We are making tomato paste at 32% solids using a concentrator operating at 0.2 b

Question

We are making tomato paste at 32% solids using a concentrator operating at 0.2 bar absolute pressure, with a capacity of 2 tonnes per hour of tomato juice at 93% moisture content. Calculate how much water will be removed per hour from the juice, and, how much steam is required per hour at 2 bars absolute pressure to concentrate the product.   Assume the juice enters the system preheated to boiling point at the concentrator pressure, and, there are no heat losses. Tips: Solve the first part of the problem by mass balance, second part by heat energy balance. Apply SI units.

Explanation / Answer

As the juice is already at boiling point, we just need latent heat to evaporate it.

Latent heat of water at 0.2 bar = L = 2358 kJ/kgK

m = water evaporated to make juice from 93% to concentrate of 32%

= (0.93-0.32) x 2 tonnes / hr
= 0.61 x 2000 kg / hr

= 1220 kg/hr ... water evaporated

= 0.34 kg/s

Q = heat required = mL

= 0.34 x 2358

= 800 kJ/s

This heat is to be supplied by steam at 2 bar

Steam will give its own latent heat to the juice = L = 2201 kJ/kg

800 = mL

= m x 2201

m = 0.3634 kg/s

= 1308.5 kg/hr steam required to convert juice to concentrate.

* Values of Latent heat taken from
https://www.thermexcel.com/english/tables/vap_eau.htm

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