New way to produce lignin from black liquor
by Michael Lake

Sequential Liquid-Lignin Recovery & Purification (SLRP) is a new, continuous process for producing lignin from black liquor and is being developed by Charleston, S.C.-based Liquid Lignin Company, LLC (LLC²). A patent by Michael Lake and John Blackburn is pending.

SLRP's development has been funded by Small Business Innovation Research (SBIR) grants from the Department of Energy. LLC² won their Phase I in November 2009 and their Phase II in August 2010; their total DOE award is $1.15M.

LLC² is currently starting up a pilot plant. They further plan to construct a prototype SLRP process within a southeastern papermaking operation in early 2013, capable of producing 3-6 tons/day of product lignin.

The basic principle underlying SLRP technology is recovering a dense-phase "liquid lignin" from a black liquor stream by reducing the pH with carbon dioxide (CO₂) and maintaining elevated temperature and pressure to keep the mixture from boiling. This allows retaining the heat of reaction (CO₂with black liquor components) in the system. The depleted black liquor stream is returned to the evaporator system at a higher temperature than the black liquor sent to the SLRP. This step alone saves significant energy compared to competitive lignin recovery processes which lower the black liquor temperature to precipitate filterable lignin. Alternative processes lose not only the heat of reaction of CO₂but also the sensible heat from cooling the black liquor feed.

SLRP's Unit Operations
Precipitating the lignin as a liquid phase is robust, since the liquid-lignin particles precipitated in SLRP naturally coalesce to form a dense bulk liquid-lignin phase that sinks within the less-dense carbonated black liquor matrix. This dense lignin phase behaves like a true liquid; in fact, mirror-like interfaces have been observed on the lignin from some black liquors.

Competitive processes--all of which operate at much lower temperature than SLRP--are likely to be much more temperature sensitive. The temperature of those processes must be maintained sufficiently high so that the skin of each lignin particle is soft enough to stick when they collide, forming agglomerates of small particles which can be filtered more easily. But the temperature cannot be so high that the particles melt and fuse, since an unfilterable mud would result. The temperature has to be maintained in a relatively narrow operating range. This optimum temperature window can change if the properties of the incoming black liquor change. SLRP has been found to be insensitive to temperature changes, and multiple black liquor feedstocks have behaved similarly under the same temperature operating conditions.

SLRP is totally continuous. The liquid-lignin phase from the carbonation stage is pumped into an acidification reactor where the lignin is reduced to pH 2-3 to shift the equilibria of sodium and other cations on the lignin to favor their partitioning in the aqueous phase. The acidification reactor is operated under pressure and temperature maintained at an elevated level, so the acid brine can be concentrated. Again the heat of reaction--a substantial heat from reaction of sulfuric acid--is retained by the SLRP system and returned to the host mill's evaporator system contained within the acid brine.

From the acidification reactor, the lignin is pumped to an ash removal system where residual acid and sulfate brine are displaced from the lignin phase by countercurrent contact with water. Like the previous carbonation and acidification systems, the ash removal system is operated under pressure and at elevated temperature. Since it is continuously generated, the dilute brine from this system can be recycled and used as diluent for the concentrated sulfuric acid stream entering the acidification reactor, significantly reducing the water requirements of the overall system. The competitive batch lignin recovery systems can do this also, but large storage tanks would be required and the temperatures would be much lower, near ambient for the brine recycle stream.

SLRP can be designed with very small tanks, essentially surge vessels in the interconnecting process lines that interface with the host mill. Because of its continuous nature, SLRP can be operated with relatively few operators.

SLRP can recover more than 50% of the lignin from the total black liquor, allowing a proportional increase of black liquor flow through a Recovery Furnace. Implementing any lignin recovery technology would give papermakers two strong benefits:

• A low-capital-cost option to achieve significant potential increase in the Recovery Furnace throughput
• Another revenue stream from sale of recovered lignin.

LLC² will participate in the "Future Products for the Pulp and Paper Industry" panel at PaperCon 2012 in New Orleans, which will be held at 1:30 PM, Monday April 23, 2012. Michael Lake will describe the science underlying the SLRP process and the future potential applications for lignin. Contact him at: michael@liquidlignin.com.

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