**2.2 Process description**

Ammonia is the basic raw material in urea production. Ammonia plants in question operate using Uhde's proprietary ammonia process that is based on the well-established Haber-Bosch process. In the first stage, the raw material natural gas is desulphurized, then cracked into its individual chemical components catalytically by adding steam to generate the hydrogen required for ammonia synthesis. This process also generates carbon monoxide, carbon dioxide, hydrogen and residues of methane from the natural gas cracking process. In the next stage nitrogen is added to the process by combusting methane, CO and H2 using air. With the addition of steam, carbon monoxide is converted to CO2 using catalytic converters and then scrubbed out of the synthesis gas formed. The selectively scrubbed CO2 is fed into the urea processing plant as the process medium together with the produced ammonia as starting material. The urea plants operate using the Stamicarbon process that was developed in the Netherlands [Uhde].

modeled plate geometries from the new technology (NT) series with larger gap velocities due to better fluid distribution over the plates could decrease fouling and increase the

To guarantee production reliability in the complex urea fertilizer manufacturing process, PHEs are installed in several process chains including CO2 cooling, residual gas scrubbing, and other process sections as were as in the primary urea production plant. Industrial processes commonly use water for cooling purposes. Open circuit cooling system is used in some processes, while closed loop system involving cooling towers is used in others. Closed loop systems usually cause less fouling than open ones, but they are more expensive (Kukulka and Leising, 2009). Cooling water normally contains dissolved or suspended solids like calcium carbonate and calcium sulphate. If the concentration of these dissolved solids exceeds certain limits, it leads to the accumulation of deposits on the heat exchanger surface (Müller-Steinhagen, 1999). These deposits create an insulating layer on the surface of the heat exchanger that decreases the heat transfer between the two fluids. The thermal performance of the unit decreases with time as the thickness of the deposit increases, resulting in an undersized heat exchanger and causing the process efficiency to be reduced (Kukulka and Leising, 2009). Deposit formation can be reduced either by changing the

Deposit formation is influenced by the heat exchanger surface and geometry, cooling medium and the operating conditions. Its composition depends on the flow rate, temperature and chemical composition of the cooling medium (Kukulka and Leising, 2009). Pana-Suppamassadu et al. (2009) studied the effect of plate geometry (contact angle) and the gap velocity on calcium carbonate fouling in plate heat exchanger. They found that an increase in the gap velocity could reduce the fouling rate on the surface of plate heat

In the present section, deposit formation on the surface of plate heat exchangers in different Egyptian fertilizer plants will be investigated. The effect of heat exchanger geometry and

Ammonia is the basic raw material in urea production. Ammonia plants in question operate using Uhde's proprietary ammonia process that is based on the well-established Haber-Bosch process. In the first stage, the raw material natural gas is desulphurized, then cracked into its individual chemical components catalytically by adding steam to generate the hydrogen required for ammonia synthesis. This process also generates carbon monoxide, carbon dioxide, hydrogen and residues of methane from the natural gas cracking process. In the next stage nitrogen is added to the process by combusting methane, CO and H2 using air. With the addition of steam, carbon monoxide is converted to CO2 using catalytic converters and then scrubbed out of the synthesis gas formed. The selectively scrubbed CO2 is fed into the urea processing plant as the process medium together with the produced ammonia as starting material. The urea plants operate using the Stamicarbon process that

configuration of the heat exchanger or by regular cleaning procedures.

flow patterns on the fouling behavior will be shown.

was developed in the Netherlands [Uhde].

availability of fertilizer plants.

**2.1 Introduction** 

exchanger.

**2.2 Process description** 

Fig. 3. Ammonia process [Uhde].

In the CO2 scrubbing process three plate heat exchangers are switched in parallel, two in operation (A and B) and one in standby (C). Figure 4 shows the three coolers with their operating conditions. The CO2 flows into the PHEs as a gas-steam mixture at 94 °C and is cooled down in a countercurrent process to 33 °C. Water at 30 °C is used as coolant. Each of the 10 tons and 3 meter high PHEs has 1000 m² of high-performance stainless steel (1.4539; AISI 904 L) VT-plates. The transferred heat capacity is 14.5 megawatts.

Fig. 4. CO2 coolers used in the scrubbing process.

Fouling in Plate Heat Exchangers: Some Practical Experience 539

the second is calcium phosphate (11%), which participated as a result of the increase of the

plate surface temperature resulting from the reduction in the cooling water flow rate.

Table 3. (a) Ashing results, (b) Elemental analysis as oxides using XRF.

increased from 0.30 to 0.42 m/s, as can be seen in Table 4.

**2.4 Technical solutions** 

**2.4.1 Redesigning of the PHEs** 

Loss at 500°C 14 % Loss at 925°C 23 % (a)

**Substrate Mass %**  Magnesium (MgO) 3 Aluminium (Al2O3) 1 Silicon (SiO2) 2 Phosphorous (P2O5) 20 Sulphur (SO3) 1 Calcium (CaO) 11 Iron (Fe2O3) 1 Zinc (ZnO) 38 Total oxides (normalized to loss 925 °C) 77 (b)

The surface area of the CO2 cooler was reduced by removing 86 plates out of 254 plates (the surface area was reduced by 34%). The average cooling water velocity inside the gaps was

Fig. 5. Deposits formed on the surface of VT-plate.

Nile river water treated by NALCO inhibitors is used in an open loop as the cooling medium for the CO2 coolers, the specifications of the cooling water used is given in Table 1.


Table 1. Cooling water specifications.

A typical analysis for Nile river water is shown in Table 2.


Table 2. Nile river water analysis.

## **2.3 Problem description and observations**

The cooling water flow rate on the CO2 coolers (HP Scrubber) dropped from 500m³/hr to 300m³/hr due to fouling on the cooling water side, which caused operation problems in the Urea plant. The CO2 outlet temperature was increasing with time and achieved about 50°C after 30 days of operation before the shutdown of the unit for mechanical cleaning. The CO2 cooler was opened for mechanical cleaning; the PHE's inlet was plugged with plastic bags and pieces of bottles. Deposits were accumulated at an area about 20cm from the plate inlet and selectively covered the plate surface, as can be seen in Figure 5. They could plug the channels and restrict the water flow over the plate. These deposits accumulated due to the reduction of the gap velocity (shear stress) which increased the surface temperature.

A sample from the deposits was taken and analysed using ashing and X-ray Fluorescence (XRF). The sample was dried at 105 °C before ashing and XRF analysis. The results are shown in Table 3.

The ashing results showed that 14% of the sample was lost at a temperature below 500°C, which represents the organic material and can be considered as normal range. The XRF analysis showed that the main element in the deposits is zinc hydroxide as ZnO (38%) and

Nile river water treated by NALCO inhibitors is used in an open loop as the cooling medium for the CO2 coolers, the specifications of the cooling water used is given in Table 1.

**CaH Alkalinity Chlorides Inhibitors** 

N-73203: 95ppm

**Nile water** 90 ppm 138 25 ppm N-7356P: 30ppm,

**Substrate Unit Chloride** 77.5 ppm **Ca** 48 ppm **Mg** 14.5 ppm **Na** 60 ppm **K** 9 ppm **Fe** 0.1 ppm **SO4** 57.5 ppm **SiO2** 2 ppm **HCO3** 180 ppm **KMnO4** 10.1 ppm **Total hardness** 172.5 ppm CaCO3 **TDS** 380 ppm **pH** 7.8 - **Alkalinity** 180 ppm CaCO3

The cooling water flow rate on the CO2 coolers (HP Scrubber) dropped from 500m³/hr to 300m³/hr due to fouling on the cooling water side, which caused operation problems in the Urea plant. The CO2 outlet temperature was increasing with time and achieved about 50°C after 30 days of operation before the shutdown of the unit for mechanical cleaning. The CO2 cooler was opened for mechanical cleaning; the PHE's inlet was plugged with plastic bags and pieces of bottles. Deposits were accumulated at an area about 20cm from the plate inlet and selectively covered the plate surface, as can be seen in Figure 5. They could plug the channels and restrict the water flow over the plate. These deposits accumulated due to the

reduction of the gap velocity (shear stress) which increased the surface temperature.

A sample from the deposits was taken and analysed using ashing and X-ray Fluorescence (XRF). The sample was dried at 105 °C before ashing and XRF analysis. The results are

The ashing results showed that 14% of the sample was lost at a temperature below 500°C, which represents the organic material and can be considered as normal range. The XRF analysis showed that the main element in the deposits is zinc hydroxide as ZnO (38%) and

Table 1. Cooling water specifications.

Table 2. Nile river water analysis.

shown in Table 3.

**2.3 Problem description and observations** 

A typical analysis for Nile river water is shown in Table 2.

Fig. 5. Deposits formed on the surface of VT-plate.

the second is calcium phosphate (11%), which participated as a result of the increase of the plate surface temperature resulting from the reduction in the cooling water flow rate.


Table 3. (a) Ashing results, (b) Elemental analysis as oxides using XRF.

## **2.4 Technical solutions**

#### **2.4.1 Redesigning of the PHEs**

The surface area of the CO2 cooler was reduced by removing 86 plates out of 254 plates (the surface area was reduced by 34%). The average cooling water velocity inside the gaps was increased from 0.30 to 0.42 m/s, as can be seen in Table 4.

Fouling in Plate Heat Exchangers: Some Practical Experience 541

standards with low investment costs, operation and maintenance. The optimized OptiWave plate design requires less heat transfer surface for the same performance. The new EcoLoc gaskets and installation methods simplify maintenance and ensure a perfect fit of the gasket and plate packs. The new plates have the advantage of higher gap velocities (shear stress)

In conventional plates the fluid velocity over the plate's width is decreasing, the more the fluid is distributed from the inlet over the whole plate width. This is due to the higher pressure drop in longer flow channels. The optimized fluid distribution channels of the NT series lead to balanced velocity over the whole plate width and an equal distribution of the

The flow channels of the NT-plates vary in their width and were optimized based on Computational Fluid Dynamics (CFD). The channels located further away from the inlet

Fewer deposits were accumulated on the NT-plates due to the asymmetric flow distribution over the channels as can be seen in Figure 8. These deposits were formed because the unit was taken into operation in parallel with the old two VT-plates units and hence most of the cooling water was flowing inside them. The NT-plates unit was designed in principle to

replace one of the VT-plates units so that the gap velocity could be increased.

Fig. 7. Velocity distribution over the NT-plate compared with conventional plates.

due to better fluid distribution over the plates and smaller gap size.

The advantages of the NT-plates at a glance:

 Low investment and service costs Optimized distribution of media

Quick and safe gasket replacement

 Non-standard materials available Leading manufacturer's know-how

Flexible solutions for special requirements

hole have bigger diameter than those closer to the inlet hole.

High heat transfer rates

Simplified handling

medium (Figure 7).


Table 4. Design modification for CO2 cooler in Helwan fertilizer plant, Egypt.

The deposits formed on the surface of the plates were decreased as a result of the increase in the shear stress and the decrease of the surface temperature from 72 to 69°C. The surface temperature was calculated from the fluids temperatures, thermal conductivities and duties on both sides. The operation time for the cooler was increased from 30 days to 43 days and the plates were cleaned after more than 40 days of operation, as shown in Figure 6.

The CO2 outlet temperature started to increase after about 23 days of operation due to the accumulation of deposits on the cooling water side which led to a reduction in the cooling water flow rate. The unit was opened after about 43 days for mechanical cleaning.

Fig. 6. Inlet and outlet CO2 temperatures as a function of time.

#### **2.4.2 New plate geometry**

A new cooler with computer-modeled plate geometry from the NT (New Technology) series was installed in parallel with the existing two coolers. The NT Series sets new economic standards with low investment costs, operation and maintenance. The optimized OptiWave plate design requires less heat transfer surface for the same performance. The new EcoLoc gaskets and installation methods simplify maintenance and ensure a perfect fit of the gasket and plate packs. The new plates have the advantage of higher gap velocities (shear stress) due to better fluid distribution over the plates and smaller gap size.

The advantages of the NT-plates at a glance:


540 Heat Exchangers – Basics Design Applications

The deposits formed on the surface of the plates were decreased as a result of the increase in the shear stress and the decrease of the surface temperature from 72 to 69°C. The surface temperature was calculated from the fluids temperatures, thermal conductivities and duties on both sides. The operation time for the cooler was increased from 30 days to 43 days and

The CO2 outlet temperature started to increase after about 23 days of operation due to the accumulation of deposits on the cooling water side which led to a reduction in the cooling

**Plates number** 254 168 **Gap velocity [m/s]** 0.30 0.42 **Surface tension [Pa]** 5.31 10.84 **Reynolds number** 3259 4599 **Surface temperature [°C]** 72 69

Table 4. Design modification for CO2 cooler in Helwan fertilizer plant, Egypt.

the plates were cleaned after more than 40 days of operation, as shown in Figure 6.

water flow rate. The unit was opened after about 43 days for mechanical cleaning.

Fig. 6. Inlet and outlet CO2 temperatures as a function of time.

A new cooler with computer-modeled plate geometry from the NT (New Technology) series was installed in parallel with the existing two coolers. The NT Series sets new economic

**2.4.2 New plate geometry** 

**Original Design After modification** 


In conventional plates the fluid velocity over the plate's width is decreasing, the more the fluid is distributed from the inlet over the whole plate width. This is due to the higher pressure drop in longer flow channels. The optimized fluid distribution channels of the NT series lead to balanced velocity over the whole plate width and an equal distribution of the medium (Figure 7).

The flow channels of the NT-plates vary in their width and were optimized based on Computational Fluid Dynamics (CFD). The channels located further away from the inlet hole have bigger diameter than those closer to the inlet hole.

Fewer deposits were accumulated on the NT-plates due to the asymmetric flow distribution over the channels as can be seen in Figure 8. These deposits were formed because the unit was taken into operation in parallel with the old two VT-plates units and hence most of the cooling water was flowing inside them. The NT-plates unit was designed in principle to replace one of the VT-plates units so that the gap velocity could be increased.

Fig. 7. Velocity distribution over the NT-plate compared with conventional plates.

Fouling in Plate Heat Exchangers: Some Practical Experience 543

plates also reduced deposit buildup in comparison to the standard stainless steel plates and were almost comparable to the coated plates. The time required for cleaning in place (CIP) with the coated plates was reduced by 70% compared to standard stainless steel plates.

Production problems, like decrease of production rate and increase in the intensity of cleaning procedure, arise in the dairy industry as a result of the deposit adhesion to the plate surface. The deposits must be removed by regular and intensive cleaning procedures in order to comply with hygiene and quality regulations for the dairy industry (Augustin et al., 2007). If not controlled carefully, deposits can cause deterioration in the product quality because milk cannot be heated up to the required pasteurization temperature. Milk deposits generally form so fast that heat exchangers must be cleaned regularly to maintain production efficiency and meet strict hygiene standards and regulations (Bansal and Chen, 2006). Energy losses, lost productivity, manpower and cleaning expenses cause immense costs (Beuf et al., 2003). In the dairy industry, fouling and the resulting cleaning of the process equipment account for about 80% of the total production costs (Bansal and Chen,

Gasketed plate heat exchangers with stainless steel plates are commonly used in the dairy industry. Stainless steel surfaces have high surface energies. The adhesion of product on solid surfaces is determined by the surface roughness and surface energy. The adhesion of deposits could be reduced by either decreasing the surface energy of the metal or by coating the metal surface with high anti-adhesion effect (low surface energy) materials, such as those made of nanoparticles (Gerwann et al., 2002). The application of nano-coatings with their anti-adhesion effects reduces the buildup of deposits on the surface of heat exchanger plates due to the reduction of adhesive forces. The operation efficiency of the plant can be significantly improved and the general hygienic situation of the product can increase. Additionally, intensity and frequency of cleaning can be substantially reduced to achieve

Beuf et al. (2003) studied the fouling of dairy product on modified stainless steel surfaces in a plate and frame heat exchanger. Different surface modifications, such as coatings (diamond like carbon [DLC], silica, SiOX, Ni-P-PTFE, Excalibur, Xylan) and ion implantation (SiF+, MoS2) were analyzed. No significant difference was found between the modified stainless steels and the unmodified one. The cleaning efficiency of plates coated with Ni-P-PTFE was the best. The experimental results of Zhao et al. (2007) showed that the surface free energy of the Ni–P–PTFE coating had a significant influence on the adhesion of bacterial, protein and mineral deposits. The Ni–P–PTFE coating reduced the adhesion of

The fouling behavior of whey protein solutions on modified stainless steel (SS) surfaces coated with diamond-like carbon (DLC) and titanium nitride (TiN) have been studied by Premathilaka et al. (2007). They concluded that fouling decreased in the order DLC > SS >

The goal of the present work is to assess new surface coatings (developed by the Institute of New Materials, INM, in Germany) with low surface energy and low roughness to avoid or minimize adhesion of deposits, simplify cleaning processes, reduce resource and chemical

the desired degree of product quality (Kück et al., 2007).

TiN and cleaning time decreased in the order TiN > SS > DLC.

these deposits significantly.

**3.1 Introduction** 

2006).

Fig. 8. Deposits formed on the surface of NT-plate.
