May 27, 2006

Ammonia: Steps Ahead

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.
A. 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.
B. Hydrogen Separation
Lechatelier's Principle states that a reaction equilibrium can be shifted by applying external forces. This offers a means of removing products from the reaction mixture to increase the conversion per pass. In reforming, experiments have been performed up to 500 0C (932 0F) and 20 bar (294 psig) using a palladium membrane to remove the product hydrogen. These experiments have results in a significant increase in methane conversion as can be seen by the following case study at
C. Isobaric Manufacturing
The primary hurdle in the isobaric method of manufacturing ammonia is the poor conversion of methane at elevated pressure. The bottleneck is the maximum permissible temperature range due to metallurgical constraints in the reformer tubes. Synthesis pressures are no longer an issue with the development of the
Kellogg Advanced Ammonia Process (KAAP), which utilizes a ruthenium-based catalyst operating at 90-100 ata (1470 psia). Thus, if the methane conversion can be increased by hydrogen separation, the process can be operated at higher isobaric pressures.

D. Selectoxo Unit
The Selectoxo unit offers several advantages for grass root designs as well as for revamps.
Selectoxo (or selective catalytic oxidation) was developed by Engelhard for oxidizing carbon monoxide while not oxidizing hydrogen. The Selectoxo process provides good energy efficiency because it minimizes carbon moxide "slip" (only about 0.03%), improved process flexibility, and higher productivity in revamps when compared to other oxidation options. The Selectoxo unit is capable of increasing a plant's capacity by 1.5-2.0%.

E. Carbon Dioxide Removal Section
Chemical absorption in the isobaric manufacturing of ammonia can be unattractive because of the very high pressure (100 ata). Therefore, major changes in the existing carbon dioxide removal technologies may be necessary. Replacement technologies may include cryogenic condensation or pressure swing absorption (PSA).

Carbon dioxide separation through PSA is offered in the Low Cost Ammonia Process (LCA). PSA is scalable an may be more economical because of efficient carbon dioxide recovery at higher pressures. However, further development in this direction is essential for the recovery of high purity carbon dioxide as desired in urea production.

Carbon dioxide separation via condensation may also become more attractive due to an increased concentration of carbon dioxide which can be realized with successful hydrogen separation through membranes. This would allow the concentration of carbon dioxide to be increased by 18 to 36 mole percent. This would allow carbon dioxide concentrations in the gas to be reduced to 15% by chilling of the 100 ata fron end gases. This method also provides high pressure carbon dioxide for urea production which will reduce the power consumption in the carbon dioxide compressor of the urea plant substantially. The remaining product carbon dioxide gas can be recovered via PSA. A combined PSA and condensation process may solve the problem of carbon dioxide purity from the PSA process.
F. Pressure Swing Absorption (PSA) Unit
PSA represents an effective means of reducing the hydrogen loss in the methanator. In this process, the product hydrogen is separated out from the raw synthesis gas and then nitrogen is added. The other benefit is the production of pure synthesis gas, which saves on recycle compression and the elimination of the losses through the purge gas stream by way of eliminating the purge itself.
G. Cryogenic Separation Process
Cryogenic separation of inert gases from the raw synthesis gas is a commonly used approach. This unit is integrated into the purge gas recovery loop from the back to the front end of the ammonia unit. It serves to recover hydrogen from the purge stream and feed it back to the ammonia synthesis loop after recompression.
H. Synthesis Catalyst
Research work on low temperature and low pressure catalysts to produce ammonia at 20-40 kg/cm2g and 100 0C is being performed at Project and Development India Ltd. (PDIL) according to their in-house magazine. The catalyst being studied is based on cobalt and ruthenium metals and has exhibited few encouraging results.
I. Ammonia Separation
The removal of product ammonia is accomplished via mechanical refrigeration or absorption/distillation. The choice is made by examining the fixed and operating costs. Typically, refrigeration is more economical at synthesis pressures of 100 ata or greater. At lower pressures, absorption/distillation is usually favored. A comparison of these two methods is presented in Table 2.
Final Word
The developments discussed here such as isobaric manufacturing, the use of gas heat reformers, hydrogen separation, carbon dioxide removal technology, product ammonia separation, and high activity synthesis catalyst can result in a significant reduction in energy consumption when compared with traditional technology.
Global demand, increased competition, and ingenuity have fueled efforts to enhance existing ammonia technology. In an industry where change is often accepted reluctantly, these technological advancements will have to prove themselves worthy before receiving industry-wide attention.
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