Crystallization of Mechanical Pulp Mill Effluents ... - ACS Publications

Chapter 7

Crystallization of Mechanical Pulp Mill Effluents through Hydrate Formation for the Recovery of Water Downloaded by EAST CAROLINA UNIV on January 4, 2018 | http://pubs.acs.org Publication Date: March 10, 1994 | doi: 10.1021/bk-1994-0554.ch007

Cathrine Gaarder, Yee Tak Ngan, and Peter Englezos Pulp and Paper Centre, Department of Chemical Engineering, The University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada

This paper presents data toward the development of a new crystallization technology for the concentration of pulp mill effluents and the subsequent recovery of water. In this process, clathrate hydrate crystal formation is the crystallization step. A screening of clathrate -hydrate forming substances was performed and two substances were chosen, carbon dioxide and propane. Experiments were carried out with these substances to measure the temperature and pressure conditions at which hydrates form in mechanical pulp mill effluent. It was determined that hydrate crystals can form in these effluents at temperatures several degrees above the normalfreezingpoint of water. The measurements were compared with similar hydrate formation data in pure water. It was found that the presence of impurities in the effluent samples did not cause any appreciable change in the equilibrium hydrate formation conditions. The pulp and paper industry requires a large amount of water to run its unit operations and, as a result, generates a substantial volume of dilute aqueous effluent that contains a variety of impurities of both organic and inorganic matter (7). In the past few years the discharge limits on these effluents have become increasingly stringent. And, as a consequence, the industry must improve the current effluent treatment technologies or develop new ones that can substantially reduce the pollution load that is now being discharged into the environment. The current technologies that are being used include primary and secondary treatment systems. Primary systems, e.g. clarifying tanks, physically remove the majority of the settleable solids in the effluent whereas secondary systems, predominantly biological, remove a large portion of the organic compounds resulting in a decrease in the biological oxygen demand, BOD, of the effluent. The mills that have secondary end-of-pipe treatment systems in place are capable of complying with the current regulations but if these regulations become more stringent, as expected, these systems may prove to be inadequate (2). 0097-6156/94/0554-0114$08.00A) © 1994 American Chemical Society

Tedder and Pohland; Emerging Technologies in Hazardous Waste Management IV ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

Downloaded by EAST CAROLINA UNIV on January 4, 2018 | http://pubs.acs.org Publication Date: March 10, 1994 | doi: 10.1021/bk-1994-0554.ch007

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Crystallization of Mechanical Pulp Mill Effluents 115

Therefore, a two-fold approach to implement innovative water management strategies that would result in zero-liquid discharge (ZLD) mills is emerging. The first part is to modify the internal process to minimize the consumption of fresh feed water and the second is to develop cost effective end-of-pipe technologies that can recover clean waterfromthe effluent. The separation technologies that are being considered for these ZLD systems include evaporation, membrane separation, and freeze concentration, also referred to asfreezecrystallization (2). Compared with other separation technologies the major advantage of a crystallization process is that at a given temperature and pressure only one component will crystallize and it will do so as a pure crystal (3). The separation process that is the focus of this paper, clathrate hydrate concentration, is a variant offreezeconcentration. Freeze concentration refers to the concentration of an aqueous solution by generating a solid phase within the liquid stream, i.e.freezingit, followed by the physical separation and melting of the resulting crystals (3). This process is currently used by the food industry, e.g. the production offrozenorange juice, and in the desalination of seawater (4). Thefreezeprocess has the potential to be made more economical if thefreezingstep is modified to generate clathrate hydrates (clathrate hydrate concentration). The reason being that clathrate hydrates (5), also referred to as hydrates, have the ability to form at temperatures several degrees above the normalfreezingpoint of water. Clathrate hydrate crystals are formed at suitable pressure and temperature conditions by the physical combination of water molecules with a large number of different molecules, e.g. argon, carbon dioxide and propane, which can be trapped within a network of hydrogen bonded water molecules (6, 7). The gas hydrate crystals contain only water and the hydrate forming substance. The concentration of aqueous streams by this process was patented by Glew (8) and was also investigated by Werezak (9). Formation of hydrate crystals in seawater was considered as the basis of a process to recover pure water. The process was demonstrated on a pilot plant stage (10, 11, 12, 13) but did not become commercial for economic reasons. The application of the clathrate process has recently become of interest in waste minimization as well as in the food products industry (4, 14, 15). In this work, we are investigating the formation of clathrate hydrates in mechanical pulp mill effluents which are aqueous streams containing organic and inorganic substances. The scope of our work is to develop a separation technology based on clathrate-hydrate crystal formation. The process will concentrate the effluent and recover clean water to be recycled back into the mill. In the present study, our objectives are: (a) to select two suitable hydrate forming substances based on a number of criteria; (b) to form hydrates in mill effluents and observe the characteristics of the crystal formation; (c) to determine the minimum pressure, at a given temperature, at which hydrate crystals are formed; and (d) to discuss a conceptual process for the recovery of clean waterfromthe mill effluent. Background Effluents from Mechanical Pulping. In mechanical pulping the wood chips are fed through parallel refîner discs which mechanically break down the wood into



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EMERGING TECHNOLOGIES FOR HAZARDOUS WASTE MANAGEMENT

individualfibres.There are several different mechanical pulping processes. For example, pulp produced by the thermomechanical pulping (TMP) process is preheated and refined under pressure whereas bleached chemi-thermomechanical pulp (BCTMP) is produced by the addition of chemicals in the preheater, refined under pressure, and then bleached. The organic composition of the wood chips depends on their age and the wood furnish used. The general composition includes extractives (neutral solvent-soluble components), lignins (acid-insoluble aromatic compounds), hemicelluloses (mild acid or alkali-soluble carbohydrates), and cellulose (strong alkali insoluble carbohydrates). The effluent that resultsfromthese processes contains the wood material that was not recovered asfibreand traces of the chemicals that are added during the process. These components are either dissolved in the aqueous media or appear as small particles or colloidal material. Clathrate Hydrate Phase Diagram. A partial phase equilibrium diagram for the propane-water system in the hydrate formation region is depicted in Figure 1. The points indicate experimental datafromKubota et al. (16) whereas the lines are interpolations of these data. Line KL is the vapor pressure line for propane. Line AQ defines the locus of hydrate-vapor (rich in propane)-liquid-w(rich in water) phase equilibrium. At a given temperature the pressure that corresponds to a point on this line is the minimum pressure required for hydrate formation. This pressure is often referred to as the incipient hydrate formation pressure. Propane hydrate is formed at conditions corresponding to pressures and temperatures on the left of line AQ. The temperature corresponding to point Q is the maximum temperature at which propane can form hydrate. This point is called the upper quadruple point. At this point four phases namely, hydrate, vapor, liquid-w and liquid-p(rich in propane) coexist in equilibrium. Line AQ extends down to another quadruple point (not shown on the plot) at 273.15 Κ where hydrate, ice, vapor and liquid water coexist in equilibrium. Line QC defines the locus of hydrate-liquid-w-liquid-p condensed phase equilibrium. Propane hydrate is formed when the pressure and temperature correspond to a point on the left of line QC. It should be noted that a similar diagram for carbon dioxide can also be prepared with datafromthe literature. It is well known that the presence of impurities such as electrolytes in the water alters the location of the three phase line. In particular, at a given temperature, the required pressure for the formation of clathrate hydrate crystals is higher or at a given pressure, the equilibrium hydrate formation temperature is lower (depression of hydrate formation). For example, in Figure 1, the incipient hydrate formation conditions in a 2.5 wt % NaCl solution are shown. These experimental points are on the line A1Q1. The deviationfromthe formation conditions in pure water depends on the concentration of the impurities. Because the mill effluents contain various substances, our objective is to determine the locus of the hydrate-vapor-liquid equilibrium with the mill effluent as the liquid phase. This will be accomplished by measuring the pressure-temperature (P-T) hydrate formation conditions. Knowledge of these three phase equilibrium conditions is necessary for the design of the clathrate hydrate process.


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GAARDER ET AL.

Crystallization of Mechanical Pulp Mill Effluents


Conceptual Clathrate Hydrate Concentration Process There are at least four unit operations used to perform a separation with the clathrate hydrate concentration process. These are shown in Figure 2. They include the formation of the crystals, the separation of the crystalsfromthe concentrate, the decomposition of the crystals, and the recovery of the hydrate former. Clathrate hydrate formation is accomplished by mixing the hydrate former with the effluent at suitable pressure and temperature conditions. The function of the separation step is to remove the clean hydrate crystalsfromthe concentrated solution. This is a difficult separation (10, 11) because surface contaminants, impurities in the concentrated solution, are attracted to the crystal surface. The degree of this attraction will determine both the ease and extent of the separation. These attractions can be minimized if the surface area to volume ratio of the crystal is small. Once the hydrates are clean they are decomposed and the resulting mixture is separated into water and the hydrate former which is then recovered and returned to the hydrate formation unit. Effluent Sources The effluent samples used in this study camefromtwo different mills. One sample was obtainedfromQuesnel River Pulp, in Quesnel, British Columbia, which produces both BCTMP and TMP, resulting in a mixed TMP/BCTMP generated effluent. The other sample came from Fletcher Challenge, in Elk Falls, British Columbia, which produces TMP. The chemicals used during the BCTMP runs at Quesnel include sodium sulphite, and DTPA (Diethylenetriaminepentaacetic Acid) which are used in the preheating stage, and caustic, sodium silicate, magnesium sulphate, DTPA, and a high consistency hydrogen peroxide which are used in the bleaching stage (Jackson, M., Sunds Defibrator, personal communication, 1992). The total solids content of the TMP effluent was found to be 710 mg/L, compared with 30,000 mg/L for the TMP/BCTMP effluent. The volatile solids portion increasedfrom240 mg/L to 12,000 mg/L for the TMP and TMP/BCTMP effluents respectively. These differences resultfromthe specific processes and operations used at the two mills. It is well known that the use of chemicals in the BCTMP process will decrease the pulp yield, in comparison to the TMP process, resulting in an increase in the solids load of the effluent. The volume of water used in the process will also affect the solids concentration. Selection of Hydrate Formers Several criteria were used to determine which of the over one hundred hydrate formers would be most suitable for this study. Thefirstcriterion compared the hydrate formation equilibrium curves in pure water to determine which hydrate formers could form hydrates at temperatures well above 0°C and at pressures close to atmospheric. This was chosen as the initial criterion because the hydrate formation temperature must be well above thefreezingpoint of the solution to give clathrate hydrate concentration an energy advantage overfreezeconcentration. The next set


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1.0 0.8

(8 OL

0.6

CO

0.4

• pure water (16) • vapor pressure (16) 2 . 5 w t % N a C l (16)

n


Φ

0.2 0.0 270

272

274

276

278

280

Temperature (K)

Figure 1. Propane-Water Partial Phase Diagram in the Hydrate Formation Region

Wood Chips CRYSTALLIZATION UNIT

Effluent

ι

Hydrate Former

Process Water

Hydrates S Ε Ρ A R A Τ Ο R

DECOMPOSITION UNIT

)

V. Concentrate

f To Evaporator Figure 2. Unit Operations for a Clathrate Hydrate Concentration Process



7.

GAARDERETAL.


of criteria were used tofindenvironmentally acceptable hydrate formers, i.e. hydrate formers that are nontoxic, have low ozone depletion potential (ODP), low global warming potential (GWP), low explosion potential, and are nonflammable. Other criteria that were considered include the solubility of the hydrate former in water, its cost and availability. The solubility aids in determining what type of a separation step, if any, is required to recover the hydrate formerfromthe water phase once the hydrates have been decomposed. The extent of hydrate recovery will depend on both the cost of the hydrate former and the degree of process water purity that is required. Based on the above selection criteria the two hydrate formers chosen for this study were carbon dioxide and propane. Table 1 lists pertinent information for these compounds. TABLE 1: Hydrate Former Characteristics for Propane and Carbon Dioxide CHARACTERISTIC

CARBON DIOXIDE 10.1 °C at 4.5 MPa

Maximum equilibrium Τ & Ρ in pure water (quadruple point) Ozone Depletion Potential (ODP) Global Warming Potential (GWP) Explosion Potential

None

Flammability

Nonflammable

Toxicity

Asphyxiant TLV 5000 ppm

PROPANE 5.3 °C at 0.542 MPa

(19)

(16)

0

0

1.0

-

Hydrate density (g/cm)

1.112

Lower level 2.1% Upper level 9.5% Autoignition Temperature 432°C Asphyxiant (no threshold level given) 0.88

Solubility in water at maximum temperature (quadruple point) Heat of hydrate formation (KJ/mol hydrate) Number of water molecules per gas molecule in hydrate

6.1 wt percent

0.06 wt percent

-55.0

-133.2

7.3

17.95

3

Propane is known to beflammableand has an explosion potential if not handled properly, but it was decided that due to its wide spread use in other


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industries, procedures that would minimize both of these risks could be implemented. Carbon dioxide was chosen despite its high hydrate formation pressures, up to 45 atm, and its high solubility in water because it is inexpensive, nontoxic and does not pose a safety hazard. Other hydrate formers that were considered include cyclopropane and the new refrigerant R-134a. It was found that the cost of cyclopropane was much higher than that for propane, and that R-134a was both expensive and difficult to obtain in small quantities for lab purposes.


Experimental Apparatus and Procedure

The experimental apparatus that was used to study hydrate formation in mill effluents is described in detail elsewhere (77, 18). The main component of the apparatus is the vessel in which the hydrates form. Two different vessels were used. The low pressure vessel was used for propane hydrate formation whereas the high pressure one was used for carbon dioxide hydrate formation. In both cases, the vessel sits inside a glycol-water bath in which the temperature is controlled by an external refrigerator/heater, and a uniform temperature maintained by a motor driven stirring mechanism. Both vessels had sight windows such that the hydrate formation process could be viewed. The procedure used to determine the incipient hydrate phase equilibrium data was the same for both of the vessels. At standard procedure to obtain such data was used(77). Results and Discussion Experimental Hydrate Formation Conditions. The objectives of the experimental

part of this study were to determine if hydrates could form in mechanical pulp mill effluents and ,if so, how would the impurities in the effluents affect the formation conditions. Initial experiments were performed to ensure that hydrates would form in mechanical pulp mill effluents by using propane or carbon dioxide. It was found that both propane and carbon dioxide could easily form hydrates in the TMP/BCTMP effluent. Subsequently, the incipient equilibrium pressure at temperatures above the normal freezing point of water were determined. The results of the incipient equilibrium hydrate formation experiments determined for the TMP/BCTMP and the TMP effluent samples in the presence of propane are shown in Figure 3. Results for pure water are also shown to give a comparison. It wasfoundthat hydrates formed in both of the effluents at pressures only slightly above those needed for hydrate formation in pure water at the same temperatures. Figure 3 also shows that hydrates will form in both of the effluent samples at very similar conditions. This is a positive characteristic when considering the implementation of the clathrate hydrate concentration process as it implies that the process can accommodate changes in effluent composition. The experiments using carbon dioxide as the hydrate former showed similar results to those with propane. The results are shown in Figure 4. Again the TMP/BCTMP effluent formed hydrates at pressures only slightly above those for pure water at the same temperatures. Also found in Figure 4 are resultsfroma run in which pure water was used. The pressure-temperature diagram that resulted from


7.

GAARDER ET AL.


0.6


CO QL

•

water (16)

•

water (17)

•

Elk Falls effluent

Δ

Quesnel effluent

m

ô tm

3 φ co

•

è

CO

Δ 0.1 273

274

275

276

277

278

279

Temperature (K)

Figure 3. Hydrate Formation in Effluent with Propane.

•

water (19) water (This work) Ο Quesnel effluent A

4i CO

CL

3Η CO CO

φ

2Η

272

274

276

278

280

282

284

Temperature (K)

Figure 4. Hydrate Formation in Effluent and Pure Water with Carbon Dioxide.


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these experiments were in good agreement with datafromthe literature thereby validating the experimental equipment and procedure used in this study. The resultsfromboth hydrate formers have shown that the impurities in the mill effluent did not significantly affect the hydrate formation conditions when compared with those in water. This implies that either their concentration is too low to cause any appreciable change in the hydrate formation conditions or the effluent contains impurities that are not hydrate inhibitors. If indeed hydrate inhibitors exist in the effluent then it is expected that as water is removed and becomes a part of the hydrate phase the concentration of the impurities will increase. This will result in an increase in the pressure needed for hydrate formation to occur. The extent that this will affect the formation conditions depends on the amount of water that is to be recovered by the process. For example, recovery of 80 percent of the water will increase the concentration of the impurities 5 times. In this case, one expects the inhibiting action would be roughlyfivetimes that in the initial effluent. However, because the inhibiting action of the impurities in the initial effluent was small the overall increase due to the concentration process should again be small. Qualitative observations about the crystals were made in experiments where considerable amount of hydrates were formed. The crystals that were formed in the presence of propane varied in colour, the ones thatfloatedon top looked to be clear, similar to ice, compared to those at the bottom and on the sides of the vessel. The reason for the colour of these crystals is not known for sure but it may be due to occlusion of concentrate in the vacant space between individual crystals. These observations point out the need for more research into the formation process (kinetics and morphology studies) before a large scale crystallization unit can be designed. Conclusions Propane and carbon dioxide were selected as suitable hydrate forming substances for the clathrate hydrate formation process. Propane was found to form hydrates in both types of mechanical effluents that were examined. Carbon dioxide was tested with only one of the effluents and was found to form hydrates readily. The pressuretemperature hydrate formation conditions (partial phase diagram) for both gases were determined. It was found that the presence of the impurities in the mill effluents altered the pressure-temperature equilibrium locus only slightly. Acknowledgments Thefinancialsupportfromthe Natural Science and Engineering Research Council of Canada (NSERC) and the Pulp and Paper Research Institute of Canada (PAPRICAN) is greatly appreciated. Literature Cited 1. Smook, G.A. Handbook for Pulp and Paper Technologists; 2nd Edition; Angus

Wilde: Vancouver, B.C., 1992 pp 45-64, 378-391.


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2. Beaudoin, L.; Dessureault, S.; Barbe, M.C. CPPA 77th Annual Meeting;

Montreal, Quebec, 1991, A235-A253. 3. Heist, J.A. Chem. Eng. 1979, May, pp 72-82. 4. Douglas, J. EPRI Journal 1989, Jan-Feb. pp 16-21.

5. Findlay, R.A. In New Chemical Engineering Separation Techniques; Scho

H.M., Ed.; Interscience Publishers: New York, 1962; pp. 257-318. 6. Englezos, P.; Kalogerakis, N.E.; Bishnoi, P.R. J. Inclusion Phenom. Mol.


Recognit. Chem. 1990, 8, pp 89-101.

7. Sloan, E.D. Clathrate Hydrates of Natural Gases; Marcell Dekker: New York, 1990, pp 1-66. 8. Glew,D.N. Solution Treatment, U.S. Patent 3,085,832, Oct. 16, 1962. 9. Werezak, G.N. AIChE Symp. Ser. 1969, 65(91), pp 6-18. 10. Knox, W.G.; Hess, M.; Jones, G.E.; Smith, H.B. Chem. Eng. Prog. 1961, 57(2), pp 66-71. 11. Barduhn, A.J. Desalination, 1968, 5, pp 173-184. 12. Barduhn, A.J. Chem. Eng. Prog 1975, 71(11), pp 80-87. 13. Tleilmat, B.W. In Principles of Desalination; Spriengler, K.S., Ed.; 2nd Edition; Academic Press: New York, 1980, pp 359-400. 14. Tech Commentary 1988, 1(1), pp 1-4 (Freeze Concentration). 15. U.S.D.Energy Report 1988, Prepared by Heist Engineering. Report available from NTIS, U.S.D.Commerce, Springfield, Virginia 22161 (Beet Sugar Refining Applications). 16. Kubota,H.; Shimizi, K.; Tanaka, Y.; Makita, T. J. Chem. Eng. Japan. 1984, 17, pp 423-428. 17. Englezos,P.; Ngan, Y.T. J. Chem. Eng. Data. 1993, 38, pp 250-253. 18. Englezos,P.; Ngan, Y.T. Fluid Phase Equilib. 1993, in press. 19. Robinson,D.B.; Mehta, B.R J. Can. Petr. Tech. 1971, 10, pp 33-35. R E C E I V E D December 1, 1993


Crystallization of Mechanical Pulp Mill Effluents ... - ACS Publications

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