How-to, Tips and Articles: Syngas Conversion to Methanol

Thursday, July 18, 2019

Syngas Conversion to Methanol | Coal Gasification Encyclopedia

Methanol is an important primary chemical product, used as a chemical feedstock for production of a range of important industrial chemicals, primarily acetic acid, formaldehyde, methyl methacrylate and methyl tertiary-butyl ether (MTBE). Methanol is also used directly as a fuel or fuel supplement. As fuel, methanol can be used to fire rapid-start utility peak-shaving combustion turbines; to substitute for or blend with gasoline to power vehicles; to be converted to gasoline via the ExxonMobil methanol-to-gasoline (MTG) process; or to be converted to dimethyl ether (DME) to power diesel engines.

Most methanol is made from syngas. Although the majority of methanol synthesis is based on natural gas as feedstock, coal-derived syngas is also used; coal/solid feedstocks are used to make 9% of the worldwide output of methanol (Gasification, Higman C., Van der Burgt M., 2003).

Process Chemistry

Catalytic conversion of hydrogen (H2) and carbon monoxide (CO) from coal-derived syngas into methanol can be done with conventional gas-phase processes, or with a liquid phase methanol (LPMEOH™) process developed by Air Products and Chemicals. The reactions of interest are:

2 H2 + CO → CH3OH

CO2 + 3 H2 → CH3OH + H2O

CO + H2O → CO2 + H2

All three reactions are highly exothermic. The conventional commercial gas-phase process carries out the conversion in fixed-bed reactors at high pressure. Depending on the catalyst supplier, the synthesis reaction is normally carried out at about 600 to 1,700 psig and 400 to 600°F. Substantial process gas recycle of H2 rich gas moderates the temperature rise across the adiabatic reactor. CO concentration at the reactor inlet is normally limited to about 10-to-15%, after dilution with recycled H2.

Catalyst systems used for methanol synthesis are typically mixtures of copper, zinc oxide, alumina and magnesia. Recent advances have also yielded a possible new catalyst composed of carbon, nitrogen, and platinum. This catalyst is based on an earlier catalyst created by Dr. Roy Periana of the Scripps Research Institute. This newer catalyst is a solid material that is suspended in sulfuric acid to aid in the catalysis. The material is easily recyclable as it can be filtered from the acid.

Of the three methanol synthesis reactions, the latter is the well-known water-gas-shift (WGS) reaction. Since the H2/CO ratio in syngas from today’s slagging gasifiers typically ranges from 0.3 to 1, extensive water gas shift is required to meet the stoichiometric H2/CO ratio of 2 for full conversion to methanol.

Examples of Technology and Plant:

Methanol production from syngas is a commercially demonstrated technology, using both natural gas and coal as feedstock. The current world-class methanol plants are typically in the order of 2,000 to 2,500 metric tons per day (t/d). Larger-scale (5,000 t/d) single train methanol process technologies are being offered. Major technology providers include:

Toyo Engineering Corporation
Lurgi Chemie GmbH
Foster Wheeler/Starchem

From 2011 to 2014, nearly 11 GWth syngas capacity for methanol production started up at several new coal or lignite gasification-based plants in China.

METHANOL PLANT PROCESS CONFIGURATIONS

Figure 1 shows a simplified block flow diagram (BFD) of a methanol (MeOH) plant based on coal feedstock. Syngas from the gasifier is cooled by generating high pressure (HP) steam in the high temperature (HT) gas cooling system before being water quenched and scrubbed to remove fine particulates. The scrubbed syngas then goes through a sour water gas shift (WGS) to adjust the H2-to-CO ratio to approximately two. Depending on the amount of CO needing to be shifted, supplemental steam injection to the sour WGS feed may be necessary. The syngas from sour WGS is then cooled in low temperature (LT) gas cooling before mercury removal, and followed with hydrogen sulfide (H2S) and carbon dioxide (CO2) removal in an acid gas removal (AGR) unit. Sweet syngas from AGR is sent to the MeOH synthesis block where it is highly compressed before going through the MeOH reactor to produce a crude MeOH product. The crude MeOH is then purified to meet product specifications via distillation. Purge from the MeOH reaction system is routed through a pressure swing absorption (PSA) unit to recover H2 for recycling back to the MeOH reactor. Net low pressure purge gas from the PSA is burned in low-Btu boilers to produce power and steam to meet in-plant power demand. Acid gas from the AGR is sent to the sulfur recovery unit (SRU) to recover sulfur (alternately sulfuric acid) as a byproduct. Since CO2 is removed and vented ahead of MeOH synthesis, carbon sequestration can be implemented by the addition of a CO2 drying and compression system.

Figure 1: Simplified Block Flow Diagram for Coal to MeOH

LPMEOH™ PROCESS

The liquid-phase methanol (MeOH) synthesis process known as the LPMEOH™ process from Air Products and Chemicals has great promise as an emerging methanol synthesis technology. It offers superior reaction temperature control and higher conversion. At the heart of the process is its bubble slurry reactor, shown in Figure 1. The process uses an inert mineral oil/powdered catalyst slurry as a reaction medium and heat sink. As the feed gas bubbles through the catalyst slurry forming MeOH, the mineral oil transfers the reaction heat to an internal tubular boiler where the heat is removed by generating steam. The ability to remove heat and the large oil slurry inventory allows the LPMEOH™ reactor to operate at isothermal (constant temperature) conditions by dampening large and rapid process changes, and when handling carbon monoxide (CO)-rich (in excess of 50%) syngas with wide compositional variations. Having the ability to handle CO-rich syngas, an upstream water-gas-shift (WGS) unit to increase the syngas H2/CO ratio is not needed, for partial MeOH production up to full utilization of feed H2. In this manner, the LPMEOH™ process can be designed for either baseload or IGCC co-production operation.

Co-producing MeOH and power in integrated gasification combined cycle (IGCC) applications using LPMEOH™ has potential as a very effective technology to convert part of the H2 and CO in the IGCC power plant syngas into MeOH via a once-through process, and using unconverted gases as fuel gas in a power cycle. Figure 2 shows a process flow diagram depicting the use of LPMEOH™ in this co-production mode; Figure 3 shows the same process in an overall plant context, block-flow diagram. Part or all of the treated syngas from gasification is routed through the once-through LPMEOH™ reactor to make MeOH. The syngas feed passes through a carbonyl guard bed, COS hydrolysis reactor and sulfur guard bed to remove trace contaminants and residual COS. The reactor gaseous effluent is cooled, entrained oil removed and cooled to condense-out crude MeOH product, before the high pressure off-gas is sent to and burned in the gas turbine for power generation. The crude MeOH product is separated and purified by distillation before being exported.

The amount of MeOH conversion through the LPMEOH™ reactor can be increased by internal recycle, carbon dioxide (CO2) removal, and/or by steam addition options. Feed compression and product expander options may also be added to increase system operating pressure for higher MeOH conversion.

Figure 3 shows a simplified block flow diagram of the co-production process.

DOE R&D advantage

The U.S. Department of Energy (DOE) helped with LPMEOH™ process development, first by housing its initial pilot plant testing at the DOE LaPorte Alternative Fuels Development facility in Houston, Texas, and later by funding the Demonstration Plant at Eastman Chemical Company's chemicals-from-coal complex in Kingsport, Tennessee. To date, the LPMEOH™ process has not been fully commercialized.