How to increase Ammonia Plant Capacity
In order to obtain the base data both for capital and operating data, material and energy balances have been prepared for each revamp concept using AspenPlus. These simulations include the process itself as well as its waste heat utilization including steam production and consumption.
Basics of Capacity Increase
Extending the capacity of a chemical process plant in general requires a larger throughput of the feedstock to be treated, i.e. larger flowrates in almost every part of the plant. The measures to be taken in a capacity revamp have to provide the following conditions for the larger flowrate.
Process Flow: If no measures are taken to increase cross sectional areas along the flowpath the larger flowrates inevitably increase pressure losses. Higher pressure drop means higher loads on compressors and drivers. A 30 % increase in flowrates associated with the envisaged capacity expansion would result in an almost 70 % higher pressure drop. Hence, it is worthwhile to look at options to mitigate the effect of higher flowrates on pressure drop.
Higher natural gas inlet pressure could compen-sate the additional pressure drop. However, the potential for pressure increase is rather limited as the equipment design pressures are usually only a little above their original operating pressure. With respect to pressure drop, the arrangement of additional equipment in parallel to existing equipment is obviously preferable compared to arrangement in series. However, parallel arrangements might require more elaborate piping arrangements.
Heat Transfer: The larger heat duties which have to be trans-ferred to or extracted from the process gas can be met via increasing the area of heat transfer surfaces, improved heat transfer coefficients or larger temperature differences. In certain areas a change in outlet temperatures caused by larger temperature differences can be tolerated, e.g. if the associated effects are only moderate drops in plant energy consumption.
Chemical Reactions: Every catalyst loses activity with time. Larger flowrates speed up this process. If no further measures are taken, reductions in service life of the catalyst charges are expected after the revamp implementation. On the other hand, a plant dating back from a time some decades ago has been designed for the catalysts available at that time. Since some catalysts have undergone significant improvements, the higher activity available today can compensate for the effect of higher throughput. In other cases, larger catalyst volume or smaller grain size might be the choice, both leading to higher pressure drop.
Separation of Undesired Species: In several process steps of an ammonia plant separation of undesired species from the process gas takes place. The most critical area is the CO2 removal. However, there are several other unit operations such as the desulphurization as well as the removal of condensate from process gas and finally the separation of the produced ammonia from the recycle gas in the ammonia synthesis.
All these process steps have been designed for certain volume flowrates. Once their capacities are exceeded, they usually respond with fast drops in performance, i.e. with effects like sharp rises in concentration of the species to be re-moved or with entrainment of liquid into the process gas.
Compared Process Concepts
As shown in the block flow diagram of the existing ammonia plant in Fig. 1, its process is fairly conventional.
The desulphurization has to cope with the higher gas throughput. The 30 % higher flow rate will reduce the lifetime of the zinc oxide bed by 30 %. Whether this is acceptable or not depends on the actually achieved lifetime and the turna-round strategy of the plant. If more catalyst vol-ume is required, the installation of a second par-allel vessel which allows catalyst replacement while the plant is kept in operation is fairly sim-ple.
The most critical and cost intensive part of the synthesis gas generation is the reforming section. The following three process alternatives have been selected for this comparison:
• Concept 1: Enlargement of existing primary / secondary reforming section,
• Concept 2: Secondary reformer operation with oxygen-enriched air,
• Concept 3: Autothermal Reformer (ATR) parallel to existing reforming section,
The main features of the concepts are discussed in detail below.
Concept 1: Enlargement of Existing Reforming Section
Fig. 2 presents a block flow diagram of the plant's synthesis gas generation section for this revamp concept. For every process unit of the reference plant it contains the modifications required for the envisaged capacity enlargement. The dashed lines indicate the unit operations which have to be modified or need an upgrade by additional equipment.
In the existing plant preheating of the feed for desulphurization and of the primary reformer feed (feed / steam mixture) is done in the waste heat section of the primary reformer. Modifica-tions of the respective heat exchanger coils are required to achieve the additional heat transfer.
Within the reforming section the split of reform-ing duties between primary and secondary re-former remains unchanged. Hence, both reform-ers have to transfer approximately 30 % larger heat duties to the process gas. In principle, the following options are available to increase the overall heat duty of the steam methane reformer (SMR):
• increased average heat flux
• larger heat transfer area by:
• larger reformer tube diameter
• longer reformer tubes
• additional reformer tubes
For the existing primary reformer of the reference plant a significant rise of the average heat flux has been ruled out. Also, new reformer tubes with a larger diameter are not an option since the reformer is already equipped with 5" tubes. Longer reformer tubes are in principle a viable path to increase the heat exchange surface of a new reformer. However, for an existing reformer this option is not practical as it would entail a complete rework of the furnace box including a relocation of the entire inlet mani-fold arrangement.
For these reasons, the addition of a suitable number of tubes was considered the only feasible option. The additional reformer tubes can either be placed in a separate oven box or integrated into the existing primary reformer. A separate box would have advantages compared to integration of the new tubes into the existing box both from the process side (lower pressure losses, lower flow velocities on the flue gas side) as well as from the construction side (easier tie-in of the revamp part, less shutdown time of the plant for tei-in). However, a completely separate new furnace box plus the associated waste heat section would be an expensive solution and render this revamp concept uncompetitive.
Enlargement of the existing oven box and inte-gration of the additional reformer tubes is a fairly complicated exercise. Nevertheless, it has been successfully carried out in the past.
To cope with the larger process gas flowrate and the associated larger heat duty, the secondary reformer has to be replaced. A new compressor must be installed parallel to the existing process air compressor to provide the additional amount of process air. An electric motor is selected as driver. The process air preheating coils in the waste heat section of the primary reformer have to be modified for the additional heat transfer. All heat exchangers in the process gas cooling train downstream of the secondary reformer have to be backed by additional parallel heat exchangers or replaced by single larger units.
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