Page 272 - Design of Simple and Robust Process Plants

258 Chapter 7 Optimization of an Integrated Complex of Process Plants
. One plant specifically delivers this stream to one downstream plant; the
stream then belongs to that plant, and the total plant reliability (including the
supply plant) should be studied and used for decision making.
. The producing plant supplies the stream to other consuming plants:
± The first option according to the design philosophy is to build a back-up facil-
ity such as a pipeline connection to an alternative source. For the alternative
source, make sure that availability and reliability are part of the contract (be
aware of all gas streams, steam and power supplies).
± The second option is to design the producing plant at a very high reliability.
± The third option is to build a load-shedding system to prioritize the consumer
plants. This last option could be additive to the first two options. Load shed-
ding is only applicable if there is limited availability.
. Shortage may occur in the supply and demand balance due to variable capa-
city operations of producer(s); in that case a back-up facility might be consid-
ered.
The site vulnerability study should include the gas feed streams.

7.5.1.2 Lower-grade product streams for internal usage
The potential of process streams between plants at a lower than commercial specifi-
cation can sometimes provide a major benefit. The potential can be attractive tech-
nology-wise for components, which are difficult to separate in the upstream plant.
In the consuming plant the product is often removed by reaction, and the impurity
is concentrated and recycled to the upstream plant. A well-known example is that of
the cumene (iso-propyl-benzene) plant. In this process, propane is recycled over the
reactor while the conversion of propylene is 100%. The option to supply a lower-
grade propylene, with propane as the major impurity, from a refinery off-gas or ole-
fin plant can be very attractive for a grass root design. This option is also exploitable
for an expansion project of the upstream plant, in order to avoid expensive invest-
ment. The back-up for these streams is mostly carried out with commercial grade
product to avoid any additional storage for the lower-grade material.
Ethyl benzene plants have the ability to process lower-grade ethylene, with ethane
as the major impurity. All ethyl benzene processes ± the aluminium chloride route
as well as the gas and liquid fixed bed, zeolite catalysed, routes ± have this option
(Netzer, 1997). The option of consuming lower-grade benzene in the ethyl benzene
process is discussed by Netzer (1999), who presents the benefits of integrating ethyl
benzene production with an olefins plant.
Other examples are the consumption of lower-grade styrene for poly-styrene, as
well as the use of wet ethylene oxide for ethylene glycol production or wet butadiene
for latex production. The removal of iso-butylene from a C 4 mixture is widely
applied to the reaction with methanol to form methyl tert-butyl ether (MTBE).
Analyzing applications which might be attractive for the utilization of lower-grade
reactants highlighted the following conditions as having potential for further evalua-
tion: