(Daily Question) Coal-to-oil and olefins forum, 2018.1.16
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3.jpg Participation: Prize of 5 Wealth； Correct answer: Reward of 10 wealth points ； Note: This post is valid for 48 hours. Question: What is the purpose of installing high and low pressure depropanization columns in an (DMTO) olefin separation unit? The feed to the depropanization columns contains large amounts of C4 and higher hydrocarbons with unsaturated bonds; at higher temperatures, these compounds tend to form polymers that cause scaling in the reboilers and lead to blockages in the tower trays. To address the issue of scaling that blocks equipment, it is necessary to reduce the pressure in the depropanization tower in order to lower the operating temperature of the system. However, considering operation at low pressure, the tower top condensation temperature also decreases accordingly, resulting in an increased demand for cooling capacity. Taking into account both the polymerization issue and the cooling capacity requirement, most olefin separation units have adopted a high-pressure and low-pressure propane removal process, which not only reduces the amount of refrigerant used but also addresses the polymerization problem. Note: The answer will be announced automatically in two days!
The lower part of the depropanization column contains heavier fractions above C4; if only one column is used, the temperature at the bottom of the column will be very high, causing these heavier fractions to polymerize and form coke. Two towers are used; the material from the bottom of the high-pressure tower enters the low-pressure tower. On one hand, this lowers the temperature at the bottom of the high-pressure tower, and on the other hand, the presence of components with molecular weights higher than C4 in the high-pressure tower is reduced, which helps to prevent coking. Although the carbon tetrahydrocarbon content in the low-pressure column is high, on the one hand the pressure is lower, and on the other hand the temperature in the reactor is also lower than in the single-column setup, which in turn reduces coking.
Reduce the coking problem present in single-tower systems
Reduce the coking problem present in single-tower systems
C3 and C4 are separated using a high-pressure column and a low-pressure column cycle, and then they are separated one by one from light to heavy in sequence. Reduce energy consumption.
Since the propylene tower is used to separate propylene from propane, and the relative volatility of propylene and propane is very close, a large number of trays are required for this separation; moreover, the purity of propylene intended for polymerization needs to be 99.6%. To separate propylene with a purity of 99.6% using a single tower, approximately 240 tray levels would be required, resulting in a tower height of around 150 meters – which is clearly unreasonable. To reduce the tower height, a two-tower system is used (even with this setup, the height of the second propylene tower still reaches 90 meters).
C3 and C4 are separated using a high-pressure column and a low-pressure column cycle, and then they are separated one by one from light to heavy in sequence. Reduce energy consumption.
The lower part of the depropanization column contains heavier fractions above C4; if only one column is used, the temperature at the bottom of the column will be very high, causing these heavier fractions to polymerize and form coke. Two towers are used; the material from the bottom of the high-pressure tower enters the low-pressure tower. On one hand, this lowers the temperature at the bottom of the high-pressure tower, and on the other hand, the presence of components with molecular weights higher than C4 in the high-pressure tower is reduced, which helps to prevent coking. Although the carbon tetrahydrocarbon content in the low-pressure column is high, on the one hand the pressure is lower, and on the other hand the temperature in the reactor is also lower than in the single-column setup, which in turn reduces coking.
To ensure that the C4 composition in the net top product meets the allowable values specified for the final C3 product