Effects of Feed Jets on Pressure Distribution in FCC Riser

Fluid Catalytic Cracking (FCC) is an important process for upgrading heavy oil in modern refining industry. In recent years, riser reactor is widely used to realize a fast cracking reaction. In the feed injection zone of FCC riser, atomized raw oil is injected into the riser from the atomizing nozzles to contact with the catalyst particles. Cracking reactions occur immediately during the mixing between oil and catalysts. Therefore, the dynamic mixing of feed jets and catalysts in this zone is very important. In most commercial FCC systems, the nozzles are pointed upward at 30^o~40^o relative to the riser axis. However, a large number of studies on the gas-solid flow and reaction have indicated that there are some disadvantages in the traditional feed injection zone. To solve these problems, many efforts have been done by researchers. According to the theoretical analysis and 3D-simulation results, a better jet-catalysts mixing was found when feed jets were downward. By this way, the disadvantages of secondary flow are expected to be minimized. What’s more, the hydrodynamic characteristics in different types of jet influence zones are need to be further investigated, especially by experiments. Therefore, in this paper, effects of different feed jets on the pressure distributions in FCC riser were analyzed by large scale cold model experiments.

To study the pressure distributions under the influence of upward and downward feed injections, a large scale cold-riser model was established based on the characteristics and operating conditions of FCC reactor-regenerator system. The riser section was 0.186 m in inner diameter and 11 m in height. Four feed nozzles were equipped at a height of 4.5 m above the gas distributor. Both upward and downward oriented nozzles were mounted in the experiment. The angles between the nozzles and the riser axis were both set to 30^o no matter the nozzles were mounted upward and downward to make a comparison.

Pressure sensors were used to measure the pressure distributions and pressure drop in the jet influence zone. Generally, comparing with the upward injection, the pressure drop in the downward feed injection zone is a little larger. Within the operating conditions, the pressure drops caused by the downward jets are about 1%~10% higher than that of the upward jets. When the velocity of jet gas increases, the pressure drop in the upward and downward feed injection zones both increases. However, it is more significant when nozzles are mounted upward. Under lower jet gas velocity condition (U_j=41.8 m/s), the pressure drop of downward injection zone is 9.88% higher than that of the upward injection zone. When the jet velocity is higher (U_j=78.5 m/s), the pressure drop is nearly the same for the two cases. Furthermore, composition of the pressure drop was analyzed. In the feed injection zone, the overall pressure drop can be divided into three parts, i.e., the static pressure drop, the accelerated pressure drop and the frictional pressure drop. The static pressure drop and accelerated pressure drop are easy to be obtained by measuring the solid holdup and particle velocity. Then the frictional pressure drop in the jet influence zone is calculated by knowing the overall pressure drop. Characteristics of frictional pressure drop under the influence of upward and downward injections are presented. Based on the experiment results, correlations between frictional pressure drop and operating parameters are obtained.