Spring 2006

Design of a thermal cracker with minimal pressure drop
and temperature gradient for removal of trace impurities
in a specialty gas stream.

Project Background

    Air Products wishes to scale up an existing heated, packed-bed reactor.  To do so, PSU

    students were asked to model the heat transfer and pressure drop that would be expected

    in the new reactor.  A section of the full scale reactor was constructed in order to measure

    pressure drop, and more importantly to record the dynamic and steady-state temperature

    of the reactor at various flow and heating rates.

Objectives

Aside from collecting data, the experiments purpose was to determine the feasibility of a

scale up operation.  Air Products provided some specifications to guide project testing:

    Target Specifications:

        Pressure Drop < 1" water column

        Temperature Gradient < 100 degrees Fahrenheit

Results

    Measurements with an inclined manometer showed the pressure drop to be lower than

0.20" WC across all tested flow rates.

    After testing multiple flow rates at variable heat inputs, numerous steady-state and

transient temperature profiles were created.  The following graph (Figure 1) shows the

temperature profiles and reactor settings which attained the 100 degree temperature

gradient requirement.  Interestingly, the introduction of a small flow (6.5cfm) resulted

in a temperature gradient comparable to zero flow (50 degree gradient), but with an

average temperature 50 degrees higher than zero-flow.  Upon further increase in flow

(10cfm), the temperature gradient remained within the 100 degree limit (~75 degrees),

but dropped nearly 130 degrees from the zero-flow case. 

The steady state behavior observed under constant heat input should prove useful in

the scale up process by determining the number of units needed to safely meet the

total volumetric flow requirement.  For example, if the overall volumetric flow is 50 cfm,

and a flow rate of 10 cfm provides the desired temperature profile, then 5 reactors

would be constructed.

Figure 1:  Steady State Temperature Profiles at 50% Variac with variable flow rates.

Conclusions and Recommendations

    It was found that reactor scale up is in fact feasible, based on the measured pressure

    and temperature data.  The recorded pressure drop (0.20" WC) was only 20% of the

    maximum allowed (1" WC).  Furthermore, the reactor was able to attain steady-state

    temperature profiles with less than a 100 degree Fahrenheit temperature gradient at

    certain flow and variac settings.  However, at the flow rate Air Products wishes to use

    we were unable to meet the temperature gradient requirements.  It is suggested that

    a smaller reactor be used, or an internal heat source be added to the reactor.

    For further experimentation, it is recommended that flow tests be run with an air

    source that is temperature regulated, due to effects of inlet air temperature on steady-

    state behavior.  By using an in-room compressor or air flow radiator, the effects of

    varying outside air temperature on the incoming air supply could be eliminated.