Annular Injection Mixer Approach Improves Evaporation of Heavy Hydrocarbons

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 216883, “How To Evaporate Heavy Hydrocarbon in a Natural Gas Stream Within a Short Distance: The AIM Concept,” by Fariz Maktar and Christian Chauvet, Wood, and John Sabey, SPE, Prosep. The paper has not been peer reviewed. Evaporating heavy hydrocarbons has long been a challenging task, especially in a limited area that requires rapid vaporization of liquified petroleum gas (LPG) fractions within a short distance. A static mixer that the authors call the annular injection mixer (AIM) has demonstrated superior performance in providing immediate and uniform vaporization of LPG fractions into natural gas. The complete paper focuses on the use of AIM in vaporizing heavy hydrocarbon (C4–C9) fractions into natural gas streams and the evaluation of evaporation performance through computational fluid dynamics (CFD). The efficacy of the AIM revolves around its capability to generate fine liquid droplets in the main gas stream. In the AIM, droplets generation takes place through a series of primary and secondary breakup processes. As the liquid phase is introduced into the AIM, the liquid phase travels along the conical wall as a thin liquid film. The difference in velocity between the liquid film and the carrier fluid, in this case natural gas, induces instability within the liquid film. Downstream of a sharp rim, called the “knife edge” by the authors, these instabilities grow further and eventually lead to the breakup of liquid film into liquid ligaments. These unstable ligaments experience further atomization and generate droplets. At higher carrier-fluid velocity, these droplets will deform and experience secondary atomization, generating much smaller droplets. This process continues until the droplets are sufficiently small and stable. The AIM is a static mixer with no moving parts (Fig. 1). It relies on the momentum of the carrier fluid to generate small liquid droplets and enhance their evaporation, resulting in 100% homogenization and full vaporization of the droplets within several pipe diameters downstream. The AIM’s design consists of a convergent conical section and a divergent conical section. Between the sections, at the vena contracta, is the knife edge. LPG in the liquid phase is introduced into the AIM through annular rings consisting of multiple opening channels just upstream of the knife edge. Because of the high natural gas velocity in this area, the LPG is spread along the conical wall, forming a thin liquid film. Once the LPG liquid film reaches the knife edge, the liquid film transforms into liquid ligaments. These liquid ligaments are unstable and experience further breakup into liquid droplets. Additionally, these liquid droplets are subjected to droplet deformation and secondary droplets break up. Conventional 1D process simulators might be able to assess the evaporation capability of LPG into natural gas; however, such software will not be able to measure the dynamics and kinetics of the evaporation process. This is where 3D CFD simulators hold an advantage over conventional 1D process simulators.