Process for scale inhibition for kraft digesters

Abstract: Inhibition of calcium scale under the extreme temperature conditions found in pulp digesters has been accomplished by employing certain mixtures of polyaminopoly(alkylenephosphonic acids) together with nonionic surfactants. Diethylenetriaminepenta­ (methylenephosphonic acid) together with a poly­ ethoxynonylphenol has been found especially effective for the purpose.

Description: Scale formation in the apparatus employed in the making of kraft papers is especially prevalent under the conditions ordinarily employed, namely the cooking of the wood chips at high temperatures in aqueous media containing caustic, sodium carbonate, and sodium sulfide. Under such conditions the calcium present in the water can react with the carbonate and produce rapid scaling from deposition of CaCO₃. Such deposits can constrict the slots of the digester screens and cause unscheduled shutdowns of the equipment. In addition to the calcium in the water, scale forming amounts of calcium, manganese, sulfur and magnesium may be leached from the wood chips as they pass through the digester. Organic constituents, found naturally occurring in the wood, are also leached in the digester and some of these organic constituents apparently have a chelating effect and actually prevent the precipitation of mineral scale, e.g. CaCO₃, at low temperatures. The metal-chelant complexes so-formed probably decompose at the higher temperatures during the cooking process, permitting the precipitation of inorganic scales. Some organic constituents of the wood also produce scale which deposits in polymeric form, and is commonly referred to as "pitch". Pitch deposit is often a problem, but this occurs most often at the lower temperatures and pH ranges encountered after the cooking step in the digester in a kraft mill. Various additives have been employed in efforts to control the deposition of scale. Low molecular weight polyelectrolytes have been employed. In particular, the use of a proprietary mixture of low molecular weight anionic polyelectrolytes and sequestrants is reported in an article in Canadian Pulp and Paper Industry, April, 1976, pp. 25-27. The article reports an improvement in cleaning frequency of from weekly before treatment to one to two months after treatment. The length of time required to clean the digester heater was also reduced. Phosphonates which have been commonly used to control scale include nitrilotri(methylenephosphonic acid) (NTMP), diethylenetriaminepenta(methylenephosphonic) acid (DTPMP) and 1-hydroxyethane-1,1-diphosphonic acid (HEDP). These compounds are used in the acid form or in the form of their water soluble salts, e.g., ammonium and alkali metal salts. In U.S. Patent 4,253,912 there is disclosed the use of certain oligomeric phosphonic acid derivatives. These are condensate oligomers prepared by reacting an alkanoyl chloride or an organic acid anhydride with a 1-hydroxyalkane-1,1-diphosphonic acid compound. In an example, phosphorous acid and propionic anhydride are reacted to produce one such oligomer; another is formed by reacting with heating phosphorous acid and decanoyl chloride and subsequently pouring the thick melt formed into acetic anhydride. These oligomeric products are said to inhibit the precipitation of calcium at threshold levels. Other additives have been employed in aqueous systems to control deposition of scale. Thus, for example, in cooling water systems and in pulp and paper mill applications phosphonate-type chelants have been used. In U.S. Patent 4,406,811 there is disclosed a composition for control of corrosion and scale in aqueous systems containing multiple metallurgies, e.g., industrial cooling equipment. Such compositions contain (a) a triazole or a mercaptotriazole, (b) and aliphatic mono- or dicarboxylic acid, (c) a nonionic wetting agent and (d) a phosphorous-containing organic scale inhibiting compound, e.g., HEDP or NTMP. It has now been discovered that certain polyaminopoly(methylenephosphonic acid) compounds together with certain nonionic surfactants provide excellent control of scale due to calcium under the extreme temperature conditions found in pulp digesters. Suprisingly, certain compositions containing polyaminopoly(methylenephosphonic acids), together with at least one nonionic surfactant, have been found to be excellent inhibitors of calcium scale under the extreme temperature conditions found in pulp digesters. Especially, the composition of diethylenetriaminepenta(methylenephosphonic acid) with a polyethoxynonyl phenol has been found effective for this purpose. Also, the use of naturally derived nonionic surfactants, such as alkyl glucosides and oligosaccharides, provides a homogeneous formulation at a substantially neutral pH. Subject matter of the present invention is a process for inhibiting scale in pulp mill kraft digesters by the addition of an aminopoly(alkylenephosphonic acid) to the pulp liquor characterized by adding a composition comprising a polyaminopoly(alkylenephosphonic acid) having the formula wherein: R is a hydrocarbon residue, having two or more carbon atoms, which can be a linear, branched, cyclic, heterocyclic, substituted heterocyclic, or a fused ring-type structure; m is 1-500; m' is 0 to 500; the E and F substituents may be the same as or different from any other nitrogen atom substituent and each R may be the same as or different from any other R; substituents A, B, C, D, E and F are independently hydrogen or wherein: X and Y are independently hydrogen, PO₃M₂ or hydrocarbon radicals having from 1 to 3 carbon atoms; n is 1 to 3, with the proviso that when n>1, each X and Y may be the same as or different from any other X or Y on any carbon atom; M is hydrogen, ammonium or an alkali metal; together with at least one nonionic surfactant which is the condensation product of an alkylene oxide and an alkyl phenol. Some specific examples of compounds which are included by the formula (I) are bis(aminomethyl)dicyclospentadienetetra(methylenephosphonic acid), bis(aminomethyl)bicycloheptanetetra(methylenephosphonic acid), ethylenediamine tetra(methylenephosphonic acid), diethylenetriaminepenta-(methylenephosphonic acid) (DETA-PMP), pentaethylenehexamineocta(methylenephosphonic acid), hexamethylenediaminetetra-(methylenephosphonic acid), piperazinedimethylenephosphonic acid and phosphonomethylated polyalkylene polyamines, having molecular weights up to about 100,000 or more, which main contain piperazine rings in the chain. The dicyclopentadiene and the bicycloheptane derivatives contain the dimethyltricyclodecane and dimethylnorbornane radicals, respectively. The molecular weight range of the unsubstituted polyamine is preferably from 100 to 300,000, more preferably from 1,000 to 2,000. Polyaminopoly(methylenephosphonic acids) which are useful in the practice of the invention are, for example, ethylenediaminetetra(methylenephosphonic acid), diethylenetriaminepenta(methylenephosphonic acid) (DTPMP), triethylnetetraminehexa(methylenephosphonic acid) (TTHMP), hexaethyleneheptaminenona(methylenephosphonic acid and the like. Others include the analogous polymethylenephosphonic acid derivatives of polypropylenepolyamines such as di-1,2-propylenetriaminepenta(methylenephosphonic acid). Nonionic surfactants within the scope of the invention include the condensation products of alkylene oxides and alkyl phenols. Thus, useful nonionic surfactants are, e.g., polyethoxyalkylphenols wherein the alkyl group contains 4 to 12 carbon atoms and wherein the number of ethylene oxide (EO) groups condensed with the alkyl phenol may range from 6 to 15. Other nonionic surfactants include naturally occurring surfactants such as mixed alkyl glucosides and oligosaccharids. The mixed alkyl glucosides also benefit the formulation by providing a homogeneous , substantially neutral pH mixture in combination with ethylene glycol, glycerine and the like polyhydroxy compounds. The weight ratio of the phosphonic acid derivative to that of the surfactant can be varied within the range of from 1/10 to 20/1. A preferred range is from 1/5 to 10/1. The following examples show the use of a screening test used in selecting those combinations which are useful in practicing the invention. Screening Test (Hot Wire Test Procedure) A nichrome wire (26 gauge), 20" (50.8 cm) in length, is bent into a helical coil, weighed on an analytical balance and connected to a variable power source with an ammeter for measuring the current through the wire. The wire is then immersed in a glass vessel containing 200 ml of black liquor obtained from a pulp mill. A 2-amp current is then passed through the wire coil for a given period of time, e.g., 30 minutes. This causes the wire to get hot and, of course, heats the surrounding liquor. The current is then cut off and the wire disconnected from the source, cooled, dipped in acetone to remove excess solution, dried and weighed to determine the amount of scale deposit. The maximum temperature of the solution during the test was 90°C. Analysis of the scale shows it to be primarily Na₂SO₄ and organics, with CaCO₃ being present in lesser amounts. Example 1 In accordance with the above screening procedure several tests are made on control samples which contain no additive. The same tests are made on solutions of the black liquor containing measured amounts of a 1 percent aqueous solution (active content) of the particular additive. The percent scale reduction was obtained by dividing the weight gain of the control minus the weight gain of the test composition by the weight gain of the control. The result times 100 gives the percent reduction. The results of comparative tests of several known commercial scale inhibitors (Examples A through F) and a composition (DTPMP + surfactant) of the invention (Example 1) are shown in Table I. SUMMARY OF SCREENING DATA COMPOUND/20 PPM AVERAGE % SCALE REDUCTION Control/No additive 0 Ex. 1 DTPMP acid 10 ppm surfactant 37 Ex. A HEDP acid -49 Ex. B Polyacrylic acid (low m.w.) -23 Ex. C Polymaleic acid -15 Ex. D Sulfonated polystyrene -44 Ex. E Blend X -15 Ex. F Blend Y 8 The following experiments are tests of scale inhibitors run in a pressure vessel in order to simulate more closely the conditions found in pulp manufacture. Example 2 A six-foot (180 cm) length of 1/8" (0.3 cm) thinwall stainless steel tubing is formed into a coil and mounted on the inside of the cover of a Parr bomb pressure vessel so that a heating medium can be passed through the interior of the coil. The weight of the coil is then determined prior to its being placed into the vessel which contains the black liquor (about 700 ml) obtained from a pulp mill. After the coil is placed into the bomb, the cover is fitted and sealed, the coil connected to a source of heat (steam) and heating begun. The black liquor is then heated to a temperature of 140°C and maintained at that temperature for period of three days (72 hrs). At the end of this time the steam is turned off, the bomb is cooled, the coil is disassembled, washed with water, dried in a vacuum oven and finally cooled and is weighed to determine the weight gain. Several replications are made to determine an average weight gain (due to deposition of scale from the liquor) with no scale inhibitor present. Additives are then tested adding various amounts of a 1 percent aqueous solution (active content) of each, as before. The percent scale reduction is calculated as previously shown. Analysis of the scale shows it to be about 95 percent CaCO₃ and 3 percent organic material, essentially the same as found in the black liquor heaters of commercial pulp mills. Results of several commercial inhibitors (Examples G through J) and various formulations of the invention (Examples 2a through 2e) are shown in Table II. Example No. Compound/Amount % Scale Reduction Blank Control/None 0 Ex. G Blend X / 20 ppm (Comp.) 8 Ex. H Blend Y/ 20 ppm (Comp.) 3 Ex. I Blend Z / 20 ppm ( Comp.) 5 Ex. J DTPMP /20 ppm (Comp.) 16.5 Ex. 2a DTPMP / 10 ppm + Tergitol NP-10 /20 ppm 51.8 Ex. 2b DTPMP / 10 ppm + Tergitol NP-10/60 ppm 59.0 Ex. 2c DTPMP /5 ppm + Tergitol NP-10/1 ppm (formulated in ethylene glycol) 47.0 Ex. 2d DTPMP /20 ppm + Tergitol NP-10/4 ppm (formulated in ethylene glycol) 58.0 Ex. 2e DTPMP/20 ppm + Tergitol Np-10/10 ppm (formulated in ethylene glycol) 65.0 Example 3 The procedure of Example 2 is followed except that a five-foot length of stainless tubing is employed, hot oil is used as the heating source instead of steam and the duration of the test is 24 hours. A commercial product is tested for comparison with a product of the invention, concentration being based on active content as before. The composition of the invention contained 24 percent DTPMP acid, 4.2 percent Tergitol NP-10, 7.0 percent Triton BG-10 and 27 percent ethylene glycol, the balance being water. The solution had a pH of 4. A commercially available high temperature calcium carbonate inhibitor employed for comparison is an aqueous solution containing an anionic surfactant (55 percent) and a polyacrylate (12 percent) used at three different concentrations (Examples K, L and M). The results, shown also at three concentrations (Example 3a through 3c), are given in Table III. Example No. Compound/Amount % Scale Reduction Blank Control/None 0 Ex. K Commcl. product / 5 5% Ex. L Same / 20 18% Ex. M Same / 40 21% Ex. 3a Chelant + surfacant / 5 26% Ex. 3b Same / 7 38% Ex. 3c Same / 13.5 89% Example 4 In a trial in a commercial pulp mill, the digester heater chosen for the test is operated for 5 days (during which time 4900 tons of pulp is produced) until the heat transfer decreases to an unacceptable level and is shut down to be acid cleaned. For the next heater cycle the scale inhibitor of the invention is added on a continuous basis to the pulp liquor. The composition [containing DTPMP (21 percent by weight), Tergitol NP-10 (4 percent by weight) and ethylene glycol (52 percent by weight), the balance being water] is added at 65 ppm. (This amount of the compositon represents about 14 ppm DTPMP and about 3 ppm surfactant based on the black liquor.) The heater runs for about 8 days (during which time 7400 tons of pulp is produced) before the heater transfer again becomes unacceptable. After discontinuing the use of the inhibitor, the heater scales up again, the heat transfer becoming unacceptable after about 4 days of exposure to the untreated black liquor (during which time only 3200 tons of pulp is produced). Example 5 In another pulp mill trial a composition of the invention is employed which contains two different surfactants. This combination provides a homogeneous composition at a neutral pH. The digester heater is operated for four days (during which time 3000 tons of pulp is produced) before scale deposition required acid cleaning. The formulation of Example 3 is added at 65 ppm based on the black liquor. This provides 9 days of operation (during which time 9000 tons of pulp is produced) before heat transfer becomes unacceptable. After discontinuing the use of the scale inhibitor, the heater scales up again in 3 days (during which time 300 tons of pulp are produced).

Claims: A process for inhibiting scale in pulp mill kraft digesters by the addition of an aminopoly(alkylenephosphonic acid) to the pulp liquor characterized by adding a composition comprising a polyaminopoly(alkylenephosphonic acid) having the formula wherein: R is a hydrocarbon residue, having two or more carbon atoms, which can be a linear, branched, cyclic, heterocyclic, substituted heterocyclic, or a fused ring-type structure; m is 1-500; m' is 0 to 500; the E and F substituents may be the same as or different from any other nitrogen atom substituent and each R may be the same as or different from any other R; substituents A, B, C, D, E and F are independently hydrogen or wherein: X and Y are independently hydrogen, PO₃M₂ or hydrocarbon radicals having from 1 to 3 carbon atoms; n is 1 to 3, with the proviso that when n>1, each X and Y may be the same as or different from any other X or Y on any carbon atom; M is hydrogen, ammonium or an alkali metal; together with at least one nonionic surfactant which is the condensation product of an alkylene oxide and an alkyl phenol. The process of Claim 1 wherein the alkyl group of the alkyl phenol contains 4 to 12 carbon atoms and the alkylene oxide is ethylene oxide. The process of Claim 2 wherein the condensation product contains form 6 to 15 moles of ethylene oxide. The process of Claim 3 wherein the condensation product is nonyl phenol condensed with 10 moles of ethylene oxide. The process of Claim 2 wherein m and m' are such that the molecular weight of the unsubstituted polyamine is from 100 to 30,000. The process of Claim 1 or 5 wherein n is 1. The process of Claim 6 wherein the polyaminopoly(methylenephosphonic acid) is diethylenetriaminepenta(methylenephosphonic acid).

Inventors: COOPER LANCE A, CRUMP DRUCE K, HOPKINS THOMAS M, COOPER, LANCE A., CRUMP, DRUCE K., HOPKINS, THOMAS M.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1988
Main classification: C02F5
IPC codes: D21C9, C11D3, C11D10, C02F5, C11D1