1576
KO,K , L
INDUSTRIAL AND ENGINEERING CHEMISTRY
equilibrium adsorption constants total number of molar active centers per unit mass of catalyst m = (1 7r K O K ) /rkLK,(K 1) no,np = moles per unit mass of ortho- and para-hydrogen in inlet gas P = -K n , 1 n, = 0.264 = =
+
+
z
=
conversion, moles per unit mass of feed
+
= total pressure = p , p, pB = bulk density of catalyst. mass per volume PF = feed density, mass per volume 0 =time r
Literature Cited
+
p,, p ,
S, TI‘
partial pressures of ortho- and para-hydrogen, respectively = space velocity, volume feed a t standard conditions: (volume of catalyst bed) (time); [the E factor of ( 1 )is a space velocity] = mass of catalyst in converter =
Vol. 45, No. 7
(1) Grilly, E. R., Rev. Sci. Instr., 24, 1 (1953). (2) Hougen, 0. A , , and W a t s o n , K. M., “Chemical Process Principles,” Part 111, pp. 918-19, 927-9, S e w York, John Wiley & Sons, 1947.
RECEIVED for review October 13, 1952. BCCEPTED April 15, 1953. Work done under contract with the Atomic Energy Commission.
Solvent Recovery from Pentachlorophenol-Treated Wood MONIE S. HUDSON Taylor-Colquiff C o . , Spartanburg, S. C.
I
iu T H E treatment of wood with the new organic solvent-sol-
uble preservatives such as pentachlorophenol (4, 7 , 8) and copper naphthenate ( 3 ) , the role of the solvent is much more important than with the water-soluble salts. Until recently these salts were the only preservatives that could be used to produce treated wood that was free from the objectionable odor and dark color of creosote and could be satisfactorily painted. Solvents of widely varying character have been used for preparing solutions of pentachlorophenol ( 4 , 6, 7,9), ranging in properties from heavy residual oils such as Bunker C, to very light distillates, such as mineral spirits or V X & P naphtha. The former are very dark and heavy and therefore afford no improve-
ment over cieosote with respect to color and paintability, the latter, being highly volatile, are quickly lost from the wood by evaporation into the air, and as this takes place the toxic chemical is brought to the surface of the wood where it crystallizes to produce “blooming.” This blooming can be prevented by the use of “antiblooming” agents, which are resinous plasticizers with a high solvency for pentachlorophenol. Use of these antiblooming agents, however, materialli increases the cost of the preservative treatment, and the blooming is often replaccd by “bleeding” of resinous exudate to the surface of heartwood, where it forms objectionable sticky deposits. For these reasons an intermediate range of solvents has been
4
5 d
r 2 Figure 1. Wood-Preserving Plant Equipped for Solvent Recovery by Vapor Process A.
E. C.
D.
E. F.
Treating cylinder Evaporator Condenser Separator Condensate tank Impregnating solution storage
July 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 3. Sections from Boards Slotted with Saw to Show Residual Stresses Straightness of prongs indicates freedom from stress TOP row. Sections from original untreated boards Middle row. Sections from impregnated and solvent-recovered boards Bottom row. Sections from boards impreg. naled only
-
more widely used for the pressure treatment of wood with pentachlorophenol-such as those specified by the American Wood Preserters’ Association ( I ) , which have the characteristics of distillate fuel oils or gas oils. The necessity for using these solvents, however, has to some extent defeated one of the purposes for which the organic solvent-soluble preservatives have been developed-to afford a material Figure 2. Combination that is nonleachable by water, Dean-Stark Soxhlet Extraction Apparatus but a t the same time produces a clean and paintable product. Even with the lightest of this intermediate range of solvents, it is necessary to wait from one to several months to allow the surface to dry, so that it can be painted without risk of discoloration or other injury to the paint. The heartwood of some species-particularly Douglas fir, about 95% of which is heartwood in the upper grades of lumber, and the heartwood and knots and other resinous areas of southern pine-exudes considerable quantities of the treating solution in standing after treatment, which renders the wood unpaintable. The extent of this bleeding is dependent on the type of impregnation process used, the condition of the wood, and a number of other factors. As the solvents appear t o be the chief cause of troubles experienced with wood pressure-impregnated with pentachlorophenol, the ideal means of applying the preservative to the wood would seem t o involve the use of the pure chemical without solvents. This might be possible if the chemical were a liquid and if only the surface of the wood were to be treated. Superficial treatment of millwork and other materials (9, 16,16),consisting of painting on or dipping in preservative, has proved satisfactory, because such items are used where exposure to wood-destroying organisms is only slight. But when wood is to be used under more severe conditions, relatively large volumes of solution, in the neighborhood of 6 to 10 pounds per cubic foot of wood to be treated, are required to convey the preservative into the cellular structure and to penetrate it thoroughly enough to protect i t from early decay. Because pentachlorophenol is not liquid a t the usual wood-treating temperatures, and the large volumes required in pressure treatment would make the cost prohibitive
1577
even if it were, there seems to be no way of avoiding the use of a solvent for its application. Another possibility would be to use a solvent for impregnation, followed by removal of the solvent to prevent the difficulties experienced when it is left in the wood. I n 1949 a development program was begun in the woodpreserving pilot plant of Taylor-Colquitt Co., directed toward recovery of solvent from wood pressure-treated with organic solvent-soluble preservatives. During first 18 months of this development, work was carried out with copper naphthenate solutions in cooperation with the Cuprinol Division of Darworth. Inh., and the past year has been devoted to work with pentachlorophenol solutions in cooperation with the Chapman Chemical Co. This program has shown that i t is possible to treat wood with these chemicals in volatile solvents and t o remove and recover the solvent, a t the same time eliminating from the surface of the wood objectionable deposits which adversely affect cleanliness and paintability. This has been accomplished by employing a modification of the vapor-drying process
(10-12). Equipment and Operation. I n Figure 1 is shown a woodpreserving cylinder, to which has been connected the necessary equipment for carrying out vapor drying. Green wood, which is placed in the pressure-treating cylinder,
A , is subjected t o the action of vapors from an organic solvent boiling in the evaporator, B. The solvent vapors are conveyed into the bottom of the treating cylinder and passing upward heat
the wood by condensation on it. Water is vaporized from the wood and is carried by an excess of the organic vapor from the top of the cylinder t o a condenser, C. Here the vapor mixture is condensed, and the condensate, composed of water and organic solvent, passes into the separator, D. I n this vessel, separation takes place by gravity, the water being dischar ed from the system and the drying agent being returned throu& the condensate tank, E, to the evaporator t o begin another pass through the system. As an example of the use of the vapor process for recovery of solvent from pentachlorophenol-treated wood: Assume that charge of 2 X 4 inch southern pine lumber is in the cylinder, that it has been impregnated with a solution of pentachlorophenol in xylene, and that the solvent is t o be recovered from it. A quantity of the same solvent, xylene, is placed in the evaporator, which is heated to its boiling point of about 280 F., and the vapor produced is conducted into the cylinder containing the impregnated wood. The temperature of the cylinder is then brought up as rapidly as possible t o approximately the boiling point of the xylene b y continuous passage of vapor t o the cylinder. This usually requires about 1 hour. During this heating cycle a large quantity of xylene is condensed on the wood, extracting excess preservative from the surface as i t drips down through the charge t o the bottom of the cylinder, from whence i t passes into the condensate tank, and from there is pumped back t o the evaporator. At the end of this hour of heating, the evaporator is closed off from the cylirider, and a vacuum is applied to the cylinder and its contents through the condenser. This vacuum, which ranges between 24 and 26 inches of mercury, serves to vaporize a considerable amount of the xylene that was put into the wood during the impregnation with pentachlorophenol solution. After about 2 cycles of alternating, 1-hour heating in vapor and 1-hour vacuum, virtually all of the xylene will have been removed from the wood. Pentachlorophenol extracted from the wood ultimatelyreaches the evaporator, where it accumulates, and when it has reached the proper concentration, the solution is transferred to the solution storage, F , where it is used as impregnating solution, while fresh xylene is placed in the evaporator for further solvent-recovery runs. O
Suitable Solvents The vapor solvent recovery process has been carried out using Stoddard solvent (mineral spirits), VM&P naphtha, xylene, toluene, high-flash coal tar naphtha, perchloroethylene,
1578
INDUSTRIAL AND ENGINEERING CHEMISTRY Table 1.
Vol. 45, No. 7
Processing Data for Recovery of Solvent from 2 X 4 Inch Lumber Pressure impregnated with Pentachlorophenol in Xylene (Vapor recovery agent xylene, b.p. 280-300° F.)
Solvent Recovery Cycle, Hours
7 LbJCu. in after Treating
Init. Change .4ir No. Lb./Sq.'Inch (1) (2)
vapor (3)
-4. 759 25 760 25 76 1 -11.1 762 -11.1 763 Atmospheric 764 Atmospheric Average Impregnated only Solvent recovered
766 765 768 767 773 774
25 25 50 50 50
A n. .
Average Impregnated only Solvent recovered
B.
Retained in Imprgenation,
vapor (4)
solution (5)
Ft.
Dry penta (6)
Final Final Pentachlorophenol Retention, ~,,i..~,,+ y".."..L Lb./Ch. F t . Retention Amount Weighted Extractives Moisture Content of Cross av. for b t h e r T h a n (Based O n Wt.) Section Outer 2nd cross Pent,a), Initial Final Penetrated, inch l/a Inch Remainder, section Lb./Cu. Ft. % % % (7) (8) (9) (10) (11) (12) (13) (14)
DOUGLAS FIRIXPREGSATED W I T H PRESERVATIVE PRESSURE O F 125 POUXDS P E R SQUARE INCH r o R 3 HOURS 5 % Pentachlorophenol in Impregnating Solution Impreg. only Sa 7.6 0.39 0.36 0.15 0.11 0.22 3.3 13.3 9.8 6 0.9 13.3 1.1 6 7.5 0.38 0.14 0.08 0.04 0.10 13.3 9.2 Impreg. only Sa 30.2 1.10 16.4 1.52 1.54 0.85 0.76 2.4 13 3 0.8 0.35 6 6 29.7 0.35 0.34 1.49 0.36 Impreg. only Sa 12.2 .5 , 1 13.5 10.6 0.62 0.55 0.19 0.36 0.28 6 4.1 13.5 1.3 0.26 6 16.3 0.82 0.26 0.17 0.33 0.81 0.28 Impreg. only 6 Impreg. only 3 Impreg. only 1.5
Sa 6 S5
3 1, 5
0.35 0.18
P I S E I M P R E G N A T E D WITH
0.49 0.31
0.42 0.25
PRESERVATIVE PRESSURE O F 100
96 78 96
0 56 0.24
10% Pentachlorophenol in Impregnating Solutioii 0.51 0.60 10.0 0.99 0.77 0.45 0.31 0.32 0.25 9.2 0.89 0.38 0.41 0.39 0.74 0.67 0.51 7.4 0.29 0.27 0.73 0.30 0.29 7.5 0.54 0.42 0.65 0.68 0.57 6.6 0.33 0.24 7.3 0.72 0.37 0.32 0.71 0.35
SOUTHERN YELLOW
0.43 0.23
::
100
3.9 1.1 3.2 0.8 5.1 1.6
7.6 7.6 7.6 7.6 7.3 7.3
5.6
0.7
6.9 1.2 6.3 2.2
95 93
100 100
100 96
0.56 0.31 POUADS PER
SQr.4RE INCH F O R 2 HOWRE
5 % Pentachlorophenol in Impregnating Solution
755
5
756 5 757 5 758 5 Average Impregnated only Solvent recovered
Impreg. only 3 Impreg. only 2
S" 3 Sa 2
12.9 11.6 12.7 14.1
0.66 0.60 0.65
0.12
0.82 0.34 0.53 0.41
0.35 0.16 0.35 0.37
0.28 0.11 0.22 0.28
0.51 0.21 0.38 0.36
0.68 0.38
0.35 0.27
0.25 0.20
0.45 0.29
7.1 0.8 6.0 5.3
10% Pentachlorophenol in Impregnating Solution 0.74 7.1 0.64 0.60 7.2 0.71 0.94 769 25 Impreg. only 0.26 0 4 0.24 0.20 2 7.2 0.71 0.32 770 25 2 7.1 0.66 0 70 0.78 0.63 8.2 0.81 771 25 Impreg. only 0.41 1.7 0.38 0.39 1 8.0 0.79 0.44 772 25 1 Average Impregnated only 0.86 0.64 0.63 0.72 Solvent recovered 0.38 0.31 0.30 0.84 a Charge after impregnation &earned 2 h hours a t 260" F. a n d then subjected t o 1-hour vacuum a t 24 inches of mercury.
15.1 15.1 16.8 16.9
14.4 3.9 15.2 6.8
12.6 2.3 13.1 4.9
61 96 84 80
96 93 95 95
and trichloroethylene. Physical operation of the process is equally satisfactory with any of these chemicals, although economic considerations favor the mixtures of compounds t h a t can be obtained as fractions with fairly narrow boiling ranges, rather than the pure compounds. There is some preference for fractions containing aromatic compounds rather than straight aliphatics for this operation, because of their better solvency for pentachlorophenol and the fact that they more readily dissolve the natural resins of the wood, whose removal in the vapor processing operation improves the quality of the surface from the standpoint of paintability. 0
strips between the boards and binding them together to prevent their separation during treatment. The first charge was impregnated with a solution of pentachlorophenol in xylene using a conventional type of pressure-impregnation cycle which would give the final retention of chemical desired. After impregnation, the charge was removed from the cylinder, and each board was again weighed and was cut a t the mid-point to remove one 2 X 4 inch block, 0.5 inch long, and two similar blocks 1 inch long for samples. The 0.5-inch blocks were used for determination of moisture, solvent, and pentachlorophenol by the following method:
Experimental Work
The blocks from the four boards were marked off, working in depth from the original surface, into outer l / 4 inch, second 1/4 inch, and the core or remainder. These zones were separated and chipped up by means of a heavy slicing device, and the zones from the four boards were combined, to give three composite samples consisting of the outer 1/4 inch, second 1/4 inch, and core. These were placed in the combination Dean-Stark water distillation unit and Soxhlet extractor shown in Figure 2, and were extracted with toluene. Water was removed by leaving the s t o p cock, A , open while distilling toluene through the apparatus, the water being received in the trap, B. After removal of the water, stopcock A was closed and extraction by the Soxhlet method was continued for about 4 hours until removal of the pentachlorophenol from the wood was complete. The toluene extract was analyzed for pentachlorophenol by the lime ignition method (g), and the extracted wood was then oven-dried. The amount of solution retained in the wood was determined by subtracting the oven-dried weight from the initial weight to obtain the total loss on extraction; subtraction from this figure of the amount of water removed gave the amount of solution present in the wood. This latter figure, of course, showed a higher solution retention
The experimental work was carried out in a 16-inch-diameter cylinder, 6 feet long The tests were run with two species of wood, Douglas fir and southern yellow pine, with dimensions of 2 X 4 inches X 6 feet. The wood was air-seasoned in the case of the fir and air-seasoned or kiln-dried in the case of the pine. Duplicate charges were made up from the same original pieces of lumber, one of which was impregnated only and the other impregnated, then solvent recovered. I n preparing charges for processing, four 2 X 4 inch pieces slightly over 12 feet long were cut at the mid-point to produce two charges consisting of four mated boards 6 feet long. At the time that this was done, a section 1 inch in length was taken from the mid-point of each for determination of moisture content and density. Each board in a given charge was measured to determine its volume and then was weighed. Charges were made u p by placing I-inch spacing
July 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY
than actual, because of natural extractives that were removed from the wood along with the pentachlorophenol solution.
were left unpainted. After 72 hours, photographs were taken of the painted panels. They then remained in the laboratory for 1 month, after which they were again photographed, and then were Of the two 1-inch blocks removed from each board, one was placed on a test fence which faced south a t an an le of inclination of about 45 to assure substantially perpendicufar impingement dusted with an oil-soluble dye to show penetration of the pentaof the sun's rays. chlorophenol solution, and the percentage of the cross section The second charge of the mated pair was impregnated in the penetrated was estimated by inspection. The other section was same manner, and after being removed from the cylinder was slotted according to the U. S. Forest Products Laboratory weighed and measured, and then returned t o the cylinder and subjected to alternate cycles of heating in vapor and periods of vacmethod for determining residual stress in wood ( 1 4 ) as shown in uum. Figure 3. The solvent used for preparing the pentachlorophenol impregOne set of the 3-fOOt sections that remained, after the 6-foot nating solution for this series of tests was xylene, and hence xylene boards had been cut to obtain the above sam les, was made up was used in the evaporator to produce the vapor. During the into a panel by nailing the four boards on two bPitcking strips, and solvent-recovery cycle, all water distilled from the charge was these panels were allowed to stand for 1 week. At the end of this time a 1-foot section of the right end of each board on the panel, recovered at the condenser and weighed in order to obtain a maafter bein thoroughly wiped with a clean cloth, was painted with terial balance. The total time of solvent recovery was varied on a k n o t - s e k g material: Western Pine Association knot sealer No. successive charges, in order to determine the minimum length of 578, which has been recommended (6)as a sealer to be used on time necessary for satisfactory removal of solvent. After the wood such as Douglas fir or southern pine heartwood to prevent damage to the paint coat b exudation of pentachlorophenol solusolvent-recovery cycle, the charge was recovered from the cylintions. The panels were d e n allowed t o stand for 24 hours for der and reweighed, the boards were sectioned to obtain samples, this coat t o dry. This 1-foot section and 1 foot at the opposite and panels were prepared in exactly the same manner as deend of each board were then painted with one coat of Sherwinscribed for the "impregnated only" charges. Williams Metallastic Red. The central sections of the boards In Table I, A , a r e shown data on the operating conditions employed, distribution of pentachlorophenol in the wood, and A. DOUGLAS FIR 6 . SOUTHERN PINE \ other pertinent data for the Douglas fir \ charges. I n order to secure a range of \ 5 % PENTACHLOROPHENOL IN XYLOL retentions of pentachlorophenol in the charges, two methods of control were employed: (1) changing the concentration of pentachlorophenol in the solution (approximately 5 and 10%) and (2) varying the initial air pressure applied to the wood during impregnation, as shown in column 2 of the table. The values in this column are counted from atmospheric pressure as zero. On charges 759 and 760 the cells of the wood in the treating cylinder were first filled with compressed air at 25 pounds per square inch and while this pressure was maintained, the wood was submerged by filling the cylinder with treating solution. Pressure was gradually increased on the treating solution until a pressure of 125 pounds per square inch was reached a t the end of 2 hours. This pressure was maintained for 1 hour, after which it was released, On release of pressure, the 25 pounds per square inch initial air trapped in the wood expanded, driving out the excess preservative solution; subsequent to this, a 1-hour vacuum period was employed to expand this air further to expel more of the treating solution. Charge 759, which was impregnated only, was then subjected to live steam for 2 hours at 240" to 250" F. and then to a vacuum of about 24 inches of mercury for 1 hour, a method commonly used in the wood-preserving industry to clean the surface of pressureimpregnated wood. In the case of charge 760 after impregnation, the treated boards were subjected to 1 hour of heating in xylene vapor at approximately 270" F., followed by an hour vacuum a t 24 inches of mercury. Cycles of 1-hour heating in vapor followed by 1-hour vacuum were repeated SLY times for a total processing time of 12 hours. O
*
*
1579
INDUSTRIAL AND ENGINEERING CHEMISTRY
1580
Vol. 45, No. '1
inch positive pressure, the next pair of charges, 763 and 764, were impregnated with initial air a t atmospheric pressure. This served to reduce materially the retention in the impregnated-only specimens from that obtained in charge 761, but in the case of the solvent-recovered charge the reluction in retention was less. Referring to column 11, it will be seen that charge 764 had a much higher final solution retention than did the other two m1vent-recovered charges. There is no known explanation for this, although it has happened in a number of charges filled a t atmospheric pressure. With the 10% pentachlorophenol solution, all of the solventrecovered charges retained approximately the desired 0.25 pound per cubic foot of preservative. For example, in charge 765 with 25 pounds per square inch initial air and a 12-hour solvent recoverv cycle, the retention of pentachlorophenol averaged 0.26 pound per cubic foot; with 50 pounds per square inch initial air and a 6-hour solvent recovery cycle, the retention in charge 767 was 0.29 pound per cubic foot (see column 10 of Table I). Final steaming of the impregnated-only charges appeared to be contributing to the formation of heavy surface deposits of pentachlorophenol crystals, as some of the solvent was evaporated by the vacuum after steaming. I n an effort to prevent this blooming, the final steaming was not used on charge 773. This improved the appearance of the lumber as it came from the cylinder, but on standing for a week the heavy blooming still took place. The initial and final moisture content values in columns 12 and 13 show some reduction of moisture content as a result of water being vaporized in the vacuum after final steaming of the impregnated-only charges. In the solvent-recovered charges, this reduction in moisture content was much greater as a result of the drying of the wood by action of the heated vapor. However, in spite of the low final moisture content, the residual Figure 5.
Red Painted Panels after 8 Months on Test Fence
Top row. Douslar fir Left. Solvent recovered Right. Impregnated only (note roughened surface) Bottom row. Southern pine Left. Solvent recovered Right. lrnpresnated only (note bleed-through on heartwood)
Table 11.
Distribution of Pentachlorophenol in 2 X Douglas Fir Lumber
4 Inch
Lb./Cu. Foot Pentachlorophenol by Analysis of Wood Weizhted ..~~ . ..~
At the end of the impregnating step, both charges had retained approximately the same amount of pentachlorophenol solution, 7.6 pounds per cubic foot in the case of charge 759 and 7.5 pounds per cubic foot in the case of 760. These solution retentions contained 0.39 and 0.38 pound per cubic foot of dry pentachlorophenol, respectively. After the steam cleaning step used on charge 759, the pentachlorophenol solution retention had decreased to 3.3 pounds per cubic foot (0.22 pound per cubic foot of dry pentachlorophenol) while in charge 760 which was solventrecovered, the solution had been reduced to 0.9 pound per cubic foot (0.10 pound per cubic foot of dry preservative). As in this particular case, the retention of pentachlorophenol in the solventrecovered material was considerably below the desired 0.25 pound per cubic foot in the treatment of the next pair of charges, an initial vacuum of 22.5 inches of mercury (corresponding as Rhown in the table to a negative pressure from atmospheric, of 11.1 pounds per square inch) was employed in order to fill the cells of the wood completely with solution to increase the retention of preservative. This served t o increase the retention of solution shown in column 5 of Table I to approximately 30 pounds per cubic foot. Final steaming of the impregnated only specimens reduced this t o 16.4 pounds per cubic foot (1.1 pounds per cubic foot of dry pentachlorophenol), whereas solvent recovery on charge 762 left a final solution retention of 2.4 pounds per cubic foot (0.35 pound per cubic foot of dry toxicant). This was above the 0.25 pound per cubic foot average desired. Having bracketed the 0.25 pound per cubic foot retention as lying between an initial negative pressure of 11.1and 25 pounds per square
Charge NO.
Board No.
779
221B
779
222B
781
225B
781
226B
790
237B
790
23QB
792
222s
Av.
785
221
785
222
782
225A
782
226.4
789 789 AY .
When Sampled 1st '/a 2nd * / 4 after Treatment inch inch Impregnated Only 0.62 0.45 Immediately 0.86 0.11 1 month 0.64 0.60 Immediately 0.13 0.87 1 month 0.35 0.25 Immediately 0.38 0.08 1 month 0.26 0.35 Immediately 0.40 0.05 1 month 0.09 0.23 Immediately 0.09 1 month 0.19 0.16 0.09 Immediately 0.03 0.26 1 month 0.08 0.12 Immediately 0.18 0.03 1 month 0.26 0.35 Immediately 0.45 0.07 1 month Solvent Recovered Immediately 0.47 0.47 1 month 0.47 0.39
Core
average for cros8 section
0.40 0.09
0.50 0 41
0.50 0.08 0.20 0.04 0.21 0.04 0.01 0.03 0.01 0.02 0.09 0.01 0.20 0.04
0.59 0.41 0.28 0.19 0.28 0.19 0.12 0.12 0.09 0.13 0.10 0.08 0.28 0.22
0.44
0.46 0.42
0.41
Immediately 1 month
0.57 0.58
0.59 0.59
0.64
0.60
Immediately 1 month Immediately 1 month
0.30 0.25
0.23 0.25
0 . 27 0.20
0.27 0.24
0.28 0.20
0.26 0.16
0.26 0.11
0.26 0.16
237.4
Immediately 1 month
0.16 0.25
0.17 0.16
0.09 0.08
0.14 0.17
239A
Immediately 1 month
0.12 0.13
0.04 0. 12
0.02 0 . 10
0.07 0.12
Immediately 1 month
0.32 0.31
0.29 0.28
0.26 0.26
0.29
,
0.47
0.55
0.29
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1953
1 ,
..
1581
the 2 X 4 inch cross sections is shown in columns 7, 8, and 9 of Table I. Column 10 is the weighted average for the cross section. The average pentachlorophenol distribution given below the four groups of data in the table is shown graphically in Figure 4. These curves represent the type of distribution that has been found through this work with organic solvent-soluble preservatives; distribution is much more uniform in the solvent-recovered than in the impregnated-only specimens. A study of the final solvent retention values shown in column 11 of Table I1 reveals the effectiveness of the vapor process in recovery of solvent from the wood. These values are total extractive, exclusive of pentachlorophenol, found in the wood after treatment, and hence the actual solvent retention was somewhat lower. Final solvent retention values below 1 pound per cubic foot probably represent natural resinous extractives removed from the test blocks by toluene extraction; in such cases the final solvent retention can safely be considered as being in the neighborhood of zero. This column indicates that a solvent recovery cycle of 12 hours’ duration is required for virtually complete removal of solvent from Douglas fir impregnated with 25 pounds per square inch initial air (see charges 760 and 765). With 50 pounds per square inch initial air this was accomplished with a recovery cycle of only 6 hours on charge 767. The 3-hour recovery cycle used on charge 774 was evidently of too short duration, since the final solvent retention was 1.6 pounds per cubic foot, I n the more permeable southern pine the final extractive value for charge 756 AFTER TREATMENT -0was 0.8 pound per cubic foot. This was -aobtained in only 6 hoursof solvent recovery in spite of the fact that the initial air pressure was just 5 pounds per square inch. The final solvent retentions of charges 770 and 772 indicate that optimum solvent recovery on southern pine can be obtained with a cycle of between 2 and 4 hours’ B. SOLVENT RECOVERED duration when 25 pounds per square inch BY VAPOR PROCESS initial air is used.
stresses in the wood were not great, which is indicated by the straightness of the prongs of the slotted stress sections shown in the middle row of Figure 3. Data for the southern pine charges are shown in Table I, B. Because of the greater permeability of southern pine than of Douglas fir, the initial air pressure required to obtain a solution retention after impregnation which would assure a final retention after solvent recovery of about 0.25 pound per cubic foot of dry pentachlorophenol was somewhat less than was used with the Douglas fir. The total length of the preservative pressure period as well as the intensity of preservative pressure was less in the case of the pine. The solventirecovery time required for the pine was also much shorter than for the fir. The results &s to pentachlorophenol retention and distribution and final solvent retention were, in general, similar to those obtained in the fir. In order to obtain a final retention of pentachlorophenol of about 0.25 pound per cubic foot after solvent recovery, it was necessary to leave 0.65 to 0.75 pound per cubic foot in the wood after impregnation. Thus, the solvent recovery operation removed 0.4 to 0.5 pound per cubic foot. Distribution of the pentachlorophenol in the various zones of IO0
I
I
I
I 0.90
I
I
SAMPLES TAKEN IMMEDIATELY
I
o ao
I
0.10
t-
2 0.M Y,
m
J
d
2 +
5%
I
1
MONTH
---mi-
0.50
0 OL
z
E
0.4C
a
1
k
I
l
1
1
0.4
0.C
.
g ‘ F
Z 0.3C
w
0.U
0.io
0
0.2
DEPTH, INCHES
Figure 6.
Distribution of Pentachlorophenol in
9 X 4 Inch Douglas Fir Lumber
Actual dimen6ions 1.64 X 3.50 inches
Surface Appearance and Paintability of Treated Wood Considerable blooming of pentachlorophenol on the surface of the wood occurred. When final steaming was discontinued, occurrence of the blooming was merely delayed. This bloomingshowed up as a heavy crystalline deposit over all the surface of the boards. On standing, the crystalline deposit gradually flattened to an amorphous coat 1/32 to ‘/le inch thick, appearing pimpled from nodules of pentachlorophenol forming craters around the openings of the horizontal resin ducts as the xylene evaporated from the solution exuding from the wood at these points. This heavy, nodulated surface deposit was difficult to remove except by scraping, but painting over it without first scraping off the deposit produced a very rough surface and the paint coat was quickly discolored. When the intermediate solvents are used blooming does not occur on the treated wood, nor does i t occur with properly formulated solutions using the more volatile solvents, It is possible t o treat wood with the latter solutions without the appearance of undesirable resin on the surface. I n this
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 45, No. 7
face of the wood had taken place, and in order to obtain quantitative information on this point, another series of charges like those recorded in Table I was run. One or two boards from each charge were cut after treatment t o obtain samples for analysis. The zones cut from different boards in a charge were not combined for analyclis as before but were analyzed individually. The %foot sections remaining after sampling were stacked in an open criblike pile in the laboratory for 1 month in order to allow any remaining solvent to evaporate. At the end of this time one set of the %foot sections from a charge was cut a t the mid-point to obtain samples for analysis, while the other set was paint'ed with one coat of Pittsburgh white primer, and 24 hours later with one coat of Pittsburgh Titanic outside white paint. The painted set was then placed on the test, fence. This modified routine was carried out on both Douglas fir and southern pine for the solvent-recovered a s well as the imprrgnatedonly specimens.
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SAMPLES TAKEN IMPIEDIATELY X T E R TREATMENT ,,
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The results of analyses for pentachlorophenol in the Douglas fir speciinens immediately after treatment and 1 month after treatment>are shown in Table I1 and the averaged data from this table are plotted in Figure 6. This figure shows t h a t most of the preservative had migrated to the surface of the impregnated-only specimens as the solvent evaporated over the period of 1 month, while in t'he solvent-recovered specimens the pentachlorophenol distribution remained virtually static:.
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AREA OF GIVEN ZONES EXPRESSED AS
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7. Comparison of Relative Distribution of Pentachlorophenol in 2 X 4 Inch Douglas Fir
work, addition of Hercules polypale resin in a ratio of 1 to 1 to the pentachlorophenol in the treating solution resulted in a surface covered with a viscous exudate which formed large blisteis, thus rendering the wood unpaintable. I t is possible that a variation in the treating schedule might have resulted in a cleaner surface. The solventrrecovered specimens were entirely free from SUI'face deposits; in fact, the resinous exudate that is usually present over knots and pitch streaks on untreated southern pine was absent, having been extracted along with the high surface concentration of pentachlorophenol by the condensing soIvent vapor. Figure 5 shows the condition of typical charges of the impregnated-only and solvent-recovered specimens after 8 months on the test fence. The upper band of paint on each board was applied after the WP578 knot sealer had been used. This did not prevent bleed-through and damage to the paint. Migration of Pentachlorophenol
It was evident from observation of the impregnated-only charges t h a t considerable migration of pentachlorophenol to t,he sur-
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Figure 8 .
White Painted Panels after 6 Months on Test Fence A,
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Solvent recovered after impregnation Impregnated only Douglas fir Southern pine
This migration of pentachlorophenol to the surface of the wood is shown somewhat more clearly by the relative distribution values in Figure 7 , 8,for the impregnated-only and Figure 7, R, for solvent-recovered specimens. I n this figure the amount of pentachlorophenol in the various zones expressed as a percentage of the amount in the whole cross section has been plotted against the area of the zones expressed as per cent of the total area of the cross section. Samples from the solvent-recovered specimens taken immediately after treatment show a relative distribution that is very close to the uniform distribution represented by the 45' line, drawn from 0 to 1 0 0 ~ o .The impregnated-only specimens taken immediately after treatment show mare diver-
July 1953
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INDUSTRIAL AND ENGINEERING CHEMISTRY
gence from the uniform distribution line. After 1 month the solvenbrecovered specimens show an almost identical relative distribution of pentachlorophenol to that found immediately after treatment, indicating that little or no movement had taken place, since very little solvent was present t o cause migration. On the other hand, the impregnated-only specimens after 1 month show a relative distribution that is much more divergent from the uniform distribution line than was the case immediately after treatment. Extrapolation of the upper line in Figure 7, A , indicates that approximately 70% of the pentachlorophenol was present in or near the surface of the wood after 1-month evaporation of solvent. Similar results were obtained with southern pine. This large scale migration of the preservative to the surface of the wood during evaporation of solvent not only is responsible for heavy blooming that is experienced when wood is treated with pentachlorophenol in light solvents and the solvent allowed to evaporate into the air, but also it probably accounts for the fact that test specimens treated with pentachlorophenol in such solvents have failed to give as good service life against wood-destroying organisms as those treated with heavier solvents ( 6 , 18). This has been variously explained; but the data herein reported indicate very strongly that in wood treated with the chemical in light solvents the pentachlorophenol migrates to the surface and is lost quickly by being washed away by rain or displaced from the surface of the wood by shrinkage and swelling stresses and other agencies, leaving the wood virtually untreated in the interior and an easy prey to the wood destroyers. Final proof of this supposition will have to await the outcome of stake decay tests th‘at are already in progress, but will requjre some years to complete. Figure 8 shows a comparison of some of the treated only and solvent-recovered boards from the second series of teats, on which white paint was used. These were photographed 6 months after exposure on the test fence and are included to show the discoloration caused by the surface deposits that were present on the wood that was impregnated only, even when an entire month was allowed for evaporation of solvent before the painting was done. This discoloration is not apparent in Figure 5, because the paint used on those panels was red.
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Summary When wood is pressure treated with pentachlorophenol in volatile solvents, most of the chemical is brought t e the surface of the wood by evaporation of the solvent, leaving objectionable deposits which interfere with paintability. Addition of plasticizing agents to the treating solution prevents formation of the crystalline deposits known as blooming, but does not stop the migration of pentachlorophenol to the surface. By employing a modification of the vapor-drying process, it is possible t o remove and recover most of the solvent vehicle that is normally lost. Usually 6 t o 10 pounds per cubic foot of solution containing 5% pentachlorophenol in the solvent is required for pressure impregnation of wood. The cost of the solvent portion of this solution amounts to from $12 t o $18 per thousand board feet of wood processed, while the value of the pentachlorophenol itself is only about $5.00 to$9.00. The solvent can be recovered by means of t h e vapor process at a cost of about $6.00 t o $8.00 per thousand board feet, which represents a decided economic advantage. I n addition to the saving obtained by removal of this solvent, the vapor process produces a finished product that is free of surface deposits of natural resins and of preservative impregnant t h a t cause discoloration and other defects of paint applied t o the wood. literature Cited (1) American Wood-Preservers’ Association, “Manual of Recommended Practice,” Standard P9-51. (2) Ibid., Standard A6-51. (3) American Wood-Preservers’ Association, Proc. A m . Wood-Preservers’ Assoc., 43, 62 (1947). (4) Carswelland Hatfield, IND. ENG.CHEM.,31,1431 (1939). (5) Chapman Chemical Co., Tech. Newsletter, No. 6 (July 1952). (6) Duncan and Richards, Proc. Am. Wood-Preservers’ Assoc., 46, 131 (1950). (7) Hatfield, Ibid.,40, 47 (1944). (8) Ibid., 45, 84 (1949). (9) Hubert, IND.ENQ.CHEM.,30, 1241 (1938). (IO) Hudson, Forrest Products Research SOC.Proc., 1 , 124 (1947). (11) Hudson, Proc. Am. Wood-Preservers’Assoc., 46, 209 (1950). (12) Hudson, U. S. Patent 2,435,218 (Feb. 3, 1948). (13) Sedziak, J. Forest Products Research SOC., 2 , No. 5 , 2 6 0 (1952). (14) U. S. Forest Products Laboratory, ANC Bull., 21 (1946). (15) Verrall, Southern Lumberman (June 15, 1949). (16) Western Pine Association, BUZZ.6 (July 1951). ACCEPTED February 9 , 1953
RECEIVED for review March 5 , 1951.
Electrolytic Preparation of Beryllium Hydroxide
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Aqueous Sodium Beryllium Fluoride as Cathode Liquor RAMAN K. PARIKH AND KARL
KAMMERMEVER
Sfufe University o f lowu, lowa City, lowo
M
UCH work has been done on the electrolysis of molten
beryllium salts ( 2 ) ,and in particular of beryllium chloride (1). Only one reference ( 4 ) seems to exist concerning the electrolysis of aqueous solutions, but no information whatever is available on operating variables. The object of the present work was to investigate the factors involved in the preparation of beryllium oxide by the electrolytic method, using an aqueous solution of sodium beryllium fluoride ( NazBeF4) as a cathode liquor with a graphite rod as a cathode. Beryl, 3Be0.AI2O3.6Si02,is the chief raw material for the manufacture of
beryllium oxide. Ores run from 10 to 12% beryllium oxide. Numerous processes have been developed, studied, and pa& ented for production of beryllium oxide, but very few have actually been successful in industry. The most important two processes now in use in industry are based on (1) fusion of beryl with lime, and (2) fusion of beryl with sodium silicofluoride or sodium ferric fluoride, called the “fluoride process.” The sodium beryllium fluoride solution for electrolysis is obtained in the fluoride process, which is described in detail by Lundin (8).
