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Ammonia Production

How to increase Ammonia Plant Capacity

| Author / Editor: Dr. Klaus Noelker / Anke Geipel-Kern

Concept 2: Secondary Reformer Operation with Oxygen-enriched Air

Fig. 3 is a block flow diagram of this process al-ternative. The basic approach of Concept 2 is to provide the additional heat duty for reforming the larger process gas flow via the secondary reformer (SR). Using ambient air to supply the required larger oxygen flow to the reactor would introduce a considerable amount of excess nitrogen into the process gas. This can be avoided via operation with oxygen-enriched air.

The oxygen required for air enrichment must ei-ther be imported from outside or provided by an air separation plant within battery limits. In both cases, its operating and capital cost may not be neglected in the economic analysis. Generating the oxygen within battery limits is associated with significant additional capital investment. However, it has the advantage that the purity of the oxygen can be adjusted to the requirements of the process and in general does not have to be very high. Also, integration of the power re-quirements of the air separation with the plant's steam system is possible. Hence, this alternative has been assumed for the cost comparison.

The primary reformer remains mostly unchanged. Also, the other modifications required in the reforming section are fairly similar to Concept 1.

The process gas of Concept 2 contains more CO and CO2 than the gas of Concept 1 due to the autothermal supply of the extra reforming duty. Hence, the loads for the CO shift converters and for the CO2 removal unit are somewhat higher compared to Concept 1.

Concept 3: ATR parallel to Existing Reforming Section

The basic approach of this alternative is an auto-thermal reformer in parallel to the existing re-forming section. Fig. 4 contains the block flow diagram of this process concept. In the same way as in Concept 2 it is operated by oxygen-enriched air in order not to exceed a reasonable amount of nitrogen in the process gas.

The autothermal reformer is a brick-lined vessel in which the two inlet streams of feed / steam mixture and oxygen are brought to reaction. It consists of a first reaction zone (or combustion zone) at the top, at the inlet of the two streams, and of a second catalyst-filled reaction zone in the bottom. Its principle design is shown in Fig. 5.

The existing reforming section remains essentially unchanged. Since the additional reforming is done entirely through autothermal reforming, this concept requires more oxygen than Concept 2. Thus the CO2 content in the process gas is even higher than in Concept 2, having an impact on the duty of the CO2 removal unit.

CO Shift and Methanation

The three different reforming concepts of course have influence on the conditions and capacity of the other process units, but in principle for all concepts identical solutions for the other process units are selected for all three reforming options. The difference in equipment size has been taken into account for in the cost evaluation.

As the CO shift reactors are sufficiently sized in the reference plant, no change is made with them and a slightly higher CO content is tolerated. Also the methanation reactor does not require changes.

The final gas cooling upstream of the CO2 removal requires an additional cooler.

Carbondixid Removal

As there are many different processes available for CO2 removal, there are also many different options for a capacity increase. Options for ca-pacity increase include:

• Change of packing material in the absorber, allowing for higher gas and liquid loads,

• Installation of an additional flash step in the desorption section of the solvent cycle for improving the solvent regeneration,

• Change of the activator in the absorption so-lution to a more effective one,

• Complete change of solvent type (e.g. from potassium carbonate to amine-based), in-volving also significant modifications at equipment.

Typically, the absorber is the bottleneck. Changes in the desorption section are easier to implement as they involve low-pressure equip-ment. If needed, additional regeneration heat can be provided by low pressure stream.

Syngas Compression and Ammonia Synthesis

Upgrading all items in the synthesis loop would be a task which would involve many modifications at the existing high-pressure piping and equipment. This would cause a high amount of modifications which would have to be executed in a small area in a short time while the plant is not in operation.

Therefore, an approach is selected where a whole new unit can be installed while the plant is in operation, and only a few tei-in points have to be connected during a shutdown of the plant. This is done by selecting the Uhde Dual-Pressure Synthesis [3]. It consists of a once-through (OT) ammonia synthesis which is added to the plant on an intermediate pressure level between synthesis gas generation and the synthesis loop as shown in Fig. 6.

For the envisaged relatively large capacity ex-pansion it can be assumed that the synthesis gas compressor is not able to cope with the signifi-cantly larger flowrate. Hence, an auxiliary com-pressor parallel to the first and second stage of the existing syngas compressor has been selected (not shown in Fig. 6), essentially taking the additional gas up to the intermediate pressure level.

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