FAIR-CT95-1195 A Static Mixer Reactor for Starch Graft Polymerisation (StaMiStAP)

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Biopolymers/Gums : FAIR Area 2.1 - Chemical and Physical Processes



Annual Progress Report 2000 - Scientific Synthesis Report

Source: Progress Report, September 2000

Consortium: This project was co-ordinated by the department of Chemical Engineering, University of Groningen, Groningen (The Netherlands), in partnership with AVEBE BA, Foxhol (The Netherlands), the department of Materials Science and Metallurgy, University of Cambridge, (UK) and BASF Aktiengesellschaft, Ludwigshafen (Germany)

SCIENTIFIC SYNTHESIS REPORT

Introduction

Grafted starch-acrylic copolymers (S-g-A-polymers) are known at laboratory scale. Due to their attractive properties for use as in products such as superabsorbent and detergents; research activities are increasing in this area. This is reflected in a great number of available patents, some of them held by the industrial partners of this project, covering this subject. However, research has not yet been focused on processing methodologies and scale up of processes. Such process engineering is important to estimate the potential market viability of the new products.

Objectives

To obtain the required information the project aimed to:

• develop of a Continuous Static Mixer Reactor (CSMR) for the production of grafted starch (meth)acrylic acid copolymers (S-g-A polymers). The CSMR is attractive because of almost ideal plug-flow behaviour and intense micro-mixing, excellent heat transfer characteristics, the ability to conveniently handle highly viscous liquids, and low energy consumption.
• test the application of products resulting from the new, continuous static-mixer process. Grafting of (meth)acrylic acid onto starch can result in promising products, such as super absorbing non-wovens, detergents or products for the paper industry.

In order to develop this continuous process for these starch polymerisation reactors, apply the static mixer reactor and to optimise the final product properties the project intends to:

• facilitate process development of the CSMR through investigation of the reaction kinetics and hydrodynamics of the reactor
• predict and analyse the material properties of the S-g-A polymers as a function of the molecular structure.

To achieve this the research is sub-divided in three tasks:

• a study of the graft polymerisation reaction
• a study of a bench-scale CSMR for production of S-g-A polymers.
• process evaluation.

Activities

During the fourth year of the project the following tasks had to be carried out:

• determination of the kinetics of the starch-acrylic acid reaction (task 1.1)
• initiation of the kinetic model (task 1.3)
• production of reaction samples and determination of the operational window of the CSMR (task 2.3)
• performing of mixing and hydrodynamics experiments and completion of the rheological analysis (task 2.2)
• characterisation of reaction samples (task 1.2 and 2.3)
• continuation of retrogradation tests (task 3.1)
• start of feasibility and marketability studies (task 3.2)
• finalisation of the analytical procedures (subtask 1.2)

Some of these tasks were carried out within the anticipated time schedule set up at the beginning of the project. However, a considerable part of these were delayed as a result of problems previously encountered in the project (analytic procedure, construction of the CSMR and health of a Ph.D. student). Activities over the reporting period were as described below.

Task 1: A study of the graft polymerisation reactor

Subtask 1.1: Kinetics of starch-(meth)acrylic acid polymerisation These activities were postponed until the amount of homopolymer could be determined accurately. Such determination is vital for the interpretation of the kinetic experiments. Due to the nature of the experiments and the analytical procedure, the kinetic research has been delayed.

Subtask 1.2: Characterisation of the samples The final problems in the analytical procedures were solved. It is now possible to determine the percentage of grafting of acrylic acid on the starch, the amount of free acrylic acid and the amount of homopolymer formed during the reaction.

Subtask 1.3: Kinetic modelling Participants: No kinetic experiments were performed as indicated under subtask 1.1. However, extra measurements and DSC experiments will be performed shortly to enable a kinetic model to be set up. The development of an exact and accurate expression for the kinetic rate equations may be obstructed by the relatively large error in the analysis. Therefore an extensive literature search was carried out in order to obtain feasible kinetic equations for the consecutive reactions of grafting and homopolymerisation.

Task 2: A bench scale CSMR for production of S-g-A-polymers

Subtask 2. 1: Construction of a bench scale CSMR Although, as reported previously, it was believed that the construction of the CSMR had been finalised, unexpected problems were encountered during initial reaction runs. The main problem occurred with the acrylic acid pump. Within half an hour from the start of the experimental runs the pump started making noises and the throughput decreased gradually. Therefore no experimental data could be obtained with this pump.

An extensive search resulted in identification of an alternative pump that worked, but limited the range of experiments that could be performed. However, it appeared to be the only feasible solution, taking into account the limited the time remaining, once the pump arrived in August . The first samples of grafted starch have now been produced.

Subtask 2.2: Hydrodynamics of the CSMR The University of Cambridge completed rheological measurements on the starch gelatinisation process and the grafting reaction. The effects of these parameters on the rheological behaviour were translated into equations describing the viscosity of the mixture in the reactor as a function of shear. These equations can be used to model the hydrodynamics of the Static Mixer Reactor.

Subtask 2.3: Study of the operation characteristics of the CSMR

No new measurements were performed due to a lack of reactor samples. The absence of reactor samples was due to problems mentioned in subtask 2.l.

Task 3: Process Economics

Subtask 3.l: Stability of the produced S-g-A polymers

The absence of reactor samples also made further retrogradation tests impossible. Due to the limited research time left and the status of the project the sub-contracting to test the biodegradability of the products will not be carried out, since biodegradability tests need a considerable running period. Instead the funds that were set aside for these tests were used to buy the new acrylic acid pump, with approval of the EC.

Subtask 3.2: Feasibility and marketability studies

As with subtask 3. 1, the lack of reactor samples resulted in no progress in this area.

Results

Progress was made in a number of task areas, as follows.

Subtask 1.2: Characterisation of the samples The final problems with the analytical procedure were solved, so that it is now possible to determine

• the percentage grafting of acrylic acid on the starch
• the amount of free acrylic acid, and
• the amount of homopolymer formed during the reaction.

The current analytical procedure consists of the following steps:

• samples are separated into two sub-samples of 5 grams and 10 grams, used to determine the amount of acrylic acid (AA) and the grafting percentage of PAA on starch
• analysis of AA -1-propanol is added to the 5 g sub-sample, which results in precipitation of the grafted starch. The sample is centrifuged and the supernatant is diluted and analysed using two special LC columns in series to determine the amount of AA and PAA.
• analysis of the amount of grafted starch - the 10 g sub-sample is treated with acid and an acetone-water mixture. After centrifugation and drying the sample is analysed using proton-NMR. To do this the sample is suspended in deuterated water (D₂O). The relation between the peak height of PAA and starch in the NMR spectrum give a good estimate of the amount of PAA grafted on the starch.

Subtask 1.3: Kinetic modelling Participants: No kinetic experiments were performed. However, extra measurements and DSC experiments will be performed shortly to enable a kinetic model to be built up. The development of an exact and accurate expression for the kinetic rate equations may be obstructed by the relatively large error in analysis, hence a literature search was carried out, as detailed above.

Subtask 2. 1: Construction of a bench scale CSMR After a few successful runs problems started occurring with the Moyno pump. No reproducible throughputs could be obtained and throughout the experiments the pump capacity became worse. After a number of checks the cause of the problem was established. A bearing had started to move and had damaged the inside of the pump. After solving this problem throughput-pressure calibration curves for starch-water mixtures were measured and found reproducible. However, then a more serious problem occurred with the acrylic acid pump. Within half an hour after the start of the experimental runs the pump started making noises and the throughput decreased gradually. Therefore no further experimental data could be obtained with this pump.

As the experimental set up of the SMR reactor is quite unique no direct alternative for the existing pump could be found. Therefore several experiments were performed to explore the working limits of this pump and try and improve its performance. These experiments were performed in consultation with BASF and consisted of:

• The treatment of the pump with a speciality polymer of BASF to prevent the polymerisation of AA in the pump.
• The addition of acetone as a cleaning fluid to the pump during the pumping of acrylic acid.
• The addition of extra inhibitor to the acrylic acid to prevent polymerisation in the pump.

None of these experiments worked and after each trial the pump had to be completely overhauled, which took a lot of time. Therefore, it was concluded that an alternative pump was absolutely necessary in order to be able to run the reactor. After an extensive search, an alternative pump was found. However, its capacity was such as to limit the range of experiments that can be performed. Use of this pump was felt to be the only feasible solution for the time remaining. The pump was obtained August, since when the first samples of grafted starch have been produced.

Subtask 2.2: Hydrodynamics of the CSMR, rheology of the starch Further analysis was completed covering the rheology of the starch and grafted starch products. From the gelatinisation experiments, it could be concluded that thermal gelatinisation differs from gelatinisation with NAOH, although NAOH obviously influenced the viscosity progress of gelatinisation. A comparison of the experimental results indicated that the behaviour on cooling is similar in all cases. The presence of NAOH in the starch slurries promotes low temperature gelatinisation of the starch granules, but weakens the resultant gel structure. The effect of the shear and temperature on the rheological behaviour of the pure starch, the reaction mixture and the product were investigated and translated into equations describing the viscosity of the mixture in the reactor as a function of shear. These equations can be used to model the hydrodynamics of the Static Mixer Reactor.

Discussion

During the fourth of the project a number of positive steps vital for a good end result of the project have been taken, solving problems with the analysis and the reactor (pump problems). As a result samples have been produced and their characterisation initiated, including investigation of the stability of the reaction products. Mixing and hydrodynamics experiments have carried out as has the rheological analysis.

The problems with the analytical procedure influenced the start of the kinetics experiments and the set up of a model. Kinetic experiments were performed from which one initiator system, the Fentons reagent, was selected. With this system more extensive experiments are still required in order to set-up a kinetic model. Therefore, the reaction will be studied with DSC and special attention will be paid to the progress of the reaction in the first five minutes. After collecting these data an equation for the grafting reaction and the homopolymerisation can be developed.

The construction of the CSMR was anticipated to take less than 1.5 years. Due to the various problems, the reactor was not available until the latter part of the fourth year. This delay has caused limitation in the performance of a number of subtasks, including hydrodynamic studies and the characterisation of samples).

It appears now that most of the experimental problems that have previously been encountered have finally been solved. Hence, the project can continue with most of the remaining tasks and try to complete them as far as possible.

Future actions

The focus in the remaining time will be on determination of the operational window of the Static Mixer Reactor and the characterisation of the resulting. This will enable the feasibility and applicability of the process and the products to be determined. If no other serious problems are encountered, it is anticipated that the work on the hydrodynamics of the SMR can also be completed. Hence, over the last reporting period the following tasks have to be performed:

• Produce reaction samples and determine the operation window of the CSMR
• Characterise reaction samples
• Start feasibility and marketability studies
• Perform mixing and hydrodynamics experiments
• Carry out retrogradation tests
• Establish a kinetic model

Extension

An extension of the project has been requested to compensate for the delays.