Space Administration, Exxon Research and Engineering Company, and the donors of the Petroleum Research Fund, administered by the American Chemical Society.
g = fluid viscosity, lb/(ft)(sec)
Nomenclature A = cross-sectional area of tube, ft2 CD = drag coefficient, dimensionless D , = particle diameter, f t Dt = tube diameter, ft
Literature Cited
f = Fanning friction factor, dimensionless gc = unit conversion factor, 32.17 (€t)(lb)/(lbf)(sec)2 L = tube length, ft n = constant defined bv ea 5 (NR,), = Dppf(uf - u,)t/pf = particle Reynolds num-
ber, dimensionless = Dppfut/pf = particle terminal Reynolds number, dimensionless A P f = pressure drop due to static head of fluid, lbf/ft2 APf, = pressure drop due to fluid-wall friction, lbf/ftZ AP, = pressure drop due to static head of solids, lbf/ft2 = pressure drop due to solids-solids friction, lbf/ft2 SP,, = pressure drop due to solids-wall friction, lbf/ft A P I = total pressure drop, lbf/ft2 q f = volumetric flow rate of fluid, ft3/sec q5 = volumetric flow rate of solids, ft3/sec uc = superficial slip velocity, ft/sec uf = fluid velocity, ft/sec u1 = adjusted terminal velocity defined by eq 17, ft/sec urns = mean suspension velocity, ft/sec uof = superficial fluid velocity, ft/sec uoi = superficial solids velocity, ft/sec us = solids velocity, ft/sec us] = (fluid-solids) slip velocity, ft/sec ut = particle terminal velocity, ft/sec
Greek Letters A = differenceoperator c = void (fluid) fraction in lift line, dimensionless (1- c ) = solids fraction in lift line, dimensionless t d = delivered water fraction, dimensionless I
pf ps
= fluid density, lb/ft3 = solids density, lb/ft3
Barton, P., Fenske, M . R . , Ind. Eng. Chem.. Process Des. Dev.. 9, 18 (1970)
Bingham, "Fluidity and Plasticity," McGraw-Hill, New York. N . Y . , 1922. Condolios, E . . Chapus, E. E., Chem. Eng.. 70 ( 1 3 ) . 93 (1963). Davies, G., Robinson, D. B., Can. J , Chem. Eng., 38 ( 6 ) , 175 (1960). Drew, T . E.,Koo. E. C . , McAdarns. W . H.. Trans. AICh€, 28, 5 6 (1932) Keenan: J. H., Keyes. F . G . , "Thermodynamic Properties of Steam," Wiiey. New Y o r k , N . Y . , 1937. Kopko. R. J.. P1.D. Thesis, The Pennsylvania State University, University Park, Pa.. 1969. Newitt, D . M . , Richardson, J. F . , Gliddon, B. J.. Trans Inst. Chem. Eng , 39,93 (1961). Orr, Ciyde, "Particulate Technology," Macmiilan, New York, N . Y . , 1966 Richardson, J. F . , Zaki, W. N . , Trans. lnst. Chem Eng.. 32,36 (1954) Wilhelm, R. H., Kwauk, M . . Chem Eng. Prog.. 4 4 , 201 (1948) Zenz, F. A . . Pet. Refiner, 36 ( B ) , 147 (1957)
Receiued f o r revieu August 23, 1974 Accepted February 20,1975 Microfilm of "Hydrodynamics of Cocurrent Countergravity Solids Transport for Liquid-Fluidized Heat Exchangers," a Doctoral Dissertation by Ronald J . Kopko, may he obtained, free of charge for specified periods, on interlibrary loan from Pattee Library, The Pennsylvania State University, University Park, P a . 16802. A purchase of the microfilm can be made through University Microfilms, Inc., Ann Arbor, Mich.
Supplementary Material Available. Experimental results (Appendix I) will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper only or microfiche (105 X 148 mm, 24X reduction, negatives) containing all of the supplementary material for the papers in this issue may be obtained from the Journals Department, American Chemical Society, 1155 16th St., N.W., Washington, D.C. 20036. Remit check or money order $4.50 for photocopy 01 $2.50 for microfiche, referring t o code number PROC-75-264.
Pilot-Plant Production of Urea-Ammonium Sulfate Gordon C. Hicks and John M. Stinson* Tennessee Valley Authority. Muscle Shoals. Alabama 35660
Urea-ammonium sulfates (6- to 12-mesh size) of high-nitrogen content (43-30% N) and containing readily available sulfur (1-13% S) were produced in pilot-plant facilities from concentrated urea solutions (99+"/0) and by-product ammonium sulfate crystals. Granulation was by processing fluid mixtures (275-300°F) of the raw materials in a pan granulator and by prilling in oil. The feed to the pan was introduced in the form of a spray: granulation temperatures were controlled with recycle. I n oil prilling, the fluid mixture was fed to a prilling cup, the prills were quenched in a lightweight mineral oil (8O-10O0F), and deoiled (1-2% oil) by centrifuging. No drying steps were required in these processes. Results of exploratory tests of air prilling also are described. All of the products had greater strength than air-prilled urea and should be more suitable for use in bulk blending because of the larger size. Granulation in a pugmill was not considered satisfactory.
Sulfur is an essential secondary plant nutrient, required for healthy growth of all crops. In the past, cultivated crops generally have received sufficient sulfur either from native soil content, from sulfur content of rain, or from the collateral sulfur content of fertilizers. The sulfur con-
tents of the fertilizers were mainly from ammonium sulfate, which was used chiefly for its nitrogen content, and ordinary superphosphate, which was used chiefly for its phosphorus content; both of these materials contain abundant sulfur in available sulfate form. In recent years, howInd. Eng. Chem., Process Des. Dev., Vol. 14, No. 3, 1975
269
Table I. Analysis of Crystalline Ammonium Sulfate Used in Pilot-Plant Tests Chemical analysis, % Source
Total N
SO4
Caprolactam Coke oven
21.1
72.0 70.8
21.0
Screen analysis, mesh %
Moisture +10 0.1 0.2
5.5
...
ever, the sulfur content of fertilizers has declined drastically because the newer nitrogen sources, such as ammonium nitrate, urea, and anhydrous ammonia, and the newer phosphates sources such as triple superphosphate and diammonium phosphate, all are essentially sulfurfree. As a result, deficiencies of sulfur are beginning to appear in some areas where the newer products are used and where lesser amounts of sulfur compounds are deposited by rainfall. Enrichment of fertilizers with sulfur, therefore, has become important for some areas. In this connection, TVA has for several years produced and distributed demonstration-scale quantities of ammonium nitrate sulfate containing 30% nitrogen and 5% sulfur. Also, pilot-plant work has been carried out on production of higher analysis material by the combination of ammonium sulfate with urea to produce granular urea-ammonium sulfates. This pilot-plant study is the subject of the present paper. Ammonium sulfate is a large-volume byproduct of several industrial processes, including the coking of coal and the production of caprolactam, and, as such, sometimes has been available a t reasonable cost for inclusion in fertilizers. Joris and Sor (1971) reported successful tests of production of urea-ammonium sulfate (33% N) by compaction and by air prilling. In the air-prilling operation, which was carried out in pilot-plant equipment, ammonium sulfate crystals were mixed with molten urea a t 270°F and the mixture was passed through spray nozzles into the prilling tower. They reported that agronomic tests of several crops showed better results with urea-ammonium sulfate than with ammonium nitrate and also that ammonia volatilization loss was greatly reduced when equal amounts of urea were applied to the soil surface as urea-ammonium sulfate rather than as urea. This paper describes pilot-plant studies of the granulation of ammonium sulfate and urea solution by processing mixtures of the two materials in a pan granulator or by prilling in an oil medium. By these procedures it was possible to make larger size urea-ammonium sulfate products that would be more suitable for bulk blending or broadcasting than either the smaller size urea prills or ammonium sulfate crystals. Granulation in a pugmill was not considered satisfactory. None of the processes required a drying step because the use of highly concentrated urea solution (99.3-99.7%) of about the concentration commonly produced by industry and evaporation within the process obviated the need of such a step. By-product ammonium sulfate from both coking operation and from caprolactam production was used. Screen analyses of the sulfates are shown in Table I. Both byproducts contained about 21% nitrogen and 24% sulfur. The sulfate from the coking process was of small particle size (essentially all -28 mesh). Over half of the material from the caprolactam process was +16 mesh in size and most of this (90%) was - 10 mesh. Products that contained from 30 to 43% total nitrogen were made while allowing sufficient overage to permit addition of 2% of anticaking agent and, in the case of oilprilled products, up to 2% of oil. Emphasis was placed on the manufacture of products of 34% total nitrogen con270
Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 3, 1975
+35
+12
+16
+28
17.1
51.7 0.3
82.2
...
1.3
10.4
...
...
...
43.8
66.4
\ ,
LIQUID
-
445 425
-
405
-
305
-
365
-
345
-
32 05
+60
’.’
i
465 405
+48
LIQUID
+
AMMONIUM SULFATE
t 0
10
20
3 0 4Q 5 0 6 0 70 U R E A , % BY WEIGHT
00
90
100
Figure 1. Phase diagram of system urea-ammonium sulfate
tent, about the same as ammonium nitrate and of 40% total nitrogen content which was intermediate between this grade and the grade of urea. In some cases, concentrated urea solution and ammonium sulfate crystals were added separately to the pan granulator. In other pangranulator tests, and in the prilling tests, the materials were first combined and fed as a fluid mixture. A 4:l weight ratio of urea to ammonium sulfate was used in preparation of material of 40% nitrogen and 4% sulfur. Such a mixture was very fluid and could be handled with little difficulty a t relatively low temperatures of 275 to 300°F. It was not a true melt, however, and from the phase diagram (Hignett, 1971) shown in Figure 1 was estimated to contain about 9% by weight of unmelted ammonium sulfate. A 1.3:l weight ratio of urea to ammonium sulfate was used in preparation of material of 34% nitrogen and 9 to 10% sulfur. This mixture was estimated to contain about 40% of undissolved ammonium sulfate a t temperatures of 275 to 300°F. Nevertheless, the mixture was fairly fluid and could be handled without appreciable difficulty if long runs of horizontal piping were avoided. Pan-Granulation Process The inclined pan (or disk) granulator has been used satisfactorily a t TVA in the production of several high-nitrogen fertilizers, including 3 C l C O ammonium phosphate nitrate and 30-0-0-5s ammonium nitrate sulfate (Meline et al., 1968; Phillips, 1962; Young and McCamy, 1967). Late in 1973, TVA began producing granular urea in a 12-ft-diameter pan granulator for demonstration purposes. These facilities also have been designed for production of urea-ammonium sulfate that contains 40% nitrogen and 4% sulfur. Process and Equipment Description. Production of granular urea-ammonium sulfate in the pan granulator
5 /I
GRANULATOR
-
t RECYCLE FINES
i
c D
Figure 2. Flow diagram of granulation pilot plant for production of urea-ammonium sulfate.
was studied in pilot-plant facilities used for development of other TVA pan-granulation processes. A flow sheet of the process is shown in Figure 2 . Highly concentrated urea solution was sprayed onto a bed of recycled undersize granules in the pan granulator. The exothermal heat of crystallization of the urea was absorbed largely by the recycled granules. Crystalline ammonium sulfate was fed either with the recycled material or as a mixture with the urea solution. For commercial-scale operation it probably would be necessary to heat the ammonium sulfate prior to mixing with the urea solution. The urea and ammonium sulfate built up a series of “onionskin” layers on the granules until they became large enough to be discharged. As a result of the natural classifying action of the inclined pan, the larger granules discharged from the lower side of the pan and the smaller granules continued to recirculate in the pan passing under the sprays of urea solution until large enough to discharge. The temperature for satisfactory granulation in the pan was usually 200” to 230°F and was controlled by the proportion of granules recycled. At the elevated temperatures nearly all the small proportion of water in the urea solution was evaporated. The granules were cooled in a rotary cooler by a countercurrent flow of air. After being cooled, the material discharged from the pan was screened to remove product-size material. The oversize material was crushed and recycled to the granulator together with the undersize and part of the onsize. Since urea solution facilities were not available, the urea solution of 99.3 to 99.7% concentration was prepared by melting unconditioned urea prills in steam-heated tanks and adding a small quantity of water. This is about the concentration of solution from evaporators in commercial urea plants, but the biuret content was higher than usual because of the longer retention time of the melting procedure. Additional biuret did not form in the other steps of the process. The steam pressure was adjusted to maintain a temperature of 275 to 300°F. At lower temperatures too much undissolved ammonium sulfate was present to allow the mixtures to be handled satisfactorily. Operation was not carried out at higher temperatures because of the increased possibility of urea decomposition. The concentrated solution was fed through a steam-jacketed pipe by a centrifugal pump to two hollow-cone-type
RECYCLE ENTERS HERE
EDGE SCRAPER
-
PAN ROTATION
Figure’ 3. Sketch of pan operation.
spray nozzles; mixture temperature a t the nozzles was about 15°F lower than in the melter. The nozzles were positioned over the rolling bed of solids as shown in Figure 3 so a s to selectively coat the smaller granules. The pan granulator was 38 in. in diameter and had vertical sides 9 in. high. It was rotated a t a speed of 34 to 36 rpm in most of the tests and was mounted a t an angle usually 61” from the horizontal. Scrapers were provided to remove buildup from the shell. The rotary cooler was 2.5 ft in diameter and 20 ft in length. A small quantity of heat from a gas burner was applied to the cooler whenever necessary to keep the temperature of the recycled material a t about 130°F to simulate expected plant-scale conditions. A minimum of air was usually used to prevent dust from escaping. The cooler was equipped with a cyclone to recover the fine particles in the air exhausted, but dust recovered in the cyclones was not recycled to the process. In commercial operation, the dust probably would be dissolved in the urea solution feed. Return of such fines to the pan granulator results in poorer granulation. Double-deck, mechanically vibrated screens were used to separate product-size granules. The sieve sizes were 5 or 6 mesh for the top deck and 8 mesh for the bottom deck. A Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 3, 1975
271
Table 11. Pan Granulation of Urea-Ammonium Sulfate Nominal grade 40-4s
34-0-0-1
OS s1-0-0-34
Test no. 1
2 ~~
Nominal production rate, ton/hr Feed rate, lb/hr Urea prills Water Ammonium sulfate With urea solution With recycle stream Urea solution concentration, % Urea solution temperature in tanks, O F Granulator product Temperature, O F Water content, % Screen analysis (Tyler), % +6 mesh -6 +8 mesh -8 +10 mesh -10 mesh Recycle Ratio, lb/lb of product Temperature, O F Screen analysis (Tyler), % +8 mesh -8 +10 mesh -10 +16 mesh -16 mesh Cooler product Temperature, "F Moisture content, yo Cyclone fines collected, % of production Onsize productb Chemical analysis, % Total N S
3
4
~~
0.5
0.5
0.5
0.5
760 10
780 10
540 5
996 10
...
222
...
99.3
463 99.7
195 99.5
...
99.5
301
299"
202 0.2
...
...
...
1.2 36.1 40.5 22.2
11.6 49.6 32.3 6.5
19.4 32.6 32.9 15.1
25.1 43.3 26.1 5.5
1.6 122
1.7 130
0.5 106
2.6 134
18.3 47.1 21.2 13.4
35.2 47.4 14.2 3.2
7.5 47.7 27.9 16.9
15.8 51 .O 19.6 13.6
141 0.2
149 0.2
...
...
6.6
2.6
4.1
7.2
41.2 4.6 0.2
41.4 4.6 0.3
3 7.4" 9 .2" 0.1
45.1" 1.1" 0.2
223
297
79
...
238
155
301" 234
163
Moisture content Screen analysis, % 0.1 0.1 0.0 3 .O +6 mesh 17.4 25.1 15.5 27.7 -6 + 7 mesh 68.9 68 .O 50.7 43.2 -7 +8 mesh 13.6 6.8 33.8 26.1 -8 mesh Bulk density, lb/ft3 46 45 ... ... Temperature of mixture of urea and ammonium sulfate. Analyses are of unconditioned products; therefore, slightly lower analysis would be expected in commercial practice. The nitrogen content was high and the sulfur content was low because of excessive urea in product as a result of using straight urea as initial solid feed (recycle)to granulator. (I
double-shaft chain mill was used to crush oversize material. Usual production rate was about 1000lb/hr. Results of Pilot-Plant Tests. Typical data from the pilot-plant tests of the pan-granulation process are given in Table 11. Tests were made with three ratios of urea to ammonium sulfate to produce materials that would contain 34, 40, and 43% nitrogen after conditioning. In most pangranulator tests, material of 40-0-0-4s grade was produced. As stated earlier, this grade was made with a 4:l weight ratio of urea to ammonium sulfate. Crystalline ammonium sulfate from the coke-oven process was used and was introduced either with the recycle or in the tanks 272
Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 3, 1975
where the urea solution was prepared. A small accumulation of crystals in the conical bottom of the tanks indicated a need for thorough agitation during the mixing operation. No other problems in handling the mixture were observed even when the solution was pumped through about 20 ft of horizontal steam-jacketed pipe. Good granulation was obtained with both procedures when sufficient recycle was added to maintain the temperature of the granulated product in the range of 200 to 225°F. With a recycle temperature in the range of 120 to 130"F, a recycle ratio of 1.5 to 1.9 lb/lb of product was required. Somewhat less recycle was needed when the am-
monium sulfate was introduced with the recycle (1.5-1.6 as compared with 1.7-1.9). Typical data from tests of the 40% nitrogen grade are shown in tests 1 and 2 (Table 11). When the ammonium sulfate was introduced with the recycle (test l), the temperature of the granulator product was 202"F, and 77% of the granulator product was in the -6 +10 mesh size range. When ammonium sulfate was introduced with the urea solution (test 2), the temperature of the granulator product was 223"F, and 82% of the granulator product was -6 +10 mesh in size. The moisture content of both onsize products was about 0.2%. When less recycle was used and higher temperatures were obtained, granulation was not satisfactory; the urea formed crystals of very small size rather than building up in layers on the granules. Premixing the ammonium sulfate with the urea was found to be slightly superior to independent introduction of the sulfate and urea in that the amount of -10 mesh particles in the granulator was much lower (7 vs. 22%). Also, the proportion of the fine particles collected in the cyclone was less (3 vs. 7%). All granules were very strong, but those made by incorporating the ammonium sulfate in the urea solution were slightly stronger than those made by feeding the ammonium sulfate with the recycle. Product to have a 34-0-0-10s grade after conditioning was formulated with a urea to ammonium sulfate weight ratio of 1.3:l; however, in the one test made (no. 3, Table E), the nitrogen content (37%) was high and the sulfur content was low because of excessive urea in the product as a result of using straight urea as initial solid feed (recycle) to the granulator. In this test, crystalline ammonium sulfate (from caprolactam production) was fed to the pan granulator with the recycled material. Granulation was only fair; the granules were rough and not well rounded, but they were very strong. The high proportion of ammonium sulfate in the feed resulted in the use of a low recycle ratio of only 0.5 lb/lb of product. This is equivalent to the solid-to-fluid ratio of 21, which is in the range used for the other grades. The temperature of the granulator product was 238°F. About 66% of the granulator product was -6 +10 mesh in size and about 21% was +6 mesh. The proportion of undersize was greater than that recycled, and this resulted in an accumulation of undersize material. Additional tests of this formulation probably would have improved the operation with this high proportion of ammonium sulfate. Adding a portion of the ammonium sulfate to the melt and the remainder with the recycle would likely have been beneficial. When all the ammonium sulfate was added to the urea melt, the mixture did not flow satisfactorily in the long horizontal pipe that conveys the solution from the melt tanks to the pan. In tests of the oil-prilling process, described later, little difficulty was encountered in handling a mixture containing this proportion of ammonium sulfate (43%); however, the mixture was handled in a different physical setup in which the urea and ammonium sulfate were mixed directly above the prilling cup. In test 4, only about 8% ammonium sulfate was added with the urea solution. Ammonium sulfate from the caprolactam process was used. Because of dilution with straight urea used as starting recycle, the product granules had a high nitrogen content (45 vs. 43%) and a low sulfur content (1.1 vs. 1.8%) as compared with the feed mixture. With a recycle ratio of 2.6 and a recycle temperature of 130"F, the granulator product had a temperature of 234°F. About 69% of the granulator product was -6 +10 mesh in size. Evaluation of all tests indicated that the recycle ratio required for granulation was nearly proportional to the urea in the formulation. Apparently, since the ammonium
sulfate was largely undissolved, it had an effect similar to recycled solids. All the urea-ammonium sulfate granules, especially those with the higher sulfate content, had a rougher surface and were less spherical than straight urea granules produced in the pan during other tests under similar conditions. The presence of the ammonium sulfate appeared to cause more rapid crystallization so that less rolling and rounding occurred before the melt solidified on the surface of the granules. The same effect had been noted earlier when potassium chloride or ammonium sulfate was fed in the granulation of ammonium nitrate in the pan.
Oil-Prilling Process Oil prilling was another method studied for granulation of mixtures of urea and ammonium sulfate. This method consists essentially in quenching droplets of melts in oil and removing the oil. Some European companies use this method to prill calcium nitrate (Vanderberg and Hallie, 1960). Recently, Hatakeyama (1971) reported on pilotplant production involving oil prilling of 28-28-0, 18-18-18, and other grades of urea-based fertilizer. TVA has carried out pilot-plant studies on. the production of urea and of urea-ammonium polyphosphates (Tennessee Valley Authority, 1968) by prilling in oil. Process and Equipment Description. A flow sheet of the oil-prilling pilot plant is shown in Figure 4. As practiced in the pilot plant, the process consisted in feeding urea and water as necessary to simulate about a 99.5% urea solution into a melter, together with ammonium sulfate to produce a hot mixture (280°F). By-product ammonium sulfate from both coking operation and from caprolactam production was used. This mixture flowed to a rotating prilling cup. Droplets of the melts were thrown from the cup and quenched in light-weight oils (viscosity of 4-7 CP a t 100°F). The oil-prill mixture passed to a trommel where most of the oil was removed and then to a centrifuge for removal of most of the remaining oil. Centrifuged prills were screened and off-size material (-12-mesh undersize and +6-mesh oversize) usually was recycled to the melter. Operation was carried out in a continuous manner at production rates of about 90 to 200 lb/hr. As mentioned earlier, the urea melter was necessary in the pilot plant because hot concentrated urea solution was not available. In a large-scale operation, the feed to the prilling cup might be prepared by mixing urea solution from a conventional concentrator with recycle material and heated ammonium sulfate. In most tests, the prilling cup used was 3 in. in diameter. It contained 24 holes of 2-mm diameter and was rotated at 300 to 400 rpm. The prilling vessel was composed of a short cylindrical section (40-in. diameter) with a cone bottom of 45" sides. The oil depth in the center of the prilling vessel was 28 in., and the retention time of the prills in the vessel about 0.5 min. Oil temperature was kept a t 80 to 100°F by circulation through external coolers using water to remove the heat. The trommel used for initial deoiling was 2 ft long by 10 in. in diameter and was made of 30-mesh screen; it was operated at about 50 rpm. Centrifuging of the prills was carried out in a vertical screening-type centrifuge operated a t a maxim Am centrifugal force of 600 to 800g. From the centrifuge the product was screened on a double-deck reciprocating-type screen. Oil from the trommel and the centrifuging step was collected in a reservoir beneath the trommel and, after being cooled, was recycled to the prilling vessel. Results of Pilot-Plant Tests. Data from the production of prills with nominal grades of 30-0-0-13S, 34-0-0-98, Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 3, 1975
273
AMMONlUM SULFATE
1 OFFSIZE FROM SCREENS
HEAT FLUID MIXTURE OF AMMONIUM SULFATE AND UREA
-
PR!LLiNG CUP
OFFSIZE TO MELTER PRODUCT (-6 I 2 MESH)
+
Figure 4. Flow diagram of oil-prillingpilot plant for production of urea-ammonium sulfate.
Table 111. Oil Prilling of Urea-Ammonium Sulfate Nominal grade 30-0-0-13S
34-0-0-9S4WS
_____
Test no. 5
6
7
120
200
90
47
114 0.5 86 43
... 24 ...
______-
Nominal production rate, lb/hr Feed rates, lb/hr Urea prills Water Ammonium sulfate Recycle Urea solution concentration, 9; Temperature of urea-ammonium sulfate mixture, "F Screen analysis of prilled product (Tyler), % 4-6 mesh -6 +10 mesh -10 +12 mesh -12 mesh Unconditioned prilled product
... 79 ... 99.8
99.3
68
99.8
280
280
300
7
