The CO 2 that is removed from the synthesis gas in an ammonia plant is released to the ambient air if it is not used by another process. This CO 2 stream can contain a considerable amount of methanol. Methanol, like other volatile organic compounds (VOC), contributes to the formation of photochemical smog which consists mostly of ozone (O 3). Since ozone adversely affects human health, limits are set in many parts of the world. Consequently, in many places (e.g. the USA) limits have been placed on the emissions of its precursors.
These limits become a challenge for ammonia producers. Methanol is generated as an unwanted by- product in the CO shift reactors. Part of it is removed from the process gas with the process condensate. The remainder passes to the CO 2 removal unit and ends up in the CO 2 stream.
This paper briefly describes the effect of methanol in the atmosphere and the process by which methanol is formed in an ammonia plant. It focuses on ways to reduce emissions by reducing the amount that is formed, by process changes and by “end of pipe” abatement.
Klaus Noelker ThyssenKrupp Uhde GmbH
John Pach Johnson Matthey plc
weather and is created from nitrous oxides and hydrocarbons in presence of ultraviolet (UV) Introduction radiation. It attacks respiratory organs and erms like summer smog and ground- harms animals and plants. level ozone are well known to many of Tus because they impact our daily life. Effect of Methanol Emissions on Many of us, for example, know the appeals to the Atmosphere avoid outdoor physical efforts under certain climatic conditions in the summer. Ozone Formation
Summer smog (also called photochemical smog, Ground-level ozone is formed from nitrous ox- ozone smog or Los Angeles smog) arises from ides under the influence of the ultraviolet (UV) the pollution of ground-level air by high con- radiation of the sun. Nitrogen dioxide (NO 2), centrations of ozone. It occurs during sunny emitted by combustion processes into the at- mosphere, is split by UV radiation into nitrogen monoxide (NO) and an oxygen atom. This atomic oxygen combines with an oxygen mole- cule, forming ozone (O 3):
NO 2 + light (λ < 420 nm) → NO• + •O• •O• + O 2 → O 3
At the same time, nitrogen monoxide (NO) de- composes ozone to form nitrogen dioxide and oxygen:
NO + O 3 → NO 2 + O 2
As ozone is simultaneously being formed and being decomposed, an equilibrium concentra- tion can be defined. During the night (no UV radiation), decomposition dominates and the ozone concentration falls.
Ozone formation is favored by volatile organic compounds (VOC, R-CH 3) because they help to form nitrogen dioxide from nitrogen monoxide in the presence of sunlight:
R-CH 3 + 2 O2 + 2 NO → RCHO + 2 NO 2 + H2O
The reaction mechanism is summarized in a simplified form in Figure 1.
Countermeasures When thinking about countermeasures against high ozone concentration, the following must be considered: • ozone cannot be attributed to a single point of emission because it is formed within the atmosphere, and
• the weather as one of the originators cannot Figure 2. Simplified flowsheet of CO shift, CO 2 be changed. removal, and condensate stripper of an ammo- nia plant As a result, measures are taken to limit the emissions of its precursor substances; NOx and By-products are formed, in small quantities, VOC from industry, traffic and households. across both HTS and LTS catalysts. Of these the largest component by far is methanol, which Short-term measures to limit ozone include is believed to be formed by the reaction of car- shutting down industry and limiting traffic when bon dioxide with hydrogen (). ozone levels are high. Longer-term measures involve modifications to facilities and vehicles, CO 2 + 3 H 2 ↔ CH 3OH + H 2O ∆H = -50 some of which are already being implemented. kJ/mol For example, on large furnaces, (catalytic) NO x removal is standard in many countries; for vehi- The chrome phase is believed to promote meth- cles, the contribution to ozone formation has anol formation across HTS catalysts (not sur- been reduced by exhaust gas catalysts by 80 to prising as early methanol catalysts were chrome 95%, reducing NO x, CO and VOC emissions. based). The presence of copper also contrib- utes. In the USA, New Source Review (NSR) thresh- old for significant emissions increase of VOC As copper is the active component in both and NOx for ozone regulation is 40 tons per methanol synthesis and LTS catalysts, it is not year . Methanol is classified as a Hazardous surprising that LTS catalyst will, to an extent, Air Pollutant (HAP). A release of more than catalyze the formation of methanol. 10 tons per year is considered a “major source” and is controlled by the EPA . Whilst the methanol concentration leaving the HTS catalyst is close to equilibrium and there- Methanol Emissions from fore independent of catalyst type, methanol formation in LTS catalysts is kinetically limited Ammonia Plants and does not usually reach equilibrium. As the methanol forming reaction is exothermic, and Methanol Formation HTS catalysts operate at higher temperature, the Figure 2 illustrates the parts of an ammonia shape of the equilibrium curve dictates that plant which are relevant to the formation of methanol formation across HTS catalysts is sub- methanol and its path through the process to the stantially less than that across traditional, non- points of emission. It also serves as the base for selective LTS catalysts (Figure 3). the discussion of reduction of ammonia emis- sions discussed later in this text.
By-product MeOH as a function of temperature Split of methanol between condensate and vapour 3000 100% 90% 2500 80% MeOH 70% 2000 Equilibrium 60% operating 50% 1500 Operating window window 40% 30 barg for MeOH formed for HTS 30% 40 barg 1000 catalyst over the LTS bed 20% 10% By-product MeOH 500 condensate in methanol %of 0% 0 0 20 40 60 80 100 120 140 160 200 LTS HTS Separator Temperature (°C) Temperature Figure 3. Effect of equilibrium on methanol Figure 5. Split of methanol between condensate formation and vapor for a modern ammonia plant
LTS catalyst is slightly unusual in that selectivi- Modern plants, and older plants retrofitted with ty increases, and by-product formation decreas- condensate strippers in the last couple of dec- es, as the catalyst ages (Figure 4). This behav- ades, use a medium pressure stripper to treat the iour is because the activity with regard to condensate. A well designed stripper meets methanol synthesis declines as the active sites most environmental standards for waste water for methanol synthesis decrease. There is no and will recycle methanol to the reformer feed. corresponding impact on the activity with regard to the shift reaction, which, in a well-structured Plants with older designs of stripper can use LP catalyst, remains high enough to maintain an stripping to remove methanol and ammonia equilibrium CO slip. from the condensate. Sometimes they are vent- ed to atmosphere along with the stripping steam, but more commonly the vent stream is con- densed and sent, for example, to a feed coil sat- urator (although sometimes a small, methanol containing stream is “purged” to atmosphere).
Methanol Remaining in Process Gas The methanol that is not removed in the process condensate separator passes into the CO 2 re- moval system. The majority of that CO 2 passes into the saturated CO product, with its fate in- Figure 4. Methanol formation with time across 2 tertwined with that of the CO product. The few an LTS catalyst 2 parts per million that remain in the treated syn- gas (stream no. 2 in Figure 2) decompose in the Process Condensate Treatment methanator. The gas leaving the LTS is cooled and conden- sate is removed via the process condensate sepa- The methanol absorbed by the CO 2 removal so- rator. The split of methanol is determined by lution in the absorber is almost completely ex- vapor-liquid equilibrium, hence the lower the pelled from it in the regeneration section. In temperature, the greater the proportion of meth- Figure 2, which represents a plant with an acti- anol that is removed via the process condensate, vated MDEA CO 2 removal unit, the CO 2 con- as illustrated in Figure 5. taining streams are the high-pressure flash gas (stream no. 7) and the high-purity CO 2 stream Reduction of Methanol Formation (stream no. 8). The HP flash gas stream is nor- mally used as reformer fuel, and the methanol The amount of by-product methanol produced in and other hydrocarbons contained within it are the LTS converter is influenced by a variety of combusted and do not appear as an emission. In factors, which are summarized in Table 2. In a typical system, about 8% of the methanol most plants, there is only limited flexibility to leaves with the HP flash gas and about 92% change operating conditions with the result that LTS catalyst selection is the only real alterna- leaves with the CO 2 stream. tive for reducing methanol by-product for- mation. Selective low methanol LTS catalysts If the CO 2 is scrubbed, cooled or compressed, a methanol containing liquid effluent can be cre- such as KATALCO JM 83-3X, reduce methanol ated. In many plants, the volumetric flow is formation by about 90% compared to conven- small enough for this either to be disregarded, or tional non-selective LTS catalysts. As the rela- to be treated by pumping it into the process tive contribution to methanol formation from the condensate stream for purification in the exist- LTS falls, that from the HTS increases, and ing process condensate stripper. methanol emissions can be grossly underesti- mated if formation across a selective LTS is Compression and cooling is the case when part, considered in isolation (Figure 6). or all, of the CO 2 stream is used in a urea plant (stream no. 10). Methanol and water condense Increase in Effect on methanol and are separated from the gas in the inter-stage make separators of the CO 2 compressor. The remain- Steam / H 2 ratio ↓ ing CO 2 is removed in the H 2 removal reactor Inlet temperature ↑ (if not at equilibrium) by catalytic conversion with O . 2 ↓ (if at equilibrium)
Pressure A methanol containing gaseous effluent stream ↑ Space velocity can be created if CO 2 gas is vented, or, as hap- ↓ pens on some food grade CO 2 plants, it passes Catalyst age ↓ through a solid adsorbent which is then regener- Catalyst selectivity ↓ ated. Table 2. Effect of various operating parameters on methanol production Methanol Emission Reduction By-product methanol from HTS and LTS catalysts Overview 3000
2500 Several options exist for reducing methanol 2000 emissions including the following:
1500 • Reducing the amount that is formed. Methanol from non selective LTS • 1000 higher than that Removal or decomposition of the methanol from HTS
within the process. Methanol By-product 500 Methanol from HTS similar to • Removal or decomposition at the point of that from selective LTS 0 emission. 200 220°C 450°C MeOH Equilibrium Non selective LTS Selective LTS conventional HTS • Combination of the above schemes. Temperature Figure 6. By-product methanol from LTS and HTS catalysts
It is critical therefore that a catalyst supplier is • Increasing the number of back wash trays at able to provide an accurate prediction of the the absorber top increases the methanol con- level of methanol that both HTS and LTS cata- centration in the CO 2 vent. lysts will make during their service life. John- • Increasing in the number of back wash trays son Matthey uses models developed with the in the LP flash does not reduce the concen- Technical University of Delft to enable detailed tration of methanol in the CO 2 vent. and accurate predictions of by-product methanol levels to be made . With a CO 2 removal system based on hot potas- sium carbonate, the inlet temperature is about Reduction of Emission by Process Measures 90 °C (190 °F). There is less flexibility to re- duce the inlet temperature, as the solution tends As illustrated by Figure 5, on most plants, less to crystallize at low temperatures and it is nor- than half of the methanol will enter the CO 2 re- mally recommended that the loaded solution moval unit, and the proportion falls as the sepa- (absorber bottom) should not fall below 70 °C rator temperature is lowered. A typical inlet (160 °F). temperature to an absorber on an amine system is 70 to 75 °C (160 to 170 °F). A lower temper- Table 3 shows the typical distribution of the ature is possible but may require a slightly taller methanol through the ammonia process. absorber or more effective packing due to the reduction in the rate of absorption of CO at 2 Sepa- Stripped Steam HP vent lower temperatures. rator conden- to flash or urea
tem- sate process to fuel plant A temperature reduction to approximately 50 °C pera- stream stream stream stream (120 °F) is possible with little effort because it ture* 5 6 7 8 can be reached by cooling the process gas with 10 °C cooling water or demineralized water. To (50 °F) 2.2% 94.1% 0.2% 3.4% achieve lower temperatures chilling is required. 30 °C Both investment and operating cost increase if (85 °F) 2.1% 89.2% 0.6% 8.0% the gas is cooled or chilled (e.g. by a chiller 50 °C 2.1% 80.2% 1.2% 16.3% connected to the refrigeration unit of the plant), (120 °F) even when part of the cooling duty is provided 75 °C (165 °F) 2.1% 63.6% 2.3% 31.7% by heat exchange with the gas for re-heating it *) separator upstream of CO absorber again to the necessary inlet temperature of the 2 Table 3. Distribution of methanol through the absorber. Cooling the gas fed to the CO ab- 2 process in percent of methanol present down- sorber has only little impact on the rest of the stream of LT shift. Stream numbers refer to Fig- process, the only material change being an in- ure 2 crease in the load placed on the condensate stripper. Streams 6 and 7 lead to decomposition within
the process. Only stream 8 leads to a potential With a two-cycle activated MDEA system, it is gaseous emission. hardly possible to influence the methanol bal- ance by process variations: • A reduction in the HP flash pressure leads to Reduction by Removal at the Point of only marginal reduction in methanol con- Emission centration in the CO 2 vent. Methanol emissions can also be reduced by modifying the CO 2 removal system with the in- tent that more methanol is trapped in the con- densate downstream of the CO 2 coolers (stream 8 in Figure 2). To lower the methanol equilibrium concentration in the gas phase, the liquid phase must also have a low methanol concentration. This means that some of the methanol-containing condensate has to be re- moved from the process and replaced with pure water. The bleed stream of methanol-containing condensate can be sent to a stripper in order to avoid enrichment of the methanol in the con- densate. The bleed flow must be fairly large even for a small reduction in methanol emis- Figure 7. Catalysts for VOC reduction sions. On the other hand, the condensate con- tains traces of the amine employed in the CO 2 Although the technology is similar to that used removal. To avoid the loss of the amine one for hydrogen removal on the CO feed to urea would prefer to return as much as possible the 2 plants, higher temperatures are necessary (Fig- condensate to the CO removal process. 2 ure 8). Since venting usually takes place up-
stream of any compression, heat of compression At the usual low pressure in the regeneration cannot be used as a source of heat. The heat section methanol tends to stay in the vapour therefore has either to be provided externally or phase so that the lowest level that can be by internal combustion (eg: in a recuperative achieved is around 80 ppmv. Using chilled wa- system). ter for regenerator overhead cooling to 10 °C
(50 °F), instead of 50 °C (120 °F), reduces this concentration by only 20 ppmv.
Removal of Methanol by Catalytic Oxidation To achieve very low emission levels in the range of a few ppm, methanol must be removed from the CO 2 vent stream. Gaseous streams can be treated by catalytic oxidation using technolo- gy for VOC removal, which has already been proven in other industries. Catalytic oxidation requires a slight excess of oxygen over the quantity required by stoichiometry. This excess can conveniently be achieved by injecting air in- to the CO 2. Figure 8. VOC conversion as a function of tem- CH 3OH + 1½ O 2 → CO 2 + 2 H 2O perature
Monolithic catalysts coated with a very thin lay- Economic Considerations on er of platinum or palladium are used on vent streams so as to minimize pressure drop (Fig- Methanol Emissions ure 7). When the CO 2 is at pressure, catalyst Besides environmental aspects, there is also an pellets are often more economic. economic driver to reduce methanol formation by the process, at least to the extent that addi- The economics of a process which absorbs the tional costs are not prohibitive. Methanol for- methanol from the CO 2 stream downstream the mation in the CO shift consumes hydrogen regenerator in the CO 2 removal unit, depends on which would otherwise be used to make ammo- the following decisions: For a simple system, nia. In broad terms, every tonne of methanol there is just a bleed stream and a make-up by that is formed (and not recovered to the reform- fresh water. Possibly, the CO 2 of stream 8 is er feed via the process condensate stripper) re- cooled below ambient temperature to condense sults in a loss of 1.1 tonne of ammonia. This as much liquid as possible. A more complex loss in its own right can be a powerful incentive system would contain an additional absorber in- for installing a selective LTS catalyst such as stalled in stream no. 9 and a desorption for ex- KATALCO JM 83-3X. ample by sending the water to the already exist- ing condensate stripper. In addition, methanol is an oxygen containing component which must be removed from the If an “end-of-pipe” system has to be designed process gas before it is fed to the ammonia syn- for the maximum possible amount of CO 2 vent- thesis. This removal is normally achieved in the ed, it will need to operate at 5 to 6% of capacity methanation by reaction with hydrogen. most of the time (basis: 2,200 MTPD ammonia plant with a 3,500 MTPD urea plant) and will Stripping of process condensate is a very eco- experience 100% load only a few times per nomical way to reduce the methanol content in year. the process condensate to make it ready for re- use as make-up water to the demineralization The favored option might be different for a new unit, thus reducing raw water consumption. project than a retrofit. In the first case, one is free to design the plant right from the beginning for low emission figures. In the latter case, it Size of the Methanol Reduction can be difficult to accommodate changes in pro- Systems cess temperatures due to limitations in space for additional equipment or availability of utilities. If the ammonia plant is coupled to an adjacent urea plant, the majority of the methanol contain- ing CO 2 stream is used as feed to the urea plant. Comparison of Operating Costs In such a situation it does not lead to emissions during normal operation of the complex. Emission reductions inevitably require addition- al investment and lead to increased energy con- If the authorities allow emissions for the limited sumption. With the exception of the benefits amount of time of the year whilst the urea plant that can be obtained through the use of use of is not in operation, an “end-of-pipe” methanol selective LTS catalysts, there is little economic reduction system as described above might be incentive to reduce methanol emissions. None designed only for the quantity of CO 2 vented of the removal or abatement systems convert the during normal operation. This allowance can methanol into a valuable product, but all of lead to a significant cost advantage when com- them consume energy, as highlighted in Table 4. pared to the systems described above which are fully integrated into the ammonia process and which ensure a low methanol concentration for the full amount of CO 2. In this case they have to be designed for the full flowrate of process gas, with no reduction in size being possible.
one should not prematurely decide on one tech- Lower temperature at More condensate to be nical solution. It is recommended to look at the condensate separator stripped. process in close co-operation with the catalyst (cooling with CW) More cooling water vendor, CO 2 removal licensor, and overall pro- needed cess licensor and contractor, in order to obtain Lower temperature at More condensate to be the best overall solution. condensate separator stripped. (cooling with chill- More cooling water References ers) needed Chilling duty is need-  Landesanstalt für Umwelt, Messungen und ed (electric power / Naturschutz Baden-Württemberg, http:// steam) www.lubw.baden-wuerttemberg.de/servlet Catalytic methanol Methanol is combust- /is/18805/ removal at vent ed and CO 2 is formed  State of California, MIR values for car ex- by this process. haust gases, 2010 Air blower needed  EU Richtlinie 2002/3/EG für „Grenzwerte (electric power) zum Schutze der Gesundheit“ (zum 11. Table 4. Energy consumption implications of Juni 2010 abgelöst durch die neue Luft- methanol removal qualitätsrichtlinie 2008/50/EG)  United States Environmental Protection Summary and Conclusion Agency, National Ambient Air Quality Standard (NAAQS), 40 CFR part 50, 73 As with other VOC emissions, methanol emis- FR 16436, 27 March 2008], cited on sions from ammonia plants are considered to http://www.epa.gov/air/ contribute to ground-level ozone. Consequent- criteria.html ly, in many places of the world, targets and reg-  United States Environmental Protection ulations exist to reduce VOC emissions. Regu- Agency, New Source Review (NSR) Im- lators are starting to pay more attention to VOC provements: Supplemental Analysis of the emissions on ammonia plants and this trend is Environmental Impact of the 2002 Final likely to increase. NSR Improvement Rules, November 2002  Clean Air Act, Section 112 Several technical options exist to reduce these  “Development of a Predictive Kinetic emissions, including the following: Model for Methanol Formation over Low • Reducing methanol by-product formation in Temperature Shift Catalysts”, P V the LTS catalyst. Broadhurst, C Park and I Johnston, Pro- ceedings of Nitrogen 2006, Vienna, 12-15 • Modifying the CO 2 removal process. • End-of-pipe solution. March 2006.
Lowest VOC emissions can be achieved only by KATALCO is a trademark of the Johnson Mat- catalytic conversion at the point of emission. they group of companies. However, by combining it with the other op- tions, the amount of methanol to be converted in the emission stream can be reduced.
Since the VOC emissions are influenced by sev- eral process parameters and catalyst properties,