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Refrigerant price 13.11 $/GJ [28] Electricity price 0.049 $/kWh [29] Fuel price 2.98 $/MSCF [30] Plant operator rate 32.74 $/h [31] Plant supervisor rate 68.13 $/h [32] Maintenance 5 % of FCI [25] Operating charges 25 % of operating labor cost [25] Plant overhead 50 % of operating labor + maintenance cost [25] General administrative 8 % of all other operating costs [25] Stream factor 0.96 — —

      The only additional equations used were those to estimate the number of workers based on the number of processing steps [28]:

      (2.2)

      where N_np, the number of non‐particulate processing steps, which is related with the number of operators per shift (N_OL), given as in Eq. (2.3):

(2.3)

      where P, the number of processing steps where particulate solids are handled.

      2.4.2.1 Changes in Fixed and Variable Costs

Additional cases were fed through the base case process design, and any needed modifications were made to meet product specifications. Therefore, fixed and variable costs were first estimated as before, and then only the change in each of these costs was used in the subsequent calculations. For fixed costs, the change was only used in cases where the costs were higher, since the need for smaller equipment does not result in any savings for an existing plant.

      2.4.2.2 Revenue

Table 2.5 shows the price of each component [33]:

Table 2.5 Price of different commodities.

Commodity	Units	Base case
Heat valuea	$/MMBtu	2.98
Ethane	$/gal	0.262
Propane	$/gal	0.632
n‐Butane	$/gal	0.691

      ^a Methane and wellhead gas are priced in terms of their heat value ($/MMBtu).

      2.4.2.3 Economic Calculations

      The next step was to calculate the return on investment (ROI) [26]:

      (2.4)

      where TCI, total capital investment.

First, the quantities in Eqs. must be calculated.

      (2.5)

      where FCI, total fixed capital investment:

      (2.6)

      where WCI, working capital investment (assumed to be 15% of FCI):

      (2.7)

      where AFC, annualized fixed cost (depreciation); FCI_S, the salvage value of the FCI (assumed to be 10% of FCI); and N, plant lifetime (assumed to be 10 years):

      (2.8)

      where AOC, annual operating cost; VC_i, total variable cost for each process unit:

      (2.9)

      where TAC, total annualized cost:

      (2.10)

      (2.11)

      where the tax rate is assumed to be 30%.

      For the other cases, the calculations are similar, except instead of fixed and variable costs only the additional costs were determined. This is done because we are considering only the additional costs for processing a new composition. Similarly instead of ROI, incremental return on investment (IROI) is calculated instead:

      (2.12)

      where ΔTCI, the change in total capital investment for a given additional case.

      Finally, the total ROI can be determined for treating multiple feeds from the following:

(2.13)

      where p, probability or likelihood of obtaining a particular feed, and the subscript f denotes feed.

      2.4.3 Safety Index Calculations

A modified version of the PRI was used to evaluate each case in order to incorporate safety considerations into the design process. These calculations were done following the method of Leong and Shariff [18], but the flow rate of each stream was incorporated as follows:

      (2.14)

      where

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