Method for fractionating a stream of cracked gas, using an intermediate recirculation stream, and related plant
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Solution Overview
Problem
Current processes for fractionating cracked gas streams from hydrocarbon pyrolysis installations are energy-intensive and costly, particularly for small ethylene production units, due to the need for external ethylene refrigeration cycles, which are expensive to install and operate, and can be challenging to start without initial ethylene supply.
Innovation Solution
A method that eliminates the external ethylene refrigeration cycle by using a double expansion of the fuel stream through dynamic expansion devices and internal heat exchangers to achieve low temperatures, allowing for ethylene recovery without external refrigerants, thereby reducing energy consumption and investment costs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an external ethylene refrigeration cycle is used to cool cracked gas to below -100°C, then high ethylene recovery rate (>99.5%) and high purity (>99.95%) are achieved, but energy consumption and investment costs significantly increase
Solution Approach 1:
The fuel stream, which would otherwise be a waste product, is used as the refrigerant to cool the cracked gas. The fuel stream undergoes expansion in dynamic expansion devices to provide the necessary cooling effect, eliminating the need for external ethylene refrigeration cycles and significantly reducing energy consumption while maintaining high ethylene recovery rates
Solution Approach 2:
The invention changes the operating parameters by using the fuel stream at different pressure levels (high pressure from crackers, intermediate pressure after first expansion, low pressure after second expansion) to create multiple thermal levels for efficient heat exchange, replacing the traditional single ethylene refrigeration cycle
2Reliability
If an external ethylene refrigeration cycle is installed, then high ethylene recovery rate is achieved, but investment costs increase significantly (up to 5% of unit equipment price)
Solution Approach 1:
The system uses its own fuel stream product as the refrigerant, eliminating the need for separate ethylene refrigeration equipment. The dynamic expansion devices utilize the pressure differential of the fuel stream itself to generate cooling, converting a waste stream into a useful refrigerant and avoiding significant capital investment in external refrigeration systems
Solution Approach 2:
The fuel stream serves dual purposes: it is both the product to be recovered and the refrigerant to cool the cracked gas. This multi-functionality eliminates the need for dedicated refrigeration equipment and reduces overall system complexity and investment costs
3Reliability
If an external ethylene refrigeration cycle is used, then high ethylene recovery rate is achieved, but the system becomes complex to start up and operate without initial ethylene supply
Solution Approach 1:
The fuel stream, which is continuously produced by the crackers, serves as the refrigerant, eliminating the need for initial ethylene charging and complex commissioning procedures. The system is self-sufficient from startup, using its own product to provide the necessary cooling function
Solution Approach 2:
The fuel stream is pre-cooled in heat exchangers before expansion, and the expansion process itself provides the necessary cooling effect. This preliminary preparation of the fuel stream ensures that the system can start up immediately without requiring external ethylene refrigeration equipment to be charged and commissioned first
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 enables high ethylene recovery rates (>99.5%) with lower energy consumption and reduced investment costs, making it suitable for small units by utilizing internal heat exchange to cool the cracked gas streams, thus eliminating the need for external refrigeration equipment.
Implementation Method 1
double expansion of the fuel stream in two successive dynamic expansion devices to provide the frigories necessary for cooling the cracked gas stream to low temperature
Implementation Method 2
The cracked gas stream is put into heat exchange relationship with ethylene circulating in an external refrigeration cycle
Data Source
Figure 1
Figure 2
AI summary
The invention relates to a method which includes the following steps: forming an expanded intermediate recirculation current (170) from a liquid (112, 128) obtained during an upstream cooling step and/or intermediate cooling step, upstream from the downstream cooling step; circulating the intermediate recirculation current (170) at least through an upstream heat exchanger (42) in order to cool an upstream stream of cracked gas (102); and injecting the reheated intermediate recirculation stream (170) back into a raw cracked gas (20) upstream from at least one compressor (36, 38) of a cooling and compression stage (24). The upstream, intermediate and downstream cooling steps are performed with no heat exchange, respectively, of an upstream stream of cracked gas (102), an intermediate stream of cracked gas (114) and a downstream stream of cracked gas (140) with an external cooling cycle, such as an ethylene cycle.