Elsevier

Applied Thermal Engineering

Volume 137, 5 June 2018, Pages 575-583
Applied Thermal Engineering

Research Paper
Dynamic simulation of liquefied petroleum gas vaporisation for burners

https://doi.org/10.1016/j.applthermaleng.2018.04.030Get rights and content

Highlights

  • A thermodynamic model is developed to investigate the vaporisation characteristics of LPG in a cylinder.

  • Transient characteristics of LPG vaporisation are analysed under various conditions.

  • The effects of various parameters on LPG vaporisation and the residues in LPG are analysed.

  • A graphical procedure is proposed to determine the design value of Wobbe Index for burners.

Abstract

This study numerically describes the transient behaviours of liquefied petroleum gas (LPG) natural vaporisation in a cylinder using an experimentally validated model. The model can be applied to easily predict transient compositions and thermophysical properties during the vaporisation process under various conditions. A simple graphical design procedure is presented to determine the Wobbe Index and the corresponding composition as design references for gas burners to avoid incomplete combustions. The effects of initial composition, surrounding temperature and discharge rate of LPG on both the LPG natural vaporisation and the residue amount in the cylinder are simulated and investigated. The results demonstrate that the present work may serve as a useful tool to evaluate the LPG behaviours throughout the vaporisation, design or retrofit gas burners, and predict the unburned residue.

Introduction

Liquefied petroleum gas (LPG) is mainly composed of propane (C3H8) and n-butane (C4H10) with some propylene (C3H6), butylene (C4H8), and other minor compositions of hydrocarbons. LPG is in gaseous form at ambient temperatures and pressures. Due to a dramatic reduction in the volume after being liquefied, LPG is commonly stored and transported in liquid form in pressurised steel cylinders. This makes LPG a popular domestic fuel in rural and remote areas, even in some urban regions away from natural gas transmission networks [1]. For instance, Chinese total LPG consumption for householders was 21.73 million tons in 2014 [2].

The common way for domestic and commercial burners to use LPG is to supply liquid LPG in commercially available cylinders without thermal insulation and then to use it in gaseous form. The liquid-to-vapour phase change occurring within the cylinder is called spontaneous (or natural) vaporisation in which process the heat for vaporisation comes from the liquid itself and/or from the surroundings and components with lower boiling points vaporise easier and earlier. The properties of both the liquid and the vapour vary continuously with the change in LPG compositions over time. The minimum inlet pressure of a gas regulator automatically modulates high-pressure gas to a safe pre-determined limit for gas burners, so the cylinder cannot be completely emptied and the remaining liquid is called as the residue. It means that if the vapour pressure in the cylinder is less than the required minimum value, the natural vaporisation process will be completed.

To estimate the physical properties of LPG and the residue amount, it is necessary to know the composition [3]. The chromatography method is usually applied to determine the composition of LPG after analysing its sample [4], [5], [6], [7]. However, it is hard to adopt this method to continuously determine the composition that changes instantaneously through the vaporisation process. Some mathematical models were developed to predict the dynamic behaviours of LPG spontaneous vaporisation in the cylinder [3], [8], [9]. However, such models are typically valid for specific and rigorous assumptions. The LPG within the cylinder was assumed to be ideal [3], [8], [9]. Furthermore, Yan [8] assumed that the change in LPG composition did not alter the values of some thermodynamic properties and the ratio of the pressure to the temperature of LPG kept constant. Tian and Jiang [3] treated the mass fraction of each component as an equivalent of its mole fraction and assumed that the LPG temperature remained unchanged throughout the vaporisation [3]. These above mentioned assumptions are in conflict with the practical natural vaporisation and may cause some limitations of the models.

This work emphasises the need to account for the non-ideal behaviour of both the liquid and the vapour phases in the pressurized steel cylinder and the importance of variable physical properties in investigating the real LPG spontaneous vaporisation process. The main objective of this study is to present a comprehensive model and explore the detailed transient characteristics of LPG natural vaporisation process for gas burners, which can provide a basis for designing or retrofitting burners to get good combustion performance and predicting the residue amount. First, a dynamic model is established which prediction accuracy is validated by using the previous experimental data. Further, the transient behaviours of LPG vaporisation are obtained and analysed, with special emphasis on variation of combustion properties of vapour LPG. Finally, the effects of initial composition, surrounding temperature and discharge rate on the natural vaporisation and the residue amount are discussed based on the simulation results.

Section snippets

Description

As shown in Fig. 1, when the user opens the reducing valve to use LPG, the vapour flows out. Then the vapour pressure in the cylinder reduces and the equilibrium no longer exists. The liquid LPG has to vaporise immediately to mitigate the drop in the pressure and to regain a vapour-liquid equilibrium (VLE) condition. Due to no temperature difference, the latent heat of vaporisation is only from the liquid itself at the beginning. The liquid temperature thus decreases, which generates the

Model validation

In this study, the presented model was validated with the experimental data obtained by the chromatography method [11] under the same operating conditions listed in Table 2. For ‘ysp118-I type’ cylinders generally used as a group in commercial sectors, the minimum inlet pressure of the regulator is 0.17 MPa [11], [13].

As shown in Fig. 4, Fig. 5, more than 89% and 78% of the calculation results are with the ±4% bandwidth of experimental results for compositions in vapour and liquid phases,

Concluding remarks

The transient behaviours of LPG spontaneous vaporisation in the cylinder were clearly studied by the presented general model. Some conclusions are obtained by this investigation.

  • (1)

    The proposed model is proved to be applicable for determining real-time compositions and physical properties of LPG throughout the discharge process.

  • (2)

    From the beginning to the end of the natural vaporisation, the WI of vapour LPG increases by 8.0%. The WI obtained by the proposed graphical design procedure can ensure

Acknowledgements

This work was done while Guo-Hua Shi was visiting at the University of Melbourne. The visit was supported by the program of China Scholarships Council [No. 201606735054]. The authors appreciate the support from the Natural Science Foundation of Hebei Province, China [E2016502027], the Fundamental Research Funds for the Central Universities, China [2017MS124] and the Scientific Research Project of Education Department of Hebei Province, China [Z2015119]. The authors also gratefully acknowledge

References (23)

  • L. Wang et al.

    Determination of composition of liquefied petroleum gas by capillary gas chromatography

    J. Instrum. Anal.

    (2009)
  • Cited by (7)

    • Suitability of part block switching of ambient air vaporizer for propane vaporization in contrast to cryogenic vaporization

      2023, Applied Thermal Engineering
      Citation Excerpt :

      Various vaporization techniques are available for supplying high-pressure gaseous propane to industrial and residential complexes. The simplest and commonly used technique is known as spontaneous or natural vaporization [19]. As the LPG tanks are not insulated, once depressurized and cooled, there will be constant heat transfer from ambient to the stored liquid.

    • A review of low and zero carbon fuel technologies: Achieving ship carbon reduction targets

      2022, Sustainable Energy Technologies and Assessments
      Citation Excerpt :

      The flame temperature of LPG is low, so it can reduce the NOX emission. In order to explore the feasibility of LPG as an alternative fuel, many scholars have studied the combustion flame characteristics of LPG in burners [85–89]. Among the researches, Elbaz et al. [89] studied the LPG combustion flame with annular swirl and outer swirl in the staged swirl burner, and found that mixing LPG into mixture could improve the stability of flame, and reasonable swirl number could reduce flame temperature, residence time and NO emission.

    • Dynamic modelling and performance evaluation of a direct-expansion solar-assisted heat pump for LPG vaporisation applications

      2019, Applied Thermal Engineering
      Citation Excerpt :

      Their experimental results showed that the three-in-one system spent no more than 3 h producing 200 L of water at 50 °C under different weather conditions in the city of Shanghai, China. Liquefied petroleum gas (LPG) is in gaseous form at ambient pressures and temperatures and is usually stored and transported in liquid form in pressurised cylinders without thermal insulation owing to the considerable reduction in volume after being liquefied [39]. As a cleaner fuel compared with raw coal and petroleum oil, it is still a main commercial and residential fuel in developing countries, especially in rural or remote areas with no access to natural gas pipelines [40–42].

    • Modeling and studying basic parameters of tank sets in combined regasification

      2019, IOP Conference Series: Materials Science and Engineering
    View all citing articles on Scopus
    View full text