Lined Pipe for Pulp and Paper Mills
A kraft mill is a chemical plant that happens to be fed with wood chips, and the hardest service in it is the bleach plant: chlorine dioxide at bleaching strength, hot sulfuric acid and chlorate at the generator, caustic and peroxide in the extraction stages, and acidic chloride-laden filtrate that pits 316 stainless in a season. Marspec Technical Products fabricates the PTFE-, PVDF- and polypropylene-lined pipe spools and supplies the lined fittings, valves and accessories that mills specify for that chemistry, and has for more than half a century. Call 251-625-4949 with the line list from your next outage.
The same liners serve tall oil acidulation, wet-end acid and alum, deinking caustic and peroxide, and the hypochlorite and coagulant feeds at the mill water and effluent plants. Contact us online with sizes, media and temperatures and we will recommend a liner for each.
Where a mill corrodes pipe
Chlorine dioxide, from generator to tower
Every bleached kraft mill makes its own chlorine dioxide by reducing sodium chlorate with methanol or hydrogen peroxide in strong sulfuric acid. Most modern generators run under vacuum at the boiling point of the liquor, so water flashes off, sodium sesquisulfate or sodium sulfate crystallizes out as saltcake, and the chlorine dioxide gas is absorbed in chilled water to a solution of roughly 8 to 10 grams per liter. The feed side moves 93 percent sulfuric acid, concentrated chlorate solution and methanol; the product side moves chlorine dioxide solution with some free chlorine, saltcake slurry and spent acid. The solution goes to the D0 stage at a pH near 2.5 and 120 to 160 F and on to the later D stages, with acidic filtrate returning through the washers. Chlorine dioxide and its filtrate attack stainless and most alloys, which is why PTFE-lined pipe, PTFE-lined diaphragm valves and vented housings are the bleach-plant standard, with PVDF where the solution stays cool. The generator acid and chlorate headers run in PTFE-lined spools with PTFE-lined plug valves.
Caustic, oxygen and peroxide
Between the chlorine dioxide stages the pulp is extracted with sodium hydroxide, usually reinforced with oxygen and hydrogen peroxide, at 160 to 190 F. Fifty percent caustic is unloaded and made down at ambient, and the hot extraction liquor with peroxide in it belongs in PTFE, which is rated to 450 F and is unaffected by oxidizers; we quote PTFE for the make-down as well, with polypropylene available where a specification calls for it. Hydrogen peroxide has a rule of its own: iron, copper and manganese ions decompose it, so peroxide storage and dosing lines are PVDF- or PTFE-lined, vented, and laid out without dead legs. Sulfur dioxide or bisulfite antichlor after the last D stage is another fluoropolymer service.
Tall oil, the wet end and the water plant
Skimmed black liquor soap is acidulated with sulfuric acid at 180 to 200 F to make crude tall oil, leaving a hot acidic brine of sodium sulfate and lignin that corrodes everything but fluoropolymer. On the paper machine, alum, sulfuric acid for pH, sodium hypochlorite and biocides for slime control run through dosing lines where PTFE takes all of them, with PVDF for the hypochlorite and polypropylene for the alum and dilute acid where a specification calls for them. Deinking adds caustic, peroxide and silicate. The mill water plant and effluent treatment feed hypochlorite, ferric or alum coagulant, sulfuric acid and caustic for neutralization, the same chemistry as a municipal water and wastewater plant and the same lined diaphragm valves.
Liners, valves and the bleach-plant package
- Chlorine dioxide solution, D-stage filtrate and chlorine-bearing vents: PTFE in vented housings, PVDF when cool
- Generator sulfuric acid, chlorate solution and spent acid: PTFE
- Hot extraction liquor and tall oil acidulation: PTFE
- Ambient 50 percent caustic, alum and dilute acid: PTFE, with polypropylene where specified
- Hydrogen peroxide and hypochlorite: PVDF or PTFE, vented, no dead legs
On the valve side, PTFE-lined diaphragm valves are the choice on chlorine dioxide solution because they have no cavity to trap solution and are easy to throttle. PTFE-lined plug valves isolate the generator acid and chlorate feeds, lined butterfly valves cover the large filtrate and caustic headers, lined ball valves serve the chemical unloading racks, and lined check and sampling valves finish the dosing skids. Patterns and linings are on our lined valves page; automated packages ship with actuators, positioners and switches mounted and tested in-house.
A typical bleach-plant spool order is 3" through 10" PTFE-lined pipe built to the tower and washer isometrics in lengths up to 20 ft, class 150 ANSI B16.5 rotating flanges, vent holes in every housing, with lined elbows, tees, reducers and laterals to match; PTFE spacers and spectacle blinds for isolating a tower during an outage; PTFE-bellows expansion joints on the hot D-stage runs; lined strainers ahead of the generator feed pumps; and sight glasses on the peroxide and hypochlorite dosing lines. Sizes and liners are on our lined pipe and fittings page and the accessories are under piping accessories.
Vacuum, venting and cold-mill tracing
Chlorine dioxide generators and washer seal legs run under vacuum, and PTFE liners are rated for vacuum by size and temperature; the ratings, along with flange torque and bolt lengths, are on our technical details page. Permeation is the other design point: chlorine dioxide and chlorine pass slowly through any fluoropolymer, so the steel housing is vented and the flange faces are checked at outages. Northern mills add a third variable. Fifty percent caustic begins to crystallize near 54 F, so outdoor caustic racks in Maine, Wisconsin or Washington carry clamp-on steam heaters or electric tracing with heat-transfer compound from our temperature maintenance line. Chlorine dioxide solution and peroxide are the opposite case and are kept cool, never traced.
Mill country
Bleached kraft and paperboard mills line the Fox and Wisconsin rivers of Wisconsin, the Kennebec and Androscoggin in Maine, the Georgia coast at Port Wentworth and Brunswick and the Savannah River at Augusta, the Alabama river system from the Mobile corridor north, the lower Columbia in Washington and Oregon, and the St. Louis River and the Rainy River in Minnesota. The chlorate and chlorine chemistry that feeds them comes from the chlor-alkali and chemical plants on the Gulf Coast and the Great Lakes.
Standards and ordering
Our lined products comply with ASTM F1545, fittings conform dimensionally to ANSI B16.5 and are welded to ASME B31.3 by welders certified to ASME Section IX, and every lined part is spark tested at a minimum of 10,000 volts before it ships. Send isometrics, field dimensions or a photograph of the spool that failed, call 251-625-4949 or contact us online. Fairhope, Alabama, ships to mills nationwide, and custom spools built to field dimensions are our specialty.
FAQ: Lined Pipe for Pulp and Paper Mills
Which liner do you recommend for chlorine dioxide solution from the generator to the D0 tower?
PTFE in a vented housing is the standard, with PVDF as the lower-cost choice where the solution stays cool. Chlorine dioxide permeates slowly through any liner, so the housing is vented and the flanges are inspected at outages.
Can polypropylene-lined pipe run the caustic in our extraction stage?
It is available for 50 percent caustic unloading and make-down at ambient temperature, but the extraction stage itself runs hot with peroxide and oxygen in the liquor, and there we recommend PTFE, which is rated to 450 F and is unaffected by the oxidizers. We quote PTFE for the make-down lines as well, since in a finished spool the liner is a small part of the cost.
The bleach-plant atmosphere rusts the outside of our pipe. What housing should we specify?
304 or 316 stainless. The liner protects the inside of the spool; a stainless housing stands up to chlorine dioxide fumes and washdown on the outside, where a painted carbon steel housing rusts under the insulation.
Can you match the lined pipe already installed in our bleach plant?
Yes. Lined pipe built to ASTM F1545 with ANSI B16.5 flanges bolts up regardless of who made the original. Send the field face-to-face dimensions, flange class and liner, and we fabricate, spark test and ship a replacement from Fairhope, Alabama. Call 251-625-4949.