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The production of liquid hydrocarbons by the Fischer-Tropsch synthesis Buck, F.A. Mackinnon 1944

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THE PRODUCTION OF LIQUID HYDROCARBONS BY THE FISCHER-TROPSCH SYNTHESIS by F. A. Mackinnon Buck, B. A. Sc. A Thesis submitted in partial fulfilment of the requirements for the Degree of MASTER OF APPLIED SCIENCE in the Department of CHEMICAL ENGINEERING The University of British Columbia April, 1944. TABLE OF CONTENTS Page I Introduction (A) (B) II Theory (A) (B) (C) (D) (E) Object of Research Historical 3-Principal Reactions Equilibrium Calculations (1) Discussion (2) Calculations for Methane (3) Extension to higher homologues The Catalyst Influence of Magnetic Field Other Factors (1) Composition of reactant gas (2) Size of reaction tube (3) Influence of catalyst concentration (4) Influence of flow velocity III Apparatus 15. (A) General Description (B) Reaction Chamber (0) Solenoid (D) Temperature Control (E) Condensers IV Experimental Work 19. (A) Operation (B) The catalyst Y Appendix (A) References (B) Heats of formation (C) Free energies of formation (D) Out-throw diagram THE PRODUCTION OF LIQUID HYDROCARBONS BY THE FISCHER-TROPSH SYNTHESIS I Introduction (A) Object of Research This research was undertaken i n an attempt to investigate a few of the factors influencing the hydrogenation of carbon monoxide to liquid hydrocarbons by the so called Fischer-Tropsch Synthesis. The investigations originally planned were: (1) The influence of different catalysts (2) The influence upon the rate of reaction and the products obtained of a superimposed magnetic f i e l d . It is evident, however, that before any conclusions regarding the effect of these two factors may be made, some sort of reaction standard must be adopted to act as a blank against which results may be compared. In other words i t was necessary f i r s t of a l l to design and build an apparatus so that a l l process variables except the two under consideration could be maintained constant; and secondly to make enough runs under normal operating conditions to establish some sort of a standard of comparison for reaction rate, products obtained, catalyst l i f e , thermal efficiency, and so forth. (B) Historical It has long been known as a result of the 1 pioneering work of Sabatier that finely divided nickel 1. w i l l catalyze the hydrogenation of carbon monoxide to methane, ffischer and Tropsch were among the f i r s t to show that with the proper selection of the catalyst the methane reaction of carbon monoxide and hydrogen may be modified so drastically that the principal product is a complex mixture of higher paraffin hydrocarbons originally known as "Synthen" but more recently given the appropriate name "Kogasin". Like the Berguis process, the Kogasin process provides a new method for the chemical conversion of coal to liquid products, and is looked upon as a partial solution to the diminishing resources of crude petroleum. Kogasin consists almost wholly of straight chain hydrocarbons. It i s unsuitable for motor fuel unless i t is blended with antiknock agents or re-formed by crack-ing. The higher fractions, however, are eminently suited to use as Diesel fuels and lubricants. It is not surpris-ing, therefore, that technological progress has been rapid, and that the Fischer-Tropsch synthesis i s already a well established industrial process in Germany. 3. II Theory. (A) Principal Reactions The Kogasin synthesis is , complicated by the fact that a single set of reactant gases will give a wide multiplicity of products, depending upon the control of the operating conditions. The following reactions w i l l indicate some of the more important possible syntheses, a l l of which have been more or less established industrially: (1) CO + H2 HCHO (2) CO + 2H CH OH ' 3 (3) CO + 3H0 CH + H O 2 4 2 (4) nCO -f- (2n)H2 ^ Cn H2n+1QH -r (n-l)H20 (5) nCO + (2n + l)H 0 =?=^  C H -t- nH?0 2 n 2n-h 2 * Fortunately the catalysts required and the equilibrium conditions for the formation of oxygenated compounds, such as formaldehyde and methanol, are such that these do not take place to any appreciable extent in the range of the Kogasin synthesis. The actual problems in this process are how to keep the formation of the Kogasin fraction (b.p. 50 - 200° C ) at a maximum and the formation of the volatile gases (i.e. methane, ethane, Sec. ) and the heavy waxes at a minimum. 4. (B) Equilibrium Calculations (l) Discussion Thermodynamic calculations w i l l be applied to the formation of hydro carbons by the hydrogenation of carbon monoxide. In particular , free energy equations are to be investigated for reactions of the following type: (1) CO + 3H 2 , CH4-t-H20(g) (2) 2C0 H~ 5H2 C -t- 2H20(g) (3) 3C0 •+ 7H2 ^ C?H8 ~t 3H20(g) or, in general, (4) nCO -+- (2m-l)H2 ^ C nH 2 n ^ 1 - H n^OCg) Carrying out a reaction of this type involves the following problems: (1) How fast can the reaction be made to take place; and (2) What is the maximum possible yield under given conditions of temperature and pressure? The rate of the reaction depends largely on the catalyst used, and hence must be determined experimentally. But before the efficiency of the catalyst can be tested i t i s necessary to find out under what donditions of temperature and pressure the reaction is thermodynamically possible of taking place, and what the maximum yield, would be given a favourable rate. It is also helpful to know the thermo-dynamics of other possible reactions, so that conditions may be adjusted to exclude undesirable side reactions. 3. (2) Calculation of Equilibrium Constant for Methane Reaction . CO -4- 3H0 CH H 0(g) 2 4 2 To calculate the equilibrium constant of the above reaction i t i s necessary to calculate the free energy change ( <3 ) as a function of the temperature. The method employed w i l l be the integration of the specific heat equations for the substances involved. The equations 2 used were determined from spectroscopy data by Bryant : 6.88 -+• .000066T -h .000000279T2 6.25 -h .002091T - .000000459T2 2 3.38 -+- .017905T - .O00004188T 2 6.89 -+- .003283T - .000000343T Combination of these equations in the usual manner leads to the following expression: = - 16.62 -+• .018 899 T •+ .000 004 909T2 To obtain an equation for the heat of reaction, recall (a) CO: ~ Cp s (b) H2: Cp 3 (c) C H C = 4 P (d) H20: Cp = and by integration * H - ^ r a T i - ^ ^ -r 3 to where <4M0is the constant of integration. To get the free energy equation, substitute for ^ H in the expression «/ (J = —  J^=i. ^  ^ ~ which gives = _ // A , ^ . 4 / ; 7-^7- , 7 4 ^ = £g? -^/z^~r ~ ^7 7 - - ^JS ^ 1^ = -^1/7 TVw 7"- T - 2 - T " * 7 (2) o where I is another constant of integration. The final equation now contains two constants. These are evaluated in the following manner: (1) To evaluate ^ /L\ is calculated for the reaction from the heats of formation of the products and reactants at 18°C and 1 atm. pressure. (see appendix 1 ) ^ z i > l = - 5 0 , 5 0 0 cal. per mol. This value is substituted in equation (l) for heat of reaction, giving = - 4 6 , 5 0 0 cal per mol. (2 ) To evaluate integration constant I The free energy change for the reaction i s calculated for 25°G by the summation of the standard free 7. energies of formation (see appendix 2 }: & & Z S 8 ~ ~ 33,blO cal. per mol. This value and the value of are substituted in equation (2), giving T(29Q) " - 33, €>/o - -+- A Sg (^S>S)(23) /oS(zs>e) - - 14,460 I = - 48.5 The final free energy equation then becomes: To get the equilibrium constant from the free energy change, use the, relation Z±6 - - RT In K or, In K - (4) (2.303) RT Example: To get the equilibrium constant for the reaction at 200°C (473° A): Erom equation (3) = - 23,200 cal per mol. Substituting this value in equation (4) K = 5.4 x 1 0 1 0 8. The variation of the equilibrium constant with temperature is shown by the following table, using results calculated from equations (3) and ( 4 ) . Temp, deg. c K 25 1.4 x 102^ 100 1.02 X 1 0 1 7 200 5.4 X 1 0 1 0 300 2.8 x 105 (3) Extension to higher homologues. Oalculations of the equilibrium constants of several different hydrocarbons have been made by 3 Smith in a paper published in 1927. Although the cal-culations for methane made in this thesis were from more recent data, the results obtained were in close enough agreement to Smith's results to justify accepting his values for the higher homologues. Eigure 1 shows graphically the variation with temperature of the calculated values of the equil-ibrium constant for the reaction. nCO + (2n-*-l)H — ^ C n H 2 n 2 ^ 2 ° Analysis of Figure 1 shows that i t becomes increasingly easier to form the higher paraffin hydrocarbons than the lower members at a l l temperatures. It should be Figure 1 Variation of Equilibrium Constants with Temperature > 10. possible, given suitable catalysts, to form any of the paraffin hydrocarbons at atmospheric pressure. The tendency to form these compounds f a l l s off rather rapidly with increased temperature. (C) The Catalyst The catalysts most suited for the kogasin synthesis are the metals iron, cobalt, and nickel - these metals have the common property of forming carbides. As 4 outlined by Fischer the reaction is catalysed by the formation of metal carbides of the formula Me C at the active centres of the catalyst. In the presence of hydrogen the carbide is converted again into metal while (CELj) rad-icals are formed. The reaction in the case of cobalt and nickel below 200°C is as follows: 2H0 -+- CO -t~Ni C Ni,C0 -f- H0 HO c y p <- c d hi C -h H. >• Ni C -h (CH } 3 2 2 3 2 Additions to the true catalytic metals are materials that have characteristic chemical action. The addition of copper to cobalt and iron catalysts activates the catalyst since i t lowers the reduction temperature for oxides; however i t acts adversely in the presence of nickel. Other additions, such as manganese and aluminum, which are simultaneously precipitated with the* catalytic metal, appear to play a part in obtaining certain configurations or lattice structures of the metallic alloy i t s e l f . The use of . 11. kieselgur greatly aids In the distribution of the actual catalyst by an increase in the surface area of the contact material. In another article , Fischer claims the best catalyst to be a mixture of Ni - Mn - Al prepared by precipitating the oxides on kieselgur. This was the catalyst chosen for the present investigation. Details for preparation w i l l be given later. (D) The Influence of the Magnetic Field According to Bhatnagar there are theoretical reasons why an impressed magnetic f i e l d should influence chemical reactions. In chemical reactions, even i f no change of valence i s involved, there may be a change in the magnetic moments of the i n i t i a l and final products. If during the reaction the magnetic moments tend to decrease, then the presence of an external f i e l d w i l l try to "conserve" the magnetism, and so retard the reaction. On the other hand, i f the magnetic moment increases during the course of reaction, then the presence of a f i e l d w i l l further help the increase and so accelerate the rate of reaction. Also, in a magnetic fie l d the atoms tend to assume certain definite positions relative to cue another. Thus the collisions with-in the f i e l d should take place in a more orderly and directed manner, and the rate of reaction should change. It seems logical, therefore, that a chemical transformation from diamagnetic to paramagnetic (or less 12. diamagnetic) state should be accelerated, and that from a paramagnetic to diamagnetic or from feebly diamagnetic to more strongly diamagnetic state should be retarded. This result can be expressed symbolically by the following: (1) X/w > ]E! X ^ then reaction is accelerated M* by f i e l d . (2) 2! X <C X then reaction is retarded by M p M« the f i e l d . where ^ X/v,p s the sum of the molecular susceptibilities of the final products. ZT X/v]R = the sum of the molecular susceptibilities of the i n i t i a l reactants. Equations (1) and (2) above would indicate the possibility of making predictions as to the effect of a magnetic fie l d on the water gas reaction. A complete analysis is not possible, however, because the molecular susceptibilities of some of the compounds involved ( in particular CO) have not been recorded in the literature. In a summary of Pascal's work on molecular 7 susceptibilities, Farquharson recalls that susceptibility may be treated as an additive property. Pascal's expression * . - Z * J a X * ^ is: 1 m where N a is the number of atoms in the molecule of susceptibility. X , and A is the constitutive constant a 13. depending on the chemical linkage. 8 In another article Farquharson calculates the molecular susceptibility of the -CHg- gorup in combination to be X • - 11.64 z 10"6 per -CHC-m 2 In view of previous reasoning, then, i t is to be expected that a magnetic f i e l d will favour the formation of smaller homologues. Pascal has also shown experimentally that any organic reaction involving the change from a -G=C- double bond to two single bonds w i l l cause a rise in diamagnetic -6 susceptibility of 3'5 x 10~ . This leads to the prediction that a magnetic f i e l d would cause a greater proportion of unsaturated compounds. The above values for molecular susceptibilities were a l l given for room temperature. At elevated temperatures the susceptibilities w i l l decrease, according to Curie's law (X) T m constant In view of the facts that molecular velocities are greater at higher temperatures, that the susceptibilities decrease, and that the effects of the magnetic moments are small even for fairly large fields, i t might be that the change in the reaction Is so small as to be practically unnoticeable. 1 4 . (E) Other Factors (1) Composition of Reactant Gases 9 It has been reported by Watanabe that the best reactant mixture is that which contains COrH^ in the ratio 1:2. A higher percentage of CO results in decreased yield and increasing olefin content, and a higher percentage results in increasing saturation and more gaseous products. The dilution of the raw gas with inert gases (Ng, COg, and etc.} causes the increased production of gaseous olefines and CO2, as in the case of the mere lack of when the velocity of the hydrogenation is decreased owing to the reduced partial pressure of H^. Moreover, the lower-ing of the partial pressure of CO by the introduction of inerts brings forth unfavourable conditions for polymeriz-ation, and consequently the ratio of o i l formation is grad-ually decreased as the inert concentration of the raw gas becomes larger. (2) Size of Reaction Tube A series of papers by the Institute of 10 Physical Chemistry (Tokyo) sums up the influence of the bore and length of the reaction tube as follows: With lOg. nickel catalyst a bore of 13mm. is most favourable. A 2Oram reaction tube must be longer than 30cms. With constant bore the decrease of reaction tube length promotes the formation of gaseous hydrocarbons. With decrease in tube length^ the most favourable reaction temp-erature decreases and the v o l a t i l i t y of the product increases. (3) Influence of Catalyst Concentration If more than a certain amount of catalyst is used (constant velocity being assumed) the reaction reaches a condition of equilibrium independent of the amount of catalyst used. The limiting amount depends on the quality of the catalyst and the conditions of the experiment. Polymerization, hydrogenation, heavy hydro-carbon formation and CQg formation are promoted by the use of too large a quantity of catalyst. When the most favourable amount of catalyst is used the formation of gaseous hydrocarbons i s at a minimum. (4) Influence of Current Velocity The optimum gas velocity is apparently /•/ 2/hr for 1 g. catalyst ( at a temperature of 197 C.) Wide variations in velocity do not markedly effect the most favourable reaction temperature, but marked changes occur in the composition of the kogasin, the formation of COg, heavy hydrocarbons, and etc. I l l Apparatus (A) General Description The essential parts of the apparatus are shown on the out-throw diagram included in the appendix. A photograph of the assembled equipment is included also. The reactant gas mixture i s forced from resevoir (c) by water from the constant head device (a). 1 7 . Entrapped air bubbles are bled off at trap (b). The rate of flow of the gas is regulated by a valve (d) and is indicated by the micro flowmeter (e). The gas then flows through three drying towers (f) containing CaCl , KOH, and finally P 90 c. The gas enters the glass reaction chamber (g) through a tube (h) which also contains a thermocouple well. The temperature is controlled by a surrounding resistance coil (j) regulated by an autotransformer. (i) is another thermocouple well. Two manometers (k, k) assist in regulating the pressure throughtout the chamber. A trap (l) catches the least volatile products, while the gases enter a condenser (m) at the end of which there is another trap. The uncondensed gas passes next through two more condensers cooled by "dry-ice" (n) and liquid air (o) respectively. The remaining uncondensed gases see, collected over water in resevoir (p). An adjustable siphon arrangement (q) removes the water from the resevoir and helps regulate the pressure throughout the system. (B) Reaction Chamber The size of the reaction chamber was chosen for a through-put of 4 l/hr. It is made from pyrex glass tubing, l8mm. inside diameter and 75cms . long. The tube i s at an angle of 1 5 ° , which is well below the maximum angle of repose of the catalyst. The catalyst is distributed evenly along the bottom of the tube, to a depth of approximately 8mm. Small glass thermocouple wells are sealed in at both 18. ends of the tube. These thermocouple wells l i e right on the surface of the catalyst and give an accurate measurement of the temperature of the surface of the catalyst. (C) Solenoid The magnetic f i e l d and the heat necessary for the control of the reaction are both supplied by a solenoid of No. 14 B. & S. bare copper wire wound on a brass tube 3-5 •^^r cms inside diameter. The wire is spaced one diameter apart and each layer is separated from the next by asbestos insulation. For measuring the strength of the magnetic fi e l d a small search coil was designed for use with W. G. Pye and Co. fluxmeter, No. 6860. The search coil has 200 turns of ffljG magnet wire with an average cross sectional area of 4.12 sq. cms. and a total resistance of 24.8 ohms. The fluxmeter was recalibrated and the correction factor was found to be l.jjb. (D) Temperature Control It was desired to keep the temperature of the reaction chamber within plus or minus two degrees. Two thin glass thermocouple wells were sealed in the reaction tube so that they rested on the surface of the catalyst. The thermocouples were made from copper-copel ^30 B.&-S. wire. The e.m.f. produced is measured on a Leeds and Northrup Student* potentiometer, using a L. Sc/V. table galvanometer with a sensitivity of 0.18 microvolts per mm. With this arrangement 19. the temperature could be quite accurately read to one tenth of one degree. (E) Condensers. The products of the reaction are separated and analyzed by liquefaction of the different fractions at different temperatures. The f i r s t trap (1) catches the heavy oils and some water vapour. The remaining water vapour and the kogasin fraction is condensed in the water cooled condenser (m). Condensers (n) and (o) are made from spirals of glass tubing and have a small trap at the bottom. The f i r s t condenser is packed with carbon dioxide snow and retains a l l hydrocarbons from propane and propylene up. The second condenser is immersed in liquid air (b.p. - 196°) and 'theoretically should condense a l l remaining hydrocarbons down to ethane (b. p. - 162°), but apparently a l l the ethane is not retained and some passes on to the receiver. IV Experimental Work (A) Operation At the time of writing nine separate runs had been made, without the superimposed magnetic field. For these blank runs a heater coil was made by winding 20 feet of fid nichrome wire on a l|-t t glass tube and insulating i t with asbestos paper. Carbon dioxide and hydrogen were mixed in the resevoir (capacity 20 litres},'the reaction chamber brought to 200 °C, and the gases passed through at the rate of 4 l/hr. The pressure in the reaction tube was kept below 20. 2 inches of water (the drop in pressure due to friction i n the tube was less than one tenth of an inch). If at the end of the five hour run there had been l i t t l e reaction, the original mixture was returned through the apparatus. However, no satisfactory results were obtained because, apparently, of the inactivitynof the catalyst. (B) The Catalyst The catalysts were made up according to 3 directions given by Fischer as follows: To a solution of 250 parts Ni(NO,)2.6H0O3 50 parts Mn(NO^)p.bHgO, 34 parts AUNO3)3.9H2O in 600 parts dis t i l l e d water there are addea in the cold b0 parts kieselgur. There i s next added a solution of 210 parts anhydrous K2CO3 in 600 parts distilled water and the precipitate which.forms (plus the whole solution) is brought to a boil and filtered from the solution. The precipitate is washed with 600 parts of hot water, and is dried in an air stream at 110°C. The dried moss i s ground to a powder in a mortar for reprducible results. Fischer makes no mention of reducing the catalyst. At f i r s t i t was attempted to reduce the catalyst in the reaction chamber with the H2 - CO mixture but with l i t t l e result, reduction was too slow and hydrocarbon form-ation was n i l . Three additional catalysts have so far been prepared,, reduction temperatures being chosen as 250, 300, and 450 degrees C. Other modifications were attempted also, such as washing more thoroughly, and using pre-boihd kieselgur None of these catalysts gave very .good results. A study of the reasons for the inactivity of catalysts indicates that their activity may be reduced by: 21. (1) sulfur poisoning (2) local overheating at the surface (3) reduction at too high a temperature The possiblility of sulfur poisoning being the cause of the inactivity was eliminated in two ways: f i r s t , absolutely pure CO was prepared from formic acid, and second, a sample of inactive catalyst was tested quantitatively for sulfur. There was none.detected. Local overheating is not possible because of the fact that the thermocouples are right at the surface of the catalyst. This seems to indicate then that the trouble lies in the method of reducing the catalyst. Work is s t i l l being carried on and this possibility is being investigated. REFERENCES 1. Sabatier-Reid, "Catalysis in Organic Chemistry", D. Van Nostrand Co., New York, 1923, p. 144. 2. Bryant, W. M. D., Ind. Eng. Chem., 23, p. 820 (1933) 3. Smith, D. F., Ind. Eng. Chem., 19, p. 801 (1927) 4. Fischer, Franz, Petroleum Refiner, 23, No. 2, February, 1944. 3. Fischer, Franz, Roelen, and Feist, Petroleum Refiner, 22, No. 12, December, 1943. 6. Bhatnagar, S. S., Phil. Mag., Series 7, Vol. 8, p. 457 (1929) 7. Farquharson, J., Trans. Faraday Soc., 32, p. 219(1936) 8. Farquharson and Sastri, Trans. Faraday Soc, 33, p. 1472 U937). 9." Watanabe, S., J. Soc. Chem. Ind. Japan, Suppl. Binding, 38, p. 328-31 (1933). 10. Sci. Papers Inst. Phys. Chem. Research (Tokyo), Chem. Abstracts 30, 8570 - 8. APPENDIX (B) Heat of Reaction from standard heats of formation C 0 ^ 3 H 2 — ^ CH4-f-H20(g) &MZ9I = A\ (products) — >^ ^ ^ (reactants) From data in Chemical Rubber Co. handbook (27th. ed.) AH.u = - 19,100 - 76,926 1- 26,428 - 50,498 A/S2S>, - - 50,500 cal per mol. (C) Free energy change from free energies of formation. CO + 3H2 *- CH4 -h H20(g) /^G - A G° (products) — IE. ^  &° (reactants) From Chem. Rubber Co. handbook, (27th. ed.) AG%^ - 11,617 - 66,124 i- 32,510 - 33,614 •AG2gg 5 - 33,610 caliper, mol. AH. CO AK - 26,428 0 - - 32,510 Co = 0 APPARATUS see p. 17 


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