*3.1. Determination of Pressure Coefficient b*

Since the brine outlet pressures are not provided the above references, alternative methods must be used to estimate *b*.

Although the study of Avlonitis et al. [29] does not give a value of the friction coefficient *k f b*, the value for this coefficient is given by the study of Senthilmurugan et al. as 2.5008 <sup>×</sup> <sup>10</sup><sup>8</sup> <sup>m</sup>−<sup>2</sup> [21]. This value is used together with the temperature and feed concentration from a selected feed condition from the 32 data points of Avlonitis et al., in this case 20 ◦C and 35 g/L are used together with Equations (35) and (19) to give the value of coefficient *b*.

For the Saehan module, the study of Mane et al. provides a value of the friction coefficient *<sup>k</sup> f b* <sup>=</sup> 5.18 <sup>×</sup> <sup>10</sup><sup>10</sup> <sup>m</sup>−<sup>2</sup> [23]. However, using this value leads to a calculated pressure drop which is much greater than the maximum pressure drop specified by the manufacturer for similar modules [40]. For this reason, the *b* value is estimated for this case using the maximum pressure drop and the highest tested flow rate in Equation (20).

The estimated values of b are in this way given in Table 2.
