An Evaluation of Mixing Effect on the Recycle Stage of Syngas Compressor Based on Numerical and Experimental Investigations

With the requirement of the chemical synthetic process, the reaction pressure of the synthesis gas is over 14MPa. Meanwhile it is necessary to employ a recycle stage to improve the synthesis efficiency. The performance of the recycle stage would be influenced by the upstream from the previous stage and the complementary gas. However, it is difficult to understand the performance of the recycle section separately because of the lack of the testing data in practice. In fact, the inlet flow distortions caused by the two streams mixing process may result in significant deterioration in compressor performance. To evaluate the impact of the mixing process on the recycle stage, a 3D numerical flow passage model consisting of last stage of high pressure compressor (HPC) section, the mixing chamber and recycle stage, has been established in the paper. True thermodynamic properties of the mixture are considered for the numerical analyses at high pressure and the results are validated with the test performance data of an onsite synthesis gas compressor. In the mixing chamber, the flow directions, pressures, temperatures and velocities of the two upstreams are different, which result in the non-uniform distribution of the flow along radial direction at the inlet of recycle stage. From the numerical results, eight different types of losses are calculated to understand the performance of the flow passage. It is found that the loss of the mixing process and blade loading loss are the two main losses. Further, the uniform conditions and mixing inlet conditions are applied separately at the impeller inlet position to evaluate the recycle stage performance. Compared to the uniform inlet, under the mixing inlet condition the recycle stage pressure ratio and efficiency decreases, and the operating range becomes narrower. In order to understand this mechanism, a fan test rig is setup with the mixing inlet structure similar to the recycle stage. The performance of the fan with the mixing inlet structure is numerically investigated. Both of the testing and numerical results illustrate the deterioration of the performance with the mixing inlet structure. The effect of the different mixture ratios on the fan performance is evaluated. The results suggest that the best efficiency point of the fan is achieved at the flow ratio (m_inlet2/m_inlet1) of 5.