Steam systems are used to generate power, and provide heat energy. They are foun
ID: 701181 • Letter: S
Question
Steam systems are used to generate power, and provide heat energy. They are found in power plants, chemical plants, refineries, and even residences. In this flowsheet, liquid water may contain impurities (dissolved solids), or may be pure. Steam is always pure water. Liquid water from two streams is fed to a Boiler: Stream 1 is from an outside source, and Stream 7 is water recovered from the system. The water in Stream 1 contains 30 wt. ppm of dissolved solids. Some of the liquid water fed to the Boiler is purged from the system in stream 2 in order to control the concentration of these impurities to 120 wt ppm and therefore prevent fouling and corrosion. The boiler vapourizes the vast majority of the water creating 100.0 kg/s of pure water steam at 300 psig. The steam is sent to the Users (could be a plant, apartment units to be heated, etc.) where it is condensed and sent to a unit operation called the Deaerator, which operates at 10 kPag. In the Deaerator, traces of oxygen are removed, but 3% of the feed to the Deaerator is vapourized and leaves the system. The remaining water is then pumped to high pressure and fed back to the Boiler Users Boiler Deaerator (plant, units, etc.) 7 Pump a) Start with the general form of the material balance. What assumption(s) are appropriate for this process and what do they mean for the material balances? b) Develop a stream table to go with this flowsheet. Include both mass fractions, and total mass flow rate of each stream. Also include the pressure and temperature for each stream. Enter the data you know from the description, but do not do any calculations. c) Identify the steam pressure in each stream, and report them in kPaa. Use the steam tables in the back of FRB to estimate the temperatures of the streams.Explanation / Answer
(a). First we list all the stream names:
Incoming feed to boiler: Stream 1,A
Incoming recovery feed to boiler: Stream 7,R
Purge from boiler: Stream 2,P
Outgoing from boiler,incoming steam to user:Stream 3,B
User to deaerator: Stream 4,C
Vaporized product from deaerator: Stream 5,D
Recovery water at less pressure from deaerator: Stream 6,L
Note stream 7 and stream 6 have same mass only change in phase may take place due to pumping.
Lets assume a time period of unit second of operation of given system: then as per given statement stream B is 100 kg.
All of this 100 kg is used by users and then at deaerator some of the oxygen which adds during usage is separated in two parts stream 6 gets 0.97 of stream 4(which is 100kg steam plus some trapped oxygen) now stream 6 is also 100 kg
This means 0.97*C=100
C=103.09 kg and stream 5 i.e. D=3.09 kg oxygen
Here important assumption is that pure steam is fed to users, then stream 6 and hence stream 7 has only pure steam of 100 kg.
Material balance for inert solids(impurities in feed):(mg)
A(0.30) + 120 = P(x), x is mass fraction of inert in purge
MB across boiler:
A + R(=100) + deposits = P + B(=100)
Deposits are 120 wt ppm or mg/l so real unkowns here A,P & x. Also boiler capacity should be given but its okay we can take B as reference to calculate deposits.
(b).
As far pressure is concerned its 300 psig for stream 3 and it condenses to stream 4 , deaerator works at 10 kpag so stream 5 and 6 are at 10 kPag. Temperatures can be found out from steam tables given pressure and mass.
(c). Suggested answer from (b)
Note 14.7 psig = 101.325 kPa
Stream 1 2 3 4 5 6 7 Overall mass(kg) Not known Not known 100 103.92 3.92 100 100 Mass fraction 0.99997 Not known (x) 1.0 0.97 1.0 oxygen 1.0 1.0