title: Pressure screening studies with wood pulp

authors: Weeds, Zuben

subjects: Wood-pulp Refining Separation

abstract: Pressure screening is used in paper making to separate components of a wood pulp slurry (water, wood fibre and solid contaminant material) and to concentrate the wood fibre. The complex solid-liquid and solid-solid mechanisms involved in pressure screening are still poorly understood. This thesis reviews the literature on pressure screening mechanisms and highlights the problems in the models used to describe pressure screening. 
Two models, based on well-mixed tanks in series, are developed to describe reject thickening behaviour of long and short screens. The first model uses a single one row of tanks to model steady flow from feed to accept and from feed to reject streams of a screen. A back-flow model, which had two rows of well-mixed tanks, one for the feedside and another for the accept-side of the screen, was also developed. The passage ratio for back-flow of fibre PB was estimated numerically by deducing the passage ratio for forward-flow of fibre PF. The model showed that PB can decrease more quickly than PF when feed consistency increases, even when overall passage ratio Pp remains constant. 
A small industrial pressure screen (diameter 203 mm, length 220 mm) was used to investigate the effect of screen length Ls and feed consistency Cf on pressure screening performance. The effects of pulp type ( eucalypt for short fibre and pine kraft pulp for short fibre), feed consistency (from 0.005% to 2% compared to approximately 1 % in industrial plants), screen length and placement along the length of the screen, rotor type (bump or step), and screen hole shape (round holes or slots) were investigated. Measurements of pulp consistency in the feed, reject and accept streams under different processing conditions were used to assess the screen models developed. Screening behaviour of screens less than 220 mm was studied extensively for the first time. 
The tanks-in-series model showed that the effect of screen length on reject thickening could be described as well-mixed tanks in series with plug flow behaviour when the screen was modelled as 100 tanks. The reject thickening factor T decreased semilogarithmically when number of tanks was decreased but increased slightly when the number of tanks was increased. . Reject thickening factor decreases when a screen with 1-mm hole is shortened from 220 mm to 27 mm. This reduction can be attributed mainly to the flow regime changing from plug to mixed flow.
The maximum volumetric accept flow rate Qmax decreased by approximately 30 % when a 55-mm screen section was moved from the feed end to the reject end. 1bis decrease in Qmax was consistent with an axial decrease in relative speed ( difference between rotor tip and pulp suspension in the feed annulus), along with an estimated 33% reduction in the negative pressure pulse PNP along the screen. Overall fibre passage ratio PP also decreased when the screen section was moved to the reject end, or when pulp was accelerated in a feed chamber upstream of the feed annulus. 
The three distinct regions and two critical feed consistencies observed when Cr increased from 0.005% to 2% are postulated to indicate when fibre accumulation on the feed and accept sides of the screen produces screen plugging. The critical Cr for the accept side is about ten times greater than that for the feed side due mainly to the fact that the conical holes have a larger diameter on the accept side so more fibre must accumulate before backflow is hindered. In Region 1 ( crowding number < 1 ), reject thickening is independent of feed consistency and remains constant regardless of rotor type (bump or step) and screen type (smooth holes or contoured slots). In Region 2 (1< crowding number< 60), reject thickening decreases with feed consistency. In Region 3 (crowding number > 60), reject thickening either . decreases with feed consistency or remains constant depending on the ratio of forward to back-flushing flow exhibited by the rotor and as predicted by the two-tanks in series model. For the step rotor with a flow ratio of 2:1 reject thickening is constant with feed consistency regardless of screen type. For the bump rotor with a flow ratio of 5: 1 reject thickening decreases with feed consistency regardless of the screen type.
Fibre-length fractionation index <I> increased with increasing Cr to a maximum at about Ct=0.7% when using a 1-mm smooth-holed screen with a bump rotor or at Ct=2% for a 2.4-mm smooth-holed screen. Fractionation index then decreased with increasing Cf. 
It is recommended that the back-flow model be further developed so that the volumetric flow ratio (ratio of forward flow to back flow) can be calculated from a residence time distribution. It is also recommended that fibre-length fractionation be attempted at various positions along the screen for medium consistency pulp (Cr of 8 to 12%) using unconventionally large holes (3-5 mm).

text: Pressure screening studies with wood pulp Pressure screening is used in paper making to separate components of a wood pulp slurry (water, wood fibre and solid contaminant material) and to concentrate the wood fibre. The complex solid-liquid and solid-solid mechanisms involved in pressure screening are still poorly understood. This thesis reviews the literature on pressure screening mechanisms and highlights the problems in the models used to describe pressure screening. 
Two models, based on well-mixed tanks in series, are developed to describe reject thickening behaviour of long and short screens. The first model uses a single one row of tanks to model steady flow from feed to accept and from feed to reject streams of a screen. A back-flow model, which had two rows of well-mixed tanks, one for the feedside and another for the accept-side of the screen, was also developed. The passage ratio for back-flow of fibre PB was estimated numerically by deducing the passage ratio for forward-flow of fibre PF. The model showed that PB can decrease more quickly than PF when feed consistency increases, even when overall passage ratio Pp remains constant. 
A small industrial pressure screen (diameter 203 mm, length 220 mm) was used to investigate the effect of screen length Ls and feed consistency Cf on pressure screening performance. The effects of pulp type ( eucalypt for short fibre and pine kraft pulp for short fibre), feed consistency (from 0.005% to 2% compared to approximately 1 % in industrial plants), screen length and placement along the length of the screen, rotor type (bump or step), and screen hole shape (round holes or slots) were investigated. Measurements of pulp consistency in the feed, reject and accept streams under different processing conditions were used to assess the screen models developed. Screening behaviour of screens less than 220 mm was studied extensively for the first time. 
The tanks-in-series model showed that the effect of screen length on reject thickening could be described as well-mixed tanks in series with plug flow behaviour when the screen was modelled as 100 tanks. The reject thickening factor T decreased semilogarithmically when number of tanks was decreased but increased slightly when the number of tanks was increased. . Reject thickening factor decreases when a screen with 1-mm hole is shortened from 220 mm to 27 mm. This reduction can be attributed mainly to the flow regime changing from plug to mixed flow.
The maximum volumetric accept flow rate Qmax decreased by approximately 30 % when a 55-mm screen section was moved from the feed end to the reject end. 1bis decrease in Qmax was consistent with an axial decrease in relative speed ( difference between rotor tip and pulp suspension in the feed annulus), along with an estimated 33% reduction in the negative pressure pulse PNP along the screen. Overall fibre passage ratio PP also decreased when the screen section was moved to the reject end, or when pulp was accelerated in a feed chamber upstream of the feed annulus. 
The three distinct regions and two critical feed consistencies observed when Cr increased from 0.005% to 2% are postulated to indicate when fibre accumulation on the feed and accept sides of the screen produces screen plugging. The critical Cr for the accept side is about ten times greater than that for the feed side due mainly to the fact that the conical holes have a larger diameter on the accept side so more fibre must accumulate before backflow is hindered. In Region 1 ( crowding number < 1 ), reject thickening is independent of feed consistency and remains constant regardless of rotor type (bump or step) and screen type (smooth holes or contoured slots). In Region 2 (1< crowding number< 60), reject thickening decreases with feed consistency. In Region 3 (crowding number > 60), reject thickening either . decreases with feed consistency or remains constant depending on the ratio of forward to back-flushing flow exhibited by the rotor and as predicted by the two-tanks in series model. For the step rotor with a flow ratio of 2:1 reject thickening is constant with feed consistency regardless of screen type. For the bump rotor with a flow ratio of 5: 1 reject thickening decreases with feed consistency regardless of the screen type.
Fibre-length fractionation index <I> increased with increasing Cr to a maximum at about Ct=0.7% when using a 1-mm smooth-holed screen with a bump rotor or at Ct=2% for a 2.4-mm smooth-holed screen. Fractionation index then decreased with increasing Cf. 
It is recommended that the back-flow model be further developed so that the volumetric flow ratio (ratio of forward flow to back flow) can be calculated from a residence time distribution. It is also recommended that fibre-length fractionation be attempted at various positions along the screen for medium consistency pulp (Cr of 8 to 12%) using unconventionally large holes (3-5 mm).

year: 2006

title: A study of the digestion and metabolism of dairy cows differing in susceptibility to bloat

authors: Carruthers, Vicki Robin

abstract: The digestive characteristics of cows of high (HS) or low (LS) susceptibility to bloat were investigated. 

Sixteen HS and 16 LS cows were offered pasture or hay when pregnant but not lactating, and pasture or 75% pasture plus 25% hay when lactating. The susceptibility groups did not differ in voluntary intake or apparent digestibility of dry matter, organic matter, nitrogen or energy on either occasion. LS cows produced more milk of a lower protein content than HS cows but yields of fat, protein and lactose were similar for both susceptibility groups.

Subsequent experiments used 4 or 5 cows of each susceptibility group, each fitted with a fistula in the dorsal rumen and offered diets of hay or pasture.

On 3 of 8 diets offered, non-lactating LS cows had less digesta, measured by removal through the fistula, in the reticulo-rumen before feeding than did HS cows. Increasing or decreasing the quantity of digesta in the reticulo-rumen after feeding correspondingly increased or decreased both the quantity of digesta subsequently lost and the quantity remaining 21 hours after feeding, or before the next feeding, for both HS and LS cows. On all diets LS cows lost less fluid from the reticulo-rumen during feeding and more after feeding, than did HS cows. Altering the concentration of Na in rumen fluid by addition of water or electrolyte solutions did not affect the loss of fluid and DM from the reticulo-rumen of LS cows relative to that of HS cows. When offered 3 pastures differing in maturity, an increase in maturity of the pasture resulted in an increase in the quantity of digesta in the reticulo-rumen before feeding, but HS and LS cows did not differ in quantity of digesta on each of the 3 pastures. The quantity of digesta in the reticulo-rumen was greater in cows when offered a diet of low K:Na ratio than when offered a high ratio diet. On both diets, however, LS cows had less digesta in the reticulo-rumen, a lower concentration of K in plasma and excreted less K in their faeces than HS cows.

Lactating HS and LS cows offered pasture ad libitum did not differ in intakes of dry matter and water, in the quantities of fluid and dry matter in the reticulo-rumen or in the rate of digestion when estimated from the loss of dry matter from nylon bags incubated in the ventral sac of the rumen.

HS and LS cows did not differ in capacity of the reticule-rumen. The rumen fluid dilution rate, estimated using Cr EDTA, was similar for both susceptibility groups in 3 experiments and higher for LS than HS cows in one experiment.

Overall, these studies revealed few differences between HS and LS cows in the parameters measured, except that under some experimental conditions LS cows had less digesta in the reticulo-rumen than did HS cows.

text: A study of the digestion and metabolism of dairy cows differing in susceptibility to bloat The digestive characteristics of cows of high (HS) or low (LS) susceptibility to bloat were investigated. 

Sixteen HS and 16 LS cows were offered pasture or hay when pregnant but not lactating, and pasture or 75% pasture plus 25% hay when lactating. The susceptibility groups did not differ in voluntary intake or apparent digestibility of dry matter, organic matter, nitrogen or energy on either occasion. LS cows produced more milk of a lower protein content than HS cows but yields of fat, protein and lactose were similar for both susceptibility groups.

Subsequent experiments used 4 or 5 cows of each susceptibility group, each fitted with a fistula in the dorsal rumen and offered diets of hay or pasture.

On 3 of 8 diets offered, non-lactating LS cows had less digesta, measured by removal through the fistula, in the reticulo-rumen before feeding than did HS cows. Increasing or decreasing the quantity of digesta in the reticulo-rumen after feeding correspondingly increased or decreased both the quantity of digesta subsequently lost and the quantity remaining 21 hours after feeding, or before the next feeding, for both HS and LS cows. On all diets LS cows lost less fluid from the reticulo-rumen during feeding and more after feeding, than did HS cows. Altering the concentration of Na in rumen fluid by addition of water or electrolyte solutions did not affect the loss of fluid and DM from the reticulo-rumen of LS cows relative to that of HS cows. When offered 3 pastures differing in maturity, an increase in maturity of the pasture resulted in an increase in the quantity of digesta in the reticulo-rumen before feeding, but HS and LS cows did not differ in quantity of digesta on each of the 3 pastures. The quantity of digesta in the reticulo-rumen was greater in cows when offered a diet of low K:Na ratio than when offered a high ratio diet. On both diets, however, LS cows had less digesta in the reticulo-rumen, a lower concentration of K in plasma and excreted less K in their faeces than HS cows.

Lactating HS and LS cows offered pasture ad libitum did not differ in intakes of dry matter and water, in the quantities of fluid and dry matter in the reticulo-rumen or in the rate of digestion when estimated from the loss of dry matter from nylon bags incubated in the ventral sac of the rumen.

HS and LS cows did not differ in capacity of the reticule-rumen. The rumen fluid dilution rate, estimated using Cr EDTA, was similar for both susceptibility groups in 3 experiments and higher for LS than HS cows in one experiment.

Overall, these studies revealed few differences between HS and LS cows in the parameters measured, except that under some experimental conditions LS cows had less digesta in the reticulo-rumen than did HS cows.

year: 1986

title: Alternative anthropologies: Kete aronui from the Waikato

authors: McCormack, Fiona

abstract: As three anthropologists working at the University of Waikato in Hamilton, Aotearoa (New Zealand), we experience anthropology in our daily work in the context of our local histories, communities and politics. While many anthropologists are familiar with the critiques of anthropology that play out in the USA or Europe, the narratives and practices of anthropology from places such as New Zealand are less well known. We argue that these local, diverse experiences of anthropology can enlarge our international understandings and imaginations of what anthropology can be, as well as the challenges it may face.

Anthropologists working in New Zealand today face the same plethora of academic pressures as those found in their counterparts in North America, Britain and Europe; pressures instigated by decades of neoliberal reform, managerialism, and the impact of new entrepreneurial and corporate models of universities that shape everyday identities and social relationships (Shore 2010). Similarly, the critique of anthropology as a discipline rooted in colonial imperatives and practices, resounds in a society whose imperial history and settler colonial present continues to imprint on educational institutions, pedagogy and research. In New Zealand, no neat historical trajectory marks a path from extractive research, wherein Indigenous knowledge and ways of life are pottled for export to the empire’s core, to one based on mutuality, co-creation and the indigenisation of anthropological knowledge.

text: Alternative anthropologies: Kete aronui from the Waikato As three anthropologists working at the University of Waikato in Hamilton, Aotearoa (New Zealand), we experience anthropology in our daily work in the context of our local histories, communities and politics. While many anthropologists are familiar with the critiques of anthropology that play out in the USA or Europe, the narratives and practices of anthropology from places such as New Zealand are less well known. We argue that these local, diverse experiences of anthropology can enlarge our international understandings and imaginations of what anthropology can be, as well as the challenges it may face.

Anthropologists working in New Zealand today face the same plethora of academic pressures as those found in their counterparts in North America, Britain and Europe; pressures instigated by decades of neoliberal reform, managerialism, and the impact of new entrepreneurial and corporate models of universities that shape everyday identities and social relationships (Shore 2010). Similarly, the critique of anthropology as a discipline rooted in colonial imperatives and practices, resounds in a society whose imperial history and settler colonial present continues to imprint on educational institutions, pedagogy and research. In New Zealand, no neat historical trajectory marks a path from extractive research, wherein Indigenous knowledge and ways of life are pottled for export to the empire’s core, to one based on mutuality, co-creation and the indigenisation of anthropological knowledge.

year: 2025