Question

Explain the production of methanol by one-step, two-step and auto thermal reforming process in detail?
b) What are the types of reactors employed in the synthesis of methanol?

1000 words each
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Answer 1

Methanol Production Technology

All commercial methanol technologies feature three process sections and a utility section as listed below:

• Synthesis gas preparation (reforming)

• Methanol synthesis

• Methanol purification

• Utilities

In the design of a methanol plant the three process sections may be considered independently, and the technology may be selected and optimised separately for each section. The normal criteria for the selection of technology are capital cost and plant efficiency. The synthesis gas preparation and compression typically accounts for about 60% of the investment, and almost all energy is consumed in this process section. Therefore, the selection of reforming technology is of paramount importance, regardless of the site. Methanol synthesis gas is characterised by the stoichiometric ratio (H2 – CO2) / (CO + CO2), often referred to as the module M. A module of 2 defines a stoichiometric synthesis gas for formation of methanol. Other important properties of the synthesis gas are the CO to CO2 ratio and the concentration of inerts. A high CO to CO2 ratio will increase the reaction rate and the achievable per pass conversion. In addition, the formation of water will decrease, reducing the catalyst deactivation rate. High concentration of inerts will lower the partial pressure of the active reactants. Inerts in the methanol synthesis are typically methane, argon and nitrogen.

Synthesis Gas Preparation

Several reforming technologies are available for producing synthesis gas:

• One-step reforming with fired tubular reforming

• Two-step reforming

• Autothermal reforming (ATR)

In one-step reforming, the synthesis gas is produced by tubular steam reforming alone (without the use of oxygen). This concept was traditionally dominating. Today it is mainly considered for up to 2,500 MTPD plants and for cases where CO2 is contained in the natural gas or available at low cost from other sources. The synthesis gas produced by one-step reforming will typically contain a surplus of hydrogen of about 40%. This hydrogen is carried unreacted through the synthesis section only to be purged and used as reformer fuel. The addition of CO2 permits optimization of the synthesis gas composition for methanol production. CO2 constitutes a less expensive feedstock, and CO2 emission to the environment is reduced. The application of CO2 reforming results in a very energy efficient plant. The energy consumption is 5–10% less than that of a conventional plant . A 3,030 MTPD methanol plant based on CO2 reforming was started up in Iran in 2004.

The two-step reforming process features a combination of fired tubular reforming (primary reforming) followed by oxygen-fired adiabatic reforming (secondary reforming). A process flow diagram for a plant based on two-step reforming is shown in Figure 1. By combining the two reforming technologies, it is possible to adjust the synthesis gas to obtain the most suitable composition

The balance required to obtain a desired value of M depends on the natural gas composition. This is shown in Figure 2 for two feed gas compositions: Pure methane (full lines) and a relatively heavy natural gas with the overall composition CH3.6 (broken lines). The heavy gas requires more steam reforming and less oxygen compared to the requirements for lean gas. The same is true for gas containing CO2.The secondary reformer requires that the primary reformer is operated with a significant leakage of unconverted methane (methane slip). Typically 35 to 45% of the reforming reaction occurs in the tubular reformer, the rest in the oxygen-fired reformer. As a consequence the tubular reformer is operated at low S/C ratio, low temperature and high pressure. These conditions lead to a reduction in the transferred duty by about 60% and in the reformer tube weight by 75 to 80% compared to one-step reforming.

Autothermal reforming (ATR) features a stand-alone, oxygen-fired reformer. The autothermal reformer design features a burner, a combustion zone, and a catalyst bed in a refractory lined pressure vessel as shown in Figure 3

The burner provides mixing of the feed and the oxidant. In the combustion zone, the feed and oxygen react by sub-stoichiometric combustion in a turbulent diffusion flame. The catalyst bed brings the steam reforming and shift conversion reactions to equilibrium in the synthesis gas and destroys soot precursors, so that the operation of the ATR is soot-free. The catalyst loading is optimized with respect to activity and particle shape and size to ensure low pressure drop and compact reactor design. The synthesis gas produced by autothermal reforming is rich in carbon monoxide, resulting in high reactivity of the gas. The synthesis gas has a module of 1.7 to 1.8 and is thus deficient in hydrogen. The module must be adjusted to a value of about 2 before the synthesis gas is suitable for methanol production. The adjustment can be done either by removing carbon dioxide from the synthesis gas or by recovering hydrogen from the synthesis loop purge gas and recycling the recovered hydrogen to the synthesis gas [6]. When the adjustment is done by CO2 removal, a synthesis gas with very high CO/CO2 ratio is produced. This gas resembles the synthesis gas in methanol plants based on coal gasification. Several synthesis units based on gas produced from coal are in operation, this proves the feasibility of methanol synthesis from very aggressive synthesis gas. Adjustment by hydrogen recovery can be done either by a membrane or a PSA unit. Both concepts are well proven in the industry. The synthesis gas produced by this type of module adjustment is less aggressive and may be preferred for production of high purity methanol. Figure 4 shows a process flow diagram for a plant based on ATR with adjustment of the synthesis gas composition by hydrogen recovery in a membrane unit.

Methanol Synthesis and Purification In the methanol synthesis conversion of synthesis gas into raw methanol takes place. Raw methanol is a mixture of methanol, a small amount of water, dissolved gases, and traces of by-products. The methanol synthesis catalyst and process are highly selective. A selectivity of 99.9% is not uncommon. This is remarkable when it is considered that the by-products are thermodynamically more favoured than methanol. Typical byproducts include DME, higher alcohols, other oxygenates and minor amounts of acids and aldehydes.

The methanol synthesis is exothermic and the maximum conversion is obtained at low temperature and high pressure. Thermodynamics, reaction mechanism, kinetics, and catalyst properties are discussed in .

A challenge in the design of a methanol synthesis is to remove the heat of reaction efficiently and economically - i.e. at high temperature - and at the same time to equilibrate the synthesis reaction at low temperature, ensuring high conversion per pass.

Different designs of methanol synthesis reactors have been used:

• Quench reactor

• Adiabatic reactors in series

• Boiling water reactors (BWR)

A quench reactor consists of a number of adiabatic catalyst beds installed in series in one pressure shell. In practice, up to five catalyst beds have been used. The reactor feed is split into several fractions and distributed to the synthesis reactor between the individual catalyst beds. The quench reactor design is today considered obsolete and not suitable for large capacity plants.

A synthesis loop with adiabatic reactors normally comprises a number (2-4) of fixed bed reactors placed in series with cooling between the reactors. The cooling may be by preheat of high pressure boiler feed water, generation of medium pressure steam, and/or by preheat of feed to the first reactor.

The adiabatic reactor system features good economy of scale. Mechanical simplicity contributes to low investment cost. The design can be scaled up to single-line capacities of 10,000 MTPD or more.

The BWR is in principle a shell and tube heat exchanger with catalyst on the tube side. Cooling of the reactor is provided by circulating boiling water on the shell side. By controlling the pressure of the circulating boiling water the reaction temperature is controlled and optimised. The steam produced may be used as process steam, either direct or via a falling film saturator.

The isothermal nature of the BWR gives a high conversion compared to the amount of catalyst installed. However, to ensure a proper reaction rate the reactor will operate at intermediate temperatures - say between 240ºC and 260ºC - and consequently the recycle ratio may still be significant.

Complex mechanical design of the BWR results in relatively high investment cost and limits the maximum size of the reactors. Thus, for very large scale plants several boiling water reactors must be installed in parallel.

An adiabatic catalyst bed may be installed before the cooled part of the BWR either in a separate vessel or preferably on top of the upper tube sheet. One effect of the adiabatic catalyst bed is to rapidly increase the inlet temperature to the boiling water part. This ensures optimum use of this relatively expensive unit, as the tubes are now used only for removal of reaction heat, not for preheat of the feed gas. This is illustrated in Figure 5, which compares the operating lines in identical service for BWRs with and without adiabatic top layer.

The installation of the adiabatic top layer in the BWR reduces the total catalyst volume and the cost of the synthesis reactor by about 15-25%. The maximum capacity of one reactor may increase by about 20%.

A boiling water reactor with adiabatic top layer will be installed in a 1000 MTPD methanol plant in China.

The last section of the plant is purification of the raw methanol. The design of this unit depends on the desired end product. Grade AA methanol requires removal of essentially all water and byproducts while the requirements for fuel grade methanol are more relaxed. In all cases the purification can be handled by 1-3 columns, where the first is a stabiliser for removal of dissolved gases.