GB2285979A - A method for forming a cellulose powder - Google Patents

Abstract

Sheets of cellulosic material are diced 1 and added 2 to a reactor vessel containing water. Concentrated acid is added to a heated mixture in the vessel and heat is added until hydrolysis is complete. The reaction product is filtered in a filter press 6 and the filter cake produced is reslurried in a first reslurry tank 9 which a base is added to neutralise the cake. An in-line pH water 16 is used to control the discharge. Reslurried neutralised material is spray dried in a dryer 30 in which cooling air is supplied directly to the top of an atomiser wheel to reduce discolouration. Air supply to the dryer is heated in three stages for maximum heat transfer efficiency. <IMAGE>

Description

"A method for forming a Cellulose Powder" The invention relates to a method for forming a cellulose powder.

Various methods are known for forming cellulose powders, either as single component powders or as powders which contain more than two cellulosic powders.

Such cellulose powders are widely used in the food and pharmaceutical industries. The chemical make up, particle size and visual appearance of the powders are key factors in producing a high quality marketable product. To achieve maximum production efficiency in methods for producing such cellulose powders it is essential to maximise process conditions to achieve maximum yield, to minimise energy usage and to reduce effluent.

This invention is directed towards providing an improved method for forming such cellulose products.

According to the invention there is provided a method for forming a cellulose powder comprising the steps of: dicing sheets of cellulosic material; introducing water and the diced material into a reactor vessel; heating the water and diced material in the vessel; agitating the contents of the vessel; adding concentrated acid to the vessel; continuing heat addition until hydrolysis is complete; venting the reactor vessel; sampling the product in the reactor vessel to determine the yield of the reaction; filtering the reaction product in a filter press; delivering the filter cake to a reslurry tank; separating air from the filtrate; extracting excess waste heat from the filtrate in a filtrate heat exchanger; recycling hot water from the filtrate heat exchanger to the reactor vessel; monitoring the turbidity of the filtrate; separating the solids from the filtrate in a separator; recycling the separated filtrate solids to the filter feed; discharging liquid filtrate effluent from the separator as determined by the turbidity of the liquid stream; re-slurrying the filter cake; neutralising the reslurried cake with a basic material; determining the pH of the slurry; recycling the slurry until the pH of the slurry is at a desired level; spray drying the slurry in a spray dryer having an atomiser wheel; and recovering by cyclone separation the dry particles of cellulose material thus formed and delivering the particles to a storage hopper.

The air supply to the spray dryer is at least partially heated by excess heat extracted from the cyclone separators.

In a particularly preferred embodiment of the invention, cooling air is supplied to the top of the atomiser wheel of the spray dryer.

Most preferably, the cooling air is supplied to the top of the atomiser wheel of the spray dryer from a dedicated cooling air supply.

In a preferred embodiment of the invention, gases from the cyclones are scrubbed and the liquid produced is recycled.

Advantageously, the liquid produced on scrubbing the gases is recycled to the filter feed.

In one embodiment of the invention filter cake prior to reslurrying is proportionately mixed with another cellulosic material. Typically, the additional cellulosic material is carboxy methyl cellulose.

In this case the dried particles of the mixed cellulose materials are recovered by cyclone separators, the gas from the cyclones being scrubbed and the liquid produced being recycled to the filter.

The invention also provides cellulose powder whenever formed by the method of the invention.

The invention will be more clearly understood from the following description thereof given by way of example only with reference to the accompanying drawing which is a schematic flow diagram of a method for forming a cellulosic powder according to the invention.

Referring to the drawing the method for forming a cellulosic powder according to the invention comprises first dicing in step 1 sheets of cellulosic material.

Water is added to a reactor vessel along the line 3 and the diced cellulosic material is introduced in step 2 into the reactor vessel. The water and diced material are heated in the vessel and the contents of the vessel are agitated. Concentrated acid, in this case concentrated mineral acid, is then added to the reactor vessel along the inlet line 4 and heat is added to the reactor until hydrolysis is complete. The reactor vessel is then vented. The product in the reactor is sampled to determine the yield of the reaction. The reaction product is then led in step 6 to a filter press. The filter cake produced along line 7 from the filter press 6 is delivered along line 8 to a first reslurry tank 9.

In the reslurry tank 9 a basic material is delivered along line 12 to neutralise the filter cake. Water is also added as required. The contents of the reslurry tank 9 are recycled by a pump 13 along a recycle line 15 having an in-line Ph meter 16 which is used to determine the pH of the slurry. When the pH of the reslurried mixture is at a predetermined level the reslurried mixture is discharged.

The filtrate from the filter press 6 is delivered to a heat exchanger 22 and excess heat is recycled along line 29 to the reactor vessel 2. The turbidity of the cooled filtrate is determined by a first turbidity meter 23 prior to delivery to a solids separator 24 which separates solids from the filtrate and delivers the solids along a recycle line 25 to the filter feed. The liquid filtrate is also monitored by a turbidity meter 26. When the turbidity has reduced to a desired level the liquid may be discharged along line 27 for effluent treatment. The use of a turbidity meter to monitor product loss in the filtrate stream has facilitated an improvement in yield, reduction of waste treatment costs and minimisation of waste. Further energy cost reductions have also been achieved by recovering heat from the filtrate stream and using this to heat the water supply to the reactor vessel.

Reslurried neutralised material is delivered to a spray dryer 30 of a type including an atomiser wheel. Cooling air is supplied directly to the top of the atomiser wheel from a dedicated air supply to reduce the charring effects during spray drying and consequently to improve the visual characteristics of the final product. The spray dried particles are delivered to a bank of cyclone separators 31, the recovered particles being delivered to a quarantine hopper 35 for packing.

The air supply to the spray dryer 30 is heated in three stages by a first heat exchanger 36 which typically heats the air to 50 to 700C, a second heat exchanger 37 which heats the air to 220 to 280"C and a third stage heat exchanger 38 which heats the air from 350"C to 4200C. This three stage heating of the air supply to the spray dryer has made a substantial contribution to optimising the energy balance of the method of the invention.

An air stream from the cyclones 31 is delivered along line 40 to a cyclone heat exchanger 41 and is then scrubbed in a scrubber 42 prior to discharge through an exhaust stack 43. Liquid material collected from the venturi scrubber 42 and stack 43 is recycled along line 45 to the filter 6.

The product delivered to the hopper 35 from the cyclones 31 is a single cellulose powder material. Another cellulose material such as carboxy methyl cellulose may be delivered to be mixed proportionately with a first cellulose material from the filter press. The mixed material thus formed is reslurried, spray dried, cyclone separated and enters a hopper in a similar manner to the single product powder described above.

EXAMPLE 1 Microcrystalline Cellulose (MCC) is produced by the partial depolymerisation of an alpha cellulose rich wood pulp at elevated temperatures in the presence of a mineral acid. This hydrolysis destroys the fibrous structure of the pulp and produces a slurry of cellulose in small particle form.

The next step is the filtration of the hydrocellulose slurry. The filtration step is the purification stage where the residual acid and the water soluble by-products (sugars) of hydrolysis are washed out using water.

The washed filter cake is either:1. Mixed proportionately with sodium carboxymethyl cellulose and subsequently dried to the required moisture content or; 2. reslurried with water and neutralised.

The neutralised aqueous slurry is fed to a spray dryer to form a crystalline product comprised of porous crystallite aggregates held together by hydrogen bonding. The powder particle size is controlled by varying the atomisation and drying conditions.

Subsequent separation of the product from the dryer air stream is accomplished in parallel cyclones. Recovery of the residual powder in the air stream is achieved in a venturi scrubber where any fine particles are removed prior to atmospheric discharge.

Waste heat in the exhaust air stream is recovered using a shell and tube heat exchanger.

Dried product is then transferred to product storage for subsequent packaging.

The advantages of the method of the invention include the following.

Recovery of heat from the filtrate stream and its application to heat the water supply to the reactor vessel has resulted in reduced energy costs.

The use of a turbidity meter to measure the level of product loss in the filtrate stream has facilitated yield improvement, a reduction of waste treatment costs, and waste minimisation.

The use of a heat recuperator on the exhaust air stream and its application to preheat the air intake stream of the dryer has resulted in a reduction in energy usage.

The use of a blower to cool the atomiser wheel of the dryer has resulted in an improved colour and more consistent purity of the product. The scrubber to remove product in the exhaust air stream has resulted in a yield improvement, and in minimising waste.

The invention is not limited to the embodiments hereinbefore described which may be varied in detail.

Claims (11)

CLAIMS.

1. A method for forming a cellulose powder comprising the steps of : dicing sheets of cellulosic material; introducing water and the diced material into a reactor vessel; heating the water and diced material in the vessel; agitating the contents of the vessel; adding concentrated acid to the vessel; continuing heat addition until hydrolysis is complete; venting the reactor vessel; sampling the product in the reactor vessel to determine the yield of the reaction; filtering the reaction product in a filter press; delivering the filter cake to a reslurry tank; separating air from the filtrate; extracting excess heat from the filtrate in a filtrate heat exchanger; recycling hot water from the filtrate heat exchanger to the reactor vessel; monitoring the turbidity of the filtrate; separating the solids from the filtrate in a separator;; recycling the separated filtrate solids to the filter; discharging liquid filtrate effluent from the separator as determined by the turbidity of the liquid stream; re-slurrying the filter cake; neutralising the reslurried cake with a basic material; determining the pH of the slurry; recycling the slurry until the pH if the slurry is at a desired level; spray drying the slurry in a spray dryer having an atomiser wheel; and recovering by cyclone separation the dry particles of cellulose material thus formed and delivering the particles to a storage hopper.

2. A method as claimed in claim 1, wherein the air supply to the spray dryer is at least partially heated by excess heat extracted from the cyclone separators.

3. A method as claimed in any preceding claim, wherein cooling air is supplied to the top of the atomiser wheel of the spray dryer.

4. A method as claimed in claim 3, wherein the cooling air is supplied to the top of the atomiser wheel of the spray dryer from a dedicated cooling air supply.

5. A method as claimed in any of claims 1 to 4, wherein gases from the cyclones are scrubbed and the liquid produced is recycled.

6. A method as claimed in claim 5, wherein the liquid produced on scrubbing the gases from the cyclone is recycled to the filter.

7. A method as claimed in any preceding claim wherein the filter cake prior to reslurrying is proportionately mixed with another cellulosic material.

8. A method as claimed in claim 7, wherein the other cellulosic material is carboxy methyl cellulose.

9. A method as claimed in claim 7 or 8, wherein the dried particles of the mixed cellulose materials are recovered by cyclone separators, the gas from the cyclones being scrubbed and the liquid produced being recycled to the filter.

10. A method as substantially hereinbefore described with reference to the accompanying drawing.

11. Cellulose powder whenever formed by a method as claimed in any preceding claim.