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Large Scale Methanol Production from Natural Gas

By Kim Aasberg-Petersen, Charlotte Stub Nielsen, Ib Dybkjær and Jens Perregaard

Large Scale Methanol Production from Natural Gas

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Abstract
The capacity of methanol plants is increasing to reduce investments, taking advantage of the economy of scale. The capacity of a world scale plant has increased from 2500 MTPD a decade ago toabout 5000 MTPD today. Even larger plants up to 10,000 MTPD or above are considered to further improve economics and to provide the feedstock for the Methanol-to-Olefin (MTO) process. A methanol plant with natural gas feed can be divided into three main sections. In the first part of the plant natural gas is converted into synthesis gas. The synthesis gas reacts to produce methanol in the secondsection, and methanol is purified to the desired purity in the tail-end of the plant. The capital cost of large scale methanol plants is substantial. The synthesis gas production including compression and oxygen production when required may account for 60% or more of the investment. In many plants today either tubular steam reforming or two-step reforming (tubular steam reforming followed byautothermal or oxygen blown secondary reforming) is used for the production of synthesis gas. However, stand-alone Autothermal Reforming (ATR) at low steam to carbon (S/C) ratio is the preferred technology for large scale plants by maximising the single line capacity and minimising the investment. ATR combines substoichiometric combustion and catalytic steam reforming in one compact refractory linedreactor to produce synthesis gas for production of more than 10,000 MTPD of methanol. The ATR operates at low S/C ratio, thus reducing the flow through the plant and minimising the investment. The ATR produces a synthesis gas well suited for production of both fuel grade and high purity methanol. The design of the methanol synthesis section is essential to ensure low investment. The optimal design andthe choice of operating parameters depend on the desired product specification. In many plants Boiling Water Reactors (BWR) are used. The use or incorporation of adiabatic reactors may be advantageous. The present paper describes the preferred technology for large scale production of methanol. The benefits of using ATR for synthesis gas production will be highlighted with emphasis on single linecapacity. A process economic evaluation will be outlined illustrating the advantages of ATR technology compared to two-step reforming at large plant capacities. The use of an adiabatic top layer in the BWR and the impact on reactor size and investment will be described. The paper also covers ongoing developments of synthesis gas and synthesis technology including reduction of the S/C-ratio in theATR to further increase single line capacity and reduce capital cost.

Introduction
The annual production of methanol exceeds 40 million tons and continues to grow by 4% per year. Methanol has traditionally been used as feed for production of a range of chemicals including acetic acid and formaldehyde. In recent years methanol has also been used for other markets such as production of DME(Di-methyl-ether) and olefins by the so-called methanol-to-olefins process (MTO) or as blendstock for motor fuels. The production of methanol from coal is increasing in locations where natural gas is not available or expensive such as in China. However, most methanol is produced from natural gas. Several new plants have been constructed in areas where natural gas is available and cheap such as in theMiddle East. There is little doubt that (cheap) natural gas will remain the predominant feed for methanol production for many years to come.

Large Scale Methanol Production from Natural Gas

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The capacity of methanol plants has increased significantly only during the last decade. In 1996 a world scale methanol plant with a capacity of 2500 MTPD was started up in Tjeldbergodden,...
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