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Ind. Eng. Chem. Res. 1996, 35, 1894-1900
SEPARATIONS Concentration of Mechanical Pulp Mill Effluents and NaCl Solutions through Propane Hydrate Formation Yee Tak Ngan and Peter Englezos* Pulp and Paper Centre and Department of Chemical Engineering, The University of British Columbia, 2216 Main Mall, Vancouver, British Columbia V6T 1Z4, Canada
In this work, recovery of water from mechanical pulp mill effluents and 2.5 wt % NaCl solutions through propane hydrate formation was investigated. A new apparatus in which hydrate nucleation, growth, separation, and melting occur in one vessel was designed and built. The emphasis of the work was on crystal separation. The average reduction in the salt content of the recovered water from the NaCl solutions was found to be 31%. Displacement with propane could increase the amount of recovered water at the same purity level. Further improvement in the purity could be accomplished with washing with water. The results with the effluents showed that the total organic carbon and the salt content of the recovered water were lower by 23 and 26%, respectively, from the levels in the effluent. Improved separation could be achieved by displacement with liquid propane. 1. Introduction Interest in exploiting clathrate hydrate formation in a variety of areas has been recently developed (Englezos, 1993; Sloan et al., 1994). These areas include biotechnology, new materials, and aqueous solution concentration (Douglas, 1989; Phillips et al., 1991; John et al., 1994). The concentration of aqueous streams by hydrate formation was patented by Glew (1962). The formation of hydrates in seawater was considered as the basis of a process to recover pure water. The process was demonstrated at a pilot-plant stage (Knox et al., 1961; Tleimat, 1980) but was never developed commercially because other processes could achieve the same purity with lower cost. The process has recently become of interest in waste minimization (effluent concentration), the food products industry, and the concentration of other aqueous solutions (Douglas, 1989; Willson et al., 1990; Gaarder and Englezos, 1995). The interest in effluent concentration arose as a result of the fact that in recent years closed-cycle plant operation is increasingly being regarded as the ultimate method for pollution prevention (Byers, 1995). It should be pointed out that closed-cycle actually denotes zero liquid discharge (ZLD) operation. In the pulp and paper industry, closed-cycle or ZLD technology is now in operation in mechanical pulp mills, and an intense effort to reduce water usage in Kraft mills with the ultimate objective to achieve closed-cycle operation is underway (Chandra, 1993; Patrick et al., 1984). An essential part of a ZLD plant is a process that will be capable of removing dissolved substances or recovering clean water from the effluent. Processes for the recovery of water include evaporation, crystallization (freeze concentration and clathrate hydrate concentration), and membrane separation. Evaporation is a mature process and operates in mechanical pulp mills (Young, 1994a,b). * Author to whom correspondence should be addressed. Telephone: (604) 822-6184. Fax: (604) 822-6003. E-mail:
[email protected].
S0888-5885(96)00001-2 CCC: $12.00
Freeze concentration refers to the concentration of a solution by generating ice crystals followed by the physical removal of the ice crystals from the solution and the subsequent melting of these crystals (Heist, 1989). In clathrate hydrate concentration, the ice formation step is replaced by clathrate crystal formation. Freeze and clathrate hydrate concentration are both based on the fact that the impurities present in the original solution are not contained within the ice or clathrate crystal structure. Because the freeze process operates below 0 oC, the potential for scaling and corrosion problems is greatly reduced compared to evaporation. Hydrate crystals can form at temperatures several degrees above the normal freezing point of water, thereby decreasing the energy requirements compared with freeze concentration. These temperatures, however, are not high enough to cause corrosion, scaling, or loss of volatile substances, as is the case with evaporation. The work on desalination through hydrate formation which was published mainly in a series of desalination symposia (Udall et al., 1965; Delyannis, 1967; Delyannis and Delyannis, 1970, 1973, 1976, 1978) indicated that the crucial step in the development of technology was a successful separation of the hydrate crystals from the concentrated brine solution. In effluent concentration, however, the purity demands for the recovered water are not as stringent and, consequently, there is a good incentive to use the hydrate process. Gaarder and Englezos (1995) studied hydrate formation in various mechanical pulp mill effluents and their concentrates. They used C3H8, CO2 and a C3H8-CO2 mixture to determine the pressure-temperature hydrate formation conditions. It was found that the clathrate process could operate 8-10 °C above the temperature of the freeze concentration process. The scope of the work undertaken in the present study was to experimentally investigate the recovery of water for reuse from bleached chemithermomechanical pulp (BCTMP) mill effluents via clathrate hydrate © 1996 American Chemical Society
Ind. Eng. Chem. Res., Vol. 35, No. 6, 1996 1895
Figure 1. Schematic of the apparatus.
formation. The specific objectives were the following: (a) to design an apparatus with the main characteristic that it uses a single vessel as crystallizer, wash column, and crystal melter; and (b) to evaluate the effectiveness of the separation of hydrate crystals from effluent concentrate in terms of the mass and the quality of the recovered water from the melted crystals. Among mechanical pulp mill effluents, BCTMP contains the highest concentration of total solids, volatile solids, suspended solids, salts, total organic carbon, and inorganic carbon. In this work, BCTMP effluent was chosen. 2. Experimental Apparatus and Procedure The new apparatus and the experimental procedure are described next. More details are given elsewhere (Ngan, 1995). 2.1. Apparatus. Figure 1 illustrates a schematic of the apparatus. The crystallizer/wash column produces hydrate crystals in a semi-batch manner. Liquid effluent feeds into the crystallizer from the top by gravity. The gas storage vessel supplies the required quantity of propane. The hydrate former enters the crystallizer/ wash column from the bottom through a 1/8 in. orifice opening. The wash water supply vessel enables us to inject wash water into the crystallizer which then operates as a pressurized wash column. Nitrogen is used in the wash water supply vessel to pressurize the water. Figure 2 displays two cross-sectional views, 90°
from each other, of the crystallizer. The main body of the crystallizer is constructed from a 51/2 in. diameter acrylic tube with a wall thickness of 3/8 in. A 9 in. diameter acrylic tube creates a cooling jacket for the crystallizer by circulating a 50-50 wt % glycol-water mixture from a Forma Scientific 2095 refrigeration unit. Two end caps enclose the 51/2 in. tube. Twelve ready rods fix the two end caps in place. O-rings seal the two end caps. The inside dimensions are 36 in. high and 51/2 in. wide. Once the crystals have formed, the concentrated effluent drains from the crystallizer through two 3/4 in. diameter outlets. Each outlet has a No. 18 stainless steel screen. A mechanical stirrer mixes the contents within the vessel. Two stirring geometries were implemented. The first configuration which was used is referred to as stirring geometry I. It had three 4 in. diameter marine impellers which were equally spaced on a 30 in. long shaft. Later, the mixing assembly was modified by increasing the length of the impeller shaft by 4 in. and adding two 4 in. diameter six-blade Rushin turbine impellers. This configuration is referred to as stirring geometry II. Copper constant thermocouples from Omega measure the temperature within the crystallizer with an accuracy of (0.1 K. The thermocouples are situated at the top and the bottom of the crystallizer, at the wash water inlet, and at the propane inlet. A Bourdon Heise gauge from Brian Engineering with a span of 300 psi measures
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Figure 2. Two cross-sectional views of the crystallizer at 90° from each other.
the pressure in the crystallizer with an accuracy of (0.5% of the span. The Gas Storage Vessel. This vessel has two chambers. The upper chamber holds nitrogen and the lower one liquid propane. The vessel uses a piston to maintain the pressure above saturation. This piston is free to move up and down, and a rod guides the path of this piston. Knowing the pressure and temperature within the vessel, published equilibrium data from
Thomas and Harrison (1982), give the molar volume of the liquid. By measuring the displacement length in the guiding rod, it is possible to calculate the quantity of propane used in each experiment within approximately (4 g. The construction material for the storage vessel is a 316 stainless steel hollow cylindrical bar. The vessel’s end caps are made from stainless steel circular plates. The piston is machined from a cylindrical aluminum bar. Twelve bolts fasten the two end caps in place. O-Rings seal the piston and the end caps. 2.2. Effluent Tests. Each solution (feed, recovered water, and concentrates) was weighted with a Mettler PE 16 scale with a readability of 0.1 g. The conductivity was measured with an Orion Model 160 meter with an accuracy of (0.5% for solutions with electrolyte content up to 199.99 mS. The meter is capable of automatically compensating for temperature variance. A Shimadzu TOC 500 was used to obtain the carbon content in the solution. This analyzer measured the total organic carbon (TOC) and the total inorganic carbon (TIC) content. TOC and TIC indicate the concentration of organic components in the samples. Solids content was obtained by measuring the total solid content (TS), the fixed solids (FS), the volatile solids (VS), and the total suspended solids (TSS). 2.3. Procedure. All the experiments except one were conducted with liquid propane because from a water recovery point of view this is preferable (Jeffrey and Saenger, 1991). The experiment began by identifying the hydrate-effluent-propane (vapor) incipient equilibrium condition at the experimental temperature by following the isothermal pressure search method (Englezos and Ngan, 1994). At this equilibrium condition there is a very small amount of hydrate crystals present in the form of tiny crystals. Subsequently, liquid propane was injected into the crystallizer in a batch manner. At the end of the injection period, a thick layer of liquid propane at its vapor pressure formed on top of the feed solution. Typically, the injection time required 15 min. The height of the liquid propane layer varied. It depended upon the target quantity of propane chosen for that particular experiment. During the hydrate growth period, the crystals were observed for their appearance. The hydrate formation process was allowed to continue until all the liquid propane was consumed. At the end of the growth, the effluent was more concentrated than the feed thus shifting the hydrate-propane-effluent equilibrium. The new equilibrium value was also determined. One experiment, E1, used a continuous supply of vapor propane to grow the hydrate crystals. The crystals were allowed to grow until the target quantity of propane was consumed. A pressure control system was implemented in the crystallizer in order to maintain constant pressure. The controller is a PID microprocessor, Model CN2001(*)-F2, from Omega. Crystal Separation. This work conducted three types of separations: (a) no washing; (b) concentrate displacement with liquid propane; and (c) washing with water. Each process began with the drainage of the concentrate from the two liquid outlets when the crystallization process was over. While the concentrate was draining, the mixing assembly was turned off. The pressure was maintained at the hydrate-propane (vapor)-effluent equilibrium pressure by the pressure controller. The drainage time was 15 min with the exception of two runs (S1 and S2) for which the drainage period was 120 min. In the no washing experiments,
Ind. Eng. Chem. Res., Vol. 35, No. 6, 1996 1897 Table 1. Summary of Operating Conditions
run
temp (K)
pressure (MPa)
S1 S2 S3 S4 S5 S6 S7 S8 S9 E1 E2 E3 E4 E5
273.7 ( 0.3 273.8 ( 0.2 272.8 ( 0.3 272.0 ( 0.1 272.1 ( 0.1 273.6 ( 0.1 273.5 ( 0.1 274.4 ( 0.1 274.3 ( 0.1 274.9 ( 1.2 276.7 ( 0.1 276.4 ( 0.1 276.5 ( 0.1 276.2 ( 0.1
0.498 ( 0.008 0.493 ( 0.004 0.484 ( 0.009 0.469 ( 0.004 0.473 ( 0.004 0.501 ( 0.004 0.503 ( 0.004 0.512 ( 0.004 0.510 ( 0.004 0.465 ( 0.005 0.545 ( 0.004 0.531 ( 0.004 0.540 ( 0.006 0.535 ( 0.004
stirring rate amount crystal average propane procedure for and torque of C3H8 growth consumption impeller crystal bed (rpm/oz. in.) spent (g) period (h) rate (g/h) geometry formation 200/25 300/35 300/35 300/35 300/35 300/105 300/105 300/100 300/103 210/45 300/90 300/96 300/90 300/100
221 288 348 514 455 342 346 401 533 236 423 412 420 423
116 119 94 22 24 6 6 8 10 336 17 6.5 6 5.5
the crystals were decomposed after the drainage by venting out the propane. The recovered water as well as the concentrate were collected, weighted, and analyzed for their characteristics by the methods that were discussed previously. In the propane displacement experiments, liquid propane was used to displace the concentrate from the bed of hydrate crystals. Liquid propane floats on top of the concentrate which can then be displaced by an additional drainage procedure. Following this drainage procedure, the crystals were melted in the same fashion as in the no washing experiments. In each experiment, the concentrate, the displaced concentrate, and the recovered water were collected and analyzed. In preparation of this separation procedure, a bed of crystals must be deposited at the bottom of the crystallizer. In the experiments with stirring impeller geometry II, almost all the crystals remained in suspension. As a result, during drainage they easily fell to the bottom of the crystallizer and formed a uniform bed of crystals (bed formation procedure A). In the experiments conducted with impeller geometry I, some of the crystals remained in suspension but a considerable amount adhered to the crystallizer’s wall, forming a hydrate plug. After drainage, the suspended crystals fell to the bottom of the crystallizer, forming a bed, but the hydrate plug remained attached to the crystallizer’s wall. These hydrates were forced downward by evacuating the coolant from the cooling jacket and thus causing a thin layer of hydrate to melt, enabling the plug to slide downward onto the rotating impeller which easily broke it. The crystals fell to the bottom of the crystallizer. Once the new bed had formed (procedure B), coolant was reinjected into the cooling jacket. We then modified the stirring assembly to geometry II as previously described and achieved better mixing which enabled almost all the crystals to remain in suspension. The washing with water experiments are similar to the propane displacement ones except that deionized water was used. By throttling nitrogen manually into the wash water supply vessel, the increased pressure inside the vessel forced the water into the crystallizer. During washing, the pressure was maintained at the hydrate-propane (vapor)-effluent equilibrium at the experimental temperature. The wash water displaced some of the concentrate from the crystals. This solution, called spent wash water, was drained from the crystallizer. Subsequently, the crystals were melted. The concentrate, the spent wash water, and the recovered water were weighed and analyzed.
1.91 2.42 3.70 18.9 19.0 57.0 57.7 50.13 53.3 0.702 24.88 63.4 70.0 76.9
I I I I I II II II II I II II II II
N/A N/A B B B N/A A A A N/A N/A N/A A A
type of separation no washing no washing washing with water washing with water propane displacement no washing propane displacement propane displacement washing with water no washing no washing no washing propane displacement washing with water
Table 2. Recovery of Water from NaCl Solutions without Washing the Hydrate Crystals run S1 S2 S6
mass or conductivity
feed
recovered water
concentrate
mass (g) conductivity (µs) mass (g) conductivity (µs) mass (g) conductivity (µs)
11 000 42 400 10 550 42 000 9 981 40 800
900 23 600 1 200 26 400 2 789 35 500
10 075 44 800 9 330 44 000 7 097 44 200
3. Results and Discussion Because BCTMP effluent is a dark liquid that contains a variety of salts and organic compounds, it is difficult to visually observe the clathrate crystallization and separation process. As a result, we also performed experiments with 2.5 wt % NaCl solutions. 3.1. Salt Solution Experiments. Prior to the hydrate concentration tests, the freezing point depression was measured and found to be 1.40 °C. The value reported in the 69th edition of The Handbook of Chemistry and Physics is 1.486 °C for a 2.5 wt % NaCl solution. Nine experiments were conducted with the salt solution, and Table 1 gives the operating conditions. Also shown in the table are the fluctuations in the temperature and pressure because of changes in the ambient temperature. Three experiments (S1, S2, and S6) were conducted with no washing, three (S3, S4, and S9) were performed with water as the washing agent, and the last three (S5, S7, and S8) used liquid propane to displace the concentrate from the crystals. The hydrate-propane-salt water equilibrium values that were obtained at the beginning of the concentration experiments were found to be in very good agreement with the data from Kubota et al. (1984). Table 2 gives the results for the experiments without washing. The recovered water was found to have a conductivity lower from that of the feed by 44, 37 and 13% in runs S1, S2, and S6, respectively. Runs S5, S7, and S8 used liquid propane to displace concentrate from the bed of crystals. Table 3 reports the results of these experiments. Using propane as a displacement agent, 1 kg of concentrate was displaced on average. The conductivity of the recovered water compared to that in the feed solution was found to be lower by 28, 39, and 29% in runs S5, S7, and S8, respectively. Run S7 had the lowest conductivity measurement for the recovered water. Comparing this run with S1 (lowest conductivity in the recovered water for the no washing experiments), the conductivity was 5% higher than run S1, but it produced 203% more water. As expected, the
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Table 3. Recovery of Water from NaCl Solutions in Experiments Which Employed Propane Displacement and Wash Water To Enhance the Separation of Hydrate Crystals from the Concentrate run S5 S7 S8 S3 S4 S9 a
mass or conductivity
feed
recovered water
concentrate
displacement agent
displaced concentrate
mass (g) conductivity (µs) mass (g) conductivity (µs) mass (g) conductivity (µs) mass (g) conductivity (µs) mass (g) conductivity (µs) mass (g) conductivity (µs)
10 300 41 700 9 973 40 500 9 598 40 850 10 122 41 700 10 300 42 500 9 432 40 850
2700 29900 1824 24600 2258 29052 2037 11190 3284 7460 3897 23600
6064 46900 6964 43900 6251 45800 8069.6 45500 5839.2 47800 5312 45900
1703a 0 665a 0 650a 0 3600b
