Feedstock Definitions
Black Liquor is defined as a solution of 65% solids and 35% water, by mass. Of the total
solid mass, 45% is defined to be lignin, the primary chemical feedstock of this process.
Of the mass of lignin, 12.5% is defined to be methoxy groups, the reactive portion of the
lignin polymer. The spent black liquor, after reacting, will be sent back to the Kraft mill
this process will be co-located with, in order for the Kraft mill to burn it in their recovery
boilers. All other portions not previously defined are assumed to be inert for the purposes
of this process. For clarity’s sake, this includes: all non-methoxy portions of the lignin
polymer, all non-lignin solid mass in black liquor, along with any and all water contained
in the black liquor.
Sulfur is defined as pure, elemental sulfur, in the liquid phase. Plant utilities will be used
to maintain sulfur in this state prior to any reactive steps in the process. All sulfur
supplied will be utilized in a reactive step, leaving no waste sulfur. Any impurities
contained in the sulfur are assumed to be inert for the purposes of this process.
Tall Oil is defined as a useful by-product of the Kraft process that is added to our molten
sulfur (in a 1% by mass ratio) to inhibit polymerization. It is obtained from the paper mill
this plant will be co-located with. It will be returned to the Kraft mill in the spent Black
Liquor stream. The tall oil and any impurities present in the tall oil are assumed to be
inert for the purposes of this process.
Sodium Hydroxide is defined as a saturated (50%) solution of sodium hydroxide in water.
It is used to control the pH of the Black Liquor, important for a reactive step in the
process. The sodium hydroxide added will be returned to the Kraft mill in the spent Black
Liquor stream. The mixed in sodium hydroxide, as well as any impurities present within
it, are assumed to be inert for the purposes of this process.
Ammonia is defined as pure ammonia, obtained from a working pipeline dedicated to its
transport. It will be obtained from the pipeline as a pressurized liquid and then vaporized
as needed. Any unreacted ammonia will be disposed of by dissolution in a water solution,
with the water being obtained from a reaction of ammonia with oxygen. Any impurities
present in the ammonia are assumed to be inert for the purposes of this process.
Conversion Technology Description
Lignin->DMS Reactor: This agitated reactor, constructed out of stainless steel, converts
the methoxy groups in lignin into dimethyl sulfide, via the action of elemental sulfur. The
elemental sulfur anion displaces methyl groups in series from a methoxy group of the
polymer in an Sn2-like reaction. When two groups have been displaced, DMS is formed
and bubbles out of the solution fed to the reactor, in a 50% yield with respect to the lignin
supplied. This reaction occurs at a temperature exceeding the normal boiling point of
water (343F), and in the aqueous solution. Therefore, some method must be employed to
keep the water in black liquor from vaporizing. Our method used is to supply steam to the
reactor at such a rate that the overall pressure in the reactor is maintained at 116psia. The
supplied steam mixes with the DMS produced in the reaction and this vapor continues on
for further processing. This reaction is exothermic, and this reactor has a cooling jacket
with cooling water used to maintain its temperature.
DMS->DMSO Reactor: This reactor, constructed out of stainless steel, converts
vaporized dry DMS to DMSO via the action of NO2. The reaction occurs in the vapor
phase, and at relatively mild conditions (atmospheric pressure and 132F). This reaction is
exothermic, and this reactor has a cooling jacket with cooling water used to maintain its
temperature. An optional condensation plate can be added to this reactor for two reasons.
First, this condensation plate gives a place for the produced DMSO to condense onto, and
secondly, the colder this condensation plate is maintained at, the less by-products the
reactor will form.
NH3 -> NO Reactor: This reactor, constructed out of Inconel 600, converts vaporized
ammonia, mixed with air containing enough oxygen sufficient to convert all ammonia to
nitrogen dioxide (as well as monoxide), is heated to 1472F via a fired heater, and passed
over a catalyst of platinum with up to 10% rhodium. The catalyst, along with the extreme
temperatures, converts ammonia and oxygen to nitrogen monoxide and water in a 9598% yield. This reaction is exothermic, and the reactor has a cooling jacket of hot oil
used to maintain its temperature. The hot oil can be further cooled in a steam generator,
generating useful steam for this process.
NO -> NO2 Reactor: This reactor, constructed out of stainless steel, converts the nitrogen
monoxide produced in the previous step to nitrogen dioxide, using the remaining oxygen
in the fed air stream. This reactor also doubles as a regeneration point for recycled
nitrogen monoxide after it has reacted with DMS. The reaction occurs in the vapor phase,
at mild conditions (203F, atmospheric pressure), but care must be taken to keep the
temperature of this reaction low, as the reaction will reverse itself at higher temperatures,
leading to the breakdown of nitrogen dioxide. This reaction is exothermic, and the reactor
has a cooling jacket of cooling water used to maintain its temperature.
Separation Technology Description
It is assumed the reader is familiar with the general working concept of flash drums and
distillation columns. Flash drums are used throughout this process to separate more
volatile components from less volatile components. An 8-tray distillation column is used
at the end of the process as the final purification step in producing DMSO. The feed is
fed at stage three, numbering stages from the top down. All flash drums as well as the
distillation column are constructed out of stainless steel.
This process uses two packed desiccation columns, to produce dry DMS from a “wet”
DMS feed obtained by decantation. The columns should be packed with a desiccant –
such as silica gel - sufficient to remove enough water to reach an industrial specification
for dryness of DMS. Two columns are used, because as one column is actively
desiccating a DMS stream, another column can be regenerating. In order to regenerate the
column, an electric heater contained in the column can be turned on.
This process also uses pressure swing adsorption (PSA) to separate various components
of a vapor mixture from each other. Four PSA columns will be used, two pairs of two.
The first pair of columns will be used to separate NO2 from the other components in the
vapor stream, and the second pair of columns will be used to separate nitrogen and argon
from the remaining components in the vapor stream. If this PSA separation was not
present, nitrogen and argon would accumulate in the system, ultimately requiring
periodic venting of hazardous gases, or overuse of relief valves.
The columns themselves will be packed with a zeolite or membrane sufficient to perform
the stated seperation. In each pair of columns, one column will be venting the effluent,
while the other column will be venting the retained gas of interest. Valves and vacuum
pumps will ensure the gases will move in the correct fashion. The nitrogen and argon will
be vented to the atmosphere.
Product Description
Dimethyl Sulfoxide (DMSO) is the product of this overall process. It will be produced at a
rate of approximately 2200 lb/hr for 8000 hr/yr. We believe we can sell this product at a
price of $1.75/lb. The DMSO produced will meet at least technical grade requirements
(99.7% purity), ranging upwards to reagent and medical grade. More information about
the various grades of DMSO can be found in Appendix 13.
Location Sensitivity Analysis
Repeating the statements made about our process location and explaining the reasoning
behind each one:
Our process will be co-located with a Kraft paper mill, in the state of Louisiana.
Co-location with Kraft paper mill: The vast majority of the operating costs of this process
derive from the cost of black liquor, and the lignin contained therein. Due to the large
mass of black liquor required (on the order of 50,000 lb/hour) for the desired throughput
of DMSO, economic feasibility demands that the price of black liquor be no more than
perhaps 10 cents per pound.
Calculations derived from bond energies put the total energy loss of black liquor as the
result of our process at approximately 15% of the energy content of the incoming black
liquor. Based off the higher heating value of black liquor, and a standard cost of energy,
this energy loss is given a value of approximately 2.2 cents per pound.
Co-location with the paper mill allows us to completely eliminate transportation costs
associated with black liquor. Furthermore, since black liquor is already burned for energy
by the paper mill, we can return our spent liquor back to them for processing in their
plant. They likely have the facilities to handle its combustion, recovery of salts used in
the production of paper, and sequestration of components deemed unhealthy for the
environment. The paper mill also receives a large tax credit for burning black liquor
instead of disposing it in other fashions.
If this process were to be truly stand-alone, we would have to pay transportation costs for
black liquor, along with the full energy costs of black liquor plus the inert gases the Kraft
mill covets, along with facilities for the proper disposal of waste components in black
liquor. All of these costs added together would likely push the price of black liquor into
the dollars per pound range, and render the process economically suicidal.
Less important reasons for co-location include the free obtainment of Tall Oil for use as a
polymerization inhibitor, along with large reserves of water available for our process.
State of Louisiana: Several reasons point towards Louisiana being a prime location. First,
certain states do not allow the manufacture of hazardous gases (in our case, DMS) by
industry. Louisiana is not one of these states. Secondly, due to the large amount of preexisting petrochemical refineries in Louisiana, and the required desulfurization of diesel
fuel, sulfur prices in Louisiana should be very low compared to the rest of the country.
Lastly, in Louisiana there are ammonia pipelines we can tap into, giving an opportunity
for elimination of transportation costs relating to that feedstock.
Turndown Ratio
A study of the minimum throughput this process can obtain is outside the scope of this
design document. That said, this process is designed for nominal start-up operation
(without recycle streams), which operate at approximately 25-35% greater flow rates than
their corresponding values at steady-state.
Compliance
All efforts have been maintained to abide by the currently understood state of the science
behind chemical engineering.
An engineering safety hazards assay was not performed as part of the production of this
process. If this process were to proceed further, this design would be improved or
modified to ensure compliance with these accepted standards and laws.