Ind. Eng. Chem. Res. 1987,26, 2389-2393 Greek Symbol ci = standard deviation of the ith normal distribution, m
Registry No. PEG-6000, 25322-68-3; victrex (200P), 2566742-9; poly(vinylpyrrolidone),9003-39-8.
Literature Cited Cabasso, I.; Klein, E.; Smith, J. K. J . Appl. Polym. Sci. 1976, 20, 2377. Chan. K.: Matsuura, T.: Souriraian, S. Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 605. Hsieh. F.-H.: Matsuura. T.: Souriraian. S. Znd. Enz. Chem. Process Des. Dev.’1979, 18, 414.‘ Kai, M.; Ishii, K.; Tsugaya, H.; Miyano, T. In Reverse Osmosis and Ultrafiltration; Sourirajan, S., Matsuura, T., Eds.; ACS Symposium Series 281; American Chemical Society: Washington, DC, 1985; pp 21-33. Matsuura, T.; Sourirajan, S. In Reverse Osmosis and Ultrafiltration; Sourirajan, S., Matsuura, T., Eds.; ACS Symposium Series 281; American Chemical Society: Washington, DC, 1985; pp 1-19. “
I
-
2389
Nguyen, T. D.; Chan, K.; Matsuura, T.; Sourirajan, S. Ind. Eng. Chem. Prod. Res. Dev. 1985,24,655. Nguyen, T. D.; Matauura, T. In Proceedings of International Membrane Conference;Malaiyandi, M., Talbot, F. D. F., Kutowy, o., Eds.; National Research Council of Canada: Ottawa, 1986; pp 99-114. Nguyen, T. D.; Matsuura, T.; Sourirajan, S. Chem. Eng. Commun. 1987a,in press. Nguyen, T. D.; Matsuura, T.; Sourirajan, S. Chem. Eng. Commun. 1987b,in press. Sourirajan, S.; Matsuura, T. Reverse Osmosis/ Ultrafiltration Process Fundamentals; National Research Council of Canada: Ottawa, 1985; Chapter 4. Tweddle, T. A.; Kutowy, 0.;Thayer, W. L.; Sourirajan, S. Znd. Eng. Chem. Prod. Res. Dev. 1983, 22, 320. Wumans, J. G.; Smolders, C. A. Eur. Polym. J . 1983, 19, 1143.
Received for review April 18, 1987 Revised manuscript received June 14, 1987 Accepted July 27, 1987
Performance Test of Absorption Air-Cooling Unit Using NH4Br-NH3 and NH41-NH3 Systems Hideki Yamamoto, Seiji Sanga, and Junji Tokunaga* Department of Chemical Engineering, Faculty of Engineering, Kansai University, Suita, Osaka 564, Japan
An absorption air-cooling apparatus using NH4Br-NH3 and NH41-NH3 systems was designed and tested in order to most effectively utilize low energy (i.e., solar energy and hot drain). This apparatus used liquid ammonia as the refrigerant and ammonium halides (NH4Bror NH41)as the absorbent, and this absorption refrigerator permitted air cooling and refrigeration. Furthermore, a rectifying tower for recovery of refrigerant from ammonia solution was not needed. Coefficients of performance (C.O.P.) of the apparatus using these two ammonia solution systems were determined for the same T,,and N operating conditions, and these values are compared and discussed. The effect of Tg, on C.O.P. was investigated under various conditions. These values were also compared with the ones calculated from the enthalpy-concentration charts for both the NH4Br-NH3 and the NH41-NH3 systems. In a recent analysis of advanced thermal energy storage systems for heating or cooling utilizing solar energy, the process of using chemical reaction of an anhydrous salt and ammonia is proposed and discussed for its practicality (Fujiwara and Sato, 1985; Jeager and Fox, 1981). The reaction products from anhydrous salt and ammonia are referred to as ammoniated salts or amine complexes whose state is either a solid or liquid. If the liquid phase is used as a working system, the rate of absorption and heat transfer is activated by agitator. The absorption cooling model proposed here consists of a generator (or absorber), a condenser, and an evaporator, and the cycles involving the generating-condensingprocess and the evaporating-absorbing one are carried out alternately. Liquid-phase systems of NH4Br-NH3 and NHJ-NH, systems (Toyoda et al., 1983) were used, where ammonium halide was the absorbent and liquid ammonia was the refrigerant. In these systems, air cooling is possible and the rectifying tower for recovery of the refrigerant from solution is not needed. This paper is concerned with the performance test of the absorption air-cooling unit using liquid phase of NH,Br-NH, and NH41-NH3 systems.
Absorption Air-Cooling Unit Used Absorption Air-Cooling Unit by Two Batch Processes. As solar heat and hot drain are low and intermittent energies, these heat sources are not suitable for an ordinary type of cooling system. Then, corresponding
to the properties of these low-energy sources, the cooling unit was designed for the intermittent operation. This absorption air-cooling unit shown in Figure 1 was driven by the alternate operation of two batch processes. One is the generating-condensing process (I) shown in Figure 2. In this process, the strong ammonia solution (where liquid ammonia is rich) in the generator heated by hot water and ammonia gas of about 100% purity is generated under high pressure. Then, ammonia gas moved to the condenser is condensed by cooling water, and liquid ammonia is stored in the condenser in this process. Finally, the pressure in the generator approaches the saturated vapor pressure of ammonia solution at the temperature of heating water. The other process is the evaporating-absorbing process (11)shown in Figure 3. The liquid ammonia in the condenser is reduced through the expansion valve, and the vaporization of ammonia occurs in the evaporator. Then, the refrigerant evaporated under low temperature and low pressure flows to the absorber and dissolves in the weak ammonia solution in the absorber under constant pressure (0.49 MPa). These two processes are repeated alternately.
Evaluation for Coefficient of Performance of This Unit The ratio of QR to QH is very important for practicality of these units. In order to compare the efficiency of these systems, coefficient of performance, E , was defined as the ratio of QR to QH, that is, 6
= (Q R /Q H )~ O O
0888-5885/87/2626-2389~01.~0~00 1987 American Chemical Society
(1)
2390 Ind. Eng. Chem. Res., Vol. 26, No. 11, 1987
Calculation of c,,~.. In this calculation, the following two conditions were assumed. (a) As enthalpy change of ammonia gas corresponding to temperature was little, ammonia gas generated from the ammonia solution has constant enthalpy in the generating-condensing process (I). (b) Ammonia gas evaporated under the constant pressure has constant enthalpy in the evaporating-absorbing process (11). Under these assumptions, the heat balance in the generator on the generating-condensing process is given by eq 2. QH was calculated from eq 2, while enthalpy values (2) Mihi + QH = Mrhf + M(G-C)NHaHNH3
Heating o r Cooling water
r
t
Elf-
of hi and hf were obtained from enthalpy-concentration charts for NH4Br-NH3 and NH41-NH3 systems (Yamamot0 et al., 1984). Q R was determined from enthalpy change of ammonia that vaporized in the evaporator ideally. Therefore, QR was expressed by eq 3. From eq 2 and 3, test was calculated by eq 4. (3) QR = M(E-A)NH~(HNH3 - hNHS) test.
Figure 1. Schematic diagram of the experimental apparatus. T.C. I CONDENSER W
TC. T
c
o
o
l
m
g water
NH3 gas
Liquid ammonia =]Ammonia
gas
Figure 2. Flow diagram of the generating-condensing process (I): T.C., thermocouple; P.G., pressure gauge.
T.
IE
CONDENSER
ABSORBER
, T.C.
I
. ...
1.L
Space to be cooled
,
?ling
water
T.C.
Figure 3. Flow diagram of the evaporating-absorbing process (11): T.C., thermocouple; P.G., pressure gauge.
where QH is the heat quantity transferred into the ammonia solution from the heating water in the generator in the first process. Q R is the heat quantity of air cooling in the evaporator at the second process.
= (QR/QH)~OO
(4)
Calculation of tabs.. On the other hand, €obs. was determined from the experimental data. Q H was obtained as the difference between the amount of supplying heat quantity (QG) and the heat loss in the generator itself (QL). Therefore, tabs. is expressed as (5) cobs. = QR/QH = ~ Q R / ( Q G - Q L ) I ~ O O where Q H was calculated from the flow rate of circulating water and the temperature change between inlet and outlet of heating water. Q R was also obtained from the flow rate of brine and the temperature difference between inlet and outlet of brine. Experimental Section Experimental Apparatus. Schematic diagram for the experimental unit was shown in Figure 1. This unit includes a generator which is also used as an absorber, a condenser, and an evaporator. The flow rate of circulating water was measured by a flow meter. The temperature of each part of this apparatus was measured by Chromel-Alumel thermocouple corrected by a standard mercury thermometer, and its accuracy was within f O . l K. The pressure in the generator and the condenser was measured by a Bourdon gauge (up to 3.0 MPa) with the accuracy of f0.1% full scale. In order to insulate the apparatus from the surroundings, this apparatus was covered with a foaming polystyrol. Details of each part of the apparatus are as follows. (a) Generator (Or Absorber). The generator, which is also used as an absorber, is shown in Figure 4. It was a 3.0 X m3 cylinder made from stainless steel and rolled coil pipe, in which heating (or cooling) water flowed. The generator contained a pressure-resistant Pyrex window (up to 3.0 MPa) for observation. Flow rate of heating (or cooling) water was controlled at 0.2 (or 0.3) L/min during the experiment. Agitator was set in the generator, and the experiment was carried out at 300 or 1000 rpm. (b) Condenser. The condenser shown in Figure 5 was also a 3.0 X m3 cylinder constructed of stainless steel and covered with acryl pipe. Cooling water was passed between the cylinder and acryl pipe at a rate of 0.4 L/min. The temperature in the condenser was kept constant throughout the experiment. The condenser also had the pressure-resistant Pyrex window (up to 3.0 MPa) for measuring the liquid level of refrigerant.
Ind. Eng. Chem. Res., Vol. 26, No. 11, 1987 2391 Table I. Initial Conditions of t h e Experiment Generating-Condensing Process (I) generating temp, K 328, 338 and 348 condensing temp, K 303
H--
Agitator
t o Condenser
Evaporating-Absorbing evaporating temp, K absorbing temp, K revolution speed, rpm
t o Evaporator
r+ Pyrex window for
-
Figure 4. Schematic diagram of the generator, which is also used as absorber. Shell
Acryl pipe
Teflon bed
dW
to Evaporator
‘to
Generator
Figure 5. Schematic diagram of the condenser: P.G., pressure gauge. ,$cry1
pipe
Inlet Of
ref rigera
Outlet o
utlet Of
efrigerant
f brine
Figure 6. Schematic diagram of the evaporator.
(c) Evaporator and Expansion Valve. Schematic diagram of the evaporator is shown in Figure 6. In the m figure, stainless steel capillary coil tube of 6 X diameter was inserted in the evaporator. Furthermore, this evaporator was covered with insulating material to minimize heat leak by either external heating or cooling. A microvalve was utilized as expansion valve. Materials. Ammonium bromide (NH4Br)and ammonium iodide (NH41)from Wako Pure Chemical Industries, Co., Ltd., were guaranteed reagents, were specified as pure grade having minimum purities of 99.5%, and were used without further purification. The powder crystals were thoroughly dried at 373 K and stored over silica gel in a desiccator. Ammonia gas of 99.99% purity was provided from Seitetu Kagaku Co., Ltd.
Process (11) 283 303 300 or 1000
Experimental Method. The weighted amount of absorbent (NH4Br or NH41) was charged in the generator, and the generator was sealed up and vacuumed for 6 h in order to removed moisture from the system. Liquid ammonia was preliminarily charged in the ammonia vessel whose volume is about 1.0 X m3 from the ammonia cylinder. By chilling the generator, liquid ammonia was allowed to be introduced into the generator from the ammonia vessel. Weight of liquid ammonia in the generator was determined by measuring the difference weight of the ammonia vessel. Then, ammoniurn hdides were dissolved in liquid ammonia by agitating the mixture. Experiment was started with the initial conditions shown in Table I. Initial concentration of ammonia solution in the generator was 59.0 wt % for NH4Br-NH3 system and 61.0 wt % for NH41-NH3 system. Final concentration was determined from the pressure of the solution in the generator. During the experiment, the measurement of temperature was carried out at several points shown in Figures 2 and 3. In the generating-condensing process (Figure 2), the ammonia solution in the generator was heated by the hot water at 328, 338, and 348 K. Flow rate was maintained at 0.2 L/min during the experiment. Temperature of cooling water in the condenser was varied from 293 to 307 K, and the flow rate of cooling water was controlled at 0.4 L/min. Stirrer speed was either at 300 or 1000 rpm. In the evaporating-absorbing process (Figure 3), the temperature of cooling water in the absorber was maintained at 293 K at a flow rate of 0.3 L/min. Water was applied as brine in the evaporator and was circulated by a pump. Inlet temperature of brine was 303 K, and the flow rate was measured accurately by use of the measuring cylinder. Furthermore, the temperature difference between inlet and outlet of brine was measured during the experiment. Speed of revolution of agitator in the absorber was 300 or 1000 rpm. Results and Discussion Temperature-time data for the NH4Br-NH3 system in the generator, the condenser, and the evaporator are shown in Figures 7 and 8. In these figures, I is the time course for the generating-condensing process and I1 is that for the evaporating-absorbing process. For NH4Br-NH3 system, the data in process I show a similar trend, regardless of the difference in agitator speed. However, air-conditioning time at the evaporator in process I1 depends on the agitation speed in the absorber. The temperature difference between inlet and outlet brine at 1000 rpm of the agitator showed a higher value than that at 300 rpm. For NH41-NH, system, similar results were obtained. It was found that the revolution speed of agitator in the absorber had a significant influence on the operating condition of this cooling apparatus. for this unit using NH4Br-NH3 and NH41-NH3 systems at various condensing temperatures is shown in Figures 9 and 10, and an test.calculation chart is also shown by dotted curves in these figures. With rising Tgand decreasing T,, cobs, for both systems increased. cobs, for the
2392 Ind. Eng. Chem. Res., Vol. 26, No. 11, 1987
1
I
NH4Br-NH3 System
350
- 340 E
3 30 0
320
3 20
o
h
0
a
E
310
c Q,
-
10
300
\
290 I
2801
0 290
295
Condensing
Time
\
300 305 310 IKI temperature TI
Figure 10. Comparison of coefficient of performance between estimated and observed value for NH,I-NH3 system at 1000 rpm in the absorber.
lhrl
Figure 7. Time course of the temperature in the experimental apparatus at 1000 rpm.
II1 N K B r -NH3 System
1
I
NHABr-NHj System Ts = 348 K
=296K N =300rpr#
rc
? ?. 320
2
E
5
310
'"1, 300
,
I
,
,
~,
,
~
290
295
300
305
:0
!HI
2 70
0 05 1.0 1 5 20 2 5 30 35 40 45 Time
IhrI
Figure 8. Time course of the temperature in the experimental apparatus at 300 rpm.
Con& nsing temperature TC
Figure 11. Relation between coefficient of performance and speed of revolution for NH4Br-NH3 system.
NHII-NH,
System
I
0 IKI
Condensing temperature Tc
Figure 9. Comparison of coefficient of performance between estimated and observed value for NH4Br-NH3 system at 1000 rpm in the absorber.
NH4Br-NH3 system was always higher than for the NH41-NH, system under the same operating conditions. Similar results for tWt,were obtained. The main reason for this is that the enthalpy difference between ammonia gas and ammonium iodide solution is always larger than
Figure 12. Relation between coefficient of performance and speed of revolution for NH41-NHBsystem.
that for ammonium bromide solution at the same concentration. That is, the heat quantity required for generation of a certain amount of ammonia in the NH4BrNH3 system was smaller than that in the NHJ-NH, system. In this study, it was found that this air-cooling unit could be driven at about 65% and 55% of test. for NH,Br-NH3 and NH41-NH3 systems, respectively.
Ind. Eng. Chem. Res. 1987,26, 2393-2397
Figures 11 and 12 show the relation between cob. and
Tcat 300 or 1000 rpm of the agitator for NH4Br-NH3 and NH41-NH3 systems. For both systems, at 1000 rpm waa superior to that at 300 under the operating conditions examined. These figures showed that the agitation speed in the absorber had a important effect on the coefficient of Derformance.
Conclusion In the performance test on the absorption cooling apparatus proposed in this work, the system of NH4Br-NH3 was superior to the NH41-NH3 system. The coefficient of performance was about 55% -65% of the estimated one calculated from the enthalpy-concentration chart. These values are not sufficient in practical use, but the usefulness for the batch-type absorption cooling apparatus for use of utilizing low-level energy has been confirmed.
2393
Q G = heat
quantity supplied from heating water in the generator, kJ QH = net heat WmtitY h n ~ f e r r e d in the generator ( 8-~83, kJ
21 et^,^^^;^,
Tg= temperature in the generator, K
Greek Symbol = coefficient of performance
t
Subscripts i = initial state of the solution f = final state of the solution E-A = evaporating-absorbing process G-C = generating-condensing process
est. = estimated value obs. = observed value Registry No. NH3, 7664-41-7; NH,Br, 12124-97-9; NHJ, 12027-06-4.
Acknowledgment
Literature Cited
We A*Kurata for his Toyoda for his experimental work.
Fujiwara, I.; Sato, M. Reito 1985, 60,24. Jeager, F.; Fox, E. C. D.O.E. Report CS-30248-T-1, 1981; D.O.E., New . - . York. - -. .. Toyoda, T.; Kurata, A.; Sanga, S. Preprints of the 48th Annual JaDan, Kyoto, Meeting of The Society of Chemical Engineers, 1983, p43. Yamamoto, H.; Kurata, A.; Sanga, S. Kagaku Kogaku Ronbunshu 1984, 10, 421.
discussion and T*
Nomenclature H = enthalpy of ammonia gas, kJ/kg h = enthalpy of ammonia solution, kJ/kg M = mass, kg N = speed of revolution, rpm QR = heat quantity of air cooling in the evaporator, kJ
Received for review March 31, 1986 Accepted August 10, 1987
Tendencies of Aromatization in Steam Cracking of Hydrocarbons Frank-Dieter Kopinke,* Gerhard Zimmermann, and Bernd Ondruschka Central Institute for Organic Chemistry, Academy of Sciences of GDR, Department of Basic Organic Materials, 7050 Leipzig, GDR
The formation of aromatics from nonaromatics during steam cracking of naphtha is described quantitatively. To get realistic data, the tracer technique was used on the basis of about 40 '%-labeled hydrocarbons as constituents of a naphtha fraction. These model compounds are representative of pyrolysis feedstocks, reaction intermediates, and reaction products. Characteristic aromatization yields are given for different types of C atoms and essential molecules. The formation of aromatic hydrocarbons from nonaromatic ones is unavoidable under the conditions of industrial steam cracking of straight-run or hydrocatalytically pretreated crude oil fractions. The liquid fractions resulting from the pyrolysis of such feedstocks are cracked gasoline and cracked fuel. Both fractions are highly aromatic. On the one side, cracked gasoline is a valuable feedstock for producing pure benzene and motor fuel; on the other side, alkylbenzenes and especially oligocyclic aromatics are precursors in the formation of coke-like deposits in the pyrolysis coils and the transfer line heat exchangers (e.g., Trimm (1983), Lohr and Dittmann (1978)). Further, aromatization reactions decrease the yield of olefins, the main object of the process. In most cases, the fuel fraction can be used for heating only. For these reasons, the extent of aromatization has to be limited by suitable means, such as special temperature profiles along the coils, short residence times, high speeds of product quenching, and optimum steam dilutions. Since the paper of Kossiakoff and Rice (1943),the very complex reaction taking place in the thermal degradation 0888-5885/87/2626-2393$01.50/0
of hydrocarbons has been interpreted on the basis of a radical chain mechanism. The generation of aromatic structures is attributed to what is called secondary reactions, about which only little relevant knowledge is available now. Recent results obtained by us on the mechanism of aromatics formation will be presented elsewhere (Kopinke et al., 1987). In principle aromatics are formed under the condition of pyrolysis via two routes: by synthesis reactions of lower hydrocarbon species and by degradation reactions. The first route is the only one in the pyrolysis of paraffins; the second path dominates in the pyrolysis of oligocyclic naphthenes (Zimmermann et al, 1985; Cypres and Bredael, 1980; Korosi et al., 1979). An analysis of published results leads to an important conclusion. Sufficient fundamental knowledge concerning thermal aromatization does not exist under conditions relevant to industrial steam cracking which would permit a kinetically and mechanistically founded quantitative description or even modeling of this important process. Accordingly, the concepts and kinetic data used in pyrolysis models like SPYRO (e.g., Dente et al. (1979)) or 0 1987 American Chemical Society
