KINETIC STUDY OF WAX DEPOSITION USING COLD FINGER APPARATUS

Transcription

1 KINETIC STUDY OF WAX DEPOSITION USING COLD FINGER APPARATUS Tunya Ketjuntiwa a, Luqman Mahir*,b, Claudio Vilas Boas Favero**,b,H. Scott Fogler***,b, Pomthong Malakul a,c a The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok, Thailand b Department of Chemical Engineering, University of Michigan, Michigan, USA c Center of Excellence on Petrochemical and Materials Technology, Bangkok, Thailand Keywords : Incipient layer, Cold finger, Deposit ABSTRACT The formation of the incipient layer of wax deposit when waxy oil is exposed to a cold surface is studied. Using a cold-finger apparatus, the evolution of deposit thickness is monitored with a video camera, allowing one to resolve the deposit layer formation from inception up to long times where deposit thickness no longer times where deposit thickness no longer grows. Characterization of wax deposits show that incipient layer has identical composition to waxy oil, indicating that deposit is not formed by a diffusion process. Modelling shows deposit formation at the investigated conditions cannot be explained by a diffusion process. *luqmanh@umich.edu,**cfavero@umich.edu, ***sfogler@umich.edu INTRODUCTION One of the most important problems in oil and gas industry is wax deposition. Wax deposition can happen in various locations during operation for instance production, transportation and storage. Crude oil is a mixture of waxes, aromatics, naphtenes, asphaltenes and resins (Huang et.al, 2011). Moreover, waxy component in crude oil referred to carbon numbers greater than 20 (Lee, 2008). It represents a group of n-alkanes which known as n-paraffins. One of very specific problem of waxy crude oil is wax deposition during transportation. At the reservoir condition the temperature is relatively high, waxes are dissolved in crude oil. However, during oil transportation from reservoir to any processing facilities, its temperature reduced continuously. Moreover, when the temperature fall below the appearance temperature (WAT) (Berne-Allen&Work, 1938), waxy components will start to precipitate out of the crude oil and from solid layer. This solid waxy layer can decrease ability of crude oil flowing through pipeline, causing reduction in oil production. In addition, it causes pressure drop increased and can there will be a safety issue. There are some methods that have been used, in order to minimize this problem. Firstly, pipeline insulation but, this solution could increase a lot of production cost. Secondly, is the most common technique that applied for long distance called pigging, the inspection gauge or pig is sent in the pipeline to scrape off the wax deposits on the wall (Huang et.al, 2011). Nevertheless, production needs to stop while doing pigging. Therefore, this method also can increase the production cost. Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 1

2 This study is performed by using the cold finger apparatus, as cold finger is immerged into the reservoir or jacket, there is a layer of deposit form immediately. There could be because of another step of wax deposition apart from molecular diffusion called gelation involved in the wax mechanism. Gelation occurred by effect of temperature gradient between bulk hot fluid in the reservoir and cold surface of cold finger. If this spectacle can be explained by this experiment, it could change the wax mechanism that has been suggested before. The purpose of this study is to determine the process of wax deposition when gelation dominates the process. The technique of video recording is introduced while experiment is running, in order to see the changed since the beginning of experiment. The thickness could be determined from the video. Moreover, deposit sample is collected to measure the weight and analyzed by the HTGC to find the carbon distribution of each sample. EXPERIMENTAL Figure 1, shows a schematic diagram of a cold finger in the University of Michigan laboratory. The deposition cell includes a stainless steel cylinder which is a cold finger probe. Cold finger is maintained to be at low temperature, then wax will deposit on its surface (Huang, Zheng and Fogler 2015). During the wax deposition, a cold finger probe is placed inside the jacket. The stir bar is put in the container or jacket, in order to introduce shear force to the system. Shear level can be adjusted by the speed button on the heat plate. The circulation system includes a heating system and cooling system. Reservoir or jacket needs high temperature, more than WAT. It provided energy from the heater. In another hand, cold finger probe should set to the temperature below WAT, which is adjusted by Chiller. The following steps will explain more in detail how to operate the experiment using cold finger apparatus. Cold Finger Chill Heat Jacke Heat Figure 1: Cold Finger Schematic Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 2

3 A. Prepare oil reservoir Firstly, tare the watch dish with a weighing paper on top by the scale. Then weight out 5% of wax which is 9.923g and place the wax in the reservoir jacket. Follow by, measure out the 220 ml. of oil with a measuring cylinder and pour into the reservoir. B. Wax Deposition Cold Finger Apparatus After sample has prepares, fixate the jacket with the chain. Then connect the two tubes between reservoir and chiller. Before open the heater, make sure that that water in chiller tank is at an appropriate level (between high and low). Turn on the heater and set the temperature to be 60 C and turn on the magnetic stir at level 3 then wait until wax is completely dissolved. For the cold finger side, turn on the cold finger water bath and set the cold finger temperature to desire temperature. After entirely wax dissolved, adjust the temperature to the desire reservoir temperature. Wait until reservoir and cold finger temperature reaches the set point. Start flow the cold fluid through cold finger to start the deposition experiment. Wait until the time achieves to the desired time for wax deposition. After the time is reached, take off the cold finger out from the reservoir then wash the wax deposition on cold finger with MEK. Tare the container for collecting deposit on the scale. Collect the cold finger deposit in prepared container and weight the amount of wax deposition on cold finger by scale. C. Cleaning Step After the experiment is done, take off two tubes between the chiller and the cooling jacket and take off the chain that around the jacket. Empty the water in the jacket and take off the stir bar from jacket by using a plier. Pour the model oil from the reservoir into the waste container. Wash the reservoir and a stir bar with detergent and water, dry the reservoir. Wash the cold finger with Toluene and Acetone. D. Characterization of wax content by Gas Chromatography The wax content in the deposit was determined using a Hewlett-Packard 6890 GC equipped. The initial temperature is at 50 C and increased to 350 C at a rate of 15 C/min and then maintained at 350 C for 20 minutes. The sample is prepared by diluting three drops with toluene until it filling up to 4 ml solution. Then inject 1.8 microliter of sample into HTGC. HTGC will give the carbon distribution of deposit. RESULTS AND DISCUSSION A. Effect of rpm changes The condition is set at Temperature of jacket, Tjac equals to 45 ºC. Temperature of cold finger, Tcf equals to 5ºC. Deposition time, t equals to 2hr. Rotational speed of stir bar, rpm is changed from 0 rpm to 135 rpm and 240 rpm. Video was recording for the whole experiment in order to calculate the thickness. It can be seen from figure 2 on the right hand side, the thickness of deposit form immediately since cold finger dipped in the jacket at any rpm. Moreover, it comes to the hypothesis that, it could be another process during wax deposition which could be gelation. Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 3

4 (a) Figure 2: Thickness VS Time at different rpm (a) from time 0 to 7200 sec (b) from time 0 to 140 sec B. Effect of T changes Condition for this experiment is set at Tjac equals to 45 ºC. rpm equals to 135 and t equals to 2hr. Tcf is changed from 5ºC to 25ºC and 35ºC. From figure 3, it can be concluded that as the temperature of cold finger increases or T is reduces, the final thickness of deposit decreases. The red curve, T= 35ºC, shows the promising reduction of thickness. (b) Figure 3: Thickness VS time at different T Figure 4, perform at Tjac equals to 45 ºC, rpm equals to 135 and t equals to 5hr. Tcf is changed from 5ºC to 35ºC. At T=10ºC, the increase of deposit mass is slower than T=40ºC. It indicate that Gelation can be dominant when T is huge. (a) (b) Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 4

5 Figure 4: (a) Deposit Weight VS time at different T (b) Mass Rate VS time C. Carbon Distribution Figure 5 represents carbon distribution at the condition of Tjac equals to 45 ºC and rpm equals to 135. Tcf are 5ºC and 35ºC. The carbon number distribution when T=40ºC for 1 min run can be compared with the model oil 5%Wax. The equality composition can be associated to gelation process. When T = 10 C for 5 hr run, the carbon distribution shifts to the right as it contains more high carbon number. The differences in carbon composition of deposit is due to the mass transfer process. Figure 5: Carbon Distribution D. Evolution of thickness with cooling rate Experiment is performed at the base case of Tjac equals to 45 ºC, rpm equals to 135, t equals to 3hr and Tcf at 5ºC. However, Tcf is decreased manually by 1 ºC from 45 ºC to 5 ºC. The cooing rate was around 0.38 ºC/min. Figure 6 represents that thickness increases almost linearly, the same trend as temperature of cold finger. It can be explained that temperature is the most important factor in gelation process, which is a driving force of the wax mechanism. Figure 6: Thickness with cooling Rate Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 5

6 CONCLUSIONS Recording the wax deposit formation on cold finger allows measurement of initial, intermediate and final deposition rates as well as final deposit thickness. Technique can be used when bulk temperature close to or above WAT of oil. The final thickness will decrease as the increasing of agitation rate, jacket temperature and cold finger temperature. Moreover, final thickness is independent of the pathway.the driving force of gelation is temperature gradient between hot bulk fluid and cold surface of cold finger. ACKNOWLEDGEMENTS The authors acknowledge the financial support from the following members of the University of Michigan Industrial Affiliate Program on Upstream Engineering: Chevron, Total and Assured Flow Solutions. In addition, the experimental instruments were contributed by Chemical Engineering Department, University of Michigan. This study would not be finished without suggestions from Mark Zheng, Luqman Mahir and Claudio Vilas Boas Favero. I would like to thank you Professor Fogler who gives me the opportunity to do the research at the University of Michigan. Finally, thank you to the Petroleum and Petrochemical College. REFERENCES Han, S., Huang, Z., Senra, M., Hoffmann, R., & Fogler, H. (2010, January 19). Method to Determine the Wax Solubility Curve in Crude Oil from Centrifugation and High Temperature Gas Chromatography Measurements. Energy&Fuels, Hoffmann, R., Amundsen, L., Huang, Z., Zheng, S., & Fogler, H. (2012, May 25). Wax Deposition in Stratified Oil/Water Flow. Energy&Fuels, Hoffmann, R., & Amudsen, L. (2013, May 9). Influence of Wax Inhibitor on Fluid and Deposit Properties. Elsevier, 107, Huang, Zhenyu, Sheng Zheng, and H. Scott Fogler. Wax Deposition: Experimental Characterizations, Theoretical Modeling, and Field Practices. CRE Press; Huang, Z., Lee, H. S., Senra, M., & Fogler, H. (2011, February 15). A Fundamental Model of Wax Deposition in Subsea Oil Pipelines. AIChE, 57(11), Singh, P., Venkatesan, R., & Fogler, H. (2000, May). Formation and Aging of Incipient Thin Film Wax Oil Gels, 46(5). AIChE, Zheng, S., & Fogler, H. (2014, July 16). Fundamental Investigation of Wax Diffusion Characteristics in Water-in-Oil Emulsion. I&EC Research, Zheng, S., Zhang, F., Huang, Z., & Fogler, H. (2013, November 12). Effects of Operating Conditions on Wax Deposit Carbon Number Distribution: Theory and Experiment. Energy&Fuels, Daniel, M., Michele, M., & Sebastiano, C. (2016, November 27). Kinetic of Waxy Gel Formation from Batch Experiments. Energy&Fuels, Petrochemical and Materials Technology Tuesday May 23, 2017, Pathumwan Princess Hotel, Bangkok, Thailand Page 6