Ind. Eng. Chem. Res. 2008, 47, 7027–7030
7027
Liquid-Phase Guard Bed for Removal of Synthesis Gas Contaminants Robert Quinn* and Bernard A. Toseland Air Products and Chemicals, Inc., 7201 Hamilton BouleVard, Allentown, PennsylVania 18195-1501
This article describes a single guard bed that is effective for the removal of all poisons from a methanol synthesis process. The guard bed consists of a methanol synthesis catalyst suspended in a nonreactive liquid (a liquid-phase guard bed). The key to making the guard bed universal is operating the guard bed at a temperature below the operating temperature of the reactor but at a sufficiently high temperature and residence time to allow achievement of equilibrium with the incoming gas. The successful use of this guard bed is demonstrated for several catalyst poisons. In addition, the use of spent catalyst for the guard bed is demonstrated. Introduction Copper-based methanol synthesis catalysts are used to produce methanol (CH3OH) from a synthesis gas (syngas) feed as in the reaction CO(g) + 2H2(g) ) CH3OH(g)
(1)
The active catalyst generally consists of a mixture of metallic copper, zinc oxide, and alumina (Cu/ZnO/Al2O3). Such catalysts tend to lose activity over time, and this is generally attributed to catalyst sintering and/or poisoning.1 Sintering, which results in a decrease in the copper surface area available for catalysis, can be minimized by operating at low temperature. The technology of methanol synthesis catalysis has improved to the point that a useful operating life of up to 5 years can be achieved in fixed-bed operation on clean syngas. Catalyst poisoning involves a loss of activity from the reaction of syngas contaminants with the catalyst and can cause premature loss of catalyst activity. Adequate catalyst lifetime and activity require removal of catalyst poisons from the feed. Among the species identified as poisons for methanol synthesis catalysts are the sulfur-containing species hydrogen sulfide,2,3 carbonyl sulfide,3-5 and thiophene;3 hydrogen chloride;1,2,6,7 iron pentacarbonyl; and nickel tetracarbonyl.4,5 More recently, additional poisons have been reported and include arsine, phosphine, methyl thiocyanate, methyl chloride, and methyl flouride.8,9 The solution to catalyst poisoning is to remove the offending contaminants from syngas. This, however, can prove to be difficult. Depending on the feedstock, syngas can contain a wide variety of contaminants, some of which might need to be removed to very low concentrations, 10 ppb or less, to prevent catalyst poisoning. In practice, contaminant removal is most often accomplished by a bulk separation followed by a trace removal process. The latter typically involves the irreversible reaction of a contaminant with an adsorbent by passage of the syngas feed through a vessel, referred to as a guard bed, packed with an adsorbent. For example, syngas containing significant H2S concentrations (140
4 5 6
been shown for a packed-bed configuration of a copper-based hydrogenation catalyst preventing poisoning of a downstream reactor containing the same catalyst.2 Similarly, a Cu/ZnO/Al2O3 low-temperature shift catalyst has been used to accomplish HCl removal. In this application, the inlet portion of the fixed-bed reactor removed HCl, thus protecting the remainder of the downstream catalyst.13 A packed bed of reduced methanol synthesis catalyst operating at ambient temperature was found to be effective for removal of H2S from syngas.14 The concept of a liquid-phase guard bed has several advantages when compared to a more conventional packed-bed approach. Because the liquid-phase guard bed uses an adsorbent in the form of a powder, mass transfer and adsorbent utilization are superior to that for a formed adsorbent in a packed bed. In addition, as the adsorbent capacity reaches its maximum, spent adsorbent can be removed from the guard bed and replaced with fresh adsorbent, and this can be accomplished without a shutdown of the bed or the need for duplicate beds, as in the case of a conventional fixedbed approach. Finally, a liquid-phase guard bed provides more efficient heat management so that all parts of the bed operate below the temperature of the reactor. Experimental Results. A series of experiments were conducted using a 50 cm3 reactor containing approximately 1.5 g of catalyst in a mineral oil slurry as a liquid-phase guard bed. Syngas was passed through the guard bed reactor into a second 50 cm3 reactor that functioned as the methanol synthesis reactor and contained 3.0 g of catalyst in a mineral oil slurry. The effectiveness of the liquid-phase guard bed was determined in two ways: (1) monitoring of the methanol synthesis rate and (2) analysis of spent catalyst. The absence of contaminant on the methanol synthesis reactor catalyst and no decline in catalyst activity above that for clean syngas are indications that the guard bed was effective for contaminant removal. Three contaminants, AsH3, H2S, and COS, were evaluated, and experimental conditions are summarized in Table 3. Experiments involving arsine as a syngas contaminant were carried out using guard bed temperatures of 140, 150, and 225 °C. The presence of 0.526 ppm arsine in the syngas feed resulted in no significant change in the methanol synthesis rate constant, implying that AsH3 was removed by the guard bed. To ensure that higher guard bed temperatures and, thus, higher methanol concen-
trations exiting the guard bed could be used without detrimental impact on the performance of the methanol synthesis catalyst, the guard bed temperature was ramped to 150, 200, and finally 225 °C. As shown in Table 4, no adverse effect on the methanol synthesis rate was observed. The overall catalyst deactivation rate was 0.016%/h, a value comparable to that for clean syngas under the same experimental conditions. Most importantly, analysis showed no detectable arsenic (limit of detection, 10 ppmw) on the methanol synthesis reactor solid following the run. The guard bed solid contained 2620 ppmw arsenic, in excellent agreement with the calculated arsenic loading of 2720 ppmw based on the AsH3 concentration and flow rates and assuming that all of the arsenic was deposited on the guard bed solid. It can be inferred that, because no arsenic was detected on the methanol reactor catalyst, the guard bed was effective for AsH3 capture even at 140 °C. In the absence of a guard bed, the presence of 0.578 ppm AsH3 in the syngas feed resulted in a continuous decline in the methanol synthesis activity with a deactivation rate of 0.328%/h indicative of catalyst poisoning.8 After 116 h of AsH3 exposure, the catalyst had lost ∼35% of its initial activity, and analysis confirmed the presence of arsenic on the spent catalyst.8 By comparison, use of the liquid-phase guard bed resulted in only a 5% activity loss over the same time and no detectable arsenic on the spent methanol synthesis catalyst. Removal of hydrogen sulfide, H2S, from a syngas feed was evaluated in a similar manner. Syngas containing 2.76 ppm H2S was fed to a liquid-phase guard bed at 140 °C over a 72.0-h period. As shown in Table 5, rate constants during the H2S exposure were unchanged. Analysis of the spent solids indicated no detectable sulfur (
