Page 195 - Fluid Catalytic Cracking Handbook

Unit Monitoring and Control 165

as preheat temperature. Depending on conversion level, catalyst
type, and feed quality, the heat of reaction can vary from 120 Btu/lb
to 220 Btu/lb.
In the unit, the heat of reaction is a useful tool. It is an indirect
indication of heat balance accuracy. Trending the heat of reaction on
a regular basis provides insight into reactions occurring in the riser
and the effects of feedstock and catalyst changes.

Example 5-6
Reactor Heat Balance

I. Heat into the reactor
6
1. Heat with regenerator catalyst = 3.169 x 10 Ib/hr x 0.285 Btu/lb-°F
6 6
x 1,309°F = 1,182.4 x 10 Btu/hr = 1,182.4 x 10 Btu/hr
2. Heat with the fresh feed:
At a feed temperature of 594°F, °API gravity = 25.2 and K factor
= 12.08, the feed liquid enthalpy is 405 Btu/lb (see Figure 5-7),
therefore, heat content of the feed is = 658,914 Ib/hr x 405 Btu/lb
6
= 266.9 x 10 Btu/hr.
3. Heat with atomizing steam:
From steam tables, enthalpy of 150 Ib saturated steam = 1,176
Btu/lb, therefore, heat with steam = 10,000 Ib/hr x 1,176 Btu/lb =
6
11.8 x 10 Btu/hr.
4 Heat of adsorption:
The adsorption of coke on the catalyst is an exothermic process;
the heat associated with this adsorption is assumed to be the same
6
as desorption of coke in the regenerator (i.e., 35.3 x 10 Btu/hr).
6
Total heat in = 1,182.4 + 266.9 + 11.8 + 35.3 = 1,496.4 x 10 Btu/hr.
II. Heat out of the reactor
6
1, Heat with spent catalyst = 3,169 x 10 Ib/hr x 0.285 Btu/lb-°F x
6
972°F = 878 x 10 Btu/hr.
2, Heat required to vaporize feed:
From Figure 5-8, enthalpy reactor vapors = 778 Btu/lb, therefore,
heat content of the vaporized products = 658,814 Ib/hr x 778 Btu/lb
6
= 512.6 x 10 Btu/hr.
3. Heat content of steam:
Enthalpy of steam @ 972°F = 1,519 Btu/lb, therefore, heat content
6
of steam = 10,000 Ib/hr x 1,519 Btu/lb = 15.2 x 10 Btu/hr.
4. Heat loss to surroundings:
Assume heat loss due to radiant and convection to be 2% of heat
6
with the regenerated catalyst (i.e., 0.02 x 304.4 = 6.1 x 10 Btu/hr)