The process for making biodiesel consists of the
following steps:
·
Preheat
used vegetable oil to remove water
·
Determine
oil pH and measure appropriate quantity of NaOH and methanol
·
Mix reactants
·
Allow
glycerin to settle out, drain glycerin and send to methanol recapture
·
Wash
biodiesel to remove trace contaminants
·
Set
in the sun to drive off traces of water
The process design
is divided into the following stages:
- pre-heat,
- processing,
- wash,
- methanol recapture,
- water treatment
- solar heat.
Preheat Stage
The role of the
preheat stage s to remove any water that might be mixed with the used vegetable
oil. Water is detrimental to the transesterification process and must be
removed as much as possible. Heating the oil to 50-54 oC
is enough to cause the water to settle out.
There are two preheat tanks. The tank volumes are 150
gal each. Using two tanks allows for the continual receiving of used vegetable
oil. Hot water flows from a propane heater and through a heating coil to heat
the oil, the cooled water is then returned to the heater.
The valve system
is designed such that hot water will flow through only one tank at a time,
likewise the vegetable oil will be pumped out of the tanks sequentially. While
one tank is pre-heating, the other is available for receiving used oil.
Figure 1 - Schematic of Preheat
Stage
Processing Stage
The processing
stage consists of two reactor tanks, each connected to its own pump for
recirculating and mixing of the reactants. The first tank, reactor A, is tall
and narrow with a conical bottom. This reactor shape provides adequate mixing
of the fluid when recirculated by the pump. The second tank, reactor B, is
bulkier and rounded on the bottom, which would create dead zones if mixed by
simple recirculation. To overcome this problem and ensure adequate mixing in
both processing tanks, a mechanical mixer with impellers was purchased. Static
mixers were also purchased to increase mixing within the pipes.
The goal of the
processing piping system is to ensure flexibility of the system while
maximizing use of only two pumps. The primary feature of this system is that it
allows for the transfer between the two reactors. Used vegetable oil can be
pumped into either tank via pump A, and then recirculated with the reactor
tanks via their respective pumps. Contents can be transferred between the two
tanks via this piping scheme. The proposed processing method is
outlined as follows:
·
Used vegetable
oil is pumped from the preheat stage into reactor B.
·
Mixed
methoxide is added to reactor B and circulated to provide initial mixing.
·
Half
of the mixture is then transferred to reactor A, and both tanks are
recirculated for the
required reaction times.
required reaction times.
· Following
the reaction, the tanks are allowed to settle and glycerin is drained out for eventual methanol recapture.
·
The
biodiesel product is pumped out towards the washing stage.
Initially mixing
all of the reactants in one container ensures homogeneity of the oil:methoxide
ration within both reactors. This removes one measuring step. Using both
reactors for processing allows for eventual expansion. The system also
maximizes the use of the two pumps, which perform all the required transferring
and recirculating.
Pump A is
significantly larger than pump B. It also has more power and the additional
feature that it can run dry without damaging. These two features make pump A
the best choice for pumping used vegetable oil into the reactor, as it doesn’t
require the operator to monitor the level in the preheat tanks.
Figure 2 - Schematic of
Processing Stage
Washing Stage
There are many
methods of washing available when processing biodiesel. An ideal washing method maximizes interaction between the fresh biodiesel and washing water, while
minimizing the risk of emulsifying. Alternatively, water and biodiesel could be
simply mixed manually. This works well
for small batches, however the manual aspect makes it increasingly
time-consuming with increase in batch size, and more difficult to control to
avoid emulsification if automated.
The most
commonly recommended method of washing biodiesel is aeration, whereby air is
bubbled through layers of water and biodiesel. The bubbles rising into the
biodiesel carry with them a thin film of water. The biodiesel contaminants
dissolve in the water. When the bubble bursts, this water falls out of the
biodiesel layer and returns to the water layer, brining the contaminants with
it. This method yields a very high water-biodiesel surface area and minimizes the
risk of emulsification. It also has a lower water requirement.
The wash tank design makes use of the following items:
·
A 1 m3 plastic tank
donated by a paint company.
·
The compressor
required for the operation of the pneumatic pumps.
·
Lengths of ½”
PVC piping for the aerator.
The aerator is constructed by drilling small holes
along the PVC pipe at 3” intervals. Four lengths of tubing, running parallel
along the bottom of the tank, are connected to a single shaft extending to the
top of the tank. The compressor connects to a regulator and then to the shaft.
The rate of air flow through the aerator, and thus the rate of bubbles, is
controlled by the regulator.
For each batch, an amount of water equal to half the
volume of biodiesel is added to the wash tank, followed by the biodiesel. The
product is bubble-washed for 6 hours and then let to settle for 1 hour. This
wash is repeated three times, each time the water is drained from the bottom
and new water is added.
One
identified problem with the aerator system is that biodiesel degrades PVC glue
over time. As such, it is important to minimize the interaction between the
aerator and the biodiesel. The paint tank has a built-in drain located at the
bottom, but an additional tap was installed a few inches higher, which is the
tap that will be used to drain the water to ensure that a certain volume of
water will remain above the aerator at all times, protecting the PVC piping.
The total volume of the wash tank is 350 gal. Given
that some airspace is required at the top, and
1/3 of the tank ΄s volume must be water, this wash tank cannot wash
biodiesel batches greater than 200 gal. Should we choose to increase
production, a second wash system would be required.
Figure 3 - Schematic of Wash Tank
Methanol Recapture
Methanol is the
most expensive input for the biodiesel process. The price is tied to the price
of oil, and as such is at risk of instability as crude prices rise.
In order to ensure that the reaction consumes all of
the vegetable oil, methanol is added in excess to force the equilibrium to the
right. This excess methanol ends up in the glycerin by-product and represents a
significant loss. Methanol in the glycerin also limits the potential for
marketing it as a product, as the combination is deemed unsafe and flammable.
Given the relatively low boiling point of methanol, it
is possible to recapture the methanol via a simple still. The mix of glycerin
and methanol, still liquid following the reaction stage, can be heated to
vaporize the methanol. These vapors can then be condensed and recycled,
maximizing use, reducing waste, and lowering overall processing cost.
An additional methanol recapture system is needed and
this could be constructed of items that
can be found easily.
Figure 4 - Schematic of Methanol Figure 5 - Photograph of a Methanol
Recapture Unit Recapture Unit
The glycerin by-product is poured in to a chemical
container through a funnel in the lid. The container must be sturdy and
air-tight. An electric heater heats the glycerin to the methanol boiling point
of 66°F.
The vapors rise through the bucket and into a length of copper tubing. The
copper tubing then coils as it enters a condenser. Cold tap water passes
through the condenser, cooling the methanol vapors to a liquid. The liquid
falls through the copper tubing where it is collected at the bottom. Once the liquid
methanol stops flowing, the process is complete and the glycerin is drained
from a tap while still liquid.
Water Treatment
In an effort to reduce the overall impact of biodiesel
production, water consumption and recycling were considered. Three wash stages
are required for biodiesel production, the first stage taking out the most
contaminants and each subsequent wash containing significantly less. Water
recycling then becomes an ideal method for reducing overall water consumption
within the process.
Each wash stage,
at maximum production capacity uses 100 gal of water. The first wash produces
water too dirty for reuse, but the water from the second and third washes are
fairly clean. To reuse the water, two interim storage tanks of 100 gallons each
will be placed near the washing station.
The water from the second and third washing stages
will be pumped into the storage tanks and then reused in the following wash
process. The general recycling process is outlined Figure 6. The water from wash 3 will be reused
for wash 2, and the water from wash 2 reused in wash 1. Once the loop is
established, the required water consumption for washing will be reduced by 2/3,
requiring only half a gallon of water per gallon of biodiesel instead of 1.5.
Figure 6 - Outline of Water Recycling Process
In addition to water recycling, the following water
treatment options is also explored, with the hopes of bringing the process
water demand to nearly zero.
This method of water purification produces almost
perfect distilled water and would remove all of the glycerin, methanol, and
dissolved solids. Solar distillation, however, is highly inefficient and would
require an extremely large surface area in order to treat the amount of water
needed for biodiesel production. Through some research and testing, it was
determined that a solar still covering an area of 4m2
could only treat about 30 gallons a week, simply not enough and anything larger
would take up too much space and be too costly to implement.
Slow Sand Filtration
Slow sand filters are very efficient at removing
organic solids and quite inexpensive to construct. The problems with this type
of system are that it may clog up if the waste water is too turbid, and it will
not remove dissolved solids. This means that the waste water from the washing
stage of biodiesel production might have to be pre-treated to decrease
turbidity in order to pass it through a sand filter. Furthermore, it remains
unclear exactly how much dissolved caustic soda would remain in the treated water
and its effects on whether the water would be acceptable for re-use. Finally,
there may be a problem with using a slow sand filter since most of the actual
water treatment is performed by the microorganisms living in the top layer of
the sand and it is still unclear how these organisms would fair with the filter
not in constant use..
Methanol Evaporation
In some of the literature examined for the biodiesel
internship there was mention of heating the unwashed biodiesel to boil off the
methanol and cause the suspended solids to precipitate out. The theory is that
methanol evaporates at a much lower temperature than biodiesel (only 60C) and
thus could safely be evaporated out which would release the suspended solids
leaving clean, useable biodiesel. This step would, in theory, altogether
eliminate the need for a wash stage and thus eliminate the
need for clean water..
Solar Heating
In further attempts to reduce the environmental impact
of biodiesel production, solar heating could be investigated. The energy
requirements for the oil pretreatment stage could easily be met using a solar
heating system.
The design
requires a pump to allow the working fluid to circulate through the element up
towards the solar panel. A debate emerged between two possible options:
indirect or direct solar heating. Direct heating involves having the used
vegetable heat directly as it flows through the coil of the solar panel.
However, this option is eliminated as it causes tube-side fouling hence resulting
in higher maintenance needs.
