Page 187 - Separation process engineering

and Q . Now L, V, , and can be calculated and we can proceed to an exact check that V and are
R
less than V max . This is done by calculating a permissible vapor velocity. This calculation is similar to

calculating u perm for a flash drum and is shown in Chapter 10. The condenser and reboiler sizes can also
be checked. If the flow rates are too large or the condenser and reboiler are too small, the existing column
will not satisfactorily do the desired separation. Either the feed rate can be decreased or L/D can be
decreased. This latter change obviously requires that the entire solution be repeated.

When other variables are specified, the stage-by-stage calculation is still trial and error. The basic
procedure remains the same. That is, calculate and plot everything you can first, guess the needed
variable, and then check whether the separation can be obtained with the existing number of stages.
Murphree stage efficiencies are easily employed in these calculations.

Simulation problems are probably easier to do using the matrix approach developed in Chapter 6, which
is easily adapted to computer use.

4.13 New Uses for Old Columns

Closely related to simulation is the use of existing or used distillation systems for new separations. The
new use may be debottlenecking—that is, increasing capacity for the same separation. With increasing

turnover of products, the problem of using equipment for new separations is becoming much more
common.
Why would we want to use an existing column for a problem it wasn’t designed for? First, it is usually
cheaper to modify a column that has already been paid for than to buy a new one. Second, it is usually
quicker to do minor modifications than to wait for construction of a new column. Finally, for many
engineers solving the often knotty problems involved in adapting a column to a new separation is an
interesting challenge.

The first thing to do when new chemicals are to be separated is clean the entire system and inspect it
thoroughly. Is the system in good shape? If not, will minor maintenance and parts replacement put the
equipment in working order? If there are major structural problems such as major corrosion, it will
probably be cheaper and less of a long-term headache to buy new equipment.

Do simulation calculations to determine how close the column will come to meeting the new separation
specifications. Rarely will the column provide a perfect answer to the new problem. Difficulties can be
classified as problems with the separation required and problems with capacity.

What can be done if the existing column cannot produce the desired product purities? The following steps
can be explored (they are listed roughly in the order of increasing cost).
1. Find out whether the product specifications can be relaxed. A purity of 99.5% is much easier to obtain
than 99.99%.

2. See if a higher reflux ratio will do the separation. Remember to check if column vapor capacity and
the reboiler and condenser are large enough. If they are, changes in L/D affect only operating costs.
3. Change the feed temperature. This may make a nonoptimum feed stage optimum.

4. Will a new feed stage at the optimum location (the existing feed stage is probably nonoptimum) allow
you to meet product specifications?

5. Consider replacing the existing trays (or packing) with more efficient or more closely spaced trays (or
new packing). This is relatively expensive but is cheaper than buying a completely new system.
6. Check to see if two existing columns can be hooked together in series to achieve the desired
separation. Feed can be introduced at the feed tray of either column or in between the two columns.