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The differential equation dP/W derived from the Ergun equation has an Ergun constant, gamma naught, multiplied by ratios of the initial temperature, moles, and pressures to the temperature, moles, and pressure down the reactor. Why does it have this form? The Ergun equation has a term for gas density, which can be related to the ideal gas law and therefore pressure, temperature, and total moles. The Ergun equation has a term for superficial mass velocity, so volumetric flow comes in there, which is determined by the ideal gas law Because total moles aren't changing volumetric flow is constant. The void fraction is greater than zero. A packed bed is simply a cylindrical pipe