lignin case study

Executive Summary

Black liquor is a common byproduct in the pulp and paper process, which has the lignin from pulp chips. Separating lignin from this mixture is important as lignin can be used as a raw material and additive to consumer products. One particular material that lignin can be added to is polypropylene (PP), which increases degradation temperatures, polymer strength and oxidation reaction time. The process to separate lignin from black liquor is a series of acids being added to the black liquor, mixing, and filtrating through three cycles. This precipitates out the lignin, then the lignin is boiled, all acids are evaporated and the lignin powder product is formed. Since there is acid and some equipment appropriate safety and relief devices will be necessary and implemented in the process design. Product lignin is stored in a water-proof container and any black or white liquor must be contained in a pressure tested vessel. Over the past ten years, the price and demand of PP have increased every year. This means that the addition of lignin to PP doesn't only add thermal and physical properties to the material but it is economical to invest in an improved plastic for future usage. Raw materials that would need to be purchased are low cost acids and can easily be obtained. Lignin is priced between 0.003 $/kg (Alibaba, 2018) and 0.50 $/kg (Sigma Aldrich, 2018), this allows maximum profit while still being a competitive market.

Background Literature

Black liquor is a major by-product of the Kraft pulping process. White liquor, a mixture of sodium hydroxide and sodium sulfide, is added to wood chips and then heated in order to break down the wood fibers into pulp. Once the pulp is separated, the resulting mixture is a black, tarry substance called black liquor. The white liquor has leached out a lot of chemicals from the wood chips, including the structural polymer lignin. Traditionally, black liquor is burned and then run through a chemical process to regenerate it back into white liquor. The scope of this project is to separate the lignin and hemicellulose components from the black liquor in order create a higher-value raw material using these natural polymers. The remaining black liquor will still be able to be regenerated.

One route for the separation of lignin from black liquor is to take advantage of the functional groups on the lignin polymer. The majority of lignin in softwoods is insoluble in acidic solutions (Olsson 2018) and can be precipitated from the liquid by dropping the pH. Black liquor is very strongly alkaline, with a pH usually around 13.4. Lignin begins to precipitate at pH’s around 11 and should be completely precipitated at pH 9 (Fatehi, 2016). This separation can be done with two stages, initially by sparging CO2 through the mixture and then with the addition of concentrated H2SO­4. This process is called LignoBoost, a common and patented process to separate lignin from black liquor. However the product is only at 70 wt% lignin which could be improved (Ohman, 2015). Upon further purification, the resultant lignin powder can have a wide range of industrial applications.

One industrial use for powdered lignin would be as an additive to thermoplastics. It is not very useful as a pure product, being prone to brittleness. Polypropylene and polybutadiene both have similar bonding mechanisms to lignin, so mixing lignin in can reduce the demand for these petroleum-based plastics and provide a route for biodegradation. One study found that by adding lignin to PP, the degradation temperatures, polymer strength and oxidation reaction time increased (Morandim-Giannetti, 2010). By adding a cross-linking agent to the lignin, it is possible to control the average molecular weight in order to adjust the final properties of the polymer blend (Saito et al. 2012). The higher molecular weight lignin will be harder and more brittle, while lower weights will be more flexible and elastic.

Process Pathway

In order to separate lignin from black liquor several assumptions need to be made. First, the black liquor is from Pacific Northwest softwoods and contains 30 wt% lignin on average with a pH of 13.4 (Fatehi, 2016). Second, organic solvents, such as methanol, will allow for separation of lignin from remaining ash and hemi-cellulose. Lastly complete separation of acid-insoluble lignin occurs at pH 9.

There are several patented processes that separate lignin from black liquor such as LignoBoost and LignoForce. The proposed process for lignin separation uses the LignoBoost process however with a few additions to increase product quality. The process flow diagram is below in Figure 1.

Figure 1. Proposed Process Diagram for lignin from kraft Black Liquor

In this process, kraft black liquor is sent to a precipitator reactor and sparged with CO2. This lowers the pH from 13.4 to 11.0, beginning the precipitation of lignin out of the black liquor. The solution is then put through a drum filter, the filtrate or "lignin slurry" is moved to a mixing vessel and effluent is recycled back into the kraft liquor recycle system. In the mixing vessel sulfuric acid is added, this lowers the pH to 9.0 which is the point at which majority lignin precipitates out. There is a possibility that the reduction of pH to 9.0 could cause the generation of H2S gas, which with a boiler could be recycled, further research is required to determine if any side reaction occurs. The precipitate then goes to a filter, the waste goes to waste treatment and the filter cake is then put in a vessel with methanol. Methanol helps separate the lignin from the solution. This solution is then filtered again, boiled to recycle the methanol and the lignin is dried as a powder.

Liquor Storage & Handling

Black liquor can be stored in pressure rated vessels that have been tested for any residue hydrogen sulfide. It is advised that when mixing Black liquor to add slowly to solution or water to decrease possibility of heat generation and spattering.

Lignin powder can be stored in waterproof drums or containers depending on amount purchaser requires.

Liquor Disposal

After black liquor solution has undergone the proposed process, removing lignin, it will be returned to the kraft recovery cycle for black liquor which with the addition of lime turns the solution back into white liquor. This white liquor is then recycled in the pulp process.

HAZOP Analysis

Possible dangers that lie in the proposed process include: toxic hazard, fire hazard and explosive hazards. For the safety and well-being of staff and equipment the following actions can prevent any accidents from occurring.

HAZOP analysis of each piece of equipment establishes the overall level of danger.

1. The precipitator vessel: Flow rate is the important parameter here. At too high of flowrates, the possibility of damaging the equipment and creating high pressures and temperatures is present. By installing a flow controller this damage and issue can be mitigated.

2. Pumps: Pump failure or clogging is a very possible situation. In order to mitigate this having a redundant system of pumps in parallel with gate valves. This allows for one pump to be turned off while the other can still pump full capacity of process.

3. Filtration: Any failure or break in a filter would cause pressure build-up and damage equipment and product. In order to control this, sensors for failure should be installed and pressure sensors in case of pressure build-up.

4. Chemicals handling: Handling of chemicals like sulphuric acid and methanol should be done in careful manner since it is corrosive. Any splashes or spills could cause serious burns to people. A spill kit should be on site and proper PPE should be worn at all times.

Economic Analysis

In order to propose a lignin separation and recycle system, there needs to be an economic opportunity for profit. As an additive in polypropylene (PP), the demand of this product is vital. PP makes up 23% of today's plastics and is used for a wide variety of consumer products.

Figure 2. Price of PP from 2005 projected to 2020 (Statistica, 2015).

Polypropylene was projected in 2015 to cost 679 $/ton however currently it is priced at 810 $/ton which shows that the plastic industry is growing and faster than was anticipated. This outlook is good for the proposed product as the demand and profit are still positive.

The price of lignin powder can be as low as 0.003 $/kg (Alibaba, 2018) or as high as 0.50 $/kg (Sigma Aldrich, 2018). This drastic gap between prices is an indication of the range in quality of product and supplier. The wide range in value allows for the proposed lignin product to have a high profit and still be competitive with quality suppliers.

This process will also require purchasing of raw materials including CO2, methanol and sulfuric acid. Though the exactly quantity of each raw material is undetermined currently their prices are as follows: CO2 is 0.0188 $/kg, methanol is 0.605 $/kg and sulfuric acid is 0.303 $/kg.

Further calculations are required to further price equipment, raw materials and labor for the process.

Appendix A: References