Lined Pipe for Chlor-Alkali and Chemical Plants

A chlor-alkali plant is the one place in industry where nearly every major stream attacks steel (dry chlorine, the exception, runs in carbon steel): saturated brine on the way in, wet chlorine and 32 percent caustic out of the cells, hypochlorite in the vent scrubbers, synthesis-grade hydrochloric acid from the burner and sulfuric acid from the chlorine dryers. Marspec Technical Products has fabricated PTFE-, PVDF- and polypropylene-lined pipe for that chemistry for more than half a century, and we supply the lined fittings, valves and accessories that go with it. Call (251) 625-4949 with a line list.

The same liners serve the chemical plants built around chlorine: vinyls, isocyanates, chlorinated solvents, epoxies and the fine and agrochemical units that use them. Contact us online with concentration and temperature and we will recommend a liner for each line.

The process, stream by stream

Brine and the cell room

Salt is dissolved to a saturated brine of roughly 300 grams per liter, treated to drop calcium and magnesium and acidified with hydrochloric acid before the electrolyzers. Depleted brine leaves the anode side near 200 grams per liter carrying dissolved chlorine, is dechlorinated under vacuum, finished with sodium bisulfite and resaturated. Chloride-rich brine pits stainless; chlorinated brine is worse. PTFE-lined pipe handles raw and treated brine and, with vacuum ratings checked by size, the dechlorination loop on one liner specification; polypropylene-lined spools are available for dechlorinated brine where a specification calls for them, since chlorine attacks polypropylene. On the cathode side, membrane cells make caustic at about 32 percent, which is evaporated to the 50 percent grade that ships. Hot caustic is a job for PTFE, because PVDF is not used in caustic at any strength; we quote PTFE for cool caustic at 32 or 50 percent too, with polypropylene available where specified.

Chlorine, hypochlorite and HCl

Chlorine leaves the cells hot and saturated with water. Until it is cooled and dried in sulfuric acid towers it is wet chlorine, which attacks nearly every metal. That section of the plant, plus the vent headers and the scrubbers that absorb chlorine into dilute caustic to make 12 to 15 percent sodium hypochlorite, is PTFE-lined territory: lined pipe and lined plug or ball valves with a vented plug or ball (on the hypochlorite side, lined diaphragm valves or ball valves with a vented ball, vent facing upstream). Dry chlorine runs in carbon steel, so the lined section ends at the dryer; the chlorine-saturated spent acid from the dryers leaves in lined pipe. Hydrogen and chlorine burned together and absorbed in water give 32 to 37 percent synthesis-grade hydrochloric acid, which we pipe in PTFE with inspected flanges, or in PVDF at moderate temperature. Hypochlorite lines are vented and laid out without dead legs, because the product decomposes and releases oxygen.

Downstream: vinyls, isocyanates, solvents and specialty chemistry

Most chlorine never leaves the fence line. In a vinyls unit, EDC cracking throws off HCl, which oxychlorination consumes; phosgenation in an MDI or TDI unit and the chlorination of solvents and intermediates also make hydrochloric acid, and the same lined systems carry it. Chloride-process titanium dioxide, epichlorohydrin, bromine and the chlorinated and phosphorus chemistry behind crop-protection actives put hot acid, wet halogens and solvents in one plant. The fine, specialty and agrochemical units inside these complexes usually run their bromination and chlorination reactors in glass-lined steel, a borosilicate glass fused to a carbon steel shell that resists acids across the concentration range, except hydrofluoric and hot concentrated phosphoric acid, and cleans easily, with PTFE-lined transfer lines and PFA-lined valves between the vessels. Glass is never used on hydrofluoric acid or fluoride-bearing streams, is kept off hot strong caustic and dislikes thermal shock. We supply glass-lined pipe and equipment, including reglassing, alongside the fluoropolymer line.

Where lined pipe is specified, and which liner

• Brine: PTFE, with PVDF where it is warm or carries chlorine, and polypropylene for dechlorinated brine where a specification calls for it
• Wet chlorine, chlorine vents and spent dryer acid: PTFE
• Caustic: PTFE whether hot or cool, never PVDF; polypropylene available for cool caustic where specified
• Hypochlorite: PTFE for bulk 12.5 to 15 percent bleach, PVDF only for dilute or on-site hypochlorite at pH 12 or below; vented, no dead legs
• Synthesis HCl: PTFE, PVDF to moderate temperature
• Dryer and sulfonation sulfuric acid: PTFE

Valves follow the liner: PFA-lined cylindrical plug valves, 1/2" through 2" full port and 3" through 6" reduced port, cavity-free, with no in-line adjustment, for brine and, with a vented plug, for chlorine gas; PFA-lined ball valves for tight shutoff on HCl; lined diaphragm valves for brine sludge; lined diaphragm valves, or ball valves with a vented ball (vent facing upstream), for hypochlorite; PTFE-lined butterfly valves on the large caustic headers; and lined check and sampling valves. Automated packages are assembled in-house, actuators, positioners and switches mounted and tested before shipment.

A typical cell-room or bleach-unit package

A cell-room order usually reads like this: 2" through 8" PTFE-lined carbon steel spools built to isometrics in lengths up to 20 ft, class 150 ANSI B16.5 rotating flanges, vent holes in every housing, and lined elbows, tees, reducers and laterals to match; PTFE spacers at the tie-ins and spectacle blinds at the battery limits; PTFE-bellows expansion joints on the long outdoor caustic and brine runs; lined Y-strainers ahead of the hypochlorite and brine pumps; and a sight glass on the bleach product line. The spools and fittings are on our lined pipe and fittings page and the rest is under piping accessories. Sizes run 1/2" through 12" in carbon steel or 304/316 stainless housings.

Fifty percent caustic begins to crystallize near 54 °F (12 °C) and the 73 percent grade from the evaporators sets up near 145 °F (63 °C), so outdoor caustic racks and rail unloading lines get clamp-on steam heaters or electric tracing with heat-transfer compound from our temperature maintenance line, sized to keep the liner inside its rating. Hypochlorite is the opposite case: it is kept cool and vented, and because it can freeze in deep cold, it is traced only to keep it from freezing and never overheated, since heat speeds its decay. For jacketed or traced service, we recommend specifying PTFE.

Where the industry is

Chlor-alkali production follows salt domes and cheap power. The largest sites are on the Texas Gulf Coast at Freeport and along the Houston Ship Channel, and in Louisiana at Lake Charles and in the Plaquemine and Geismar stretch of the Baton Rouge corridor; Alabama makes chlorine at McIntosh on the Mobile chemical corridor. The Kanawha Valley of West Virginia grew up on brine wells, and Niagara Falls makes chlorine on hydropower. The vinyls, isocyanate, solvent and agrochemical plants cluster along the same corridors, beside the refineries and petrochemical plants and the water and wastewater plants that use the same liners.

Standards and how to order

Our lined pipe and fittings 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 spool and fitting passes a spark test at a minimum of 10,000 volts, which catches pinholes before they become chlorine leaks. Liner temperature limits, liner thicknesses, flange dimensions and torque tables are on our technical details page. Send isometrics or a line list, call (251) 625-4949 or contact us online. We ship nationwide from Fairhope, Alabama, and fabricate custom spools in our own shop.

FAQ: Lined Pipe for Chlor-Alkali and Chemical Plants

Why do lined pipe housings have vent holes?

Small molecules such as chlorine and hydrogen chloride permeate PTFE slowly, and faster as temperature and concentration rise. Every spool and fitting housing has drilled vent holes as standard; they relieve that permeant pressure between the liner and the housing to prevent liner collapse. We also recommend periodic inspection of the flange faces.

Can polypropylene be used on 50 percent caustic in a chlor-alkali plant?

It is available wherever a customer specification requires it, but we recommend and quote PTFE: it covers the hot caustic from the evaporators and the cool caustic on the same liner, and in a finished spool the liner is a small part of the cost. PVDF is not recommended for caustic at any concentration you will meet in a cell room.

What liner do you recommend for brine dechlorination and other vacuum services?

PTFE, with the vacuum rating checked against the pipe size and temperature. Tell us the size, temperature and vacuum level and we will confirm the rating.

Do you supply glass-lined equipment for the specialty chemical units on a chlorine site?

We do: glass-lined pipe and fittings with split-ring flanges in ANSI B16.5 class 150 and 300 or DIN 2873 drilling, glass-lined reactors, columns, receivers, heat exchangers and Nutsche filters, and reglassing of existing vessels. Glass is not for hydrofluoric acid, fluoride-bearing streams or hot strong caustic; for those services, and for the transfer lines between glass-lined vessels, we quote fluoropolymer-lined pipe and valves.