How to Evaporate Heavy Hydrocarbon in a Natural Gas Stream Within a Short Distance -The AIM Concept

Abstract

Abstract Evaporating heavy hydrocarbons has always been a challenging aspect, especially in a limited space area that required rapid vaporization of these LPG (liquified Petroleum Gas) fractions within a short distance. Annular Injection Mixer (AIM) has proven to have superior performance in providing immediate and uniform vaporization of LPG fractions into natural gas. This article focuses on the use of AIM in vaporizing heavy hydrocarbon (C4 to C9) fractions into natural gas streams and the evaluation of evaporation performance via computational fluid dynamics (CFD). By means of CFD, the thermal and dynamic aspect of the evaporation of heavy hydrocarbon was tackled in two steps. The first step was the determination of droplet size distribution. Multiphase modelling was conducted with the Volume of Fluid (VoF) model. An in-house script was developed to instruct the CFD software to generate droplets size based on the incoming liquid characteristics such as liquid volume, surface tension, and liquid velocity. The second step was modelling the droplet secondary breakup and evaluating the droplet evaporation. Based on each individual hydrocarbon fraction properties such as the latent heat of vaporization, molecular diffusivity, saturated vapor pressure, critical pressure, critical temperature, and other local flow conditions surrounding each droplet, droplet break up and later evaporation into natural gas were evaluated. Quasi-steady droplets evaporation model with Ranz Marshall correlation was used for the evaluation. Droplets evaporation length and homogeneity of heavy hydrocarbon vapor within the natural gas were among other crucial criteria evaluated. Position of each liquid droplets were track explicitly with the position of last droplet before being evaporated were measured over time. In the case study presented, with the worst operating condition used, all droplets have managed to evaporate before reaching the heat exchangers. Comparison checks were performed with conventional wafer type static mixer. Based on this comprehensive understanding of the droplet evaporation phenomenon within the AIM, whenever necessary, an appropriate recommendation can be provided to ensure that complete droplet evaporation is achieved within the required distance. The proven AIM technology of evaporating heavy hydrocarbons in LPG streams offers significant cost benefits for plant development, as it could prevent the installation of an evaporator and increase the market value of the product.