Satsha Management Services provides engineering support to various manufacturing companies such as paper & pulp mills, caustic manufactures, cotton textile firms and others to adopt cleaner energy alternatives by switching over to Natural Gas based power generation. Currently, most of them are producing power from petcoke, fuel oil or coal. These fuels produce excessively high quantities of harmful SOx and NOx gases in the exhaust gases and pollute the environment.

Switching over to Natural Gas would enable them to meet stricter regulatory norms which Central Pollution Control Board (CPCB) has fixed recently in terms of reduced SOx and Nox emissions, <600 mg/m3 for SOx and <300 mg/m3 for NOx. On the other hand, it can provide several other benefits. For example, fuel cost per unit of power production can be optimized relative to the existing cost depending upon the power generation capacity by choosing right configuration, due to better thermal efficiency of Natural Gas based power plant and lower price of natural gas fuel. It would also save the handling and storage costs associated with  the existing fuel. Further, the space which was being used for storage and handling of the existing fuel can now be used productively. Thus, overall saving in various costs could more than off-set the capital cost on modification of existing plant and machinery depending upon their existing configuration and capacity resulting into overall net saving.

Other alternative of installing SOx and NOx removal facilities could be uneconomic for the smaller capacities of power generation which these firms need.

Satsha Management Services has complete know-how  for modification of existing power plants based on petcoke, coal or fuel oil to natural gas based power plant which includes definition of air compression parameters, fixing flue gas composition and temperature, optimizing net isentropic and actual work produced from gas turbine, fixing the parameters for steam turbine, optimizing steam conditions, optimum definition of steam turbine parameters.

It provides complete engineering solution (based on its own know-how) in the form of techno-commercial feasibility study, design & engineering, and project management consultancy. It suggests the most optimum configuration in terms of steam turbine, gas turbine or combined cycle depending upon the existing configuration, power generation capacity and steam requirement of the plant so as to maximize the net benefit thus creating value to its customers.

Satsha Management Services has designed for one of its esteemed clients the shell-and-tube heat exchanger that handles a fluid with viscosity as high as 17000 cP at 40 ºC. The exchanger cools molasses, a by-product of sugar manufacturing process, from 50 ºC to 40 ºC with cooling water available at 37 ºC. The Process conditions of hot fluid (Molasses) are as under:

Heat duty 83720 KCal/hr
Flow rate 15 m3/hr
Inlet temperature 50 °C
Outlet temperature 40 °C
Density 1400 kg/m³
Viscosity at 40ºC  17000 cP
Viscosity at 50ºC  6000 cP
Specific heat 2500 J/kg K
Thermal Conductivity 0.3 W/mK
Maximum pressure drop 2 bar

Design Constraints

The given Process conditions set following constraints for the design of the exchanger:
i) The extremely high viscosity of molasses (17000 cP at 40 deg C) makes the design of the exchanger pressure-drop-limited.
ii) As molasses side is controlling the heat transfer and its heat transfer coefficient is extremely low due to extremely high viscosity, the exchanger design is limited by heat transfer also. It is to be noted that such a high viscosity leaves little room to increase the velocity due to high pressure drop. Thus, it also causes very low velocity on molasses side. The low velocity reinforces the impact of high viscosity on the heat transfer coefficient. The combined impact of the two factors makes the molasses side heat transfer coefficient very low.
iii) As approach (temperature difference between molasses outlet and cooling water inlet) is very low (3 deg C), it makes the exchanger MTD (mean temperature difference) limited too. In fact, shell and tube heat exchangers are rarely designed for an approach less than 5-6 deg C. As below this limit, a shell and tube heat exchanger becomes highly inefficient from heat transfer point of view. Yet, as per the client’s requirement, Satsha Management Services designed the shell-and-tube exchanger with lowest possible heat transfer area.

Design Philosophy

As molasses side heat transfer coefficient is very low (due to excessively high viscosity) and it is controlling the heat transfer (the other side being cooling water with high heat transfer coefficient), the overall heat transfer coefficient becomes very low. This requires large heat transfer area for the exchanger even for a small heat duty of 83720 kCal/hr.

As molasses has extremely high viscosity, it has been placed on shell side. Putting on shell side and providing 45○ tube lay out (rotated square pitch) creates the tendency for turbulent flow leading to higher heat transfer coefficient and lower required heat transfer area as compared to placing it in tubes. Putting it on tube side would make the flow laminar resulting into very low heat transfer coefficient and making the exchanger excessively large.

In order to ensure a reasonably high velocity of cooling water in tubes, the number of tube passes has been increased to 8. The high velocity in tubes minimizes the fouling tendency of cooling water.
As molasses is supposed to have fouling tendency, U-tube exchanger has been selected to facilitate mechanical cleaning of outer surface of the tubes by way of removal of tube bundle. A minimum cleaning lane of 0.25 inch as required by TEMA has been ensured by specifying tube pitch 1 inch for ¾ inch tube outer diameter.
A 45○ tube lay out (rotated square pitch) has been selected which provides dual advantage in this case. Firstly, it creates induced turbulence on shell side and hence improves shell side and overall heat transfer coefficient particularly when viscosity of molasses is so high. Secondly, it facilitates mechanical cleaning of outer surface of tube bundle.

Double segmental type of baffle has been used to cope up with high pressure drop on molasses side due to its extremely high viscosity. With double segmental baffle, cross flow in shell is divided and hence shell side pressure drop is reduced substantially.