502
Znd. Eng. Chem. Res. 1991,30,502-507
Removal of Trace Iron and Nickel Carbonyls by Adsorption Timothy C.Golden,* Thomas H. Hsiung, a n d Karen E.Snyder Air Products and Chemicals, Inc., Allentown, Pennsylvania 18195 Five commercially available adsorbents were screened for selective trace carbonyl adsorption from various gas streams. Among the five adsorbents tested, activated carbon and H-Y zeolite showed the best iron carbonyl adsorption capacity. The activated carbon (Calgon BPL) and the zeolite (Linde H-Y) were further evaluated for nickel carbonyl adsorption. For both adsorbents, the capacity for iron carbonyl exceeded that for nickel carbonyl. The effects of temperature, pressure, and carrier gas compasition on carbonyl adsorption were determined in a systematic study. Thermal regeneration of both adsorbents was also studied. For iron carbonyl, BPL carbon showed a decrease in capacity after regeneration with nitrogen a t 120 "C while the H-Y zeolite showed consistent capacity after three adsorption/regeneration cycles. For nickel carbonyl, the capacity of the BPL carbon was completely restored after the same regeneration procedures. Introduction One recently developed indirect coal liquefaction technology used to make methanol is the liquid phase methanol (LPMEOH; a trademark) process (Roberts et al., 1985). The principal benefit of this process is that synthesis gas from a coal gasifier or partial oxidation unit can pass directly into a three-phase slurry reactor to form methanol. Commercial copper-based methanol catalysts are used. In the process, the solid catalyst is suspended in an inert liquid, typically a paraffinic mineral oil. The inert, liquid provides an excellent heat-transfer medium within the reactor achieving isothermal catalytic reaction conditions. Thus operating temperatures can be selected that are high enough to produce substantial reaction rate without causing catalyst deactivation due to hot spots. This allows high synthesis gas conversion per pass. Air Products has been aggressively developing this process, originally invented by Chem Systems, under the sponsorship of the Department of Energy (DOE) and with the support of the Electric Power Research Institute since 1982. The commercial copper-based methanol catalysts are subject to poisoning by certain process impurities including metal carbonyls. During the development of the LPMEOH process which focused on the use of CO-rich synthesis gas, Air Products experienced catalyst deactivation in the process development unit (PDU) at LaPorte, TX, due to the presence of iron and nickel carbonyls (Final Report to DOE, Contract No. DEAC22-81PC30019). The choice of suitable metallurgy has since minimized these problems. Catalyst deactivation by carbonyls was further verified in laboratory autoclave experiments (Brown et al., 1985). Even at low parts-per-million (ppmv) levels, these carbonyls were found to be detrimental to the methanol catalyst. Iron and nickel carbonyls are present in many industrial gas streams, particularly those with high concentrations of carbon monoxide (Brief et al., 1971). Iron carbonyl is formed in the presence of carbon steel (Sirohi, 1974) and 1/2% Mo steel (Inouye and DeVan, 1979), while nickel carbonyl is formed over h e y nickel catalyst, 304 stainless steel (Ludlum and Eischens, 1973), and 316 stainless steel (Inouye and DeVan, 1979). In addition, the presence of steam in the CO-containing gas can enhance the formation of both iron and nickel carbonyl over 316 stainless steel (Hsiung, 1987). Coal-derived synthesis gas, before water-gas shift, is usually rich in carbon monoxide. Table I lists typical gas compositions of several modern coal gasifiers. The potential for carbonyl formation is high in these gas streams. The most likely metal carbonyls in coal gases are iron 0888-5885/91/263O-O5O2$O2.5O/O
Table I. Typical Composition Gasifiers Texaco" HZ,vol 7% 35.1 c o , vol % 51.8 coz, vol 7% 10.6 CHI, V O ~% 0.1 Ar + Nz, vol % 0.9 other balance
of Oxygen Blown Coal
Shell" 30.1 66.1 2.5 2.5 0.4 balance
BGC-Lurgi' 28.6 54.9 3.4 3.4 4.4 balance
Dod 41.4 38.5 18.5 0.1 1.5 trace
"Kuo (1984). bFisackerly and Sundstrom (1986).
pentacarbonyl, Fe(CO)5,and nickel tetracarbonyl, Ni(CO)& Thermodynamic functions of these two carbonyls were reported by Ross et al. (1964). The rate of formation of these carbonyls is kinetically favored by high CO partial pressure and high temperatures. However, the thermodynamics of carbonyl formation is favored at low temperatures. Normally, the optimum temperature for these carbonyls to form is around 200 "C. Any methanol process that uses the conventional copper-based catalyst requires both the iron and nickel carbonyl concentration in the feed gas to be below ppm levels. Commercially, methanol licensors design to less than 10 parts per billion (ppb). The removal of these carbonyls from a coal gas stream where the potential to form these poisons is high has become a critical step to ensure the success of the methanol synthesis process. The purpose of this study was to identify physical adsorbents for the removal of iron and nickel carbonyls from coal-derived synthesis gas. The results of this study are being considered in the design of a front-end temperature swing adsorption (TSA) system for the LPMEOH process. More specifically, pertinent data needed to design a TSA system include (1)the equilibrium capacity and the length of the mass-transfer zone required for impurity removal, (2) the effect of process conditions such as temperature, pressure, and feed composition on adsorbent performance, and (3) the ease and effect of thermal regeneration on the adsorbents. Experimental Apparatus and Procedures Equilibrium and kinetic adsorption properties were measured volmetrically in a recirculating adsorption apparatus (Figure 1). The apparatus is constructed of stainless steel including 1/4-in. tubing, mixing vessels, various ball valves, and a metal bellows recirculating pump. Adsorption measurements were conducted by placing a known concentration of carbonyl in the recycle or bypass loop of the apparatus at a known temperature, pressure, and system volume. A Hewlett-Packard 5890A gas chroQ 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No. 3,1991 503 PREHEATER
ADSORBENT BED
Nads (mmole/g)
DI
1
.....................
.................P. . . . . . . . .
DRY TEST
METER
INDICATOR FEED
QAS
(/'PORT SEPTUM
SEPTUM PORT$
RECIRCULATINGPUMP
I 2
'0
0
6
4
10
12
16
14
Concentration (ppm)
Figure 1. Schematic of the recirculating adsorption apparatus. Table 11. Physical Properties of Adsorbents for Fe(CO)5 Adsorption surf. bulk area. densitv. av mre adsorbent manufacturer m2/g kg/m5' diak,A BPL carbon Calgon 1100 480 20 760 740 22 silica gel, grade 40 DaGson 340 720 60 activated alumina, 5-100 Alcoa methanol catalyst, UnitedCatelysta 100 1280 EPJ-19 H-Y zeolite, LZY-72 Linde 900 670 10
matograph (GC) equipped with an electron capture detector was used to analyze both the nickel and iron carbonyl concentration. The GC packing employed was 10% Squalane on 100/120 mesh Chromosorb. The carbonyl in the recycle loop was then directed to the adsorption bed which contained regenerated adsorbent, and the concentration decrease was monitored over time yielding data on the carbonyl uptake rate. Once sufficient time was allotted for a constant carbonyl concentration (1-2 h), the final equilibrium concentration was noted and the extent of adsorption was calculated by difference. Other adsorption or desorption points were obtained by either increasing or decreasing, respectively, the carbonyl concentration in the recirculating loop.
Safety The primary hazard associated with studying this application experimentally is the toxic nature of Fe(CO)6and Ni(CO)& These carbonyls have respective TLVs (threshold limit values) of 0.1 and 0.05 ppm. In addition, Ni(CO)4 is a suspected carcinogen. In order to avoid personnel exposure to these compounds, a number of precautions were taken. These precautions included (1)placing the recirculating adsorption apparatus in a walk-in hood, (2) equipping the system with a low-pressure shut-off in case of a system leak, (3) keeping the poison concentration and total gas volume low, (4) treating all gases vented from the system with a heated alumina bed to decompose the carbonyls, and ( 5 ) employing an air-line respirator in all Ni(CO)4experiments. Results and Discussion A. Adsorption of Fe(CO)s. 1. Equilibrium Capacity of Various Adsorbents at 38 O C . A number of commercially available adsorbents were screened for their Fe(C0l6 adsorption capacity at 38 OC and 377 kPa (6% CO/SS% Nz).These adsorbents included an activated carbon, a silica gel, an activated alumina, a H-Y zeolite, and a went Dowdered methanol catalvst. Some of the pertinent ph&d properties of these adbrbente are given in Table 11.
Figure 2. Fe(COI5 adsorption isotherms at 38 O C , 377 kPa (5% CO, 95% N,) on various adsorbents: (4-) BPL carbon; (-A-) Linde Alcoa S-100;(-*-) EPJ-19. H-Y; (--O--) Davison grade 40; (-+-) Nods (mmole/g)
.....
0.6
-..e.
......
.......
......
0' 0
2
4
6
._..-........ *.........
0
..........................................
10
12
11
I
16
Concentration (ppm)
Figure 3. Effect of C02partial pressure on Fe(CO)6adsorption on BPL carbon and H-Y zeolite at 38 O C , 377 kPa. 95% N2/5% C O (-*-) BPL; H-Y. 73% C02/26%N2/l% C O (--o--) BPL; (--A--) H-Y.
The Fe(CO)&adsorption isotherms on the five adsorbents screened initially are given in Figure 2. As clearly noted from the isotherms, the activated carbon and the zeolite show significantly higher adsorption capacities per unit weight than silica gel, alumina, or spent methanol catalyst. The adsorption capacities per unit weight on silica gel, alumina, and spent methanol catalyst were also less then that of the active carbon and zeolite at higher C02partial pressures. Consequently, the activated carbon and H-Y zeolite were further evaluated for Ni(CO)4removal. Several variables including the effect of COz partial pressure, temperature, thermal regenerability, and total carrier gas pressure on the adsorption properties were investigated. The results in Table I1 and Figure 2 show that the Fe(CO&capacity of the adsorbents increase as the surface area of the adsorbent increases. This is true for adsorbents with very different surface polarities like carbons and zeolites. This suggests that adsorption of Fe(CO)6on these adsorbents is dominated by the nature of the pore structure and less affected by the chemical nature of the surface. Of course, the value of BET surface areas can be misleading, particularly with exclusively microporous adsorbents like zeolites. Nonetheless, the high capacities of the polar zeolite and nonpolar carbon indicate the importance of pore structure over the chemical nature of the surface. 2. Effect of COz on Fe(CO)& Adsorption. The composition of carrier gas has a large effect on the adsorption of trace compounds. Aa the strength of adsorption between adsorbent and carrier gas increases, the extent of adsorption of the trace impurity is decreased. Of all the compounds present in dry synthesis gas, COz ia the most strongly physically adsorbed. Consequently, the effect of
504 Ind. Eng. Chem. Res., Vol. 30, No. 3, 1991 Table 111. Asenrent Heats of Fe(C0). Adsomtion BPL carbon H-Y zeolite R. mmol/a a. kcal/mol R, mmol/a Q, kcal/mol 0.40 21.2 0.10 12.5 0.60 20.1 0.20 10.6 0.70 19.1 0.30 9.6
Nods (mmoie/g)
~
COScarrier gas on the adsorption of Fe(COI5on activated carbon and H-Y zeolite was investigated. Figure 3 shows the effect of COz on the adsorption of Fe(CO)6on BPL carbon and H-Y zeolite. The reason H-Y zeolite was chosen over other zeolites such as 13X for this study is that its affinity for C02 is much less. It is clear from the isotherms that the presence of COz does reduce the Fe(CO)6capacity of both adsorbents. The capacities of BPL carbon and H-Y zeolite are reduced by about 30% and 50%, respectively, at an equilibrium concentration of 5 ppm Fe(CO)& The reduction in capacity is greater for the zeolite because COP,which is a polar molecule, is adsorbed more strongly on the polar zeolite than the nonpolar carbon. 3. Apparent Heat of Fe(CO)6Adsorption. The effect of temperature on Fe(C0)5adsorption on H-Y zeolite and BPL carbon was investigated. From adsorption isotherms at two different temperatures, the isosteric heat of adsorption was calculated by use of the Clausius-Clapeyron equation. The Clausius-Clapeyron equation is strictly thermodynamically correct for pure components only. Hence, the heat of adsorption calculatedfrom mixture data is an "apparent" heat of adsorption. Table I11 gives the apparent heat of Fe(CO), adsorption on BPL carbon and H-Y zeolite as a function of surface coverage from adsorption isotherms at 38 and 22 OC. The total carrier gas pressure was 377 kPa and ita composition was 73% COz/26% Nz/l% CO. Table I11 shows that the apparent heat of Fe(C0)6adsorption on H-Y zeolite under the given conditions is 12.5 kcal/mol at a surface coverage of 0.10 mmol/g and reduces to 9.6 kcal/mol at 0.30 mmol/g. Firstly, the magnitude of the apparent heat of adsorption is rather large. This indicates that the adsorption of Fe(CO)6is very sensitive to temperature. Thus, the extent of Fe(C0I6 adsorption can be significantly modified by changing the adsorption temperature. Secondly, the decrease in appqent heat of adsorption with increasing surface coverage indicates adsorbent heterogenity with respect to Fe(CO)s adsorption. Adsorption of Fe(C0)6 on BPL carbon shows greater temperature sensitivity than that on H-Y zeolite as noted by its higher apparent heat of adsorption. Fe(CO)5is adsorbed more strongly on BPL carbon than H-Y zeolite as evidenced by ita higher adsorption capacity, Henry's law constant, and heat of adsorption. Hence, due to the high heat of adsorption of Fe(C0)6on both H-Y and BPL, the adsorption capacity can be significantly enhanced by reducing the adsorption temperature. In addition, the decrease in Fe(CO)s heat of adsorption with increasing surface coverage again denotes adsorbent heterogeneity. 4. Thermal meneration Studies. Thermal regeneration experiments were carried out to determine (1)if the adsorptive capacity of the adsorbents is recovered after thermal regeneration, (2) what temperature is required to regenerate the adsorbent, and (3) if all the carbonyl impurity is desorbed as such or if metal species are deposited on the adsorbent surface. Figure 4 shows Fe(COIs adsorption isotherms on BPL carbon at 22 OC from 377 kPa of carrier (73% COz/26% Nz/l% CO) on successive runs following regeneration in Nzat 120 "C. The boiling point of Fe(CO), is 103 "C. The
04
02
[i
01
I 2
4
6
8
10
12
14
16
Concentration (ppm)
Figure 4. Effect of thermal regeneration (N2at 120 "C) on the Fe(CO)Scapacity of BPL carbon a t 22 OC, 377 kPa (5% C0/95% N2).(-A-) First run; (- -0--) second run; (-O-) third run.
capacity of BPL for Fe(CO), is reduced on each successive run following the regeneration. The adsorption capacity at 5 ppm decreases from 0.96 to 0.77 mmol/g following two regeneration cycles. To elucidate the cause of this adsorbent deactivation, the gas effluent during regeneration was monitored for Fe(C0)6concentration. By carefully monitoring the effluent Fe(CO)6concentration as a function of time, the total amount of Fe(CO)5desorbed can be calculated. Since the Fe(CO)6loading of the adsorbent prior to regeneration was known and the total quantity of Fe(C0)5desorbed can be calculated from the desorption curve, the amount of iron remaining on the carbon surface can be calculated by difference. In addition, the amount of iron species remaining on the adsorbent can be determined gravimetrically. On the basis of the above, a number of points can be made concerning thermal desorption of Fe(C0I6 from active carbon. Firstly, very little adsorbed Fe(C0I5 is desorbed as Fe(CO)& The desorption curves indicate that less than 0.1 w t % of Fe(C0)6is desorbed as such. This means a large fraction of some iron species remains on the carbon surface. It is strongly suspected that the iron species is iron metal even though the regeneration temperature of 120 "C is below the decomposition temperature of Fe(C0)6(149 "C). Previous studies on the adsorption of Fe(CO)Salso indicate that elemental iron is left on the adsorbent surface following thermal regeneration in this temperature range (Nagy et al., 1979). Another interesting observation that suggests elemental iron is formed is the ferromagnetic nature of the spent adsorbent. In addition, both CO and C02 are evolved during the thermal decomposition step. Elemental iron can catalyze the Boudouard reaction at low temperatures (100 "C) (2CO COz + C) (Dwyer and Somorjai, 1978). Thus, CO evolved during decomposition of Fe(C0)6 is presumed to react in the presence of elemental iron and form COz and deposit carbon. Thermal regeneration studies on active carbon following Fe(C0)5 adsorption demonstrate (1) Fe(CO)s adsorption capacity is reduced following thermal regeneration and (2) thermal regeneration leads to deposition of elemental iron on the surface of the carbon which is most likely the reason for the loas of adsorption capacity. Similar thermal regeneration studies were carried out on H-Y zeolite. For three successive adsorption/regeneration cycles as described above, the zeolite demonetratas the same adsorptive capacity. The thermal desorption curve for H-Y showed that significantly more Fe(CO)s is desorbed from the zeolite than the carbon. However, the total quantity of Fe(CO)6desorbed is equal to about 30% of that adsorbed. Once again the iron species on the
-
Ind. Eng. Chem. Res., Vol. 30, No. 3, 1991 606 Fractional W a k e (%)
Nods (mmole/g) 1.2
..e........ ...................
1
.........
I
;/
....p.. ........ .......... ..............
0.8
...... ......
0.6
40
-;
.......
0.4 20
0
-
: 0.2
2
6
I
10
0
2
"spent" adsorbent seems to be elemental iron as indicated by gravimetric measurements and the ferromagnetic property of the spent adsorbent. This result agrees with previous investigations of Fe(CO), adsorption and decomposition on H-Y zeolite (Nagy et al., 1979). The experimental data suggest that some Fe(CO), is decomposed on the zeolite surface, yet its equilibrium capacity remains unchanged after three regeneration cycles. A possible explanation for these findings is that the adsorbed Fe(CO), in the zeolite pores migrates into the macroporous binder of the zeolite during thermal regeneration. The macropores of the zeolite do not contain significant adsorption capacity. However, diffusion in pelletized zeolites is limited by macropore diffusion. The primary reason for this is the much smaller size of the microporous zeolite crystals (1-10 pm) versus the size of the macroporous binder network (1-2 mm). Hence, if deposition occurs in the macropores, the rate of Fe(CO)& adsorption should decrease. Figure 5 shows the fractional approach to equilibrium as a function of time for three successive adsorption/regeneration cycles. As depicted in the f v ,the approach to equilibrium is slowed with each successive adsorption/regeneration cycle. In addition, Nagy et al. (1979) have shown that the iron crystallites present in H-Y zeolite following thermal regeneration of Fe(CO)s can be larger than 5 nm. Species this large in size will not fit in the micropore network of H-Y zeolite. This suggmta that at least a portion of the deposited iron species is outaide the zeolite structure. The character of the thermal desorption effluent from H-Y zeolite deaervea comment. During thermal regeneration of H-Y zeolite, the Fe(CO)s concentration in the effluent stream rises significantly above the inlet concentration. Therefore, the thermal regeneration effluent will have hQh concentrations of Fe(CO)s particularly at the oneet of regeneration. Since this effluent is potentially toxic, the syetem deeign and eepecially the downstream disposal method must deal with this safety h u e properly. In addition, a further safety consideration is that thermal regeneration of Fe(CO)s-laden adsorbenta leads to deposition of elemental iron on the adsorbent. It is well-known that finely divided, supported metallic species can be pyrophoric. Thus, when an adsorption bed is to be replaced, the possible pyrophoric nature of the 'spent" adeorbent muat be taken into consideration. This pyrophoric property ie particularly a concern with reepect to finely divided metallic speciee preeent on carbonaceous surfacee. However, good design and operations practice will effectively manage thie problem. 6. Effect of Carrier Gas Pressure on Fe(CO)sAdsorption. It ie important to investigate the effect of
8
10
12
14
Concentration (ppm)
Time (minutes)
Figure 5. Effect of thermal regeneration (N2at 120 "C) on the Fe(CO)I uptake on H-Y Zeolite at 22 "C, 377 Wa (5% C0/95% N2). (-A-) First run;(--O--) second run, 120 "C regeneration; (-O-) third run, 120 "C regeneration.
6
4
Figure 6. Effect of carrier gaa pressure on Fe(CO)&adsorption on BPL carbon and H-Y zeolite at 22 "C. 377 kPa: (-*-) BPL; (-O-) H-Y. 722 kPa: (--0--) BPL; (--A--) H-Y. Nods (mmole/g) 0.25
0"' 0
I 2
4
6
6
10
Concentration (ppm)
Figure 7. Mixed Langmuir prediction of Fe(CO)&adsorption on H-Y zeolite at 22 OC, 722 kPa (85% C02/14% CO/l% NJ. (-O-) Experimental; (- -0-) theoretical.
pressure on trace impurity removal. In general, as the pressure of the carrier gas increases, the adsorption capacity for trace impurity decreases. This is because at higher adsorption pressures the competition for adsorption sites between the trace impurity and the bulk carrier is enhanced. Thus, the much more prevalent carrier gas molecules inhibit the adsorption of the trace impurity. Clearly, from the results of this study, the Fe(CO), capacity is reduced when the carrier gas pressure is increased. Figure 6 depicts Fe(CO), adsorption isotherms on BPL carbon and H-Y zeolite at 22 O C from 377 (73% co2/26% Nz/l% CO) and 722 kPa of carrier (85% COZ/14% Nz/ 1%CO). At an equilibrium concentration of 5 ppm, the Fe(CO), capacity drops from 0.96 to 0.62 mmol/g an the carrier gas pressure increases from 377 to 722 @a. In the zeolite case, the equilibrium capacity at 5 ppm decreases from 0.31 to 0.18 mmol/g. 6. Mixed Langmuir Prediction of Fe(CO), Adsorption. The mixed Langmuir technique (Markham and Benton, 1931) was used to predict Fe(CO), adsorption on H-Y zeolite at 22 OC from 722 P a of carrier (85% co2/14% CO/1% N2). The equilibrium adsorption coefficients for COz, CO, and Nz were obtained from pure-component isotherms and that for Fe(CO)swas obtained from 377-Wa-carrier data. The comparison of mixed Langmuir theory and the experimental data is shown in Figure 7. The mixed Langmuir underpredicta by about 40% the amount of Fe(CO)sadsorption. At an equilibrium concentration of 5 ppm under the given conditions, the amount of Fe(CO)s adsorbed is 0.18 mmol/g versus 0.13 mmol/g predicted by mixed Langmuir theory. B. Adsorption of Ni(CO)4. 1. Adsorbents Screened. On the basis of the results obtained with Fe(COIs, adsorption isotherms were measured for Ni(CO)4with BPL
506 Ind. Eng. Chem. Res., Vol. 30, No. 3, 1991 Nods (mmole/g)
Nods
0.7
0
I
5
10
15
2c
Concentration (ppm)
0' 0
I
5
10
15
2c
Concentration (ppm)
Figure 8. Ni(CO), adsorption isotherms at 22 OC, 377 kPa (5% C0/95% N2) on BPL carbon (-O-) and H-Y zeolite (- -0-).
Figure 9. Effect of carrier gas pressure on Ni(CO), adsorption on 722 kPa. BPL carbon at 22 "C. (-n-) 377 kPa; (--O--)
Table IV. Apparent Heats of Ni(CO), Adsorption BPL carbon H-Y zeolite n, mmol/g q, kcal/mol n, mmol/g q , kcal/mol 0.20 17.4 0.05 12.4 0.30 13.8 0.10 11.7 0.40 11.7 0.20 9.6
sorbent was regenerated in flowing N2 at 120 OC. As opposed to the Fe(CO)&adsorption/regeneration case, the Ni(C0)4capacity remains intact after three adsorption/ regeneration cycles. This is most probably due to the smaller capacity and weaker adsorption of Ni(CO)4than for Fe(CO)&demonstrated by BPL carbon.
carbon and H-Y zeolite only. Figure 8 shows Ni(CO)4 adsorption isotherms on BPL carbon and H-Y zeolite at 22 "C from 377 kPa of carrier (95% N2/5% CO). The capacities at an equilibrium concentration of 1ppm Ni(CO)4for BPL carbon and H-Y zeolite are 0.22 and 0.07 mmol/g, respectively. The corresponding values for Fe(CO)s adsorption under the same conditions are 0.60 and 0.24 mmol/g. Thus, Ni(CO)4 is adsorbed less strongly on these adsorbents than Fe(CO)S, which is expected from the lower molar volume and higher saturation vapor pressure of Ni(CO)& 2. Apparent Heat of Ni(C0)4Adsorption. The effect of temperature on Ni(CO)4adsorption was investigated. Table IV gives the apparent heat of Ni(CO)4 adsorption as a function of surface coverage on both BPL and H-Y determined from isotherms at 38 and 22 OC from 377 kPa of carrier (73% C02/26% N2/l% CO). As was the case with Fe(CO),=, adsorption, the magnitude of the heat of adsorption is large. At surface coverages of 0.05 and 0.2 mmol/g for H-Y and BPL, respectively, the heats of adsorption are 12.4 and 17.4 kcal/mol. The higher heat of adsorption on the carbon is manifested in higher adsorption capacity and Henry's law constant. In addition, the decrease in heat with increasing surface coverage indicates both adsorbents demonstrate heterogeneity with respect to Ni(CO)4 adsorption. As in the case of Fe(CO)s adsorption, reducing adsorption temperature provides an effective means of enhancing the adsorptive capacity for Ni(CO)& For example, the Ni(CO)4capacities on H-Y at 1 ppm are 0.022 and 0.070 mmol/g at 38 and 22 "C, respectively. 3. Effect of Carrier Gas Pressure on Ni(CO)4Adsorption. Figure 9 shows the effect of carrier gas pressure on the adsorption of Ni(CO)4on BPL carbon. As was the case with Fe(CO)sadsorption, higher carrier gas pressures decrease adsorption of the trace impurity. At 1 ppm Ni(COI4,the adsorption capacity of BPL carbon decreases from 0.20 to 0.06 mmol/g as the pressure increases from 377 to 722 kPa This diminution is greater than that noted with Fe(CO)& This is expected as the effect of carrier gas pressure increases when the capacity for the trace impurity decreases. 4. Thermal Regeneration Studies. Following adsorption of Ni(CO)4on BPL carbon at 22 OC and 377-kPa carrier gas pressure (73% co2/26% N2/l% CO), the ad-
Conclusions Removal of catalyst poisons such as iron and nickel carbonyls from coal-derived synthesis gas is an important part of the methanol synthesis process. One possible technique for the removal of carbonyls is to use a temperature swing adsorption system before the methanol reactor. Both activated carbon and H-Y zeolite demonstrate higher Fe(CO)s adsorption capacities on a weight basis then silica gel, alumina, or the methanol catalyst itself. It was determined that the Fe(COIS adsorption capacity of both active carbon and H-Y zeolite was decreased by enhancing the carrier gas pressure, increasing the COz concentration in the carrier gas stream, and increasing the adsorption temperature. The heat of Fe(CO)s adsorption on both adsorbents was quib large (- 13-20 kcal/mol), so that reducing the temperature can greatly enhance the extent of Fe(CO)Sadsorption. The effect of thermal regeneration in N2at 120 OC showed that Fe(COI5 decomposes on the surface of both adsorbents at these conditions. The extent of decomposition is greater on carbon than on the zeolite. The decomposition results in the deposition of elemental iron on the adsorbents surface which reduces the Fe(C0Is capacity of the carbon due to blockage of micropores. The zeolite adsorbent shows a reduced uptake rate of Fe(CO)Sfollowing regeneration but consistent capacities, and it is hypothesized that iron is deposited in the macroporous binder. Adsorption of trace Ni(CO), on the zeolite and carbon showed capacities less than those noted for Fe(COI5, meaning larger adsorption beds. As in the case of Fe(CO)5 adsorption, the carbon showed higher adsorption capacity than the zeolite and both adsorbents exhibited high heats of Ni(C0)4adsorption (-10-17 kcal/mol). Thus, Ni(CO)4 adsorption capacity can be significantly enhanced by reducing the adsorption temperature. Finally, thermal regeneration studies showed that the Ni(CO)4capacity of the carbon was reestablished following regeneration in N2 at 120 "C. Thus, a temperature swing adsorption carbonyl removal system could consist of an adsorbent bed staged with both H-Y zeolite for Fe(CO)s removal and active carbon for Ni(CO)4removal. The zeolite would be situated at the feed end for Fe(CO)s removal despite its lower adsorption capacity. Since regeneration of Fe(C0)6-ladencarbon results in iron deposition and loss of capacity, the zeolite can be
Ind. Eng. Chem. Res. 1991,30,507-516
considered a guard bed protecting the carbon from Fe(CO)& The advantages of using the zeolite for Fe(CO)6removal include retention of capacity following thermal regeneration, higher bulk density than carbon, and that iron-laden zeolite is less combustible than iron-laden carbon. The active carbon located behind the zeolite would affect the required Ni(C0)4removal. Thermal treatment of the mixed bed in N2 at 120 O C is needed for satisfactory adsorbent regeneration.
Acknowledgment This work was supported by the Department of Energy under Contract No. DE-AC22-87PC90005. Registry No. Fe(CO)5, 13463-40-6; Ni(CO)4, 13463-39-3;
MeOH, 67-56-1; C, 7440-44-0.
Literature Cited Brief, R. S.; Blanchard, J. W.;Scala, R. A.; Blacker, J. H. Metal Carbonyls in Petroleum Industry. Arch. Environ. Health 1971, 23, 373-384. Brown, D. M.; Hsiung, T. H.; Rao, P.; Greene, M. I. Catalyst Activity and Life in Liquid Phase Methanol. Presented at the 10th Annual EPRI Clean Liquid and Solid Fuels Contractor’s Conference, Palo Alto, CA, April 1985. Dwyer, D. J.; Somorjai, G.A. Hydrogeneration of CO and C02Over Iron Foils. J. Catal. 1978, 52, 291-301.
507
Fisackerly, R. H.; Sundstrom, D. G. The DOW Syngae ProjectProject Overview and Status Report. Presented at the 6th Annual EPRI Conference on Coal Gasification, October 15-16, 1986. Hsiung, T. H. Air Products and Chemicals, Inc., unpublished information, 1987. Inouye, H.; DeVan, J. H. Formation of Iron Carboxyl Between a 1/2 Pet Mo Steel and High-pressure Gases Containing Carbon Monoxide. J. Mater. Energy Syst. 1979, 1, 52-60. Kuo, J. C. W. Tutorial Lecture on Indirect Coal Liquefaction at the 188th National Meeting of the American Chemical Society, Phildelphia, August 1984. Ludlum, K. H.; Eischens, R. P. Carbonyl Formation in Stainless Steel Infrared Cells. Surf. Sci. 1973, 40, 397-398. Markham, E. C.; Benton, A. F. The Adsorption of Gas Mixtures by Silica. J. Am. Chem. SOC.1931,53,497-503. Nagy, J. B.; Van Eunoo, M.; Derovane, E. G. Highly Dispersed Supported Iron Particles from the Decomposition of Iron Carbonyl on HY Zeolite. J. Catal. 1979,58, 230-237. Roberta, G.W.; Dicciani, N. K.; Kosek, J. The LPMEOH ProcessAn Efficient Route to Methanol from Coal. Presented at the Conference on Coal Gasification and Synthetic Fuels for Power Generation, San Francisco, CA, April 1985. Ross, L. W.; Haynie, F. H.; Hochman, R. T. Thermodynamic Functions of Nickel Carbonyl and Iron Pentacarbonyl. J. Chem. Eng. Data 1964, 9 (3), 339-340. Sirohi, V. P. Carbonyl Formation in Coal Gasification Plants. C. F. Braun & Co. report to ERDA under Contract No. E(49-18)-2240, 1974.
Receiued for review April 18, 1990 Revised manuscript received August 31, 1990 Accepted September 26,1990
Multicomponent Batch Distillation. 3. Shortcut Design of Batch Distillation Columns Mohommad S. Al-Tuwaim and William L. Luyben* Department of Chemical Engineering, Lehigh University, 111 Research Drive, Bethlehem, Pennsylvania 18015
As the trend to speciality chemicals continues to emphasize batch processing plants, the need for a quick, easy-to-use method for designing batch distillation columns becomes more important. Both the non-steady-state conditions and the nonlinearity of batch distillation make it difficult to derive analytical design formulas except for very basic and overly simplified systems. This paper presents a method for determining preliminary shortcut economic designs for both binary and ternary batch distillation columns. Design correlations are given that can be used to easily read off the optimum number of trays and the optimum reflux ratio for a given separation: specified relative volatilities, product purities, energy cost, and material of construction.
Introduction In many chemical plants, batch processes are becoming more important as the trend to speciality small-volume, high-value chemicals continues. The preference for batch over continuous processes is based on economic and operational criteria. Batch distillation columns play an important role in batch processes. Batch distillation provides outstanding flexibility: a single column can handle any number of components. To achieve the same result using a continuous distillation system, NC - 1 columns would typically be required for a system of NC components. The batch distillation process is characterized by unsteady-state conditions. This means that the compositions are functions of time. A large number of design and operating parameters must be optimized in order to design and operate the column. Furthermore, the policy of operating the column should be chqeen such that the process
* Author to whom correspondence should be addressed.
will be as simple as possible. Most of the work on batch distillation has been limited to binary systems. Luyben (1971), Bauerle and Sandal1 (1987), Coates and Pressburg (1961), Treybal(l970), Guy (1983), Kerkhof and Vissers (19771, and Featherstone (1976) all considered binary mixtures. On the other hand, ternary batch distillation was studied by Stewart et al. (1973), Van Dongen and Doherty (1985), and Luyben (1988). The work previously presented on the design of binary systems is usually with no tray holdup, an infinite number of trays (corresponding to minimum reflux), or a fixed number of trays. Moreover, previous work was carried out mainly to select an optimum policy of operating the column. Bauerle and Sandal1 (1987) presented analytical equations for the design of batch distillation columns (binary mixtures) with an infinite number of plates, neglecting any holdup in the column. Luyben (1971) showed that batch distillation can be significantly affected by design and operating parameters such as tray holdup, reflux drum holdup, and number of trays. He proposed a method to design batch distillation columns by using the
0888-5885f 91f 263O-ObQ7$02.50f 0 Q 1991 American Chemical Society
