Halbach array is extensively used in process control industry targeting NMR (Nuclear Magnetic Resonance) technology as it has several advantages compared to non Halbach array methods; such as uniform magnetic field, light weight, low cost, and concentrated magnetic field. In this paper, we propose to design and assess a Halbach array targeting NMR application for a flowing multiphase flow process passing through a 2″ pipeline. The design methodology is based on Particle Swarm Optimization (PSO) algorithm, combined with a three dimensional (3D) finite element method-based software. The aim of the design is to generate simultaneously a concentric, uniform, and high intensity magnetic field inside the target sensing area in order to provide the most accurate results, while using a cheap and reasonably light weight hardware. Results of extensive 3D simulation results using an array of 12 permanent magnet elements of size (20mm × 20mm × 50mm size) indicate that an optimized magnetic field distribution of 0.9 Tesla maximal intensity and 606 ppm homogeneity could be achieved for an area of 40 mm diameter, which is adequate for our application.