Cryogenic Biogas Purification With External CO2 Solidification
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Solution Overview
Problem
Current biogas purification processes are inefficient in separating and liquefying methane and carbon dioxide with minimal methane loss and require multiple operations, especially when recovering cold used in CO2 solidification is not possible.
Innovation Solution
A combined facility and process involving cryogenic separation and liquefaction of biogas, utilizing a distillation column with a cold section and hot section, external containers for CO2 trapping, and a refrigeration circuit to recover and reuse cold, allowing for simultaneous separation and liquefaction of methane and carbon dioxide in a single operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If absorption, permeation or adsorption techniques are used for biogas purification, then CO2 removal is achieved, but additional modules are required and CO2 content remains too high for efficient liquefaction
Solution Approach 1:
The patent combines the purification function with the liquefaction function into a single integrated system. The cryogenic distillation column simultaneously removes CO2 from biogas and produces liquid biomethane, eliminating the need for separate purification modules followed by separate liquefaction equipment. This merging of functions directly addresses the contradiction by reducing device complexity while maintaining effective CO2 removal.
2Quantity of substance
If cryotrapping with reversible exchangers is used, then CO2 solidification and separation is achieved, but cold recovery is not possible and multiple exchangers are needed for continuous production
Solution Approach 1:
The patent implements a feedback mechanism where the cold stream produced during CO2 solidification in the distillation column is not discarded but fed back into the system. Specifically, the cold methane stream from the top of the distillation column is used to pre-cool incoming biogas or to drive the refrigeration cycle, thereby recovering the cold energy that would otherwise be lost. This feedback loop eliminates waste and improves overall system efficiency.
Solution Approach 2:
The patent utilizes phase transitions of CO2 (from gas to solid during separation, then solid to liquid/gas during regeneration) to drive the separation process. The phase change of CO2 releases or absorbs latent heat, which is harnessed to maintain the cryogenic temperatures needed for separation without requiring external energy input for cooling, thus recovering cold energy inherently through the phase transition mechanism.
3Quantity of substance
If multiple operations are used for separation and liquefaction, then purification is achieved, but methane loss increases and operational complexity rises
Solution Approach 1:
The patent merges separation and liquefaction into a single cryogenic distillation operation. The distillation column separates CO2 from methane based on their different volatilities at cryogenic temperatures, and the condensed overhead stream is directly collected as liquid biomethane. This single integrated operation minimizes the number of transfer steps and intermediate storage requirements, thereby reducing methane losses that would occur during multiple handling operations.
Solution Approach 2:
The patent establishes continuous operation where biogas is continuously fed into the distillation column, CO2 is continuously removed, and liquid biomethane is continuously produced. The system operates without interruption, with the cold stream and heat stream continuously exchanged within the column. This continuity eliminates idle periods and repeated start-stop cycles that would increase methane loss and operational complexity.
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 method enables efficient separation and liquefaction of biogas components with minimal methane loss and energy recovery, facilitating the transportation of biomethane and reducing operational costs by utilizing thermal integration and recycling of cold and methane.
Implementation Method 1
a cryogenic distillation step for separating a methane-rich stream and a carbon dioxide-rich stream from the biogas
Implementation Method 2
a means E02 for liquefying the methane produced at the top of the column
Implementation Method 3
at least two containers V04 A/B external to the distillation column, for placing in contact the liquid from the cold section and the vapour rising from the hot section and for trapping all the solid CO2
Implementation Method 4
a means M3 for expanding and heating the CO2-enriched liquid recovered at the bottom of the column and for recovering the cold from the CO2-enriched liquid
Data Source
AI summary
A combined plant for cryogenic separation and liquefaction of methane and carbon dioxide in a biogas stream, including a mixing means, a compressor, a first exchanger, a distillation column, a second exchanger, a separating means, an expanding means, and a separator vessel. Wherein, the mixing means is configured such that the recycle gas is the overhead vapour stream, and the first exchanger and the expanding means are combined.