Week 6 Challenge

Challenge - 6 Thermal Design of Heat Exchanger

Question 1. Generate TEMA datasheet for the shell and tube heat exchanger with 20 percent over design for the following

a. Hot side fluid – Debutanizer column vapor

b. Cold side fluid – Cooling water

c. Exchanger type – Condenser

d. Vapor flow rate – 2500 kg/hr

e. Vapor composition

i. Propane – 0.1 mol %

ii. i-butane – 0.4 mol %

iii. N-butane – 0.5 mol

f. Operating pressure for hot side fluid – 3 barg

g. Cooling water inlet temperature – 30°C

h. Cooling water outlet temperature – 45°C

i. Cooling water inlet pressure – 1 barg

j. Allowable shell side pressure drop – 0.1 bar

k. Allowable tube side pressure drop – 0.2 bar

Answer

In order to create TEMA sheet with 20% over thermal design of heat exchanger, it is important to first calculate the process design using the above specification and find the unknown parameters in the process.

Scenario – Debutaniser column vapor with the given composition has to be condensed with the help of cooling water in the condenser heat exchanger. Based on the specifications in the table, assumption must be made in order to calculate the duty of the condenser.

However, when the vapor fraction of condensed vapor is assumed as 0 which means all the vapor is going to condense in the liquid state, the temperature of it is calculated as 28.7°C which is lower than the cooling water inlet temperature and there appeared to be a temperature cross. To avoid the temperature profiles crossing, the cooling water inlet and outlet conditions are slightly modified to get the minimum approach temperature at least greater than 10°C.

Process Stream / Column Vapor	Utility Stream / Cold Water
Vapor Flowrate = 2500 kg/h Operating Pressure = 3 barg Vapor Composition = 0.1 Propane, 0.4 i-butane and 0.5 n-butane Since this is vapor stream it will be on shell side and the allowable pressure drop = 0.1 bar Assume inlet temperature = 45°C which gives vapor fraction = 1 Assume vapor fraction of outlet = 0 (since this is a condenser) which gives the tepmerature = 28.7°C	Cooling water Temperature (inlet) = 15°C Cooling water outlet temperature = 30°C Cooling water inlet pressure = 1barg Allowable tube side pressure drop = 0.2 bar

Given composition is noted.

Provided all the input data that is slightly modified cooling water inlet and outlet temperatures to avoid close temperature ranges.

Thermal Design on EDR - (Exchanger Design and Rating)

After Inputting all the process data by importing the process data file, the process data input gets automatically filled up. After importing the process design base case, select the Rating/checking mode so that we have a control over the other parameters and hence achieve the 20% over design.

Next, input the heat exchanger geometry, by trail and error method with the objective of getting the percentage over design only 20%. There are number of possibilities to get the 20% over design while following the TEMA guidelines.

The TEMA standards to keep note here inorder to avoid error warnings:

• Baffle %cut should be in the range of 10-20% and this will also affect the E stream percentage.
• Baffle spacing, baffle number, Tube length, shell ID and Tube number are all interdependable.
• No vibration and no (ρv^2) violation.
• Allowable shell side and tube side pressure drop criteria must be followed.

In the flow analysis

• the B stream should be at least 80%
• A stream should be 5-10%
• E stream should be 15-20%

In an attempt to increase the E stream to 15-20%, the baffle cut percentage is reduced. This has increased the E stream to 10-12% but the A stream also increasing and as a result, it is reducing the B stream to 0.74.

Therefore, the optimisation stopped here and once agian checked all the specifications provided with 20% over design and then generated the TEMA sheet which is attached below.

TEMA Sheet