Effect of Lower Bed Height on Collapse Velocity in the Two-Stage Bubbling Fluidized-Bed with a Standpipe for Solid Transport

고체 수송관이 있는 2 단 기포 유동층에서 붕괴 속도에 대한 하단 층 높이의 영향

Abstract

The effect of lower bed height on the collapse velocity was investigated for a two-stage bubbling fluidizedbed (0.1 m in diameter, 1.2 m high) connected with a standpipe (0.025 m in diameter) for solid transport. Air was used as fluidizing gas and mixture of coarse (< $1000{\mu}m$ in diameter and $3625kg/m^3$ in apparent density) and fine (< $147{\mu}m$ in diameter and $4079kg/m^3$ in apparent density) particles as solid particles. Mixing ratio of fine particles, height of the lower bed and the distributor of the upper bed were considered as experimental variables. The collapse velocity increased with static height of the lower bed. However, the effect decreased as the mixing ratio of fine particles increased. The effect seemed to be attributed to the increase in height of the dense layer of coarse particles that prevented the gas from flowing into the standpipe, not in pressure drop for the standpipe, as the bed height increased. The collapse velocity decreased a little as the pressure drop of the distributor of the upper bed increased. An improved correlation was proposed for predicting the collapse velocity.

고체 수송관(standpipe, 내경 0.025 m)으로 연결된 2 단 기포 유동층(내경 0.1 m, 높이1.2 m)에서 붕괴 속도에 대한 하단 층 높이의 영향을 조사하였다. 기체로는 공기를 사용하였고, 고체로는 입도가 큰 입자(< $1000{\mu}m$, 겉보기 밀도 $3625kg/m^3$)와 입도가 작은 입자(< $147{\mu}m$, 겉보기 밀도 $4079kg/m^3$)를 혼합한 입자를 사용하였다. 작은 입자의 혼합비, 하단 유동층의 층 높이, 상단 유동층 분산판을 실험 변수로 고려하였다. 붕괴 속도는 하단 유동층의 정체 층 높이가 증가할수록 증가하였다. 그러나 작은 입도의 혼합비가 증가하면 이 효과가 감소하였다. 이 효과는 층 높이 증가에 따른 고체 수송관 압력 강하의 증가 때문이 아니라, 수송관 출구를 막는 농후상 굵은 입자 층 높이의 증가 때문으로 보였다. 상단 유동층 분산판 압력 강하의 증가는 붕괴 속도를 조금 감소시켰다. 붕괴 속도를 예측하는 개선된 상관식을 제안하였다.

Fig. 1. Two-stage fluidized-bed system. 1. Air compressor 7. Cyclones 2. Filter 8. Solids hopper and feeder 3. Pressure regulator 9. Solids drain hopper 4. Mass flow meter 10. Pressure transducers 5. Lower fluidized-bed (FB1) and upper fluidized-bed (FB2) 11. Data logger 6. Standpipe 12. Personal computer

Fig. 2. Size distribution of feed particles.

Fig. 3. Minimum fluidizing velocity of coarse particles.

Fig. 4. Density and voidage of bulk solids.

Fig. 5. Pressure distribution of the fluidized-bed process (30% fines). FB1: 1. Windbox, 2. Distributor in bed, 3. Standpipe bottom, 4. Freeboard. FB2: 5. Windbox, 6. Distributor in bed, 7. Standpipe top, 8. Freeboard.

Fig. 6. Collapse velocity with variation in mixing ratio of fine particles.

Fig. 7. Collapse velocity with variation in mixing ratio of fine particles and static bed height of FB1.

Fig. 8. Comparison in collapse velocity between measured and calculated.

Table 1. Properties of particles

Table 2. Properties of particles used in the study of Youn and Choi [2]

Table 3. Properties of particle mixture

Table 4. Properties of particle mixture used in the study of Youn and Choi [2]