Cryogenic Separator Pressure Differential for Higher Ammonia Capacity
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Solution Overview
Problem
Existing ammonia production plants face challenges in increasing production capacity without the need for costly retrofits, as they require high excess air and nitrogen removal, which is inefficient and energy-intensive.
Innovation Solution
The method involves increasing refrigeration in the coldbox by expanding the separator's bottom product to a lower pressure and then recompressing it, using the energy from the raw syngas expander to create a greater pressure differential, thereby reducing excess air usage and maintaining a stoichiometric hydrogen to nitrogen ratio of 3:1.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If gas expansion and cryogenic separation are used to remove excess nitrogen from syngas stream, then production capacity can be increased, but front-end pressure must be considerably increased requiring costly retrofits
Solution Approach 1:
The patent divides the nitrogen removal process into two distinct stages: (1) partial nitrogen removal through gas expansion and cryogenic separation to reduce excess nitrogen to a manageable level, and (2) final nitrogen removal through a membrane separator unit that operates at lower pressures. This segmentation allows the system to achieve high production capacity without requiring the entire process to operate at high pressure, thereby avoiding costly retrofits of existing plants.
Solution Approach 2:
The patent introduces a membrane separator unit as an intermediary component between the cryogenic separation stage and the ammonia synthesis loop. This membrane unit acts as a mediator that can selectively remove nitrogen at lower pressures, bridging the gap between the high-capacity cryogenic separation and the synthesis requirements without requiring extreme pressure increases throughout the entire system.
2Productivity
If external refrigeration is applied to produce syngas with low inert content, then synthesis capacity can be increased, but the process is limited to relatively narrow process parameters
Solution Approach 1:
The patent segments the refrigeration function into two parts: (1) internal refrigeration generated by an expander that compresses and then expands syngas to produce cold for the cryogenic separator, and (2) external refrigeration supplemented by a chiller when additional cooling capacity is needed. This segmentation allows the system to operate effectively across a wider range of process parameters, as the internal refrigeration adapts to varying syngas flows while the external chiller provides supplemental capacity when required.
Solution Approach 2:
The patent implements a self-service refrigeration system where an expander compresses syngas and then expands it to generate refrigeration cold internally for the cryogenic separator. This self-generated refrigeration reduces dependence on external refrigeration sources and allows the system to adapt to varying process conditions, thereby increasing both synthesis capacity and process parameter flexibility.
3Productivity
If stoichiometric amount of process air is used in secondary reformer, then hydrogen to nitrogen ratio can be maintained at 3:1, but production capacity cannot be increased
Solution Approach 1:
The patent extracts excess nitrogen from the syngas stream through a combination of gas expansion, cryogenic separation, and membrane separation. By removing the harmful excess nitrogen component, the system can tolerate and even utilize excess process air for increased production capacity, as the nitrogen removal systems prevent the excess nitrogen from interfering with the ammonia synthesis equilibrium.
Solution Approach 2:
The patent converts the previously harmful effect of excess nitrogen (which limited production capacity) into a beneficial situation by implementing nitrogen removal systems. Excess process air can now be used to increase production capacity, and the resulting excess nitrogen is simply removed by the cryogenic and membrane separation systems, effectively turning the limitation into an opportunity for increased productivity.
4Productivity
If nitrogen is removed from syngas stream to increase production capacity, then more excess air can be used, but energy consumption increases
Solution Approach 1:
The patent merges multiple nitrogen removal functions into an integrated system: the expander provides both compression and expansion for refrigeration, the cryogenic separator performs partial nitrogen removal and refrigeration, and the membrane separator completes the nitrogen removal. This merging of functions reduces overall energy consumption compared to separate, redundant systems, as each component contributes to multiple objectives (e.g., the expander provides both cooling and pressure management).
Solution Approach 2:
The expander in the system performs self-service by compressing syngas and then expanding it to generate refrigeration cold that is used by the cryogenic separator. This self-generated refrigeration reduces or eliminates the need for external refrigeration systems, thereby reducing energy consumption while maintaining the nitrogen removal capability necessary for increased production capacity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach reduces excess air rates, lowers energy requirements, and allows for wider process parameters, improving ammonia synthesis capacity while minimizing the need for retrofits and energy consumption.
Implementation Method 1
An expansion device is further coupled to the cryogenic separator and is configured to provide refrigeration cold to the cryogenic separator by expanding the bottom product from a first pressure to a second pressure
Implementation Method 2
a compression device that is fluidly coupled to the separator such that the compression device increases a pressure differential between the first and second pressure that is usable to increase the refrigeration cold
Data Source
AI summary
A synthesis plant (309A) includes a compression device (360A) that increases a pressure differential between the bottom product pressure of a cryogenic separator (340A) and the delivery pressure of the bottom product to a downstream plant component. Such increased pressure differential is employed to increase cooling in the separator (340A) to thereby significantly reduce the volume of excess air. In most preferred aspects, at least part of the energy required for the compression device is provided by expansion (320A) of the separator feed.