Question

Explain the start-up issues in a distillation column. Also, explain the function of reboiler and give its types.

Answer 1

A variety of performance problems can arise in distillation towers during startup. Many are caused by equipment, hardware or process conditions. Hardware issues may be related to instrumentation, tower internals or ancillaries. Instrumentation malfunctions are also common during startup and are predominantly related to liquid level indicators, online analysis of key components and in some cases, flowmeters. If not properly compensated, flowmeters can mislead the operator on streams operating outside the normal design envelope. Vessel-isolation blinds, valve alignments or unexpected obstructions in valves can also restrict or misdirect flows. Any of these issues could lead to hazardous conditions and ultimately process incidents. Anomalies in tower internals, such as obstructions, mechanical damage or poor installation can also cause towers to perform poorly. Meanwhile, issues related to process conditions could arise in all operations conducted from the point of shutdown to getting the tower ready for startup, during the startup itself, and ultimately during steady-state operation. The presence of unexpected impurities, such as water, are also known to cause problems during distillation, or in some cases may result in pressure surges.

Classification of reboilers

There are three major types of reboilers:

i. Thermosyphon natural circulation reboiler: The boiling occurs inside the tubes in vertical thermosyphon reboiler and inside shell in horizontal thermosyphon reboiler . In vertical thermosyphon reboiler, the liquid circulation occurs due to density difference between vapor-liquid mixture (two phase) in the exchanger from the reboiler and the liquid through the downcomer to the reboiler.

Advantages: most economical because no pump is required.

Limitations: not suitable for heavily viscous fluid; high construction cost for the installation of the column base at suitable elevation to get thermosyphon effect; not suitable for low temperature difference processes due to boiling point elevation imposed by static head.

ii. Forced circulation reboiler : The liquid is fed by means of a pump. Forced circulation reboilers with vertical or horizontal tubes boiling may be designed. Forced circulation reboilers are similar to vertical thermosiphon reboilers, except the pump is used for the circulation of the liquid and the hot liquid flows inside column. To calculate the heat transfer coefficient it is generally assumed that, heat is transferred only by forced convection. The usual method of shell and tube exchanger design can be used.

Advantage: suitable for viscous and highly fouling fluids.

Disadvantage: high pumping and maintenance cost; pump is required to circulate the boiling liquid through the tubes and back into the column.

iii. Kettle reboiler : The tube bundle is immerged in a pool of liquid at the base of the column in an oversize shell . Kettle reboiler is also called a “submerged bundle reboiler”. The height of the tube bundle is usually 40-60% of the shell ID. The submergence of the tube bundle is assured by an overflow weir at height of typically 5-15 cm from the upper surface of topmost tubes.

Advantage: suitable for vacuum operation and high vaporization rate up to about 80% of the feed.

Limitations: low heat transfer rate than other types as there is no liquid circulation (low velocity); not appropriate for fouling fluids; kettle reboiler is not suitable for heat sensitive materials as it has higher residence time.