Studies On Flow Characteristics And Stall/Surge Of Recycle Stage In Syngas Compressor With Dual Inlets And A Mixing Chamber

In the coal chemical industry, the synthetic gas centrifugal compressor is one ofthe key equipments used in synthetic ammonia or methanol plant. The operating statusof the compressor directly affects the production and economic benefits of thecompany. With the requirement of the chemical synthetic process, the reactionpressure of the synthesis gas is around tens to hundreds of atmospheric pressure. It isnecessary to employ a recycle stage to improve the synthesis efficiency. However, it isdifficult to understand the performance of the recycle section separately because ofthe lack of the testing data in practice. The flow rate ratio of the recycle gas to thefresh gas is in the range of3～5. The thermodynamic properties including pressure,temperature and the composition of the two streams are not the same. The space of themixing chamber (with diameter of about54mm and length of55mm) and the mixingtime (with incoming velocity of more than70m/s) is limited. The mixing process inthe mixing chamber is strongly unsteady and the flow is non-uniform. Inlet flowdistortions caused by fluid mixing may result in significant deterioration incompressor performance.For the validation process of the compressor design, numerical study on the firststage of the high-pressure casing in a synthesis ammonia compressor is presented.Detailed flow field comparisons are made between impeller/stage models. Comparedto the results from stage model, the impeller model results indicate that the predictedaerodynamic performance is higher, and operating range is wider in both stall andchoke side. Under the same inlet volume flow rate, the blade pressure coefficients,Mach number and flow angle in the blade passage for both models are nearly the same,suggesting that the flow field data in the rotating impeller is to some degree crediblefor stage performance prediction. However, as the impeller model neglects thematching effect with the downstream stationary parts, there needs some correction forstage working range with stable operation. Besides, the internal flow fields of stageusing air and syngas mediums are compared respectively. Results indicate that theaerodynamic performance and operation range are different for both mediums becauseof different density and gas constant. For the flow field of the whole stage, largediscrepancy occurs in the leading edge of the return channel under the same inletvolume flow rate. It suggests that the existing air model stage couldn’t be directlyused for the syngas compressor and needs redesigning.To evaluate the influence of mixing effect on the recycle stage, a3D numericalflow passage model consisting of last stage of high pressure compressor (HPC)section, the mixing chamber and recycle stage has been established. Numerical resultsare validated with the field test data. Different mixing length of25mm,55mm,85mmand flow ratio ranging from0.5to7are investigated. In the mixing chamber, the flowdirection, pressure, temperature and velocities of the two upstreams are different, which result in non-uniform distribution along circumferential and radial direction.With the development of flow, strong secondary flow is detected on the cross section.There exists flow parameters gradient in the mixing chamber. The pressure andvelocity gradually increases from shroud side to hub side. Compared to the uniforminlet, under the mixing inlet condition the recycle stage pressure ratio and efficiencydecreases, and the operating range becomes narrower. The flow rate ratio of the dualinlets, which is mainly related to the area ratio, almost keeps constant when adjustingthe back pressure. Under the high pressure condition, the increase of mixing lengthhas little influence on the static pressure rise of the flow, and the impact on efficiencyis no more than1%. The total pressure loss showed quadratic relationship with thetotal inlet flow rate. There exists a critical flow ratio at which the efficiency is thehighest and the velocities of the dual inlets at the intersection place are the same. Withfurther increase of the flow ratio the impeller efficiency almost keeps constant.Furthermore, a one-dimensional model associated with the geometriccharacteristic of the mixing chamber is derived based on momentum equation topredict the static and total pressure of the sidestream. The model is compared to theCFD results and they agree very well. It can be used for rapid assessment of thedesign of mixing chamber. Based on the model result, the larger of the curvatureradius Rs at the sidestreams outlet, the larger of the outlet pressure of the returnchannel. At the design flow rate point, nine different types of losses are calculated toassess the performance of the flow passage. It is found that the mixture loss in themixing chamber, secondary flow loss in the blade passage and vaneless diffuser lossare the three main losses.To partially solve the test difficulty of the mixing chamber in the syngascompressor under the high pressure condition, an experimental test rig is setup for anindustrial fan with mixing inlet structure similar to the recycle stage and performanceof the fan is numerically investigated. Effect of different mixture ratios and mixinglength on fan performance is evaluated. Compared to the uniform inlet condition, atthe design point the average efficiency and pressure rise is decreased by6.5%and203Pa, respectively. The non-uniformity propagates to the rotating impeller and volute,and results in quite different mass flow rate in each blade passage. The maximum andminimum mass flow rate is0.1997kg/s and0.0272kg/s, both located near the volutetongue area. In comparison, the maximum and minimum mass flow rate underuniform inlet condition is0.068kg/s and0.086kg/s, respectively. The fan efficiencyand pressure rise drops with the increase of mixing length. At the design point, theefficiency and pressure rise decrease by an average of2%and70Pa in increments of100mm mixing length, respectively. At the constant total incoming flow rate, the bestefficiency point of the fan is achieved at the flow ratio (minlet2/minlet1) of5. Withfurther increase of the flow ratio the fan efficiency keeps nearly constant.With the development of large-scale trends in the synthesis gas compressor, the stall and surge problem of an industrial centrifugal compressor with casing treatmentis investigated. A series of experimental investigations are conducted to study theeffects of casing treatment on steady and transient performance of the compressor.Dynamic pressure is monitored under stable operating point and surge occurrencecondition at following positions: two ends of the treated holes,37%-chord impeller tipposition and the vaneless diffuser. By use of the casing treatment, the surge margin ofthe compressor is increased by about10%Qdesignwith negligible decrease in efficiency.Pulsating pressure change is taken as an indicator for evaluating the flow direction inthe hole. Near surge point, when the static pressure of the bleeding port is larger thanthe turning point, a sudden increase in pulsating pressure indicates that the bleedingflow occurs. Near choke side, when the static pressure of the turning point is largerthan the bleeding port, a severe change in pulsating pressure indicates that the bypassflow occurs. Signal analysis of the dynamic pressure at surge occurrence shows thatthe compressor stalls with a modal inception rather than a spike wave no mattercasing treatment is applied or not. The travelling speed of the stall cell found in thecompressor with casing treatment is faster than that with solid casing. The occurrenceof modal precursor wave is delayed with casing treatment.