Page 78 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

some commonsense heuristics may be used to choose a good base case or starting point. The following
heuristics are modified from Douglas [1].
• If the impurities are not present in large quantities (say, <10–20%) and these impurities do not
react to form by-products, then do not separate them prior to feeding to the process. For
example, the hydrogen fed to the toluene HDA process contains a small amount of methane (5
mol%—see Stream 3 in Table 1.5). Because the methane does not react (it is inert) and it is
present as a small quantity, it is probably not worth considering separating it from the hydrogen.
• If the separation of the impurities is difficult (for example, an impurity forms an azeotrope with
the feed or the feed is a gas at the feed conditions), then do not separate them prior to feeding
to the process. For example, again consider the methane in Stream 3. The separation of methane

and hydrogen is relatively expensive (see Example 2.3) because it involves low temperature
and/or high pressure. This fact, coupled with the reasons given above, means that separation of
the feed would not normally be attempted.
• If the impurities foul or poison the catalyst, then purify the feed. For example, one of the most
common catalyst poisons is sulfur. This is especially true for catalysts containing Group VIII
metals such as iron, cobalt, nickel, palladium, and platinum [7]. In the steam reformation of
natural gas (methane) to produce hydrogen, the catalyst is rapidly poisoned by the small
amounts of sulfur in the feed. A guard bed of activated carbon (or zinc oxide) is placed
upstream of the reactor to reduce the sulfur level in the natural gas to the low ppm level.
• If the impurity reacts to form difficult-to-separate or hazardous products, then purify the feed. For
example, in the manufacture of isocyanates for use in the production of polyurethanes, the most
common synthesis path involves the reaction of phosgene with the appropriate amine [8].

Because phosgene is a highly toxic chemical, all phosgene is manufactured on-site via the
reaction of chlorine and carbon monoxide.
CO + Cl2 → COCl2
phosgene
If carbon monoxide is not readily available (by pipeline), then it must be manufactured via the
steam reformation of natural gas. The following equation shows the overall main reaction
(carbon dioxide may also be formed in the process, but it is not considered here):
CH4 + H2O → CO + 3H2
The question to ask is, At what purity must the carbon monoxide be fed to the phosgene unit? The
answer depends on what happens to the impurities in the CO. The main impurity is hydrogen.
The hydrogen reacts with the chlorine to form hydrogen chloride, which is difficult to remove
from the phosgene, is highly corrosive, and is detrimental to the isocyanate product. With this
information, it makes more sense to remove the hydrogen to the desired level in the carbon
monoxide stream rather than send it through with the CO and cause more separation problems in
the phosgene unit and further downstream. Acceptable hydrogen levels in carbon monoxide
feeds to phosgene units are less than 1%.

• If the impurity is present in large quantities, then purify the feed. This heuristic is fairly obvious
because significant additional work and heating/cooling duties are required to process the large
amount of impurity. Nevertheless, if the separation is difficult and the impurity acts as an inert,
then separation may still not be warranted. An obvious example is the use of air, rather than
pure oxygen, as a reactant. Because nitrogen often acts as an inert compound, the extra cost of
purifying the air is not justified compared with the lesser expense of processing the nitrogen
through the process. An added advantage of using air, as opposed to pure oxygen, is the heat
absorbing capacity of nitrogen, which helps moderate the temperature rise of many highly
exothermic oxidation reactions.