Feasibility of recovering methanol from cooking and evaporative emissions

About cooking and evaporative emission

The amount of methanol in the non-condensable gas during cooking and evaporation is 11.4 to 53.4 times that of the other components. The full recovery of methanol may be a new economic growth point for modern pulp mills.

The reaction of displacement cooking is the same as that of conventional cooking delignification, so the total amount of material discharged is also generally balanced. From the hot spray, it is changed to cold (about 80 °C), and the non-condensable substances such as flashing during hot spraying are transferred to evaporation, and the evaporation of the above substances is increased. There is no problem of flashing and hot sewage in the replacement cooking pot, which provides great convenience and simple and feasible for fully recovering the non-condensable gas generated by cooking. Continuous cooking is easier to recycle than conventional batch cooking.

The amount of various substances in the non-condensable gas varies depending on the type of seed, the type of pulp, the cooking method and the process conditions. In the kraft pulp mill, the chemical reaction between the chemicals used in cooking and the raw materials produces non-condensable gas containing various components. Some of the gases have a strong odor, and their components are hydrogen sulfide (H2S), methyl mercaptan (CH3SH), methyl sulfide (CH3SCH3), dimethyl disulfide (CH3SSCH3) and other sulfides, that is, total reduction. Sulfide (TRS). Such a large amount of methanol is produced during the cooking and evaporation process, so the recovery and utilization of methanol should be given importance and worthy of research and discussion.

Now, according to the concentration and volume of the non-condensable gas generated in different parts of the production process, it can be divided into two types: non-condensable gas with high concentration and small volume (HCLV) and low concentration and large volume (LCHV).

In HCLV, the volume of TRS is approximately 10% by volume. In LCLV, TRS accounts for about 0.1% of the volume fraction.

The possibility of recovering methanol

The modern pulp mill adopts two processes of HCLV and LCHV non-condensable gas, which are respectively sent to the alkali recovery furnace for combustion, recovering heat energy and thenardite, but methanol is not separately recovered.

Methanol is an important raw material for fine chemicals and organic synthesis, and it is also one of clean energy. It has a broad market and application space.

Non-condensable gas (NCG) is an industry idiom, but methanol can be dissolved in water at a water temperature below 65 ° C, rather than non-condensable, but generally it is also classified as non-condensable. In actual production, a large amount of methanol is dissolved in low-temperature water, but the content in HCLV is small.

Conduct a practical investigation of production 

The qualified enterprise can conduct an investigation on the actual amount of methanol in the exhaust and waste liquid of the enterprise. 

Identify the plan and implement recycling 

Based on the survey results and the basic nature of methanol, a recycling program was developed. The focus is on the recovery of the spent and evaporated waste liquid and the methanol in the exhaust gas. The exhaust gas containing more methanol is cooled to a liquid methanol mixed with a waste liquid having a higher methanol concentration, and commercial methanol is obtained by distillation. The exhaust system of this recycling system can be combined with the existing HCLV, LCHV combustion system to achieve the purpose of green, circulation and income increase.

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