Heat generated during thermo-mechanical pulping (TMP) is wasted unless a suitable
means of recovery is provided. This heat appears as steam and is usually from two
sources:
1) Cyclone under pressure, and
2) Atmospheric vents from refiners and stock chests.
The cyclone sources have pressure up to 20 to 30 psig and are fairly free of air
entrainment. These cyclones are normally located in the discharge lines from pressurized
refiners--usually in the primary refiners. Pressurized steam can also come from
secondary refiners or from pressurized stone grinders.
Atmospheric steam can be collected from secondary refiners reject refiners, and various
stock chests. This steam is usually accompanied by 5 to 20 percent air by volume, thus
reducing the maximum condensing temperature to 200 to 208 degrees F.
The heat recovery is best accomplished by condensing the various steam flows. How the
steam is condensed depends upon the heat sink available. Either direct contact or
indirect contact condensers can be used.
When the steam is condensed, its latent heat is given up and can be utilized for heating
process water. Since the steam can be contaminated by fiber and volatile wood
components, the method of condensation of the steam will be determined by the use for
the heated process water.
In direct contact condensers, the process water is showered into the steam, and as the
steam condenses, the shower water is heated.
The volatile components from the wood in the steam will also be condensed, thus the
heated process water will contain these contaminants. Also, any pulp fiber entering the
condenser with the steam will be washed from the steam. Water heated in a direct
contact condenser can be used for pulp dilution, for wash water on chemical pulp
washers, etc., where the contaminants and fiber can be tolerated.
Indirect contact condensers can be used when the heat is to be added to a process
stream that cannot be contaminated by condensed volatiles or fibers. These
contaminants remain with the steam condensate and can be maintained separate from
the process water.
Examples of process waters which may be used as heat sinks for indirect contact
condensers are:
• Boiler feed water,
• Water or glycol Systems for transfer of heat to air systems, or
• Recirculation water for flashing to a vapor recompression unit.
The steam recompression of vapor flashed from hot water is particularly attractive when
pressurized steam is available from a cyclone. In this application, either a thermal or
mechanical compressor can be used to increase steam pressure from 30 to 60 psig. The
higher pressure steam can then be used in paper machine dryers or returned to the
appropriate mill steam header.
Auxiliary equipment that can be included in TMP Heat Recovery Systems include steam
scrubbers, after condensers, and mist entrainment separators.
A steam scrubber is utilized to wash pulp fibers and dust from the steam prior to condensation. In
order to minimize condensation of steam in the scrubber, the scrubbing medium is hot water at
the steam saturation temperature. Fiber filters are then utilized to remove the fiber from the
recirculating scrubber water. The only water makeup to the system is that required to replace the
fiber filter backwash. Since the scrubber is operated at steam temperature, volatile compounds in
the steam will not be condensed.
Depending upon the operating pressure of the system, a backwash receiver in equilibrium
with the steam can be used to prevent flashing across the filter control valves. The flash
is subsequently taken on a more suitable valve used as the receiver level control valve.
An after condenser can be utilized to condense excess steam leaving the primary
condenser. The operation of the primary condenser, either direct contact or indirect
contact, may be at a temperature that is too great to adequately condense all steam prior
to discharge from the system. Removal of all excessive water vapor in an after
condenser will reduce the size of an induced draft fan and will reduce the condensation
plume from a vent stack.
Prior to an induced draft fan, a mist entrainment separator can be used. This unit
removes water droplets from the gas stream and improves operation of the fan.
The actual system selected for TMP Heat Recovery requires consideration of the total mill
heat balance and the unique requirements of each installation. Not only must the amount
of steam generated be defined, but heat sinks must also be identified.
TMP Heat Recovery System Flow Diagram
DL-4568
TMP Heat Recovery System Flow Diagram
DL-4569
TMP Heat Recovery System Flow Diagram
DL-4570