Backmixing in channel reactors with high-strength bottom gas injection
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of gas injection. The injector tip diameter ranged between 0.21 and 0.32 cm. Various combinations of liquids were used to represent the light and heavy phases, respectively. Some of these combinations were water and kerosene, tetrachloroethylene and water, brine and kerosene, brine and heat-bodied linseed oil, brine and alkali refined soybean oil, and tetrachloroethylene and brine. The physical properties of these liquids are shown in Table I. The effects of various physical properties and experimental conditions on the residence time distribution (RTD) behavior of the channel reactor were determined experimentally. These parameters included densities, viscosities, flow rates and bath heights of the two liquid phases, injector diameter, and distance between injectors. The tracer RTD studies were conducted using two different tracers depending on the combination of heavy and light liquids. For the water/kerosene system and the tetrachloroethylene/water system, the RTD study was carried out using an on-line conductivity probe and a concentrated potassium chloride solution as the tracer. In the case of the experiments with brine as the heavy liquid, the potassium chloride tracer had little effect on the conductivity of the solution and its progress could not be detected at the exit. Thus, pH change caused by the sulfuric acid tracer was the measured parameter. Further details of these experiments can be found in References 8 through 10. A typical exit-age distribution curve observed for the light and heavy liquid flows is shown in Figure 2. The backmixing analysis was carried out on the experimentally observed RTD for the light and heavy liquids in the channel reactor with high-strength bottom gas injection. The concentration of the tracer in a channel reactor can be described by tTJ
O0
Oz2
Oz
The dimensionless group D/uL, the vessel dispersion number, is the parameter which represents the extent of axial dispersion (backmixing). For plug flow reactors (PFRs), D / u L is 0, while it is infinity for continuously stirred tank reactors (CSTRs). For a closed vessel with plug flow in and out of the vessel, which is the case in this work, the expression for D / u L can be given as 17j
O~oo=
7
= 2 uL
2\uL/
(1
e -~/~
[2]
For the channel reactor, the o~0 varies between 0.05 and 0.32 in the heavy phase and between 0.09 and 0.15 in the light liquid. These values correspond to the degree of backmixing for 3 to 20 CSTRs in series for the heavy liquid and 6 to 11 CSTRs in series for the light liquid. This suggests that there is a moderate amount of backmixing for the light liquid, while in the case of the heavy liquid, the degree of backmixing can have a wide range depending on the operating conditions. VOLUME 25B, AUGUST 1994--619
LightLiquid
HeavyLiquidk
'] Flowmeters Compressed Air --4~---
AirManifold
I
Fig. l - - S c h e m a t i c diagram of experimental setup.
Table I. Physical Properties of Liquids Used in Experiments Liquid Water Brine Water + sodium carboxy methyl cellulose Tetrachloroethylene Kerosene S
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