Cleaning equipment

To understand the effect of varying fluid parameters on cleaning, studies were done at laboratory and pilot scale on a variety of equipment.

3.5.1. Ab Aziz PIV Rig

This laboratory scale cleaning rig was described in Ab Aziz (2007) and the principles referred to in Section 2.4. In brief, this rig consisted of a stainless steel horizontal duct (width 32 mm, height 7 mm), with a test section positioned midway along the length of the horizontal duct (1250 mm). The base of this test section was made of stainless steel with a glass side and top and a circular gap in the base of the test section, in which a coupon of surface diameter 26 mm diameter was secured. Flow rates up to 7 l min-1 were possible. Water was heated in a coil heat exchanger. The coupon was fouled with a layer of toothpaste (0.8 g) and glued in to the duct. The duct was then secured to the heat flux sensor unit described in Section 2.4. Images were captured from above the glass section containing the coupon, and heat flux data captured from beneath the coupon during the cleaning experiments. A picture of the rig is shown in Figure 3.9.

Figure 3.9: Image of Ab Aziz (2007) cleaning rig, fouled coupon is positioned in circular opening in the centre of the horizontal pipe, on top of a heat flux block described in Section 2.4. Cleaning fluid is then passed through the system to clean the fouled coupon.

This rig was used to allow comparison with other deposits from previous works. Paste N toothpaste was studied at flow rates of 1.5, 5 and 7 l min-1, corresponding to a mean velocity for a clean duct of 0.11 m s-1, 0.37 m s-1 and 0.5m s-1 and temperatures of 30, 50, 70°C to allow direct comparison of these materials with those used by Christian (2003) and Ab Aziz

61

(2007), and allow the study of different cleaning mechanisms from circular vs. square

coupons to be captured.

3.5.2. Coupon Rig

This set-up is referred to as the Coupon Rig in this thesis. This rig is based on the principle of the previous horizontal ducts used in cleaning studies at the University of Birmingham [Ab

Aziz (2007), Christian (2003)]. The horizontal duct (15 mm x 35 mm z 140 mm) has a

hydraulic diameter as defined in equation 3.3.

dh = 4 A / p , dh = 0.021 m (3.3)

where dh = hydraulic diameter (m), A = area section of the duct (0.000525 m2), p = wetted perimeter of the duct (0.1 m)

This equipment was however designed to be more flexible than the previous rigs, enabling different size square or rectangular base coupons (normal size coupons 0.025 m, large different surface coupons, 0.035 m) to be used. The base is removable to enable ease of cleaning for the glass section and avoid the need to invert the equipment to glue in the coupons. This redeveloped rig allowed side imaging. The schematic of this rig is shown in Figure 3.10.

62 Drain V7 Chemical solution tank Water tank Temperature controller Pump V1 V3

Test section with fouled coupon in place.

Tc4

Tc5

Tc6

C

Signal conditioning unit

PC, Labview V2 V4 V5 V6 Flow meter

Heating tank with three 3kW heaters Mixer 1 Tc1 Camera Tc2 Tc3 MHFS

Copper stub with Tc2,

Tc3and MHFS in place

Figure 3. 10: Schematic of the new cleaning rig, known as the Coupon Rig in this work (diagram from Goode et al., (2010)), a coupon with foulant on is secured into the base of a horizontal duct in a glass section which allows observation from above and side, the coupon is positioned above a heat flux sensor allowing the removal of the foulant from the coupon to be monitored. The duct is attached to a heat exchanger reservoir and pump and these are used to supply cleaning fluid to the cleaning rig.

The rig was developed by Dr. Konstantia Asteriadou, University of Birmingham, with workshop assistance from Unilever HPC and the workshop at the University of Birmingham. The coupon rig comprised of a heating tank, a centrifugal pump, a flow transmitter PD 340, rectangular flow channel with test section, a conductivity and temperature meter at the return (LMIT08), thermocouples and data logger system. The test section held the fouled stainless steel coupon, (1.3 g toothpaste) which sat upon the heat flux sensor unit. It was surrounded by a clear quartz type glass to allow observation of cleaning from both top and side view. Readings from thermocouples and MHFS (Micro Heat Foil Sensor) are collected by a signal conditional unit (SC-2345) and fed into Labview (NI 4.4.1) software on a standard P.C.

63

Cleaning progress is characterised by the increase in heat transfer coefficient (U) as the insulating deposit layer is removed, until the value is similar to that of a clean surface. The calculations for heat transfer coefficient are given in Section 2.4and presented by Christian

(2003). The camera used was a Canon EOS 30D, which was manually focussed, with the

flash turned off, and positioned on a tripod stand. The time intervals were set with a timer unit. The experimental protocol is given in Appendix 4. The cleaning rig experiments were monitored by sequential images which were then analysed using the software Image J. This translated to a measurement profile which had a high initial value corresponding to a fully fouled surface, which then reduced over time until a clean surface is achieved. A typical profile is presented in Section 4. 2.

Experimental program (Coupon Rigs):

Square coupon system using Paste T, (surface L, 1.30 g ± 0.06 g) at temperatures (20°C, 40°C and 50°C) and velocities (0.25 m s-1, 0.37 m s-1, 0.5 m s-1), velocities based on a clean duct

Different pastes were investigated using surface L, at temperature 40°C, and velocity 0.37 m s-1 based on a clean system, where the coupon was coated with 1.30 g ± 0.06 g of paste, which was placed on the coupon as evenly as possible. This was to allow comparison between pastes.

Di

fferent surfaces: stainless steel, glass, acrylic and PTFE, (0.025 m and 0.035 m square) as well as different levels of surface finish for the stainless steel (coupon size), have been investigated at cleaning conditions of 40°C, and 0.25 m s-1 (based on the velocity of a clean tube).

Additionally the Coupon Rig was adapted to fit a 1 m section of 23.9 mm diameter pipe. This was connected by triclamps in the place where the normal horizontal duct section was fitted. The line was converted for the Kemtrac turbidity meter by including expanders and reducers to the pipe line to allow the 47.7 mm diameter meter to be added via some flexible tubing and triclamp fittings. This was to allow a „scale- up‟ set-up with the laboratory equipment parameters which could be compared with results on the pilot plant.

64 Surface properties for Coupon Rig study:

For the small surface study conducted on the Coupon Rig. The stainless steel coupons, A-E, were all the same size (25 mm square) and coated with 1.30 g ± 0.06 g of Paste T, for the different types of surfaces, the surfaces were 35mm square with a surface coating of 2.3 g ± 0.06 g. This resulted in the same surface covering per area for both coupon sizes (deposition mass 0.002 g mm2)). The surfaces have been characterised by surface roughness measurements and indicative values of contact angles gained from work by Ahktar (2010).

The surface roughness measurements were performed by Alan Saywell (University of Birmingham) using a Talysurf machine, and the results reported in Table 3.1. The measurements are sensitive to 0.00015 μm. The laser beam resolution is 10 nm on measurements up to 6 mm.

Table 3. 1: Surface roughness measurements for coupons used in cleaning studies on the Coupon Rig. Surface measurements performed by Alan Saywell (University of Birmingham) using a Talysurf machine .

Surface Types Surface Finish (Ra µm) ± 0.15 µm

Stainless steel 1.05

Polypropylene 0.77

PTFE 1.36

Glass 0.01

Acrylic 0.01

Stainless steel (A) 0.50

Stainless steel (B) 0.85

Stainless steel (C) 1.23

Stainless steel (D) 1.65

Stainless steel (E) – mirrored 0.05

Stainless steel (L) 0.17

Indicative values for contact angles for the different surface types have been gained from work done by Akhtar (2010) for sorbitol (a key toothpaste ingredient) and water as presented in Figure 3.11.

65

Figure 3.11: Contact angles for different surface types as reported in Akhtar (2010) for both sorbitol (a key toothpaste ingredient) and water.

Contact angles are indicative of the wetability of a surface, if the contact angle is less than 90°, then the surface if considered hydrophilic, i.e. the water will spread out over the surface, if the contact angle is greater than 90° the surface is considered hydrophobic. From this contact angle data from Akhtar (2010) it is possible to see that PTFE and glass are hydrophobic surfaces but when sorbitol is placed on glass then it has a significantly reduced contact angle around 100° which is similar to the wetting seen with stainless steel.