Hydrocarbon Processing - December 2021 - 43

Process Optimization
Feeds
TABLE 1. Parameters impacting regenerator cooling options
Product
Raw gasoline
Light hydrocarbons +
gas O2
Light hydrocarbons +
adsorbed O2
Syngas + solid O2
Natural gas + gas O2
Natural gas +
adsorbed O2
High-sulfur gasoline
Confidential
FCC products
CxHyOz
CxHyOz
Gasoline
Olefins
Olefins
Low-sulfur gasoline
Confidential
% coke
Base
<< Base
<< Base
>> Base
The development of the Z-Sorb low-sulfur gasoline process
is an excellent example of how collaboration between research
and engineering-and taking some calculated risks-can expedite
the time to market of a new process. The author was
part of the team that designed, built and successfully demonstrated
a 6,000-bpd unit within 16 mos of the project award, allowing
ConocoPhillips to license the process before the short
window of opportunity disappeared. Engineering started on
Day 1, based on limited bench-scale data, and progressed while
bench-scale research continued in parallel.
If the research at the bench scale is done correctly, then all
the information needed to design a commercial, high-fines
Group A reactor can be obtained. The only reasons for using a
pilot plant is to build confidence (an admittedly valid reason)
and to verify issues that cannot be addressed at the bench scale.
Some of these issues may be addressed at much lower cost using
cold mock-ups.
CFD capabilities in modeling gas/solids systems have increased
tremendously and will continue to do so. CFD is a useful
tool for supporting successful scale-up of new, large-scale
fluid bed reactors using Group A powders. Unfortunately,
some organizations rely on CFD results too heavily, without
asking if the results make sense.
Many articles have been written on how solids behave inside
a riser and how this may impact reactor yield and gas back-mixing.
Many such articles were written years ago as bubbling beds
were being introduced. Obviously, the design of the riser exit
can cause recirculation of solids, and therefore gas. As for the
rest of the riser, the higher the solids concentration of a riser, the
more " well-behaved " it becomes. In addition, although chemical
plants are designed to operate at full capacity, they sometimes
must operate at reduced rates for extended periods of
time. If a new process cannot handle turndown using riser technology,
then perhaps the use of a riser is not the best choice?
The hydrocarbon processing industry witnesses an occasional
paradigm shift that allows novel and untried approaches
to become viable. However, it is also true that almost every process
used to produce a chemical via the application of fluid beds
has already been examined in the past, in some form or another.
Recipe for successful scale-up. A list of lessons learned
for the successful commercialization of a new, large-scale FFB,
Solids circulation
Base
~ Base
> Base
>> Base
>> Base
>> Base
<<< Base
<<< Base
Regenerator airflow Regenerator temperature
Base
<< Base
<< Base
<< Base
<<< Base
<<< Base
Riser solid hold, vol%
500
100
150
200
250
300
350
400
450
4
5
6
7
8
FIG. 2. Riser solid holdup.
CFB or CFFB process for chemical synthesis includes:
* Catalyst with acceptable characteristics is available
(this is a given)
* Bench-scale research is carried out correctly; this is easier
said than done. Many organizations do not obtain data
correctly. The challenges we face are highlighted by the
use of partial oxidation reactions represented by Eq. 1:
CxHy + O + (N, Cl, etc.) = Product + H2
O + COx (1)
Partial oxidation reactions are favored by reduced
pressure. The lower the operating pressure and the lower
the concentration of hydrocarbon in the total feed, the
higher the per-pass conversion and per-pass yield. Testing
such a process using a low-pressure or an atmospheric
pressure unit, and focusing on adjusting only the
concentration of the hydrocarbon, masks this effect.
This results in lower-than-expected yields when the unit
is scaled up in the pressurized pilot/commercial plant.
Some partial oxidation reactions can also suffer
from the formation of " color bodies. " These are trace
byproduct(s) formed in ppm concentrations or less
when the catalyst is not at its optimal condition.
Color bodies must be removed, sometimes with great
difficulties, to ensure that the end-user product meets
the required color specifications (hence " color bodies " ).
The conditions under which color bodies are formed can
be found only through testing at the lab or pilot plant.
FIG. 3 shows the impact of reactor size on yield for a
Hydrocarbon Processing | DECEMBER 2021 43
9 10
11
12
Riser velocity, m/sec
13
14 15
16
es = 1.1%
es = 1.5%
es = 2.8%
es = 3.7%
es = 5.7%
es = 8.3%
es = 10.4%
~ 700°C
< 450°C
< 450°C
-
-
> 700°C
< 450°C
~ 600°C
Solid flux, kg/m2
/sec
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