Ind. Eng. Chem. Res. 1994,33,2996-3003
2996
Process for Converting Hydrogen Chloride to Chlorine H.Y. Pan2 R. G. Minet2 S. W. Benson,' and T. T. Tsotsis*pt Department of Chemical Engineering and Department of Chemistry, University of Southern California,
Los Angeles, California 90089
There exists, worldwide, a strong incentive for the development of efficient processes that permit the recovery of Cl2 from HCl waste byproducts, to avoid the need for disposal of this toxic and corrosive material. In this paper we will describe such a process that has been developed by our group and which is currently being tested at the pilot plant scale. The process uses a salt mixture impregnated on a high surface area support alternately chlorinated and oxidized in two fluidized-bed reactors in series. The process development included basic studies of the mechanism of the HCl oxidation reaction over a broad range of temperatures (25-400 "C) and pressures (lov5Torr, 1atm). This paper will present a n overview of the results of these studies. On the basis of these investigations, a preliminary economic evaluation was completed and a pilot-plant-scale unit was designed and constructed. 1. Introduction
In recent years environmental pressures have grown to limit the shipment of potentially hazardous and toxic chemicals, such as Cl2 and HC1, on public carriers and highways. A number of large-scale chemical processes worldwide involve the use of chlorine as a primary raw material and the accompanying production of HC1 as a byproduct which must be dealt with by shipment, sale, or disposal by chemical means. Processes to recycle Cl2 have attracted renewed scientific interest. A number of processes are capable of producing Cl2 from HC1. These include (1)Electrolysis of HC1 with production of Cl2 and H2 (Roberts, 1950; Hine et al., 1984). (2) Direct oxidation of HC1 with various inorganic agents (Bostwick, 1976; Schreiner et al., 1974; van Dijk and Schreiner, 1973; Johnstone, 1948). (3)Oxidation of HC1 by air or 0 2 in the presence of a catalyst, e.g., Deacon-type processes (Deacon, 1875; de Jahn, 1940; Balcar, 1940; Johnstone, 1948; Belchetz, 1952; Banner and Perrin 1954; Reynolds, 1957; Engel et al., 1962; Feurke, 1968; Allen and Clark, 1971; Wattimena and Sachtler, 1980; Mallikarjunan and Zahed Hussain, 1983; Kiyoura et al., 1989; Itoh et al., 1989). Deacon-type processes remain interesting because of their relative ease of application and low electrical power and thermal requirements when compared with other approaches. The original process was brought to commercial reality by Deacon, who in the late 1800s received over 20 patents. At that point the Deacon process replaced the commonly practiced Weldon process in which aqueous solutions of HC1 were oxidized with MnO2 to produce MnC12, which was then reconverted to MnO2 by treatment with air in the presence of lime. In the Weldon process about half the chlorine introduced as HC1 was wasted as CaCl2 in the regeneration step. The Deacon process involves the equilibrium-controlled oxidation of HC1 by air (oxygen) to produce Cl2 over a catalyst. The gas-phase reaction, represented as
Department of Chemical Engineering. Department of Chemistry. 0888-5885l94l2633-2996$04.50lO
is exothermic and reversible. Deacon suggested the use of manganese, copper, and iron salts on various inert porous supports. His preferred catalyst consisted of copper chloride placed on pumice. The operating temperature range was 430-475 "C. Unfortunately, his catalyst was not particularly active, and the unreacted HC1 in the presence of H2O caused corrosion problems. Copper chlorides, furthermore, start evaporating at an appreciable rate above 400 "C, resulting in rapid catalyst deactivation. Many efforts have been undertaken since then for developing more effective catalysts culminating with the development of the Shell catalyst (Engel et al., 1962; Feurke, 1968; Wattimena and Sachtler, 1980)and MT-Chlor processes (Kiyouraet al., 1989; Itoh et al., 1989). The Shell catalyst contains CuClz and KC1 in an equimolar ratio impregnated on a Si02 support. The catalyst also contains salts of rare-earth metals. In a preferred commercial composition the Shell catalyst is reported to contain 5% Cu-5% Di-3.1% K-86.9% SiOa. Di is a relatively inexpensive mixture of rare-earth metals obtained &er isolation of Ce from a monazite sand. The use of Di had been suggested in a series of patents issued to AIRCO about 20 years earlier (Balcar, 1940). The Shell process made use of a single fluidized-bed reactor. The catalyst was reported to be very active in the temperature range 350-365 "C attaining close to equilibrium conversions. Even so the presence of unreacted HCl (under stoichiometric conditions when thermodynamic equilibrium conversions are attained, over 20% of HC1 remains unreacted) resulted in reactor corrosion problems especially in the presence of steam, which is also a product of the reaction. The Shell process is currently inactive. Recently Mitsui Toatsu Chemicals has proposed (Kiyoura et al., 1989; Itoh et al., 1989)a process (MT-Chlor) utilizing a crystalline chromic oxide catalyst (20-90% wt Cr203) supported on Si02. Crystalline chromic oxide was in the past thought to be inactive toward the Deacon reaction. In fact, prior workers went to great pains t o prevent amorphous chromic oxide from becoming crystalline. This happens by inadvertent heating over 500 "C for extended periods of time (Banner and Perrin, 1954). The process utilizes a fluidized-bed reactor operating in the temperature range 370-420 "C and OflCl ratios in the range 0.3-0.75 (in stoichiometric large excess of 0 2 since the catalyst is deactivated
0 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 2997 when large excess of HC1 is present). The chromic oxide catalyst is very sensitive to the presence of Fe (and to a lesser extent Ni and Ti) so that the Si02 support and the reactor wall must be free of Fe (made, for example, by materials with a Fe content of less than 1%).It is unclear how problems similar to those encountered by the Shell process are overcome by the MT-Chlor process. 2. Thermochemistry and Reaction Mechanism
With the development of efficient catalysts attaining the corresponding equilibrium conversions the thermodynamics and thermochemistry of copper compounds attracted renewed interest (Fontana et al., 1952a,b; Arnold and Kobe, 1952; Ruthven and Kenney, 1968; Wattimena and Sachtler, 1980). The reported thermodynamic data, of course, pertain to bulk compounds. The rationale has always been that since most copper compounds of relevance to the process are in a molten state, at the temperatures typically found in industrial practice, such bulk thermochemical properties have a true bearing on the reaction mechanism. The early mechanistic work on the reaction has been discussed by Allen and Clark (1971), and the more recent work by Mallikarjunan and Zahed Hussain (1983). There appears t o be a general agreement that the Deacon process is described by the following overall reaction scheme: 2CUCl,(S) = 2CUCl(S) 2CuCl(s)
Cu,OCl,(s)
+ Cl,(g)
+ 1/z02(g)= CU,OCl,(S)
+ 2HCl(g)
2CuCl,(s)
+ H,O(g)
(2)
HCl sorption: CuO(s)
---L
CU,OCl,(S)
+ H,O(g)
-- CuO(s) + CUCl,(S)
valency change: CUCl,(S)
+ CUO(S)
-
(4)
H20(a) H,O(g)
(7) (8)
= CUCl(S) + l/,Cl,(g) (9)
(10)
This mechanism appears to be consistent with surface investigations (Moroney et al., 1981; Vishnu Kamath, 1984; Prabhakaran et al., 1986; Prabhakaran, 1990) of the interaction of 0 2 and Cl2 with Cu single-crystal surfaces using XPS and EELS. Such investigations have uncovered the existence of chlorohydroxy surface intermediate species on the solid metal surfaces (bulk copper chlorohydroxy compounds are also known to exist and to decompose upon heating to the corresponding copper oxychloride compounds and water). For example Moroney et al. (1981) expose a Cu(ll1) surface to 0 2 at 295 K and subsequently cool the surface down to 80 K at which point the surface is exposed to HC1. Using XPS, Moroneyet al. (1981),observe that the initial O(1s) peak a t 529.8 eV corresponding to chemisorbed oxygen is replaced upon exposure to HC1 by three O(ls) peaks. One of these peaks is the original 529.7 eV peak, the second has a binding energy of 531.5 eV, assigned by Moroney et al. to hydroxyl species, and the third a t 533 eV is due to molecularly adsorbed water. Scanning the C1 (2p) binding energy region (198-201 eV) indicated the presence of chlorine adatoms. Moroney et al. (1981) believe that chemisorbed oxygen is involved in hydrogen abstraction reactions through “strong hydrogen bonding”. They invoke the formation of surface intermediates of the type Od-* *HC1in which the adsorbed oxygen adatoms have a net negative change pointing away from the copper surface. For the overall reaction they write
(3)
Cu(OH)Cl(s)
2Cu(OH)Cl(s) ---L Cu,OCl,(s)
CUCl,(S)
+ O(a) OH(a) + Cl(a) OH(a) + OH(a) - H,O(a) + O(a)
+ l/zO,(g) = CUO(S)
+ HCl(g)
+
HCl(g)
Hisham and Benson (1989)have developed techniques for the accurate estimation of the thermodynamic properties of metal-oxychloride and metal-chlorohydroxy intermediate compounds. This has enabled them t o study in detail the thermochemistry of the Deacon process. On the basis of the knowledge of the thermochemistry of the process and the properties of the intermediate reactive species, they have proposed (Benson and Hisham, 1992) an alternative more detailed mechanism for the Deacon process, which involves the following reaction steps: CuO formation: CUCl,(S)
catalyst regeneration: 2CUC1(2) l/,O,(g)
-
(11) (12)
(13)
Their mechanism parallels that suggested by Benson and Hisham (1992). The mechanism described by reactions 5-10 has also so far been found to be in general agreement with our own kinetic investigations (see discussion to follow). One should be cautioned, however, that the agreement between thermochemicaY thermodynamic considerations and the reaction mechanism for the Deacon process might be a fortuitious one. Though, for example, the metal salts involved might exist in a molten state it is probable that for the most part they will behave as a 2-D fluidlike system spread on the underlying “inert”support surface. The influence of rare-earth and alkali metals on the thermochemistry of copper compounds is, furthermore, not well understood. Ruthven and Kenney (1968) have reported, for example, that the ternary (CuC12-CuCl-KCl) and quarternary (CuC12-CuCl-KCl-LaC13) systems behave as nonideal mixtures (due to nonideal entropies of mixing). CuC12-CuC1 and CuCl2-CuCl-ZnCln on the other hand behave essentially as ideal mixtures. It is likely that alkali metal chlorides form stoichiometric compounds with both CuClz and CuCl of the composition MCuCl3, M2CuC14, MCuC12, MzCuC13. Much more remains to be learned, and work in this area is continuing. In the mechanism described by reactions 5-10 the copper complex enters into the reaction in both the oxide and chloride forms. The oxide form is necessary for the HC1 uptake step (reactions 6-81, while the chloride form participates in the Clz release step (reactions 5, 9, 10). The HC1 uptake step is exothermic and thermodynami-
2998 Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994
cally favored (but also kinetically fast) at low temperatures. The Cl2 release step, on the other hand, is endothermic and is favored at high reaction temperatures. Negligible Cl2 release is expected below 300 "C in the absence of 0 2 . These observations would indicate that the overall HC1 oxidation reaction would be carried out more efficiently as a two-stage process. The HC1 uptake step to produce the chloride form of Cu should be carried out at low temperatures in the range 100250 "C. The release of the Cl2 and the oxidation of the catalyst should be performed at elevated temperatures, in the range 300-360 "C. These conditions would eliminate the unfavorable equilibrium limits for HC1 uptake associated with the classical Deacon process and would considerably improve the conversion of HC1 to Cl2. In addition the resulting product gas effluent would consist primarily of water vapor from the HC1 sorption step and mostly Cl2 (and NZand 0 2 ) from the oxidation step. Two-stage type processes are, of course, not unknown in this area (Engel et al., 1962) with some of the earlier processes going back to Deacon's times. Mond (1889) for example proposed passing HCl first over MgO mixed with KC1 and China Clay a t temperatures of 400-600 "C. Once the solid is saturated with HC1, dry air is then passed over it in the temperature range 450-500 "C to recover the Cl2. Bale (1893) in addition to MgO proposed the use of Mn2O3. The Mond process was commercialized by Weldon and Pechiney but was eventually abandoned due to problems with the presence of HC1 during the second stage of the reaction. Grosvenor Laboratories in the 1940's (Grosvenor and Miller, 1940) proposed a process utilizing FeC13 impregnated on a silica support. KC1 was also added as a volatilization suppressant together with CdClz as an activity booster. The process proposed by Grosvenor Laboratories uses fixed-bed reactors. In its simplest configuration the catalyst is loaded in a fixed bed-reactor and an HC1 stream in N2 is allowed t o pass over the catalyst (after it has been oxidized t o Fe203) in the temperature range 250-300 "C. The reaction is stopped just before complete conversion is attained. Subsequently the bed temperature is raised to 490-510 "C and dry air or 0 2 is passed over the material to produce Cl2 and regenerate Fe2O3. Dow Chemicals (Pye and Joseph, 1951) proposed a moving-bed process, which utilizes the catalyst developed by Grosvenor Laboratories. The Dow reactor consisted of three parts. The top part acts as a preheater, the middle as the chlorinator operating between 400-475 "C, and the bottom part as the oxidizer operating between 475 and 520 "C. The catalyst material moves from top t o bottom while the gases move in a countercurrent direction. A 35 tonslday plant built by Hercules utilizing the Grosvenor and Dow patents encountered problems, as one might have expected with temperature control and catalyst deactivation due to volatilization of the FeCl3 compound. A patent for a similar moving-bed process utilizing FeCl3 was issued to Standard Oil in 1948 (Murphree and Summit, 1948). Patents for a two-stage process issued to Socony Vacuum Oil (Gorin, 1947a,b) claim the use of copper chloride melts. In the first reactor, packed with an inert material, copper chloride melt enters from the reactor top at a temperature of 350-400 "C. Air flows countercurrently, its flow rate adjusted so that the melt leaves the reactor bottom a t 425 "C. The melt subsequentlyly enters a second reactor containing heated
graphite tubes, coated with silicon carbide, where a t a temperature of 475-550 "C is contacted by a HC1 stream to produce H20 and Cl2, presumably according to reaction 4. In a variation of the process the melt is contacted in the first reactor in a temperature range 350-400 "C by HC1 and 0 2 . The melt leaving the first reactor is subsequently fed into a second reactor, where it is heated t o 500-600 "C to produce Cl2, presumably according to reaction 2 above. Obviously given the volatility of copper salts above 400 "C the commercial feasibility of the Socony Vacuum Oil process appears somewhat questionable. It would appear that the previous two-stage catalytic HC1 oxidation processes have been plagued by a number of problems. These include (i) the use of inefficient catalyst systems; (ii) improper reactor design, (packed or moving beds are ill-suited for carrying out the Deacon process), and (iii) a poor understanding of the reaction mechanism and thermochemistry of the process, which has resulted for the most part in inappropriate choices of reactor operating conditions. In our laboratories, for the past few years we have, ourselves, studied the feasibility of a two-stage, HC1 oxidation process. As a result of these studies, we have proposed and patented (Minet et al., 1991; Benson and Hisham, 1992) a new process for converting HCl to Cl2. The process is called "the catalytic carrier process for chlorine production". It uses a two-stage procedure, in which HC1 is reacted first with CuO, and the resulting complex is transported to a second reactor, where it is oxidized back to CuO releasing the Cl2. Though extensive investigations of the reaction kinetics of the Deacon process are available relatively little has appeared in the open literature (Ruthven and Kenney, 1967; Fontana et al., 1952)concerning the individual HC1 sorption and Cl2 release steps. We have, therefore, undertaken an investigation of the detailed reaction mechanism and kinetics of these processes. On the basis of these studies a preliminary economic evaluation was completed and a pilot-plant scale unit was designed and constructed. 3. Experimental Results 3.1. Preparation of the Catalyst. The catalytic carrier process involves two distinct gas-solid reactions: the chlorination reaction in which copper oxide reacts with HC1 to produce water and copper chloride, and the oxidation reaction in which copper chloride reacts with oxygen to release Cl2 and regenerate the oxide form of Cu. The catalyst material used in our laboratory investigations reported here contained no rare-earth metals. It was prepared by wet impregnation using an equimolar aqueous copper chloride and sodium chloride mixture on support materials suitable for fluidized-bed operation. The amount of Cu metal was typically in the range of 5-15% per total weight of catalyst. Several support materials have been tested (alumina, silica, zeolites). The results reported here are with catalysts supported on a silica obtained from Davison Inc (pore volume 1.15 cm31g;pore diameter average 190 A; surface area 340 m21g;mesh size 60-200 mesh). The resulting slurries were dried at about 100 "C until reaching constant weight. The recovered materials thus obtained were first dried at 150 "Cin a fluidized-bed reactor for 3 h and were then treated in a Nd02 atmosphere at 350 "C for 4 h. This results in a complete conversion of the chlorided form of Cu into CuO. 3.2. Experimental Apparatus. Two types of apparatus were used in the experiments. For the experi-
Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 2999
SI.lnlcrrSlcd fl*npe
Absorption Rubblcn
Figure 1. The Knudsen cell.
;;% ...
... ... ...
ooc
Figure 3. Fluidized-bed reactor system
Figure 2. (a) Reactant delivery system; (b) the low-pressure reactor system.
ments in the low-pressure regime (less than Torr) our reactor was a pyrex glass Knudsen cell (see Figure 1). The cell consists of two chambers separated by a sliding glass seal. The solid reactant is loaded in the lower chamber. By means of the sliding glass one is able to bypass, if so desired, the lower chamber without exposing the solid reactant to the flowingreactant gases. Measurement of the gaseous reactants and of the reaction products is achieved by using a Molecular Beam UTI lOOc Quadrupole mass spectrometer. Molecules that exit the 3 mm aperture of the Knudsen cell form a molecular beam, which is collimated by passing through holes into two consecutive differentially pumped chambers (see Figure 2). The left chamber is pumped by a Balzers turbomolecular pump with a pumping speed of 240 Us;the right chamber is pumped by a Leybold-Heraeus turbomocular pump with a pumping speed of 140 Us. Most of the gas molecules from the reactor are pumped away through the left chamber under molecular flow conditions. The molecular beam
is modulated mechanically by using a tuning fork chopper a t 200 Hz, before it enters the ionization chamber of the mass spectrometer to eliminate the background interference. Both the output of the mass spectrometer and the tuning fork beat frequency are fed into a lock-in amplifier (Ithaco Dynatrac 3), and only the rectified, modulated portion of the ion signal is retrieved from the lock-in amplifier. This phase sensitive detection of the molecular beam ensures that only that portion of the total signal which has a frequency of 200 Hz is monitored. A glass vacuum system is used for gas handling (Figure 2). It is pumped by a CVC oil diffusion pump and a Sargent Welch mechanical roughing pump. The pumping system is isolated from the vacuum line by a removable liquid Nz cold trap. Three glass bulbs are used for storing the gaseous reactants connected to three buffer volumes. The reactants are leaked into the buffer volumes through three needle valves. The other end of the buffer volumes is connected to a set of three helically coiled lengths (100 em) of capillary tubing with diameters of 0.023, 0.035, and 0.051 cm, respectively, which roughly correspond to a ratio of flow rates (at the same buffer pressure) of 1:1030, respectively. The pressures of the buffer volumes are measured by Validyne pressure transducers and are used to calibrate the flow rates through the capillaries. The fluidized-bed experimental apparatus used in this study consisted of the following major parts: the reactant handling and delivery section, the catalytic fluidized-bed reactor and its control hardware section, and the product collection and measurement section. A schematic diagram of the overall apparatus is shown in Figure 3. The reactant gases utilized were HCI (technical grade, Matheson), 02 (ultra-high pure grade, Matheson), and Nz (ultra-high pure grade, Matheson). Before being sent to the reactor, the gases from the compressed gas cylinders were treated by passing through purifying colums, which contained indicating drierite (CaS04)to remove water vapor, and molecular sieve and activated charcoal to remove hydrocarbons along with any trace of oil and heavy hydrocarbons.
3000 Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994
A detailed schematic of the reactor is also shown in Figure 3. The reactor section consisted of a preheater, a fluidized-bed reactor and an electric furnace. The preheater was made using a coil of quartz tubing with an 0.d. of 8 mm and i.d. of 6 mm. The coil was connected to the reactor by fusing it to its base. The reactor was constructed from a quartz tubing with an outside diameter of 25 mm and inside diameter of 22 mm. Its length was approximately 80 cm. The reactor assembly was placed inside a tube furnace equipped with an Omega-CN-2000 temperature controller. A multipoint chromel-alumel thermocouple was placed inside the thermowell, so that the temperature of the reactor base, center, and the top portions were monitored. Some quartz wool was packed around the reactor to seal the annular spacing between the reactor wall and the furnace wall, t o reduce the loss of heat by convection. When the catalyst bed was fluidized a uniform temperature profile along the reactor length was achieved. The experiments in the fluidized-bed reactor were carried out in the following manner. During the study of the HC1 reaction with CuO, the reactor was first heated t o the reaction temperature while being purged continuously with a N2 stream. When the temperature stabilized, the HC1 gas flow was started. An HCVNz gas mixture entered the reactor from the reactor base and passed up through the fluidized bed, reacting with the solid reactant. The reacted gas mixture from the top of the fluidized bed reactor then passed through two bubblers in series, which contained a basic solution to detect HC1. The reaction run was continued until HCl was detected in the outlet of the reactor, i.e., until HC1 breakthrough. The time required for HC1 breakthrough, i.e., the time required for complete saturation of CuO with HC1 depended on the weight of catalyst and its copper content. After the chlorination reaction was complete, the catalyst bed was ready for the oxidation run. To initiate this run the HC1 stream was shut off and only N2 gas was passed through the catalyst bed. The temperature of the bed was raised to 320370 "C. When the temperature reached the desired level, 0 2 gas mixed with Nz was introduced into the fluidized-bed reactor, and the oxidation reaction was started. The Cl2 concentration in the effluent gas was continuously monitored by taking liquid samples from the two bubblers, containing KI solution t o absorb the Cl2 from the reacted gases, and analyzing the samples by titration. After the oxidation reaction was over, the 02 flow was stopped. The temperature of the reactor was then lowered to 150-250 "C under flow of N2, and the chlorination step was repeated, followed by the oxidation step at preselected conditions. 4. Experimental Results and Discussions
4.1. The Reaction of HCl with the Supported CuO. The chlorination step was studied at atmospheric pressures in the fluidized bed in a temperature range from 150 to 300 "C. In the series of experiments described here the total flow rate for all runs was kept at 430 mumin and the inlet HC1 concentration was set at 24 vol % in N2. The total amount of solid reactant used (10 wt % Cu) was 50 g. The efficiency of HC1 sorption was determined by measuring the time for HC1 breakthrough from the fluidized bed. Once HC1 was detected at the reactor exit the HC1 flow was immediately stopped, the reactor temperature was raised to 350 "C, and the catalyst bed was oxidized for 1h in
-
0.6-
.-a o . o
0
0.3-
x
0
"
"
6
'
12
18
24 30 36 Time (min.)
42
48
54
60
Figure 4. Oxidation rate as a function of time. 02 concentration 60% vol in Nz, T = 350 "C.
a 60% vol02 in N2 flowing mixture. At these conditions the chlorination reaction was very rapid. At temperatures below 250 "C, increasing the temperature had no effect on the breakthrough time. Above 250 "C, increasing the reaction temperature results in a decrease in the breakthrough time. During the chlorination, condensed water vapor was noticed in the exit tubing, when the bed was close to saturation. These observations are consistent with the proposed reaction mechanism (reaction step 7) and the process thermochemistry. Since the HC1 sorption step a t atmospheric pressure conditions is very fast, we did not find it convenient to measure its reaction rate in the fluidized-bed reactor system. Studies of the reaction kinetics for this step were conducted in the low-pressure reactor system previously described equipped with a UTI lOOC quadrupole mass spectrometer. A brief overview of these studies follows. Further detailed accounts can be found elsewhere (Pan, 1993). At temperatures lower than 80 "C the reaction order with respect to HC1 was found to be close to 1. Above 80 "C, however, the reaction order becomes larger than 1,the rate shifting from first order to second order. The observed reaction activation energies also depend on temperature. In the lower temperature range (
