Online Furnace Cleaning with Polarchem
(NOTE THAT DUE TO OUR CONFIDENTIALITY, WE CAN NOT DISCLOSE THE NAME OF THE STATION) Work year is 2012
EXECUTIVE SUMMARY: The ethylene cracking furnace convection area was substantially dirty and was not cleaned for almost 20 years. Polarchem L2K injections were done and compressed air was purged following the injections.
The actual operation lasted for 5 days in which only steam was run in the tubes. Very favorable positive trend was obtained at bridge and stack temperatures of the flue gas before and after the injections at comparable conditions. Some of the spot calculations indicated that there is about 2.8 % savings in the fuel usage for ton of crude hydrocarbon in the tubes. This should also reflect in the increase of end product as well. The draft values were also improved after Polarchem applications reaching down the negative levels.
EHYLENE CRACKING FURNACE ON LINE CLEANING:
OBJECTIVE: Online cleaning of the tube surfaces of the convection area of A Ethylene cracking Furnace without stopping the operation.
POLARCHEM L2K OPERATION: Polarchem technology is an on-line injection of magnesium based solution into the fireside of a furnace to oxidize the unburned carbon in the fouling. It also neutralizes the SO3 in the flue gas. As the unburned carbon gets oxidizes, the fouling becomes porous and slowly detaches from the tube surfaces. Entry points are always defined prior to the injections to have the most effect of the chemical, depending on the places of the fouling. The fouling in this case was mostly at the convection area. Therefore, the injections were made under the convection area as described in Table II.
The injections were done by using a 100 liter capacity spray tank which is connected to the lances via metal reinforced hose. All the equipment and connections were prepared one day before the injections started. Solution of L2K with water were prepared in a 1000 liter tank. For each injection, 100 liter of this solution was transferred into spray tank which later pressurized with a compressed air. The pressurized tank content was then injected into the furnace.
The injections were done very carefully and slowly so that L2K goes into the flue gas not on the tubes. The chemical reactions oxidized the unburned carbon. When this happened, the fouling started skinning out of the tube surfaces in a more flyable manner. Intensive compressed air injections were applied to get rid of these skin like dry, flyable fouling in increments as of third day of the operation.
During the air injections, we were able to follow, visually, the removal of dirt like butterfly. Also, during the Polarchem L2K injections, we could see the sparks of instant oxidation of unburned carbon as we watched the process. Before, we started injecting, the surface of the convection tubes were covered with layered fouling in such a way that one could not see the separations between them. As the injection and air purging continued, one could start observing these separations.
The data from the furnace on the fuel, charge, steam, bridge wall, convection and stack temperatures were obtained 6 days before the Polarchem applications to see the condition of the furnace.
The furnace, starting on the 26th February 2012, was switched to have only steam in the tubes rather than crude and steam mixture. Also, the steam level was gradually increased till the 28th February. Normally, we try to keep the charge, or steam and the fuel levels constant during our operation to follow the temperature profiles. In this situation, steam levels changed in the first two days. Still, we could follow the drop of the flue gas temperatures at the end of our operation.
The damper opening was 25% when Polarchem application started and prior to that as well. During the injections, we kept 25%, yet during the purging with compressed air, we fully opened the dampers to purgue the dirt which came out of the tubes . Some burners did not work during the trial.
Injections continued for 5 days. On the 5 th day, after very intensive air purging and some injections we closed the damper to its position before we started up (25%) and waited for about 45 minutes to obtain data when the steam was still inside the tubes.
After, we left, the station had to shut down and do some work on this furnace. They restarted up on the March 11, with Hydrocarbon crude and steam together inside the tubes. It takes about 2 sometimes 3 days to reach to stable operation conditions. We obtained the data afterwards from the station.
RESULTS AND DISCUSSIONS:
1-The furnace convection area was significantly dirty. The LOI in the dirt was reported to be about 14% most likely majority of it, due to unburned carbon some trapped. The color of the dirt was sort of brown to light brown, covering the surface of the convection area tubes in such a way, we could not clearly see the separation lines between the tubes. These tubes were not properly clean for almost 20 years. After our injections, these tubes surfaces can clearly be seen and became quite clean.
2-Radiation area vertical tubes were cleaner, yet of course, we could not see the narrow passage areas between the tubes. Since this area is exposed to very high temperatures all the time, usually, one does not see unburned carbon as a fouling.
3-After 2 days of L2K (15/85 ratio) injections, we were able to see drop in Bridge wall temperature. We could also visually observe the sparks of unburned carbon oxidation on the tube surfaces.
4-On the third day, the compressed air was blown onto 3 layers of convection tubes. After this operation, we could see a drop in the stack temperatures as well. We also observed the removal of the fouling, falling out of the tubes like butter fly flakes.
5-On the 4 th day, more injections were made. Followed by the air purging.
6-On the 5th day, we could see already drop of all the flue gas temperatures. We continued intensive air purging, and in the afternoon, all the windows were closed and damper was brought to 25% opening. After about 45 minutes of the completion of the work, new data was obtained. Figures 1 and 2, show the Bridge temperatures and Stack temperatures for the furnace when only steam was operated. This data is very important since the conditions were almost constant enough one can compare the level of cleaning before and after Polarchem application. Since there was absence of crude and only steam usage makes the comparison technically easier. Although, these were spot values, they were taken at almost similar conditions of the steam level and damper opening. The Stack temperature dropped about 18° C . Usually, this type of drop indicates the efficiency improvement of about 1% for steam uncirculated boilers and furnaces.
7-The data obtained after 11th March indicates the crude and steam passing inside the tubes. The Table I shows also temperature drops of 12° C in the stack gas for the comparable conditions, draft and crude levels when one compares with the data from Feb. 22-26th (See the blue numbers in the Table l). One also presumes the type of crude passing inside tubes were kept constant.
8-Draft changed from positive pressures to some negative pressures on March 14. Change of about 1.80 mmH2O in favorable direction. Normally, we always find the actual air usage is too high after Polarchem cleaning. Many times, the refinery people would tell us about the reading of oxygen index. Yet, this index reading will show still much higher than required amount of oxygen. In this cracking units, one can cut the air volume further, in the furnace after Polarchem cleaning, manually. One can observe that this reduction will not interfere with the burner functions to a certain degree.
9-We noticed that the steam to crude ratio was about 0.37-0.38 before Polarchem, this ratio decreased to 0.34.
10-If we just focus on the actual fuel to crude ratio and compare these values before and after Polarchem, we could obtain about 2.8% savings in the fuelgas usage for ton of crude process. Generally, we look at the each unit as a Separate Entity and do energy calculations for the individual furnace or boiler. Since we do not have the actual calorific values of the fuel gas, presumed to have the same values before and after the Polarchem application. We took actual ton usage of the fuelgas. The values which we looked at similar steam charges inside the tubes and took the values of the fuelgas and crude. Here again, we have made assumption of crude type to be the same before and after Polarchem application.
11-Also, one has to never forget that the entire POLARCHEM operation has been applied without shutting down and cooling the furnaces. There is no cooling or reheating, and no acid/base materials are used. Each shutdown , acid/base treatment and a mechanical cleaning give more damage to the tubes in the long run. POLARCHEM is a soft and dry technology.
1-We recommend these furnaces can be further treated with Polarchem system to clean it more. Since we obtained a very positive trend in a such a short time, couple of intensive injections can bring the damper to more negative values. Polarchem can be injected during the normal operation conditions as long as the furnace is under normal pressure or under vacuum .
2-We also recommend to decrease the total air even though their oxygen index might show relatively okay value. We have done this almost in all furnaces and results have been very satisfactory to the refinery people.
3-After couple of more injections, more tuning can be done in the system. Like, increase the crude amount, or decrease the fuel gas, decreasing the air. Some of these values are on automatic control system and might not allow these adjustments to be done. Yet, these could be done and readjusted to the new conditions in the setup of the automation.
4-We also recommend regular periodic injections to keep the tubes clean. After couple of more intensive injections and air purging, it will be helpful to make an injection once a week with small airpurge to keep the tubes clean. Refinery people will be able to determine when to do these injections.
In ethylene cracking furnaces, due to the deposits inside the tubes and some design parameters, or unscheduled shut downs, create fouling zones at the convection areas (fireside) due to uncompleted combustion of the fuel gas. Polarchem periodic injections will prevent forming of the fouling.
FIG. 1: Bridge temperature showed drop during the first two days, later increased is due to cleaned flakes out of the tubes were cumulated in convection areas. The last day, very intensive air purging was carried out in all levels of the convection area leading to the drop of all the flue gas temperatures. The last reading is received after Damper set up was the same as in beginning of the application.
Hours indicate from the start of Polarchem operation from 27th Feb. till March 3, 2012 18.45.
FIG. 2: Stack temperatures increased in the first three days. After the gradual air purging and the intensive air purging during the last day, temperatures dropped about 18C . (Conditions are same as described in Fig. 1)
FIG.3: The fuel gas consumption per ton of Crude was calculated before Polarchem between 22-26 th February. The values after Polarchem are obtained from March 13-14. Between, scale 8 to 10 in the X axis is the time where only steam ran in the tubes. The last two points indicate 2.8 % drop in F/C ratio.