Lyophilization for Water Recovery

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Lyophilization for Water Recovery
Eric Litwiller, Martin Reinhard Stanford
University Michael Flynn, John Fisher NASA Ames
Research Center
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Project Goal
Develop a lyophilizer that recovers water from
highly contaminated aqueous wastes,
including Appropriate wastes
Are very contaminated and/or very viscous
Contain volatile or reactive compounds
Contain enough water to make processing
worthwhile
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Lyophilization (or Freeze-Drying) General
Process Description
Lyophilization is a batch or semi-batch process,
consisting of the following steps

• Freezing -- Aqueous material is frozen at
  atmospheric pressure, separating ice crystals
  from contaminants.
• Primary Drying -- Moderate vacuum (lt200 Pa) is
  applied, causing the ice to sublime. The water
  vapor condenses onto a cold surface.
• Secondary Drying -- When ice crystals have been
  removed, the remaining solid is heated further,
  vaporizing residual, bound water.

Primary drying is the most time- and
energy-intensive step In the process.
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Water Phase Diagram
1 atm
Primary Drying Conditions
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Heat and Mass Transfer During Drying
Dried area of solid
Heat sink, 15C
4
Frozen area of solid
5
3
TECs
2
9
1
Heating plate, - 15C
8
Condenser, - 25C
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1. Heat conducts from heating plate to ice 2.
Heat conducts through ice in waste 3. Ice
sublimes at surface 4. Water vapor flows through
dry layer 5. Water vapor flows through vacuum
chamber 6. Water vapor turns to ice on
condenser 7. Heat conducts through ice on
condenser 8. TECs pump heat from condenser to
heating plate 9. Waste heat (IV from 8) is
removed to heat sink by additional TECs
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Laboratory Scale System
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Conclusions from Lab Scale Experiments
Dried area of solid
Heat sink, 15C
4
Frozen area of solid
5
3
TECs
2
9
1
Heating plate, - 15C
8
Condenser, - 25C
7
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Heat transfer from heating plate to ice in waste
(steps 1 and 2) is the rate-limiting resistance.
The dried layer that develops between plate and
ice has poor thermal conductivity. Solution
compress waste against heating plate to increase
thermal conductivity and decrease thickness of
this layer.
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Heating Block and Condenser
Heating Block
Condenser
Heat Sink
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Closed End of Condenser
Tubes supply coolant to heat sinks
Watertight seal over one end of condenser
Vacuum/water recovery tube
Mylar (not airtight) prevents ice from forming on
outside of condenser
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Vacuum Chamber
Lids incorporate piston and bellows
Mylar insulation
Set screws suspend block in chamber
ePTFE bag of waste compressed by piston
Electrical fluid feedthroughs
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Lid with Pneumatically-Actuated Piston
Piston incorporates screen at surface and holes
to permit vapor transfer
Piston is thermally isolated from bellows
Pneumatic cylinder for experiments at different
compression pressures
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System Overview
Unit rotates in stand so that either end may face
up
Vacuum pump runs if pressure exceeds setpoint
(usually off)
Lines for air to cylinders and position sensors
Chiller supplies water at 40F
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Ice on Condenser
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Ice on Condenser, Detail
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Ice Deposition on Condenser
Resistance to heat flow through ice should cause
uniform deposition on condenser
Vapor pressure at surface of thicker area is
higher by approximately 4/mm
Possible Explanation and Solution
Temperatures at Start of Drying

• In the absence of nucleation sites, vapor
  supercools
• Ice eventually forms at most favorable sites,
  corners of fins at condenser opening
• Additional ice grows from these sites rather than
  forming new nuclei
• Solve by treating surface, or by retaining some
  water from previous run on surface

T K
t min
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Pilot System Status Checklist
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Heating block is well-insulated within
chamber After drying, maintaining condenser at
1C consumes 4W of electrical power Condenser
is well-insulated within heating
block Resistance to heat flow within heating
block is low Measured DT between condenser TECs
and heat sink TECs is lt 3C Chamber maintains
vacuum with pump off every time lids are
closed Bags contain waste without leaks while
allowing passage of water vapor Pistons compress
waste without rupturing bags Waste can be dried
fully Average of three runs using monkey feces
98 of initial water removed, feces dried to 5
water (and other volatiles) by weight Water
collects at condenser rather than exiting through
vacuum pump Water can be melted in condenser and
collected through tube (with gravity) 95-100
of mass removed from feces is collected through
tube Resistance to heat flow between heating
block and waste is low Resistance to vapor flow
between waste and condenser is low Materials
with low eutectic points can be dried
successfully without fully freezing
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