Comment pubs.acs.org/EF
Response to “Comments on ‘The Effect of Operating Temperatures on Wax Deposition’ by Huang et al.” Zhenyu Huang† and Scott Fogler*,‡ †
Multiphase Solutions Kenny, Incorporated, Houston, Texas 77084, United States Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States
‡
We express our concern on the recent paper entitled “Comments on ‘The Effect of Operating Temperatures on Wax Deposition’”,1 which is referred as the “Comment” in this response. The Comment has made presumptuous claims and misstatements on a previous study that was carried out by Huang et al. entitled “The Effect of Operating Temperatures on Wax Deposition”,2 which is referred as the “original study” in this response. Not one of the claims in the Comment is correct, and our responses are listed as follows: (1) (Page 3965, Paragraph 2, Line 4) “To the best of our knowledge, this assumption relating to a variable (and increasing) oil−deposit interface temperature has not been validated experimentally”. Response: It is not possible to directly measure the oil−deposit interface temperature during a wax deposition experiment without significantly interrupting the flow and heat-transfer profile. However, the increase of the oil−deposit interface temperature with time is due to the insulating effect resulting from the buildup of the deposit layer. This effect has been validated from the substantial increase of the outlet temperature as well as the decrease in the coolant temperature in the test section, which has been observed multiple times by the University of Tulsa’s 50 m flow loop:3−5 (Matzain,3 pages 46−48 and Figures 7−10; Lund,4 pages 58−63 and Figures 4.8−4.15; and Hernandez,5 pages 82, 84, and 91 and Figures 4-39, 4-41, and 4-49). These widely cited results are direct evidence of the insulation effect by the thickening wax deposit layer during deposition. Any basic heat-transfer textbook that discusses radial heat transfer can be used to show and conclude that the oil−wax interface temperature must increase during deposition. Consequently, the claim in the Comment as well as in another study by Bidmus and Mehrotra6 that the oil− deposit interface temperature should stay constant [at the wax appearance temperature (WAT)] during wax deposition is a direct violation of heat-transfer principles and is also against many previous experimental observations. In some occasions, the oil bulk temperature is less than the WAT; if the oil−deposit interface temperature is still at WAT as the authors claimed, it means that the oil absorbs heat from (rather than losses heat to) the coolant, which obviously does not make sense and is physically incorrect. In addition, it should be noted that Bidmus and Mehrotra’s claim comes from their experimental device with a deposition section of only 10.16 cm long (2.54 cm inner diameter).6 This L/D ∼ 4 deposition device creates a heat-transfer profile that is not even fully developed and © 2012 American Chemical Society
would be inevitably drastically different from real pipe flows. These experimental drawbacks have greatly jeopardized their conclusions. (2) (Page 3965, Paragraph 1, Line 1) “The model predictions for North Sea oil deposition reported by Huang et al.1 in their Table 1 show certain inconsistencies... It would be incorrect to assume that Twall would remain constant” (until the end of the page 2). Response: There are absolutely no inconsistencies, as we have explicitly stated in multiple locations in the original study (page 5183, paragraphs 4 and 6) that the calculation is made at a pre-deposition state. We have never assumed T wall to be constant during wax deposition. (3) (Starts from Page 3964, Paragraph 3, Line 16, and continues to Paragraph 4) “These wax crystals could deposit or ‘stick’ to the cooling surface as well as possibly remain suspended in solution, depending upon the cooling approach used. Huang et al.1 did not clarify this important issue”. Response: As early as 2000,7 it has been shown experimentally that precipitated wax crystals do not contribute to deposition on the pipe wall. The authors of the Comment seem to be well aware of this finding because it is one of the principles used in one of their studies to design cold flows:6 (page 3185, paragraph 5, line 7). “‘Cold flow’ is an alternative approach that has been proposed to control and reduce solids deposition problems during the flow of waxy crude oil. This type of flow occurs when a slurry (precipitated wax) is formed at cooler temperatures, which is transported through the pipeline under stable conditions with no deposition of the solids.” Consequently, one questions the purpose of the authors of the Comment by raising this question to which they know the answer. (4) (Page 3964, third to last line of the page) “Moreover, precipitation of wax crystals will also lower the WAT of the residual liquid phase of the oil”. Response: This accusation further demonstrates the disadvantage (with little mass-transfer analysis) in Bidmus and Mehrotra’s study,6 using only heat transfer to model wax deposition. This “lowering of the WAT as wax precipitates because of the decrease of temperature” Received: October 2, 2012 Revised: November 29, 2012 Published: December 3, 2012 7390
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more accurate and fundamental description of the effect of the temperature on wax deposition. (7) (Page 3964, Paragraph 2, Line 8) “The solubility data were obtained at a relatively fast cooling rate of 1 °C/ min, for which the supercooling effects in the crystallization process might be more dominant”. Response: The cooling rate of 1 °C/min is sufficient to obtain the solubility curve of the model oil because we did not find any significant difference using a smaller cooling rate. It is universally accepted that a solubility curve is indispensable in analyzing the deposition characteristics of an oil. Nevertheless, in Bidmus and Mehrotra’s presentation of their wax deposition model with the model oil,6 no solubility curve was provided. To validate our model analysis on their data, we have purchased the exact same oil−wax system used in their experiments and carried out a differential scanning calorimetry (DSC) measurement of the solubility curve. With this information, we were able to predict not only the “cold flow” results but also the “hot flow” results in their study. (8) (Page 3964, Paragraph 3, Line 22) “Without a complete description of the experimental protocol. It is difficult to understand and reproduce their experiments”. Response: There was no need to repeat the details of the experimental procedures because they were described in detail in the study by Hoffmann et al.,8 which is properly referenced in the original study (page 5181, section 2.A).
is exactly the physical significance of a wax solubility curve of an oil, which we applied on different oils in our studies. We have shown in the original study that the different solubility curves result in different behaviors of the deposit thickness with changes in the temperature. This analysis shows that simply using heat transfer alone cannot predict this result. (5) (Page 3964, Paragraph 2, Line 13) “When both sets of solubility data are plotted using the same scales, as shown in Figure 2 of this Comment, the solubility curve of the model oil is seen to be actually more temperaturesensitive (i.e., having a larger slope) than that from the North Sea oil sample”. (Page B, Paragraph 2, Line 17) “Thus, the solubility data by Huang et al. do not support their own observations and the associated conclusions”. Response: It is not the magnitude of the gradient (slope) but the change in the gradient (slope) that is important in predicting whether or not the wax thickness will increase or decrease. One notes from their Figure 2 that the gradient (slope) of North Sea oil A goes from a maximum value at 5 °C to essentially zero at 30 °C, while the slope of the model oil is constant below 33 °C. This point has been stated in multiple locations in the original study (page 5185, section 4.A.3 and page 5186, section 5) (6) (Page 3963, Paragraph 2, Line 5) “Huang et al. did not distinguish correctly between the results from two sets (i.e., cold flow and hot flow) of wax deposition experiments with the model oil”. (Page 3963, Paragraph 3, Line 1) “The effect of the thermal driving force on wax deposition has not been interpreted correctly in sections 1.B and 1.C of the paper by Huang et al.”. (Page 3964, Paragraph 1, Line 16) “Thus, contrary to observations in sections 1.B and 1.C of the paper by Huang et al., the wax deposition, under ‘hot flow’ conditions, has been shown to decrease with an increase in Toil and/or Tcoolant”. (Page 3964, third line from the end of Paragraph 1) “Huang et al. examined the results from ‘cold flow’ experiments while not distinguishing them from ‘hot flow’ experimental results”. (Page 3964, Paragraph 3, Line 4) “This trend (i.e., for oil A, wax deposition increases with a decrease of the thermal driving force) is opposite of that observed by Bidmus and Mehrotra for ‘cold flow’ experiments but is in agreement with the ‘hot flow’ deposition experiments”. Response: The driving force has indeed been interpreted correctly. Because the governing equations for fluid flow and heat/mass transfer are identical for these two cases, it does not provide additional scientific insight to categorize deposition experiments with “hot flow” and “cold flow”, and we chose not to analyze the “hot flow” case in the original study. However, just for the record, because the Comment implied that the results would be different, we have analyzed Bidmus and Mehrotra’s so-called “hot flow” data6 shown in the Appendix with the same method that is carried out in the original study. As expected, we predicted with their data that the deposit thickness decreases with an increasing thermal driving force. This prediction further demonstrates that, instead of using solely heat transfer and categorizing wax deposition experiments with “hot flow” and “cold flow” conditions, our analysis to combine the solubility curve and mass transfer with heat transfer is a
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SUMMARY In conclusion, the Comment has made presumptuous and groundless claims on our study, which might have resulted from an careless reading of the original study and a lack of application of mass transfer fundamentals to study wax deposition. We hope that with our clarifications in this response the real academic value of the original study2 is not tarnished by the erroneous claims in the Comment.
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APPENDIX Because the Comment implied (see response to 8 above) that results would not hold for “hot flow”, we have carried out this analysis with the same method as in the original study. The method of calculation has been clearly documented in the original study (section 4.C). Table 1 shows the analysis of Bidmus and Mehrotra’s hot flow results6 using the characteristics of mass flux for wax deposition as introduced in the Table 1. Comparison of the Parameters for the Characteristics of Mass Flux for Wax Deposition, Jwax, for the “Hot Flow” Experiments Carried out by Bidmus et al.6 parameters Toil (°C) Tcoolant (°C) Qoil (m3/h) Twall (°C) Dwo,wall (×10−10, m2/s) Coil(eq) (wt %) Cwall(eq) (wt %) Coil(eq) − Cwall(eq) (wt %) Jwax (×10−10, m s−1 wt %) 7391
value 35.0
38.5 25.0 0.36
28.0 3.53 6.00 4.55 1.45 201.73
29.6 4.07 6.00 4.93 1.07 171.28
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original study, where the characteristic mass flux of wax deposition, Jwax, can predict the “hot flow” results by Budmus and Mehrotra6 (the deposit thickness decreases with an increasing thermal driving force, Toil − Tcoolant) The behavior of Jwax is consistent with the change of the deposit thickness with respect to the change in the thermal driving force. This value of Jwax involves not only heat transfer (Toil and Tcoolant) but also mass transfer (the diffusion coefficient, Dwo,wall), as well as the solubility curve of the oil [Coil(eq) and Cwall(eq)]. The consistency of this analysis further demonstrates that Bidmus and Mehrotra’s approach6 to rely solely on heat transfer and to oversimplify the deposition results with hot flow and cold flow are not sufficient to fully describe the impact of the operating temperature on wax deposition.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Bidmus, H. O.; Mehrotra, A. K. Comments on “The Effect of Operating Temperatures on Wax Deposition” by Huang et al. Energy Fuels 2012, 6, 3963−3966. (2) Huang, Z.; Lu, Y.; Hoffmann, R.; Amundsen, L.; Fogler, H. S. The Effect of Operating Temperatures on Wax Deposition. Energy Fuels 2011, 25, 5180−5188. (3) Matzain, A. Single phase liquid paraffin deposition modeling. M.S. Thesis, University of Tulsa, Tulsa, OK, 1997. (4) Lund, H. Investigation of paraffin deposition during single-phase liquid flow in pipelines. M.S. Thesis, University of Tulsa, Tulsa, OK, 1998. (5) Hernandez, O. C. Investigation of single-phase paraffin deposition characteristics. M.S. Thesis, University of Tulsa, Tulsa, OK, 2002. (6) Bidmus, H. O.; Mehrotra, A. K. Solids deposition during “cold flow” of wax−solvent mixtures in a flow-loop apparatus with heat transfer. Energy Fuels 2009, 23, 3184−3194. (7) Singh, P.; Venkatesan, R.; Fogler, H. S.; Nagarajan, N. R. Formation and aging of incipient thin film wax−oil gels. AIChE J. 2000, 46, 1059−1074. (8) Hoffmann, R.; Amundsen, L. Single-phase wax deposition experiments. Energy Fuels 2010, 24, 1069−1080.
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