Assignment-2

1) Ethane, Propane, Butane & C5+ (Pentanes & Hexanes) hydrocarbon mixture (feed
temp- 35C) needs to be separated using Distillation columns (multiple). This plant is
located in India with ambient temperature of 40C (cooling water temp @ 35C).
- Draw a scheme (without use of Trays/Internals) in order separate C2, C3, C4 & C5+
& rough Column temperature & Pressure
- Provide explanation for the sequence of the columns
- Provide Water cooled condenser or refrigeration section as required.
- No need to provide capacity
- Assume feed Pressure
 The liquids extracted from natural gas are an important source of feedstock for a
number of other petrochemical processes. The ethane and propane are used as feed to
ethylene plants and a refinery's alkylation unit uses the iso-butane. To separate these
compounds from the natural gas liquids (NGL), a series of distillation towers is used
to separate the methane and heavier streams into individual pure product streams.

Schematic diagram:
  The demethanizer column is operated at a pressure of 60bar since the mixture of
methane(feed) condenses at 0C at this pressure. The mixture then enters deethanizer
at an operating pressure of 37bar as these conditions will condense ethane out at
21C.the mixture then moves to the depropanizer operating at a pressure of 12bar, the
overflow product moves to a refrigerator(as the dew point of propane at this pressure
is greater than that of the cooling medium temperature of water at 35C) which
condenses out propane. The mixture then moves to debutanizer, product overflow
goes to a C4 splitter giving out n-butane and iso-butane product. The underflow of the
debuthanizer gives C5+ products.
 Since the various hydrocarbons in the NGL stream are easily separated by their
boiling point, the NGL feed moves from one distillation (fractionator) tower to
another. Each tower separates one of the hydrocarbon products and sends the
remaining product on to the next tower. The absorption process is carried out
relatively higher pressure and low temperature. The product moves from the
demethanizer tower where the methane is removed as the overhead product stream to
the deethanizer tower for the ethane and on to the depropanizer tower for the propane.
Since there are two types of butanes in the NGL stream, it takes two towers to
separate them into pure product streams. The first step is the debutanizer that
separates the total butanes (iso-C4 and the normal-C4) from the remaining NGL
stream. The iC4 and nC4 are then separated from each other in the C4 splitter tower.
The C5and heavier components remaining in the NGL stream is typically left as a
C5+ product stream to be used in other processes such as refinery gasoline blending.

2. Debutanizer column to be designed to separate C4s & C5s. This same column to be
provided at two locations (as listed below). For the same feed & product specifications,
at which location column will be operated at low pressure & why?

Location 1 – India with amb temperature of 40C (cooling water temp @ 35C)

Location 2 – Russia with amb temperature of 20C (Cooling water temp @ 15C)

 The debutanizer column must be operated at low pressure conditions in Russia which
has an ambient temperature of 20C (Cooling water temp @ 15C). When the feed is
supplied at an intermediate stage in the debutanizer operating at a lower pressure (and
correspondingly lower temperature) matching that of the low pressure depropanizer.
 This allows the use of one slightly larger distillation column in place of the two large
columns. Also when the overhead temperature is reduced it reduces pressure in the
column and improves relative volatility of the materials being separated which in turn
reduces reboiler duty.
 The reboilers and almost all of the trays in the debutanizer are a subject to fouling and
catalytic polymerization of unsaturated hydrocarbons. This rate of fouling doubles for
every 18C increase in temperature. Thus, reducing operating pressure and temperature
fouling is reduced and also operation and maintenance costs are saved. Also, if
debutanizers are operated at high pressures it leads to formation Gas hydrates which
cause pipeline plugs, which lead to safety and operational issues in the oil and gas
industries.2 Furthermore, gas hydrates can plug tower trays and valves, resulting in
tower flooding. This problem can be overcome by dehydration and low pressure and
low temperature operation.