function f=smoker(xb,Ns,alpha,xd,R,zf,q)
% This function uses analytical solution for binary system
% For a given Ns, it solves for xb using "fzero" or "fsolve"
% alpha - relative volatility
% R     - reflux ratio
% xd    - distillate comp. mole fraction
% xb    - bottom comp. mole fraction
% zf    - feed composition
% q     - feed thermal condition
%E.g. opt=optimset('Display','Iter')
%e.g. fsolve('ass6_2b',0.04,opt,6,3.8,.98,1.7113,.45,.55)
% Solve for (xi,yi) and store in vector xi

a(1,1) = R/(R+1); a(1,2) = -1; b(1) = -xd/(R+1);
a(2,1) = q/(q-1); a(2,2) = -1; b(2) = zf/(q-1);
xi=a\b';

% Striping section

m=(R*zf+q*xd-(R+q)*xb)/((R+1)*zf+(q-1)*xd-(R+q)*xb);
b= (1-m)*xb;

% Compute coefficients of quadratic.

c(1) = m*(alpha-1);
c(2) = m + b*(alpha-1) -alpha;
c(3) = b;

% Determine the roots K1, K2 and number of trays
temp = roots(c); K1 =min(temp); K2=max(temp);

f = Ns-log(((xi(1)-K1)/(xb-K1))*((K2-xb)/(K2-xi(1))))/ ...
   log((1+(alpha-1)*K2)/(1+(alpha-1)*K1));