LOW-EMITTING FIBER COMPOSITE MANUFACTURING PROCESS

Abstract

A novel green manufacturing process for medium and high-density fiberboard (MDF and HDF) production is disclosed, where the green manufacturing process refers to a novel low-emitting manufacture of MDF and HDF in terms of HAP emission. The novel manufacturing process comprises a preconditioning unit operation for raw wood furnish material, where blends comprising two or more woody materials having disparate moisture contents are held in a preconditioning vessel for up to 48 hours under controlled temperature conditions, where the moisture content is homogenized to produce a blend having substantially uniform moisture content. The blend is preconditioned in the novel manufacturing process to facilitate moisture homogenization kinetics, and to uniformly increase the material temperature above the lignin glass transition temperature. Subsequent process steps, such as defibration and fiber drying, require lower temperatures to produce and dry wood fiber, thus lowering HAPs emission.

Claims

1. A low VOC- and HAP-emitting medium- and high-density fiberboard manufacturing process, comprising the steps of: (i) loading into a preconditioning vessel a batch of a raw wood furnish blend, comprising a proportional blend of high-moisture content wood particles and low-moisture content wood particles, wherein the total moisture content of the raw wood furnish blend is substantially equal to the proportional sum of the water content of the high-moisture content wood particles and the low-moisture content wood particles; (ii) preconditioning the raw wood furnish blend in the preconditioning vessel, wherein the raw wood furnish is held for up to 48 hours, wherein the moisture content of the raw wood furnish is substantially homogenized; (iii) refining the preconditioned wood furnish preconditioned in the preceding step, wherein pressurized steam is injected into the refining unit at steam temperatures of 160° C. or less, and wherein wood fiber is discharged from the refining unit; and (iv) drying the wood fiber discharged from the refining unit, wherein the inlet temperature of the drying unit is 140° C. or less, and the outlet temperature of the drying unit is 60° C. or less.

2. The low VOC- and HAP-emitting medium- and high-density fiberboard manufacturing process of claim 1, wherein the step of preconditioning the raw wood furnish blend comprises metering of water into the preconditioning vessel onto the blended.

3. The low VOC- and HAP-emitting medium- and high-density fiberboard manufacturing process of claim 1, wherein the step of loading into a preconditioning vessel a raw wood furnish blend comprises prewetting the low-moisture content wood particles, wherein water is metered onto a batch of said low moisture-content wood particles (and an equilibration time is allowed to ensue), then loading the batch of prewetted low-moisture-content wood particles into the preconditioning vessel to form the raw wood furnish blend with the high-moisture content wood particles.

4. The low VOC- and HAP-emitting medium- and high-density fiberboard manufacturing process of claim 1, wherein the step of preconditioning the raw wood furnish blend in a preconditioning vessel comprises preconditioning the raw wood furnish between 12 hours and 36 hours at temperatures ranging between ambient and 95° C.

5. The low VOC- and HAP-emitting medium- and high-density fiberboard manufacturing process of claim 1, wherein the step of preconditioning the raw wood furnish blend in a preconditioning vessel comprises homogenizing the moisture content of the raw wood furnish blend to at least 20% by weight.

6. The low VOC- and HAP-emitting medium- and high-density fiberboard manufacturing process of claim 1, wherein the step of preconditioning the raw wood furnish blend in a preconditioning vessel comprises homogenizing the moisture content of the raw wood furnish blend to levels ranging between 26% and 30% by weight.

7. The low VOC- and HAP-emitting medium- and high-density fiberboard manufacturing process of claim 1, wherein the step of preconditioning the raw wood furnish blend in a preconditioning vessel comprises homogenizing the moisture content of the raw wood furnish blend to levels ranging between 35% and 40% by weight.

8. The low VOC- and HAP-emitting medium- and high-density fiberboard manufacturing process of claim 1, wherein the step of drying the wood fiber discharged from the drying unit comprises drying the wood fiber wherein exit moisture content ranges between 10% and 15% by weight based on oven dried wood.

9. A manufacturing system for a low VOC- and HAP-emitting medium- and high-density fiberboard manufacturing process, comprising: (i) A preconditioning vessel for preconditioning a raw wood furnish blend, said raw wood furnish blend comprising a proportioned mixture of high-moisture content wood particles and low-moisture content wood particles, the preconditioning vessel adapted to hold the raw wood furnish blend for up to 48hours; (ii) a refining unit operation comprising a defibrator, the refining unit adapted to receive pressurized steam, wherein the pressurized steam is injected at temperatures ranging up to 160° C., said refining unit further adapted to receive discharged preconditioned wood furnish blend from the preconditioning vessel; and (iii) a fiber drying unit operation comprising a fiber dryer, the fiber dryer having inlet temperatures of 140° C. of less, and exit temperatures of 60° C. or less, the fiber drying unit operation adapted to receive fiber discharged from the refining unit operation.

10. The manufacturing system for a low VOC- and HAP-emitting medium- and high-density fiberboard manufacturing process of claim 9, wherein the fiber dryer has an inlet temperature range of 130° C. to 140° C., and an exit temperature range of 55° C. to 60° C.

11. The manufacturing system for a low VOC- and HAP-emitting medium- and high-density fiberboard manufacturing process of claim 9, wherein the preconditioning vessel is adapted to hold the raw wood furnish blend for residence times ranging between 12 hours and 36 hours.

12. The manufacturing system for a low VOC- and HAP-emitting medium- and high-density fiberboard manufacturing process of claim 9, wherein the preconditioning vessel is adapted to homogenize the moisture content of the raw wood furnish blend to moisture levels ranging between 20% and 40% by weight.

13. The manufacturing system for a low VOC- and HAP-emitting medium- and high-density fiberboard manufacturing process of claim 9, wherein the preconditioning vessel is adapted to homogenize the moisture content of the raw wood furnish blend to moisture levels ranging between 26% and 30% by weight.

14. The manufacturing system for a low VOC- and HAP-emitting medium- and high-density fiberboard manufacturing process of claim 9, wherein the preconditioning vessel is adapted to homogenize the moisture content of the raw wood furnish blend to moisture levels ranging between 30% and 35% by weight.

15. The manufacturing system for a low VOC- and HAP-emitting medium- and high-density fiberboard manufacturing process of claim 9, wherein the preconditioning vessel is adapted to receive pressurized steam injected at temperatures up to 140° C.

16. The manufacturing system for a low VOC- and HAP-emitting medium- and high-density fiberboard manufacturing process of claim 9, wherein the preconditioning vessel is adapted to hold the raw wood furnish between 12 hours and 36 hours at temperatures ranging from ambient to 95° C.

17. The manufacturing system for a low VOC- and HAP-emitting medium- and high-density fiberboard manufacturing process of claim 9, wherein the drying unit operation is adapted to dry the discharged fiber to a moisture content ranging between 10% and 15% by weight based on oven dried wood.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIG. 1. Flow chart of a first embodiment of the innovative green MDF manufacturing process.

[0030] FIG. 2 Flow chart of the state-of-the-art MDF manufacturing process.

[0031] FIG. 3. Flow chart of a second embodiment of the innovative green MDF manufacturing process.

DETAILED DESCRIPTION

[0032] The innovative green (low-emitting) MDF manufacture process is described by the process flow diagram 100 presented in FIG. 1. Beginning with step 101, the wood furnish is prepared by proportionate blending of wood chips and saw dust, which may comprise other small wood particles having sizes between sawdust particles and wood chips. The materials may be sourced from lumber mills, logging operations and other suitable sources, and comprise material from various species of hardwoods and softwoods. Typically, the moisture content varies greatly with the type of material. If the sawdust is green, meaning raw and unprocessed, it may have a moisture content greater than 60% by weight. By contrast, previously kiln dried wood for dimension lumber may have a low moisture content, typically ranging between 7-8%, or generally less than 10%.

[0033] As the wood furnish may comprise both sawdust and small wood particles, as well as wood chips and small quantities of bark, the blend is typically inhomogeneous in terms of particle size and moisture content. As the heat capacity of the wood particles is a function of particle size and moisture content, these two characteristics have a strong bearing on the time-dependent temperature rise of the wood particles when subject to high-temperature unit operations as described in the above paragraphs. Thus, particles with high moisture content have a greater heat capacity than those with low moisture content for a given size. Conversely, for a given moisture content, the larger the size of the particle, the larger the heat capacity.

[0034] Using the example of the wood furnish blend just described, subjecting such a wood furnish without further conditioning to high temperatures of 160° C. and greater, using pressurized steam, will cause rapid temperature rise in the low moisture content wood chips, promoting thermal degradation as the chips reach pretreatment temperatures. As described above, such thermal degradation generates HAPs for which MACT technology is required. Small particles may not reach such temperatures in the pretreatment operation preceding defibration, but their high moisture content may cause the release of HAPs during downstream drying processes, such as the fiber drying operation carried out after the defibration step, which may use higher temperatures to adequately dry the fibers. As in the state-of-the-art manufacture, the moisture content of fiber output from the defibration operation is substantially disparate, the high dryer temperatures cause further release of HAPs from the dry fibers, and as the wetter fibers dry out, they will rapidly heat to dryer temperatures, which will engender additional HAPs emission.

[0035] The proportionate blending comprised by step 101 of the innovative MDF manufacture process allows a homogenized controlled moisture content for the entire wood furnish to be achieved, as the moisture content of the individual components of the blend are known. The total moisture content of the blend may be adjusted to specific ranges by adjusting the composition of the blend, thus the proportional blending. By way of example, in one embodiment, a blend consists of 40% green sawdust, having an average moisture content of 60%, and 60% dry wood chips, having an average moisture content of 7-8%. In step 102 of theinnovative MDF manufacture process of FIG. 1, the wood furnish blend from step 101 has been charged into a holding silo so that the preconditioning operation or stage may be carried out. In this embodiment, the blend may spend up to 48 hours in the holding silo, but not less than 4 hours. In some embodiments, the blend may reside for 20-28 hours; in further embodiments, the holding time may vary between 12-36 hours.

[0036] The length of the holding period may be established as the time required for the blend to reach a substantially homogeneous moisture content. This in turn generally depends on the kinetics of water uptake by the woody material comprised by the blend. Temperature is a factor here, and is thus important in determining the rate of uptake of water by the dryer component. Therefore, the operation may be carried out at elevated temperatures in some embodiments, although embodiments in which ambient preconditioning temperatures are employed due to the extended pre-conditioning periods in the innovative process may sufficient to permit water uptake kinetics to complete homogenization of moisture content within 48 hours.

[0037] Step 102 induces the water content of the wetter material to be re-distributed to the dryer material, mainly in the form of water vapor uptake, from water evaporation from the wetter material by the high temperatures of the unit operation. In this embodiment, the atmosphere surrounding the wood particles humidifies during the operation as water vapor is released from the wetter, and eventually is absorbed by the dryer material, where the moisture content may rise to levels exceeding 20%, up to 40%, for which the fiber saturation points of the particular wood materials are met or exceeded. The moisture content of the wetter particles may concomitantly decrease to similar levels. Thus, the blend becomes homogenized, wherein the moisture content of the blend components has now become substantially homogeneous. In some embodiments, the preconditioned wood material may undergo further pre-heating stage prior to entry in the refining unit operation to ensure completion of pre-conditioning with respect to temperature and moisture content. Preferred ranges of homogenized moisture content resulting from the innovative MDF manufacture for a wood furnish may range from 26-30% by weight in some embodiments, and 30-35% by weight in further embodiments. In still further embodiments, homogenized moisture content may range from 35-40% by weight.

[0038] In other embodiments, water is metered into the preconditioning vessel wherein a batch of dry woody material, such as wood chips, has been precharged. The amount of metered water is calculated to raise the moisture content of the dry material to a specific level when the water is substantially absorbed by the dry material. The contact time with the metered water depends on the uptake kinetics that depends the temperature, which may be raised above ambient in some embodiments.

[0039] In step 103, the transferred pretreated wood furnish is defibrated in the fiber refining operation. In this step, pressurized steam is injected into the defibration unit at temperatures ranging from 120°-160° C. In comparison with state-of-the-art MDF manufacture, the steam temperatures needed in the innovative MDF process are relatively moderate, as discussed below, due to the innovative preconditioning step, step 102. Due to step 102, refining conditions are less extreme in the innovative MDF manufacture process as compared to the state-of-the-art MDF manufacture processes currently widely practiced. As a result of the lower steam temperatures injected into the defibration unit, generation of HAPs is minimized, as steam temperatures remain below 160° C., above which HAPs release is greatly augmented. In some embodiments, temperature of the pressurized steam injected into the defibration unit operation is 130° C., while in other embodiments, pressurized steam temperatures used in step 103 may range between 120°-150° C. In all embodiments, the temperature of the pressurized steam injected at step 103 may fall between 100°-160° C. As indicated in FIG. 1, emission control requirements to cap HAPs evolved in step 103 are concomitantly reduced, saving capital and operational costs and expenses. An estimated cost savings may be $1.5 million per year for an MDF plant of 60,000 msf/year capacity—¾″ basis, a typical size for modern MDF plant. Energy input requirements for the unit operation embodied in step 103 are also substantially reduced.

[0040] As the preconditioned wood furnish is delivered to the refining unit operation 103 above the lignin glass transition temperature, leaving the wood material in a softened state, fibers are more easily separated than is the case in the state-of-the-art process. In the latter process, the wood furnish is not uniformly heated and softened in the short time spent in the pretreatment unit operation (see discussion below). Mechanical shear requirements to achieve a clean separation of fiber are thus necessarily lower in the innovative MDF process in comparison to the state-of-the-art MDF manufacture. As a result, the quality of the fiber produced in the defibration step is generally higher than in the state-of-the-art manufacture, as fiber fracture and production of short fragments (fines) is minimized.

[0041] Moreover, the innovative preconditioning step 102 prepares the wood furnish with a substantially uniform and an overall lower moisture content relative to state-of-the-art MDF manufacture, thus the moisture content of the refined fiber issuing from defibration step 103 is also substantially uniform. As a result, lower fiber dryer outlet temperatures, 55°-60° C. typical in most embodiments, are needed in the subsequent drying step 104, minimizing HAPs release in this step. In conventional state-of-the-art MDF (and HDF) manufacture, the fiber drying unit operation has also been identified as a source of HAPs. In the innovative process, HAP production in step 104 is minimized due to the lower drying temperatures required as a result of steps 102 and 103.

[0042] Emission controls requirements for HAPs production in step 104 are concomitantly reduced as well, indicated in FIG. 1, again reducing energy consumption and saving capital and operational expenses, as described in the above paragraphs. The lower temperature range at the outlet of the dryer in the innovative process prepares the wood fibers to a moisture content in the range 10-15% moisture based on oven-dried wood. The generation of HAPS at much higher temperatures (>130° C.) is negligible until the moisture content is lowered to 5-8%. The temperature effect does not become critical until the moisture content is reduced below approximately 26%-10% (depending on wood species) because of the evaporative cooling effect of drying.

[0043] Subsequent to step 104 are the remaining unit operations to complete manufacture of the MDF and HDF products. These comprise, among others, fiber mat forming, pre-pressing, trimming, hot pressing, cooling, sawing, sanding and trimming, painting or laminating, and packaging, all of which are embodied in step 105, leading to the finished product in step 106. In some embodiments, step 105 in the innovative process may be substantially the same as in conventional MDF and HDF manufacture, and HAPs that may be produced in these operations are not significant as temperatures used for the remaining production are generally substantially lower than 160° C., and the residual HAP content of previously kiln-dried wood materials is significantly lower than never-dried wood.

[0044] For comparison, a conventional MDF (and HDF) manufacture process flow chart 200 is shown in FIG. 2. In step 201, a wood furnish is prepared by random blending of sawdust and small particle wood material with larger wood chips. Again, these materials may be sourced from lumber mills and logging operations, and may comprise several species of hardwoods and softwoods. The moisture content of the woody materials is disparate, as the sawdust may be substantially wetter than the chips, as indicated in FIG. 2. The disparate wood blend is delivered to the pretreatment operation in step 202. However, as discussed above, the residence time within the pretreatment unit operation is typically 90seconds, necessitating high temperatures for the pretreatment, where pressurized steam is injected at temperatures between 170°-200° C. to pre-heat and soften the woody material.

[0045] At these temperatures, significant thermal degradation of the wood furnish may occur, releasing a large quantity of HAPs. Even under these conditions, a uniform attainment of lignin glass transition in the wood is not achieved for softening, and conditions in the following defibration operation (step 203) are of necessity also extreme to effect efficient fiber separation. As the temperature of a large portion of the woody material is not in the lignin glass transition temperature range, this portion of the woody material is brittle and resistant to clean fiber separation. In step 203, high shear is often required to obtain sufficient fiber separation.

[0046] The high shear creates a large degree of fractured fibers and production of fines, resulting in a lower quality fiber mass. In an attempt to ameliorate this situation, pressurized steam is injected into the defibration unit at high temperatures, typically over 160° C., to adequately soften the woody material that was not adequately heated in pretreatment step 202. Here, the HAPs emission is severe due to extensive thermal degradation. The large HAPs release in both steps 202 and 203 due to the high temperature has been found to be the largest source of HAPs in the entire MDF and HDF manufacturing chain. NESHAPs requirements necessitate substantial MACT-compliant emission control in order to contain the pollutants, engendering significant capital and operating costs. In addition, energy costs are significant as well, as the energy input is high.

[0047] This scenario is contrasted with the cost savings of the greener innovative MDF/HDF manufacturing process shown in FIG. 1, and discussed above. As a further point, fibers discharged from the defibration operation in step 203 have a disparate moisture content, since the moisture content of the overall wood furnish was not homogenized. Thus, drying in step 204 typically requires higher temperatures in comparison to the innovative process at both the inlet and outlet points of the indirectly heated dryer. For conventional MDF/HDF manufacture processes, such as that indicated in FIG. 2, dryer inlet temperatures can be at 140-150° C. (280°-300° F.) and 65-68° C. (150°-155° F.) at the outlet. In embodiments of the instant process, the inlet temperatures are lower, ranging from 130°-140° C. (265°-285° F.) and 55°-60° C. (130°-140° F.) at the outlet. Both temperature ranges are governed by the drying load.

[0048] The instant innovation reduces that load to a range wherein the lower and more moderate temperature ranges indicated may be employed to remove the moisture from the wetter fibers in the wood furnish blend. In embodiments of the novel process, the exit moisture content may be controlled to 10-15% based on oven-dried wood. At these moisture content levels, HAPs generation is greatly suppressed as the wood temperature remains below critical temperatures where HAPs generation begins. Again, high temperatures in the drying operation engender significant HAPs release as well, with consequential MACT compliant emission control. Secondarily, energy input requirements for fiber drying step 204 are higher than for the same stage in the innovative MDF manufacture process (step 104 in FIG. 1).

[0049] As a further embodiment of the innovative green MDF (and HDF) manufacture process, an example modified process flow chart 300 is shown in FIG. 3. An exemplary wood furnish comprising low-moisture content woody material, having 7-8% moisture content, is introduced in step 301. In some cases, a relatively dry wood furnish may be available as a woody feed stock, such as dry wood shavings or chips. In step 302, a preconditioning unit operation is embodied, where a holding silo is charged with the wood furnish. As in FIG. 1, the holding times may range from 4-48hours, depending on the moisture uptake kinetics of the type of wood material.

[0050] Contrasting with the earlier embodiment of the innovative manufacture shown in FIG. 1, in this embodiment water is metered into the holding unit in proportion to the amount of wood furnish contained in the holding unit (vessel or silo). The quantity of water metered into the unit is to be absorbed by the woody material, and is calculated to bring the moisture content to a minimum of 26%, ranging up to 40% when absorbed. The operation may be performed at elevated temperatures, such as those described for the earlier embodiment (FIG. 1). Typical temperatures for this operation may range from ambient to 95° C. Following the modified preconditioning step (302), steps 303-306 are the same as steps 103-106 in FIG. 1.

Example Process

[0051] A proportionally blended feed stock furnish consisting of green (unprocessed) sawdust having a 60% moisture content is blended in a 40:60 ratio (respectively) with dry planar shavings having a moisture content of 7-8%, representing a typical disparate raw material blend. The moisture conditions cannot be efficiently mitigated in a conventional pretreatment tube, without using extreme conditions that engender high emission of HAPs and VOCs.

[0052] The disparate furnish is introduced into a chip silo for a holding period of 28hours, and permitted to re-distribute the moisture content from the wetter material to the dryer material. The final moisture content of the blend is thus homogenized to 29% after 28hours. The homogenized furnish in transferred to the refining, or defibration unit operation, where it is subject to pressurized steam injected at 120° C. High quality fiber output from the defibrator unit is transferred to the fiber drying unit operation, and dried at 60° C.

[0053] The embodiments described herein constitute several examples of the novel green MDF manufacturing process, and are by no means to be construed as limiting the innovation to these examples. Skilled practitioners of the art will recognize that the innovation may be manifest in a multitude of equivalent variations, and when practiced do not depart from the scope and spirit of the innovation, as claimed in the claims that follow.