Elsevier

Computers & Chemical Engineering

Volume 122, 4 March 2019, Pages 293-305
Computers & Chemical Engineering

Crystallization of calcium carbonate and magnesium hydroxide in the heat exchangers of once-through Multistage Flash (MSF-OT) desalination process

https://doi.org/10.1016/j.compchemeng.2018.08.033Get rights and content

Highlights

  • Dynamic fouling model was developed to predict the crystallization of calcium carbonate and magnesium hydroxide in MSF-OT.

  • For the first time, the model considered the mass diffusion rate and kinetic rate to predict the crystallization of magnesium hydroxide.

  • The results revealed that the fouling in MSF-OT is less than in MSF-BR.

  • The results showed that possible prevention of fouling can be achieved by minimising the divalent ions such as Ca2+ and Mg2+ from the intake seawater.

Abstract

In this paper, a dynamic model of fouling is presented to predict the crystallization of calcium carbonate and magnesium hydroxide inside the condenser tubes of Once-Through Multistage Flash (MSF-OT) desalination process. The model considers the combination of kinetic and mass diffusion rates taking into account the effect of temperature, velocity and salinity of the seawater. The equations for seawater carbonate system are used to calculate the concentration of the seawater species. The effects of salinity and temperature on the solubility of calcium carbonate and magnesium hydroxide are also considered. The results reveal an increase in the fouling inside the tubes caused by crystallization of CaCO3 and Mg(OH)2 with increase in the stage temperature. The intake seawater temperature and the Top Brine Temperature (TBT) are varied to investigate their impact on the fouling process. The results show that the (TBT) has greater impact than the seawater temperature on increasing the fouling.

Introduction

Once-through multistage flash (MSF-OT) process is an earlier version of thermal seawater desalination process that is known for its simplicity and low capital investment compared with the conventional multistage flash desalination with recycle brine (MSF-BR). Since the seawater used in MSF-OT process is less saline than that used in the MSF-BR process, it consumes less thermal energy because of the lower Boiling Point Elevation (BPE) (Baig et al., 2011). However, due to the missing of the recycle brine stream, MSF-OT process consume a large amount of intake seawater that has to be treated with antiscalant to overcome the tendency of the scale of formation resulting in a high amount of consumed chemical additives which increases the operating costs. As a result, most of the desalination plants were replaced by MSF-BR process. However, the situation has changed in recent years resulting in cheap and very effective antiscalant, hence, there is no justification to favour MSF-BR process on the account of the chemical expenses (Husain et al., 2004). In addition, understanding the fouling phenomenon can reduce large amount of the antiscalant used.

Fouling is the accumulation of undesired solid materials at the heat transfer surface. The continuous build-up of the fouling film leads to an increase in the thermal resistance and deteriorates the performance of the plant. In thermal desalination process such as MSF, the phenomenon of fouling is mainly caused by crystallization of alkaline such as calcium carbonate (CaCO3), and at higher temperature, magnesium hydroxide (Mg(OH)2). The (HCO3) normally break down to form CO32− at temperature above 45 °C causing the crystallization of CaCO3 once its solubility limit is exceeded. Shams El Din and Mohammed (1994) conducted experimental study and found that the CaCO3 starts to form above 65 °C and reaches its maximum value at 80 °C while Mg(OH)2 starts crystallizing around 75 °C and increases steadily with temperature. The fouling, in general, is an extremely complex process that may be explained by mass, heat transfer and chemical reaction equations with respect to the properties of the scale material and the water. At heated surfaces, the fouling process undergoes five stages as follows (Kazi, 2012):

  • Initiation: slow nucleation of the fouling species at the surface to prepare the heated surface for more unsteady state growth of scale formation.

  • Transport: it is the transport of the fouling species to the surface by diffusion process due to concentration difference between the bulk phase and the liquid-solid surface. Particle size and the velocity of the bulk play an important role in accelerating or decelerating the transport process.

  • Attachment: it is the accumulation of the fouling species on the surface. Density, elasticity and the roughness of the surface material play an important role in sticking these species on the surface and thus, not all the transported species have to be deposited.

  • Removal: the disengagement of the fouling species from the surface into the bulk phase due to higher velocity, shear force and the roughness of the surface, and,

  • Aging: after a period of time, the strength of the deposited scale can vary with time resulting in break off of the scale into parts.

Although a good amount of studies were carried out on the experimental study of fouling only a handful of such studies focused on the modelling (or attempts to modelling) of fouling in MSF process (Al-Anezi and Hilal, 2007, Hawaidi and Mujtaba, 2010, Mubarak, 1998). Most of these models except (Hawaidi and Mujtaba, 2010, Al-Rawajfeh et al., 2008, Al-Rawajfeh et al., 2014, Alsadaie and Mujtaba, 2017, Said et al., 2012) have been developed and studied on their own but have not been a part of the MSF process models. Al-Rawajfeh et al. (2008) studied the deposition of calcium carbonate in flash chambers in MSF-OT and MSF-BR processes by correlating the deposition of calcium carbonate to the released rate of carbon dioxide in a steady state model based on coupling of mass transfer with chemical reaction. Al-Rawajfeh et al. (2014) extended the work of Al-Rawajfeh et al. (2008) to include deposition of calcium sulphate with calcium carbonate inside the tubes and flash chambers in MSF-OT and MSF-BR processes. However, though the authors pointed out the possible effect that the removal rate could have, their study only considered the rate of deposition but neglected the removal rate. Hawaidi and Mujtaba (2010) developed a linear dynamic model for brine heater fouling to study the impact of fouling with seasonal variation of seawater temperatures using MSF-BR process. The model was lumped and no real focus on the chemistry of water was paid. Said et al. (2012) extended Hawaidi and Mujtaba's study to include the effect of fouling in the stages by development of steady state model of MSF-BR process. Again, the fouling was considered as lumped parameter and no details on the CaCO3 or Mg(OH)2 was presented. However, all the above studies considered the effect of one parameter (temperature) only. The effect of the variation of velocity due to the decrease in the cross section area of the tubes and also the effect of salinity were neglected. Moreover, note that during the accumulation of fouling, the MSF process experiences a continuous change in the tubes surface temperature. Alsadaie and Mujtaba (2017) presented very detailed fouling model that considered the attached and removal rate of calcium carbonate and magnesium hydroxide and also it considered the effect of temperature, velocity and salinity. Though the model considered a combination of kinetic and mass diffusion of calcium carbonate, it neglected the diffusion rate of magnesium hydroxide. Moreover, the model was applied on MSF-BR process and no comparison on MSF-OT was made.

The MSF-OT process operates at low salinity and thus the tendency of fouling is expected to be lower in MSF-OT process than in MSF-BR process inside the condenser tubes and brine heater. Moreover, with the availability and at lower cost of corrosion resistant materials and antiscalant, the world of water desalination is expected to move back to MSF-OT technique for future plants for its simplicity and less energy consumption. Thus, it becomes very essential to study the crystallization behaviour of calcium carbonate and magnesium hydroxide inside the condenser tubes of MSF-OT process.

Therefore, in this work, a dynamic fouling model considering the crystallization of calcium carbonate and magnesium hydroxide is developed to investigate the behaviour of fouling in MSF-OT process with a continuous change in velocities and temperature due to the accumulation of fouling. The mass diffusion of magnesium hydroxide has been neglected in all previous developed models and thus, the proposed model here considers the combination of kinetic and mass diffusion of both calcium carbonate and magnesium hydroxide. The effect of seawater flowrate, seawater, temperature, seawater salinity and top brine temperature was investigated.

Section snippets

Process description

The MSF-OT process, as illustrated in Fig. 1, is an applied desalination method particularly known for its simplicity and a small number of components. The intake seawater at the cold inlet temperature is pumped into the inside of condenser tubes of the last flashing stage in right side (stage 21). The seawater gradually gets heated as it passes through the condenser tubes (heat exchangers) from one stage to another by exchanging the thermal energy from the flashing vapour in each stage.

Fouling mechanism

The calcium carbonate and magnesium hydroxide are known in practice as the alkaline scales. With the increase of the seawater temperature entering the MSF plant, a number of reactions take place as reported by several researchers:2HCO3CO2+CO32+H2O

El Din et al. (2002) and Mubarak (1998) mentioned that the previous reaction can occur in two sequence steps namely:HCO3CO2+OH

Followed by fast acid neutralization stepOH+HCO3CO32+H2O

Segev et al. (2012) and Ø. Olderøy et al. (2009) reported

Fouling model

The proposed dynamic model considers the growth and removal rate of the fouling. The net deposit rate then can be calculated as the difference between the total deposition rate and the removal rate. The model also takes into account the physical properties of the seawater and the soluble species in the seawater.

In mathematical term, the net deposition rate may be estimated as the different between the total deposition rate and the removal rate.dmdt=dmddtdmrdt

Results and discussions

The proposed dynamic fouling model is coded using gPROMS model builder v4.2 and then embedded into MSF-OT process dynamic model. The seawater temperature, top brine temperature, flowrate and salinity concentration of the intake seawater are varied to observe their effect on the fouling behaviour. The chemical composition of the intake seawater that used in this work is presented in Table 1.

The simulation of MSF-OT process is run for a period of 100 days (assuming no use of antiscalant) to

Conclusions

In this work, dynamic fouling model was developed to predict the rate of deposition of calcium carbonate and magnesium hydroxide in MSF-OT process. Short description of the process and the expected fouling mechanism were presented. Several parameters such as temperature (Top Brine Temperature and Seawater Temperature), flowrate and salinity were altered to investigate their effect on the scale formation in MSF-OT plant.

The outcome results indicated that the crystallization of CaCO3 and Mg(OH)2

References (28)

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