What is the quality of the recycled isobutane coming out of the alkylation deisobutanization column?
Thread Content
Everyone, please share your experiences regarding the separation efficiency in the isobutane removal tower. What is the purity level of the isobutane? If the purity is high, could you share your operational experiences and process parameters? Thank you! We only have 79–80% right now
The design purity of the top of the isobutane removal column is 85%, and the designed top pressure is 0.65 Mpa. On my end, the pressure at the top of the tower is 0.6 Ma, with a purity of 83%. One of the operational challenges in the isobutane tower is the top pressure, which is difficult to control due to changes in the temperature of the top water cooler; we have resolved this issue, and the key lies in controlling the temperature after water cooling.
If the temperature is difficult to control after water cooling, consider adding control valves to the circulating water circuit!
Your tower pressure is 0.6 – what is the backflow rate, and what should the bottom temperature be set at?
Being able to control it at 83% means that you must already be doing a great job with parameter adjustment
The higher the purity, the better. Purity depends not only on the operating procedures but also on design factors such as the gas velocity in the column, the number of trays in the column, its diameter, and the amount of cooling applied to the gas phase. Our isobutane removal column is over 100 meters tall, has 122 trays, and a diameter of six meters; it can handle more than 700 to 800 tons of material, with a purity of over 75%, even when dealing with isobutane from the alkylation process whose diameter is two meters. Simple operations only occur in specific environments.
Low pressure at the top of the tower increases the difficulty of separating isobutane, as a lower pressure results in a lower boiling point; to purify isobutane, some butane has to be sacrificed. The actual operating values are determined with reference to the parameters provided by the design agency, and laboratory data are used for monitoring operations. There are many factors that affect the pressure at the top of the tower, including the opening degree of the pressure control valve and its installation location, the amount of product taken from the reflux tank, the reflux rate, changes in the composition of the feed, and the bottom temperature, among others. Actual operations and laboratory test results are required as the primary reference data.
It’s the same with us; mainly, we’re dealing with an overwhelming volume of work. The purity of isobutane remains quite high at low loads; the isobutane column is designed to be too small
If a unit has already been designed and built, the purity of the top isobutane can be improved by adjusting the operating conditions, but there is a limit to this； Currently, alkylation units often experience a situation of excess capacity, which inevitably leads to overloading of the downstream systems. There are several suggestions for alkylation units: one is to reduce the alkyene to olefin ratio when the conversion rate is 100% and both the reaction and product quality remain normal, thereby reducing the content of isobutane in the reaction products and thus lowering the load on the towers. However, this method of adjustment is also limited; it can only provide relief. Another aspect is the replacement of high-efficiency separation trays; this is the key to making changes. All adjustments to the process parameters are based on the load on the equipment, and it’s definitely not feasible to overload the equipment.