Manufacturing Processes

6.2.1. Traditional Spray-Drying Process

In the first step of the traditional spray-drying process (Fig. 76) a slurry of thermally stable and chemically compatible ingredients of the detergent is prepared. Solid and liquid raw materials are drawn off from silos or tanks and introduced batchwise into scales. Water is added as required to maintain a manageable viscosity. The liquid mixture and the solids are taken from the scales and mixed to form a slurry in a crutcher or by using some other type of forced mixer. The slurry is transferred to a stirred storage vessel from where the process runs continuously.

The slurry is transported by a low- or medium-pressure pump (up to 80 bar).

Changes in pressure in the high-pressure portions of the system are compensated for by using an air vessel. The slurry is sprayed into the tower by two spraying levels connecting a series of nozzles. The number and type of nozzles must be designed such that overlapping of spray cones is avoided. Dried tower powder flows off from the tower at a temperature of 90 – 100 C. To prevent lumping, an airlift is used for cooling.

Table 39. Detergents manufacturing process alternatives

Process Drying step Densifying step

Tower process Spray drying Dry mixing/milling

Nontower (mixer(s))-process – fluidized bed

– chemical binding with e.g. zeolite, soda ash

– neutralization with soda ash and chemical binding

– integrated in the high granulation step or

– one part is spray dried – another part is granulated with

water and/or surfactants – third part is dry mixed

– without densifying

– integrated in the high shear gran-ulation step, or

– mixture is . extruded . tableted . roller compacted

Compound process (mixer(s)) Compounds are – fluidized bed dried – roller compacted – granulated with surfactants

– integrated in the high shear gran-ulation/extrusion/roller compac-tion step

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The spray-drying process can be run cocurrently or, most common, countercurrently [159]. If particles and air move cocurrently and the slurry enters the hottest zone of the tower, a rapid evaporation of water occurs, and blown-up, relatively light particles (beads) result. In the countercurrent process, drying starts in an area of high humidity and lower temperature, and beads are obtained that are characterized by thicker walls.

The lower free-fall speed due to the countercurrent airstream results in a higher number of particles in the tower at any time. Agglomeration products resulting from a countercurrent system are heavier and coarser than those resulting from cocurrent spray drying. Figure 76 shows a countercurrent operation. The air is directly heated by exhaust gases from an oil- or gas-fueled burner and led into the tower at about 300 C through a ring channel. Appropriate design of both the air inlet zone above the cone and the air outlet at the tower dome is important. Air entering the tower is “swirled”, i.e., it is accelerated tangentially and vertically. This causes homogeneous heat transfer between air and product. Water vapor, combustion gases, and drying air are withdrawn from the tower by means of a ventilating fan.

Figure 76. Traditional spray-drying process

a) Storage tanks for liquid raw materials; b) Storage silos for solid raw materials; c) Liquids weighing vessel; d) Solids weighing vessel; e) Mixing vessel; f ) Intermediate tank; g) Booster pump; h) High-pressure pump; i) Air vessel;

k) Nozzles; l) Airlift; m) Storage bunker; n) Belt conveyor scales; o) Powder mixer; p) Sieve; q) Packaging machine;

r) Air inlet fan; s) Burner; t) Ring channel; u) Spray tower; v) Top filter; w) Exhaust

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Fine particles are drawn off from the tower along with the exhaust gas and collected in a filter. In modern plants this filter is installed on top of the tower so that the separated fines and dust fall down into the agglomeration zone of the tower [160]. The resulting tower powder has a low content of fine particles. Heat exchangers are installed to use the exhaust gas energy for preheating the burner air. Another way is partially recycling the exhaust air (about 50 %) and mixing it with fresh air from the atmosphere [161]. In doing so, energy savings of about 10 – 20 % can be achieved. Plant control is possible by installation of an expert system [162].

6.2.2. Superheated Steam Drying

Environmental issues of spray drying using hot air are:

Emission of dust, normally 5 10 mg/m³

Emission of odor and volatile organic matter in very small amounts, normally 5 50 mg/m³

Potential dust-explosion and fire hazard Loss of vaporization heat

These problems can be solved by switching over to drying with superheated steam instead of air [163]. The usage of superheated steam allows the heat of vaporization to be recovered [164]; the system is closed and no exhaust gas escapes from the tower [165]. Presently, this technology is being adapted in a pilot project [166].

The bulk density of steam-dried products is significantly higher and the water content is lower compared to hot-air spray drying.

6.2.3. Nontower Agglomeration Process

The nontower agglomeration process is carried out in a continuous mixer such as the Ballestra Kettemix Reactor or the L
dige CB-Mixer or in a combination of high- and low-shear mixers. Zeolite is applied in powdered form to bind surplus water, especially as zeolite P or X or as superdried zeolite A plus soda ash. In the first step the powder components are continuously added. Metering is carefully controlled at the entrance of the reactor. In this zone all liquids, such as nonionic surfactants, polymer solutions, fatty acids, sodium silicates, and linear alkylbenzenesulfonic acid, are separately dosed [167].

Due to the high energy input granules are formed and at the same time neutrali-zation with sodium carbonate or sodium hydroxide takes place. The granules can take up limited amounts of nonionic surfactants. Optionally the granules are surface-treated with zeolite (“powdered”) in an additional step to reduce product stickiness, which is mainly due to the nonionics. If the water binding capacity of zeolite and soda ash is not sufficient, e.g., in the case of neutralization with sodium hydroxide solution, added

ProductionofPowderDetergents

water and reaction water have to be removed from the product in a further step. This is done in a fluidized-bed dryer, where the granules are dried with hot air (140 C) in the entrance zone. In the outlet zone the granules are cooled down, normally using cold air (5 – 15 C). The drying process is controlled by measuring the product water content and temperature. Depending on the formula the product temperature has to be maintained between 50 and 110 C (preferably between 60 and 80 C).

If the water balance is fitted by using small amounts of water and/or by neutral-ization with carbonates the granules are only cooled in the fluidized bed and not dried.

The fluidization air is transported in a cycle with a cooling/heating step to ensure a constant water content of the air.

After sieving and milling, the granules are transported to the postaddition step.

Quite a few limitations with respect to the nontower agglomeration process exist.

Firstly, the neutralization heat increases the granulation temperature. Therefore, the amount of acids in the feed material is strongly limited. Secondly, the water level must be lower than 14 %. The nonionic surfactant content depends on the detergent com-position. At higher nonionics contents the granules become too soft and sticky. A typical nontower agglomeration process with one mixer and a fluidized-bed dryer is shown in Figure 77 as a block diagram. The more advanced variant with two mixers and a fluidized-bed cooler is illustrated in Figure 78 as a flow sheet diagram. The process uses a high-shear/low-shear mixer combination. Neutralization is effected by using 50 % caustic soda.

Figure 77. Nontower agglomeration process

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6.2.4. Nontower Compound Technology

Another way of realizing a nontower process is application of compound technology.

A finished laundry detergent is produced by blending the following components [168]:

Nonionic surfactant agglomerate 13.4 wt %

Anionic surfactant agglomerate 32.5 wt %

Layered silicate compacted granules 10.1 wt %

Granular percarbonate 22.7 wt %

Tetraacetylethylenediamine agglomerate 7.8 wt %

Suds suppressor agglomerate 6.5 wt %

Perfume encapsulate 0.1 wt %

Granular soil release polymer 0.4 wt %

Granular sodium citrate dihydrate 3.5 wt %

Enzymes 2.0 wt %

Each component is produced separately, in most cases by using the granulation technology. By applying different densification technologies an inhomogeneous product aspect results after blending of the compounds. Compound blending is presented in Figure 79.