Ultrasonic process tomography (UPT) has great application potential in multiphase flow online monitoring due to its noninvasive, radiation-free and visualization features. Existing UPTs with individual transducers suffer from the small number of projection data, which results in low imaging accuracy. This article presents a novel modular and scalable UPT system, i.e., an ultrasonic phased array tomography (UPAT) system in a digital and modular design, which includes a master control board and up to eight 16-channel ultrasound transceiver boards that are capable of expanding to 128-channel ultrasound transceivers. The beamformers on each transceiver board can flexibly synthesize the sound beams with adjustable shape and direction, while parallelly collecting and processing 16-channel ultrasound signals. To deal with high amount data transmission, the serial rapid IO (SRIO) data exchange protocol with a maximum bandwidth of 4 Gbps is applied between the master board and the transceiver boards. Static imaging experiments with a novel tomographic sensing strategy validate the system performance, where the size and location of complex inclusion distribution are reconstructed at a high spatial resolution. The results show that this UPAT system meets the requirements of multiphase flow online monitoring.