Page 316 - Fluid mechanics, heat transfer, and mass transfer

HEAT EXCHANGERS 297

. What are the major cost increasing factors due to & Factors affecting fouling in crude oil preheat ex-
fouling in the operation of a plant? changers include process conditions (temperature,
& Reduced flow: Due to decreased flow areas. pressure, and flow rate), exchanger and piping
configuration, crude oil composition, and inorganic
& Increased pressure: Due to increased pressure drops.
contaminants such as salts.
& Reduced production: Due to shutdowns consequent
& More often, the fouling mechanism responsible
to fouling.
for the deterioration of heat exchanger performance
& Cleaning and disposing of toxic wastes resulting
is flow velocity dependent. Maldistribution of flow,
from cleaning operations.
wakes, and eddies caused by poor heat exchanger
& Use of costly chemical additives to prevent fouling
geometry can have detrimental effect on fouling.
deposits.
& Uneven velocity profiles, backflows, and eddies gen-
& Increased safety hazards: Due to particularly, flow
erated on the shell side of a segmentally baffled heat
and pressure variations with consequent equipment
exchanger results in higher fouling and shorter run
leaks and failures leading to fires and toxic
lengths between periodic cleaning and maintenance
releases.
of tube bundles.
& Increased investment: Use of switchable exchangers.
. What are the differences in the resistances offered for
➢ Providing excess heat transfer surfaces.
heat transfer between a new heat exchanger and an
➢ Costs of antifouling equipment. exchanger in operation for a period of time for the same
➢ Costs of cleaning equipment, chemicals, and process application?
waste disposal. & Resistances get added due to fouling caused by
➢ Increased costs of maintenance. deposits.
& Equipment replacement. & Resistances get added due to formation of oxide films
due to corrosion.
. List out streams that normally do not foul and foul
. What is the overall effect of fouling of a heat exchanger
heavily.
tube on DP?
& Streams that Normally do not Foul:
& A tube with fouling deposits will increase DP in two
➢ Refrigerants.
ways:
➢ Demineralized water.
(i) Roughness resulting from fouling will offer more
➢ LNG.
resistance for flow.
➢ Olefin-free nonpolymerizing condensing gases.
(ii) Fouled tube will decrease effective flow cross
& Streams that Normally Foul Heavily:
section and offer more resistance for flow.
➢ Crude oil. . What are the different types of fouling on heat transfer
➢ Crude distillation overheads. surfaces?
➢ Amines. & Precipitation or Crystallization Fouling: Dissolved
➢ Hydrogen fluoride. solids get supersaturated at heat transfer surfaces and
➢ Coal gasification fluids. crystallization takes place from solution. Crystals
adhere to surfaces.
➢ Cooling water that is not properly maintained.
➢ Crystalline ionic salts deposit on heat transfer
. Give an example of the consequences of fouling in the
surfaces.
operation of a refinery.
➢ Normal solubility salts (inorganic/organic) precip-
& Most fouling arises from asphaltene deposition from
itate on colder surfaces.
the crude oil onto the metal surfaces of the preheated
➢ Inverse solubility salts precipitate on hot surfaces.
train of heat exchangers. This fouling leads to a
Examples are calcium salts such as calcium car-
decline in furnace inlet temperature, by perhaps as
bonates and sulfates. These are less soluble in hot
much as 30 C, and a subsequent need to burn extra

water than in cold water. Buildup of such salts on
fuel in the furnace to make up the temperature
heat transfer surfaces start on nucleation sites such
necessary for efficient distillation.
as scratches and pits. Such scales are hard and
& Generally, the fouling deposits that occur as high
require vigorous mechanical or chemical means
molecular weight polymers are formed in the crude
for their removal.
preheat systems. Products of corrosion and inorganic
& Particulate Fouling: Accumulation of finely divid-
salts mix with the polymers and increase the volume
ed particulates (silt, mud, sand, insoluble products,
of the fouling deposits.