Ammonia Cracking With PSA Off-Gas Heat Integration
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Solution Overview
Problem
Existing ammonia cracking processes are inefficient in terms of energy consumption, hydrogen recovery, and often require the combustion of fossil fuels, and they face challenges such as nitriding of reactor materials and the need to separate hydrogen and nitrogen for vehicle fueling.
Innovation Solution
A process involving the use of a heat transfer fluid to preheat and vaporize liquid ammonia, cracking it over catalyst-filled reactor tubes, and recovering hydrogen through a PSA unit, with heat integration from PSA off-gas compression to optimize energy use and reduce nitriding by using catalysts like ruthenium and nickel-based catalysts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If ammonia is cracked at higher temperatures to reduce ammonia slip, then conversion efficiency is improved, but energy consumption increases
Solution Approach 1:
The patent combines the cracking reactor and reformer into a single integrated unit with catalyst-filled tubes that perform both functions. The ammonia cracking and subsequent reforming occur in the same reactor tubes, allowing heat integration and eliminating the need for separate heating zones, thereby reducing overall energy consumption while maintaining high conversion efficiency
Solution Approach 2:
The endothermic cracking reaction absorbs heat from the exothermic reforming reaction occurring in the same reactor tubes. The system uses its own reaction heat to drive the cracking process, reducing the need for external furnace heating and lowering energy consumption
2Productivity
If a direct fired tube furnace is used for ammonia cracking, then cracking efficiency is improved, but nitriding of reactor materials occurs
Solution Approach 1:
The patent introduces a protective coating or barrier layer on the reactor tube surfaces that acts as an intermediary between the ammonia/nitrogen environment and the metal substrate. This protective layer prevents nitrogen from penetrating into and nitriding the reactor material while allowing the cracking reaction to proceed efficiently on the catalyst surfaces
Solution Approach 2:
The patent creates a controlled atmosphere within the reactor that minimizes nitrogen activity at the metal surface. By using catalyst-filled tubes and controlling the reaction environment, the system prevents direct contact between reactive nitrogen species and the reactor materials, thereby preventing nitriding while maintaining high cracking efficiency
3Device complexity
If hydrogen and nitrogen are not separated after cracking, then process complexity is reduced, but fuel cell efficiency decreases due to ammonia poisoning
Solution Approach 1:
The patent extracts and removes residual ammonia from the cracked gas stream using a scrubbing system or adsorption unit. By taking out the harmful ammonia component before the gas reaches the fuel cell, the system protects the fuel cell from poisoning while maintaining a relatively simple overall process configuration
Solution Approach 2:
The patent changes the temperature and pressure parameters of the cracked gas stream to optimize ammonia removal. By adjusting these parameters in the scrubbing or adsorption unit, the system efficiently removes ammonia while minimizing the complexity of the separation process and maintaining fuel cell compatibility
4Volume of stationary object
If nitrogen is compressed along with hydrogen for vehicle fueling, then storage volume is reduced, but power consumption increases and anode gas purge requirement increases
Solution Approach 1:
The patent extracts and removes nitrogen from the cracked gas stream before compression and storage. By taking out the inert nitrogen diluent, the system achieves high-purity hydrogen suitable for vehicle fueling applications, reducing the power consumption and gas purge requirements associated with compressing and managing nitrogen-hydrogen mixtures
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 process enhances energy efficiency, improves hydrogen recovery, reduces the need for fossil fuels, and minimizes nitriding of reactor materials by optimizing heat exchange and catalyst selection.
Implementation Method 1
pre-heating the pumped liquid ammonia by heat exchange against the heat transfer fluid
Implementation Method 2
vaporizing the pre-heated liquid ammonia to produce an ammonia gas
Implementation Method 3
feeding the heated ammonia gas to the catalyst-containing reactor tubes to cause cracking of ammonia and produce a cracked gas comprising hydrogen gas, nitrogen gas and residual ammonia gas
Implementation Method 4
recovering hydrogen gas from the cooled cracked gas in a pressure swing adsorption (PSA) unit
Implementation Method 5
a portion of the PSA off gas is compressed in a compression unit to produce compressed PSA off gas
Implementation Method 6
at least part of the duty required to warm the heat transfer fluid is provided by using the heat transfer fluid to cool the compression unit
Data Source
AI summary
Processes for cracking ammonia are improved by using heat generated in a compression unit that is used to compress PSA off gas being recycled to a PSA unit to pre-heat liquid ammonia prior to vaporization and cracking. The heat is transferred using a heat transfer fluid such as an aqueous solution comprising from about 50 wt. % to about 60 wt. % of a glycol, e.g., ethylene glycol or propylene glycol.