914
Ind. Eng. Chem.
Process Des. Dev. 1988, 25, 914-918
Experimental Investigation on Dust Collection Efficiency of Straight-Through Cyclone wlth Air Suction by Means of Secondary Rotational Air Charge Tetsuo Akiyama" and Taketoshi Marul Department of Chemical Engineering, Shizuoka Universl;
natsu 432, Japan
Motomi Kono Ako Company Ltd., Chiba 272-01, Japan
This paper is concerned with an experimental investigation of the effect of gulde vanes on dust colledion efficiency
for a straight-through cyclone, with the suction of dust-laden air by means of a secondary rotational air charge. Four kinds of dusts were used for the experiments of dust collection efficiency. It was found that the present type of straight-through cyclone is capable of achleving a comparable perfmnce wlth the reverseflow cyclone and is especially suited to removing dust from the hot exit gas from furnaces.
The cyclone separator is widely used as a means of dust removal from gases in a variety of engineering applications, and various types of cyclones are in use in accordance with their specific requirements (Straws, 1966). The centrifugal force on particles in a swirling gas stream is much greater than gravity and, therefore, is more effective in removing smaller particles than gravitational settling and requires less space to handle the same gas volumes. On the other hand, the pressure drop in a cyclone is greater, and power consumption is much higher. In general, cyclone designs fall into two groups: the straight-through cyclone and the more common conventional reverse-flow cyclone. In the latter, the gases spiral down from a tangential inlet toward the apex of a conical section where particles are deposited and the direction of the flow is reversed. It then proceeds upward through a vortex finder to the axial gas exit. Numerous studies have been done on the performance of reverse-flow cyclones. Studies on straight-through cyclones, however, are relatively few. The advantage of the straight-through cyclone over the reverse-flow cyclone is that a large volume of gas can be handled. The drawback is the lower dust collection efficiency, which is attributable to the insufficient swirl flow in practical operations. The swirl flow in the straight-through cyclone is usually generated by fixed vanes or impellers, but sometimes turbocompressors can be used. A list of studies (up to 1966) on swirling jets issuing from vane swirlers are listed in the work by Mathur and Maccallum (1967) who discussed designs of vane swirlers for efficient directing of the air. A comparative study of the guide-vane-type cyclone with the reverse-flow-type cyclone was made by Jotaki (1954). Decay of swirling flows in circular ducts has been studied by a number of authors (Talbot, 1954; Kreith and Sonju, 1965; Murakami et al., 1975; It0 et al., 1980). Recently experiments have been performed on swirling air flow in a sudden expansion by Hallet and Gunter (1984). In order to secure a high dust collection efficiency, strong swirling flow is required. In the present study a sufficiently
* To whom all correspondence should be directed. 0 196-4305/86/ 1125-0914$01.50/0
strong swirl slow was produced by introducing the secondary air tangentially into a cylindrical duct. This swirling flow then induces the suction of dust-laden gas through an inlet duct. The method to produce swirling flow by introducing air tangentially into a cylindrical chamber (instead of using guide vanes) was used in the rotary flow duct collectors (Ogawa, 1984) and in other studies (Ullrich, 1960; Chigier and Beer, 1964), but the tangential input in those works was not intended to suck the dust-laden gas as is the case in this study. Another important role of the secondary tangential air flow is the protection of the cyclone wall from heat when used for a high-temperature gas. In fact, the usefulness and the most important feature of the present straight-through cyclone resides in its capability of handling the furnace flue gas, whose temperature is commonly in the range of 10001200 K. No conventional cyclone can handle the hot gas in this temperature range. Conventional cyclones made of steel will not withstand the operating temperature which is above 700 K. Even cyclones made of stainless steel will be of no practical use, because cracks occur at the welded parta when the cyclone is cooled down after use with a hot gas. Heat-resistant material needs to be used to cope with this problem. However, it leads to an increase in construction as well as maintenance costa. More importantly, even if the cyclone withstands high temperature, the fan through which a hot gas passes will not bear the heat when the temperature exceeds, say, 700 K. In practice, therefore, the hot exhaust gas from furnaces is cooled down either through the use of heat exchangers or through the injection of cool air or water before it is dealt with in a cyclone. The cyclone we will study in this paper does not require this precooling of hot gas and is free from the materials problem of the fan since the tangential air injection induces the exhaust gas flow toward the cyclone. Incidentally, heat-resistant stainless steel SUS31OS can be used for guide vanes, but welding should be avoided for its placement because the guide vanes are exposed to hot gases. In fact, the present type of straight-through cyclones are now being operated successfully on a commercial scale, for the removal of dust from high-temperature furnace flue gases. 0 1986 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 4, 1986 915 Valve-1
Table 11. Physical ProDerties of Dust true av diam, density, fim kdm' dust pine pollen 35.0 1050 glass beads 12.5 2480 fly a s h 4 15.0 2000-2300 fly ash-10 5.1 2000 2300
Valve-2
win dd
-
O.IO(
Figure 1. Schematic diagram of experimental apparatus.
-
I
00486
0
0135 0216
A
bulk density, kdms 367 1300-1500 682 682
I
0
0 1 0 4 4 rad
u)
E L
d
L - w Typ. A
TYP. 8
TYp.
c
Figure 2. Details of guide vanes. Table I. Guide Vanes tvDe vane d i m , m A 0.185 B-1 0.120 B-2 0.120 B-3 0.120 C 0.100
vane anale, rad 0.52 0.44 0.52 0.61 0.44
-0
8.. vane no. 8 6 6 6 8
In the preceding paper in this issue, a fundamental study was made on the fluid mechanics of swirling pipe flow with air suction in conjunction with the design of straightthrough cyclones. This paper is concerned with an experimental investigation of the effect of guide vanes on dust collection efficiency for a straight-through cyclone with the suction of dust-laden air. Experimental Apparatus and Procedures The schematic diagram of the experimental apparatus is shown in Figure 1. The main duct of the present cyclone is identical with that of the preceding study except that a catcher was set at the duct exit. Three catchers with different annular widths (S in Figure 1)were tested. A duct of 0.1 m in diameter was chosen for the inlet duct as it was found (in the preceding study) to give the most air suction, yet holds the gage pressure as well as the axial velocity (near the duct center) negative in the region close to the cyclone exit. Three times the length of the main duct diameter was chosen for the duct length in the present dust collection study, since the rate of air suction has been found to depend only slightly on the length of the main duct when greater than 0.6 m. A vibration feeder was used to provide dust a t the concentration level of 0.003 kg/m3 in the primary air flow. Several kinds of guide vanes, whose specifications are listed in Table I, were used to give swirling motion to the primary air in the upstream of nozzles. Details of guide vanes are illustrated in Figure 2. Dust is driven toward the wall by swirling fluid motion and is caught in the annular space of the catcher. Then it is sucked into a bag filter by means of a vacuum pump (approximately a t the rate of 1.5
002
004
006
008
QZ I mYt1 Figure 3. Dependence of the rate of air suction on guide vanes and annular width of catcher.
m3/min). Two kinds of experiments were carried out on the dust collection efficiency. One is to collect dust using the secondary air charge only. The other is to use, in addition to the secondary air charge, a suction fan to give a supplementary driving force for the suction of the dust-laden air. The latter experiment is carried out to simulate the working condition: the gage pressure within a furnacemay show approximately 60 Pa, which would give rise to 10 m/s of exit gas velocity. The physical properties of the dust used are listed in Table 11.
Results and Discussion Effect of Guide Vanes. When the type-C guide vanes were used, the magnitude of the negative gage pressure was observed to increase near the duct center, and the gage pressure increased slightly near the wall. This tendency was more pronounced when type-A guide vanes were used. Although it is not shown here for the sake of brevity, the increase in the magnitude of tangential velocity was noticeable in the entire range. On the other hand, the axial velocity became smaller in the whole range especially in the central region of the cross-sectionalarea. Furthermore, it was observed that the effect of guide vanes decreases at large axial distances. The dependence of the primary air flow on guide vanes as well as the width of the dust outlet, SIR,, is illustrated in Figure 3, where Q1 and Q2 refer to the primary and secondary air flow rates, respectively. The dashed line at Q1= 0.041m3/s, which gives the average axial air velocity within the inlet duct Wb= 5.2 m/s, refers to the flow rate below which dust (glass beads, fly ash) accumulates on the duct wall. The figure indicates that Q1is proportional to Q2, and the smaller width of the dust outlet leads to a larger suction rate for given Qz regardless of the existence of guide vanes. The figure also indicates that when the guide vanes of type B are used, nearly twice as much
916
Ind. Eng. Chem. Process Des.
Dev., Vol. 25,
No. 4, 1986
0
601
0.06
0.05
0.06
0.07
0216
012
4
0.08
02 [m3/sl
Figure 4. Effect of annular width of catcher on dust collection efficiency. Table 111. Dependence of Dust Collection Efficiency on the Types of Guide Vanes without Forced Air Suction (Glass Beads, 9 = 0.44 rad) q % for Qz, - m3/s . guide vanes 0.0550 0.0639 0.0712 none 77.0 67.8 63.0 B-3 86.9 77.4 70.5 B-1 78.3 77.0 Table IV. Dust Collection Efficiency without Guide Vanes for Various Dusts under Given Experimental Conditions ( S I R . = 0.0486.0 = 0.44 rad) 11 % for 61, 62,m3/s dust 0.0499. 0.0436 0.0408. 0.0336 0.0322, 0.0265 pine pollen 87.0 90.5 93.3 glass beads 77.5 84.8 86.2 fly a h - 5 73.0 79.6 fly ash-10 52.0 51.3
secondary air charge as that without guide vanes is required to secure a given rate of air suction. Experiments with the guide vanes listed in Table I indicated that the smaller the vane angle and the larger the number of guide vanes, the more friction loss the guide vanes generate. Dust Collection without Forced Air Suction at the Main Duct Exit. The dependence of the dust collection efficiency on the annular width of the dust outlet is shown in Figure 4. The dust collection efficiency, 9,refers to the percent of the dust collected by the dust catcher from the primary dust-laden air. This figure indicates that as the rate of secondary air charge is increased, q increases for SIR, = 0.216, whereas it decreases for S/R, = 0.0486 and 0.135. This suggests that when there are no guide vanes, the dust-laden air is not sufficiently endowed with the centrifugal force by the secondary air charge alone, so that dust escapes from the duct exit without being caught for small SIR,. Note that Q2/Q1 is 0.83, 1.1, and 1.5 when SIR, is 0.0486,0.135, and 0.216, respectively, and Q2/Q1 yields a smaller value for a smaller SIR,. The dependence of the dust collection efficiency on the type of guide vanes is illustrated in Table Ill glass beads are used as dust. The table shows that for the given rate of secondary air charge, the dust collection efficiency increases as guide vanes are used, and the larger the guide-vane angle is, the better the efficiency becomes within the experimental range studied. In effect, the installment of guide vanes increases the ratio of the rate of secondary air charge to that of the primary air flow, Q2/Q1. The dust collection efficiencies for various experimental conditions with four kinds of dusts are listed in Table IV in the case SIR, = 0.0486. The table indicates that the smaller the rate of secondary air injection, the better the
Figure 5. Dust collection efficiency with guide vanes and forced air suction a t duct exit. without Guide-vans H . 0 . 5 5 5 m Gbss-bcadr
Cb-0.1 00 m 8 = 0 4 4 rad
-
70t
o
o
a I
I '
0
I
501 0
I
05
10 QZIQ~
I 0.216 1
0.1 2
,
15
[-I
Figure 6. Dust collection efficiency without guide vanes under forced air suction at duct exit. Table V. Dependence of Dust Collection Efficiency on the Types of Guide Vanes with Forced Air Suction (Glass Beads, 0 = 0.44 rad) n % for 8,+ Q,. m3/s guide vanes 0.17, q% 0.16, 11% 0.13, q% B-2 88.5 89.5 91.0 B-3 88.0 88.5 90.5 C 89.5 90.0 91.5
collection efficiency becomes within the present experimental range, which is in accord with the results in Figure 4. Dust Collection Efficiency with Forced Air Suction at the Duct Outlet. When the forced air suction is imposed on the cyclone at the outlet, Q1can be varied independent of the value of Q2. Thus, dust collection efficiencies with and without guide vanes are plotted against Q2/Q1, having Q1 Q2 as a parameter in Figures 5 and 6, respectively. These figures illustrate that the dust collection efficiency increases in accordance with the increase of Q2/Q1 until Q2/Q1 reaches approximately 0.6 and then it levels off for larger Q2/Q1 regardless of the value of SIR,. Also, the larger SIR, yields better efficiency in the whole range, provided guide vanes are installed. However, the difference in SIR, does not appear to have much influence on the efficiency when guide vanes are not used. This differs from the features in Figure 4 when SIR, has a significant effect on the dust collection efficiency. The forced air suction appears to have some influence on the velocity profiles within the main duct: it seems to have some adverse effect on the dust collection efficiency when the width of S I R = 0.216 is used. In reality, however, the outlet gas from the furnace usually reaches 10 m/s, so that the use of the suction fan is unnecessary. Therefore, it can be stated, in view of the
+
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 4, 1986 917
high dust collection efficiency in Figure 5 , that the straight-through cyclone of the present flow system can be successfully used to separate dust from the exhaust gas, provided proper guide vanes are installed and sufficient tangential velocity is applied. The dependence of dust collection efficiency on the types of guide vanes is illustrated in Table V in the case of SIR, = 0.216 glass beads were used as the dust. The efficiency does not appear to depend on the types of guide vanes. This is somewhat different from the data in Table 111,when a smaller width of the dust exit was used without forced air suction. Comparison of the Straight-Through Cyclone with a Conventional Reverse-Flow Cyclone. The pressure drop within a standard reverse-flow cyclone may be calculated from (Perry, 1984) Pw b 2
AP = 7.41-
2
(1)
The energy loss is
where S b is the cross-sectionalarea of the inlet duct. With the straight-through cyclone of the present flow system, we have obtained as much volumetric flow rate of the primary flow as that of the secondary air charge, i.e.,
81= Qz with s,/sb N 0.1, where S, is the total cross-sectionalarea of the nozzles. This implies that u, = low, Thus, the kinetic energy of the secondary air charge is 1
E = (100)z p w b 3 s b
(3)
This is, in fact, 100 times the kinetic energy of the inlet air flow, Ei (4)
Therefore, from the viewpoint of giving the dust-laden air the kinetic energy of Ei,the method of the secondary rotational air charge to suck the air is quite inefficient. Obviously, it is less energy consuming if air is sucked directly by a suction fan. In this respect, the present flow system has no advantage over the reverse-flow cyclone. However, when the exhaust gas velocity from the furnace reaches as much as 10 mls, the energy to suck the air into the cyclone becomes of little importance. And, the comparison between the two cyclones can be made by assessing the pressure drop or energy loss within the cyclones. Experimental data of the preceding study together with the present one make it possible to estimate the energy loss within the straight-through cyclone. The energy loss can be represented approximately by eq 5 where the 1 (0.75 + 0.50 1.50 + 0.74 2.16)-pwb3sb = 2 1 (5.65)-pwb3sb (5) 2 coefficient (number) in parentheses on the left-hand side of eq 5 signifies the loss due to the wall friction, sudden expansion from the inlet duct to main duct, guide vanes, catcher and tangential expansion from nozzles, respectively. When the last term was derived, the nozzle velocity was assumed to be 3 times the wb, and the coefficient d l increase in accordance with the increase of the ratio
+
+
U J Wk The comparison of eq 2 and 5 demonstrates that the energy loss within the straight-through cyclone is comparable with (and can be made smaller than) the standard reverse-flow cyclone. Conclusions Experimental investigation was made on the effect of guide vanes and the dust collection efficiency of a straight-through cyclone with the suction of dust-lade air. The air suction was induced by the secondary rotational air charge into the main duct. In addition to the secondary air charge, a suction fan was used to simulate the working condition in some experiments. It was found that the installment of guide vanes is essential to achieve a high dust collection efficiency even with the secondary air charge, but it reduces the rate of primary air flow by approximately half under practical operations. The dust collection efficiency is dependent on the annular width of the dust catcher, as well as the types of guide vanes (such as the angle and number of guide vanes), but it tends to depend less on the types of guide vanes as the width of the annulus is increased. To use the secondary rotational air charge solely for the purpose of generating the primary air flow was found to be inefficient from the viewpoint of energy consumption. However, the energy loss within the present straightthrough cyclone was found to be comparable (and can be made smaller) with the reverse-flow cyclone. The most important feature of the present straight-through cyclone resides in ita ability to handle the furnace flue gas, whose temperature is commonly in the range of 1000- 1200 K. No conventional cyclones can handle the hot gases without precooling. The secondary air charge is conveniently used to protect the cyclone wall from the heat and to drive the dust to the wall, while inducing the dust-laden gas flow toward the cyclone and eliminating the material problem of the fan. Thus, not only the cyclone itself but also the auxiliary equipment must be taken into account in the evaluation of the overall energy efficiency or running and construction costs. The present straight-through cyclone can be more advantageous than the conventional cyclones when used for the removal of dust from high-temperature furnace flue gases. Nomenclature D, = diameter of main duct, m Db = diameter of inlet duct, m H = length of main duct, m P = pressure, Pa Q1= rate of air suction, m3/s Qz = rate of secondary air charge, m3/s R, = radius of the main duct, m S = annular width of catcher, m sa,s b , s, = cross-sectionalarea of main duct, inlet duct, and the nozzles, respectively, mz U,,= air velocity from nozzles, m/s W , = average axial velocity in the main duct, m/s wb = average axial velocity in the inlet duct, m/s 2 = axial distance from nozzle, m Greek Letters B = nozzle angle, rad p = air density, kg/m3 Literature Cited Chigier, N. A.; Beer, J. M. J . Basic Eng. 1984, 86, 788. Hallett, W. L. H.; Giinter. R. Can. J . Chem. Eng. 1884, 62, 149. Ito, S.; Ogawa, K.; Kuroda, C. J . C h e m . fng.Jpn. 1980, 13, 6 . Jotaki. T. Klron 1954. 20, 604. Krekh, F.; Sonju. 0. K. J . f/uH Me&. 1985. 22, 275. Mathur, M. L.; Maccallum, N. R. L. J . Inst. Fuel 1987, 40, 214.
Ind. Eng. Chem. Process Des. Dev. 1986, 25, 918-925
918
Murakaml, M.; Klo, S.; Katayama, H.; Ilda, Y. K. Ron. B . Jpn. 1975, 4 1 , 1793. Ogawa, A. Separatlon of Particles from A& and Gases; CRC Boca Raton. FL. 1984 Vd. 2. Chanters 1 and 2. Perry, R. H.'Che&I EIiglnsefs' Handbook, 6th ed.; McQraw-HIII: Slngapore, 1984; Chapter 20. StraUSS, W. Industrkrl Gas Cleaning; Pergamon: New York, 1966; Chapter 6.
Talbot, L. J . Appl. Mech. 1954, 21, 1. Ullrlch, H. Forsch. Geb. hgenkunves. 1960, 26, 19.
Received for review July 22, 1985 Revised manuscript receiued December 17, 1985 Accepted March 23, 1986
Effect of Hydrogen Pressure on Catalytic Hydrodenitrogenation of Quinoline F. Glola" Dlpartlmento dl Ingegner& Chlmica, Universifg dl Napoli, Piazzaie Tecchio, I 80 125 Napoli, Italy
v. Lee Chemical Engineering Department, Unlverslty of Delaware, Newark, De& ware 19716
Hydrodenitrogenation of qulnoline has been investigated experimentally and theoretically. The behavior to hydrotreating of this chemical is hopefully representative of that of many nitrogen heterocycles contained in the products of coal liquefaction processes. The experimental runs were conducted in a stirred batch reactor at constant temperature ( T = 350 "C)and in the presence of a p r e s u l f i catalyst (Ni-Mo/Al,O,). The hydrogen pressure, kept constant during each run, was varied in the range 10.5 bar Ipn, I151.6 bar. The experimental results show that a necessary condition for the elimination of the nitrogen atom IS that hydrogenation of the rings containing this element be completed first. The results also permit identification of the reaction network and of its modifications with hydrogen pressure. Finally, with the help of Langmulr-Hinshelwood theory, it is possible to gain an Insight into the mechanism of the single reactions which form the network. Rate equations are proposed for all these reactions, and values of the parameters are calculated.
The products of coal liquefaction processes require upgrading treatments if they are to be used as effective substitutes of petroleum. Upgrading is mainly required to enhance the ratio H/C and to remove nitrogen-containing and sulfur-containing compounds which are mostly present as large aromatic heterocycles. A way of achieving the latter task is by hydrotreating, i.e., by reacting the liquid coal products with hydrogen at high pressures in the presence of suitable catalysts. Sulfur and nitrogen are removed as H2S and NH3. In a review article by Katzer and Sivasubramanian (19791, the existing chemistry and technology for hydrodenitrogenation is reviewed and catalyst and process needs for synthetic feedstocks and heavy petroleum liquids are examined. The present work follows a line of research in which the hydrotreating of single heterocycles is thoroughly studied. In particular the influence of hydrogen pressure on the hydrodenitrogenation of quinoline is investigated. The choice of quinoline was made on the assumption that its behavior to hydrotreating is similar to that of most nitrogen heterocycles contained in the products of coal liquefaction processes. Hydrodenitrogenation of quinoline has been investigated by several authors, among them Shi et al. (1977), Bhinde (1979), Cocchetto and Satterfield (1981), Satterfield et al. (1978, 1981a-c), Satterfield and Yang, (1984), and Yang and Satterfield (1984). The results of the present work, however, provide an advancement over those of the pre-
* Author to whom correspondence should be addressed. 0196-4305/86/1125-0918$01.50/0
vious investigators for two reasons: first, because all identified reaction products and intermediates are linked quantitatively in a complex network, without simplifications, and second, because the effect of hydrogen pressure has been investigated in greater detail. In fact, the reaction has been run over a range of pressures wider than previously reported. Consequently, it has been possible to shed more light on both the network and the mechanism of the reactions involved in the process and to obtain an estimate of the hydrogen pressure dependence of the kinetic constants of the single reactions. Experimental Procedure The apparatus used in this work is similar to that described by Shih et al. (1977). A syntetic description of a typical reaction run is as follows: The catalyst, a commercial HDS-SA grounded and sieved to 150-200 mesh, was sulfided at 400 "C. The sulfiding took place by contacting the catalyst with a flowing H,S/H, mixture (10% H2S) for 2 h. The catalyst was then put into the loader together with quinoline, CS2, and hexadecane (about 25 mL). Then the loader was assembled and connected to the apparatus. At this time the heating of the reactor, a 300-mL autoclave with magnetic stirring (Autoclave Engineers) previously loaded with about 200 mL of hexadecane, was started. When the required temperature was 4 which would drop to 350 "C after loading reached ( ~ 3 6 "C the cool reactant and catalyst), a sample was collected from the reactor to monitor the hexadecane decomposition products. 0 1986 American Chemical Society
