolymerization and radical chain synthesis. The principal application of photochemistry has been in the fields of free-radical chlorination, sulfochlorination, sulfoxidation, and nitrosation. In addition, photochemical reactions are being utilized on an increasing scale for the synthesis of vitamins, drugs, and fragrances. The latter is a shift of interest from light-induced chain reaction to photoreactions with quantum yields, 4, less than or equal to unity, which may be economically feasible if they are able to shorten conventional synthesis considerably or if the products are of unusual structure and/or of high economic value. The quantum yield, 4 of a reaction is defined as moles of products obtained/einstein of light absorbed and is measured in terms of monochromatic light. In conventional photochemical systems, G 1. If 4 > 1, then a chain reaction has occurred in which a species produced by a photochemical process initiates a number of non-photochemical reactions. In photochlorination of hydrocarbons, quantum yields of as high as 2500 are obtained. Free-radical reactions
The photochemical chlorination of methane has been investigated by Coehn and Cordes and John and Bates.’ Coehn and Cordes observed that the quantum yield of the order of magnitude of 103-lo4 at temperature lOO-170°C could be obtained. The products were found to be CH,Cl, CH2C12. CHCl, and Ccl,. Jones and Bates who studied chlorination of methane in presence of oxygen postulated the following mechanism: Cl2 + hv = Cl + Cl Cl+CH,=CH;+HCl CH ; + Cl, = CH,Cl + Cl. Mechanisms of various photochemicals
reactions have been well documented.
Chlorination
Photochemical initiation of radical chain generally permits relatively low working temperature, an essential condition for numerous syntheses involving free radicals. Compared with radical chain initiation by thermal decomposition of initiators such as
Production of fuels and chemicals using solar photothermochemistry
313
peroxides and azo-compounds, light-induced radical generation is of particular advantage in cases where the radical chains are very short and very many initiating radicals are accordingly required. The following chlorination reactions are of industrial significance. Herschkindl’ of Dow Chemical describes a continuous manufacturing process for the chlorination of methane. Photochlorination can be conducted in both gas and liquid phase and is economically feasible because of the high quantum yield (> 100). Mercury lamps are used as a source of light (300-5OOnm).
CH,(g) + Cl,(g) 2 CHJl
Trichloroethane,
+ CHICll + CHCIJ + Ccl, + HCl.
a degreasing solvent is also manufactured
Cl,CHCH,(g)
by photochlorination:’
+ Cl,(g) 1: CH,CCl:, + HCl
The Phillips Petroleum Co.’ ’ conducted studies on the liquid phase chlorination of normal paraffins (C,,-C,,) using thermal, catalytic, and photochemical reactors and selected photochlorination for commercialization. The reaction is conducted at 32°C with a residence time of 14 set and uses mercury lamps for the source of U.V.radiations. Tennessee Products and Chemical Corporation, l2 Monsanto Chemicals and N.V. Chemische Fabriek, Netherlands” report the photochlorination of toluene at 40- 100°C for the production of benzyl chloride, benzal chloride, and benzotrichloride. These compounds are used to manufacture other chemicals used for bactericides, pharmaceuticals, and perfumes. CH,CI
CH, + Cl, A
Q
664 Benzyl chloride
+
CHC&
+
Benzal chloride
Ccl3
Benzo trichloride
Ethyl Corporation reports the development of a two-stage photoreactor for the chlorination of benzene to benzene hexachloride (BHC). Quantum yield of about 2500 and chemical yield of about 15% are reported. l4 Light of wavelength less than 480nm is used.
+
Cl2 -
Sulfochlorination
hv
cl-
Cl, + other isomers
and sulfoxidation
Sulfochlorination
of n-paraffins produces random substitution
RH + SO2 + Clz 1: RSO&l
+ HCI.
Quantum yields of 2000 are obtained. The sulfonyl chlorides are treated with ammonia and then with chloracetic acid and subsequently are used in the textile industry and as emulsifiers and anti-corrosion agents. In sulfoxidation, the chlorine is replaced by oxygen and a sulfonic acid is the product:
RH + SO2 + 1/202 1: RS03H.
V. K. MATHIJRand E. H. WONG
314
Beermann of Farwerke Hoechst AG (Germany and France)15 reports the sulfoxidation of n-paraffin for the production of biodegradable detergent by using a slightly different chemical route: RH + SO2 + O2 1: RS0200H RS0200H
+ Hz0 + SO1 -. RS03H + H$O,
The plant capacity is 4.5 x lo4 tons/yr with the process being ca 12% as efficient as sulfochlorination. Photonitrosarion
There are several processes for the commercial production of caprolactam, an intermediate for the manufacture of nylon-6. Toray Engineering, Japan, has developed a direct photonitrosation process for the conversion of cyclohexane in one step to the intermediate cyclohexanone oxime:
0+
NOCI + HCI -
hv
0 =
NOH
.2HCl
cyclohexane A quantum yield of about 0.7 over a wide range of wavelengths from 350 to 6OOnm is obtained.16*” A total plant capacity of about 160 x lo3 ton/yr is reported.‘* Fine chemicals
Except for the free-radical reactions and the synthesis of cyclohexanone oxime, application of photochemistry these days is rather limited to high-value products. Most of the vitamin conversion of D produced in the world is made by the photochemical 7-dehydrocholesterol. l9 Vitamin A acetate is produced by BASF, W. Germany, by photoisomerization of a stereoisomeric mixture of precursors into the all trans-vitamin. Other applications of photochemistry are in the fields of perfumes and drugs. Fragrance rose oxide is produced on a small scale by photo-oxygenation of citronellol with a rose bengal photosensitizer. The synthesis of dydrogesterone (developed by Philips and Dupher) is also reported in the literature.5
THE
SOLAR
CHALLENGE
IN PRODUCTION
OF SUN
FUELS
AND CHEMICALS
All industrial photochemical processes use mercury lamps to generate desired radiations (250-54Onm). It will be a technical and economic challenge to replace the photoreactors by reactor systems operated on solar energy. In the sunlight there is less than 4% radiation in the U.V.region and their wavelengths are more than 300nm. They do not possess enough energy to provide the desired energy to activate chemical reactions. In addition, transfer of solar radiations into a chemical reactor will be a major challenge in itself. Last, but not least, there is the problem of intermittent supply of sunlight. Beam splirting
One of the key elements for the success of producing fuels and chemicals by solar energy would be to use the entire spectrum of the sunlight. This can be achieved by splitting solar radiation into its long- and short-wavelength components, the long-wavelength component can be used for thermal processes (e.g. raising steam, generating power or carrying out chemical reactions) and the short-wavelength component can be used for quantum processes (e.g. photochemical reactions). Splitting the beam in effect decouples the thermal from the quantum processes leading to higher efficiencies since the penalty for removing the heat from the quantum process is eliminated.
Production of fuels and chemicals using solar photothermochemistry
315
Photosensitizers Photochemical reactions can be influenced to a great extent by the addition of relatively small amounts of certain materials. These additions permit a photochemical reaction to occur at wavelengths at which it ordinarily does not take place. These additives act by absorbing photons in the sensitizing region of the spectrum and then transferring the resultant excitation energy to the reactants.
Hybrid solar reactor
Since there is no way to store photons, one can take two routes to solve the problem of non-availability of photons at night for commercial processes. One route could be to select the production of such a chemical that can be manufactured in sufficient quantities during the day, and sold at a high price making the process cost-effective. These chemicals can be such as vitamin D, vitamin A acetate, aldosterone acetate, dydrogesterone, and perfumes. Another approach could be to use a hybrid photoreactor system, i.e. operate the reactor system on sunlight during the day and mercury lamps at night. The economic viability of keeping one system idle at any given time is usually questioned. In the photochemical synthesis the situation is very unique. The mercury lamps have a limited life. When they are not in use there is a saving of power costs, and an increase, by one-third, in the life of the lamps. It is most likely that there is an economic advantage of using sunlight for 8 hr particularly if some of these processes can piggyback on the thermal part of the solar energy. Efforts should be made to design photoreactors which can operate on both sources of radiation.
SOLAR
PHOTOTHERMOCHEMICAL
RESEARCH
AT UNH
Mathur and Wang*’ at the University of New Hampshire are investigating the possibility of producing chemicals using the hybrid photothermochemical technique where the entire solar spectrum is used. One of the main criteria for the selection of these chemicals which can be produced by this technique is the cost factor. The cost of these chemicals must be high enough to absorb the cost of solar energy but not so high that energy cost component become insignificant. In addition, there should be high demand for these chemicals. One such chemical under study is caprolactam, an intermediate in the manufacture of nylon& Solar caprolactam production
Caprolactam is a white, hygroscopic, crystalline solid at ambient temperature, with a characteristic odor. It is one of the most widely used chemical intermediates. However, almost all the annual production is consumed as the monomer for the manufacture of nylon-6 fibers and plastics. The world-wide annual production capacity of caprolactam is estimated to be 3,065,OOOtons, out of which 590,000 tons are expected to be manufactured in the U.S.A. The major world caprolactam producers are the Allied Chemical, BASF, Dutch State Mines, SNIA Viscosa, and Toray, Japan. Among these manufacturers Toray has a unique process. It is based on the photonitrosation of cyclohexane for the direct conversion of cyclohexane to cyclohexanone oxime hydrochloride by reaction with nitrosyl chloride in the presence of mercury light. This photochemical reaction reduces the number of chemical steps as compared to other processes, and thus reduces considerably the cost of raw materials. However, the Toray process has failed to find application outside the company’s own plant mainly due to the high cost of lamp replacement and power. The process has outstanding potential for dominating caprolactam production in the world if a cheaper source of energy can be found. Solar energy with its unique property of effecting photochemical reduction is an idea1 alternative source of energy.
V. K. MATHUR and E. H. WONG
316
Processes for caprolactam production
Caprolactam is manufactured by various processes around chemical steps involved in most processes are listed below:
the world. The major
(1) Original process phenol + cyclohexanol + cyclohexanone + cyclohexanone
oxime + caprolactam.
(2) Allied Chemical phenol process phenol + cyclohexanone + cyclohexanone
oxime + caprolactam.
(3) Cyclohexane process, via cyclohexanone cyclohexane + cyclohexanol + cyclohexanone + cyclohexanone oxime + caprolactam. (4) Toray photonitrosation process cyclohexane -+ cyclohexanone oxime + caprolactam. (5) SNIA Viscosa toluene process toluene + benzoic acid + cyclohexane carboxylic acid + caprolactam. (6) Union Carbide process, via caprolactone cyclohexanone + cyclohexanol + cyclohexanone (7) Du Pont process, via nitrocyclohexane cyclohexane + nitrocyclohexane -+ cyclohexanone (8) TechniChem process cyclohexane + cyclohexanol + cyclohexanone acid -+ aminocaproic acid + caprolactam.
-+ caprolactone -+ caprolactam. oxime + caprolactam.
+ nitrocyclohexanone
+ nitrocaproic
The distinctive feature of the Toray process lies in the photochemical synthetic reaction in the first stage. Since the nitroso group is introduced directly into cyclohexane to form the oxime, the reaction process is the shortest of those listed above. The conventional process, which uses phenol or cyclohexane as the starting material, requires at least four steps whereas the photochemical process has made it possible to reduce the reaction steps to two. In the conventional method, four to five times as much ammonium sulfate in weight is produced as that of caprolactam. In contrast, the photochemical process has reduced the by-product ammonium sulfate to about half the amount in the conventional process. It also uses inexpensive nitrosyl chloride as a nitrogen source of the lactam in place of costly hydroxylamine. Thus it has played an important role in improving the economics of the manufacture of caprolactam. Solar energy-an
alternative to mercury lamps
Operating cost of main caprolactam processes has been estimated and shows the negative effect of high cost of power and lamp replacement. 21 Most of the data used in the analysis was published in the late sixties or early seventies. Though no recent cost analysis is available the serious adverse effects on the process economy due to the nine-fold increase in energy price during the last 12 yr is almost a certainty. The high cost of power may be one of the main reasons why no plant based on Toray technology has ever been built outside of the company in Japan. Since photonitrosation of cyclohexane is carried out at wavelengths of 360- 540 nm, solar radiation (300-700 nm) will be an ideal alternative to the radiation from mercury lamps. Conceptual solar-powered caprolactam plant
A conceptual scheme for the production of caprolactam using solar photothermalcoupled reactions is shown in Fig. 1. This is an adaptation of the Toray process to the use of solar energy.20
Production of fuels and chemicals using solar photothermochemistry
GECKYANN
P”OTOREACllON
REARRANGEYEN
PURFICATK*)
MyE
VESSEL
HYDROGEN CHLOAlDE RECYCLE
RE?%E
SUN
SEPARATOI -
NEUYRALIZEII -
317
CYCLOHEWNE f
HYM10GE N CNLDRIOE
AMYOWIUY SUI LFATE
AMMONIA
AYWNIA
NITRDSE
3iiGCF -CDNVERTER -
CRYSYALLIZER
NDCI
GAS
GEMit%
Fig. 1. Conceptual production of caprolactain using solar photo-thermal
coupled reactions.
The overall photochemical reaction is the conversion of cyclohexane into cyclohexanone oxime hydrochloride and can be expressed as:
0
+NOCl
+
0 =
NOH . HCl.
Cyclohexanone oxime is then converted directly to caprolactam with the evolution of hydrogen chloride gas by a Beckmann rearrangement in the presence of oleum. Hydrogen chloride evolved in this stage is recovered and recycled to the nitrosyl chloride generator. Nitrosyl chloride required for the above reaction is obtained as follows: ammonia is burned in air to produce nitrogen oxides: 2NHJ + 302 -. NO*NO,
+ 3Hz0
nitrogen oxides are absorbed in sulfuric acid to form nitrosyl sulfuric acid: 2H2S04 + N0.N02 hydrogen chloride:
chloride is introduced
NOHS04
+ 2NOHS04
+ Hz0
into the nitrosyl sulfuric acid to produce
nitrosyl
+ HCl -, NOCl + H2S04
The recylcle of H2S04 and HCl permits the availability of the nitrosating compound without considerable acid consumption and ammonium sulfate production. Oleum used to rearrrange the oxime corresponds to a final production of 2.3 kg of ammonium sulfate per kg of caprolactam. This process can be adapted to utilizing photo-effective and thermal radiation from the sun as shown in Fig. 1. A beam splitter can be used to concentrate and direct radiation up to 54Onm to the photoreactor whereas thermal radiation can be concentrated and directed to preheat ammonia gas en route to the converter. Any surplus heat from the beam splitter can be used to generate steam or power depending upon the economics of the overall process. Beam splitters can be holograph or dichroic mirror types. EGY
12:3/4-J
318
V. K. MATHURand E. H. WONG FUTURE
PROSPECTS
The research work conducted at UNH on photothermochemical technique for the production of chemicals is likely to be the most cost-effective as it uses both photo-effective as well as i.r. portion of the sun’s spectrum. However, to find such chemicals where this technique can be successfully applied will need sustained efforts. The three types of reactions deemed worthy of further investigation are the photoamidation, photoaldol, and photo-oxidation reactions. The photoamidation reaction transforms an olefin and formamide into amides permitting functionalization into c+aminoacids and derivatives. The photoaldol reaction may allow the coupling of base-sensitive aldehydes to give condensation products in the absence of base. Potentially, products like ephedrine and pseudoephedrine may be prepared. Photo-oxidation converts aldehydes to peroxyacids and may be used to prepare organic reagents like meca-chloroperoxybenzoic acid. Of these, the photo-amidation is potentially the most general and deserves further investigation as to generality and practicality. Acknowledgement-This study is funded by the Solar Energy Research Institute, Golden, Colo. under subcontract No. XX-505026. Thanks are due to Dr R. G. Nix, Solar Energy Research Institute for his comments and suggestions.
REFERENCES R. S. Davidson, Chem. Ind. 180 (March 1978). M. Pape, Pure uppl. Gem. 41, 535 (1975). Chemical Technology Encyclopedia, Ed. Kirk-Othmer, Vol. 4, p. 193. Wiley, New York (1982). K. H. Proertner, J. Photo&em. 25.91 (1984). M. Fischer, Angew. Chem. Ind. Ed. Engl. 17, 16 (1978). D. R. Arnold and P. de Mayo, C/tern. Tech. 615 (Oct. 1971). R. W. Lepn, Chem. Engng 95 (Jan. 1982). A. Coehn and H. Cordes, J. phys. Chem. 9B, 1 (1930). L. T. Jones and J. R. Bates, J. Am. them. Sot. 56.2282 (1934). W. Hirschkind, Ind. Engng Chem. 41, 12.2749 (1949). T. Hutson and R. S. Logan, C/tern. Engng Prog. 68, 5, 76 (1972). W. H. Shearon, H. E. Horace and J. E. Steven, Ind. Engng Chem. 41, 9, 1812 (1949). H. G. Haring and H. W. Knol, Chem. Process Engng 560 (Oct. 1964). L. G. Govemak and J. T. Clarke, Chem. Engng Prog. 52, 7, 281 (1956). C. Beerman. Eur. them. News-Normal Poraflns Suppl. (1966). K. Fukuzawa and H. Miyama, J. phys. Chem. 72(l), 371 (1968). H. Miyama, N. Harumiya, Y. Ito and S. Wakamatsu, J. phys. Chem. 72(13), 4700 (Dec. 1968). N. Yoda, Toray Industries, Inc., personal communication (April 1985). Chemical Technology Encyclopedia, Ed. Kirk-Othmer, Vol. 4, p. 193. Wiley, New York (1982). V. K. Mathur and E. H. Won& “Solar Photochemical Production of Fuels and Chemicals”, UNH Report SER1/5-05026-1 (1985). 21. M. Taverna and M. Chiti, Hydrocorb. Process. 49, 137 (1970).
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
036&5442/87
1987
53.00
+O.OO
krgnmon JournalsLtd
PRODUCTION OF FUELS AND CHEMICALS SOLAR PHOTOTHERMOCHEMISTRY
USING
V. K. MATHURand E. H. WONG Department of Chemical Engineering, University of New Hampshire, Durham,
NH 03824, U.S.A.
Abstract-Use of solar energy for the production of fuels and chemicals by a photothennochcmical technique where the entire solar spectrum can be used is discussed.The solar radiation is split into long and short wavelengths, the long wavelengths are used for a thermal process and the short wavelengths arc utilized for a quantum prcess.
INTRODUCTION The U.S. Department of Energy has for a number of years supported the development of solar thermal energy in the generation of electricity, industrial process heat and production of fuels and chemicals. The most attractive feature of solar energy is that its cost is free of the vagaries of OPEC politics and world’s supply as is in the case of nuclear and fossil fuels. In addition, it is clean and inexhaustible. The research work for the utilization of sunlight for the production of fuels and chemicals has been mostly restricted to coal gasification, wood pyrolysis, shale oil retorting, methane reforming and hydrogen production. In all these processes, the sun’s energy has been used basically as a source of heat. There is a growing concern that the special characteristics of sunlight namely, photoeffective radiation and high heat flux have not received as much attention for the production of fuels and chemicals. The possibility of applying photochemical methods for the manufacture of various chemicals has excited the chemical engineers ever since these methods of increasing reaction rates and causing selected reactions to occur were reported. There is a vast amount of literature on this subject. Most of it concerns the role of photochemistry in plant growth or deals with mechanism or rate of photochemical reactions. Relatively little information is available on the technical and economic feasibility of the production of fuels and chemicals on an industrial scale. It is obvious to consider the feasibility of using a portion of sunlight as a source of photo-effective radiation for industrial photochemistry. Although this possibility appears to be very attractive, there are many important factors to be considered. The portion of the solar spectrum which is useful for organic photochemistry lies in the U.V.and visible range 300-700nm which corresponds to only about 35% of the total energy falling on the earth. Plants are able to use sunlight because chlorophyll absorbs most of the visible portion of sunlight (400-650 nm) and uses the energy obtained this way in photosynthesis. The visible light which is effective in photosynthesis is in a region where energy levels correspond to less than that involved in most chemical bonds. Since relatively few reactions of industrial interest other than chlorination are activated by visible light, extensive use of sunlight in industrial photochemistry will depend on the development of methods for rendering the visible wavelengths effective. Furthermore, since the total energy of sunlight falling on the United States averages about 1 Cal/cm2 min, it would be necessary to concentrate this in order to use it effectively. Methods of concentrating the energy of sunlight are rather well developed as a result of studies for the utilization of sunlight as a heat source. Most of the photochemical reactions including industrial operations are carried out using mercury lamps for the generation of U.V.radiations. These lamps to some extent can be custom-made to suit a photoreaction and reactor. The design of a photoreactor operated on sunlight will present a challenging problem because of the limited availability of radiations of desired wavelength. Variations in light intensity and absorption with position 311
312
V. K. MAIHIJR and E. H. WONG
can affect the rate of reaction, cause concentration gradients, and induce diffusion. Last, but not least, the problem of intermittent supply of sunlight would cause further technical as well as economic problems. However, some of these problems can be solved by the use of photosynthesizers, beam splitting, and proper choice of high-cost chemicals. There are excellent reviews on the industrial applications of photochemical synthesis.‘-’ Factors influencing solar energy conversion include spectral quality and intensity, reactor configuration, temperature, sensitizers, and inhibitors. The majority of endothermic photochemical reactions require U.V.light for activation. The solar spectrum, however, contains only 4% of such wavelengths, the shortest being about 300nm. This seriously limits the overall efficiency of photochemical solar energy conversion, even under optimal laboratory conditions to about 10%. By comparison, solar to thermal energy conversion is at 60-65% efficiency. On the other hand, chemical systems provide the advantage of indefinite storage. Recovery of absorbed energy is achieved by reversing the initial endothermic reaction. Photochemistry has other advantages over thermal and catalytic methods. These include selective activation and reactivity of reactants, low thermal load and exact control of radiation. In this paper technical and economic feasibility of the production of chemicals using solar energy is discussed. PHOTOCHEMISTRY
IN CHEMICAL
PRODUCTION
Most practical applications of photochemistry lie in areas in which the effect of a photochemical primary process is amplified by a large number of thermal consecutive reactions. This is particularly true of photography, but also applies to photopolymerization and radical chain synthesis. The principal application of photochemistry has been in the fields of free-radical chlorination, sulfochlorination, sulfoxidation, and nitrosation. In addition, photochemical reactions are being utilized on an increasing scale for the synthesis of vitamins, drugs, and fragrances. The latter is a shift of interest from light-induced chain reaction to photoreactions with quantum yields, 4, less than or equal to unity, which may be economically feasible if they are able to shorten conventional synthesis considerably or if the products are of unusual structure and/or of high economic value. The quantum yield, 4 of a reaction is defined as moles of products obtained/einstein of light absorbed and is measured in terms of monochromatic light. In conventional photochemical systems, G 1. If 4 > 1, then a chain reaction has occurred in which a species produced by a photochemical process initiates a number of non-photochemical reactions. In photochlorination of hydrocarbons, quantum yields of as high as 2500 are obtained. Free-radical reactions
The photochemical chlorination of methane has been investigated by Coehn and Cordes and John and Bates.’ Coehn and Cordes observed that the quantum yield of the order of magnitude of 103-lo4 at temperature lOO-170°C could be obtained. The products were found to be CH,Cl, CH2C12. CHCl, and Ccl,. Jones and Bates who studied chlorination of methane in presence of oxygen postulated the following mechanism: Cl2 + hv = Cl + Cl Cl+CH,=CH;+HCl CH ; + Cl, = CH,Cl + Cl. Mechanisms of various photochemicals
reactions have been well documented.
Chlorination
Photochemical initiation of radical chain generally permits relatively low working temperature, an essential condition for numerous syntheses involving free radicals. Compared with radical chain initiation by thermal decomposition of initiators such as
Production of fuels and chemicals using solar photothermochemistry
313
peroxides and azo-compounds, light-induced radical generation is of particular advantage in cases where the radical chains are very short and very many initiating radicals are accordingly required. The following chlorination reactions are of industrial significance. Herschkindl’ of Dow Chemical describes a continuous manufacturing process for the chlorination of methane. Photochlorination can be conducted in both gas and liquid phase and is economically feasible because of the high quantum yield (> 100). Mercury lamps are used as a source of light (300-5OOnm).
CH,(g) + Cl,(g) 2 CHJl
Trichloroethane,
+ CHICll + CHCIJ + Ccl, + HCl.
a degreasing solvent is also manufactured
Cl,CHCH,(g)
by photochlorination:’
+ Cl,(g) 1: CH,CCl:, + HCl
The Phillips Petroleum Co.’ ’ conducted studies on the liquid phase chlorination of normal paraffins (C,,-C,,) using thermal, catalytic, and photochemical reactors and selected photochlorination for commercialization. The reaction is conducted at 32°C with a residence time of 14 set and uses mercury lamps for the source of U.V.radiations. Tennessee Products and Chemical Corporation, l2 Monsanto Chemicals and N.V. Chemische Fabriek, Netherlands” report the photochlorination of toluene at 40- 100°C for the production of benzyl chloride, benzal chloride, and benzotrichloride. These compounds are used to manufacture other chemicals used for bactericides, pharmaceuticals, and perfumes. CH,CI
CH, + Cl, A
Q
664 Benzyl chloride
+
CHC&
+
Benzal chloride
Ccl3
Benzo trichloride
Ethyl Corporation reports the development of a two-stage photoreactor for the chlorination of benzene to benzene hexachloride (BHC). Quantum yield of about 2500 and chemical yield of about 15% are reported. l4 Light of wavelength less than 480nm is used.
+
Cl2 -
Sulfochlorination
hv
cl-
Cl, + other isomers
and sulfoxidation
Sulfochlorination
of n-paraffins produces random substitution
RH + SO2 + Clz 1: RSO&l
+ HCI.
Quantum yields of 2000 are obtained. The sulfonyl chlorides are treated with ammonia and then with chloracetic acid and subsequently are used in the textile industry and as emulsifiers and anti-corrosion agents. In sulfoxidation, the chlorine is replaced by oxygen and a sulfonic acid is the product:
RH + SO2 + 1/202 1: RS03H.
V. K. MATHIJRand E. H. WONG
314
Beermann of Farwerke Hoechst AG (Germany and France)15 reports the sulfoxidation of n-paraffin for the production of biodegradable detergent by using a slightly different chemical route: RH + SO2 + O2 1: RS0200H RS0200H
+ Hz0 + SO1 -. RS03H + H$O,
The plant capacity is 4.5 x lo4 tons/yr with the process being ca 12% as efficient as sulfochlorination. Photonitrosarion
There are several processes for the commercial production of caprolactam, an intermediate for the manufacture of nylon-6. Toray Engineering, Japan, has developed a direct photonitrosation process for the conversion of cyclohexane in one step to the intermediate cyclohexanone oxime:
0+
NOCI + HCI -
hv
0 =
NOH
.2HCl
cyclohexane A quantum yield of about 0.7 over a wide range of wavelengths from 350 to 6OOnm is obtained.16*” A total plant capacity of about 160 x lo3 ton/yr is reported.‘* Fine chemicals
Except for the free-radical reactions and the synthesis of cyclohexanone oxime, application of photochemistry these days is rather limited to high-value products. Most of the vitamin conversion of D produced in the world is made by the photochemical 7-dehydrocholesterol. l9 Vitamin A acetate is produced by BASF, W. Germany, by photoisomerization of a stereoisomeric mixture of precursors into the all trans-vitamin. Other applications of photochemistry are in the fields of perfumes and drugs. Fragrance rose oxide is produced on a small scale by photo-oxygenation of citronellol with a rose bengal photosensitizer. The synthesis of dydrogesterone (developed by Philips and Dupher) is also reported in the literature.5
THE
SOLAR
CHALLENGE
IN PRODUCTION
OF SUN
FUELS
AND CHEMICALS
All industrial photochemical processes use mercury lamps to generate desired radiations (250-54Onm). It will be a technical and economic challenge to replace the photoreactors by reactor systems operated on solar energy. In the sunlight there is less than 4% radiation in the U.V.region and their wavelengths are more than 300nm. They do not possess enough energy to provide the desired energy to activate chemical reactions. In addition, transfer of solar radiations into a chemical reactor will be a major challenge in itself. Last, but not least, there is the problem of intermittent supply of sunlight. Beam splirting
One of the key elements for the success of producing fuels and chemicals by solar energy would be to use the entire spectrum of the sunlight. This can be achieved by splitting solar radiation into its long- and short-wavelength components, the long-wavelength component can be used for thermal processes (e.g. raising steam, generating power or carrying out chemical reactions) and the short-wavelength component can be used for quantum processes (e.g. photochemical reactions). Splitting the beam in effect decouples the thermal from the quantum processes leading to higher efficiencies since the penalty for removing the heat from the quantum process is eliminated.
Production of fuels and chemicals using solar photothermochemistry
315
Photosensitizers Photochemical reactions can be influenced to a great extent by the addition of relatively small amounts of certain materials. These additions permit a photochemical reaction to occur at wavelengths at which it ordinarily does not take place. These additives act by absorbing photons in the sensitizing region of the spectrum and then transferring the resultant excitation energy to the reactants.
Hybrid solar reactor
Since there is no way to store photons, one can take two routes to solve the problem of non-availability of photons at night for commercial processes. One route could be to select the production of such a chemical that can be manufactured in sufficient quantities during the day, and sold at a high price making the process cost-effective. These chemicals can be such as vitamin D, vitamin A acetate, aldosterone acetate, dydrogesterone, and perfumes. Another approach could be to use a hybrid photoreactor system, i.e. operate the reactor system on sunlight during the day and mercury lamps at night. The economic viability of keeping one system idle at any given time is usually questioned. In the photochemical synthesis the situation is very unique. The mercury lamps have a limited life. When they are not in use there is a saving of power costs, and an increase, by one-third, in the life of the lamps. It is most likely that there is an economic advantage of using sunlight for 8 hr particularly if some of these processes can piggyback on the thermal part of the solar energy. Efforts should be made to design photoreactors which can operate on both sources of radiation.
SOLAR
PHOTOTHERMOCHEMICAL
RESEARCH
AT UNH
Mathur and Wang*’ at the University of New Hampshire are investigating the possibility of producing chemicals using the hybrid photothermochemical technique where the entire solar spectrum is used. One of the main criteria for the selection of these chemicals which can be produced by this technique is the cost factor. The cost of these chemicals must be high enough to absorb the cost of solar energy but not so high that energy cost component become insignificant. In addition, there should be high demand for these chemicals. One such chemical under study is caprolactam, an intermediate in the manufacture of nylon& Solar caprolactam production
Caprolactam is a white, hygroscopic, crystalline solid at ambient temperature, with a characteristic odor. It is one of the most widely used chemical intermediates. However, almost all the annual production is consumed as the monomer for the manufacture of nylon-6 fibers and plastics. The world-wide annual production capacity of caprolactam is estimated to be 3,065,OOOtons, out of which 590,000 tons are expected to be manufactured in the U.S.A. The major world caprolactam producers are the Allied Chemical, BASF, Dutch State Mines, SNIA Viscosa, and Toray, Japan. Among these manufacturers Toray has a unique process. It is based on the photonitrosation of cyclohexane for the direct conversion of cyclohexane to cyclohexanone oxime hydrochloride by reaction with nitrosyl chloride in the presence of mercury light. This photochemical reaction reduces the number of chemical steps as compared to other processes, and thus reduces considerably the cost of raw materials. However, the Toray process has failed to find application outside the company’s own plant mainly due to the high cost of lamp replacement and power. The process has outstanding potential for dominating caprolactam production in the world if a cheaper source of energy can be found. Solar energy with its unique property of effecting photochemical reduction is an idea1 alternative source of energy.
V. K. MATHUR and E. H. WONG
316
Processes for caprolactam production
Caprolactam is manufactured by various processes around chemical steps involved in most processes are listed below:
the world. The major
(1) Original process phenol + cyclohexanol + cyclohexanone + cyclohexanone
oxime + caprolactam.
(2) Allied Chemical phenol process phenol + cyclohexanone + cyclohexanone
oxime + caprolactam.
(3) Cyclohexane process, via cyclohexanone cyclohexane + cyclohexanol + cyclohexanone + cyclohexanone oxime + caprolactam. (4) Toray photonitrosation process cyclohexane -+ cyclohexanone oxime + caprolactam. (5) SNIA Viscosa toluene process toluene + benzoic acid + cyclohexane carboxylic acid + caprolactam. (6) Union Carbide process, via caprolactone cyclohexanone + cyclohexanol + cyclohexanone (7) Du Pont process, via nitrocyclohexane cyclohexane + nitrocyclohexane -+ cyclohexanone (8) TechniChem process cyclohexane + cyclohexanol + cyclohexanone acid -+ aminocaproic acid + caprolactam.
-+ caprolactone -+ caprolactam. oxime + caprolactam.
+ nitrocyclohexanone
+ nitrocaproic
The distinctive feature of the Toray process lies in the photochemical synthetic reaction in the first stage. Since the nitroso group is introduced directly into cyclohexane to form the oxime, the reaction process is the shortest of those listed above. The conventional process, which uses phenol or cyclohexane as the starting material, requires at least four steps whereas the photochemical process has made it possible to reduce the reaction steps to two. In the conventional method, four to five times as much ammonium sulfate in weight is produced as that of caprolactam. In contrast, the photochemical process has reduced the by-product ammonium sulfate to about half the amount in the conventional process. It also uses inexpensive nitrosyl chloride as a nitrogen source of the lactam in place of costly hydroxylamine. Thus it has played an important role in improving the economics of the manufacture of caprolactam. Solar energy-an
alternative to mercury lamps
Operating cost of main caprolactam processes has been estimated and shows the negative effect of high cost of power and lamp replacement. 21 Most of the data used in the analysis was published in the late sixties or early seventies. Though no recent cost analysis is available the serious adverse effects on the process economy due to the nine-fold increase in energy price during the last 12 yr is almost a certainty. The high cost of power may be one of the main reasons why no plant based on Toray technology has ever been built outside of the company in Japan. Since photonitrosation of cyclohexane is carried out at wavelengths of 360- 540 nm, solar radiation (300-700 nm) will be an ideal alternative to the radiation from mercury lamps. Conceptual solar-powered caprolactam plant
A conceptual scheme for the production of caprolactam using solar photothermalcoupled reactions is shown in Fig. 1. This is an adaptation of the Toray process to the use of solar energy.20
Production of fuels and chemicals using solar photothermochemistry
GECKYANN
P”OTOREACllON
REARRANGEYEN
PURFICATK*)
MyE
VESSEL
HYDROGEN CHLOAlDE RECYCLE
RE?%E
SUN
SEPARATOI -
NEUYRALIZEII -
317
CYCLOHEWNE f
HYM10GE N CNLDRIOE
AMYOWIUY SUI LFATE
AMMONIA
AYWNIA
NITRDSE
3iiGCF -CDNVERTER -
CRYSYALLIZER
NDCI
GAS
GEMit%
Fig. 1. Conceptual production of caprolactain using solar photo-thermal
coupled reactions.
The overall photochemical reaction is the conversion of cyclohexane into cyclohexanone oxime hydrochloride and can be expressed as:
0
+NOCl
+
0 =
NOH . HCl.
Cyclohexanone oxime is then converted directly to caprolactam with the evolution of hydrogen chloride gas by a Beckmann rearrangement in the presence of oleum. Hydrogen chloride evolved in this stage is recovered and recycled to the nitrosyl chloride generator. Nitrosyl chloride required for the above reaction is obtained as follows: ammonia is burned in air to produce nitrogen oxides: 2NHJ + 302 -. NO*NO,
+ 3Hz0
nitrogen oxides are absorbed in sulfuric acid to form nitrosyl sulfuric acid: 2H2S04 + N0.N02 hydrogen chloride:
chloride is introduced
NOHS04
+ 2NOHS04
+ Hz0
into the nitrosyl sulfuric acid to produce
nitrosyl
+ HCl -, NOCl + H2S04
The recylcle of H2S04 and HCl permits the availability of the nitrosating compound without considerable acid consumption and ammonium sulfate production. Oleum used to rearrrange the oxime corresponds to a final production of 2.3 kg of ammonium sulfate per kg of caprolactam. This process can be adapted to utilizing photo-effective and thermal radiation from the sun as shown in Fig. 1. A beam splitter can be used to concentrate and direct radiation up to 54Onm to the photoreactor whereas thermal radiation can be concentrated and directed to preheat ammonia gas en route to the converter. Any surplus heat from the beam splitter can be used to generate steam or power depending upon the economics of the overall process. Beam splitters can be holograph or dichroic mirror types. EGY
12:3/4-J
318
V. K. MATHURand E. H. WONG FUTURE
PROSPECTS
The research work conducted at UNH on photothermochemical technique for the production of chemicals is likely to be the most cost-effective as it uses both photo-effective as well as i.r. portion of the sun’s spectrum. However, to find such chemicals where this technique can be successfully applied will need sustained efforts. The three types of reactions deemed worthy of further investigation are the photoamidation, photoaldol, and photo-oxidation reactions. The photoamidation reaction transforms an olefin and formamide into amides permitting functionalization into c+aminoacids and derivatives. The photoaldol reaction may allow the coupling of base-sensitive aldehydes to give condensation products in the absence of base. Potentially, products like ephedrine and pseudoephedrine may be prepared. Photo-oxidation converts aldehydes to peroxyacids and may be used to prepare organic reagents like meca-chloroperoxybenzoic acid. Of these, the photo-amidation is potentially the most general and deserves further investigation as to generality and practicality. Acknowledgement-This study is funded by the Solar Energy Research Institute, Golden, Colo. under subcontract No. XX-505026. Thanks are due to Dr R. G. Nix, Solar Energy Research Institute for his comments and suggestions.
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