Cooling water facilities at phosphoric acid plants

PHOSPHORIC ACID TECHNOLOGY

Cooling water facilities at phosphoric acid plants

Wet process phosphoric acid plants require reliable cooling water facilities. Jan Tytgat, engineering manager, De Smet Agro, shares his insights on the design and operation of cooling water networks, pumps and towers.

A line of eleven cooling tower cells at a phosphoric acid plant, each cell having its own belt-driven, six-blade fan.

PHOTO: DE SMET AGRO

The increasing importance of cooling towers

Cooling water is required at various sections throughout the phosphoric plant (see box). It can be sourced in one of two ways – either being directly taken from nearby open water (such as the sea, a lake or river) or supplied using a dedicated cooling tower.

Hot sea, lake or river water is simply returned to its original source. Hot cooling tower water, in contrast, is sent back to the cooling tower. The temperature of returning hot water is then brought down – thanks to large quantities of air carried by cooling tower fans – enabling it to be pumped back as cold cooling water to the various users in the plant.

In recent times, cooling towers at phosphoric acid plants have become increasingly prevalent, as the popularity of seawater or river water cooling has declined due to their associated negative environmental impacts. This article offers practical guidelines for cooling water use at wet phosphoric acid plant and highlights:

• Cooling water quality
• Cooling water networks
• Cooling water pumps
• Cooling tower design.

Cooling water quality

In phosphoric acid plants, direct contact condensers are widely used for cooling purposes downstream of the reaction flash cooler, the vacuum filter and concentration evaporators (see box). Cooling water in these condensers will partly adsorb the fluorine present in phosphoric acid when this is released as vapour under vacuum. Almost inevitably, cooling water will pick up entrained droplets of P2 O5 as well.

In many phosphoric acid plants, extra equipment is therefore provided – mainly at the concentration section – to catch the carry-over of P2 O5 droplets and reduce the fluorine content of the cooling water. For example:

• The installation of a P2 O5 droplet separator downstream of the evaporator will contribute to a concentration section P2 O5 efficiency of 99.5-99.9 percent.
• Single or double fluorine absorption towers can recover fluorine as fluosilicic acid (FSA). These are sometimes followed by an extra FSA droplet separator installed upstream of the condenser.

The installation of the above equipment, and proper blow down control, can reduce cooling tower maintenance and operational headaches by keeping cooling water P2 O5 content and fluorine content as low as possible. Reducing fluorine content lowers fluosilicate deposition on cooling tower piping. This is beneficial as the resulting scaling is very hard and difficult to remove.

Cooling water is typically acidic (pH <2), contains 0.5-1 percent fluorine and 1-2 percent solids, together with traces of P2 O5 and chloride. This is the reason why phosphoric acid plants have their own dedicated cooling towers – and do not use a common cooling tower with a sulphuric acid plant, for example.

Due to the returning water’s acidic nature, cooling via a counter flow forced draft is highly recommended. This uses a fan in a vertical position located at the bottom of the cooling tower. A classic induced draft counter flow or cross flow cooling tower is not recommended. This is because, with the fan in a horizontal position on top of the cooling tower, the fan blades would be subject to an acidic wet air stream and acidic drift loss.

The acidic cooling tower system is completely integrated into the water balance of the phosphoric acid plant. Make-up water (needed to compensate for evaporation loss, blow down and drift loss etc.) can be provided, for example, by re-using the vacuum pump outlet seal water. The blow down – which is required to control and limit the amount of dissolved salts and the presence of fluorine in the cooling water – is not lost. It can re-used in the process as part of the cake washing water, for example.

Cooling water networks

The heat load for a standard DH plant is typically split between one-third for the reaction-filtration section and two-thirds for the concentration section (i.e., for a single-crystal Di-Hydrate process producing 28% P2 O5 filtered acid and 54% P2 O5 concentrated acid). The cooling tower is usually installed close to the concentration units to minimise the cooling water network.

Cold water is pumped from the cold cooling water basin, located near the cooling tower, to the various condensers in the plant via an above-ground or underground piping network. Hot cooling water, on the other hand, can be returned to the cooling towers via channels or piping – or a combination of both – depending on the layout constraints.

In a classic scheme, all the plant’s hot cooling water is returned to the hot cooling water basin by gravity via a common underground brick lined concrete channel (Figure 1). This arrangement (Case 1) is ideal for smaller phosphoric acid plants (less than 500 t/d P2 O5 ) where the reaction, filtration and concentration areas are usually located close together. Pumps are used to move the hot cooling water from the hot water basin to the inlet nozzles of the cooling tower cells.

However, in larger phosphoric plants – where the reaction-filtration area is more isolated and located far away from the concentration section – a combination of gravity piping and hot water return piping & pumps can be used (Figure 2). In this arrangement (Case 2), the concentration condensers and seal tanks are elevated well above the top level of the cooling tower. This allows hot cooling water to flow down from these seal tanks under gravity through a large pipe towards the cooling tower inlet nozzles. The hot cooling water from the reaction-filtration area is pumped back to join the flow through the large gravity pipe towards the cooling tower. This arrangement is very valuable if underground channels are not desirable or possible due to lay-out constraints, e.g. where overhead pipe racks are used to avoid crossing roads with channels.

Fig. 1: Cassic hot water return scheme via channels (Case 1)

Fig. 2: Hot water return piping scheme (Case 2)

Table 1: Three different hot cooling water return arrangements, advantages and disadvantages.

Of course, an arrangement combining channels (Case 1) and return piping (Case 2) is also possible. In this hybrid configuration (Case 3), concentration condenser seal tanks are kept at ground level and use a hot water return channel to link with the hot cooling water basin. Hot cooling water from the remotely located reaction-filtration area, meanwhile, is pumped back via piping to join the hot water return channel from the concentration section.

Each arrangement (Case 1, Case 2 and Case 3) has advantages and disadvantages, as summarised in Table 1.

The Case 2 arrangement has the following drawbacks, leading to extra costs, compared to Case 1 and Case 3:

• The concentration building needs to be elevated by 12-14 metres because the condenser seal tank, and hence the condenser and barometric leg (the down pipe between the condenser outlet and seal tank), needs to be elevated to enable gravity flow from the seal tank towards the cooling tower inlet nozzles.
• The cold cooling water pumps feeding the concentration condensers need a higher head (15 to 20 mLC) and hence larger motors.

On the other hand, depending on the phosphoric acid plant’s specific layout constraints, Case 2 could offset these extra costs by avoiding the need for:

• Costly brick lined concrete channels with handrails or covers
• A hot water basin
• Hot water pumps & motors.

Cooling water pumps

The reaction-filtration and concentration areas can both use a common cold water network and common cold water pumps when the condenser seal tanks are all located at the same level. In other plant layouts, dedicated cold cooling water pumps and piping are necessary for each section. For example, a dedicated system for the concentration condensers is recommended where the concentration condenser seal tanks are elevated due to a different head.

In plants with more than two or three parallel concentration lines, it is very convenient to select and match cold cooling water pump capacity to the condenser’s cooling water requirements. Then, if one concentration line is placed offline, the corresponding cold cooling water pump can be shut down too.

Arranging hot and cold cooling water pumps in a manifold line-up maximises operational flexibility. If desired, one standby pump can be easily installed via a common header.

Vertical or horizontal centrifugal pumps can be used to distribute cold cooling water and bring hot cooling water back to the cooling tower inlet nozzles. Vertical and horizontal pumps both operate at a similar efficiency of about 78-84 percent.

Vertical pumps, although usually more costly, are simpler to install as no suction piping is required (Figure 3). Horizontal pumps, meanwhile, can be installed on the basin bottom level (Figure 4). This arrangement does need an extra concrete pump pit, although it avoids the dedicated priming system required when pumps are located on the ground floor (Figure 5). Each of these cooling water pump arrangements have their own advantages and drawbacks, as summarised in Table 2.

Fig. 3: Vertical cooling water pumps.

Fig. 4: Horizontal cooling water pumps – installed at basin bottom level.

IMAGES: DE SMET AGRO

Fig. 5: Cooling water pumps – installed at ground level.

Fig. 6: Forced draft cooling towers cells in a back-to-back layout.

IMAGES: DE SMET AGRO

Table 2: Three different cold cooling water pump arrangements, advantages and disadvantages

Fig. 7: A seal between the cooling tower cell’s cold water outlet and channel to the sump prevents air loss

Cooling tower design

Specific construction materials are used to combat the acidic nature of the cooling water. The cooling tower structure consists of reinforced concrete with anti-acid protection. The anti-acid lining at the bottom of the tower and the base of the walls (a few bricks high) is protected with carbon bricks. A timber, concrete or polyester structure inside the tower support the grids or packing, the distribution channels and the drift eliminators. The grids are usually made of PP and are hung with 904L metallic wire to avoid corrosion. UV resistant PVC wave-type sheets located on top of the cooling tower cells are used for the drift eliminators.

Cooling towers consist of several cells arranged in parallel. Each typically receives a water inlet flow of about 500-1,000 m3 /h, the exact value depending on the cell size selected. The provision of one spare cell is generally recommended.

As already noted for the cold cooling water pumps, for plants with more than two or three parallel concentration lines, it is convenient if the cooling water required for one concentration line is equal to the flow sent to an exact number of cooling tower cells, e.g. 2-4 cells per concentration line. Then, when one concentration line is shut down, the corresponding cooling tower cells can be isolated and their fans stopped.

Cooling tower cells can be placed next to each other in one single line. Alternatively, they can be placed in two back-toback rows if there are space constraints (Figure 6). In either case, it is very important that enough free area is provided in front of the fan – a minimum distance of double the fan height – and that no tall structures are installed near the cooling tower. Hot saturated air leaving the cooling tower must be able to disperse as freely as possible, as any recirculation to the inlet of the fan stacks would decrease cooling tower performance.

Hot cooling water needs to distributed as equally as possible between cells. To this end, each cell is equipped with a manual valve whose position is set and checked during cooling tower start-up. Internally, water also needs to be distributed equally over the entire cell. There are several ways to do this. A concrete channel can be positioned in front of each cell, for example, as shown in Figure 6. Several perpendicular and parallel FRP (fibre-reinforced plastic) channels bring the water from this channel to spray nozzles located below. Alternatively, closed plastic piping can also be used to feed the spray nozzles. Channels are, however, more maintenance-friendly as visual inspection and cleaning is relatively easy. The fan stacks surrounding the fans blades are fixed to the concrete wall. These are covered with a stainless-steel mesh to prevent injury when the fans are in operation.

The loss of air through the cooling tower’s cold water outlet can be prevented by using a seal between the cell and the cold water channel that connects with the sump (Figure 7).

Conclusions

This article highlights the different options for the cooling water facilities at wet process phosphoric acid plants. This includes the type of cooling water network – channels, piping or both? – the configuration of cooling water pumps, as well as cooling tower design. Exactly what these facilities will look like depends mainly on the plant layout and its constraints. For phosphoric acid plant designers and producers, it is also important to understand the design parameters used by cooling tower suppliers when selecting the optimal cooling tower for your plant.