Open Discussion Forum Question 8
8. Indian Ethylene Producer (August 2006)
We have a gas cracker unit of capacity X MT per annum of Ethylene based on Ethane-Propane feed. We would like to share with you the problem of high differential pressure in water Quench Tower, we have been experiencing for more than last 2 -2 ½ years. We had inspected Quench Tower & oil water separator in the last shut down of July ‘05 and observed that in QT, all primary / secondary quench water spray nozzles & pre-saturator section spray nozzles / splash baffles were intact and were also found clean. All 18 trays in the tower were also intact & clean this time as tower was opened after gasoline wash prior to shutdown.
However, We found good quantity of tar deposit in oil water separator 1st compartment which was partially cleaned as complete cleaning was not possible due to material being very hard & not coming out even by water hydro jetting. We restarted our plant on July 22, 2005 & we were puzzled to note that when we did not find any problem in tower / its internals then why Quench tower pressure drop is increasing over a period of just 1 ½ months and once again reached 0.189 kg/cm2 from 0.09 kg/cm2 at start of run during the month of Sept 05.
In the month of Nov.05, we did exhaustive cleaning of complete circuit (Quench tower + all QW exchangers + connected piping etc) with Pyrolysis gasoline. Subsequently during operation, we had kept a close watch & control on various Quench tower parameters and were able to sustain Quench tower operation for around 08 months. But again we have been facing problem of high differential pressure for last 15 days subsequent to start up of the plant after short shutdown in July, 06. We have observed that the quantity of tar getting separated/ removed remains on lower side i.e. 10-15 kgs/day as against 100 kg/day used to be earlier and characteristic of tar is sometimes thick paste & sometimes powdery. (August 2006)
We are really slightly surprised. Ripple Trays have had good service in quench towers. This is only the third problem that I am aware of in Quench Towers that have ripple trays. Given the challenge of quench towers only to have three issues is good; One in Japan, the second in Singapore, and now your issue.
We recommend that trays are not utilized in the bottom of the first quench tower. From your description it shows that you have splash baffles in the bottom of the tower. We prefer splash baffles, followed by grids and then trays or structured packing can be above the grids. Trays should not be utilized in high fouling areas of quench towers. Trays have higher residence time than grids and lead to fouling.
The Singapore case was presented in a conference. They presented pictures that showed heavy fouling on the tray. It took several extra days to complete the maintenance work on the column.
The challenge of trays in a quench tower is that some of the products are almost too light to leave in the bottoms, and almost too heavy to be removed in the overhead. Therefore some of the hydrocarbons can have very high residence times, even if you have a middle draw. When you add a place for the hydrocarbons to pool, for example on a tray or a pan distributor, fouling can and will start.
Ripple trays are better than pan distributors and random packing, but we prefer baffle trays, followed by grids, then other devices can be utilized above the grids such as ripple trays, conventional cross flow trays, or structured packing. This configuration has less efficiency than other configurations, but will have a very good run length. In the upper section the choice between ripple trays and cross flow trays can be the vendors choice.