Extraction Fundamentals: Impact on Extractor Design & Operation

2013
Tim Kemper
Global Technical Director,
Solvent Extraction

view
a single cell

Bodies

the flake is surrounded by a bath of solvent.
Solvent Bath
Oilseed

Flake

through the cell walls of the outer layer of cells.

wall, it rapidly goes into solution with the oil bodies

inside the cell to form miscella within the cell.

of the miscella diffuses outward.

neighboring cells.

outward diffusion continues toward the center of the flake.

bath surrounding the outside of the flake.

of the miscella inside the cells of the flake comes

into equilibrium with the concentration of the miscella bath surrounding the flake.

Miscella Bath

Extracted Flake

RATE
SOLVENT RETENTION
NUMBER OF STAGES
Six Extraction Parameters

sides of each particle in the material bed.

Contact time is

time spent soaked by miscella, not just residence time !!

It is very important to flood the material in a bath of solvent at each stage to maximize contact time.

Contact Time

CONTACT TIME
“FLOODING”
GOOD
CONTACT

takes 5 minutes of contact time in each stage to

reach equilibrium.

If contact time is insufficient in any given extraction stage, then that stage will not reach equilibrium.

Residual oil in meal will increase !!

Contact Time

decrease the time required for the miscella in the cells

to reach equilibrium with the surrounding miscella by over 20%, reducing contact time needed from 5 minutes to under 4 minutes per stage.

20% thinner flakes require about 1.5 kwh per ton additional energy to create, adding significantly to operational costs.

Flake Thickness

decrease the miscella flux rate (downward miscella velocity) through the

flake bed by 40%, significantly moving back the location where the miscella exits the material bed through the screen floor.

In summary, 0.38 mm thick flakes reach equilibrium in 5 minutes of contact time, and minimize flaking operational costs. The extractor should be sized large enough to accommodate 0.38 mm thick flakes.

Flake Thickness

the miscella diffuses through cell walls,

and the faster the cells come into equilibrium with the surrounding miscella.

At the azeotropic temperature of 61-62°C, surface moisture and solvent evaporate off together. Surface moisture evaporating off also helps improve extraction efficiency.

Temperature

the extractor is operated too close to the boiling range,

excessive evaporation and over-pressurization will take place, losing significant solvent.

When balancing extraction efficiency with safety and environmental concerns,
the extractor should ideally be operated at or just under the azeotropic temperature at 60-62°C.

Temperature

descends down through the material bed. How the material is

prepared has a big impact on the miscella flux rate:

Good quality expanded soybean pellets
49 m3/h/m2 or 1.71 m/min down through expandates

Good quality 0.38 mm soybean flakes
19.5 m3/h/m2 or 0.58 m/min down through flakes

Good quality 0.30 mm soybean flakes
12 m3/h/m2 or 0.34/min down through flakes

Miscella Flux rate

material bed is designed to be in the

ideal location to catch the proper miscella stage.
If miscella flux rate reduces significantly, the miscella will descend to a later miscella collection hopper and contaminate a weaker miscella stage, causing residual oil to increase.

Miscella Flux rate

soybeans
Material Rate to Extractor 1958 kg/min
Material to Extractor 0.38

mm dehulled flakes
Slurry-filled Material Density 480 kg/m3
Miscella Flux Rate 19.5 m3/h/m2
Miscella Flux Rate as Velocity 0.325 m/min

Material Rate in Extractor 1958 kg/min / 480 kg/m3 = 4.08 m3/min
A = Area of Basket 4.89 m2
Basket Material Volume 4.89 m2 x 2.9 m = 14.2 m3
Horizontal Velocity 14.2 m3 / 4.08 m3/min = 3.47 min/basket

Open Area in Material Bed 4.89 x (1120-480)/1120 = 2.79 m2
V1 = Velocity above Material 0.325 m/min
V2 = Velocity within Material 4.89/2.79 x 0.325 m/min = 0.57 m/min
T1 = Head Time 0.15 m / 0.325 m/min = 0.47 min
T2 = Bed Time 2.9 m / 0.57 m/min = 5.08 min
Total Vertical Drain Time 0.47 + 5.08 = 5.55 min

Drainage Angle 5.55 min / 3.47 min/basket = 1.6 baskets
Drainage Angle 1.6 baskets x 20 deg/basket = 32 degrees
Minimum Stage Pump Flow 2 x 4.89 m2 x 19.5 m3/h/m2 = 191 m3/h

0.15 m

2.9 m

V1

V2

A

1.6
BASKET
DRAIN
ANGLE

32°
ORIENTATION
DRAIN
ANGLE

flakes

surface moisture in flakes
can be adjusted for with lower valve

manifold to miscella pumps

Flakes

Solvent

Full
miscella

Spent
flakes

Present Reflex Extractor Miscella staging for lesser quality flakes

going to the extractor through good flaking mill aspiration and good expander pellet drying improves the miscella flux rate.

Minimizing fines going to the extractor through more uniform grain drying, adequate tempering time, coarser cracking and proper material handling improves the miscella flux rate.

Miscella Flux rate

oil (18-20%) will fill up with the weakest stage of

miscella (0.5% oil) and be carried to the DT. Additional miscella will also be soaked into the cell, and will also coat the surface of the flake going to the DT, increasing solvent retention to 30%.

The lower the solvent retention to the DT, the less oil (0.5% oil with solvent) will be carried along,
thus reducing residual oil in the meal.

Solvent Retention

in the extractor is the most cost effective means of minimizing solvent retention.

Increasing the percentage of expanded pellets
increases the percentage of ruptured cell walls
to further reduce solvent retention down to 26%. However, expanders also increases upstream
operational costs.

Solvent Retention

white flakes,
with 30% solvent retention, the last, weakest miscella

concentration can not exceed 0.5% oil.

If an extractor had 1 miscella stage, then full miscella would need to be 0.5% oil to achieve 0.5% oil in white flakes. This would require a tremendous fresh hexane
flow and would be cost prohibitive in distillation energy.

Number of stages

equilibrium in 4 miscella stages, it can reach a weakest

miscella concentration under 0.5% with a full miscella concentration of 25% and achieve 0.5% oil in white flakes.

In the real world, it is difficult for every miscella stage
to reach complete equilibrium, so extractors need at least 5 stages (one excess). More stages increase safety factor at the expense of higher pump energy and maintenance.

Number of stages

RATE
SOLVENT RETENTION
NUMBER OF STAGES
Six Extraction Parameters