Adaptations to Support Future Process Optimisation

Two additional features have been introduced to the process design model to support future process optimisation. The two features are located in the SNG-upgrading subsystem.

The first is a revision of the compressor which re-compresses the gas of the SELEXOL flash for recirculating it to the feed of the absorber column (stripper column and recompression not illustrated in figure 2.5). This recirculation accumulates the less soluble gases in the loop to increase the recovery rate in the main outlet of the absorber column. Since the pressure difference between absorption and regeneration of the SELEXOL may later be subject of optimisation and the corresponding pressure ratio pabsorber

pflash may be rather high, the applied

compression system was revised. The revision allows a more realistic representation of the amount of heat and power utilised and of the temperature at which the heat streams are available. The original representation of this compression was a single stage compressor which resulted in available heat at high temperature after the compressor if a large compression ratio was required. Ulrich and Vasudevan [108, p. 158] state that for a compression ratio larger than a factor 4 the compression units must be staged. With compression ratios 4 and larger the isentropic discharge temperature of a diatomic ideal gas reaches 200◦C. Temperatures higher than that can cause serious damage to lubricants, seals, and other sensitive materials [108, p. 158]. Therefore, an integrated selection between a 1-stage, 2-stage, and 3-stage

2.3. Process Design Model for the Case Study

compression solution with intermediate cooling has been modelled as illustrated in figure 2.7. This will improve the confidence level of the heat integration calculations because in a 3-stage compression system, the total amount of heat is available at a lower temperature than in a single stage compression system. Depending on the pressure ratiorp= p_pout_in, one of the

compressor lines is utilised so that each compressor has a maximum compression ratio of factor 4. Two stream splitters (S1 and S2) split the input stream depending on which pressure ratio is requested. The split fractionss1ands2are defined by eq. (2.3), respectively. To allow a

smooth transition similar to a rapid switch between two technologies, a transition factor ftris

applied. This transition factor is dependent on the difference between the desired pressure ratio and the actual pressure ratio∆rp=rp,desired−rp.

s=ftr·0.9999+0.00001=

erf(70·∆rp)+1

2 ·0.9999+0.00001 (2.3)

The factor 70 in eq. (2.3) controls the width of the transition range observable in figure 2.7. Increasing this factor reduces the width of the transition and decreasing the factor widens the transition.

For the split fractions1the desired pressure ratio is set to 4 and for the split fractions2it is set

to 16. This means that unless the pressure ratio exceeds the factor 4, the single compressor line is the utilised. If it exceeds the factor 4 and is lower than 16, the 2-stage compressor line is the main compression route. For every pressure ratio larger than 16, the 3-stage compressor line is used. According to eq. (2.3) the split factor has a minimum of 0.00001. Consequently, all three compressor lines are actually utilised simultaneously. However, two of the three lines are operated with a very low mass flow so that they have only negligible influence on the final result. Economic calculations may be applied in correspondence to the primarily utilised compression line. The presented solution of the 3-lane compression system may be generally applied to all process streams which require a compression over a large pressure range.

The second feature which was implemented into the superstructure concerns the pressure adjustment of a subsystem’s feed stream. The applied solution is an integrated pressure ad- justment of the gas upgrading system’s feed stream. This feature utilises a combination of a compressor and a pressure release valve, see figure 2.8a. To illustrate the reason of this implementation one has to recall that the superstructure is an independent set of process unit models. If an output of one unit model has to be combined with the input of another, the intensive variables such as temperature and pressure are passed from the output to the input stream. This takes place independent of whether the pressure of the first process unit is lower than the pressure of the second process unit or higher. Thus, a pressure and a temperature correction has to be applied on every input stream, so that the required temperature and pressure in the corresponding unit technology can be guaranteed. A temperature correction via a heat exchanger was previously already applied. Also a pressure adjustment was con- sidered, however, only a compressor was utilised. Situations in which the outlet pressure of

S1 S2 M1 p_in p_out rp=ppout_in rp≤4 rp>4 rp≤16 rp>16

(a)Process flowsheet of the 3-lane compres- sion system. ∆rp=rp,desired−rp Transition Facto r ftr -0.1 -0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08 0.1 0 0.2 0.4 0.6 0.8 1 -0.1 -0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08 0.10 0.2 0.4 0.6 0.8 1 erf(70·∆rp)+1 2 1−(erf(70·∆₂rp)+1)

(b)Transition factor for smooth transition between two com- pressor lines.

FIGURE2.7:Application of a self-adjusting 3-lane compression system which applies a 1-stage, 2-stage, or 3-stage compression line.

the previous subsystem is higher than the required pressure in the subsequent subsystem were previously not handled in an integrated manner. The pressure had always to be adjusted in a pre-computation step which required to know the outlet pressures of previous process units a priori. Thus, the pressures in the corresponding streams had to be defined as decision variables. This is detrimental if pressure drop equations are going to be implemented in the future. The combination of a compressor and a valve in series is suitable concept to avoid the described issue.

Figure 2.8a illustrates the solution which should be applied to adjust any input of a subsystem in the superstructure. It depicts the use of a compressor operated in series with a pressure release valve and a heat exchanger. Ideally, the depicted single compression stage may be exchanged with the above presented 3-lane compressor system if large pressure ranges are expected. Figure 2.8b depicts how a transition between pure pressure relief and compression is achieved. To allow a smooth transition the pressurepmof the middle stream between the

compressor and the pressure relief valve is evaluated in dependence of the applied pressure difference∆p=pset−pin, see eq. (2.4).

pm=   ∆p 2 + s µ∆_p 2 ¶2 +0.0005  +pin=padj+pin (2.4)