Abstract
The aspiration for blending hydrogen (H2) into natural gas (NG) in gas transmission systems is high and is happening globally. However, the mechanics and details of blending the two streams are not well developed or perfected. There is a need to arrive at the best technique and approach to achieve perfect blending to minimize the potential adverse impact on the operation of downstream facilities as well as on the end-users. The challenge is primarily driven by the fact that NG and H2 have vastly different properties, principally densities, that may lead to possible stratification, short circuiting, and pockets of undesirable high concentration of H2 in the blended stream. The paper documents Computational Fluid Dynamics (CFD) simulation results conducted on five different concepts of mixer/blending designs. These mixer designs are: i) single or multiple side entries, ii) dual spiral ribbon (DSR) type mixer, iii) venturi mixer, iv) hybrid mixer of DSR inside a venturi, and v) NC5 perforated tube bundle type mixer. An example of an NPS 12 (DN300) ultrasonic meter run with an NPS 20 (DN500) header was assumed throughout the analysis. It was found that the venturi mixing concept with a single side entry is the optimum design due to its simplicity, cost effectiveness, and relatively low pressure drop. With this simple design, 99% mixing efficiency is achieved within 13D at maximum flow, where D is the main header diameter downstream of the mixing station. The pressure drop coefficient for this design is estimated to be approx. 3.1, which amounts to ∼6 kPa at maximum flow, which is relatively low. However, mixing will halt at coefficient of variance = 0.2 (80% mixing efficiency) at very low flow rate of a turndown ratio of 20:1. Final selection of a mixer design from the five designs investigated depends on the tradeoff between mixing efficiency, pressure drop and cost.
