The choice of process route and the nature of equipment used during the manu- facture of detergents depend on the detergent product. Commercial detergent products are formulated as liquids or solids (powered form).
1.4.1 Powdered Detergents
They are produced by spray drying, agglomeration, dry mixing or a combination of both.
1.4.1.1 Spray Drying
A significant portion of powdered detergents is manufactured by spray drying. The process is made up essentially of the following steps: (i) selection of formulations, (ii) slurry preparation, (iii) slurry atomization, (iv) drying, (v) conditioning of the product, (vi) post additions and (vii) packaging. Theflowchart of the process is shown in Fig.1.6.
Materials to be selected for the formulation depend on the nature of product required and on the processing route and equipment to be adopted. Once the formulation is determined, the next and the most important step in the spray drying process is the slurry preparation. The slurries are prepared in crutching units. The process may be batch, semi-batch or continuous. To deliver a product of consistent quality, the order of addition of the raw material, the concentration of solids in the slurry, slurry temperature, viscosity and aeration must all be carefully controlled.
Solid levels of the slurry are kept between 64 and 72% to reduce the heat requirement during the subsequent treatment in the spray tower, and also to Table 1.11 Typical
ingredients found in toilet bowl cleansers
Type I: Liquid toilet bowl cleansers
Hydrochloric acid, nonylphenol polyethyleneglycol ethers, cetyl dimethylbenzylammonium chloride, sodium silicate and water.
Type II: Powdered toilet bowl cleansers
Sodium hydrogen sulfate (bisulfate), linear alkylbenzene sulfonate, sodium chloride and sodium carbonate/bicarbonate.
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maximize the tower output. After the slurry is prepared, it is charged to a homogenizer where any large lump of materials or gritty particles in the slurry are desized and screened to prevent plugging of the spray tower nozzles during atomization.
Subsequently, the slurry is deaerated in a booster pump and thereafter pumped through nozzles placed at the top of the spray tower to be atomized. Pump pressures between 31 and 124 atmospheres are employed during atomization. A spray tower is essentially a contained heat source that removes moisture at a uniform rate dependent upon the quality and quantity of feed to the dryer. If feed levels are not controlled, variability in the density, moisture level, aging and/or packaging char- acteristics increase.
As shown in Fig.1.7, the atomized droplets of the slurry enter at the top of the spray tower where either in a countercurrent or concurrent mode, it is contacted with a hot gas which dries the droplets by evaporating water from them. The slurry feed rate is determined by the slurry feed facilities, solid level and drying capacity.
In general, a countercurrent tower will produce a higher density detergent particle than a concurrent tower. In most towers, the airflow rate and the temperature are adjusted at the entry ports to maintain balanced air and temperature patterns. There are numerous advantages to be accrued from the use of the spray towers to produce powdered detergents. For instance, the product density can be varied, the detergent granule solubility is considerably improved, increased production rates are Fig. 1.6 Flowchart for
making powdered detergents by spray drying
1.4 Manufacture of Detergents 31
achievable and multiple formulations can be obtained using the same equipment.
The only major disadvantages of the spray tower are its initial high capital cost, the energy-intensive nature of the process and the relative inflexibility of some spray tower units. The product from the spray tower is usually hot in the temperature range of 75–120 °C and has free surface moisture levels of between 1 and 3%. If the powder is left in this state caking or productflow problems will develop. To avoid this, the hot powder isfirst conditioned or cooled to an appropriate temperature. As afinal step, other additives like perfume and heat-sensitive ingredients, for instance, enzymes, surfactant and bleaches are added to the conditioned product. The post addition is accomplished by metering out the additives and the product from the Fig. 1.7 An illustration of the spray drying chamber. The hot air and the feed are fed in a countercurrent mode
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spray dryer into a rotary drum or some other mixing device where they are thor- oughly mixed to a uniform product before packaging. The post addition ingredients usually comprise about 2–15% of the total detergent product.
1.4.1.2 Agglomeration Processing
Agglomeration consists of blending dry raw materials with liquid ingredients in the presence of a binder to form attractive granular product with enhanced aesthetic and flow characteristics. The essential steps in this process are shown in theflowchart of Fig.1.8.
The dry raw materials are first premixed and thereafter metered into an agglomeration unit where it is sprayed with surfactants or silicates to begin building the agglomerated particles. The wet product from the agglomerator is subsequently conditioned to remove excess moisture and thereafter screened to remove oversized particles. Finally, it is blended with additives such as fragrance, bleaches, enzymes and dyes under agglomeration conditions.
Equipment used for agglomeration include horizontal ribbon and vertical mixers, rotary drums, zig-zag mixers and pan agglomerators. Horizontal ribbon mixers are used primarily for‘dry mix’formulation, where only limited agglomeration occurs.
The operation could be batch or continuous. Vertical mixers use high-speed agi- tation to intimately mix and uniformly blend liquids and dry raw materials prior to
Fig. 1.8 Flowchart for making powdered detergents by agglomeration processing
1.4 Manufacture of Detergents 33
discharge. The contact time is less than 5 s. Vertical mixers are preferred due to low space requirements and high efficiency of operation. The rotary drum units use baffles that‘roll’or‘lift’the product for uniform dispersion of liquid feeds. Pan and zig-zag agglomerators are used for small-scale specialty product manufacture.
The vertical and rotary drum agglomerators are preferred in the detergent industry because of improved agglomeration, increased liquid feed dispersion levels and optimized product uniformity. These units can also be used for post addition of raw materials.
1.4.1.3 Dry Mixing (Dry Blending) Processing
This process is used to prepare detergents from dry raw materials by mixing them in a suitable unit. The raw materials arefirst metered and fed into a blender where they are thoroughly mixed to a uniform product. A little amount of water is sometimes added to facilitate the blending process.
1.4.2 Preparation of Liquid Detergents
Liquid products contain approximately 50–60% water, with the remainder being a combination of surfactants, builders, foam regulators, hydrotropes, anti-redeposition agents, whitening agents, corrosion inhibitors, colorants and perfume. They may sometimes also contain antistatic and fabric-softening ingre- dients. To achieve a stable product, careful selection of ingredients and blending of the raw materials are required. For liquid detergents, the following variables must be carefully controlled:
Viscosity: The product must be pourable and must have consistent viscosity characteristics from batch to batch. This can be achieved by selecting the proper solvent of hydrotrope.
Clear-cloud point: Hazing of product and separation of the product upon storage in cool temperature should be avoided. This can be achieved if the composition has sufficient solubility.
Freeze–thaw stability: The formulation should be such that it does not freeze, nor phase separate at freezing temperatures.
Liquid detergents are produced in relatively simple equipment batchwise or by continuous methods. The raw materials, solid and liquids are carefully metered into a blender where they are thoroughly blended. In order to ensure uniformity and stability of the finished product, stabilizers are added during the manufacturing process. In-line or static mixers are the main equipment used. High concentrated liquid detergents are produced by using high-energy mixing processes in combi- nation with stabilizing agents.
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