Jul-1997

Reforming exchanger system in large-scale methanol plants

By eliminating a tubular fired furnace, the Kellogg Reforming Exchanger System (KRES), used in the production of methanol, is proving to be profitable and energy efficient against conventional steam reforming

Robert V Schneider III and Girish Joshi
M W Kellogg Company (Now KBR Technology)

Article Summary

In most existing methanol production facilities, synthesis gas is furnished by a tubular fired-steam reformer that uses natural gas as a feedstock. Of all synthesis gas produced from both ammonia and methanol plants, well over 80 per cent is produced in a conventional reforming furnace. Steam reforming in a conventional sense, however, requires a considerable investment in both capital equipment and maintenance. Furthermore, this operation requires heat recovery in the form of steam, forcing the hand of the designer with respect to machinery driver selection.

The M W Kellogg Company has investigated alternatives to the conventional approach with a view towards developing a process for methanol production that would be less expensive to construct, easier to operate and more reliable over the course of long-term operation. In October 1994 the company commercialised its first synthesis gas generation facility based on the concept of heat exchange reforming. This plant was built for Ocelot Ammonia Company (now Pacific Ammonia, Inc) at Kitimat, British Columbia, Canada. It produces incremental synthesis gas for an existing purge gas-based ammonia synthesis loop. The new syngas process is called the Kellogg Reforming Exchanger System (KRES). Based on the successful application of this new technology to a plant that produces ammonia, an obvious extension of this knowhow is to apply such an approach to the production of methanol.

Reforming exchanger technology goes considerably beyond combination reforming in the modification of the methanol flowsheet. With the utilisation of KRES, a stoichiometric makeup gas for methanol synthesis is produced as with combination reforming, but the tubular fired furnace has been completely eliminated. Increasing the reforming pressure reduces the makeup gas compression requirement. In the case where a gas turbine is included in the process design, a substantial improvement in energy efficiency is realised, since the usual limitation of 33–34 per cent steam cycle efficiency has been removed.

A schematic process flowsheet describing the KRES-based, 1500mtpd methanol plant is shown on Figure 1. Natural gas feedstock having the following composition enters the battery limits of the plant and is compressed in a single-stage, motor-driven centrifugal compressor to 45kg/cm2 gauge.

Typical composition for natural gas feed:
Component      Mol%
CH4                 94.5
C2H6               4.0
C3+                 0.2
N2                   1.1
CO2                0.2
Total               100.0

Sulphur 25ppmv (design)
Following compression, a small recycle of synthesis gas is added to the feed for hydrogenation of any organic sulphur compounds that may be present. The feed is then hydrotreated and desulphurised over a conventional CoMo/ZnO system operated at 350°C. The dry-feed preheat duty in this case will be provided by a coil in the convection duct of the gas turbine generator. Alternatively, this preheat can be provided by a stand-alone fired heater.

The desulphurised hydrocarbon feed is mixed with sufficient MP 45kg/cm2(g) steam so that an inlet steam to carbon (S:C) ratio of 3.4 is achieved. This mixed feedstream then passes on to the shell side of the feed/effluent exchanger, where reforming exchanger system effluent preheats the mixed feed stream to 621°C (1150°F). The mixed feed stream then passes in parallel to both the reforming exchanger and autothermal reformer in a ratio of approximately 25 per cent and 75 per cent, respectively. This ratio was chosen as appropriate to balance autothermal heat release with the reforming exchanger duty requirements and produce a final synthesis gas that was approximately stoichiometric in composition, while staying within constrained temperature limits in both the autothermal reformer combustion zone and the autothermal reformer final effluent (Figure 2).

To achieve this balance, it is necessary to promote partial combustion of the feed in the autothermal reformer through the addition of pure oxygen. To make oxygen available, it is necessary to provide an air separation plant. Alternatively, “over-the-fence” oxygen can be used in plant locations where the oxygen can be supplied at the plant battery limit.

Calculations indicate that about 0.53 tons of oxygen per ton of refined product is required, resulting in the need for an air separation facility capable of producing 800mtpd of gaseous oxygen. Product oxygen from a cryogenic separation facility will be available at just over atmospheric pressure, thus necessitating a compression step prior to O2 introduction into the process.

A stand-alone oxygen compressor system could be furnished as a subcontract by a specialist supplier, or it can be quoted as part of the turnkey package from the air separation plant supplier. In any event, the oxygen compressor is a special piece of equipment envisioned as a five-stage, two-case, motor-driven machine that will consume between 2MW and 3MW of power.

Compressed oxygen at 38kg/cm2(g) and approximately 100°C is mixed with a small quantity of steam and preheated to 260°C (500°F) in a shell and tube steam heater. The preheat temperature of 260°C is currently the maximum preheat temperature considered safe for an oxygen/steam mixture.

The preheated oxygen is injected into the top of the autothermal reformer through a special proprietary injector designed by Kellogg. This unit is typical of those designs used successfully in previous methanol and hydrogen plants designed by the company in which oxygen injection was utilised. Autothermal reformer feed (about 75 per cent of the total feed flow) and oxygen combine in the upper zone of the autothermal reformer, where partial combustion will result.