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Ammonia: The Next Step

Nov 08 2010 01:20 PM | Guest in Process and Reactions ***--

Steam reforming of hydrocarbons for ammonia production was introduced in 1930. Since then, the technology has experienced revolutionary changes in its energy consumption patterns.

Ranging from an early level of 20 Gcal/tonne (79.4 MBtu/tonne) to about 7 Gcal/tonne (27.8 MBtu/tonne) in the last decade of the 20th century.

The energy intensive nature of the process is the key driving force for improving the technology and reducing the overall cost of manufacturing Looking further ahead, we'll review some potentially significant developments and concepts that may impact the manner in which ammonia is produced. Some of these manufacturing routes are being tested or employed at a few plants around the world, but have yet to be fully developed into commercial processes. We'll also review more traditional approaches to ammonia manufacturing along the way.

Reforming Section Developments

In the conventional process, steam reforming is carried out in a fired furnace of the side fired or top fired type. Both need large surface areas for uniform heat distribution along the length of the catalyst tubes.

Overall Layout of a Steam Reforming Plant for Ammonia Synthesis
Figure 1: Overall Layout of a Steam Reforming
Plant for Ammonia Synthesis (Click to Enlarge)

This process has several disadvantages. For example, it is a thermally inefficient process (about 90% including the convection zone) and there are mechanical and maintenance issues. The process is difficult to control and reforming plants require a large capital investment.

Gas Heated Reformers

Future technologies include the use of Gas Heated Reformers (GHR), which are tubular gas-gas exchangers. In the GHR, the secondary reformer outlet gases supply the reforming heat. Though it is not presently being used widely, GHR has certain advantages over fired furnaces. Table 1 shows a list of these advantages. Kellogg's Reforming Exchanger System is an example of GHR technology. Although GHR results in reduced energy consumption, a comprehensive energy conservation network should be established to maximize the benefits of a GHR system.

Table 1: Advantages of Gas Heated Reformers

Fired FurnaceGas Heated Reformers
Large volumesSmaller volumes
Larger surface area and heat lossReduced surface area and heat loss
Complex instrumentationSimplified instrumentation
High maintenance costsLow maintenance costs
Large convection zoneNo convection zone
Stack lossesNo stack losses
High fixed capital costsLow fixed capital costs
Reduced catalyst tube loss from high temperature and uneven heat distributionLonger tube life due to uniform heat distribution
Increased downtime required for shut downReduced downtime required for shut down
Well established processYet to gain wide acceptance

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