Ind. Eng. Chem. Res. 1991,30, 126-129
126
offers greater potential for tailoring the catalyst properties to fit the application relative to the ceramic system. Conclusions The performance attributes of catalysts used for the oxidation of carbon monoxide from cogeneration plants have been discussed. Mass-transfer and pressure drop correlations are developed for a Camet metal monolith catalyst. The performance of such catalysts is compared to that of conventional ceramic catalysts. Camet metal monolith catalysts were found to have a higher masstransfer-limited activity than conventional ceramic catalysts at a fixed pressure drop. The design flexibility of metal monolith catalysts is discussed. Nomenclature Dab = bulk diffusion coefficient, ft2/s Dh= hydraulic diameter, in. f = friction factor h = corrugation height, in. jD = Colburn factor k, = mass-transfer coefficient, ft/s 1 = corrugation length, in. L = monolith length, in. SV = space velocity, l / h u = superficial velocity, ft/s x = conversion Greek Letters (Y
= geometric surface area per volume, l/ft
gas density, lb/ft3 gas viscosity, Ib/(ft.s) t = void fraction aP = pressure drop, in. of water Registry No. CO, 630-08-0; Pt, 7440-06-4; stainless steel,
p = p =
12597-68-1; ceria, 1306-38-3.
Literature Cited Cordonna, G. W.; Kosanovich, M.; Becker, E. R. Gas Turbine Emission Control. Platinum and Platinum-palladium Catalysta for Carbon Monoxide and Hydrocarbon Oxidation. Platinum Met. Reu. 1989, 33, 46-54. Deiber, J. A.; Schowalter, W. R. Flow Through Tubes with Sinusoidal Axial Variations in Diameter. AIChE J. 1979,25,638-645. Hegedus, L. L. Effects of Channel Geometry on the Performance of Catalytic Monoliths. Prepr.-Am. Chem. SOC., Diu. Pet. Chem. 1973, 18, 487-502. Jung, H. J.; Becker, E. R. Emission Control for Gas Turbines. Platinum-Rhodium Catalysts for Carbon Monoxide and Hydrocarbon Removal. Platinum Met. Rev. 1987, 31, 162-170. Kays, W. M.; London, A. L. Compact Heat Exchangers; McGrawHill: New York, 1964; pp 109-110. Mondt, J. R. NOx Catalyst Performance Comparison of 304 Stainless Steel, Inconel, and Monel in a 10'-herringbone Foil Configuration. AIChE Symp. Ser. 1976, 73,169-177. Pereira, C. J.; Plumlee, K. W.; Evans, M.Camet Metal Monolith Catalyst System for Cogen Applications. In 1988 ASME Cogen Turbo; Serovy, G. K., Fransson, T. H., Eds.; ASME: New York, 1988; pp 131-136. Receiued for review November 3, 1989 Revised manuscript received July 6, 1990 Accepted July 25, 1990
MATERIALS AND INTERFACES Scale-up Studies on an Alumina Aerogel Catalyst Support Anthony J. Fanell&*$+ Satyajit Verma,* Ted Engelmann,$ and Joan V. Burlewt Allied-Signal Inc., Research & Technology, Morristown, New Jersey, 07960-1021, a n d Allied-Signal Inc., Engineered Materials Sector, Polyolefins Plant, Baton Rouge, Louisiana 70805
Attempts to prepare alumina aerogel in a 215-gal (814-L) pressure reactor are described. The material had lower specific surface area and altered surface morphology compared to alumina aerogel prepared in the laboratory. The altered properties are believed to have been caused by the shearing stresses imparted by the fixed rate, high-speed agitator in the reactor. Introduction Aerogels are a unique class of catalyst supports which provide exceptionally high pore volume and surface area (Ayen and Iacobucci, 1988: Armor and Carlson, 1987). However, in spite of their unique physical properties, they have enjoyed limited commercial success. This is largely due to the fact that most aerogel preparations have been conducted in small laboratory-scale reactors for research purposes. A continuous process for preparing silica aerogel is described in a 1959 patent (Sargent and Davis, 1959). Although the purpose of the process appears to be pro-
'* Allied-Signal Inc., Morristown, NJ. Allied-Signal Inc., Baton Rouge, LA.
duction of silica aerogel on a commercial scale, the venture was apparently discontinued. A continuous process capable of producing 2 kg/day in a pilot-scale reactor has recently been reported (Yamanis, 1989; Haig and Yamanis, 1989). The process for producing slabs of silica aerogel intended for insulation appears to be the largest current aerogel manufacturing process (VonDardel et al., 1983; Henning, 1986). The slabs are made in a 98-L reactor in a batch-type process. This paper summarizes the results of three runs carried out in a 215-gal (814-L) pressure vessel and the physical properties of the alumina aerogel product. To the authors' knowledge, these runs represent the largest batch-type production scale-upever attempted for an aerogel material.
0888-5885/ 9 1/ 2630-0 126$02.50 / 0 @ 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No. 1, 1991 127 190 RPM
w
L
Oil out
Table I. Run Conditions for the Alumina Aerogels Prepared in the 215-gal (814-L) Reactor run 1 run 2 run 3 AIP charge, lb 118.5 118.5 118.5 methanol, lb 628 556 589 water, lb 31.5 31.5 31.5 aerogel recovered, lb 12 50 59 condensate, lb 559 633 308 47 total unrecovered, lb 458 97 heat-up to 255 OC,min 410 735 1275 0 hold at 255 O C , min 45 60 150 vent, min 435 615 0 540 cool-down, min 270 420 aerogel removal, min 120 120 agitation time, min 420 120 120 190 190 agitator speed, rpm 190 474 482 oil temp, max O C 482 1900 1750 1900 max batch pressure, psig
,,,{ CV”
~
Baffles not shown Turbine has 14 blades Figure 1. Schematic diagram of the 215-gal (814-L) pressure vessel.
This scale-up of alumina aerogels was attempted because of their interesting properties as olefin polymerization catalyst supports (Fanelli et al., 1989). The alumina aerogel was found to differ markedly from material prepared in the laboratory in several important respects, including pore volume, surface area, and surface morphology.
Experimental Section A diagram of the jacketed reactor is shown in Figure 1. The reactor’s internal dimensions were 65 in. (165 cm) in height and 36 in. (91 cm) in diameter. The agitator contained two sets of turbine stirrers spanning a diameter of 18 in. (46 cm) on a single shaft. Each set contained 14 vertical blades, 6 in. (15 cm) in height. The two sets of blades were 15 in. (38 cm) apart. The reactor had four baffles (not shown) extending vertically along the inside wall. Access to the inside of the vessel was possible through the 1.5-in. (3.8-cm) rupture disk and by removal of the stirring assembly flange, which provided an opening of about 12 in. (30 cm) in diameter. This reactor was rated for a pressure of 2300 psig (15.8 MPa) and an internal temperature of 287 “C. The rupture disk was rated at 2100 psig (14.5 MPa). The materials charged to the reactor are identified in Table I. Alumina was prepared via hydrolysis of aluminum isopropoxide (AIP):
+
2A1(C3H70)3 3H20
CHPH
A1203(hydrated)
The degree of hydration of the product could be controlled by the amount of water used. From related work, it was known that, under conditions of limited water supply, dehydrated forms of alumina could be produced directly (Fanelli and Burlew, 1986). When preparing aerogel materials, sufficient solvent must be provided to ensure achieving critical density conditions in the reactor; otherwise, all of the solvent would vaporize during heat-up without reaching critical density even though the critical pressure may be exceeded at sufficiently high temperature. For methanol, the critical constants are T,240 “C, P, 79.9 atm (8.1 MPa), and p c
LEGEND Run 1 0 Run 2 A Run 3
0 ‘
5
10
,
15 20 Time, hours
25
30
Figure 2. Internal temperature profiles for the three large-scale runs.
0.272 g/cm3. The quantity of methanol used in each run (Table I) was above the minimum required to satisfy the critical density condition. Three runs were made in the large reactor. The batch period ranged from 14 to 27 h from heat-up to termination venting. The product was removed by means of a vacuum cleaner. No difficulty was encountered in removing the product via the stirrer flange. Complete removal of the product was not possible when only the 1.5411. (3.8-cm) rupture disk opening was used. A summary of the data compiled for the three runs is tabulated in Table I. In the first of the runs, only 12 lb of aerogel was recovered. Because of its low density, much of the product was lost during venting. In the two subsequent runs, the venting rate was reduced and product loss was minimal. The internal temperature profiles for the three runs are shown in Figure 2. To achieve a reactor temperature in the vicinity of 250 “C, a large temperature differential had to be maintained between the reactor contents and the heating oil even with continuous stirring. Throughout most of the runs, the oil was heated to 400 “C.
Results and Discussion Armor and Carlson (1987) have prescribed procedures for preparing alumina aerogels in the laboratory. In their description, the precipitate from hydrolysis of metal alkoxide was transferred to a 300-cm3nonstirred autoclave and the solvent (methanol) vented at temperatures above 260 “C and pressures above 1750 psi. The physical properties of the current materials are compared to those of a “good” laboratory alumina aerogel (Armor and Carlson, 1987) in Table 11. The low surface areas and parallelism between low surface area and low
128 Ind. Eng. Chem. Res., Vol. 30, No. 1, 1991 Table 11. Physical Properties of the Alumina Aerogels Prepared in the 215-gal (814-L) Reactor typical lab scale alumina run 3 run 1 run 2 aerogel property 115 161 surface area, m2/g >400 111 0.017 0.026 bulk density, g/cm3 -0.06 0.023 3.45 2.56 pore volume, cm3/g >5 3.09 7.93 7.41 7.94 carbon content, wt % 6-8
bulk density are particularly unusual. In normal aerogels, low bulk density is associated with high surface area. As shown in Figure 2, the duration of stirring and venting differed among the three runs. In spite of these variations in processing conditions, the physical properties of the product from each of the three runs was similar. Thus, the physical properties of the product were set by the conclusion of the least severe conditions employed (run 1). Although the alumina aerogels prepared were found to have the same film-type morphology as laboratory aerogels when examined by transmission electron microscopy (TEM) (Fanelli and Price, 1984), scanning electron microscopy (SEM) revealed a drastically different surface texture. The SEM photomicrographs are compared in Figure 3. Agitation alters the surface area and pore volume by changing the nature of the aerogel morphology. In the absence of stirring, the aerogel has a sponge-like appearance. This morphology is destroyed by excessive agitation, which produces a surface terminating in a platelet-like structure. The appearance is most adequately described in familiar terms as a "corn flake" surface morphology. The poor handling properties and resistance to flow of the aerogel prepared in the plant is considered to be a result of its surface texture. Movement of the material from one vessel to another during evaluation was difficult. The material would adhere to the spatulas and side walls of the vessels and resist transfer. The powder behaved as if it were highly statically charged. However, its handling characteristics were not improved by use of methods to dissipate charge, e.g., exposure to ionizing radiation. This altered surface texture is attributed to the high shear imparted by the agitator. At 190 rpm, the agitator has a linear tip speed of 14.9 ft/s (454 cm/s). We have produced aerogel materials having physical properties like those of large-scale aluminas in a 1-L autoclave by using various combinations of run time and stirring rate. A comparable tip speed is produced by the 1.5411. (3.8-cm) diameter blade in our 1-L autoclave rotating at 2100 rpm. The aerogel properties can be destroyed by using high stirring speeds for short run times as well as moderate stirring rates over long run times, as shown by the data in Table 111. The first entry in Table 111is a control run in which stirring was used during the hydrolysis step but not during heat-up and venting. This run followed the practice of typical laboratory aerogel preparations. One sample (third entry in Table 111) was found to have the altered corn flake surface texture when analyzed by SEM.
Figure 3. SEM photomicrographs comparing the surface morphology of alumina aerogels prepared in the laboratory and in the 215-gal (814-L) reactor.
Entry 2 in Table I11 reveals that some degree of stirring can be tolerated during the heat-up period. Note that the
product from this run was not adversely affected by the low stirring speed/short run time conditions. It is expected that product in the large reactor would have been acceptable had stirring been similarly limited. This could not be tested in the 215-gal reactor because the internal temperature could not be maintained when stirring was stopped.
Conclusions The data in Table I11 suggest that in a commercial aerogel manufacturing process stirring must be avoided a t the higher temperatures when liquid is no longer present. This means that for a commerical process the conventional large volume reactor with stirrer and external heating jacket will not be suitable for aerogel manufacture unless a specially designed stirring system is used (e.g., a large, slow-moving paddle) or heat is supplied internally. Alternatively, a process that simulates the large surfaceto-volume ratio of a small reactor would be more attractive. The latter condition could be accomplished by a contin-
Table 111. Effect of Stirring Rate on the Properties (1-L Autoclave) of Alumina Aerogel max temp during stirring, run time, h (approx) "C stirring rate, rpm tip speed, ft/s surface area, m2/g S
4 5 14
65 257 260 250
500 500 2260 400
-.
50
3.5 3.5 16.0 2.8
-
0.35
292 350 174 131
bulk density, g/cm3 0.069 0.077 0.036 0.020
129
Ind. Eng. Chem. Res. 1991,30, 129-136 uous or semi-continuous process (Yamanis, 1989;Haig and Yamanis, 1989). Registry No. Alumina, 1344-28-1.
Literature Cited Armor, J. N.; Carlson, E. J. Variables in the Synthesis of Unusually High Pore Volume Aluminas. J. Mater. Sci. 1987,22,2549-2556. Ayen, R. J.; Iacobucci, P. A. Metal Oxide Aerogel Preparation by Supercritical Extraction. Reu. Chem. Eng. 1988, 5 (Nos. 1-4), 157-198. Fanelli, A.; Burlew, J. Preparation of Fine Alumina Powder in Alcohol. J. Am. Ceram. SOC. 1986, C-174. Fanelli, A. J.; Price, A. K. US. 4,478,987, 1984.
Fanelli, A,; Burlew, J.; Marsh, G . The Polymerization of Ethylene over TiC14 Supported on Alumina Aerogels: Low Pressure Results. J. Catal. 1989, 116, 318-324. Haig, S.; Yamanis, J. A Continuous Aerogel Process for the Production of Fine Grain Oxide Powder. Proceedings of the 1989 Annual Meeting of AIChE, San Francisco, CA, Nov 1989. Henning, S. A. Large Scale Production of Airglass. Springer Proc. P h p 1986, 6, 38-41. Sargent, N. A.; Davis, W. M. US.2,868,280, 1959. VonDardel, G.; Henning, S. A.; Svensson, L. P. G . U S . 4,402,927, 1983. Yamanis, J. US.4,845,056, 1989.
Received for review February 26, 1990 Revised manuscript received June 4, 1990 Accepted June 12,1990
PROCESS ENGINEERING AND DESIGN Adaptive Control of the Hydrogen Concentration in Anaerobic Digestion Denis Dochain* Laboratoire d'Automatique, Dynamique et Analyse des Systcmes, Universitd Catholique de Louvain, Bdtiment Maxwell, Place du Levant, 3, 1348 Louvain-la-Neuve, Belgium
Michel Perrier and A n d r e P a u s s Znstitut de Recherches en Biotechnologie (CNRC),6100 Avenue Royalmount, Montrdal, Canada H4P 2R2
In this paper, the use of hydrogen concentration as a controlled variable in anaerobic digestion processes is investigated. A simple nonlinear adaptive controller is proposed. The control scheme is based on the nonlinear structure of the process and does not require any analytical expression for the fermentation parameters. The stability properties of the closed-loop system are analyzed. The controller's performances are illustrated by simulation. 1. Introduction Anaerobic digestion is a biological treatment process of organic wastes in which methane gas is produced (Mc Carty, 1964). It is usually operated in CSTRs (continuous stirred tank reactors). The importance of implementing efficient control systems for anaerobic digestion processes clearly appears from the following two points: (1) Anaerobic digestion is intrinsically a very unstable process: variations of the input variables (hydraulic flow rate, influent organic load) may easily lead the bioreactor to a wash-out, i.e., a state where the bacterial life has disappeared. This phenomenon takes place under the form of acid accumulation in the bioreactor (see Binot et al. (1983) and Fripiat et al. (1984)). It is therefore important to implement controllers that can stabilize the process with a carefully designed control strategy. (2) If the process is used for waste-treatment purposes, the control objective consists of maintaining the output pollution at a prescribed level despite the fluctuations of the input pollution (organic load). From the above two comments, it is clear that control strategies would particularly concentrate on the control 'Presently adjunct professor a t the Ecole Polytechnique de MontrCal, MontrBal, Canada H3C 3A7. and author to whom correspondence should be addressed. 0888-58851911 2630-0129$02.50/0
of the substrate concentration (which characterizes the pollution level and the presence of acids). However, intricate difficultiesinherent to the process make the control problem very hard to solve. Anaerobic digestion is a very complex process in which many different bacterial populations intervene. Its kinetics are basically nonlinear and nonstationary, and they are far from being completely understood. Moreover, the concentrations of the different bacterial populations are not available from any direct measurement, even from off-line analyses. A simple nonlinear adaptive control solution, based on the nonlinear structure of the process, has been proposed in Dochain and Bastin (1984). Its theoretical properties have been studied (Dochain, 1986; Bastin & Dochain, 19881, and it has been experimentally validated on a pilot bioreactor (Renard et al., 1988). But the applicability of this control solution to industrial plants may be appear limited by the need of on-line COD (chemical oxygen demand) (or equivalent substrate concentration) measurement. In this context, the use of the hydrogen concentration as a controlled variable appears to be very promising. As it has been recently emphasized (Mosey, 1983; Pauss et al., 1990), hydrogen plays an important role in the kinetics and stability properties of anaerobic digestion, particularly in the presence of an organic substrate mainly composed of glucose (e.g., waste from the sugar industry). Moreover, 0 1991 American Chemical Society
