Thermal Cracking of Endothermic Hydrocarbon Fuel in Regenerative

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Thermal Cracking of Endothermic Hydrocarbon Fuel in Regenerative Cooling Channels with Different Geometric Structures Fuqiang Li, Zaizheng Li, Kai Jing, Li Wang, Xiangwen Zhang, and Guozhu Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00531 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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Thermal Cracking of Endothermic Hydrocarbon Fuel in Regenerative Cooling Channels with Different Geometric Structures Fuqiang Lia, Zaizheng Lia, Kai Jingb, Li Wanga, b, Xiangwen Zhanga, b, Guozhu Liua, b* a

: Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China b

: Collaborative Innovation Center of Chemical Science and Engineering, Tianjin University, Tianjin 300072, People’s Republic of China

* Corresponding Author: Guozhu Liu E-mail address: [email protected] Postal address: 92 Weijin Road, Nankai District, Tianjin, P.R.China Tel./fax: +862285356099

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Abstract The chemical heat sink of Endothermic Hydrocarbon Fuels (EHFs) is generally dependent on its thermal cracking in the cooling channel, which is accompanied and limited by the formation of carbon deposit. In this work, HF-1 (A kerosene-based EHF) was electrically heated in the rectangular, square and circular channels with the same cross-section area under 3.5 MPa to study the effect of cooling channel geometric structures on the thermal cracking and carbon deposition behaviors. It was found that on the similar conditions (inlet flow rate of fuel, pressure, outlet temperature), conversions of HF-1 in both rectangular and square channels were slightly higher than that in the circular one with high selectivity to methane but lower selectivities to the primary cracking products (such as 1-hexene and 1-heptene, etc.). In addition, more carbon deposits were formed in the rectangular and square channels, especially around the corners of channels. Based on the CFD simulation, the possible reasons should be ascribed to the difference in the gradient uniformity near the wall of different channels. The higher temperature and lower velocity in the boundary layer of the quadratic channels might cause the thermal cracking slightly severer and the rapid secondary reactions forming carbon deposit.


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1. Introduction Hydrocarbon fuel is one of the few practical coolants on board for the thermal management system of the advanced aircrafts1. Flowing in the cooling channel, the fuel firstly absorbs the heat through temperature rise or phase transition, and then, after the fuel temperature exceeding the initial pyrolysis temperature (Tip), it further absorbs heat via endothermic reactions, such as thermal cracking. Endothermic Hydrocarbon Fuels (EHFs) are used to define the hydrocarbon fuels with available endothermic reactions. However, the use of chemical heat sink is generally accompanied and limited by the formation of carbon deposit. Therefore, it is crucial for the optimal design and better use of EHFs to understand the complex thermal cracking and carbon deposition behavior in the cooling channels. During the past thirty years, much work has been done towards better understanding the thermal cracking and coking (carbon deposition) of EHFs using various model compounds or kerosene fuels. Huang et al.2 measured heat sinks of JP-7, JP-8+100 and n-octane in tubular reactor with resistive heating method and reported an overall heat sink of JP-7 up to 3838 J/g, which was higher than that of JP-8+100. Goel et al.3 numerically studied jet fuel decomposition in tubular reactor using thermal cracking of n-dodecane as a model compound, and found that the outlet bulk temperature and unreacted fuel fraction could be well predicted by an one-step global model. Ward et al.4-5 performed thermal cracking experiments of n-decane and n-dodecane in tubular reactor and proposed proportional product distribution method to describe the products of mildly (conversion 650 °C) grows. The relative gap of conversion is calculated according to Eq. 9. Taking square and circular channels for example, the conversion gap (700 °C) grows to 17.07% at 4 g/s comparing to 10.59% at 2 g/s, and the gas yield gap is 31.17% at 4 g/s from 22.42% at 2 g/s.

∆X=

Xsqaure-Xcircular Xcircular

(9)

The variations of products distributions as a function of flow rates are also obtained from the thermal cracking of HF-1. The selectivities to methane at 1 g/s and 2 g/s of inlet flow rates are presented in Figure 12(a) and (b). Other molecules like ethane, ethylene, propane and propene present no obvious variations among those three geometries no matter what the flow rates are. When the flow rate is at 1 g/s, methane seems to share equal selectivity among those three geometries. While with the flow rate increasing, the value in circular geometry falls behind that of the other two channels. Also, as shown in Figure12 (c-f), the selectivities to 1-hexene and 1-heptene in circular channels grow to higher values than that


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of quadratic channels as flow rate increases. As for 1-octene, 1- nonene and aromatics, notable changes are not observed. The gaps of conversion and selectivity can be also influenced by temperature and velocity distribution. The distributions of temperature and velocity in the outlet cross-section at both 1 g/s and 4g/s, are shown in Figure 13 and Figure 6. From 1 g/s to 4 g/s, temperature of core fluid is decreased and that near the wall is increased, with the average velocity also enhanced. This is because an increase of inlet flow rate leads to higher fluid velocity and heat load, and in result the wall temperature rises. The variations of fluid temperature and velocity along different lines at 1 g/s and 4 g/s are presented in Figure 14 and Figure 7. Comparing rectangular and circular channels, the fluid temperature in the long centerline direction (in the range of 0%-20% and 80%-100%) is eventually beyond that along the diameter of the circular channel from 1 g/s to 4 g/s, meaning that fluid temperature in boundary layer near the short side wall is higher at 4 g/s than 1 g/s. As for the square and circular channels, the fluid temperature along the centerline is initially lower than that along diameter at 1 g/s but reverses this condition as the flow rate rises up to 4 g/s, revealing that boundary layer has much higher temperature. Considering the velocity, for square and circular channels at 1 g/s, they have the same fluid velocity at 24.4% both in the diagonal and diameter direction. While at 4 g/s, the percentage is decreased to 16.0%. And comparing rectangular and circular channels at 1 g/s, the velocities at 20% of the diagonal and diameter are 3.54 m/s and 3.75 m/s indicating the gap is 6%. When flow rate is enhanced to 4g/s, the values are increased to 14.1 m/s and 14.8


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m/s at the exit, with the gap 5%. That is, the boundary layer with low velocity in quadratic channels is narrowed compared with that of circular geometry, which is not beneficial for widening the gap of conversion. As mentioned in the introduction, the conversion is co-decided by temperature and velocity and the former one takes the leading role. In our results, though the boundary layer of low velocity in quadratic channels is narrowed compared with circular geometry, the fluid temperature especially at corners is much higher than that in circular geometry. And the integrated effect is that the gap of conversion is widened as well as selectivities to certain products.

4. Conclusions The geometric structure effect of cooling channels on thermal cracking of HF-1 was investigated in rectangular, square and circular channels. It demonstrated that at the same outlet temperature, HF-1 conversions in both rectangular and square channels were higher than that in the circular one. For example, the relatively gap of conversions between square and circular channels at 700 °C, 4 g/s of inlet flow rate was 17.2%. Meanwhile, higher selectivity to methane but lower selectivities to the primary pyrolysis products (such as 1-hexene and 1-heptene, etc.) were also found in quadratic channels compared with circular geometry. In addition, more carbon deposit was observed in quadratic channels especially at their corners than that in circular channels, with carbon thickness at corners of rectangular channel nearly 67% more than that at circular channel. With the help of simplified CFD simulation of fluid temperature and velocity distribution


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in the outlet cross-section, it is clear that gradient uniformity of fluid temperature and velocity shows difference in three channels. In rectangular and square channels, the fluid temperature in the boundary layer (mainly below 20% along the centerline) is higher and the fluid velocity is lower than that of the circular channel. This difference causes further extent of pyrolysis conversion and secondary reactions, and correspondingly the carbon deposit is formed more severely in quadratic channels. Our work indicates that structure design of cooling channel should pay attention to the shape of its circumference to make a balance between promoting thermal cracking and avoiding carbon deposition.

Supporting Information Grids of three channels (Figure S1-S3); Simulation results with chemical model (Figure S4-S5); Additional SEM images of the outlet cross-sections of three channels (Figure S6).

Acknowledgments The authors sincerely acknowledge financial support from the National Natural Science Foundation of China (21522605 and 21776210)

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Table captions Table 1. Properties and major components of HF-1. Table 2. Parameters of channel geometries.


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Table 1. Properties and major components of HF-1.

Parameters

Value

Mean molecular weight

149.9

Density (20 °C), g/cm3

0.7915

ASTM distillation, °C

Method

GB/T 1884 ASTM D2887

IBP, °C

190.9

10%

202.3

20%

203.8

50%

204.4

90%

205.1

FBP, °C

217.9

PONA, wt%

SH/T 0606-2005

Normal paraffin

46.5

One-ring cycloparaffin

34.5

Two-ring cycloparaffin

18.0

Three-ring cycloparaffin

0.4

Aromatics

0.6

Flash point, °C

69.0

ASTM D93

Total sulfur, ppm

Thermal Cracking of Endothermic Hydrocarbon Fuel in Regenerative

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