Preparation for high activity, high density carbon

Lignocellulosic carbonaceous material is activated to produce a high activity, high density gas-phase activated carbon under conditions which effectively alter the particle pore volume size distribution to optimize the carbon's mesoporosity. A novel process is disclosed for producing the carbon.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to activated carbon and methods for preparing 
same. Particularly, this invention relates to new carbons useful in vapor 
adsorption and methods for their production. More particularly, this 
invention relates to activated carbon derived from lignocellulosic 
material prepared by chemical activation and shaping to produce carbon of 
high density and high activity. 
2. Description of the Prior Art 
Activated carbon is a microcrystalline, nongraphitic form of carbon which 
has been processed to increase internal porosity. Activated carbons are 
characterized by a large specific surface area typically in the range of 
500-2500 m.sup.2 /g, which permits its industrial use in the purification 
of liquids and gases by the adsorption of gases and vapors from gases and 
of dissolved or dispersed substances from liquids. Commercial grades of 
activated carbon are designated as either gas-phase or liquid-phase 
adsorbents. Liquid-phase carbons generally may be powdered, granular, or 
shaped; gas-phase, vapor-adsorbent carbons are hard granules or hard, 
relatively dust-free shaped pellets. 
Generally, the larger the surface area of the activated carbon, the greater 
its adsorption capacity. The available surface area of activated carbon is 
dependent on its pore volume. Since the surface area per unit volume 
decreases as individual pore size increases, large surface area is 
maximized by maximizing the number of pores of very small dimensions 
and/or minimizing the number of pores of very large dimensions. Pore sizes 
are defined as micropores (pore width&gt;1.8 nm), mesopores (pore 
width=1.8-50 nm), and macropores (pore width&gt;50 nm). Micropores and 
mesopores contribute to the adsorptive capacity of the activated carbon; 
whereas, the macropores reduce the density and can be detrimental to the 
adsorbant effectiveness of the activated carbon, on a carbon volume basis. 
The adsorption capacity and rate of adsorption depend to a large extent 
upon the internal surface area and pore size distribution. Conventional 
chemically activated lignocellulose-based carbons generally exhibit 
macroporosity (macropore volume) of greater than 20% of the carbon 
particle total volume. Gas-phase activated carbon macroporosity of less 
than 20% of the carbon particle volume would be desirable. Likewise, a 
high percentage of mesoporosity (i.e., above 50% of total particle volume) 
is desirable. 
Commercial activated carbon has been made from material of plant origin, 
such as hardwood and softwood, corncobs, kelp, coffee beans, rice hulls, 
fruit pits, nutshells, and wastes such as bagasse and lignin. Activated 
carbon also has been made from peat, lignite, soft and hard coals, tars 
and pitches, asphalt, petroleum residues, and carbon black. 
Activation of the raw material is accomplished by one of two distinct 
processes: (1) chemical activation, or (2) thermal activation. The 
effective porosity of activated carbon produced by thermal activation is 
the result of gasification of the carbon at relatively high temperatures 
(after an initial carbonization of the raw material), but the porosity of 
chemically activated products generally is created by chemical 
dehydration/condensation reactions occurring at significantly lower 
temperatures. 
Chemical activation typically is carried out commercially in a single kiln. 
The carbonaceous material precursor is impregnated with a chemical 
activation agent, and the blend is heated to a temperature of 
450.degree.-700.degree. C. Chemical activation agents reduce the formation 
of tar and other byproducts, thereby increasing yield. 
A "hard active carbon of high adsorptive power in the shaped or moulded 
state" is taught in U.S. Pat. No. 2,083,303 to be prepared by impregnating 
pulverized organic raw material, such as "sawdust, peat lignite or the 
like" with "known activating agents, such as zinc chloride or phosphoric 
acid" and heated to 100.degree.-200.degree. C. for one to one and a half 
hours producing a partially carbonized state wherein the material is 
somewhat plastic. Without reducing the temperature, the material is molded 
under pressure to a desired shape. The shaped material then is activated 
in a rotary activating retort and brought to a temperature of 
450.degree.-600.degree. C. for about four hours. 
Similarly, U.S. Pat. No. 2,508,474 teaches a gas mask activated carbon to 
be prepared by impregnating low density cellulosic material, such as 
finely divided wood in the form of wood shavings or sawdust, with 
concentrated zinc chloride, and heating to 120.degree.-145.degree. C. 
while agitating for not less than fifty minutes. The reacted mass then is 
compacted into "forms of appreciable size;" said forms are dried at 
160.degree.-300.degree. C.; the dried forms are crushed into granular 
particles; the granules are calcined at 675.degree.-725.degree. C.; and, 
after leaching out of the particles a greater portion of residual zinc 
chloride, recalcining the activated carbon product at 
1000.degree.-1100.degree. C. for at least thirty minutes. 
These representative techniques have produced activated carbon of adequate 
activity and density for many gas-phase applications, especially for 
purification and separation of gases as in industrial gas streams, in odor 
removal in air conditioning systems, and in gas masks. However, older 
technology gas-phase activated carbons have not proven entirely 
satisfactory in some applications for recovery (not just removal) of 
organic vapors which involves adsorption onto the carbon surface followed 
by desorption from the carbon for recapture. In fact, due to environmental 
concerns and regulatory mandates, one of the largest single applications 
for gas-phase carbon is in gasoline vapor emission control canisters on 
automobiles. Evaporative emissions vented from both fuel tank and 
carburetor are captured by activated carbon. 
Fuel vapors, vented when the fuel tank or carburetor is heated, are 
captured in canisters generally containing from 0.5 to 2 liters of 
activated carbon. Regeneration of the carbon is accomplished by using 
intake manifold vacuum to draw air through the canister. The air carries 
desorbed vapor into the engine where it is burned during normal operation. 
An evaporative emission control carbon should have suitable hardness, a 
high vapor working capacity, and a high saturation capacity. The working 
capacity of a carbon for gasoline vapor is determined by the 
adsorption-desorption temperature differential, by the volume of purge air 
which flows through the carbon canister, and by the extent to which 
irreversibly adsorbed, high molecular weight gasoline components 
accumulate on the carbon. 
Because of various economic considerations and space limitations in placing 
the carbon canister on-board a vehicle, this particular application of 
granular or shaped activated carbon requires higher activity and higher 
density properties than typically produced by the older technology noted. 
One method to control product density is taught by published European 
Patent Application 0 423 967 A2. The applicants note "a number of problems 
inherent in the use of wood as a raw material to produce directly a 
chemically activated pelletised form," claiming it to be "impossible to 
produce a high density activated carbon from a wood flour material" for 
lack of sufficient natural binding agent. An improved product (of 
substantially increased density) is claimed by use of, as a starting 
material, a "young carbonaceous vegetable product" having a "high 
concentration of natural binding agent." Such materials include nut shell, 
fruit stone and kernel, and in particular olive stone, almond shell, and 
coconut shell. 
Also, U.S. Pat. Nos. 5,039,651 and 5,118,329 teach densification of 
activated carbon product from cellulose materials including coconut 
shells, wood chips, and sawdust by pressing after initially heating to a 
relatively low temperature, followed by extrusion and calcination. Yet, 
with this improved processing the patentees could produce only carbons 
that were measured to have a volumetric working capacity (in terms of 
butane Working capacity, or BWC) of up to 12.3 g/100 cm.sup.3, although 
BWC values up to 15 g/100 cm.sup.3 are claimed. 
These prior art gas-phase carbons may have been satisfactory for limited 
volumes of vapors emitted from the carburetor and fuel tank. Because of 
impending environmental regulations requiring capture of greater amounts 
of fuel vapor emissions, it is anticipated that the volume of these 
additional vapors, combined with the space limitations and economic 
considerations which limit expansion of the size of canister systems, will 
require activated carbons with higher densities, higher activities, and 
higher volumetric working capacities than disclosed by the prior art 
(e.g., BWC&gt;15 g/100 cm.sup.3). 
Recently, co-pending and commonly assigned U.S. patent application Ser. No. 
853,133 claimed a method for making a high activity, high density 
activated carbon suitable for gasoline vapor adsorption applications which 
involved chemically activating lignocellulose fragments with phosphoric 
acid at and acid:sawdust ratio of 3:1 to 1:3, preferably 1.6, and heating 
for a time such that the discrete particle nature of the fragment was 
preserved before spheronizing the individual particles, heating again to 
thermoset, and subjecting to activation temperatures. While this method 
reported the production of seven runs which produced BWC values from 16.1 
to 18.2 g/100 cm.sup.3 and averaged a BWC of 17.1 g/100 cm.sup.3, it is 
but one method of producing such activated carbon material. 
Therefore, it is an object of this invention to provide a novel chemical 
activation process for producing activated carbons of high activity and 
relatively high density suitable for solvent and vapor capture and 
recovery, without sacrificing carbon density. It is a further object of 
this invention to produce the carbon without preserving the discrete 
particle nature of the lignocellulose fragment starting material. 
SUMMARY OF THE INVENTION 
The above objects of the invention are achieved by the chemical activation 
of a carbonaceous material, preferably lignocellulosic material, with a 
chemical activation agent in a manner to produce a viscous fluid product 
which is dried and heated to initial transition from plastic to thermoset 
intermediate product, granulated, and the granules are densified in a 
spheronizer to effectively minimize the macropore structure of the 
activated carbonaceous material. Densification is followed by increasing 
the temperature of the shaped product at a controlled rate to from 
425.degree. C. to 650.degree. C., preferably from 450.degree. to 
590.degree. C., and most preferably from 480.degree. to 510.degree. C. 
The novel high activity, high density gas-phase activated carbons produced 
are characterized by butane working capacities from above 15 to about 25 
g/100 cm.sup.3, preferably from about 17 to about 25 g/100 cm.sup.3, and 
more preferably from about 19 to about 25 g/100 cm.sup.3, a butane 
activity of from about 50 to about 80 g/100 g, preferably from about 60 to 
about 80 g/100 g, and more preferably from about 70 to about 80 g/100 g, 
and a density of from about 0.25 to about 0.40 g/cm.sup.3, preferably from 
about 0.27 to about 0.40 g/cm.sup.3, more preferably from about 0.30 to 
about 0.40 g/cm.sup.3. 
Preferably, such an activated carbon material also would exhibit a mesopore 
content of greater than about 50%, preferably greater than about 60%, and 
more preferably greater than about 70%, based on the total particle 
volume, and a macropore content of less than 20%, preferably less than 
18%, and more preferably less than 15%, based on the total particle volume 
.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The raw material precursor in the invention chemical activation may be any 
of the carbonaceous material of plant or mineral origin earlier recited. 
Preferred precursors primarily are lignocellulosic materials of plant 
origin and include wood-based materials such as wood chips, wood flour, 
and sawdust, as well as nut pits and nut shells such as coconut shell. 
Chemical activation agents which may be equally effective include: alkali 
metal hydroxides, carbonates, sulfides, and sulfates; alkaline earth 
carbonates, chlorides, sulfates, and phosphates; phosphoric acid; 
polyphosphoric acid; pyrophosphoric acid; zinc chloride; sulfuric acid; 
and oleum. Most preferred is phosphoric acid. 
The invention methods for producing the novel carbon can be described 
generally by the following sequence of steps: 
1. Activating agent/lignocellulose material blending 
2. Stage 1 heat treatment (lignocellulose degradation) 
3. Stage 2 heat treatment (drying and plasticization to thermoset) 
4. Shaping and densifying 
5. Activation 
The activation typically occurs in a rotary kiln in which the temperature 
of the thermoset shaped mixture is raised, preferably, to from about 
425.degree. to about 650.degree. C. This basic process normally is 
followed with washing (to remove residual phosphoric acid activating 
agent) and drying steps. 
The method discovered for producing the invention activated carbon product 
involves blending a chemical activating agent, preferably phosphoric acid 
or zinc chloride (which, optionally, may be pre-heated at a temperature of 
80.degree.-120.degree. C.) , with a lignocellulose material, preferably 
wood chips, sawdust (or, wood dust), or wood flour, at a ratio of 
.gtoreq.1.6:1, respectively, preferably .gtoreq.2:1, with agitation for up 
to one hour at a temperature of from about 35.degree. C. to about 
95.degree. C., after which the mixture material is converted to a viscous 
fluid product which is spread on glass trays and heat treated at from 
about 70.degree. to about 130.degree. C., first to dry the material and, 
subsequent to drying, to heat the dried material to the point of 
transition from plastic in nature to thermoset (up to about 20 hours). The 
resultant solidified material is granulated and the granules are subjected 
to a densification step which involves processing through a compressive 
shaping device such as a spheronizer. A commonly used spheronizer is a 
Marumerizer.RTM.. (Optionally, after granulation and before densification, 
the granulated material may be thermoset at a kiln temperature of from 
200.degree. to 220.degree. C., but no particular benefit appears to be 
derived therefrom.) After shaping/densification, the material is activated 
by gradually increasing the temperature to a final temperature of from 
425.degree. to 650.degree. C. 
The degradation, or (at least partial) solubilization, of wood with 
phosphoric acid produces a viscous fluid in which the discrete particles 
of the original lignocellulose may or may not be identified. In the 
solubilization process, the initial viscosity of the slurry mixture is 
very close to that of the phosphoric acid alone. As the temperature rises, 
the viscosity of the mass increases as the wood elements thereof degrade. 
If the viscosity increases too fast during this stage 1 heat treatment, 
water can be added to maintain sufficient fluidity for continued mixing 
under heat at, preferably, from about 80.degree. C. to about 120.degree. 
C. 
The surprising improvement in butane working capacity of the new carbon 
product reflects a major increase in mesoporosity of the individual carbon 
particles, at the expense of macroporosity. 
A standard determination of surface area of activated carbon usually is by 
the Brunauer-Emmett-Teller (BET) model of physical adsorption using 
nitrogen as the adsorptive. This was the method employed in calculating 
the invention carbon surface areas, based on nitrogen adsorption isotherm 
data in the range of 0.05 to 0.20 relative pressure. 
In the case of granular activated carbon, the density is an important 
feature of the effectiveness of the adsorbent, as many applications of 
granular or shaped activated carbon involve a static active carbon bed of 
fixed volumetric size. The apparent density of the invention activated 
carbon is measured according to the method ASTM D 2854. Measurements of 
apparent density of carbon in a packed bed of particles reported herein 
were based on 10.times.25 mesh carbon materials, unless otherwise noted. 
The density of the individual carbon particles was determined by 
displacement of mercury using a Micromeritics Pore Sizer 9300 instrument. 
The density is based on the mass of a particle and its volume including 
pores smaller than 35 micrometers. 
Butane activity of the invention carbons was calculated by placing a 
weighed sample of the dry activated carbon, approximately 15 ml in volume, 
in a 1.45 cm diameter tube and admitting butane gas therein. The amount 
adsorbed at saturation at 25.degree. C. is weighed and reported as butane 
activity in grams of butane per 100 grams carbon (g/100 g). The tube then 
is purged with air at 25.degree. C. at 250 ml/min. for 40 minutes, and the 
amount of butane removed is reported as butane working capacity (BWC) in 
grams of butane per 100 ml of carbon (g/100 cm.sup.3). The carbon mass to 
volume conversion is made on the basis of the measured value of the carbon 
apparent density. In view of the interrelationship of butane activity, 
BWC, and density, for carbons of a density from about 0.25 to about 0.40 
g/cm.sup.3, a BWC &gt;15 g/100 cm.sup.3 can be achieved with butane activity 
values of at least 50 g/100 g. 
Porosity in pores larger than 50 nm (macroporosity) was determined using a 
Micromeritics Pore Sizer 9310 which measures the volume of mercury forced 
into pore under the influence of pressure. The distribution of pore volume 
with pore size is calculated using the Washburn equation, a standard 
model. 
Porosity in pores smaller than 50 nm was determined using a Micromeritics 
DigiSorb 2600. Adsorption isotherm data for nitrogen, measured at a 
temperature of about 77.degree. K., are used with the Kelvin and Halsey 
equations to determine the distribution of pore volume with pore size of 
cylindrical pores according to the standard model of Barrett, Joyner, and 
Halenda. For the purposes of the examples and the invention claimed 
herein, macroporosity consists of pore diameters greater than 50 nm, 
mesoporosity consists of pore diameters of from 1.8 to 50 nm and 
microporosity consists of pore diameters of less than 1.8 nm. 
The invention method for producing the activated carbon product are 
disclosed in the following examples. 
EXAMPLE 1 
An activated carbon product was prepared by heating 698 g of 85% phosphoric 
acid to 105.degree. C. Sawdust in a total amount of 300 g (dry basis) was 
added (causing the acid temperature to drop) and mixed as the temperature 
of the mixture was raised to 75.degree. C. Mixing continued for 57 minutes 
with periodic addition of sufficient water to maintain fluidity. The 
viscous fluid product then was transferred to glass trays and heat treated 
to transition from plastic to initial thermoset (at a temperature of 
120.degree. C. for 16 hours). The resultant solidified product was 
granulated (in an Osterizer.RTM.) and the granules were processed in a 
Marumerizer for 13 minutes converting them to smooth, spherical particles. 
Finally, this product was activated in a direct fired, rotary kiln by 
heating to 480.degree. C. The resultant activated carbon had the following 
product properties: 
______________________________________ 
Butane Working Capacity 
17.6 g/100 cm.sup.3 
Butane Activity 71.8 g/100 g 
Apparent Density 0.29 g/cm.sup.3 
Particle Density 0.46 g/cm.sup.3 
Macropore Content 13% 
Mesopore Content 55% 
______________________________________ 
EXAMPLE 2 
For a comparison of the effects of the spheronization step and of the order 
of the steps of spheronization and activation, the samples described in 
FIG. 1 were prepared as follows: 
(1) Sawdust (moisture=44.2%) in the amount of 538 grams was blended (in 
four charges over two minute period) with 697.7 grams of phosphoric acid 
(concentration=86%, and pre-heated to 105.degree. C.) in a mixing bowl and 
stirred (Kitchenaid.RTM. Proline blender with SS flat blade) under heat 
(.about.75.degree. C.) for .about.40 minutes, after which 650 cc of hot 
tap water (.about.95.degree. C.) was added over two minute period and 
stirring was continued for a total mix time of .about.1.0 hour; 
(2) The viscous, tarry mass of material (with some unsolubilized sawdust 
visible) was transferred to glass drying trays, smoothed to form layers of 
a thickness of .about.1.0 inch, and dried in an oven at .about.120.degree. 
C. for 16-18 hours to thermoset; and 
(3) The thermoset material was granulated in an Osterizer.RTM. to 
6.times.25 mesh, heated further in a lab kiln to 205.degree. C., and 
divided into three samples, each of which was further processed according 
to FIG. 1. 
FIG. 1 disclosed that the sample subsequently activated, washed, dried, and 
screened (for 10.times.25 mesh) exhibited marginally acceptable (&lt;70 g/100 
g) butane activity, conventional BWC (&lt;15 g/100 cm.sup.3), and low 
apparent density. The sample which was activated prior to spheronizing 
showed the benefits of significantly enhanced density and an improved 
(though still conventional) BWC value, but suffered an even lower butane 
activity value. Finally, the invention process benefits are disclosed in 
the sample which is spheronized prior to activation. Analysis of this 
sample discloses attainment of a non-conventionally high BWC value, i.e., 
&gt;15 g/100 cm.sup.3, a significantly improved and a preferred butane 
activity value, i.e., &gt;70 g/100 g, both of which are achieved with a 
significant improvement in density (versus no spheronization). 
EXAMPLE 3 
For a comparison of the effects of optional subsequent kiln thermosetting 
at temperatures up to 220.degree. C. and of the effects of granular size 
on product properties, the samples described in FIG. 2 were prepared as in 
Example 3, with the exception that the granulated material was divided 
into two samples prior to the further lab kiln thermosetting step. These 
two samples were treated further as disclosed in FIG. 2. 
One sample is the same as the invention sample from Example 2/FIG. 1. The 
remaining sample, which was not heated to a higher temperature in the lab 
kiln, was screened to the desired 10.times.25 mesh size prior to 
spheronization and activation. As a result, the BWC and apparent density 
values were enhanced to preferred ranges while maintaining butane activity 
also in the preferred range for use in gasoline vapor adsorption. 
In the above examples, activated carbon of surprisingly high butane working 
capacity is produced by increasing surface area without sacrificing 
material density. This has been achieved by increasing carbon particle 
mesoporosity. In most instances the increase in mesoporosity has been 
created, unexpectedly, at the expense of the carbon particle's 
macroporosity. 
While the invention high activity, high density carbon has been described 
and illustrated herein by references to various specific materials and 
procedure, it is understood that the invention is not restricted to the 
particular materials, combinations of materials, and procedure selected 
for that purpose. With the disclosure herein of the concepts employed to 
produce the novel carbon, numerous variations of such details can be 
employed, as will be appreciated by those skilled in the art.