Pyrolysis Effluent Cooling via Segmented Heat Exchange
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Solution Overview
Problem
The steam cracking process for producing light olefins generates high quantities of tar, which fouls heat exchange equipment and renders conventional cooling methods ineffective, particularly in liquid crackers, leading to inefficiencies and increased costs due to the need for complex and expensive primary fractionators.
Innovation Solution
A method involving passing the gaseous effluent through primary and secondary heat exchangers to cool and condense tar, followed by separation in a knock-out drum, eliminating the need for a primary fractionator and minimizing fouling, using a sequence of heat exchangers and a tar knock-out drum to efficiently recover heat and separate tar from the olefinic product.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If conventional heat exchangers are used to cool pyrolysis effluent from liquid crackers, then heat recovery is achieved, but rapid fouling occurs due to tar accumulation and thermal degradation on heat exchanger surfaces
Solution Approach 1:
The cooling process is divided into multiple temperature zones using a series of heat exchangers arranged in sequence. The first heat exchanger operates above the hydrocarbon dew point where fouling is minimal, the second operates between dew point and tar condensation temperature, and the third operates at or below tar condensation temperature. This segmentation allows each heat exchanger to operate in a regime where fouling is manageable, resolving the contradiction between heat recovery and fouling resistance.
Solution Approach 2:
The invention changes the operating temperature parameters of each heat exchanger to match specific fouling regimes. By carefully controlling the temperature ranges in each heat exchanger (above dew point, between dew point and tar condensation, and at/below tar condensation), the system optimizes heat recovery while minimizing tar accumulation and degradation on surfaces.
2Temperature
If water quench is used to cool pyrolysis effluent from liquid crackers, then cooling is achieved, but stable oil/water emulsions form making separation difficult and increasing environmental disposal costs
Solution Approach 1:
The invention extracts and removes the heavy oils and tars from the effluent stream before the water quench step by using the third heat exchanger operating at or below tar condensation temperature. This pre-removal prevents these components from mixing with water to form stable emulsions, thereby simplifying the subsequent separation process and reducing environmental disposal complexity.
Solution Approach 2:
The system performs preliminary cooling and tar condensation in the third heat exchanger before the water quench step. By condensing and separating tar and heavy oils at this earlier stage, the invention prevents the formation of difficult-to-separate emulsions that would otherwise occur during water quench, thus simplifying downstream separation operations.
3Ease of manufacture
If primary fractionators are used to separate tar from pyrolysis gasoline, then separation is achieved, but capital and operating costs increase significantly
Solution Approach 1:
The invention merges the cooling and tar separation functions into the heat exchanger system itself. The third heat exchanger operates at or below tar condensation temperature, condensing tar and heavy oils directly from the effluent stream. This integrated approach eliminates the need for a separate primary fractionator, reducing both capital and operating costs while maintaining effective separation.
Solution Approach 2:
The heat exchanger system performs multiple functions: heat recovery (in the first and second heat exchangers) and tar condensation/separation (in the third heat exchanger). By making the heat exchanger system multi-functional, the invention eliminates the need for dedicated separation equipment like primary fractionators, thereby reducing process complexity and costs.
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 effectively maximizes heat recovery while preventing fouling, reduces capital and operating costs, and simplifies the cooling process by eliminating the need for a primary fractionator, minimizing coke accumulation, and reducing flammable liquid hydrocarbon inventory, thereby enhancing safety and operational efficiency.
Implementation Method 1
passing the gaseous effluent through at least one primary heat exchanger, thereby cooling the gaseous effluent and generating high pressure steam
Implementation Method 2
passing the gaseous effluent from step (a) through at least one secondary heat exchanger having a heat exchange surface maintained at a temperature such that part of the gaseous effluent condenses to form a liquid coating on said surface, thereby further cooling the remainder of the gaseous effluent
Implementation Method 3
the fouling tendency is high. In this regime, the heaviest components in the stream condense. These components are believed to be sticky and/or viscous, causing them to adhere to surfaces. Furthermore, once this material adheres to a surface, it is subject to thermal degradation that hardens it and makes it more difficult to remove
Implementation Method 4
separating the condensed tar and the gaseous effluent
Data Source
AI summary
A method is disclosed for treating the effluent from a hydrocarbon pyrolysis process unit to recover heat and remove tar therefrom. The method comprises passing the gaseous effluent to at least one primary heat exchanger, thereby cooling the gaseous effluent and generating high pressure steam. Thereafter, the gaseous effluent is passed through at least one secondary heat exchanger having a heat exchange surface maintained at a temperature such that part of the gaseous effluent condenses to form in situ a liquid coating on said surface, thereby further cooling the remainder of the gaseous effluent to a temperature at which tar, formed by the pyrolysis process, condenses. The condensed tar is then removed from the gaseous effluent in at least one knock-out drum.