Document QykaYzyVMMOB0ZnzZQ8E5bZE

"FINDING THE PROPERTIES OF HYDROGEN MIXTURES" Copyright 1956 Gulf Publishing Co., Houston, Texas Reproduced from "Petroleum Refiner" by permission of the publishers EB 61-11 7-7-61 Here are data you need for solving those fluid flow and heat transfer prob lems. This short-cut method helps determine physical properties. How to Cope Hydrogen With Finding the Properties of Hydrogen THIS SPECIAL report on hydrogen presents a wealth of useful data and information. While it does not give the answers to all problems associated with hydrogen it does help in process design, me chanical design and maintenance. The effect of hydrogen on the physical properties of gas mixtures is out of proportion to its size and weight. The determination of physical properties of these mixtures is essential in process calcula tions. The influence of hydrogen is especially great in fluid flow and heat transfer problems. This special report presents theoretical and practical equations for solving these everyday problems. The collection of graphical data represents hours of literature searching. On* major problem associated with hydrogen attack is the recognization of the particular type. Only after suitable classification can the seriousness of the attack be determined and preventative main tenance planned. The theory of hydrogen attack and the classification of the various types are pre sented in this special report. Hydrogen embrittlement is a very insidious phenomenon. Without any previous warning it may exhibit itself with a dramatic equipment failure. This report describes some empirical observations useful in determining hydrogen embrittlement. All hydrogan problems cannot be solved by the use of high-powered alloys. This report tells the uses and limitations of the presently available alloys and steels. This knowledge is money. Re ported here are the characteristics imparted to a steel by the use of various alloying elements. Proper use of these materials can solve the problems of hydrogen blistering, decarburization and cracking. Mixtures Viscosity Thermal Conductivity Specific Heat Density Frank L. Rubin Downingfon Iron Works, Downington, Penn. HEAT TRANSFER and fluid flow calculations for mixtures containing hydrogen can be made in normal fash ion provided one can evaluate properly the viscosity, ther mal conductivity, specific heat, and density of the mixture. This article presents methods for making such calculations. Process calculations involving mixtures containing hy drogen. present problems to engineers. Hydrogen has the lowest molecular weight of any substance. Thus the mix ture containing a very low weight percent of hydrogen contains a significant volume percentage of hydrogen. The physical properties of hydrogen differ considerably from those of the hydrocarbons and more common gases. Specific heat is approximately 3.5 as compared to a more conventional range of 0.25 to 1.0. Thermal conductivity is approximately 4 to 10 times greater than that of the more common process fluids. Viscosity of hydrogen is somewhat lower than that of the other common fluids. Petroleum Refiner--Vol. 35 No. 3 How to Cope With Hydrogen Specific H fo | 8 tu /lp The Prandtl number is dimensionless and is equal to cz (2.42) /k. The Prandtl number for hydrogen and sev eral other common gases2 at various temperatures are shown in Figure 13. The Prandtl number for hydrogen is of comparable magnitude to that of the other gases even though specific heat and thermal conductivity are rela tively high. These factors tend to balance out in the Prandtl number relationship. Process calculations involving mixtures of hydrogen with liquids involve fluid flow for two-phase systems. The method of Chenoweth and Martin18 is recommended for these problems. Both viscosity and thermal conductivity are transport properties. These are dependent upon forces between molecules. The energy of interaction between polar mole cules is quite different from that of non-polar molecules. Hydrogen has a non-polar molecule. Recommended methods of determining the proper March, 1956--Petroleum Refiner ties of hydrogen mixtures for heat transfer and fluid flow are described here. Viscosity and thermal conductivity calculations by different methods are shown. The simplest solutions, which appear to give results suitable for engi neering calculations are those of Bromley and Wilke4 for viscosity aajl the author's presentation for thermal con ductivity. The density of hydrocarbon vapors is determined from the ideal gas law with an appropriate correction factor, n = PV/RT. The correction factor is plotted as a func tion of the reduced temperature (T/Tc) and reduced pressure (P/Pc).7 For hydrocarbon mixtures the pseudocritical temperature and pressure should be used. The average boiling point method17 is recommended to calcu late the pseudo-critical properties. When gases (H2,H20, H2S, etc.) are present in a mix ture of hydrocarbon vapors, the reduced pressure of the 141 Properties of Hydrogen Mixtures ... hydrocarbon portion should be based upon the effective pressure of P V X(,c for the hydrocarbon portion. The cor rection factor, n, for the other gases should be also deter mined at an effective pressure equal to the total pressure multiplied by the square root of its mole fraction. The molal volume is then calculated by Amagat's Law.T RT V = p (xhc nb, + x.n. + ... + x.n.) Usually the correction factors for gases are equal to 1.0. The specific heat of hydrogen as a function of pres sure and temperature has been presented in a convenient. graphical form1 (Figure 1). The effect of pressure upon the specific heat is slight. The temperature effect is unusual. At low pressures the specific heat increases at a decreasing rate and then in creases at an increasing rate. At high pressure, the specific heat decreases at a decreasing rate and then increases at an increasing rate. The point of inflection occurs near 500 F. The specific heat of hydrogen is approximately thirteen times that of the other common gases as shown in Figure 2.2 The specific heat of a mixture of gases at low pressure is equal to the sum of the products of the individual weight fractions times the respective specific heats. Cm = X.C. -f- XbCb + . . . X.C. (1) The specific heat of a mixture of gases at high pressure is cal culated from enthalpy data. The heat load in the system under consideration should be deter mined -by multiplying the individ ual weight fractions times the respective enthalpy changes. The specific heat of the mixture is equal to the enthalpy change di vided by the temperature change. H = x.(h, -- h.) -f Xb(h, -- h.) + ... + x.(h1 --h.) (2) c-- tl -- U (3) 0.5 0.6 0.8 1.0 1.5 2.0 2.5 46 Reduced Temperature, Tr*T/Tc FIGURE 4--Generalized pressure correction to viscosity. The viscosity of hydrogen as a function of temperature is shown in Figure 3. The effect of pressure was expected to be small.1 The recently published data of Hilsenrath & Touloukian2 shows the ef fect of pressure on viscosity at 100 atmospheres to be about 1 percent or less for temperatures of 272 F and higher. The viscosity of hy drogen (nitrogen, oxygen, carbon dioxide, and steam) as a function of temperature (--238 to 2192 F) and of pressure (to 100 atmos pheres) are given by these au thors.2 The effect of pressure and temperature upon viscosity has been developed by Uyehara and Watson.3 A more convenient plot4 of their data is given in 8 10 Figure 4. The ratio of viscosity at pressure, P, to the viscosity at one atmosphere, Zp/zatm, as a func tion of reduced temperature for 142 Petroleum Refiner--Vol. 35 No. 3 How to Cope With Hydrogen TABU 1 Viscosities of Hydrogen and Nitrogen Under Pressure,. Pressure Atmospheres 1......................... 50......................... 100......................... 150......................... 200......................... PSI 14.7 735 1470 2205 2940 VISCOSITY IN CENT1POISES Temperature 122 F Temperature 212 F Hydrogen ! Nitrogen Hydrogen Nitrogen .00938 .00952 .00963 .00973 .00983 i .01896 > .01968 .02072 . .02202 t .02350 .01030 .01040 .01049 .C1060 .01068 .02109' .02169 .02250 .02351 .02464 From Hiroji Jwaaaki* various Lines of constant reduced pressure is shown. EXAMPLE--Calculate the viscosity of hydrogen at minus 234 F and 1470 psia. Given viscosity at this temperature and at one atmosphere is 0.0481 centipoise. Critical temperature is minus 400 F. Critical pressure is 175 psia. -234 + 460 V T,= . --400 + 460 . 226 60 = 3.77 1470 175 -= 8.4 From Figure 4 the viscosity ratioj is 1.15 zP -- 1.15 X 0.0481 = 0:0553 centipoise The viscosity shown by Hilsenrath and Touloukian1 is 0.0534 centipoise. i The viscosity of hydrogen and nitrogen mixtures' at moderate temperatures (122 and 212 F) and at pressures up to 200 atmospheres are presented5 in Figures 5 and 6. The data for the pure gases (Table 1) by the same author are in close agreement with Hilsenrath.2 Iwasaki5 presents a method for calculating the viscosity of these mixtures. The viscosities6 of binary mixtures of hydrogen with ethane, carbon monoxide, oxygen, nitrous oxide, methane, propane and nitrogen as determined by experiment and calculation are given in Table 2. It is,of particular inter- \ Mol. F \ 1 68 EX................ CG................ 212 EX.............. CG................ 392 EX................ CG................. 482 EX.............. CG................ \%Hj F. \ 81 EX*.............. CG*................ CD*................ 260 EX................ CG................ CD................ 440 EX................ CG................. CD................ 530 EX................ CG................. CD................ \ MoL F. 81...................... 260........................ 440........................ 530........................ TABLE 2] Viscosities of Binary Mixtures Viscosity In Centlpolses VISCOSITY Hs-CsHe 0.0 '45.00 85.15 100.0 .00909 .00909 .01142 .01139 .00987 .00986 .01208 .01213 .00993 .01C05 1 .01189 .01197 .C0876 .01033 .01409 .014C1 .01467 .01469 .01412 .01411 .01213 .01526 .01522 .03583 .01588 .01511 .01513 .01265 VISCOSITY Ha-COa 0.0 19.93 41.29, 78.50 .01493 .01493 .01493 .01501 .01507 .01509 .01506 .01508 .01372 .01944 .01920 .0)920 .02353 .02301 .02301 .01945 .01926 .01928 .02358 .02302 .02303 .01933 .01913 .01918 i '.02321 .02275 .02280 .01713 .01698 .01990 .01998 .02556 .02479 .02479 .02542 .02478 .02479 .02506 .02450 .02137 VISCOSITY Hi-Oa 0.0 18.35 39.451 60.30 .02057 .02064 .02064 .02019 .02021 .02024 .01925 .01934 .01939 .01784 .01774 .01782 .02568 .02567 .02567 .02507 .02506 .02509 .02381 .02388 .02394 .02192 .02178 .02186 .03017 .03015 .03015 .02950 .02940 .02943 .02790 .02795 .02802- .02556 .02541 .02551 .03220 .03224 .03224 .03147 .03140 .03146 .02978 '.02986 .02993 .02733 .02714 .02721 . 100.0 .00889 .01081 .01065 .01228 .01228 .01308 78.08 .01494 .01524 .01531 .01858 .01857 .01865 .02158 102158 .02168 .02288 02302 .02313 . 100.0 .00889 .00889 .00889 .01087 .01065 .01065 .01259 .0)228 .01228 .01381 .01308 .01308 V MoL VISCOSITY Ht-NaO P. \ 1 0.0 39.89 59.61 ,78.57 100.0 81 EX.............. CG............... .01488 .01489 .01481 .01451 .01509 . .01483 .01348 .01376 .00891 .00889 260 EX........ CG.............. .01943 .01936 ..01907 ' .01849 .01932. .01876 .01684 .01710 .01081 .01065 440 EX......... CO............... .02355 .02338 .02292 .02311 .C2206 .02229 .01990 .02009 .01256 .01228 530 EX.......... \. CG............... \ MoL .02555 .02525 .02477 .02489 .02376 .02392 .02137 .02150 VISCOSITY II2-CH4 .01341 .01308 E.'\ 0.0 28.08 48.55 60.22 92.23 100.0 68 EX................ . .01087 CG................ .01092 CD................ .01092 .01099 .01102 01105 .01098 .01096 .01101 .01086 .01083 .01088 .00955 .00952 .00955 .00876 .00876 .00876 212 EX................ CG................ CD................ .01331 .01327 .01327 .01337 .01329 .01333 .01328 .01313 .01318 .01306 .01291 .01297 . .01132 .011)5 .01119 .01033 - .01021 .01021 392 EX.;............ CG................ CD................ .01603 .01588 01588 .01602 .01583 , .01585 ..01587 .01557 .01561 .01551 .01525 .01531 .01338 .01300 .01303' .01213 .01185 .01185 482 EX................ CG................ CD................ V MoL F. \ .01725 .01709 .01709 0.0 ' .01718 .01701' .01704 "01699 .01670 .01675 .01662 . .01634 .01641 . VISCOSITY-Ha-CaHa .01423 . .01389 .01393 .. .37.04 78.82 92.25. 100.0 .01296 .01265 .01265 L 81 EX................ .00817 CG.............. .00874 . .00985 .00892 .00995 .00970 * .00891 .00980 .00889 CG..-____ . .. .01233 .01241. .01194 .01194 .01061 .01065 440 EX................ CG................ 530 EX................ CG.......... \ MoL "F-X .01422 *.01418 0.0 .01366 .01459 .01462 .01392 .013S9 .01478- .01566 . .01499 * .01566 .01485 .01482 VISCOSITY Hs-Ns 25.00, 50.00 75.00 .01256 , .01228 .01347' .01308 100.0 --312 EX................ .00544 : .00540 .00524 - .00493 CG.............. .00564 ' .00564 ' .00552 .00508. .00362 .00356 66 EX.'............. .01746 CG................ 1 .01744 .01700 .01609 .01699 . .01605 .01396 .01393 .00882 .00874 . From Hirschfelder, Bird and Spots0. - EX-^Experimental data; CG--Calculated, using a geometric mean E; CD--Cal culated using an E calculated from diffusion. , u _. est to note that the viscosity .of many of these,mixtures is greater than the viscosity ;of- either of the components! Hirschfelder, Bird, and Spotz6 calculated the viscosities (indicated with "CG" arid "CD") by using force con stants. The author used the Bromley arid Wilke4 method to calculate the viscosities. The. experimental viscosities as reported6 were used for the pure components. The latter method is much less tedious and gives results of a com parable accuracy. ' The Viscosity of -a Mixture of gases , can be deter mined by the following equation by Maxwell :T' XiZi Vmi + Xjz, Vmj... + x. z. Vm.. ,Zm=-------- ---------------------------------:------- (4) Xi Vn>i + Xj Vmj + ... + x. Vm, In a series of papers4-8-9 Wilke and others developed a method to determine the viscosity <of a gas mixture. These equations are: --Zi Zm -- + 1+HXip*12 1+-T-021 X* <(5) )'.(:::) 7 r 0u : (6) 021 = 4V2=Lr 14--JmS,i-Jl14 (7) Generalized plots of Function <f> for- gas mixtures are March, 1956--Petroeeum Refiner 143 Properties of Hydrogen Mixtures ... ically symmetrical potential functions. The most useful potential function is the Lennard-Jones (6-12) potential. The transport phenomena of molecules with spherically symmetric force fields are well understood. For polar gases and for molecules, which lack spherical symmetry, more work must be done. l | The coefficient of viscosity of a pure gas is given by _ 266.93 VmK D' W 10* (8) The Force Constants for the Lennard-Jones potential are given in Table I-A10 for various gases. In this article we refer to the collision diameter (in angstroms) as D and the potential parameter (in degrees Kelvin) as E. The appropriate integral, designated as W for viscosity and thermal conductivity calculations is given in Table I-M.10 It is necessary to determine the Reduced Temper ature, T*, to utilize Table I-M.10 T* = K/E (9) FIGURE 5--Viscosity of hydrogen-nitrogen mixtures at 122 F. shown in Figures 7 and 8. EXAMPLE--Calculate the viscosity of a 59.61 percent hy drogen-nitrous oxide mixture at 81 F. Viscosity (centipoises) at 81 F. Molecular weight Hydrogen 0.00891 2.016 SOLUTION--by the Maxwell equation (4) Nitrous Oxide 0.01488 44.02 _ (0.5961) (0,00891) V2.016+(0.4039) (0.01488) V4iW 0.5961 VT0I6 + 0.4039 V44.02 = 0.01345 centipoise. Experimental result' is 0.1451 centipoise. Calculated result is 7.3 percent lower. SOLUTION--by the Bromley and Wilke method mi mi 2.016 44.02 0.0459 Z. .00891 Zi .01481 = 0.600 From Figure 4 = 2.5 m* 44.02 mi 2.016 -=21.8 Zi .01481 Zi ~~ .00891 -= 1.67 From Figure 8 <hi = 0.19 By equation (5) .00891 .01488 ____ z. = , .4039 + , .5961 = .01494 centipoise. 1 +^96T(2.5) 1 +^039'(0.19) Calculated result is 3.0 percent higher than experimental result Hirschfelder, Curtiss and Bird10 present the develop ment of a rigorous kinetic theory for monatomic gases. The final result of this development is the expression of the various transport coefficients in terms of a set of integrals. These depend upon the force law which is assumed for the molecular interaction. The integrals are evaluated and the calculations are made of the transport coefficients-- including viscosity and thermal conductivity--for spher 144 FIGURE 6--Viscosity of hydrogen-nitrogen mixtures at 212 F. Equation 8.2-19 of the text10 introduces a correction factor, which varies from zero to 0.8 percent. EXAMPLE--Calculate the viscosity of methane at 350 F. m = 16.04 K = 450 K From Table I-A" the force constants for methane are E = 137 K and D = 3.882 angstroms. T* = K/E = 450/137 = 3.28 From Table I-M" W = 1.016 By Equation (8) z _ 266.93 V( 16.04) (450) = 0.0148 centipoise (3.882)' (1.016) (10)' The viscosity of a binary mixture can be determined from __266.93 V2mqnJC/(mi -f- ms) Z" 101 Du' Wu (10) where zi2 is the viscosity of a hypothetical substance of molecular weight 2m,m,/(mI + m2) and the molecules interact according to a potential curve specified by the Petroleum Refiner--Vol. 35, No. 3 How to Cope With Hydrogen .01 M2 FIGURE 7--Generalized plot of function <t> for gas mixture viscosities. FIGURE 8--Generalized plot of function <t> for gos mixture viscosities. interaction parameters Du and W12. EXAMPLE--Calculate the viscosity of a mixture of 23.37 mole percent nitrogen and 76.63 mole percent CO at 226.9 C. SOLUTION: at 226.9 C, K = 500.1 Kelvin. March, 1956--Petroleum Refiner Gas N. CO Ni-CO (Angstroms) (Kelvin) Di = 3.681* Ei =.91.5* Source MoLWt. Table I-A* 28.02 Ds = 3)706 Ei = 88.0 Table I-A" 28.01 Do -- 3.694 En - 89.7 See below Do = 0.5 (3.681 + 3.706) = 3.694 angstroms Eo = V (9.15) (88.0) = 89.7 K T* = K/E . W*(from Table I-M") Ti* = 500.1/91.5 = 5.446 T* = 500.1/88.0 = 5.683 To* = 500.1/89.-7 = 5.557 . 0.9127 0.9060 0.9093 From Table I-N1* A,,* = 1.102 * Data used in "Molecular Theory of Gases and Liquids''1* page 569, al though these values are recommended tor use below 300 Kelvin. Bqnation (8) Z, ==_266.93 V.(28.02) (SOOJ), = 0.02555 centipoise (3.681)'(0.9127) 10* , ^ 2, =^M3V.(28.0_L)_(500111 = q 02539 centipoisc (3.706)' (0.9060) lO' ^ Equation (10) z _ 266.93 V~(28.02) (28.01) (500.1)/(28.02 + 28.01) " (3.694)' (0.9093) 10' = .02546 centipoisc The viscosity of the binary mixture is given by Equa tion (11). . ." _j_____-x*+-Y'-x r-i+.(XL/x.).-| (z-,,), - .l+z. L'-'l + Z. J 145 Properties of Hydrogen Mixtures . . . TASLf 3 Thermo! Conductivity Thermal Conductivity In BTU/hr. sq. ft. F*/ft PRESSURE--Poasds/Sqvere Inch Absolute F 14.22 1422 2844 4266 S68S 7110 Hydrates....................... 69 .1014 .1042 .1080 .1103 .1111 1116 212 .1222 .1236 .1262 .1274 .1283 .1288 392 .1458 .1467 .1486 .1496 .1503 .1504 672 .1693 .1700 .1718 .1726 .1729 .1731 Nitrated......................... 69 .0145 .0163 .0211 .0251 .0273 .0308 212 .0178 .0184 .0219 .0254 .C272 .0306 392 .0213 .0217 .0238 .0263 .0278 .0307 572 .0249 .0250 .0265 .0286 .0298 .0323 Air................................... 68 .0149 .0161 .0220 .0262 .0292 212 jam .0178 .0217 .0249 .0271 356 .0209 .0211 .0236 .0443 .0281 vne......................... 58 .0194 .0261 .0379 .0437 .0473 .0487 212 .0262 .G284 .0358 .0404 .0432 .0442 392 .0344 .0350 .0405 .0423 .0445 .0451 CerboB Dioxide............ 127 J)1U .0323 .0485 185 .0126 .0374 .0460 212 .0135 .0196 .0353 .0444 302 .0161 .0178 .0283 .0370 392 .0185 .0191 .0259 .0335 Pram StotyuoT, IpoUev tod Teodorovich11. x,= 2x,x, (*l)l + (z,,), + x,1 (z,). (11A) (iSirf+rafe)! < 146 Solving Equation (11), using data from the proceeding example, gives the concentration dependence of the vis cosity of N2 -- CO mixtures at 226.9 C ________ 16613x,* + 33214x,x,+ 16611x, z-'*-- (10*) (6.5023* + 13.0455x,x, + 6.5423x,') For X, ^ 0.2337 x, = 0.7763 Zmii - - .02542 centipoise Experimental data give 0.02550 centipoise. Thermal conductivity of hydrogen as a function of temperature has been presented in a convenient form1 and is reproduced in Figure 9. The effect of pressure was ex pected to be small and recently published11 Russian data, extending to some 485 atmospheres, confirm this as shown in Table 3. There is a 10 percent increase in conductivity at 59 F and a 2.2 percent increase at 572 F. The thermal conductivity of nitrogen, air, methane, and carbon di oxide under pressure were also investigated and the re sults are presented. Pressure greatly effects the thermal conductivity of these gases. The effect of pressure and temperature upon thermal conductivity has been developed by Lenoir, Junk, and Comings:""The ratio of thermal conductivity at pressure, P, to the thermal conductivity at one atmosphere, kp/k,tln, as a function of reduced pressure for various lines of constant reduced temperature is shown in Figure 11. The Russian data extend considerably beyond the range of this figure, (eg. The reduced pressure of hydrogen at the highest tabulated value is 40, while the curve extends to a reduced pressure of 7.) The thermal conductivities of gaseous mixtures con- Petroleum Refiner--Vol. 35 No. 3 How to Cope With Hydrogen 0 I .2 3 .4 . 5 6 .7 . 8 9 1.0 Mole Fraction Of Light Constituent FIGURE 11--Dimensionless factor, a. for the thermal conductivity of a gas mixture containing hydrogen. Reduced Pressure Recent work indicates that this Figure predicts volues approximately 40 percent too high For ethane and 60 percent too high for propane in some regions. The behavior of the more complex organic compounds is more complicated than is shown by this figure. FIGURE 10--Generalized pressure correction to thermal conductivity. taining hydrogen are presented13 in Table 4. Lindsay and Bromley14 developed the following rela tionship for the thermal conductivity of mixtures. k, 1 -f An' : 0.25 j I + zi /miV 1 ( 1 + T ) zi \mi/ ( 1 + 't"")- The same expression gives A=1 when the subscripts are interchanged. S = 1.5Tb S = 142 Kelvin for hydrogen, deuterium, helium S,, = VSjSi for nonpolar mixtures Sn = 0.733 VSi.51 for polar mixtures with steam or ammonia Brokaw13 developed a simpler relationship for the ther mal conductivity of a nonpolar gas mixture. km --: a ksu -f- (1 -- a) kau ksK = X, k, + x, k. _1_________ Xi_ . X; knM k, + k, (14) (15) (16) The dimensionless factor, a, is presented in Figure 11. The author has developed an extremely simple equation The dimensionless factor, f, is presented in Figure 12. This empirical relationship represents all the data in Table 4 with an accuracy of 3.3 percent and a maximum deviation of 8.6 percent. Hirschfelder, Curtiss, and Bird10 present the following methods for determining the thermal conductivity of a TABU 4 Thermal Conductivities of Binary Mixtures, Containing Hydrogen Gas Pair ffa-COa............................ Literature Refereace 14 Temperature F 32 Ha-Nj.................................... IS 32 Ha-NaO................................ 14 32 Ha-CO................................... 14 32 Ha-CaHs.............................. IS 1 Hj-COa.................................. 14 77 77 Ha-COa.................................. 14 32 Mole Fractioa Hydrogea 0 0.142 0.355 0.50 0.75 0.901 1.00 0.0 0.159 0.390 0.652 0.803 1.00 0.0 0.209 0.386 0.599 0.812 1.00 0.0 0.163 0.272 0.566 0.634 0.794 1.00 0.0 0.1698 0.3140 0.5137 0.6110 0.6649 1.00 0.0 0.047 0.193 0.496 0.9059 0.9638 1.00 0.0 0.1701 0.369S 0.6068 0.8346 1.00 Thermal Caaductirity Btu/Hr. Sq. Ft. *P./Fl 0.00871 .01380 .02420 .03265 .0549 .0762 .0978 0.01331 .01936 .03073 .0469 .06215 .6978 0.0092 .01718 .02590 .0411 .0658 .0978 0.01283 .01936 .02492 .0436 .0506 .0653 .0978 0.01277 .02062 .02778 .0409 .04985 .0796 .1058 0.00987 .01072 .01833 .0366 .0847 .0973 .1058 0.00821 .0147 .02503 .0417 .0677 .1058 Mnrrh (05A--PrT!'fl! M P.Fnxr!; 147 Properties of Hydrogen Mixtures ... FIGURE 12--Dimensionless factor, f. Temperature- *F 0 1000 2000 gas and of a gaseous mixture. 0.0482 VK/m T 4 C 3 "I D'W L 15 ` R + 5 J (18) [--j4y --C -f- 3 -1j is a factor for the transfer of energy between translational and internal degrees of freedom. C, the heat capacity per mole, is equal to 3R/2 for monatomic gases. Thus, the factor reduces to 1.0 for monatomic gases (He, Ne, A, Kr, Xe.) A higher approximation introduces correction factors to increase the accuracy of the calculation. These factors vary from plus 1.0001 to 1.0125 and are ignored in this presentation. Refer to equation 8.2---32 of the text.10 Hirschfelder10 presents the ratios of calculated to ex perimental conductivities as shown in the Tables 8.4 -- 10!C and 8.4--11.10 Also tabulated are the factors for the transfer of energy. 148 EXAMPLE--Calculate the thermal conductivity of hydrogen at 32 F. Use Equation (18). K = 273.16 Kelvin m = 2.016 From Table I-A" D -- 2.968 angstroms E = 33.3 Kelvin r 4C From Table 8.4-10 and 8.4-11" 3 J"1=' 1255 T* = K/E = 273.16/33.3 = 8.203 From Table I-M" W = 0.8506 k --0-0*82 V273.16/2.061 (, 355) -- 0.0939 Btu/hrs. sq. ft. F/ft. (2.968)' (0.8506) The thermal conductivity of a mixture is determined from equation _0.0482 VK(mi + mi)/2mimi D' W11 where ki2 is the thermal conductivity of a hypothetical substance of molecular weight 2m1m2/ (m, 4- m2) and the molecules interact according to the potential curve specified by the interaction parameters D22 and W12. (Refer to example for viscosity of a mixture for methods of determining D12 and W12.) In terms of this quantity and the thermal conductivities of the pure components, the. thermal conductivity of a mixture of monatomic gases may be written as 1 Xt + Y (Kml.)l _ 1+Z* (20) There is at present no rigorous method for calculating the thermal conductivity of mixtures of polyatomic gases. However, an empirical method is suggested by Hirsch felder, et al.10 First, calculate for the mixture the quantity given by the above Equation (19). The result is denoted by kmoo. If the experimental values of k for the pure gases are known, then for each of the components com pute the ratio .. . k> expt r K-)-r-ao-a-- An empirical expression for the mixture is . . ki.ii = km., (xii-i -(- x>ri) (21) (22) x,=- x (k,)i ^ 2x,xi (k) 1 + Xi (M. (20A) Yk = Xi* (ki)i UU> 4- 2xixi (k)i U<'> +- (ki)i -yen .trim I .. TTfO I (20B) (20C) u<`> Hr An* -IT (-T B"* +1 )lt+T (mi -- mi)* minis (20D) u`!> ~11F A,'*"T2~(t'B,'* + `)mT+ 1 (mi -- mi)' 2 miitii (20E) XT(,,_^A [~ (m- + TM*H~1 (M.1 _ u 15A" L 4mmn J (k,), (k,)i J_____fii . \(m, --m,)' 12 V 5 B" + l)~ 32 A*\ 5 Bn 5/ man, (20F) [((mi + mi) w (k,,)i 4m,m. A (ki). (k.*)-']- (k, (20G) EXAMPLE--Calculate the thermal conductivity at 77 F. of a 61.1 percent hydrogen-ethylene mixture. Properties-- xi= 0.611 x, = 0.389 z, = 0.00953 z, = 0.0098 Petroleum Refiner--Vol. 35 No. 3 How to Cope With Hydroger, m, = 2.016 mi = 28.05 Tbi = 37 Rankin S, = 1.5 X 37 = 55.5 Tbi = 305 Rankin S,= 1.5 X 305 = 457 k, = 0.1058 k, = .01277 Solution by Lindsay and Bromley method. By Equation (13) Au = 0.25 ( + ,00953 .0098 / 28.05 V V 2.016 ) (i+wy (+ VI 55.5) 537 (457)> ) ('+- 55.5 537 Au = 2.72 An = 0.25 < 1 + .0098 .00953 I IS ( 1 +"457 ' / 2.016 A 537 , V 28.05 ) (+#). (> + V (457) (55.5) 537 (+- 457 537 An = 0.388 By Equation (12) 0.1058 km = 0.01277 = .0465 Solution by the Brokaw Method. By Equation (15) ksn = (0.611) (0.1058) + (.389) (0.01277) : .0695 By Equation (16) 1 0.611 0.389 kau 0.1058 + 0.01277 ~ 36 28 ku. = .0276 From Figure 11, "a" = 0.50 By Equation (14) k,, = 0.50 X 0695 + (1 -- 0.50) .0276 = .0496 By the author's method--Equation (17) From Figure 12 f = 1.092 0.611 X 2.016 **"(0.611 X 2.016) + (0.389 X 28.05)" 0'101 x,= 1.0 --0.101 =0.899 k. = 1.092 0.611+0.101 ) (0.1058) + [( J' 0.389 + -899 ) (.01277) = 0.0500 Btu/hr. - sq. ft. - F/ft. Thermal conductivity of mixture. Table 4 0.04985 Equation (12) 0.0465 Equation (14) 0.0496 Equation (17) 0.0500 NOMENCLATURE A -- dimensionless constant (Lindsay & Bromley equation) A* -- the quantity A* for calculating the transport coeffi cients of mixtures for the Leonard-Jones (6-12) potential a -- dimensionless factor (Brokaw equation) B* -- the quantity B* for calculating the transport coeffi cients of mixtures for the Lennard-Jones (6-12) potential C -- specific heat at constant volume, calories per gram mole per degree Kelvin c -- specific heat at constant pressure--Btu/lb. F. D -- collision diameter in angstroms E -- potential parameter in degrees Kelvin f -- dimensionless factor (author's equation) H -- enthalpy change--Btu h -- enthalpy--Btu/lb. K -- temperature--degrees Kelvin March. 1956--Petroleum Refiner k -- thermal conductivity--Btu/hr. $q. fc F./ft. m -- molecular weight P -- pressure--Ib./sq. in. absolute R -- gas constant, calories/degrees Kelvin-gram mole or lb./sq. in.-cu. ft./mole per degree Rankin S -- Sutherland constant, degrees Rankin T -- temperature--degrees Rankin T* -- reduced temperature--dimensionless (T*-- K/E) t -- temperature--degrees Fahrenheit V -- moial volume--cu. ft./mole W -- integral for calculating viscosity and thermal conduc tivity (as well as other properties) for the LennardJones (6-12) potential. W = fl*.** in "Molecular Theory of Gases and Liquids."* X -- weight fraction x -- mole fraction z -- viscosity--centipoises 4> -- function of molecular weights and viscosities Subscripts: a,b,n -- components a,b,n atm -- one atmosphere (14.7 lb./sq. in.) B -- boiling point at one atmosphere c -- critical point he -- hydrocarbon m -- mixture p -- absolute pressure, lb./sq. in. r -- reduced (temperature or pressure) RM -- reciprocal mixing SM -- simple mixing 1,2 -- temperatures 1 and 2, or components 1 and 2 12 -- interaction of components 1 and 2 LITERATURE CITED v Granet, Irving, Petroleum Refiner, Vo!. 33, May, 1954, 205. * Hilsenrath, J., and Y. S. Touloukian. Trans. ASME, Vol. 76, August. 1954, 967. * * Uyehara, O. A., and K. M. Watson, Natl. Petroleum New*, Vol. 36, R764 (1944). ^Bromley, L. A., and C. R. Wilke, Ind. & Eng. Chem., Vol. 43, 1641 * Iwasaki, Hiroii, Science Reports oC the Research Institutes, Tohoku University (Sci. Rep. Ritu, A-Vol. 6, No. 3), Sendai, Japan, June. 1954. Page 296*307. * Hirschfelder, J. O., R. B. Bird and E. L. Spotz, Chem. Rev., 44, 205, (1949). T Maxwell, J. B., Data Book on Hydrocarbons, Van Nostrand Co. (1950). * Buddenberg, J. W., and C. R. Wilke, Ind. & Eng. Chem., Vol. 41. 1345 (1949). Wilke, C. R., J. Chem. Phys., Vol. 18, 517 (1950). 10 Hirschfelder, J. O., C. F. Curtiss and R. B. Bird, Molecular Theory of Gases and Liquids, John Wiley and Sona (-1954). u Stolyanov, E. A., V. V. Ipatlev and V. P. Teodorovich, Zhur. Fit. Khim., Vol. 24 166*76 (1950). UuuMr, J1953 ' *ni^ W. Comings, Chem. Eng. Progress, u Brokaw, R. S., Ind. & Eng. Chem., Vol. 47, 2398 (1955). 14 Lindsay, A. L., and L. A. Bromley, Ind. 8c Eng. Chem., Vol. 42, 1508 18 Ibbs, T. L., and A. A. Hirst, Proc. Roy. Soc. (London), A 123, 134 (1929). ^^Kornfeld, G., and K. Hilferding, Z. Physick. Chem. Bodenstein-Festband, IT^mith, R. L., and K. M. Watson, Ind. & Eng. Chem., Vol. 29, 1408 (1937). u Chenoweth, J. M., and M. W. Martin, Petroleum Refiner, Volume 34 151 (1955). ** Getman, F. H., and F. Daniels, "Outlines of Physical Chemistry." Seventh Edition, P. 77, Wiley (1943). Meet the Author FRANK L. RUBIN has a background of some 15 years with the chemical, petrochemical, petroleum and power industries. Most of that time has been spent in the design of heat-transfer equipment. With Catalytic Construction Company T>gfore joining Dowington he served three years as senior job engineer with special emphasis on heattransfer problems. His work was on AEC projects, chemi cal plants and refineries. He also had been with The Lummus Company, in its Heat Exchanger division, and with Stauffer Chemical Company, assigned to Research division. He is a registered professional engineer in Penn sylvania and New York. He holds a B.A. degree from Brooklyn College, a chemical engineering degree from Cooper Union School of Engineering, and has completed graduate studies at The Polytechnic Institute of Brooklyn. 149 Rvlletin RR-5 Paper No. 53-PET-25 FACTORS INFLUENCING THE PERFORMANCE OF INTERNAL INSULATING LININGS IN PRESSURE EQUIPMENT by J. J. MURPHY Development Engineer The M. W. Kellogg Company New York, N. Y. C. M. Y0GRH Section Engineer The M. W. Kellogg Company New York, N. Y. Advance Copy Released for publication upon presentation Contributed by the Petroleum Division for presentation at the Eighth Annual American Society of Mechanical Engineers, Petroleum Division Conference, Houston, Texas, September 28-30, 1953. I. INTRODUCTION HUGE: 1 The successful use of Internal Insulation linings in pressure ves sels and pressure piping in recent years has played an important part in the practical application of new processes involving increased combinations of pressure, temperature and size. Present indications are for an increasing and more exacting need for such linings. It is the purpose of this paper to outline factors which should be considered toward assuring satisfactory per formance of particular linings. These are presented from the point of view of the designer and user as associated with the experience of the authors' company with the primary objective of promoting discussion and interchange of backround. In conmon with the design of the pressure equipment itself, lin ings can be effectively designed provided complete knowledge of process requirements not only for normal service but also for start-up, emergency and any other conditions which may exist are considered. Similarly other factors which may affect heat flow and consequently metal temperatures must be evaluated and provided for. Unusual conditions require careful consid eration - e.g. sudden depressuring which may dislodge or loosen the insula tion or rapid cycles of pressure or temperature which may accelerate its deterioration. The subject matter has been organized for presentation tinder the following four basic subjects: Conductivity of Insulation - which treats the heat transfer aspects of the insulation materials along with effects of environment. Deterioration in Service - which covers the factors contri buting to structural changes, mechanical or chem ical effects which result in loss of strength or actual disintegration of the insulation. Performance in Service - which presents factors, (other than those which affect the properties of the insula tion material,) which may influence the operating effectiveness of internal insulation. Design Considerations - which reviews the engineering problems attendant to the design assumptions, selection of materials and design details. II. CONDUCTIVITY OF INSULATION Data on basic conductivity are ordinarily obtained from the manu facturer or literature and represents performance under controlled labora tory conditions at atmospheric pressure and in the presence of air or flue gas. These values may be significantly increased in some services by fac tors which affect the density of the insulation or the composition and density of the gas in the pores or by the deposition of solid matter with in the pores so that laboratory values may not be representative of average installation particularly in the case of insulating concrete. In general PAGE: 2 low conductivity is achieved by the creation of many small voids, utilising the lower conductivity of air and reducing the overall heat flow so that, for each material, conductivity is approximately proporti ial to the density. Replacement of the ambient pressure air in the pores of the insulation by a denser or more conductive gas or hy solid material raises conductivity. A. Effect of Internal Pressure and Type of Gas on Conductivity The literature contains little information of this subject but tests have been made which show a very significant effect of hydrogen on lew density insulations. The authors have found the following ap proach logical and useful for design purposes: (1) k k]_ + xkg where is the evacuated Insulation conductivity (no gas in pores) at the operating temperature x is the degree of porosity of the Insulation (fractional per cent) kg is the conductivity of the gas in the pores under operating conditions k is the combined effective conductivity of the insulation and gas at operating conditions The value of kg depends on the type of gas and its density and pressure. The authors use the following relation which correlates reasonably well with such data as can be found in the literature. (2) kg = Ci(T) 3/2 T + q2 X log 10C3P (BTU/sq.ft/hr./F/in) where p is the absolute pressure, psi, T the absolute temperature, F, and Cl, C2 and C3 are constants for each particular gas. Suggested values of these constants for air and hydrogen are Cl Air .0054 c2 c3 225 38.3 Hydrogen .0342 16?-' 52.2 Due to its high conductivity compared to air hydrogen can increase the conductivity of low conductivity relatively porous insulations 100/6 or more. It is readily apparent from equation (1) that the effect of the gas composition and pressure and temperature has a lesser percentage effect where the material has a higher evacuated conductivity, less porosity or both. PAGE: 3 Although the thermal conductivity of mixtures of gases cannot be properly calculated by the ordinary arithmetic mixing rule it is suggested that it be applied when using equation (2) for mixtures of two gases as kK = Pi P2 * ---- 1---- kel ---- T---- kv2 Pi + P2 81 PI P2 g It is doubtful whether the conductivity so calculated would ser iously effect the design. Values of "k", must be obtained by solution of equation (l) using known values of k with air at atmospheric pressure in the pores, the known value of x and "kg" from equation (2). The apparent porosity, as reflected by the density of the insu lation, is not always the proper value to use for "xM since some pores may be sealed off which would make them insensitive to gas composition and pres sure, However it is & safe assumption for design investigation; in addition it is possible that due to operating pressure or after long service these cells may rupture. B. Effect of Deposition of Solids in Pores Conductivity will generally be increased by the infiltration of additional material such as coke, tar, corrosion products, catalyst dust etc. No simple evaluation of these effects can be made. Coke deposition in raising the effective conductivity of the insulation should be less where the material has a higher evacuated conductivity, less porosity or both. The exact effect may be quite variable however since the coke de posit may vary widely in its density (probably similar to thermal cracking where coke conductivities from 0.25 to 10 have been reported) and in the manner in which it is laid down. In some cases insulating brick or con crete thoroughly blackened shows little if any increase in density. In services where alternate reaction and regeneration is involved the black ened area will exist only in areas remote from the flow. The authors have not been able to detect any great effect from the coking of insulating con crete in reactor service in analyzing observed metal temperature. Obviously the effects of gas composition, pressure and solid deposition compete for the same void volume and their separate influences are not directly additive. This should be considered when establishing a design factor for all effects. III. DETERIORATION IN SERVICE Many factors may cause deterioration in service so that is is not practicable to fully discuss each at length in this paper. Certain major factors must be considered when choosing materials and design de tails. They may be listed as: 1. Suitability of materials 2. Application variables PAGE: 4 3. Mechanical deterioration 4. Structural deterioration 5. Chemical deterioration 6. Carbon deposition and similar effects 7. Short or long time failure due to overloading Materials must be selected with proper regard for their temper ature stability, inertness to the operating atmosphere, and corrodents which may be present on stream or shut-down. The importance of careful installation cannot be over-emphasized since many failures can be directly associated with faulty application. The important safety function of insulating linings and maintenance econ omics warrant the additional care and cost in preparing detailed instruc tions for the installation and for the adequate supervision and inspection necessary to have these specifications followed. Mechanical deterioration may take the form of spalling, cracking opening of joints, erosion or failure due to support details or vibration. Spalling is largely associated with thermal shock, either during initial curing or in service. Obviously rapid heating or cooling such as contact of hot surfaces by water sprays should be avoided. Cracking may be the resuli of shrinkage, thermal stress within the Insulation or differential ex pansion between the insulation and the shell or other metal parts. Concrete linings, because of the low tensile strength of concrete, may be expected to show hair line fissures which open upon cooling down. Without growth these are not significant but under cyclic operation and perhaps under the wedging action of infiltrated solids the cracks may grow and lead to spal ling. Opening up of joints, cracks or failure due to support or reinforc ing details may be due to ineffective design, metal warpage or growth or fabrication failures. Metal reinforcement of concrete, if made too stiff, may cause rupture in the plane of the reinforcement because of differential expansion. Vibration may initiate or accelerate mechanical failure. Where erosion resistance is necessary the materials and details must be selected accordingly. In fluid catalyst vessels, insulating and refractory concrete linings have shown adequate resistance. Avoidance of structural deterioration requires the selection of materials and cements which remain stable under the maximum temperature and do not suffer undue loss of strength or undergo significant change in volume. Sane insulations containing asbestos become friable and are gradually reduced to powder with prolonged heating at 850P and higher. In addition all unfired castables containing Portland or alumina cement as the bind*- are structurally weak between 1200 and 1800 F. Chemical deterioration includes corrosion, oxidation, chemical reaction, or other chemistry change as a result of exposure to service con ditions. Corrosion is possible in certain atmospheres such as flue gas where even minute amounts of sulphur, oxidized to acid may attack nonresistant materials or aggregates. This may occur during off-stream per iods. PAGE: 5 The effect of carbon deposition on conductivity has been discussed in Section II. In the case of brick or concrete containing iron oxides operating in an atmosphere containing carbon monoxide, the liter ature shows that carbon deposition may be accompanied by disruption of the concrete or brick. In this case a chemical decomposition of the carbon monoxide occurs in the presence of iron oxides as a catalyst and the re sulting carbon is deposited in such a way that it exerts a mechanical disruptive force. The favorable temperature range for this reaction is be tween 800 and 1800 F. Curiously however, no deterioration of this kind has been observed to the authors knowledge in fluid catalytic cracker re generator service, and iron-free aggregates and cements are not at present considered necessary for that service. Carbon deposition which is not the result of such a chemical reaction does not seem to affect the +ructural integrity of the refractory and may enhance it. Operating variables include overheating, abrupt pressure or tea> perature changes and cyclic operation. Overheating has been a cause of insulation deterioration and eventual failure where the material was sel ected ignoring occasional temperatures above the design level. Experience shows that if a process is exothermic or if there is any possibility of excess temperature due to operational mistakes, emergency or start-up or shut-down operations the materials should be selected with a proportionate safety margin. Manufacturers service temperature ratings are usually based on so-called hot face temperatures and not for the limiting temperature through the entire thickness; in addition the ratings are usually optimis tic and correspond to the most favorable conditions. The effect of over heating varies; with some materials limited periods can be sustained with out significant damage; others undergo a gradual breakdown while still others may suffer sudden complete disintegration. A given insulation may respond differently in an oxidizing than in a reducing atmosphere. Failure due to overstress, short time or gradual creep is usually the result of improper support design or overheating. IV. PERFORMANCE IN SERVICE In this section the authors will discuss factors other than those covered in Sections II and III which markedly affect performance. These are: 1. Vapor flow through or behind insulation 2. local turbulence or pressure drops 3. Support arrangement 4. Effect of external insulation __ Vapor flow through or behind the insulation will seriously de stroy its effectiveness. Accordingly the design and materials chosen must effectively guard against it. The possibility of such flow increases with the pressure drop per foot of the flowing medium and with porosity or lack of resistance to flow within the insulatlor PAGE: 6 Three situations can be considered: 1. Vessels or piping subject to internal flow without internals. 2. Vessels with fluid catalyst beds. 3. Vessels with fixed catalyst beds. In the authors' experience vessels with no internals operating at low velocities have given no particular trouble of this nature. Piping operating at considerably higher velocities and with higher proportionate area of insulation to total cross-section, has given trouble particularly when low density insulations are used. In some cases insulation has been swept out. Vessels containing fluid catalyst beds have occasionally shown evidence of flow through or behind the insulation due probably to the com bination of unit pressure drop and static differential head. Vessels containing fixed catalyst beds usually involve higher unit pressure drop, which is a function of velocity and catalyst particle size and shape, and vapor flow through or behind the insulation must be carefully guarded against. When trouble of this kind has been encountered it has been overcome by incorporation of better details to assure good con tact of the insulation with the shell, or by the use of denser materials or both in some cases; other cases have required the installation of a tight metal shield. Measures to prevent or control vapor flow through or behind insulation are discussed in Section V. Experience to date has not been entirely consistent and is difficult to interpret satisfactorily, but it is the authors' opinion that when the insulating material is concrete or brick with a density of 60 lbs per cubic foot or more, vapor flow between the insulation and the shell rather than flow through the pores of the in sulation itself is the factor which must be guarded against to prevent overheating. The lesser problem with fluid as compared with fixed catalyst beds is probably due to the lower superficial velocity and the fact that the greatest part of the unit pressure drop goes to support the weight of the suspended particles; the tendency to deposit material at locations of directional change, horizontal runs or de-aerated areas with consequent plugging also helps Local overheating due to vapor flow behind the insulation has occurred at areas of high local pressure drop and turbulence, in all applications, when the details did not provide adequate local shielding. Typical locations are tee connections in pipe lines, nozzle entrances in pressure vessels and around the edge of-^rids in fluid catalyst ves sels. A typical instance of vapor flow behind an insulating concrete lin ing occurred recently in a fluid catalytic cracking regenerator directly above the grid resulting in roving hot spots. The insulation on suc cessive inspections appeared in excellent condition; hcwever it was pos sible to demonstrate by means of an air hose that air flow behind the insulation was possible. It is logical to suppose that similar gaps existed elsewhere on the vessel lining yet the localized pressure drop across the grid seemed to be the initiating influence of the flow causing hot spots. PAGE: 7 The manner in which the insulation is supported greatly influences the possibility of vapor flow behind the insulation. Details which provide sectional support and which maintain the insulation in close contact with the shell generally provide the best performance. This is discussed further in Section V. Vessels which have both internal and external insulation would be expected to be more sensitive to overheating than vessels which have only internal insulation, and experience seems to bear this out. The blanket, block or plastic external insulations normally used are fairly efficient even in minimum practical thicknesses. Insulation for internal use on the other hand must be fairly rugged and is generally less efficient thermally. With such combinations the shell metal temperature runs much higher than with the bare shell and a given percentage increase in the internal conduc tivity generally results in a greater metal temperature rise. In addition with a bare shell there is generally a greater temperature margin available for overheating before the shell strength is seriously reduced and such areas can also be readily detected and provided with remedial measures. When using external insulation therefore even greater attention should be given to the design. V. DESIGN CONSIDERATIONS A. Selection of Internal Insulation Materials The insulation material must be selected with due consideration to the iactors previously discussed considering mis-operation and short time emergency conditions as well as nominal design conditions. The major factors are conductivity, including the effect of gas and pressure, cor rosion, erosion, porosity, coking, spalling and cracking tendencies, as well as possible deterioration, including CO-attack. When used in contact with catalyst its effect on the insulation and vice-versa should be checked. The answer in some cases may be a combination of materials, a low conduc tivity material against the shell and a stronger and more rugged material on the inside. Fran the remarks previously made it is clear that the con ductivity of the nominally more efficient but more porous insulations is likely to be more seriously affected by coke, hydrogen, pressure or vapor permeation. These materials are usually subject to greater deterioration due to overheating. A careful and comprehensive evaluation should there fore be made. Insulating and refractory concrete linings suitably installed have given good service in the temperature range of 900 to 1150F. in pet roleum vessel applications with short time operational upset temperatures of 1400 F and higher. In fluid catalyst vessel service the denser grades of insulating concrete appear to possess satisfactory erosion resistance except in localized high velocity zones. B. Metal Inclusions Metal inclusions due to their high conductivity can appreciably increase the overall conductivity of an insulation assembly. Support PAGE: 8 studs, vapor barrier ring etc. usually extend through part of the insu lation thickness or to the inside and over this depth affect heat flow in proportion to their area arid conductivity. Usually these parts are intimately welded to the shell arid the following relation for the com bined conductivity, while not precise, gives satisfactory results for de sign purposes: ( \ ( An ) * ' 1 (Ai f aJ + K"^A1 + Ajj), where k = conductivity and A = area; the subscripts 1 arid m denote insulation and metal respectively. Metal attachments of substantial cross-bection going through the insulation exert local effects and require special attention as noted in section K below. C. External - ve - No External Insulation This item was discussed briefly in Section IV in which the economic and|safety advantage of omitting external insulation was pointed out. It is re-emphasized that external insulation should be avoided when ever possible in the interest of safety. External insulation is sometimes necessary to|prevent a shell temperature which would result in an unaccept able corrosive condensate condition on the inside; however, toleration of slight corrosion is preferable when the condition is mild and consideration should be given to protective linings in more severe cases. Eternal insu lation should not be used merely for protection of operators from hot sur faces; expanded metal shields can be provided as necessary and retain the the advantage of full view of the external surface. D. Corrosion of Shell due to Condensate As mentioned above, if the shell metal temperature falls below the dewpoint of the internal vapor, condensate will form which may or may not be corrosive depending on the service. No trouble of this kind has been encountered to date in fluid catalytic cracking regenerator or reactor service despite the almost universal absence of external insula tion. This may be due to the low desulphurization properties of the cat alyst, minimizing oxidation,of sulphur fractions. In a few cases a 1" thick layer ojf dense gunnite concrete with wire mesh reinforcing has beer applied to the inside of the shell.as additional`protection against con densate corrosion. Infixed bed hydrofomer service condensate corrosion has been encountered in flue gas lines at-low temperature stagnant flow areas, and tne reactors, in which both reaction and regeneration takes place, have been externally insulated to guard against such corrosion. However, the rate of corrosion is undoubtedly connected with the sulphur content of the feed and the desulphurization properties of the catalyst; on some of the current reformer and fluid hydroformer vessels, external insulation is omitted. PACK: 9 . Support Details As mentioned in Section IV Bectionally supported construction has given the best performance. Typical insulating concrete installations are shown in Figures 1 and 2, Figure 2 involving the use of a hexagon pat tern steel surface-retaining grating. Sectional support is provided by flash welded studs which are on close centers, usually 9 to 12". Vapor barrier rings, when used, provide additional support. No expansion joints are necessary in the insulating concrete but the inner hex steel reinforced refractory layer of Figure 2 is generally panalled with expansion gaps be tween panels. (See following Section F). Other sectionally supported construction which has been suc cessfully used involves the use of block insulation supported on rings at regular intervals, wired to the shell and covered with a supported tile facing; however, the softer insulation is more easily eroded or possibly damaged by vibration where such effects are present. Linings consisting of an insulating or refractory brick column supported from the bottom head are prone to permit vapor by-passing, de pending on the unit pressure drop, since a gap may open up at the top of the column and also between the outside diameter of the brick column and the inside diameter of the shell. Such brick columns, however, have given very satisfactory performance as the inner facing for a sectionally sup ported block or insulating concrete lining, particularly for temperatures over 2000F. In addition to providing insulation support, the design should also maintain the insulation in intimate contact with the shell, as insu lating concrete or plastic insulations will not maintain a satisfactory bond with steel. The excellent performance of the detail of Figure 2 is due in part to accomplishing this objective; the Figure 1 detail is not as effective. F. Metal Grid at Inner Surface The use of a 3/4" deep x 14 gage hexagonal steel grating within the surface layer of refractory or insulating concrete has been very effec tive in providing low maintenance linings in fluid catalytic cracking re generator and reactor service at temperatures up to U50F. Expanded metal has also been tried but without equal results. As used by the authors1 company the hex 3teel is applied in panels 2' x 3' or 3' x 3'-4" usually with a 1/8" expansion gap between panel edges. In some cases the panel edges have been banded by welding on flat bars but the performance of un banded panels has also been good so that the^cost of banding is not justi fied. The first installation of this type has given maintenance free ser vice in a regenerator for eight years; however the steel grating now shows serious oxidation loss and may require replacement in the near future. When used in panels the grating shows a tendency to curl at the edges in service. This curling occurs at the ends of the long bars (strong direc tion of grating) but causes no difficulty provided the panel is adequately PAGE: 10 attached to |the studs located near the ends. Some installations have, been made with larger panel sizes and with reduction or elimination of expansion gaps and other installations have had the ends of.each steel panel voided or otherwise1 fastened to each other to, provide a monolithic inner facing. The authors do not have information relative, to performance of this type installation which should be judged only after several years and cycles of service. 1 The major practical benefit of the steel grating surface seems to be that it locks the surface refractory concrete in position and prevents surface cracking and spalling. It is also less subject to extensive failure due to faulty installation. Even so when placing the refractory concrete a gap of approximately l/4" should be provided behind the grating so that the refractory will flow behind .and obtain an added key effect. G. Vapor Shields arid Barriers ' When pressure drop relations become severe an effective metal vapor shield| will provide positive protection against vapor flow through or behind the insulation so long as it can be maintained tight in service. Experience indicates that when the pressure drop per foot exceeds about 0.25 psi in fixed bed catalyst service consideration should be given to such shielding. The authors ;do not wish to imply that satisfactory design cannot be developed for higher pressure drops without shields, as concrete linings without shields have performed satisfactorily at pressure drops np to about 1 psi per foot; however experience to date is mixed and inadequate. Therefore when a shield can be satisfactorily provided it seems the best choice. i Toj.be fully effective a metal shield must be sealed to the shell, at one end and run in front of the insulation for the full length of the high pressure drop zone or to the start of the next shield. The other end of the shield is left open to provide,fpr differential expansion and equalize vapor pressures across the sheild. A typical installation used for 1400 andjl800F. superheated steam piping service shown in Figure 3 This construction may also be used in vessels or a single cylindrical shield may be used as shown in Figure 4., The design of the shields oust be carefully)worked odt considering differential expansion, sudden de- pressuring which may cause collapse,.venting and drainage. Injlieu of vapor shields as described above, barrier rings seal welded to the shell are sometimes used at regular intervals, usually about 2 feet, to inhibit vapor-flow. Such rings are not very effective against flow through the insulation and will be only partially effective in re ducing flew behind the insulation should the details permit a gap to form. They should riot be considered an effective^substitute for a vapor shield. H. Local Shields ' Local vapor tight, shields should be provided-at nozzle entrance and exit points. Metal shields are also useful at points of local thermal shock e.g. near internal water sprays where there is danger of water im pingement. 1 HUGE: 11 J. Nozzle Construction Nozzle connections often involve a transition from internally insulated to uninsulated construction. The details must provide the desired temperature reduction. Figure 5 shows a typical welding type transition. In Case 2 of the Appendix, a formula is derived for estab lishing a nozzle length which will provide the desired temperature trans ition and avoid local overheating of the pressure shell. K. Through Metal for Internals Vessel details often require heavy metal connections through the insulation for internal parts. A typical example is the cyclone ple num chamber in catalytic cracking regenerators. To avoid local over heating of the shell these metal parts must be suitably insulated. In Case 1 of the Appendix a formula is derived for calculating the local temperature of the shell at such details. L. Design Margin against Overheating No guiding rule can be suggested for this item. It is never theless an important subject. The design metal temperature selected should at least provide sufficient allowance to take care of the probable increases in conductivity due to gas, pressure and included metal plus a suitable margin for deterioration, coking, etc. A minimum assumption which the authors have used is to assume the internal conductivity of insulating concrete to be at least 50$ higher than the manufacturers rating when used in flue gas or equivalent service and at least 100$ higher when used in hydrocarbon service involving high hydrogen or coking conditions. The factor used should be in step with the porosity and ruggedness of the insulation and the severity of the service. Individual factors for coking and hydrogen and pressure are not warranted as they should not be directly additive as mentioned in Section II. For equivalent safety a higher de sign temperature margin would be needed when external insulation is used than when it is omitted; in addition the externally insulated vessel should be provided with much more elaborate metal temperature instrumentation. The question as to whether there should be some minimum metal design tem perature as a percentage of the internal temperature or what alternate consideration there should be against the possibility of a partial loss of a sizable segment of the lining is an open one; at present designs are treated individually. VI. ACKNOWLEDGEMENT The authors gratefully acknowledge the assistance of their Company associates in the preparation of this paper. PAGE: 12 FIGURE 2 Welding Studs