How to increase Ammonia Plant Capacity
As the revamp parts are fully integrated into the existing plant it is not possible to give individual figures for the energy consumption of the revamp parts alone.
Capital Cost (CAPEX)
The capital cost for the revamp is estimated by first estimating the cost of the main equipment and then reflecting other costs for the imple-mentation by the factor method.
That means, first the equipment cost is deter-mined for the all the three process concepts using the process data determined in the simulations. Then the cost of all other contributions (piping, instrumentation, electrical, civil, engi-neering, procurement, erection and commissioning) is added, assuming that these can always be expressed by multiplication of the “pure” equipment cost by a certain factor. These factors are known from experience.
Concepts 2 and 3 require oxygen-enriched air for operation. It shall be noted that for the sake of a fair comparison the oxygen stream is not treated as a readily available utility but that the investment and operating cost of the air separation unit is included in the data.
Although the cost determined by the factor method includes cost for construction, it does not include the cost of lost production due to downtime for the revamp implementation. This is an important part of the real cost of a capacity expansion.
Obviously, erection time for the revamp extends over several months. During this time, the existing plant can maintain in operation. A complete shutdown of production is needed only to carry out the final tei-ins and for commissioning of the new sections.
However, an analysis of the revamp concepts reveals significant differences between them with respect to the activities for their final implementation. As the concepts vary mainly in the reforming section, the main differences are related to this equipment:
• Concept 3 requires only tie-ins at relatively cold and therefore non-critical piping.
• Concept 2 requires the installation and tie-in of a new secondary reformer. Hence, it is assumed that it demands at least one additional week for this work.
• Concept 1 requires difficult structural work to enlarge the box of the existing steam reformer. Even with a considerable amount of preassembling it seems likely that this work would prolong the scheduled shutdown by four weeks compared to Concept 3.
In the ammonia synthesis, the installation of the once-through section is proposed. This offers the same advantage of short time requirement for the tie-ins which Concept 3 offers for the re-forming as described above
The additional shutdown periods for Concepts 1 and 2 have been turned into capital costs via the assumption of an ammonia sales price of 400 USD/t and energy cost of 4.0 USD/MMBTU. This leads to the individual implementation cost for each revamp concept listed in Table 3.
Table 3 shows that Concept 1 is the one with the lowest erection cost. However, due to its complicated nature, the lengthy shutdown period adds significant cost by loss of production to it.
In overall cost, Concept 3 is the most attractive one. The difference between the overall costs of all concepts is 7 %. Certainly, there is some degree of inaccuracy in the cost data, but as the same methods of estimation were used for all concepts, it is believed that the data represent the correct ranking between the concepts.
CAPEX / OPEX Comparison
Finally, an economic comparison of all revamp concepts is made, considering their CAPEX and OPEX.
The comparison is made by determination of the specific production cost, that means the cost of production per ton of ammonia. (Alternatively, also the net present value of all concepts could be determined.)
This requires converting the investment cost into an annuity by an economic model, consisting of interest rate and required payback period. As it is always the case, the result of the comparison can strongly depend on the economic model used.
To illustrate the influence of the model, different scenarios have been evaluated. The first one (low interest rate and long payback period) in principle favors capital intensive plants with low specific energy consumption The second one (high interest rate and short payback period) just favors the opposite, i.e. plants with compara-tively low investment and higher energy con-sumption. The scenarios are combined with two different specific energy costs. The scenario parameters are listed in Table 4.
For operating cost, it is assumed that they are sufficiently covered by the contributions for the streams shown in Table 2. The assumption is justified that the other components related to e.g. personnel or maintenance are equal for all concepts. As the aim is only to determine the ranking between the concepts, they can be neglected. The study is made for two different gas cost as shown in Table 4. The cost for steam and electricity is derived from the gas price.
The resulting specific production cost (CAPEX plus OPEX) are summarized for different com-binations of parameters in Table 5. It shall be mentioned again that these figures shall serve only to determine the ranking between the con-cepts. As the energy consumption of the plant used as the basis is not optimized, some figures appear high.
The ATR-based revamp Concept 3 with the lowest specific energy consumption (see Table 2) is also the one with the lowest investment cost. Hence it is not surprising that it also shows the lowest overall production costs.
For all concepts, CO2 is emitted by the following three sources:
• flue gas from reformer stack (ISBL)
• flue gas from steam generation boiler stack (OSBL)
• CO2 stream from CO2 removal unit. Practically all carbon in the natural gas is finally ending up as CO2 in one of the above streams.
Also to the electricity consumption a (virtual) CO2 emission can be assigned because electricity production (inside or outside the plant) is linked to CO2 emission. As the electricity consumption of all three process concepts is similar and fairly small compared to the natural gas consumption, for sake of simplicity, no CO2 emission equiva-lent is assigned to electricity consumption. Table 6 shows the thus determined CO2 emission per ton of ammonia.
While the first two of the above listed streams from the stacks contain CO2 in a concentration of only approx. 10 % at ambient pressure, the third one is more than 99.5 % pure CO2 at slightly elevated pressure. Only this CO2 can readily be used for production of urea fertilizer.
Considering the stoichiometry of urea formation from ammonia and CO2 and the available amounts of the reactants, Concepts 2 and 3 show an increase of about 6 % in maximum possible urea formation compared to Concept 1 (see Table 6).
If one would like to achieve the same urea pro-duction by the other two concepts, their amount of usable CO2 would have to be raised. Basically two options are available to do so:
• CO2 can be washed out of the flue gas of re-former or boiler by absorption and desorption. The result is an almost pure CO2 stream which could be added to the existing CO2 stream. This solution adds high investment cost and a little operating cost to the process.
• More gas can be fed through the plant up to and including the CO2 removal unit. This would produce more CO2 in the process which is consequently separated in the CO2 removal unit, thus increasing the CO2 export stream. The surplus synthesis gas downstream of the CO2 removal is fed to the reformer burners. This solution makes the reforming and CO2 removal sections of the plant a little larger, increasing investment a little, but adds significant operating cost to them. Both concepts have been applied already to re-vamps and new plants.
A detailed investigation has been carried out to assess the economic feasibility of three different concepts for a capacity increase by 30 % of an existing ammonia plant. The main difference be-tween the three concepts is in the reforming section.
The aim of the investigation is to establish an economic ranking between the revamp concepts. Concept 3 which is based on an autothermal reformer (ATR) turns out to be the most attractive solution. The other concepts (enlargement of the existing steam reformer resp. operation of the secondary reformer with oxygen-enriched air) have higher overall costs.
Responsible for this ranking are mainly the advantages of the ATR-based concept in overall capital costs compared with the other concepts. The process calculations show only moderate differences between the individual energy con-sumption figures.
The overall capital costs must include all costs associated with revamp implementation, including loss in production by longer plant shutdown. Especially Concept 1 (enlargement of the existing steam reformer) requires considerably more complicated implementation work which con-tributes to its higher overall cost.
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San Diego, California (2002)
 F. Kessler et al., First application of Uhde’s dual pressure ammonia process for revamping of the Duslo ammonia plant, Nitrogen + Syngas In-ternational Conference 2006, Vienna (2006)
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