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Himes 224 I Hydrochloric Acid Table 116: Continued IPlont component Process air dampers, dry blower dampers Regenerate dampers Steam dampers Solvent separator Solvent pump Loading Motarwl 403 K (130 C), condensing phenol resin duroplast steam, condensing HCC, HCI 403 K (130 C), condensing steel/PTFE--sheathed steam, condensing HCC, HCI steam, 403 K (130 C) steel/PTFE--sheathed 303 K (30 C), condensed HCC, phenol resin duroplast HCI condensed HCC stainless steel Table 116: Materials for a stripper unit to remove hyd rochIoroca rbons [235] The mechanical properties of PP change on long-term temperature loading in dilute aqueous hydrochloric acid (up to 5 %) in the same way as in pure water. For example, the decrease in the strain in the tensile test after contact with dilute acid is only slight. In 10 % acid and at 373 K (100 C), the strain decreases by 30 % over a period of three months. Stress cracks are initiated at higher temperatures in hydrochloric acid concentrations above 30 % [203]. This behavior is covered in the design by higher reduction factors (Table 114). Vinyl polymers Polyvinyl chloride (PVC) PVC-U (hard PVC) is resistant in all hydrochloric acid concentrations up to 333 K (60 C). However, the material turns white in concentrated acid at 313 and 333 K (40 and 60 C) and exhibits blistering on the surface at 333 K (60 C). No changes of the mechanical properties were found up to 313 K (40 C). PVC-P (soft PVC) has only limited resistance at 293 K (20 C) in concentrated hydrochloric acid [203]. In the media lists of the German Institute for Building technology, PVC-U is permitted in concentrated hydrochloric acid up to 333 K (60 C) with a reduction factor of 1.0 [217]. This documents its superiority regarding its chemical resistance compared to polyolefins. Experience gained in practice is interesting: the resistance of PVC increases in dilute aqueous solutions of acids, bases and salts up to intermediate concentrations [222]. This is attributed to the fact that only water, but not inorganic compounds, can diffuse into PVC. Because the water activity decreases with increasing concentration, the chemical aggressiveness of the aqueous solution also decreases [203]. The upper operating temperature of PVC can be increased by glass-fiber reinforcement. If used as a liner in vessels, temperatures of up to 368 K (95 C) are permissible if the coatings are free of pores [220]. PVC-U has only slight susceptibility to stress cracking. mn Vinylpolymers 1225 In contrast to polyolefins, PVC is resistant in chlorine-containing hydrochloric acid. However, it cannot be used in hydrochloric acid containing hydrochlorocarbons with a separate solvent phase on account of diffusion and swelling [220]. The dominating role of PVC-U for components, equipment and machine parts in many industrial branches is well-documented in the literature. With regard to its resistance to hydrochloric acid, which is the main focus of this report, this dominance is based on its relatively good resistance, its high compressive and tensile strengths and its simple processability by adhesive bonding, welding, thermoform- ing and cutting. With regard to adhesive joints in aqueous hydrochloric acid, it must be taken into consideration that ordinary PVC joints are not sufficiently resistant in hydrochloric acid concentrations above 25 %. An adhesive based on PVC-C in methylene chloride is used instead of one based on PVC-U in tetrahydrofuran [236]. Further important application engineering information is also given in [237]. PVC has been used as a classical coating and lining material for many applications from an early stage. A useful listing with older literature references is given in [47]. Successful examples of a PVC-based material engineering are reported for their use in many different types of pumps in the processing industry and electroplating industry [227], in a unit used for hydrochloric acid recovery in surfactant production plants [238], in filling equipment for corrosive products [239] as well as in chloralkali and HCl electrolysis plants that have to handle media containing hydrochloric acid and chlorine under extremely different conditions [240]. PVC-U and PVC-C have proved to be suitable for this at temperatures below 313 and 323 K (40 and 50 C), respectively. The favorable properties with regard to the corrosion resistance are contrary to the behavior of PVC in the event of a fire. Although it does not burn, it releases HCl as a result of thermolysis. Plasticizers support these processes as they are combustible. Decomposition starts at 413 to 523 K (140 to 250 C), depending on the type of PVC. At 573 K (300C), 30 to 85 % HCl is liberated and at 773 to 873 K (500 to 600 C) the entire chlorine is liberated as HCl [241]. The HCl combines with the steam, which is also released by the fire, to form hydrochloric acid and can thus lead to enormous corrosion damage, particularly of electrical and electronic equipment, whereby the main damage is to metallic and other inorganic materials. HCl contamination in the event of PVC burning requires a great effort in damage evaluation and in remediation measures, which are summarised in [242]. The cleavage of HC1 from chlorinated polymers that are used as binders for coatings which are subjected to high temperatures has also caused problems [243]. The formation of hydrochloric acid is also a problem in waste incineration because HCI has to be eliminated from the flue gases. Fluoropolymers Fluoropolymers are one of the most chemically resistant groups of organic materials. Their thermal, oxidative and photooxidative resistances are excellent. The intro- r it rms. gh dis 226 I HydrochloricAcid duction of thermoplastically processable range of available fluoropolymers. A copolymers considerably increased the ties, the resistance to chemicals summarising overview of (tabulated) and comments on the physical proper- ing of these materials are given in [244, 245, 246] as well application engineeras in a series of more recent corporate publications [164, 206, tion on fluoropolymers is given in the 209, 222, 225, 247 -- 251]. Further informa- well as in those dealing with coatings following specialised sections on and linings (D1) and seals (D2). elastomers, as Fluoropolymers are first-choice vated temperatures and materials for hydrochloric acid equipment at ele- ric acid in the concentrations. They are particularly resistant presence of chlorinated hydrocarbons (HCC), as in hydrochlo- recovery units. As already described in the section on PP, the found in solvent activated carbon used for absorption in solvent recovery processes is regenerated with steam, whereby hydrochloric acid is produced lowing fluoroplastics are due to hydrolysis of the hydrochlorocarbons. The fol- solvent in accordance recommended for with Table 117 [252]: plant components in contact with the rialont (amponent Vessels, piping for solvents, water separator Condensers and coolers Condensate valves and air valves Seals and hoses Table 117: Materials for a solvent recovery plant [252] Material PVDF 1 glass PTFE sheaths vinylidene fluoride-hexafluoropropylene copolymer (Viton) theAcpohnedneonsl-efrosrmanadldceohoyldeersr[e2s3i5n].coating or graphite can be used instead of glass for The upper temperature limits for the use of fluoropolymers in the presence of hydrochloric acid and HCC are given in Table given in more detail; however, the effect of 118 [220]. The type of HCC is not of solvents and the aggressiveness are individual comparable. representatives of this group Material Polytetratluoroethylene Perfluoroalkoxy copolymer Tetrafluoroethylene / hexafluoropropylene copolymer Abbreviation PTFE PFA FEP Upper temperature limit K ('-C) 400 (127) 400 (127) 369 (96) Polyvinylidene fluoride Ethylene / chlorotrifluoroethylene copolymer PVDF ECTFE 338 (65) boiling point of HCC e corresponding Tacaibdlea1n1d8:hydUropcphelrotreomcaprebroantusr(eHlCimCi)ts12f2o0r]the use of fluoropolymers in the presence of hydrochloric ema Sveen r TRL Vinylpolymers (227 The stress cracking of thermoplasts decreases in the order PP, PE, PVC, PVDF and ECTFE [222], so that fluoroplastics also have excellent properties in this field. Polytetrafluoroethylene (PTFE) PTFE is the most chemically resistant fluoropolymer. It has no thermoplastic properties and must therefore be processed by means of pressure sintering, powder extrusion and paste extrusion. Its decomposition temperature lies above 673 K (400 C), and the permissible operating temperature lies between 73 and 523 K (-- 200 C and + 250 C). However, exact upper operating temperatures in concentrated hydrochloric acid that go up to the limit of loading are not given in the literature. This is probably due to the high pressures needed for the determination which makes this too complicated. However, the information supplied by the chlor-alkali industry that PTFE is resistant in damp chlorine (thus containing hydrochloric acid) up to 473 K (200C) can be assumed to be reliable and can be directly applied to aqueous hydrochloric acid [240]. The upper operating temperature of 523 K (250C) applies to pure thermal loading without mechanical loading and exposure to chemicals and thus only applies to conditions without these additional loads. It is important that the creep strength of PTFE is low ("cold flow"), and this must be taken into consideration in apparatus engineering. Therefore, its main application fields are linings of vessels and pipelines in which metallic materials act as the mechanical support [253, 254] or in special components in which the mechanical strength can be ensured in other ways, e.g. valves [255]. The successful use of PTFE in shell-and-tube exchangers for hydrochloric acid separation in chlorinating plants at temperatures up to 423 K (150 C) is described in [256]. PTFE linings and coatings are described in detail in Section D 1. PTFE has no chemical resistance only in a few very special media, such as fused alkali metals, fluorine, chlorine trifluoride. Similar to all fluoropolymers, they have a high resistance to stress cracking [204]. PTFE is susceptible to permeation by gaseous HCl and HE, and this has to be taken into account in concentrated acids. Reference [209] reports a permeation coefficient for concentrated hydrochloric acid at 343 K (70 C). It lies approximately 20 % above the values for inert gases, oxygen and nitrogen. Pyrolysis starts at temperatures above 473 K (200C) and can lead to the formation of traces of hydrogen fluoride and thus to'problems for fluoride-sensitive materials if they are in contact with PTFE. For example, PTFE seals in titanium equipment can have a corrosive effect on the contact surfaces of this material at high temperatures because fluorides are a problem for transition metals such as titanium, zirconium and tantalum. Polyvinylidenefluoride (PVDF) In contrast to PTFE, this material can be processed thermoplastically and can be welded and adhesively bonded. Therefore, it is one of the most frequently used 2281 Hydrochloric Acid fluoropolymers. Its resistance properties lie between those of PTFE and polyolefins. The operating temperature range is reported as extending from 213 to 413 K (-60C to + 140 C). However, an upper limiting temperature of 373 K (100 C) should be reliable for concentrated hydrochloric acid (Table 112). This also agrees with the information given in the media lists published by the German Institute for Building Technology which report a maximum operating temperature of 373 K (100 C) with a reduction factor of 1.6 for PDVF in concentrated hydrochloric acid [217]. A value of 393 K (120 C) is reported for its use as a lining material in pumps [257]. The pressure/temperature diagram (Figure 146) takes account of the mechanical loading capacity of PVDF. Since this diagram only applies to water, the pressure must be reduced in the presence of hydrochloric acid; however, no qualitative information is given for this [250]. Analogous diagrams (for other materials) are also published elsewhere [206, 258]. Working temperature, K 273 293 313 333 353 373 393 413 18 16 14 -CI,t) 12 z %`".,.3' 10 Ccri 8 b- 6 4 2 0 0 Figure 146: 20 40 60 80 100 120 140 Working temperature, C Operating pressure / operating temperature diagram for PVDF in water [2501 PVDF reacts sensitively to hot alkaline solutions by stress cracking. This must be taken into account for the neutralisation of hydrochloric acid with sodium hydroxide solutions. The permissible loading limit in alkalis is only pH = 12 for 313 K (40 C) [222]. A number of practical applications of PVDF in the paper industry and chemical industry are listed in [259]. The use of this material in the pump industry is especially advantageous [260]. Vinyl polymers 1229 Tetrafluoroethylene / hexafluoropropylene copolymer (FEP) FEP has a similar chemical resistance to PTFE; however, it can be processed like common thermoplastics by injection moulding and extrusion. It has a high resistance to stress cracking [205, 209]. Perfluoroalkoxy copolymer (PFA) PFA consists of a PTFE backbone linked to fluorinated side-chains via oxygen atoms so that this polymer can be thermoplastically processed like FEP. The thermal and chemical resistances are very good [205, 209]. PFA is a standard material used in plastic-lined chemical pumps [2571. Tel /fluorinated cyclic ether copolymer This amorphous, highly transparent material is a combination of tetrafluoroethylene with a fluorinated cyclic ether and was launched onto the market in 1989 as Teflon AF. The structure contains covalent C-F and C-O bonds which give rise to a high deflection temperature and a high resistance to attack by chemicals. The possibility of dissolving Teflon AF in fluorinated solvents establishes new application fields for coatings [205]. Ethylene / tetrafluoroethylene copolymer (ETFE) In addition to PTFE, the copolymer contains approx. 25 % PE that has to be protected by stabilising additives against thermal and photochemical degradation. ETFE was the first fluorinated plastic that could be reinforced with glass fibers. It exhibits a very good chemical resistance [205, 209]. Ethylene / chtorotrifluoroethylene copolymer (ECTFE) Because of its easy processability, this polymer is used in pore-free, electrostatic powder coatings and as woven filters. Its good mechanical behavior is maintained even in cryotechnical applications, 233 K (-- 40C) [205, 209]. ECTFE has a chemical resistance superior to PVDF [222]. The resistance of a series of fluoropolymers, such as FEP, PFA, ETFE and ECTFE, to concentrated hydrochloric acid at 368 K (95 C) was proved by absorption tests over a period of 50 days [209]. Since saturation was already reached at low concentrations of HCl after 20 days, it was possible to extrapolate for longer times so that the behavior in concentrated hydrochloric acid can be classified as being very good.