Methanol purification method and apparatus

Abstract

The invention relates to a method and apparatus to recover and purify methanol from gases produced in the digester during the kraft pulping process. The gas is typically recovered as a foul gas (called stripper off gas or SOG) comprising methanol, water and various other contaminants. The gas is then treated with successive decanting and distillation steps to remove impurities, thereby producing highly purified methanol.

Claims

1. A method to recover and purify methanol from a stripped off gas stream, comprising the steps of: obtaining, at a controlled rate, a foul gas feed comprising no more than approximately 40 wt % methanol; condensing said foul gas feed; removing immiscible contaminants from said condensed foul gas feed; heating said condensed foul gas feed in the presence of an acid to evaporate volatile components, leaving a contaminated methanol feed, said acid being supplied at an entry point below an input point of said condensed foul gas feed; refining said contaminated methanol feed by heating to evaporate methanol from said contaminated methanol feed to produce purified methanol and impure condensate.

2. The method of claim 1 further comprising the step of diverting excess foul gas to a disposal system prior to said condensing step.

3. The method of claim 1 further comprising the steps of cooling and collecting said evaporated methanol.

4. The method of claim 1 further comprising the step of stripping fusel oils from said contaminated methanol feed during said refining step.

5. The method of claim 1 further comprising the step of storing said contaminated methanol feed prior to said step of refining said contaminated methanol feed.

6. The method of claim 1 further comprising the step of recycling said condensate to said step of refining said contaminated methanol feed.

7. The method of claim 1 wherein said step of removing immiscible contaminants comprises decanting said immiscible contaminants.

8. A method to recover and purify methanol from a stripped off gas stream, comprising the steps of: obtaining, at a controlled rate, a foul gas feed comprising no more than approximately 40 wt % methanol; condensing said foul gas feed in a condenser; removing immiscible contaminants from said condensed foul gas feed in a decanter; heating said condensed foul gas feed in a first distiller in the presence of an acid to evaporate volatile components, leaving a contaminated methanol feed, said acid being supplied at an entry point in the first distiller, said entry point located below an input point in the first distiller to receive said condensed foul gas feed; refining said contaminated methanol feed by heating to evaporate methanol from said contaminated methanol feed in a refining section to produce purified methanol and impure condensate.

9. The method of claim 8 further comprising the step of diverting excess foul gas to a disposal system prior to said condensing step.

10. The method of claim 8 further comprising the steps of cooling and collecting said evaporated methanol.

11. The method of claim 8 further comprising the step of stripping fusel oils from said contaminated methanol feed during said refining step.

12. The method of claim 8 further comprising the step of storing said contaminated methanol feed prior to said step of refining said contaminated methanol feed.

13. The method of claim 8 further comprising the step of recycling said condensate to said step of refining said contaminated methanol feed.

14. The method of claim 8 wherein said step of removing immiscible contaminants comprises decanting said immiscible contaminants in the decanter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The preferred embodiment of the invention will be described by reference to the drawings in which:

(2) FIG. 1 is a graph showing the dissociation fractions for hydrogen sulphide and methyl mercaptan at various pH levels;

(3) FIG. 2 is a schematic of the topping section of the invention;

(4) FIG. 3 is a schematic of the rectification section of the invention; and

(5) FIG. 4 is a schematic of an alternative layout of the rectification section of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(6) Stripper off gas (SOG), typically containing about 40 to 70 wt % methanol, is produced in an existing foul condensate steam stripping column. The SOG is directed to a methanol purification system 10, being diverted from a kiln, boiler, incinerator or other incineration system 12, as shown in FIG. 2.

(7) Vapour 14 from the existing stripping column is introduced to a dedicated reflux condenser 16; this vessel may be of any suitable type, such as a falling film type shell and tube evaporator effect. The heat from the stripping system may be utilized in the evaporator system, but use of a dedicated vessel allows sufficient control over the system to ensure stable qualities and quantities of SOG are produced under all evaporator operating conditions. Pressure is maintained by throttling the flash vapour from the system.

(8) SOG is introduced to the methanol purification system 10 at a controlled flow rate, with any excess gas being diverted to the incineration system 12. This helps to maintain the methanol entering the purification system 10 at an optimal content of approximately 40 wt % or less.

(9) The topping column system 18 strips out low boilers and non-condensables from the SOG, including malodorous sulphur compounds, ammonia, and some ethers, ketones and aldehydes. When SOG is introduced to the topping reflux condenser 20, the low boilers and non-condensables are vented 22 back to the incineration system 12 while the condensate is drained 24 to the topping red oils decanter 26.

(10) Topping red oils pump 28 moves the decanted red oils to a turpentine recovery system (not shown), if available. The underflow 32 from the decanter 26 is moved to the topping column 34 by any suitable means, such as topping reflux pump 36. A topping reboiler 38 may be used to provide heat to the topping column 34, evaporating the volatile contaminants in a stream 42, which can be returned to the topping reflux condenser 20 or otherwise disposed of.

(11) Sulphuric acid may be added to topping column 34 by any suitable means, such as feed pump 44. Preferably the acid is added about the mid-point of the column, or at any rate at an entry point 46 below the input point 48 of the condensed underflow feed from the topping reflux pump 36. The separation between the feed input point 48 and the acid entry point 46 allows any highly volatile ammonia present in the underflow feed to be stripped out in the upper section of the topping column 34 before it has a chance to react with the acid, thereby avoiding the formation of ammonium sulphate precipitates. The acid reduces the pH in the lower section of the topping column 34, releasing dissociated hydrogen sulphide and methyl mercaptan, which will rise to the upper section of topping column 34, where it can be removed as part of volatile contaminant stream 42.

(12) The underflow 50 from the topping column 34 flows to the surge tank 52, with some being recycled to topping reboiler 38. As the flow and concentration of SOG can vary significantly depending on the operation of the existing stripping system, the surge tank can smooth out the flow and concentration of the feed to the methanol rectification column system 54.

(13) The feed enters rectification system 54 from surge tank 52, such as by rectification feed pump 56. The rectification column system 54 comprises two sections, namely a bottoms stripping section 97 and a top rectification section 99, as shown in FIG. 3. The feed is introduced to the stripping section 97 of column 55 and flows down through the packing, countercurrent to the stripping steam 57, which may be supplied by a rectification reboiler 59. The volatile component, including methanol, moves upward to the top rectification section 99, while the less volatile component, which is mainly water along with other high boilers, is removed as the underflow 63.

(14) The feed may also comprise intermediate boilers, such as some higher alcohols (primarily ethanol), higher ketones, etc. These components, often referred to as fusel oils, are drawn off from the bottoms column 55, preferably at a point 65 located below the feed introduction point 67. The fusel oils can be recovered separately, or may be combined with the underflow 63 from the column 55, passing to effluent treatment through rectification bottoms pump 69.

(15) The overhead vapour flow 61, comprising methanol and other volatiles, from upper rectification section 99 is condensed in a rectification reflux condenser 71, located above column 55. Any low boilers and non-condensables 73 may be vented to the incineration system 12.

(16) The remaining product, which is approximately 99.85 wt % methanol, is drawn off in a stream 75, preferably located slightly below the top of the packing in top rectification section 99, and moved to a methanol cooler 77 by suitable means such as by methanol pump 79, where it can be moved to storage. The methanol product is preferably drawn off in sufficient quantities to maintain the methanol profile in the column.

(17) Alternatively, the two sections of rectification column system 54 may be supplied in two separate columns, the rectification bottoms column 60 and the rectification top column 62, as shown in FIG. 4. The feed is introduced 64 to the stripping section of the bottoms column 60 and flows down through the packing, countercurrent to the stripping steam 66, which may be supplied by a rectification reboiler 68. The volatile component, including methanol, is removed into the overhead vapour flow 70, while the less volatile component, which is mainly water along with other high boilers, is removed as the underflow 72.

(18) In this embodiment, the fusel oils are drawn off from the bottoms column 60, preferably at a point 74 located below the feed introduction point 64. Again, the fusel oils can be recovered separately, or may be combined with the underflow 72 from the column 60, passing to effluent treatment through rectification bottoms pump 76.

(19) The overhead vapour flow 70 from rectification bottoms column 60 is directed to the lower section of the rectification top column 62. Any condensate 80 collected in the bottom of the top column 62 may be returned by an intermediate rectification pump 82 to introduction point 84 of the bottoms column 60. Vapour 86 from the top column 62 is condensed in a rectification reflux condenser 88, located above top column 62. Any low boilers and non-condensables 78 may be vented to the incineration system 12.

(20) The remaining product, which is approximately 99.85 wt % methanol, is drawn off in a stream 90, preferably located slightly below the top of the packing in top column 62. Again, the methanol product is preferably drawn off in sufficient quantities to maintain the methanol profile in the column and moved to the methanol cooler 94 by suitable means such as by methanol pump 92, where it can be moved to storage.

(21) It will be appreciated by those skilled in the art that other variations to the preferred embodiment described herein may be practised without departing from the scope of the invention, such scope being properly defined by the following claims.