May 27, 2006

Ammonia: Counting Energy

The actual performance of any chemical process plant can be measured by only two terms, energy consumption level and on-stream/reliability factor. These are the two overall indicators of the plant performance out of which reliability factor also contributes to the energy consumption level. Hence, it becomes inevitable in an energy intensive unit like fertilizer industry, to keep a close look on this aspect of plant performance. This in turn will affect the productivity and profitability.

The expected annual energy consumption in ammonia production is of the order of 108 gigacalories in India alone that is equivalent to ~11 to 12000 MSm3 of Natural Gas or around 10 million tons of naphtha. Hence, it is most likely today to focus on proper analysis of bottlenecks & deficiencies in an existing plant while absolute analysis of technology at grass root level is also crucial with great care in its selection.

This study has been divided in four major parts, the overall loss structure, brief analysis of sectionwise losses & their remedies, losses through cooling water, and finally the role of catalysts in overall energy efficiency of the plant.

The Bares
It is only indicative to mention here the theoretical minimum requirement of energy, i.e. the thermodynamic requirement to produce one ton of ammonia is ~4.47 gigacalories. Table-1 shows the minimum requirement of various inputs per ton of ammonia.

Table-2 shows the energy inputs to the different steps of ammonia production, versus energy outflow from those steps. The figures depicted in this table are for a NG based ammonia complex with conventional steam reforming process of manufacturing.

The total estimated typical losses as shown in Table-2 are of the order of 4.0 Gcal/ton of ammonia, which is around 90% of theoretical requirement, i.e. we need almost the double of thermodynamic requirement in the current trends. This indicates the need for proper energy management, analysis, monitoring, & operation for an efficient ammonia producer.

Overall Loss Structure

The losses distribution diagram as shown in Figure-1 represents the major losses in ammonia plant. Another diagram (see Figure-2 and Table-3) shows losses in terms of process efficiency at different steps.

Obviously, the synthesis section & reforming section are the two most inefficient operations because of irreversibility of the concerned process steps involved.

Figures 3 & 4 (also see Table-4 & 5) for losses in synthesis & reforming sections are indicating the need of new developments in these sections, to avoid the major chunk of energy loss in cooling water.

The very first reason of ~71% of total energy loss going to the cooling water is that the low-level heat can not be recovered efficiently in absence of cooler streams. Moreover, conventional ammonia manufacturing technology does not have many margins to do so. Therefore, we are left with the option to have minimum process streams of low level energy that can be utilized for preheating cooler streams.

The second big contributor is the energy loss through surface condensers of steam turbines. For all three big compressors namely synthesis, process air & ammonia refrigeration compressor, generally steam turbines are being used in the present scenario. The condensing load of these three surface condensers is ~1.3 Gcal/ton which is ~32% of total energy losses. Therefore, it is essential now to consider some alternate drives for these compressors.

The third contributor to the higher energy consumption in an ammonia plant is the conversion efficiency of different catalytic reactors by virtue of which we have to play with bulk recycle streams. Developments towards increase in the conversion efficiency at reduced cost will make a difference in the near future.

Sectionwise Analysis

Reforming Section

a. In the conventional process, steam reforming is done in a fired furnace heater. This increases the heat loss from reformer surface to ambient through air convection. The energy loss is predicted as around ~0.16 Gcal/ton of ammonia (Table-5). This loss however, can not be reduced economically unless the technology eliminates the use of fired furnace type primary reformers e.g. the use of GHR & CAR, where this loss is minimal (See my article - Ammonia : Steps Ahead).

b. Massive amount of low level heat, from the stack of convection zone of primary reformer, is rejected into atmosphere. This loss of heat from flue gases is unavoidable because of dew point limitation of sulphur & nitrogen oxides. However, it should be lowest according to the fuel specification beyond which it is an additional loss. The same is the case with the stack of fired furnace steam super-heater.

The estimated typical design loss for a particular plant is 9.0 Gcal/hr that is equivalent to around 0.130 Gcal/ton of ammonia. Note that each 10°C reduction in stack temperature costs around ~0.012 Gcal/ton of energy. The stack losses can not be eliminated as long as fired furnace type heaters are there in existence. However, in an existing plant this can be reduced to some extent by using some energy analysis tool like pinch or exergy analysis and by considering the following options:

o Reducing the heat duty of primary reformer by shifting its load on secondary reformer up to certain optimized extent permitted by PGR unit in the back end.
o Explore the possibility of installation of new heat recovery coils.
o Explore the possibility of expanding surface area of existing coils.

The recent improvements eliminate the fired steam superheater in this section to minimize the losses due to inefficient combustion process. The high-pressure steam, generated in the ammonia plant, is now being superheated by the process gas itself downstream of the RG boiler.

c. Another Loss of ~0.2 Gcal/ton in this section is through air coolers provided to cool the treated overhead vapors of process condensate with the old system of LP stripper. This loss has been already eliminated in the newer versions of the ammonia plant by incorporating MP stripper in place of LP Stripper.

Shift Section
The water gas shift reaction is a desirable one for CO2 production, which is used as raw material for urea production. This section doesn't have any significant energy loss except through surface losses from the reactors.

CO2 Removal Section
This section of ammonia plant is the major energy consumer after cooling water system (See Table-2). This is because of the involvement of the thermally very inefficient process of distillation. This is mainly because of low-level heat dissipation at various stages of the process either in cooling water or in air coolers.

a. In this process, the solvent is recycled back after cooling for absorption. The cooler stream of GV for polishing section of the absorber is cooled in air coolers where huge amount of low level heat is dissipated in the atmosphere.

The estimated amount of energy loss is around 13.0 Gcal/hr or 0.19 Gcal/ton of ammonia. Part of this heat can be recovered by developing an appropriate scheme for an existing plant. It is estimated that around 5.0 Gcal/hr of energy can be recovered with a payback period of less than 6 months. This is equivalent to energy savings of around 0.07 Gcal/ton of ammonia. However, it can be eliminated also if a feed gas saturator is considered, which has a very good payback of ~1.5 years.

b. The second major loss in this section is through the cooling of product CO2 by cooling water. The estimated energy loss in the cooling water through CO2 is ~0.45 Gcal/ton of ammonia. This loss can not be eliminated because of low-level heat content of CO2 stream.

Synthesis Section
The major part of the losses, i.e. ~88% in this section is through the cooling water, which we are going to discuss in the next section. The remaining part is because of surface losses through the synthesis converter.

Losses Through Cooling Water
It is well known that ~71% of the total energy consumed in the conventional ammonia manufacturing process is lost through cooling water. That indicates a direct energy loss of ~2.9 Gcal/ton of ammonia. The break-up of this energy loss is given in Table-2.

The major contributor to this 2.9 Gcal/ton of energy loss is through the surface condensers of three major compressors, which have a cooling load of ~1.3 Gcal/ton of ammonia. Therefore, an alternate option is needed either for compression method or for reduced steam input or the minimal use of condensing type turbines.

The use of gas turbine, in place of steam turbine, for process air compressor in the new plants is an effective way to reduce these losses to some extent. However, this device can not be used for all compressors because of total steam balance in the complex and especially for the synthesis compressor. However, the surface condenser losses for the ammonia compressor can be compared based on the economic viability of vapor absorption refrigeration cycle vs. mechanical refrigeration cycle. If it becomes economically feasible then the total saving of ~0.4 Gcal/ton can be obtained including ~0.3 Gcal/ton from process air compressor on gas turbine.

Another contributor to energy loss in this category is the use of interstage coolers of compressors. This loss through inter coolers is of the order of ~0.64 Gcal/ton of ammonia out of which ~0.46 Gcal/ton is through the synthesis compressor alone, i.e. ~72% of all interstage coolers. This loss, however, will not be eliminated unless the compression device is changed, but it can be thought of to recover this energy by process streams e.g. condensate stream before treatment as feed preheating. Nevertheless, the overall system does also require the cooling water, as during start-up these process streams will not be available.

Role of Catalyst
The performance of catalysts employed in the plant is the major factor towards overall energy efficiency of any process plant and should be considered as and when new high conversion efficiency catalysts come to the market. The catalyst activity is the property by virtue of which we have to play with bulk recycle streams; consequently, it leads to recurring expenditure on energy cost. If full replacement of any catalyst is not permitted then we should consider an optimum blend of high activity catalyst & cheaper catalyst. The higher energy cost today has completely changed the scenario of options, which were not supposed to be considered in the 1980's or 1990's.

Finally, it is the manufacturer who has to decide his limit of investment & limit of his returns. One should be careful in analyzing isolated options because one option combined with the other may be more useful & economical than both separately. Moreover, at times the effect of one option on the other may be negative as both of them serve in the same direction particularly when the plant is operating at its peak level of capacity. Hence, this is the right time for each manufacturer to look into his process again, for identifying potential energy savers using robust tools of energy audit.

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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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