Is it true that in industrial practice a distillation column rarely consists of only a single type of trays throughout the column?
Your professor exaggerates a little.
Most new columns are designed with only one type of trays, although different sections in the column can have different tray spacing and/or different number of downcomers.
New columns designed with only packing can have the same packing througout the column, but it can also be the same sort, but different type number in different parts, for example: IMTP50 in the top and IMTP70 in the bottom, or Mellapak 350Y in the top and Mellapak 250Y in the bottom.
Through the years however certain sections in a column can become a bottleneck when the plant capacity is to be increased, and those sections can then be equipped with the latest type of trays or packing, which is different from the older stuff in the rest of the column. Sometimes corroded trays are to be replaced and it is decided that a newer type is to be used so as to make future capacity increase easier. As a consequence older columns have more chance of having different types installed than new columns.
Some new columns, like fractionators, are indeed designed from the start with different types of trays, or a combination of trays and packings, but those are a minority on the total number of columns in operation.
Choice for different tray and/or packing types in one column has to do with very different vapor and liquid loadings, different viscosity, different surface tension, different corrosivity, different fouling/coking tendency, different .... (whatever), in different sections of the column.
Edited by PingPong, 26 July 2014 - 10:46 AM.