Kraft pulp mills are notorious for their distinctive smell. The odor is due to the use of sodium sulfide in combination with sodium hydroxide for the digestion of wood chips and eventual conversion into pulp. The combined effect of sodium sulfide and sodium hydroxide (white liquor) allows the delignification of the wood in a relatively short period of time, while allowing the cellulose fibres to keep their strength. For the process to be economical, the chemicals used must be recovered and recycled. Indeed, recoveries of over 99.9% are achieved in most pulp mills today. Unfortunately, however, sodium sulfide will combine with organic compounds and form organosulfur compounds, mostly organic sulfides and mercaptans which have an intense odor. In fact, many years ago when "water gas", a combination of carbon monoxide and hydrogen, was used domestically for cooking and heating, mercaptan was intentionally added to warn people of the toxic but otherwise odorless water gas.
Sulfur oxides are well known for causing severe health problems and were emitted in large quantities by power plants before more stringent regulations were enacted. In the United States and the European Union these emissions of sulfur oxides have been regulated for over 30 years. On the other hand, the emission of reduced sulfur compounds such as hydrogen sulfide and mercaptans was not so widespread, and regulation came much later. The control of these emissions has impacted Kraft pulp mills, since it is these plants that represent the bulk of these emissions. In the case of pulp mills, even minute emissions of hydrogen sulfide and mercaptans, formed when sulfides react with acids or organic compounds, can produce health problems in the employees and surrounding residents, as well as public relations problems.
Prado & Associates staff was involved in the design of an alkaline scrubber for the removal of TRS emissions in a Kraft pulp mill in Canada many years ago. Then, in the year 1985 the State of Florida became one of the first states in the USA to enact legislation requiring the control of TRS emissions. Prado & Associates designed the first alkaline scrubber for Georgia-Pacific Corporation in Palatka, Florida. This was a very successful project, and several others have followed. Since then Prado & Associates has become well know for its design of alkaline scrubbers for the removal of hydrogen sulfide and organic sulfides.
In addition to scrubbers, there are alternative technologies for dealing with TRS gases in pulp mills. One consists of burning them in either the recovery boiler or the lime kiln. This will successfully dispose of the TRS problem but at the expense of creating a sulfur oxide problem. This sulfur oxide can be scrubbed with a solution of calcium hydroxide. However, this alternative needs to be studied at each mill to determine its economic feasibility.
Oil refineries have been dealing with "sour gases" for many years. These are hydrocarbons containing varying amounts of hydrogen sulfide and mercaptans. One of their strategies consists of flaring these gases if the amount of TRS is small. Since the balance of the gases are combustible, they can be burned in a flare. This option is not available to pulp mills, since the non-condensable gases have limited fuel value and generally will not support combustion. In oil refineries where the amount of TRS is substantial it is possible to use either the Clauss process or amine scrubbers for the removal of TRS. This will result in the production of either solid sulfur or liquid sulfides. This option is not attractive to pulp mills since it requires a high TRS content to justify the capital investment, with sulfur as a by-product. On the other hand, oil refineries do not find alkaline scrubbers attractive, since their operation results in the production of a sodium sulfide solution by-product, which would create a disposal problem for them. However, pulp mills would welcome sodium sulfide, since it is one of the ingredients in the production of white liquor.
So, for kraft pulp mills, the use of alkaline scrubbers is probably the most desirable technology due to its low capital and operating costs. However, the strategy for the design of such an air cleaning device is tricky. All Kraft pulp mills have an ample supply of white liquor. It would be desirable to use this material as a scrubbing agent. However, due to its temperature and sulfidity content, its use is limited. Prado & Associates has resolved this issue by combining two scrubbers in series. The first one uses white liquor to remove the bulk of the TRS, or about 90% of the total. This level of removal, however, is not sufficient in most cases. We follow with a sodium hydroxide scrubber to remove 99.9% of the remaining TRS. The waste solutions from both scrubbers are sent back to the white liquor storage of the pulp mill. For removal beyond 99.9%, the use of activated carbon absorption is required. Carbon is expensive, but the use of an alkaline scrubber first will minimize its use and cost.
The materials of construction for the scrubber can be either stainless steel type 316 or Fiberglas Reinforced Plastic (FRP). The use of FRP is less expensive. However, the use of FRP in alkaline solutions is more complicated and requires a special design specified by Prado & Associates and a well qualified fabricator. If the reliability of the FRP fabricator cannot be assured, then it is better to fabricate all the equipment out of stainless steel.
In summary, the use of alkaline scrubbers is a well established technology available to kraft pulp mills. It is cost effective and compatible with the chemistry already in use. Also the safety precautions it requires are familiar to mill personnel. Thus it is friendly technology acceptable to existing and new mills. The use of alkaline scrubbers not only meets most environmental regulations but also helps in resolving the long-existing public relations issue of odor emissions. This technology has been embraced by most pulp mills in North America and the European Union.
Organization name: Prado & Associates
Street address: P.O. BOX 274206
City: Tampa
Postal code/ZIP: 33688
Country: USA
Phone: 813-961-1072
E-mail: tino@pradotec.com
Web: http://
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