EVAPORATION w. L. BADGER, ANN ARBOR,MICH. R. A. LlNDSAY, THE
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DOW CHEMICAL CO., MIDLAND, MICH.
The past year has not presented any unique development in the theory of evaporation or the design of evaporators. The design of units for the concentration of food products and their operation are discussed frequently in the current literature. If we can judge by the amount d work reported, the problem of scale, particularly in the sugar industry, continues to be a problem although the use of ion exchange shows promise. Considerable work has been done recently on the problem of evaporation from droplets and the theory of spray evaporation and partial pressure evaporation has been explored. The need for easily obtained figures for rapid calculation has been recognized by articles presenting a quick method for the evaluation of coeffkients and an approximation of maintenance costs.
is stated for organic liquids and the corrections for various evaporator types and pressure are outlined. The problem of maintenance costs is discussed by Leonard (31). Evaporator costs are described as being moderate when compared to other pieces of chemical equipment and consist primarily of tube replacement, repair of condensers, jets, tail pipes, transfer pumps, and salt removal apparatus. I n noncorrosive spvice, it is estimated a t $500 to $600 per year for each effect containing 1000 square feet of area. It is difficult to estimate the costs in corrosive service, but they will be in the range of $200 to $1500 per year on the same basis. Costs for glass-lined pan evaporators for pharmaceutical and fine chemical service are equivalent to about $800 per 1000-gallon capacity per year and are made up of a relining charge every 2 years a t one half to one third the original cost. The rate of evaporation of liquids from droplets is discussed thoroughly in recent literature. Tverskaya (67) has developed the following formula to express the rate:
A
VERY complete study of the theory of the solar evaporation of brines is presented by Bloch et al. (8). It is shown that with shallow layers of brine 35% of the total solar energy, principally in the visible part of the spectrum, passes through the brine unabsorbed. The proper use of dye was shown to increase by40% theproduction of a plant operating on Dead Sea brine. If sufficient dye is used, the evaporation rate is independent of the brine depth. The amount of 2-naphthol green required to accomplish this was 3.5 grains per cubic foot of brine. The authors present a formula for the solar evaporation of water from brines developed from the formula on water. The problem of calculating steam film coefficients in horizontal tube evaporators gets a boost from Peds and Reddie (40). The authors discuss a modification of the Nusselt equation to allow for the acceleration effect. The resulting semiempirical relation is shown as a result of work on six organic liquids and literature data from 16 investigators. Yoder and Dodge (61) present data on the boiling film coefficients for Freon 12. Calculated and experimental work is shown for a temperature range of -60" to 100' F. in a vertical tube 8 feet long by 0.954 inch in internal diameter. A curve is shown for the evaluation of the boiling film coefficient when the amount of vapor in the evaporator is known and the heat flux is constant. Byran and Quaint (6) show heat transfer Coefficient data for boiling of Freon 12 and Freon 11. The data indicate no correlations between the coefficients and the operating variables such as pressure drop; however, the data for Freon 11 and Freon 12 can be correlated and a method is suggested as a comparison between different refrigerants. Data are presented that indicate that at a given ratio of vapor to liquid in the evaporat'or feed the heat transfer coefficient is directly proportional to a dimensionless number. A discussion of methods for increasing the capacity of vertical tube evaporators is presented by Martynov (33). The need for rapid methods of estimating answers to questions regarding evaporator problems has been recognized by many authors. Myers and Jarzombek ( 3 4 ) have prepared a nomograph for the rapid determination of the boiling point rise of aqueous solutions of inorganic salts when the concentration and total pressure is known. Twenty-two common inorganic solutions are pinpointed on the graph and a method is explained to expand the data on any other solution. If the concentration, boiling point rise, and total pressure data are known for two points, it is possible to use the nomograph to expand the data to the entire concentration range at any pressure. Davies (16)discusses the rapid approximation of heat transfer coefficients including those encountered in evaporator practice. A chart is shown for vertical tube evaporators presenting the over-all coefficient times the liquor viscosity, as a function of the over-all temperature difference. A general approximation
dmldt where
m = mass of the droplet r = radius of droplet
coefficient of diffusion molecular weight gas constant temperature, absolute media factor vapor pressure at surface of droplet = vapor pressure in media
A = = = = = Ek =
M R T f
-
t
- ( 4 r A M / R T ) ( E k- e)rf
e
The relationship between the radius and saturated vapor pressure of droplets is presented by Tsuji (66). A complete discussion by. Luchak and Langstroth (38) covers the application of diffusion theory to evaporation rates from droplets and flat surfaces. The radius change with evaporation and its effect on the rate were investigated and applied to the rate equation. Droplets 1 to 2 mm. in diameter were investigated by Langstroth et al. (89). Ranz and Marshall (46, 46) cover the evaporation rate from- droplets containing dissolved or suspended solids. SCALE
A general discussion of scale in sugar evaporators during the 1949-50 and 1950-51 seasons is presented by Bottger (4). It is reported that careful control of secondary saturation prolongs the period between boil outs and the benefits of oil exchange are covered in this regard. The C.E.P.I. electromagnetic equipment is mentioned for scale control. Burdick and Allen (7) have patented a method for the removal of scale-forming materials from citrus juice prior to ev.aporation. Lime is used to precipitate pectin, acids, and other compounds which may form scale. Three years' .experience with organic scale inhibitors in the treatment of evaporator feed in sugar factories is reported by Gaddie (84). A mixture of seaweed extract containing 12% .of sodium alginate and an equal weight of sodium tripolyphosphate defi-
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INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
56
nitely decreases scale. The use of tetraphosphoglucosate doe3 not decrease the number of boil outs but the scale is nearly all deposited in effect numbers 3, 4, and 5 . Honig ( 2 7 ) presents a summary of the current practices in the control of scaling of evaporators in raw sugar mills. Tests on the use of tetraphosphoglucosate as a descaling compound are described by Pichardo and Romero (42 ). Good results are reported if the juice is well clarified prior to addition and if the addition is made in two stages. Rawlings ( 4 7 ) covers the control of evaporator scale by sodium alginate, which acts as a protective colloid and inhibits precipitation of scale-forming solids. The use of ion exchange on the thin juice in a sugar plant is reported by Werner (60) as giving excellent results in preventing scale. Its application in a particular campaign showed a 13y0 fuel saving as compared to a previous campaign where phosphate was used. The analysis of scale from a q u a d r u p 1e-ef f ec t sugar evaporator in Natal is reported by Dymond (19). H o n i g ($8)p r e s e n t s a method for the calculation of the heat transfer coefficient through scales formed on evaporator walls in the sugar industry. FOOD INDUSTRY
Vol. 45, No. 1
juices and heavy puree (30% solids) coefficients &B high as 500 to 600 B.t.u. per square foot per O F. per hour are obtained in the evaporator with little fouling of the tube surfaces. The unit produces full-flavored concentrates and requires less floor area and fewer materials of construction than standard units. Heat transfer data on a large number of runs on apple juice are reported and it is claimed that the unit pasteurizes, sterilizes, and accomplishes enzyme inactivation, deaeration, deodorization, essence stripping, and aoncentration. An experimental falling-film evaporator for the preparation of juices and puree concentrates is described by Walker and Patterson (69). The unit is all-glass apparatus designed to study behavior of the material and the effects of concentration on flavor and quality. The data should show the threshold temperature for the preservation of the flavor and the proper rate of evaporation. The unit can be operated from 0.23 to 0.67 pound per square inch corresponding to saturated vapor temperatures of 57" to 175' F. No data are given for particular products, but the apparatus is completely described. SUGAR INDUSTRY
h thermodynamic conScott ( 5 1 ) presents a reCOURTESY D I A M O N D A L K A L I CO., HOUSTON, TEX. sideration of the concentraview of evaporating and tion of sugar solutions is predrying operations in the Figure 1. Dowtherrn-Heated Caustic Evaporator sented by Bortolini ( 3 ) . dairy industry. The deThe equations for the calvelopments in 1951 in the culation of energy losses due to the irreversibility of the evapcevaporation of fruit juices, salt, and sugar are reported by Coulration of binary systems containing a nonvolatile compound son (IO). TheoreticaI advances, special units, and low temperaare derived and applied to practical examples. The irreversiture evaporators are covered with 20 references. A unique bility of heat transfer in the evaporation of water with steam at graphite evaporator for acid is described under an illustration 120" C causes a loss of 27.4 kcal. per kg. of steam. The miniin this article. Coefficients of 500 to 600 B.t.u. per square foot mum irreversibility in the concentration of sugar sirup from 60" per O F. per hour are reported for unit. Coulson (9) further to 87' Brix at 190 mm. of mercury (64.6' C.) is 20.5 kcal. and the reviews developments with particular regard to the food industry, actual loss is 55.7 kcal. per kg. of steam. The isobaric evaporacovering 28 references. Evaporators in the frozen food industry tion by heat transfer and the adiabatic evaporation by expansion are discussed by Praschan ( 4 2 ) . Historically, these units were are discussed. Various data are inspected by the Xuchasenko multipass falling-film recompression evaporators with five passes formula by Alaguzzi-Valeri (1). The time required for crystalin the later designs and as many as 14 in the eaqlier models. lization in various types of sugar boiling equipment is deterThe design went through several variations to the present-day mined. Neuville (36) has developed an empirical formula heat pump units, sometimes using a medium other than steam as expressing the vaporization changes in sugar heaters with multiplethe heat transfer agent. Some present-day units operate with effect units: liquid temperatures as low as 55" F. Cross (2 1-14) has patented E KS(P-p) a low temperature unit for food uses, including milk and fruit juice, where with ammonia and Freon, covered as refrigerants, in the condenE = vaporization per effect sers for the water vapor effluent. S = surface in effect The evaporation of heat-sensitive materials such as foods K = constant using high frequency dielectric heating is described by Nyrop ( P - p ) = pressure drop (38). Calculations for the proper field density to prevent overThe control of pan boiling in the sugar industry is discussed heating are shown, The water vapor is collected from the units by Rabe (44). The use of the Cuitmeter conductivity control, by use of an absorbent. Brown et ak. (6) describe a pilot plant with recorder and equipped to compensate for fluctuating voltage, evaporator for heatrsensitive materials with a preheater, evaporaimproves the quality and increases the product yield. Faulty tor, and cooler that has a total holdup time of 1 second. The boiling resulting in excessive conglomeration and false grain, feed is heated to 300" F., concentrated, and cooled to 80" F. is eliminated. Sofronyuk ( 6 8 ) improves the color of beet A steam injection heater is used to superheat the liquid; it is sugar by the addition of aodium dithionate. The addition is then flash vaporized into a steam jacketed tube. With fruit
January 1953
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INDUSTRIAL AND ENGINEERING CHEMISTRY
made by injection into the sugar pans at intervals before seeding and after formation of crystals. An apparatus and process for the concentration and crystallization of sugar by evaporation are disclosed by Saitzew (49). Speyerer (63) discusses the calculation of the heat transfer area required in sugar evaporators and juice heaters. Steam consumption determinations are also shown. Steam consumption is further discussed by Tegze (M), who states that the principal causes of variation in steam consumption in vacuum pans are the changes in heat transfer coefficient due to changes in concentration during boiling and the discontinuous intake of sirup. Graphs from factory tests are given in which the amounts of condensate produced, the temperature difference between steam and juice, and the electrical conductivity .of the sirup are plotted against time. The electrical conductivity apparatus used in Czechoslovakia is recommended for the control of boiling, together with a more uniform intake of sirup. Corrosion in sugar evaporators is noted by Degrez (16)as being most severe a t the bottom of the tubes close to the lower tube sheet. Results are given on test work with various tube materials, indicating varying succem. The corrosion was correlated with the concentration of carbon dioxide and ammonia in the condensate; it increases with higher carbon dioxide concentrations and decreases with higher ammonia concentrations. MISCELLANEOUS
'I
A laboratory evaporator with a capacity of 20 liters; designed for operation at low temperatures for the concentration of biological extracts and water solutions where frothing is a problem, is described by Vere-Jones (68). The liquid is preheated and fed to the bottom of a jacketed silver tube which is operated like a recirculating liquid long-tube vertical evaporator. Another unit for biologicals is presented by Dunoyer (18). Various factors such as vacuum operation, evaporator size, temperature of operation, and amount of surface, and their effect on heatsensitive materials are discussed. Regestad (48) has reviewed the literature information on the evaporation of black liquor in the paper industry from 1930 through 1950. The review deals with the physical properties of black liquor (specific gravity us. per cent solids, boiling point rise, viscosity, specific heat, and heating value). Also covered are: chemical analyses of black liquor, the corrosion problem, and the relation between scale and corrosion. The causes and occurrence of corrosion in black liquor evaporators are discussed along with the composition and behavior of various corrosion-resistant materials. Scale is covered in relation to its composition (sodium sulfate, calcium carbonate, and sodium aluminum silicates), its effect on the heat transfer coefficient, and ita relation to the dry solids content. Odors from different sources are discussed, as are air oxidation and its effect on corrosion, sulfur retention, and odor reduction. The various evaporator systems such as desk and cascade types, the Inka system, Kestner-type multiple effects m d their variations, falling-film, and heat pumps are discussed. Also reviewed is the Bergstrom-Trobeck system for firing dry solids. General heat balance calculations are made and general operational problem are presented from 64 references. Recent developments in the evaporation and combustion of waste liquor are presented by Edling (20). The heat balance in a strong sulfite mill which evaporates and burns its waste liquor is discussed. Edling shows that the economic use of waste liquor is so complex that each mill has to work out its problem separately. The replacement of calcium oxide with magnesium oxide in the sulfite process is described by Callahan (8). The plant operated by Weyerhaeuser using a quintuple-effect stainless steel evaporator with submerged forced circulation heaters operating at 4.6 pounds of evaporation per pound of steam is described. Flourney (21) discusses corrosion-resistant equipment. Stainless steel types 317, 316, 304, and 347 are indicated as satisfac-
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tory for steep water evaporators. Cast iron evaporators and copper tubes have not been satisfactory. The best life was shown by welded 316 tubes, linings, and tube sheets. The use and installation of evaporators in caustic service and the economics of caustic evaporation are described by Niewiadomski (36). Dow ( 1 7 ) discloses a pretreatment of the feed to a high temperature caustic concentrator. The complete evaporation of electrolytic caustic is difficult because the chlorates normally present are extremely corrosive to nickel. A small amount of sucrose is added to destroy the chlorate and oxygen. The product contains 1%water and is chlorate-free. The mathematical theory of the falling-film evaporator is discussed by Frenkel(2.2) and its application to boiler house practice is explained. Guastoni and Go. (26) discloses an apparatus for the preparation of soft water from sea water. The preparation of solid calcium chloride by partial pressure evaporation a t 120' to 200' C. from a solution containing 60 to 72% is r e vealed by Hedley (26). Furumi ( 2 3 )has calculated the compression power requirements for the indirect thermocompression evaporator in which a heating medium is compressed, condensed, and vaporized by expansion in a circulating circuit. Numerical examples are given to compare the use of trichloroethylene (and other chlorinated hydrocarbons) as a heating medium on the one hand, with direct thermocompression on the other hand. This same type of unit is the subject of a patent issued to Lauro and Pellegatti (30). Tube cleaning and the equipment required are the subject of an article by Nikolaev (37). Partridge (39)describes a rotary disk film-type laboratory evaporator to handle liquids that foam on vacuum boiling. An Italian patent has been issued to Teatini (64) covering the design of an evaporator for crystallizing solutions. Samuelson (60) discloses an apparatus for theevaporation of solutions containing materials whose solubility decreases with increasing temperature-for example, sulfite waste liquor. Pujol(4.9) describes a new process which has been named Partialdruck Verdampfung (P.D.V.). At approximately atmospheric pressure an aqueous solution is heated to 70" C. in a heat exchanger and then pumped to the evaporator where it is brought in contact with hot air. The exit air and vapor is heat-exchanged with the incoming air and it is suggested that any residual heat may be used elsewhere. A two-stage countercurrent unit is described. The advantages claimed for this system over vacuum evaporation are less wall thickness in the evaporator, less heat transfer area, no scale or heating areas, no vacuum, and no cooling water. Where air cannot be used, an inert gas in closed cycle with condensers is suggested. LITERATURE CITED
(1)Alaguzzi-Valeri, G.M.,Industria succar. itul., 43,261-76 (1950). (2) Bloch, R.,Farkas, L., and Spiegler, K. S., IND.ENQ. CHEM., 43, 1544-53 (1951). (3)Bortolini, P., Ricerche sez. sper. zuccheri, Univ. studi. Padova, 1st. chim. id.,5, 125-41 (1951). (4) BBttger, S., Zucker, 4,212-18 (1951). (5) Brown, A. H., Lazar, M. E., Waaserman, T., Smith, G. S., and Cole, M.W., IND.ENG.CHEM.,43, 2949-54 (1951). (6) Bryan, W. L., and Quaint, G. W., Refrig. Eng., 59, 67-72 (1951). (7) Burdick, E.M.,and Allen, J. S., U. S. Patent 2,563,705(Aug. 7, 1951). (8) Callahan, J. R.,Chem. Engr., 56, No.2, 137-9 (1949). (9) Coulson, J. M.,Food M a n u f . , 31,438-42 (1951). (10) Coulson, J. M.,Intern. Chem. Eng. & Process I d s . , 33, 127-30 (1952). (11) Cross, J. A.,U. S. Patent 2,570,210(Oct. 9,1951). (12)Ibid.,2,670,211, (13)Ibid., 2,570,212. (14)Ibid., 2,570,213. (15) Davies, G.F., Chem. Eng., 58, No.8,122-4 (1951). (16) Degrez, R., Bull. centre belge dtude et document. eaux (Libe), NO.11, 47-51 (1951). (17) Dow Chemical Co., Brit. Patent 665,007(Jan. 16, 1952). (18) Dunoyer, L.,Compt. rend., 232, 1080-2 (1951). (19) Dymond, G.C., S. A f r i c a n Sugar J.,35,301 (1951). (20) Edling, G.,Das Papier, 4, 438-45 (1950).
58 (21) (22) (23) (24)
INDUSTRIAL A N D ENGINEERING CHEMISTRY Flourney, R. W., Corrosion, 7,129-33 (1951). Frenkel, AM.S., Eng. and Boiler House Rev.,66, 198-205 (1951). Furumi, T., Chem. Eng. ( J a w n ) , 15, 173-7 (1951). Gaddie, R. S., Proc. A m . SOC. Sugar Beet Technol. 6, 532-7
(1950). (25) Guastoni and Co., Ital. Patent 461,101 (Jan. 15, 1951). (26) Hedley, A. G. M., U.S. Patent 2,556,184 (June 12, 1951). (27) Honig, P., Proc. Intern. SOC. Sugai-Cane Technol., 7, 529-49 (1950). (28) Honig, P., Proc. K o n i n k l . Ned A k a d . Wetenschap., 54B, No. 2 110-19 (1951). (29) Langstroth, G . O., Diehl, C. H. H., and Winhold, E. J., Can. J . Research, 28A, 580-95 (1950). (30) Lauro, G., and Pellegatti, O., Ital. Patent 456,242 (March 29, 1950). (31) Leonard, J. D., Chem. Eng., 58, No. 9, 149-53 (1951). (32) Luchak, G., and Langstroth, G. O., Can. J . Research, 28A, 574-9 (1950). (33) Martynov, M. I., Sakharnaya Prom., 24, No. 11, 28-32 (1950). (34) Myers, J. E., and Jarzombek, R., Chem. Ew., 58, No. 11, 156-7 (1951). (35) Neuville, P.. I d s . ayr. et aliment ( P a r i s ) , 67, 233-5 (1950). (36) Niewiadomsbi, S., and Wieczffifiski, K., Prremysl Chem., 29, NO. 6, 333-7 (1950). (37) Nikolaev, E. P., Sakharnaya Prom., 24, No. 11, 34-5 (1950). (38) Nyrop, J. E., U. S. Patent 2,585,825 (Feb. 12, 1952). (39) Partridge, S. M., J . S c i . Instr., 28, 28-9 (1951). ENG.CEEM.,43, 2926-31 (40) Peds, R. E., and Reddie, W. A., IND. (1951).
me$
Vol. 45, No. 1
Pichardo, G. M., and Romero, J. J . L., Sugar Abstr., 11, 178 (1949).
Praschan, V. C., Chem. Eny. Progr., 47, 325-30 (1951). Pujol, M. P., I o n , 11, 202-5 11951). Rabe, A . E., S. African Sugar J . , 35,461 (1951). Rani, W. E., and Marshall, W, R., Jr., Chern. Eng. Progr., 48, 141-6 (1952). Ibid., pp. 173-80. Rawlings, F. N., Proc. Am. SOC.Sugar Beet Technol., 6, 528-31 (1950). Regestad, S. O., Svensk Paperstidn., 54, 35-51 (1951). Saitzew, J., Brit. Patent 585,683 (Feb. 12, 1947). Samuelson, H. O., Swed. Patent 131,798 (May 29, 1951). Scott, D., Australian J . Dairy Tecknol., 5, 53-94 (1950). Sofronyuk, L. P., Sakharnaya Prom., 25, No. 8 , 26-8 (1951). Speyerer, H., Zucker, 4, 293-306 (1951). Teatini, D., Ital. Patent 457,279 ( M a y 12, 1950). Tegze, M.,Cukoripar, 3, 282-6 (1950). Tsuji, M., Geophys. Mag., 22, 11-20 (1950). Tverskaya, N. P., Izvest. A k a d . N a u k S.S.S.R., Ser. Geonraf. i Geojiz., 15, 74-81 (1951). Vere-Jones, N.W., -Vew Zealand J . Sci. Teciinol., 31B, S o . 3, 1 4 (1949). Walker, L. H., and Patterson, D. C., IND. ENQ.CHEW,43, 5346 (1951). Werner, E., Zucker, 4,467-70, 454-7 (1951). Yoder, R. J., and Dodge, B. F., Refrig. Eng., 60,156-9 (1952).
SOLVENT EXTRACTION
ROBERT E. TREYBAL
NEW YORK UNIVERSITY, NEW YORK 53, N. Y . The year's recorded progress in liquid extraction is considerable, with the work on the fluid dynamics of spray and packed towers, and on formation and behavior of liquid drops especially worthy of note. No innovations in equipment design were introduced, but minor variations were proposed for all major types. In the application to specific processes, sulfur removal and the separation of aromatic hydrocarbons from light distillates were given much attention in the petroleum industry, and many new solvents and techniques were proposed for the separation of oxygenated compounds from hydrocarbons in connection with the Fischer-Tropsch and related processes. In leaching relatively little fundamental work was reported, but there were many improvements proposed in techniques and equipment for the processing of oil seeds, sugar beets, and metal-bearing ores.
T
HE volume of the literature in both liquid and solvent
extraction has continued a t the high level of the past several years. This review is therefore not an exhaustive treatment of the year's developments, but is necessarily limited t o a consideration Qf the major contributions and to representative samples taken from among the remainder. LIQUID EXTRACTION
Several reviews of broad coverage have appeared. In addition to the annual review of this series (a%), Von Berg and Wiegandt (180)have discussed briefly all phases of the subject, and Weisz (286)has reviewed the special problems of fractional countercurrent extraction. Maloney has summarized in chart form the principal considerations in choosing liquid extraction as a separation method (f78). EQUILIBRIA
Systems whose ternary liquid equilibria have been fairly thoroughly investigated, a t least to the extent of determination of several complete tie line compositions, are listed in Table I. Those systems for which only the distribution of a solute between immiscible phases has been determined are listed in Table 11. Table 111 lists the binary liquid systems fw which mutual solu-
bility data have been measured. In addition, thelimits of miscibilitywithin the ternary system castor oil-ethyl alcohol-water have been established (168), and the system succinonitrilewater-ethyl alcohol was used to illustrate the effect of changing temperature on the appearance of new liquid phases (188). An extensive compilation of ternary systems which separate into two liquid layers has also been made (90). Bono and Brusset (26) have developed graphical methods of treating thermodynamic data to obtain binary nonelectrolytic liquid equilibria. The numerical calculation of tie lines and initial directions of binodal curves in 3- or &phase systems, including those containing solid components, are outlined by Meijering (186). Brancker (SO) has illustrated his method of computing quaternary equilibria from ternary data using the system acetone-water-vinyl acetate-acetaldehyde. Othmer and Thukar (606)have demonstrated that the effect of temperature on distribution coefficients can be shown by a linear pIot of the coefficients against the vapor pressure of water a t the corresponding temperature on logarithmic paper. METHODS OF CALCULATION
Klinkenberg (145) has presented simple derivations for the relation between number of stages, extraction factor, and the ratio of amounts of extracted solute in extract and raffinate, with and without reflux, for countercurrent, stagewise extraction with constant distribution coefficient. The results are also applied to the separation of two components by a double-solvent system (fractional extraction) (146). Mathematical treatment of the data resulting from a Craig-
